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. 2022 Aug 24;118(3):425–441. doi: 10.1002/jeab.795

Peripheral vision in matching‐to‐sample procedures

Live Fay Braaten 1,✉, Erik Arntzen 1
PMCID: PMC9805225  PMID: 36053794

Abstract

Eye‐tracking has been used to investigate observing responses in matching‐to‐sample procedures. However, in visual search, peripheral vision plays an important role. Therefore, three experiments were conducted to investigate the extent to which adult participants can discriminate stimuli that vary in size and position in the periphery. Experiment 1 used arbitrary matching with abstract stimuli, Experiment 2 used identity matching with abstract stimuli, and Experiment 3 used identity matching with simple (familiar) shapes. In all three experiments, participants were taught eight conditional discriminations establishing four 3‐member classes of stimuli. Four different stimulus sizes and three different stimulus positions were manipulated in the 12 peripheral test phases. In these test trials, participants had to fixate their gaze on the sample stimulus in the middle of the screen while selecting a comparison stimulus. Eye movements were measured with a head‐mounted eye‐tracker during both training and testing. Experiment 1 shows that participants can discriminate small abstract stimuli that are arbitrarily related in the periphery. Experiment 2 shows that matching identical stimuli does not affect discrimination in the periphery compared to arbitrarily related stimuli. However, Experiment 3 shows that discrimination increases when stimuli are well‐known simple shapes.

Keywords: peripheral vision, eye‐tracking, matching‐to‐sample, contingencies on eye movements, university students

When participants learn conditional discriminations in a computerized MTS procedure, a trial starts with a sample stimulus presented on the screen. Participants have to respond to the sample stimulus, and immediately two or more comparison stimuli appear on the screen. Whether the comparison stimuli appear in a row on the bottom of the screen or in the four corner positions, participants must respond to a comparison stimulus that matches the sample stimulus. Participants learn the relation between stimuli due to reinforcement contingencies that the experimenter arranges. For example, in arbitrary matching, reinforcement is provided for responding to stimulus B in the presence of stimulus A and to stimulus C in the presence of stimulus B, resulting in the establishment of AB and BC relations. Usually, comparison stimuli are randomly positioned on the screen across trials to avoid creating a location preference. Without knowing where the comparison stimuli's location will be on the screen, a general assumption is that participants have to do a visual search to locate the stimuli.

Eye movements have been referred to as observing responses (e.g., Tomanari et al., 2007). Observing responses have been described as behavior that affects the sensing of stimuli or behavior that affects the amount of stimulus energy on the receptor cells (e.g., Dinsmoor, 1985). Sensory contact with the environment is essential for establishing stimulus control (Dinsmoor, 1985). Eye movements are one way to get sensory contact with the environment and can be measured with eye‐tracking technology (e.g., Hansen & Arntzen, 2015).

About 20 research articles investigating stimulus control in some form in the behavior analytic literature have measured eye movements. The topics have included reinforcement schedules (Holland, 1957; Rosenberger, 1973; Schroeder & Holland, 1969), simple discrimination (Huziwara et al., 2015; Pessôa et al., 2009; Schroeder, 1969; Schroeder, 1970, 1997; Schroeder & Holland, 1968), conditional discrimination (Huziwara et al., 2016; Kirshner & Sidman, 1972), selective attention (Dube et al., 2006; Dube et al., 1999; Perez et al., 2015) and stimulus equivalence (Hansen & Arntzen, 2018; Sadeghi & Arntzen, 2018; Steingrimsdottir & Arntzen, 2016). Eye movements are measured to reveal aspects of stimulus control arranged in the experiments and contribute to a detailed stimulus control analysis. Despite these publications, there is still a need for a more comprehensive understanding of what is measured using this equipment, if and to what degree eye movement measures correspond with conventional measures of stimulus control, and the eye as a sense organ to best utilize eye‐tracking technology.

Eye‐tracking technology calibration processes aim to measure foveal vision gaze. There is full acuity in the fovea area, and foveal vision accounts for 2° of the human visual field (Holmqvist et al., 2011). Humans move their eyes to get the high‐resolution foveal vision to acquire information from the environment. Vison outside the fovea is peripheral vision, which accounts for more than 99.9% of human vision (Rosenholtz, 2016). Foveal vision is not required to perceive stimuli. Color vision deteriorates in the periphery, but many visual tasks do not require high acuity. Foveal vision and eye movements are not necessary to perceive and discriminate objects. Research investigating peripheral sensitivity has shown that peripheral vision has a more significant role in different visual tasks than commonly assumed (Rosenholtz, 2016).

Also, stimulus control researchers measuring eye movements have found that participants rely on peripheral vision when responding. Schroeder (1970) reported, “Ss did not fixate each stimulus on every trial. Ss without fail were looking at the feedback lights in the center of the display when the next slide appeared. In many cases, Ss chose the correct stimulus without scanning” (p. 122). Perez et al. (2015) also reported on the possibility of peripheral observation, writing that “The results…suggest that the participants were ‘paying attention’ to both S+ components despite the fact that they only fixed their eyes on one component”(p. 88). They suggested that future studies using eye‐tracking should change the size and the position of the stimuli to decrease discrimination in peripheral vision. However, the authors were not specific about the size and position the stimuli should be. Other researchers have seen similar eye movement patterns and address that “it is crucial to consider the confounding effect of peripheral vision” when analyzing eye‐movement data (Hansen & Arntzen, 2018, p. 16). Therefore, studying the role peripheral vision plays in an MTS procedure can reduce the potential confounding effects and refine the operational definitions when measuring eye movements.

One way to investigate peripheral vision in MTS procedures is to measure peripheral sensitivity using a setup similar to the MTS procedure. In general, measuring peripheral sensitivity has been done in numerous ways and is often arranged by presenting stimuli at different points in the visual field and evaluating participants' responses (Strasburger et al., 2011). The stimuli used in such peripheral sensitivity tests vary from simple stimuli like spots of lights (e.g., Wang et al., 2011), gratings (e.g., Rovamo et al., 1982), detecting contrast (e.g., Legge & Kersten, 1987), colors (e.g., Abramov & Gordon, 1977), lines (Carrasco et al., 1995), and flicker frequencies (e.g., McCarthy et al., 1994) to more complex stimuli like letters (Anstis, 1974; Staugaard et al., 2016; Zegarra‐Moran & Geiger, 1993) and natural or natural‐like objects (Dill & Edelman, 2001; Mäkelä et al., 2001; Thorpe et al., 2001).

In letter detection, the stimulus size and distance from the fixation point (eccentricity) affect peripheral acuity. Anstis (1974) investigated letter‐size detection thresholds in peripheral vision. Two participants were asked to fixate in the middle of a tangent screen and respond when letters in different sizes were visible enough to be identified. Then, the letters were moved slowly from the outer corner towards the fixation point. If the participant moved their eyes, they had to tell the experimenter, and those trials were excluded from the analysis. Anstis measured letter‐size thresholds at 4° to 55° eccentricity from the fixation point and eight different angles (0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315°). The two participants' results showed that the threshold letter size (the smallest sized letter to be detected) increased linearly with increased distance (up to 30° eccentricity) from the fixation point.

Zegarra‐Moran and Geiger (1993) explored participants' ability to discriminate or detect letters in two different sizes and more complex stimuli in the periphery when presenting the stimuli in different eccentricities from the fixation point. The experiment's complex stimuli were drawings of well‐known symbols (for example, a cup, glasses, a house, a duck, a fork). Zegarra‐Moran and Geiger found that stimulus identification decreased as a function of increased eccentricity from the fixation point for all stimuli (2.5°, 5°, 7.5°, 10°, 12.5°, and 15°). Participants more readily identified big letters than small letters in the periphery and identified the symbols more accurately than the small letters but worse than the big letters. The differences between the stimuli remained the same. However, peripheral identification of stimuli decreased with increasing eccentricity. Staugaard et al. (2016) also found similar results: Eccentricity and the size of the stimuli affect the ability to identify letters in the periphery.

Experiments investigating peripheral sensitivity are usually arranged in two different ways. In the first arrangement, participants fix their gaze on a point in the middle. Then, stimuli are presented for a short time in the periphery. Participants must report if, where, or what they observed. In the second arrangement, participants are asked to gaze or fixate on a stimulus. Then, other stimuli are presented in the periphery and stay until the participants respond while the gaze is fixated on the initial stimulus. Researchers rely on participants' verbal responses on whether they moved their eyes or not in the latter arrangement. However, such verbal reports of one's own eye movement might be unreliable because it is difficult for the participant to identify such movements due to a limited experience reporting on eye movements. Therefore, eye‐tracking equipment is a helpful tool in peripheral sensitivity research (e.g., Boucart et al., 2016; van Asselen & Castelo‐Branco, 2009; Vater et al., 2017) because researchers can quickly identify when a participant's gaze moves away from the fixation point.

Computerized MTS procedures are analogous to peripheral sensitivity procedures regarding the stimulus layout and how stimuli are presented on the screen. One main difference between the two procedures is that in peripheral sensitivity procedures there is no relation between the stimulus the gaze fixates on and the stimuli participants identify peripherally. The fixation stimulus is a dot in peripheral sensitivity procedures and not a stimulus related to other stimuli, such as the sample and comparison stimuli are in MTS procedures. Also, participants in peripheral sensitivity research are not asked to choose different stimuli positioned in the periphery depending on a stimulus on the screen, as is done in MTS procedures. As far as we know, no previous experiments have investigated peripheral sensitivity in a more complex cognitive task such as conditional discrimination with identity or arbitrary matching performance, nor have these two procedures been compared. In identity matching, participants are asked to respond to the comparison stimuli that are physically similar to the sample stimulus. Identifying the same shape in the periphery as the gaze fixates might be easier compared to arbitrary matching, where participants must respond to physically different stimuli that are arbitrarily related because of previous reinforcement history.

Hence, the MTS format is well suited to test more complex discrimination tasks to investigate how and if that affects peripheral vision and sensitivity. Therefore, the primary purpose of the present study is to investigate to what degree participants trained on the MTS procedure can discriminate comparison stimuli in the periphery when varying the size and position of the stimuli and if and to what degree variations of the procedure affect such responding. To serve this purpose, participants were exposed to a series of peripheral tests in which they had to fixate their gaze on the sample stimulus while responding to comparison stimuli in the periphery. Before the series of peripheral vision tests, the participants underwent a training phase in which a contingency was in place based on the recorded eye movements. For example, if participants kept their gaze on the sample stimulus while responding to a comparison stimulus, verbal praise followed that response. On the other hand, if the gaze went outside the sample stimulus, a verbal prompt to regain fixation on the sample stimulus was given on the subsequent trial. The stimuli varied in size and position in 12 test phases. Three experiments were conducted to examine if arbitrary and identity MTS affect discrimination in the periphery by arranging arbitrary MTS in Experiment 1 and identity MTS in Experiments 2 and 3. Finally, we also explored if different types of stimuli, abstract (Experiments 1 and 2) or simple shapes (Experiment 3), affect peripheral discrimination in an MTS procedure.

Experiment 1

Experiment 1 was designed to investigate to what degree participants can discriminate stimuli in the periphery in a computerized MTS procedure in which they learn conditional discrimination with arbitrarily related abstract stimuli. MTS procedures with arbitrarily related abstract stimuli are a common arrangement in experiments investigating conditional discriminations or stimulus equivalence, as well as in experiments investigating eye movements (e.g., Dube et al., 2006; Hansen & Arntzen, 2018; Huziwara et al., 2016; Sadeghi & Arntzen, 2018). In the present experiment, participants were exposed to 12 peripheral vision tests after an MTS procedure. Throughout the peripheral vision tests, participants were asked to fixate their gaze on the sample stimulus and respond to a comparison stimulus in the periphery with a mouse click. The size and positions of the comparison stimuli varied in each peripheral test phase. In the end, all participants were tested on the relations trained without any requirements to keep their gaze on the sample stimulus to ensure that the conditional discriminations were intact.

Method

Participants

Eight university staff members and students with an age range of 22–42 years participated in this experiment. They had self‐reported normal or corrected‐to‐normal vision. All participants had previous experience with MTS training procedures. However, none of the participants had previous experience with eye‐tracking equipment. Participants read and signed a consent form before the experiment. Here, they were informed about the experimental situation, their rights, and the experiment's approximate duration (1.5 hr). After the experiment, the participants were fully debriefed and shown their data. Participants were urged not to inform other potential participants about the experiment.

Setting and Materials

The experiment was conducted in a 12 m2 room. The room was furnished with four tables and chairs, one up against each wall. The experimental computer was on one table, and the eye‐tracking computer was on a second table. The third table was for participants to sit and read the consent form, and the last table stored experimental equipment. A chinrest was attached to the table, 60 cm in front of the experimental computer. Opaque curtains covered a window on one wall, and the room was lightly illuminated to reduce the light disturbance on the eye‐tracking equipment. An A3 poster with 12 different letters in four different sizes, 3 cm, 1.5 cm, 0.7 cm, and 0.3 cm (three letters in each size), hung on the wall in the experimental room. The poster was used to test participants' vision, ensuring they could identify letters in the sizes they later would be presented with in the test condition.

Apparatus and Stimuli

Twelve abstract symbols were used in the computerized training and testing condition to form four 3‐member classes (see Fig. 1). All stimuli were in four different sizes on the screen: 3 cm, 1.5 cm, 0.7 cm, and 0.3 cm. The four stimulus sizes were chosen based on two factors. (1) The largest size had to be similar to other research using eye‐tracking in MTS procedures (e.g., Huziwara et al., 2016; Sadeghi & Arntzen, 2018). (2) By dividing the size three times, the smallest stimuli had to be seen in the foveal vision but very difficult to see in the peripheral vision. Pilot tests found that sizes 3 cm, 1.5 cm, 0.7 cm, and 0.3 cm met the two criteria. Eight stimuli made up of numbers and written numbers (1, 2, 3, 5, ONE, TWO, THREE, and FIVE [words translated from Norwegian: EN, TO, TRE, FEM]) were used in the periphery training condition, see Figure 1 (stimuli X and Y). These numbers were chosen because they were known to the participants, and the words created two pairs of words that are the same length in Norwegian. EN (one) and TO (two) are two‐letter words, and TRE (three) and FEM (five) are three‐letter words.

Figure 1.

Figure 1

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Stimuli used in Experiments 1 and 2 Note. The letters represent the stimuli classes, and the numbers represent class members. The Y‐stimuli are written numbers in Norwegian: EN = one, TO = two, TRE = three, and FEM = five.

The experiment was conducted on a custom‐built computer with Windows 7 Professional (32‐bit) (Computer 1) that was connected via AVerKey 300 to an ISCAN® computer running Windows 7 (Computer 2). The MTS training was conducted on Computer 1 with custom‐made MTS software, registering participants' responses. The eye‐tracker was an ISCAN® head‐mounted pupil/corneal reflection eye‐tracking system (ISCAN Corp., Burlington, MA; http://www.iscaninc.com), measuring the movements of the left eye. The head‐mounted eye‐tracker was connected to Computer 2 with an ISCAN® DQW 1.2 program. This program processed data at 60 Hz. (16.5 milliseconds) and sent eye‐tracking data to Computer 1. A custom‐made software (ETAnalyzer) on Computer 1 analyzed the MST software data with the eye‐tracking data. The LCD screen on Computer 1 had a 1280 × 1024 resolution. There was a target area around each stimulus of 212 × 212 pixels. A mouse click inside this area was considered a response “on” the stimulus. The same area was also the area of interest (AOI), defining fixation time and rate. The mouse pointer, shaped like a white arrow, was 0.6 cm long and 0.4 cm wide. The mouse pointer did not reset to a specific position for each trial; it followed the movement of the corded computer mouse throughout each trial in both training and testing phases, including the intertrial interval.

There was one computer screen outside the experimental room duplicating the screen of Computer 2, allowing the experimenter to follow the calibration and eye‐tracker live. If, for some reason, the eye‐tracker was not measuring correctly during the MTS training condition, the experimenter would do a recalibration, ensuring correct eye measurement. The screen was turned off when the experimenter was in the experimental room (e.g., during calibration or answering questions).

Design

A within‐subject A–B–A design, with 12 different phases of the B condition, was used in the present experiment. The last A condition was included to ensure that the trained relations remained intact after repeated testing without any programmed consequences. Additionally, the last A conditions were included to verify that any changes in responding in the different B conditions were due to the manipulations in each phase and not due to fatigue or loss of stimulus control.

Procedure

Vision Test

After reading and signing the consent form, participants were told to stand 60 cm from a poster hanging on one wall of the experimental room. This poster had different letters in four different sizes on it. Three and three letters were on a row, and each row had letters in different sizes. Letters A, F, and G were 3 cm, letters H, J, and K, were 1.5 cm, I, S, and L were 0.7 cm, and E, O, and C were 0.3 cm. The sizes of the letters were of the same size as the stimuli in the test conditions. Participants were asked to read the letters aloud to ensure that participants could identify letters in different sizes and exclude that reduced vision affected test results.

Calibration

Participants were asked to sit in front of the computer with their chin on the chinrest and to adjust the table and the chair for a comfortable position before the head‐mounted eye‐tracker was placed on their heads. During the eye‐tracker calibration process, the participants were asked to look at five dots on the screen: one in the middle of the screen and one in each corner positioned 60% out from the middle dot towards the outer edge of the screen. The calibration process took 3–10 min and ended when the eye‐tracker point‐of‐regard (real‐time eye movement measurement) accurately represented the participant's visual gaze. To ensure reliable measurements, recalibration was done if the point‐of‐regard was off during training or between testing conditions.

Matching‐to‐Sample Training. Instruction

Nonnative Norwegian participants read the following instruction on the screen, and native Norwegian participants read a Norwegian translation:

"A stimulus will appear on the screen. You must click this with the mouse. Four other stimuli will appear; choose one of the stimuli you think is correct. If you choose the correct one, "good," "super," etc., will appear on the screen. If you press incorrectly, "wrong" will display on the screen. During the experiment, the computer will no longer provide feedback on whether your choices are correct or incorrect, but you can get all the tasks right based on what you've learned. Try to get the most correct anyway. Good luck!"

Participants had to press “START” under the instruction to advance to training.

Matching‐to‐Sample Training Condition

The purpose of the training condition was to establish eight conditional discriminations with abstract stimuli arbitrarily related. All stimuli in this condition measured either 3 cm tall or 3 cm wide, dependent on the stimulus shape. A sample stimulus was presented in the middle of the screen, and with a mouse click on the stimulus, four other stimuli appeared in corner positions on the screen, 12 cm from the middle of the sample stimulus. The sample stimulus remained on the screen when the comparison stimuli were presented, allowing for simultaneous matching performance. Participants had to choose one of the comparison stimuli with a mouse click. If participants chose the experimentally defined correct stimuli, words like “good,” “correct,” and so on appeared in the middle of the screen. If they chose the stimuli defined as incorrect, the word “wrong” appeared on the screen. The programmed consequences were on the screen for 0.5 s and were followed by a blank screen for 0.5 s, resulting in a 1‐s intertrial interval before the subsequent trial. Comparison stimuli appeared randomly in one of the four corners for each trial. The baseline relations were established concurrently in blocks of 24 trials, where each relation was presented randomly three times. Participants were trained using a one‐to‐many training structure and learned A1B1, A2B2, A3B3, A4B4, A1C1, A2C2, A3C3, and A4C4 relations. The letter denotes the class members, and the number denotes the classes. In this training procedure, the A stimuli were the sample stimuli on all trials, and the comparison stimuli were the B and C stimuli. A 90% mastery criterion was required for all blocks to advance in training. After blocks with 100% programmed consequences, the probability of programmed consequences was reduced to 75%, 25%, and 0% in consecutive blocks depending on meeting the mastery criterion. During the training condition, the experimenter sat outside the experimental room.

Peripheral Vision Training. Instruction

Non‐native Norwegian participants read the following instruction on the screen, and native Norwegian participants read a Norwegian translation:

"A stimulus will appear on the screen. Keep your eyes on this stimulus. You must click this with the mouse. In this task, you must always keep your eyes on the stimulus in the middle. Four other stimuli will appear. While keeping your eyes on the stimulus in the middle, choose one of the stimuli that appears that you think is correct. You will not get feedback if the one you choose is correct or not. It is more important that you keep your eyes still than get the answer right. Try to get the most correct anyway. Good luck!"

Peripheral Vision Training Condition

This condition's purpose was to familiarize participants with the task of responding to comparison stimuli without moving their gaze from the sample stimulus and practicing mouse‐clicking on stimuli without gazing at the cursor. One of the words EN, TO, TRE, or FEM appeared as a sample stimulus, and immediately after a mouse‐click on the sample stimulus, four comparison stimuli appeared, while the sample stimulus remained on the screen. Participants had to choose one of the numbers, 1, 2, 3, and 5 without moving their gaze from the sample stimulus. A 1‐s intertrial interval followed a response to a comparison stimulus before the subsequent trial began, without programmed consequences. Participants had to perform simultaneous arbitrary matching only using their peripheral vision. The four training trials, X1Y1, X2Y2, X3Y3, and X4Y4, were presented 12 times in random order with a total of 48 trials. For each trial, the four comparison stimuli appeared randomly in the four corners of the screen. In this condition, the experimenter sat inside the experimental room, in front of Computer 2, prompting participants to look at the sample stimulus and reinforcing with positive verbal praise such as “yes” and “good” when they kept their gaze on the sample stimulus. Verbal praise was given regardless of whether the response to the comparison stimuli was correct because the purpose of this training phase was to practice gazing at the sample stimulus while responding to a comparison stimulus. The first two correct trials in the training block or the first two correct after an error with gaze constant on the sample stimulus were reinforced. After a trial in which the participant's gaze went outside the sample stimulus, the verbal prompt “look at the stimulus in the middle” was given once.

The experimenter loaded the parameters for the following condition of the experiment. At the same time, participants sat in the chair with their chins on the chinrest. The following condition started without a new round of calibration.

Peripheral Vision Tests. Instruction

The instruction was the same as the previous condition for the first test condition. For the following test conditions, the instructions were, “Continue as you have done. Have your gaze on the stimulus in the middle while answering.”

Peripheral Vision Test Condition

The test condition's purpose was to test the previously trained conditional discriminations while participants kept their gaze on the sample stimulus when responding to the comparison stimuli. The sizes and positions of the comparison stimuli varied across phases. The stimuli were presented in four different sizes, 3 cm, 1.5 cm, 0.7 cm, and 0.3 cm, and placed in three different positions on the screen, 6 cm, 12 cm, and 18 cm from the center of the sample stimulus, which represents 5.7, 11.4, and 17 degrees of eccentricity from the sample stimulus, respectively. This amounted to 12 different test phases presented randomly (see Table 1). All participants were exposed to the same order of test phases. Each phase contained 24 test trials. The sample stimulus was 3 cm wide or broad as in the training condition in all phases. After clicking on the sample stimulus, four comparison stimuli appeared in the screen's four corners while the sample stimulus remained on the screen. The positions and the sizes of the stimuli varied depending on the test phase. Participants were instructed to gaze at the sample stimulus while responding to a comparison stimulus. Participants were tested for A1B1, A2B2, A3B3, A4B4, A1C1, A2C2, A3C3, and A4C4 relations. There were no programmed consequences in the test phases. As in the previous condition, the experimenter sat inside the experimental room, in front of Computer 2. If the point‐of‐regard moved away from the sample stimulus on two trials in a row, the experimenter verbally prompted the participant to fixate the gaze on the sample stimulus (repeated the instruction). Participants kept their heads in the chinrest between phases while the experimenter started the following phase on Computer 1. All participants were informed that they could take a break between phases, and if necessary, a recalibration was done. After the last peripheral vision test phase, the eye‐tracking equipment was recalibrated before the last experimental condition.

Table 1.

An Overview of Order of Stimulus Position and Size in the Testing Conditions

Test Conditions Position from Sample Stimulus (cm) Size (cm)
1 12 3
2 6 0.5
3 18 3
4 12 0.5
5 12 0.7
6 18 0.7
7 18 0.3
8 6 0.3
9 18 0.5
10 6 3
11 12 0.3
12 18 0.7
MTS Test 12 3
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Matching‐to‐Sample Test Condition With Free Eye Movements

The purpose of the MTS test condition was to test whether the conditional discriminations learned in training were still intact. Participants were instructed to move their eyes freely on the screen in this condition. A1B1, A2B2, A3B3, A4B4, A1C1, A2C2, A3C3, and A4C4 relations were tested three times each, with 24 trials. All stimuli were 3 cm wide or tall, as in the training condition. The sample stimulus appeared in the middle of the screen, and after a mouse‐click on the stimulus, four comparison stimuli appeared in the four corners of the screen, 12 cm from the center of the sample stimulus. The sample stimulus remained on the screen until participants responded to a comparison stimulus. The subsequent trial started after a 1‐s intertrial interval. All test trials were randomized, and comparison stimuli appeared in random order on the screen for each trial. The experimenter sat inside the experimental room in front of Computer 2 without prompting or reinforcing eye movement behavior during the condition.

Response Measurements and Data Analysis

Percentage Correct Responses

The dependent variable was matching responses. This was measured and summarized as correct or incorrect responses by the computer and presented as correct percentage responses in MTS training and last MTS test, and percentage correct responses of all trials meeting the criteria for correct eye movement for each phase in the peripheral vision tests. See below for the definition of correct eye movement.

Eye Movement Measurements

A single test trial was excluded if the point‐of‐regard was more than 100 milliseconds (ms) outside the AOI (sample stimulus). Eye blinking during test trials was expected and gave a rapid eye movement away from the fixation point; 100 ms is within the range of an average eye blink duration (Wang et al., 2011). If the number of removed trials in one block exceeded 50% of trials with correct eye movement, an entire test phase was removed from the analysis.

Results and Discussion

Vision Test, MTS Training Condition, and Periphery Vision Training Condition

All participants responded correctly in the vision test, showing the ability to see and discriminate all stimuli with foveal vision. In the MTS training condition, participants used an average of 255 trials (range of 168–336 trials) to establish and maintain the conditional discriminations. Unfortunately, due to a software error, P17203 was recalibrated after five training trials, and the MTS training condition was restarted from the beginning, resulting in five additional training trials. In the periphery training condition, all participants received 48 trials and were instructed to focus on the sample stimulus while responding to one of the four comparison stimuli.

Peripheral Vision Test and MTS Test Conditions

Individual data from the peripheral vision test are displayed in Figure 2. Each graph shows the percentage of correct responses for each test phase. The results in all the graphs in Figure 2 were re‐ordered for analysis purposes based on the stimulus size and the positions from sample stimulus. Participants were exposed to the test phases differently than shown in the graphs (see Table 1). The scores from the test phases were the percentage of correct trials in each phase after trials, in which eye movements outside the AOI were removed. All participants responded above 90% correct when stimuli were 3 cm in size regardless of position. P17203, P17205, P17206, P17207, and P17209 also responded above 90% correct when stimuli were 1.5 cm regardless of position. The remaining three participants (P17208, P17225, and P17226) responded above 90% correct when comparison stimuli were 1.5 cm and positioned 6 cm and 12 cm from the sample stimulus but had a lower percentage correct when comparison stimuli were 1.5 cm and positioned 18 cm from the sample stimulus.

Figure 2.

Figure 2

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Percentage of Correct Responding for Individual Participants in Each Test Condition in Experiment 1 Note. The graphs show the percentage of correct responses in the peripheral test phases and the MTS test in Experiment 1 for each participant. The x‐axis presents the position of the comparison stimuli in cm from the middle of the sample stimulus and the size of the comparison stimuli in the different peripheral test phases. Largest = 3 cm, Large = 1.5 cm, Small = 0.7, and Smallest = 0.3 cm. The asterisk denotes conditions in which participants had too many trials with eye movements outside the fixation point.

All participants responded above 90% correct in the phase in which the comparison stimuli were 0.7 cm (small) in size and positioned 6 cm from the sample stimulus. Further, accuracy reduced as stimulus positions for the 0.7 sized stimuli increased. P17203, P17205, P17206, P17208, P17225, and P17226 (see six first graphs in Fig. 2) showed similar decreasing accuracy patterns regarding the smallest stimuli size (0.3 cm) as the position from the sample stimulus increased. Additionally, when stimuli were 0.3 cm, P17207 and P17209 (see the two graphs at the bottom of Fig. 2) had lower accuracy with stimuli positioned 12 cm from the sample stimulus compared to 6 cm, and an increase in accuracy when stimuli were positioned 18 cm from the sample stimulus.

All participants met the mastery criterion on the MTS test condition, where they could freely move their eyes (see the last bar in Fig. 2). The average was 99.5% correct. The high accuracy scores in this condition confirm that the stimulus classes established in training were intact throughout the 12 peripheral test phases.

Summary

Overall, these results show that adult participants, to a large extent, can identify and discriminate stimuli in the periphery. The present results also support previous results from peripheral sensitivity research. Discrimination of stimuli in the periphery decreases as a function of eccentricity and size, similar to research on peripheral sensitivity (e. g., Anstis, 1974; Staugaard et al., 2016; Zegarra‐Moran & Geiger, 1993).

There was an added layer of complexity of the task in the present experiment compared to more conventional peripheral sensitivity research described earlier. In the more traditional experiments, the fixation point is often a dot or a cross, and the task is to identify light, lines, letters, or simple shapes. This contrasts with the present experiment in which the participants had to respond to previously trained conditional discriminations of arbitrarily related abstract stimuli. The comparison stimuli to which the participants had to respond looked different from the sample stimulus that the participant was instructed to fixate on. As far as we know, no other experiment has used such a complex task in periphery sensitivity research. Hence, it is challenging to evaluate if the result of the present study is due to a peripheral sensitivity threshold regardless of the complexity of the task, or if a less complex task would yield different results. Identity matching can be considered a less complex task. Here, participants had to respond to comparison stimuli identical to the sample stimulus. Using identity matching instead of arbitrary matching might influence discrimination of abstract stimuli in the periphery because participants only have to identify the physical features that are the same as the stimulus they fixate on.

Experiment 2

The purpose of Experiment 2 was to examine the peripheral discrimination of abstract stimuli in an identity MTS procedure. Using similar abstract stimuli as both samples and comparisons allows for an investigation into whether the task's complexity affects the discrimination of stimuli in the periphery. In identity matching, participants must respond to a comparison stimulus that is physically identical to the sample stimulus. Locating and identifying an identical stimulus in the periphery might yield higher accuracy than with arbitrarily related stimuli. In Experiment 2, participants were exposed to 12 peripheral test phases in which the size and position of the stimuli were varied after training on an identity MTS training procedure.

Method

Participants

Four university staff members and students participated in this experiment. The age range was 25–41 years. All reported normal or corrected‐to‐normal vision. As in Experiment 1, all participants had previous experience with MTS training procedures, though not with eye‐tracking measurement research. Participants were informed about the experimental situation, their rights, and the experiment's approximate duration (1 hr) in a consent form, which they signed. All participants were fully debriefed, shown their data after the experiment, and urged not to inform other potential participants about the experiment.

Setting, Materials, and Apparatus

The setting, materials, and apparatus were the same as Experiment 1.

Stimuli

Four abstract stimuli and four written numbers were used as stimuli in this experiment, respectively, stimuli A and Y in Figure 1. As in Experiment 1, the four abstract stimuli were presented in four different sizes and positions on the screen during peripheral vision tests. In addition, the written numbers were used in the peripheral training condition.

Design

The design was the same as in Experiment 1.

Procedure

Vision Test and Calibration

The vision test and the calibration process were the same as Experiment 1.

Matching‐to‐Sample Training

In this training condition, participants were to perform identity matching with four abstract stimuli. Participants learned A1A1, A2A2, A3A3, and A4A4 baseline relations. Each block consisted of 24 trials, and programmed consequences were reduced stepwise as in Experiment 1. The instruction and the rest of the procedural setup in terms of stimulus presentations, timings, mastery criterion, and programmed consequences were the same as in Experiment 1.

Peripheral Vision Training

The instructions and the setup of the procedure were identical to those in Experiment 1, except that in this condition, participants were to do identity matching of the Y stimuli in the periphery. The four trials were Y1Y1, Y2Y2, Y3Y3, and Y4Y4.

Peripheral Vision Tests

Participants were exposed to 12 different conditions testing A1A1, A2A2, A3A3, and A4A4 relations in the periphery, varying the stimuli size and position. The order of the test conditions is shown in Table 1. The instruction, the setup, and the size and position of the stimuli were the same as in Experiment 1.

Matching‐to‐Sample Test Condition With Free Eye Movements

The baseline relations A1A1, A2A2, A3A3, and A4A4 were tested in this condition. The instructions, setup, and size and position of the stimuli were the same as in Experiment 1.

Response Measurements and Data Analysis

Response measurements, the criterion for eye movements, and data analysis were the same as in Experiment 1.

Results and Discussion

Vision Test, MTS Training Condition, and Periphery Vision Training Condition

All four participants managed the vision test. Three participants used 96 training trials to learn the conditional discriminations in the training condition, and one participant (P17216) used 120 trials. In addition, all participants received 48 trials in the peripheral training condition.

Peripheral Vision Test and MTS Test Conditions

Figure 3 shows percent correct in the 12 different peripheral vision test conditions for individual participants. The results show that all participants had more than 90% correct responding for the largest (3 cm) and large (1.5 cm) stimuli regardless of position. In the condition with large stimuli positioned 18 cm from the sample stimulus, one participant (P17215) scored below the mastery criterion (80%), and the three other participants responded above. All participants responded correctly on all trials with small stimuli (0.7 cm) positioned closest to the sample stimulus (6 cm). Two participants (P17212 and P17214) responded with more than 90% correct when stimuli were 0.7 cm (small) and positioned 12 cm from the sample stimulus, whereas the two other participants (P17215 and P17216) responded 76.9% and 61.9% correct, respectively. When the small stimuli were positioned 18 cm from the fixation point, none of the participants responded above 90% correct. Regarding the smallest stimuli (0.3 cm), none of the participants responded above 50% correct on any of the positions, except P17212, who scored 87% correct when the smallest stimuli were positioned 6 cm from the sample stimulus. Four phases were removed from the P17215 dataset due to too many trials with eye movement outside the AOI. The removed phases are marked with an asterisk in the graph. Overall, the results show that discrimination of comparison stimuli in the peripheral vision decreases when the size and position of the stimuli from the sample stimulus increases.

Figure 3.

Figure 3

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Percentage of Correct Responding for Individual Participants in Each Test Condition in Experiment 2 Note. The graphs show the percentage of correct responses in the peripheral test phases and the MTS test in Experiment 2 for each participant. The x‐axis presents the position of the comparison stimuli in cm from the middle of the sample stimulus and the size of the comparison stimuli in the different peripheral test phases. Largest = 3 cm, Large = 1.5 cm, Small = 0.7, and Smallest = 0.3 cm. The asterisk denotes conditions in which participants had too many trials with eye movements outside the fixation point.

All four participants responded 100% correctly in the MTS test condition, displayed in the last bar of Figure 2. These results confirm that the stimulus classes were intact and inaccuracy in the peripheral test phase was not due to a dissolution of the stimulus classes trained but due to the manipulation done in the peripheral test phases.

Summary

Only four participants were recruited for Experiment 2 because of the consistent response pattern between the four participants. The experimenters viewed the probability of a very different outcome with four more participants as very low. All participants responded within the criterion on the post‐MTS test, confirming that erroneous responses were not due to a lack of stimulus control but a reduced ability to identify the stimuli in the periphery.

The results from Experiment 2 are very similar to those of Experiment 1. Therefore, there seems to be no difference between participants' arbitrary and identity matching performance regarding discrimination when only using their peripheral vision. In Experiments 1 and 2, the stimuli were abstract, and the participant had no history with them before entering the experimental situation. Still, it is difficult to say if the level of peripheral sensitivity to stimulus size and position found in Experiments 1 and 2 is a general threshold or if other variables affect peripheral performance, for example, the type of stimuli. Most peripheral sensitivity research has used simple stimuli like light flickers or well‐known shapes like letters. Therefore, one way to test if there would be different results depending on the type of stimuli used is to replicate Experiments 1 and 2 using well‐known, simple shapes.

Experiment 3

In Experiment 3, participants were exposed to an identity MTS task with simple shapes and asked to discriminate stimuli in the periphery in 12 consecutive test phases in which the comparison stimuli varied in size and position. Experiment 3 was conducted to see if changes in the stimulus would affect participants' responding in the periphery or if the results of Experiments 1 and 2 were due to a sensory threshold independent of other environmental variables, such as the type of discrimination tasks or stimulus shapes. Since Experiments 1 and 2 showed no difference between arbitrary and identity matching of abstract pictures, the present study used an identity matching procedure because it requires less training. The rest of the experimental setup and analysis were identical to Experiments 1 and 2.

Method

Participants

Four university students participated in this experiment, with an age range of 21–29. All four participants had previous knowledge of matching‐to‐sample procedures but no experience with eye‐tracking equipment. Participants signed the same consent form as those in Experiment 2 and were thoroughly debriefed at the end of the experiment.

Setting, Materials, and Apparatus

The setting, materials, and apparatus were the same as in Experiments 1 and 2.

Stimuli

Four well‐known stimuli and four written numbers were used in this experiment, respectively, stimuli D in Figure 4 and the Y stimuli in Figure 1. Similar to Experiments 1 and 2, the four D stimuli were presented in four different sizes and positions on the screen during peripheral vision tests. In addition, the written numbers were used in the peripheral training condition.

Figure 4.

Figure 4

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Stimuli in Experiment 3 Note. The letter D denotes the class, and the numbers denote class members.

Design

The design was the same as in Experiments 1 and 2.

Procedure

Vision Test and Calibration

The vision test and the calibration process were the same as in Experiments 1 and 2.

Matching‐to‐Sample Training

In this training condition, participants performed identity matching with the four D stimuli in Figure 4. Participants learned D1D1, D2D2, D3D3, and D4D4 baseline relations. Each block consisted of 24 trials, and programmed consequences were reduced stepwise as in Experiments 1 and 2. The instruction and the rest of the procedure setup were the same as in Experiments 1 and 2.

Peripheral Vision Training

The instruction and the setup of the procedure were identical to those in Experiments 1 and 2, except that in this condition, participants were to perform identity matching of the Y stimuli in the periphery. The four trials were Y1Y1, Y2Y2, Y3Y3, and Y4Y4.

Peripheral Vision Tests

Participants were exposed to 12 conditions testing D1D1, D2D2, D3D3, and D4D4 relations in the periphery, varying the stimuli size and position. The order of the test conditions is shown in Table 1. The instructions, setup, and size and position of the stimuli were the same as in Experiments 1 and 2.

Matching‐to‐Sample Test Condition With Free Eye Movements

The baseline relations D1D1, D2D2, D3D3, and D4D4 were tested in this condition. The instructions, setup, and size and position of the stimuli were the same as in Experiments 1 and 2.

Response Measurements and Data Analysis

Response measurements, eye movement criterion, and data analysis were the same as in Experiments 1 and 2.

Results and Discussion

Vision Test, MTS Training Condition, and Periphery Vision Training Condition

The four participants responded satisfactorily to the vision test by correctly identifying all letters in all sizes. One participant, P17219, required 120 trials to establish and maintain the baseline relations in training, whereas the other three required 96 trials. Three participants required 96 training trials to learn the conditional discriminations in the training condition, and one participant (P17216) required 120 trials. All participants received 48 trials in the peripheral training condition.

Peripheral Vision Test and MTS Test Conditions

Figure 5 shows the results of the peripheral vision test condition for each participant in four graphs. All participants responded with 100% accuracy when the stimuli were largest, large, and small regardless of position. On average, 98.8% accuracy was achieved when the stimuli were smallest and positioned 6 cm from the sample stimulus. Two participants (P17221 and P17222) responded within the 90% correct criterion with the smallest stimuli positioned 12 cm from samples. P17219 and P17220 responded with 72.2% and 68.2% correct, respectively. None of the participants responded within the criterion in the condition with the smallest stimuli positioned furthest away (18 cm) from the sample stimulus. P17219 had 64.3% correct, P17220 had 40% correct, P17221 had 85% correct, and P17222 had 52.6% correct. All four participants responded 100% correctly in the MTS test condition in Experiment 3, displayed in the last bar of Figure 3, showing that the stimuli classes trained in the beginning were intact throughout the peripheral test phases.

Figure 5.

Figure 5

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Percentage of Correct Responding for Individual Participants in Each Test Condition in Experiment 3 Note. The graphs show the percentage of correct responses in the peripheral test phases and the MTS test in Experiment 2 for each participant. The x‐axis presents the position of the comparison stimuli in cm from the middle of the sample stimulus and the size of the comparison stimuli in the different peripheral test phases. Largest = 3 cm, Large = 1.5 cm, Small = 0.7, and Smallest = 0.3 cm.

Summary

Overall, this experiment shows that participants can discriminate simple and well‐known shapes in the periphery to a great extent. A negative effect of eccentricity only applies when the stimuli are very small. In addition, the accuracy data from Experiment 3 shows that the type of stimuli plays a more significant role when discriminating and identifying stimuli in the periphery than the type of conditional discrimination, when compared to Experiments 1 and 2.

General Discussion

The primary purpose of the present study was to investigate to what degree participants can discriminate and identify stimuli in the periphery in an MTS procedure on a computer. Generally, the three experiments show that participants can discriminate stimuli in the periphery to a large extent and that simple stimuli are more discriminable than abstract stimuli. The stimuli had to be extremely small, 0.7 cm and 0.3 cm, before participants started to make errors in the peripheral tests for abstract and simple stimuli, respectively. There was no difference between arbitrary and identity matching of abstract stimuli regarding accuracy when discriminating in the periphery, comparing Experiments 1 and 2. This might indicate that once the conditional discriminations with arbitrary relations between stimuli are established, the stimulus control is similar when the comparison stimuli are within gaze and when they are in the periphery.

The results from the present experiments replicate those of other experiments in that eccentricity and stimulus size affect peripheral discrimination (Anstis, 1974; Staugaard et al., 2016; Zegarra‐Moran & Geiger, 1993). Comparing Experiments 1 and 2 with abstract stimuli and Experiment 3 with simple shapes shows that the type of stimuli used affects peripheral discrimination, supporting the results of Zegarra‐Moran and Geiger (1993). They found that adult participants more readily discriminated letters than symbols of the same size.

Many participants in the current study found the peripheral tests challenging, so employing eye‐tracking technology in this experiment had several benefits. First, because we recorded eye movements, participants did not have to respond to the researcher when they failed to keep their gaze on the sample stimulus, which could be an additional stressful task. Second, the eye‐tracker and the recordings made it easy for the experimenter to exclude trials with eye movements outside the fixation point after data collection, precluding the need for a second observer in the experimental setting and interscorer agreement. Third, the experimenter saw the participant's eye movements in real‐time due to how the equipment was set up, which made it possible for the experimenter to give direct verbal consequences and prompts dependent on eye movement in the peripheral training phase. Because, as mentioned, several participants found the task to be demanding, they had good use of the training phase to familiarize themselves with the task.

In the present experiments, we taught participants not to move their gaze from the sample stimulus when responding to the comparison stimuli by reinforcing no movements outside the sample stimulus and prompting that behavior after trials with erroneous eye movements. Placing a contingency on eye movements allows for an operant analysis of eye movements. Madelain and colleagues have studied operant control of eye movements, training faster saccades in human adults (Vullings & Madelain, 2019) and smooth pursuits in infants (Darcheville et al., 1999) based on reinforcement contingencies. Future research should continue treating eye movements as the primary dependent measure and subject to reinforcement contingencies. It is possible to create software in which immediate changes occur on the screen depending on the participants' eye movements. Based on the present experiments, one may, for example, have programmed for comparison stimuli to disappear off the screen if the participants moved their gaze away from the sample stimulus. 1 In such a setup, eye movements will be followed by an immediate consequence and force the participants to keep their gaze on the sample stimulus to sense the comparison stimuli; thus, they have to use their peripheral vision when choosing comparison stimuli.

A potential limitation of the present experiments is the use of mouse clicking to respond to stimuli, especially to the comparison stimuli. For example, most people are used to moving their gaze with the mouse cursor to observe if it hovers on the screen over the object on which they will click. Although the peripheral training phase was put in place for participants to practice this, they were not trained to respond to stimuli in the different positions that were used in the test phase. Using specific numbers on the keyboard that represent the four different stimulus positions/directions or a joystick might have made the peripheral task easier and reduced the number of trials that had to be excluded from the data analysis and should be considered in future research.

What do the present results on peripheral vision say about observing, and how do they contribute to research using eye‐tracking technology? The present experiments show that participants can discriminate relatively small stimuli in the periphery when forced to. However, that does not necessarily mean that participants do that in an MTS procedure when measuring eye movements. Hansen and Arntzen (2018) discuss peripheral vision concerning changes in eye movements (fixation rates and fixation duration) throughout training and test. They discuss that clear‐cut fixations might not be necessary at the end of training when the conditional discriminations are learned. However, peripheral vision can potentially be actively used at the beginning of training, as seen in Perez et al. (2015). Also, Sadeghi and Arntzen (2018) examined eye movements when groups of participants were trained on MTS procedures with three different training structures. One measure reported in Sadeghi and Arntzen is transition rates, defined as eye movement between fixated comparison stimuli (fixation defined as 200 ms within the AOI). Regardless of training structure, the transition rate was, on average across participants, below one at the beginning of training. Hence, participants could gaze directly at one comparison stimulus and not between comparison stimuli, indicating that they relied on periphery vision when learning the conditional discriminations.

For some research, the main goal of using eye‐tracking technology is to use it as a measure of observing responses, as a type of precurrent behavior, or as a necessary condition for stimulus control. In that case, one should seek to reduce the possibility of sensing the stimuli in the periphery. The data measured with eye‐tracking equipment is foveal vision and not equal to stimulus energy on receptor cells in the eye, as Dinsmoor (1985) defines observing responses. Based on the present and previous experiments on peripheral vision, procedural changes can be made to reduce or avoid the use of peripheral vision in MTS procedures. First, stimuli can be smaller than what has usually been used. For example, stimuli could be 1.5 cm if they are abstract or 0.3 if they are simple, well‐known shapes. Second, the stimuli can be positioned in the outer corners of the screen; that is, far from the sample stimulus. A disadvantage with positioning the stimuli in the outer corners of the screen is that the angle between the pupil and the cornea reflection, which may be too large with some eye‐tracking equipment measures, and might result in less accurate measures. Therefore, the stimuli could be positioned closer to the sample stimulus but reduced to 0.7 cm to decrease peripheral vision.

Hiding the stimuli is also a way of reducing peripheral discrimination. For example, research on crowding, in which the main stimulus is placed between two other stimuli, has been shown to greatly influence the identification and discrimination of stimuli in the periphery (e.g., for a summary, see Rosenholtz, 2016). In stimulus control research, crowding could easily be done when tracking eye movements to avoid peripheral discrimination.

In conclusion, eye‐tracking technology in behavioral research requires an in‐depth understanding of what is measured and that eye movements are not necessarily equal to stimulus control. If participants respond to some stimuli, they must have seen or observed them somehow, despite fixation data. Even though there probably is a high degree of correspondence between what is measured by the eye‐tracking equipment and stimulus control, it is not a one‐to‐one correspondence. However, this does not exclude eye‐tracking and fixation as measures in research, though it forces well‐designed experiments and thoughtful analysis and interpretations of the eye‐tracking data.

Endnote

1

Based on a comment by Professor Iver Iversen at the authors' symposium at the 9th European Association for Behaviour Analysis Conference in Wurzburg, Germany, September 2018.

Editor‐in‐Chief: Mark Galizio

Associate Editor: Karen Lionello‐DeNolf

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