Interocular suppression in primary visual cortex in strabismus: impact of staggering the presentation of stimuli to the eyes

Abstract

Diplopia (double vision) in strabismus is prevented by suppression of the image emanating from one eye. In a recent study conducted in two macaques raised with exotropia (an outward ocular deviation) but having normal acuity in each eye, simultaneous display of stimuli to each eye did not induce suppression in V1 neurons. Puzzled by this negative result, we have modified our protocol to display stimuli in a staggered sequence, rather than simultaneously. Additional recordings were made in the same two macaques, following two paradigms. In trial type 1, the receptive field in one eye was stimulated with a sine-wave grating while the other eye was occluded. After 5 s, the occluder was removed and the neuron was stimulated for another 5 s. The effect of uncovering the eye, which potentially exposed the animal to diplopia, was quantified by the peripheral retinal interaction index (PRII). In trial type 2, the receptive field in the fixating eye was stimulated with a grating during binocular viewing. After 5 s, a second grating appeared in the receptive field of the nonfixating eye. The impact of the second grating, which had the potential to generate visual confusion, was quantified by the receptive field interaction index (RFII). For 82 units, the mean PRII was 0.48 ± 0.05 (0.50 = no suppression) and the mean RFII was 0.46 ± 0.08 (0.50 = no suppression). These values suggest mild suppression, but the modest decline in spike rate registered during the second epoch of visual stimulation might have been due to neuronal adaptation, rather than interocular suppression. In a few instances neurons showed unequivocal suppression, but overall, these recordings did not support the contention that staggered stimulus presentation is more effective than simultaneous stimulus presentation at evoking interocular suppression in V1 neurons.

NEW & NOTEWORTHY In strabismus, double vision is prevented by interocular suppression. It has been reported that inhibition of neuronal firing in the primary visual cortex occurs only when stimuli are presented sequentially, rather than simultaneously. However, these recordings in alert macaques raised with exotropia showed, with rare exceptions, little evidence to support the concept that staggered stimulus presentation is more effective at inducing interocular suppression of V1 neurons.

INTRODUCTION

Diplopia results from strabismus because images are cast upon noncorresponding locations in the retinas. In children who develop strabismus early in life, this noxious sensation is prevented by suppression of the image in one eye. One of the great challenges remaining in the field of visual physiology is to elucidate the neural mechanism of diplopia suppression.

In a recent study we recorded from 500 neurons in the primary visual cortex of two awake macaques raised with exotropia induced by surgical weakening of the medial rectus muscle in each eye (1). In normal animals with intact stereopsis the response of most neurons is augmented by binocular stimulation at the optimal retinal disparity (2–5). In the two strabismic monkeys no net facilitation or inhibition occurred during stimulation of both eyes’ receptive fields. This breakdown in binocularity has been reported previously in strabismic animals (6–13).

Evidence has emerged that in strabismus, the timing of stimulus presentation may have a critical impact on receptive field interactions. In anesthetized strabismic cats, Sengpiel and Blakemore (14) reported two major observations. First, they found absence of interocular suppression when gratings were presented simultaneously to the receptive fields of both eyes. Second, they subsequently tested neurons by driving a response with a drifting grating in the dominant eye. After a lapse of 5 s they introduced a second grating, drifting in the receptive field of the other eye. This paradigm caused strong and sustained suppression of neuronal firing, which was independent of the relative spatial phase or orientation of the second grating (15). The investigators suggested that this phenomenon might account for the suppression of double vision in strabismus.

As mentioned earlier, we found no net inhibition or facilitation in awake, strabismic monkeys from stimulation of the receptive field in one eye versus simultaneous stimulation of the receptive fields in both eyes (1). In this respect, our results confirmed the first observation reported by Sengpiel and colleagues. Here, we now report data addressing their second observation, namely, the intriguing effect they discovered by staggering the presentation of gratings to the two eyes. Our goal was to replicate this finding in primates.

In exotropia the perception of signals emanating from the peripheral temporal retina of each eye is suppressed (Fig. 1) (16–19). There is a corresponding reduction in levels of cytochrome oxidase (CO), a metabolic enzyme, in ocular dominance columns of the ipsilateral eye representing the peripheral nasal visual field (20–23). To maximize the likelihood of encountering suppression, we selected neurons for recording that 1) were dominated by the ipsilateral eye and 2) had receptive fields located in the peripheral nasal visual field.

Figure 1.

Suppression in exotropia. A: with one eye occluded no suppression is evoked: a stimulus presented anywhere in the visual field of the viewing eye is perceived. The grating shown here drives a neuron with its receptive field 25° in the nasal field, just below the horizontal meridian. B: with both eyes viewing, the peripheral temporal retina in each eye is suppressed (gray shading). Each eye perceives a separate portion of the visual scene (left eye = blue; right eye = red). The grating in A is now suppressed in the right eye, and instead, perceived via a noncorresponding location in the left eye. To maximize the likelihood of encountering neurons showing interocular suppression while recording from right V1, neurons were selected that were right eye dominant with peripheral nasal receptive fields.

METHODS

Experimental Design

Experiments were conducted in two male Rhesus monkeys with an alternating exotropia induced by cutting the tendon of the medial rectus muscle in each eye at age 4 wk (24). The main findings from these monkeys have been reported previously, along with a full account of the methods (1). The recordings described in this present report were conducted after the main data set from 500 neurons was acquired. The goal of these supplementary recordings was to address explicitly the effect of staggering the timing of stimulus presentation to each eye.

Recordings in both animals were made in the right calcarine cortex, where the peripheral left visual hemifield of each eye is represented. All surgical and experimental procedures were conducted with approval from the Institutional Animal Care and Use Committee at the University of California San Francisco. Buprenorphine was administered for analgesia for at least 2 days after any surgical procedure.

Each monkey viewed through lenses providing optical correction for distance during all recordings, as determined by cycloplegic refraction. The exotropia in Monkey 1 measured 15°–20°. The exotropia in Monkey 2 was 10°–15°, with a slight increase in upgaze. In both animals, contrast sensitivity as a function of spatial frequency was determined for each eye through performance of a two-alternative discrimination task (25, 26). The plots were normal in both eyes, indicating that neither animal had amblyopia (see Fig. 4 in Ref. 1).

Eye Tracking

Each monkey was seated in a primate chair with the head fixed by an implanted post (27). The positions of the eyes were monitored independently using two infrared eye trackers (SensoMotoric Instruments, Teltow, Germany). Each eye could be occluded by an infrared filter, moved into position by a pneumatic piston under computer control. Eye and stimulus positions were sampled at 120 Hz by a Power1401 data acquisition and control system (Cambridge Electronic Design, Cambridge, UK). The monkey was rewarded with a blend of biscuits, fruit, and juice.

Neuronal Recording

Recordings were made via a titanium recording chamber (28) implanted over the right striate cortex using tetrodes controlled with a Mini Matrix microdrive system (Thomas Recording, Giessen, Germany). Data were recorded digitally at 25 kHz and saved to disk for off-line spike sorting to identify single units (Spike 2 software, Cambridge Electronic Design, Cambridge, UK).

The monkeys were trained to fixate a 0.25° spot rear-projected onto a large tangent screen at 57 cm. The fixation spot appeared at either of two positions straddling the midline. The two positions were separated by a distance corresponding to the animal’s strabismus, so that only a small saccade was needed to alternate fixation when the spot switched from one side of the screen to the other.

The tetrode was advanced into the right calcarine cortex, searching for well-isolated, responsive cells. When a promising unit was identified, it was checked immediately for receptive field location and ocular dominance. A unit was accepted for further testing only if 1) eccentricity was >12° for Monkey 1 and >8° for Monkey 2 and 2) ocular dominance strongly favored the right eye. For cells satisfying these criteria, the receptive field and optimal orientation were mapped interactively by presenting stimuli controlled via a computer mouse.

Responses were then tested to stimuli consisting of achromatic sine-wave gratings, 0.5 cycles/°, 100% contrast, drifting at 4°/s, presented at the optimal orientation. They were displayed within a circular aperture having a diameter of 6°–10°. When gratings were displayed to both eyes, their orientations were identical.

Testing Paradigms

Trial type 1 began with occlusion of one eye (Fig. 2). A fixation spot was displayed to the open eye. The monkey took a variable amount of time to fixate the spot, sometimes ranging up to several seconds. Starting 250 ms after the monkey acquired the fixation spot, a grating was displayed in the receptive field of the open eye. After 5 s the occluder was raised, exposing both eyes to the same grating for another 5 s. The monkey was rewarded periodically for maintaining steady fixation during the entire epoch of visual stimulation. After the 10 s trial was concluded the screen became isoluminant gray for 2 s.

Figure 2.

Staggered sequence of stimulus presentation. Trial type 1 began with occlusion (gray square) of the left eye (blue dot). Once the monkey fixated a target (green cross) with the right eye (red dot), a grating was displayed in the nasal field, beyond the midpoint (vertical dashed line) between the centers of gaze. After 5 s, the left eye was uncovered, exposing both fundi to the grating. Presumably, these conditions induce suppression of the grating image in the right eye to prevent diplopia. After a 2 s blank period trial type 2 began with presentation of a target to the same eye. Once it was fixated, a grating was displayed to the receptive field in the eye for 5 s, followed by appearance of a grating in the nonfixating eye’s receptive field for 5 s. Repeated sets of trials were presented in rotation: right eye fixating, trial type 1 then 2; left eye fixating, trial type 1 then 2.

Trial type 2 (Fig. 2) began with presentation of a fixation spot to the same eye that viewed the fixation spot in trial type 1. After a variable period the monkey fixated the spot. After 250 ms of sustained fixation, a grating was displayed in the receptive field of the fixating eye. After 5 s of viewing binocularly this single grating, a second grating was placed in the receptive field of the nonfixating eye, and both gratings were viewed binocularly for another 5 s. A 2 s blank period ensued before the next trial. Trial types 1 and 2 always occurred sequentially, with the eye chosen for fixation during each trial set alternating between right and left. In trial type 2, as for trial type 1, the monkey was rewarded intermittently for maintaining steady fixation. Trial type 2 duplicated the test conditions that reportedly induce strong interocular suppression (15).

Data Quantification and Statistical Analysis

Three indices were computed for each unit to compare responses under different stimulus conditions. In results, each index is diagrammed and explained in the context of the data. Each index could span a value from 0 to 1:

Ocular dominance index.

This index compared the response of a neuron to stimulation of the contralateral (left) versus the ipsilateral (right) eye, with the fellow eye occluded.

$$\mathrm{O}\mathrm{D}\mathrm{I}=\frac{R_{\mathrm{r}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\hspace{0.33em}\mathrm{e}\mathrm{y}\mathrm{e}}}{R_{\mathrm{r}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\hspace{0.33em}\mathrm{e}\mathrm{y}\mathrm{e}}+R_{\mathrm{l}\mathrm{e}\mathrm{f}\mathrm{t}\hspace{0.33em}\mathrm{e}\mathrm{y}\mathrm{e}}}$$

For calculation of the ocular dominance index (ODI), the neuronal response (R) was defined as the mean firing rate following stimulus presentation minus the baseline firing rate. The baseline firing rate was derived from the 100 ms period before stimulus onset. In occasional cells, the response during monocular stimulation for a given eye was lower than the spontaneous firing rate, resulting in a negative value for R. In such cases, the firing rate to that eye was set to 0 spikes/s, to avoid confounding the ODI calculation. The ODI was divided into seven equal bins from 0 (complete left eye dominance) to 1 (complete right eye dominance) to form a traditional ocular dominance histogram (29).

Peripheral retina interaction index.

This index quantified the response of neurons during trial type 1. It compared stimulation of the right eye’s receptive field with the left eye occluded (R_{bestmonocular}) to the response to the same stimulus with both eyes open (R_best).

$$\mathrm{P}\mathrm{R}\mathrm{I}\mathrm{I}=\frac{R_{best}}{R_{best}+R_{best\hspace{0.17em}\hspace{0.17em}monocular}}$$

The peripheral retina interaction index (PRII) was 0 for maximal suppression, 0.5 for no effect, and 1 for maximal facilitation.

Receptive field interaction index.

This index quantified the response of neurons during trial type 2. It compared the response to stimulation of the receptive field in the dominant right eye (R_best) to the response during subsequent stimulation of the receptive fields in both eyes (R_binocular). Under both stimulus conditions, both eyes were open while the dominant right eye fixated.

$$\mathrm{R}\mathrm{F}\mathrm{I}\mathrm{I}=\frac{R_{binocular}}{R_{best}+R_{binocular}}$$

The receptive field interaction index (RFII) value ranged from maximum suppression (RFII = 0), to absent suppression (RFII = 0.5) to maximum facilitation (RFII = 1). It was compiled from data obtained only during right eye fixation trials.

Anatomical Correlation

After recordings were concluded each monkey was euthanized with pentobarbital and then perfused transcardially with 1 L normal saline followed by 1 L of 0.5% paraformaldehyde in 0.1 M phosphate buffer. The primary visual cortex was unfolded, flat-mounted, cryoprotected in 30% sucrose, and sectioned tangentially at 60 μm with a freezing microtome (30). Each section was dried on a slide and processed to reveal the pattern of CO activity (31).

RESULTS

Reduced Metabolic Activity in Columns Driven by the Ipsilateral, Temporal Retina

In both monkeys the CO activity in the ocular dominance columns serving the ipsilateral eye’s temporal retina was reduced relative to the CO activity in the ocular dominance columns of the contralateral eye’s nasal retina. An example is shown in Fig. 3, a single right cortex section from Monkey 2 processed for CO histochemistry. The flat-mount is imperfect, with a split at the hinge between opercular cortex (representing the central 8°) and calcarine cortex (representing 8° to the far periphery). Nonetheless, in calcarine cortex where the section passes through layer 4 C, one can detect a subtle pattern of alternating light and dark columns. The light columns, which are thinner than the dark columns, belong to the ipsilateral right eye. This assignment was established previously in exotropic monkeys by injecting one eye with [³H]proline (23). Columns are also present in opercular cortex, from 3°–8°, but they are even lower in contrast.

Figure 3.

Single flat-mount section from the right primary visual cortex of Monkey 2 processed for CO activity showing light (right eye) and dark (left eye) columns in layer 4. Arrows denote examples of dark columns, which were most evident in peripheral cortex beyond 8°, but also faintly visible between 3°–8°. Damage to the cortex from tetrode penetrations is visible just above the split in the tissue section, but the calcarine cortex [8° to the monocular crescent (mc)] where recordings were made is in good condition. Oval, approximate location of the left eye’s blind spot representation.

Before these recordings commenced, we knew from earlier work that CO activity would be reduced in ocular dominance columns serving the peripheral temporal retina of the right eye (23). This change in cortical metabolism corresponds to the suppression of the far nasal visual field that occurs in exotropes during binocular viewing (Fig. 1; 19). It was the rationale for targeting our recordings to the calcarine sulcus (where peripheral visual field is represented) and to neurons dominated by the ipsilateral eye. We reasoned that this population of neurons was the most likely to show evidence of interocular suppression.

Typical Responses of V1 Cells in Strabismic Monkeys

Figure 4 shows the ocular dominance profile of the cells recorded in this study. Virtually all fell into category 7, because this was a criterion for their selection. Twenty-six cells were recorded in Monkey 1 and 56 cells in Monkey 2. The data were merged because results in the two animals were similar.

Figure 4.

Ocular dominance index (ODI) showing ocular dominance profile of the 82 neurons recorded in the right calcarine cortex. Neurons were selected for recording only if they were strongly dominated by the right eye, for reasons explained in the text. contra, Contralateral; ipsi, ipsilateral.

The two trial types entailed six different stimulation conditions and four different transitions between stimulation conditions (see methods). Figure 5 shows the responses of a typical unit to a complete set of test paradigms. The background firing rate was 26.2 spikes/s. With the left eye occluded, presentation of a grating to the receptive field in the right eye resulted in an increase in the mean firing rate to 67.7 spikes/s (Fig. 5A). The spike rate modulated in synchrony with the drift rate of the grating, revealing this unit to be a simple cell (32). When the left eye was uncovered after 5 s, there was no change in the cell’s discharge. The PRII was 0.50, indicating that exposure of the grating to the left eye did not inhibit or facilitate the response driven by the right eye. Inhibition, had it occurred, would have explained the loss of CO activity in the right eye’s ocular dominance columns (Fig. 3) and would have provided a mechanism for the suppression of diplopia. This is an example of trial type 1.

Figure 5.

Peristimulus time histograms showing responses of a typical neuron. A: trial type 1, right eye fixating. With appearance of the grating, the discharge rate modulates in phase with the drifting grating. No change occurs when the left eye is uncovered after 5 s. B: trial type 2, right eye fixating. During the first 5 s, conditions are the same as during the last 5 s of the previous trial, and the neuron’s response is similar. Adding a second grating after 5 s makes no difference in the firing rate. C: trial type 1, left eye fixating. No response occurs because the cell is right eye monocular. D: trial type 2, left eye fixating. The cell’s discharge rate remains at the background rate until the second grating is added at 5 s, driving the right eye’s receptive field. Like most neurons, this unit shows no change in response from switches in fixation or from induction of stimulus conditions expected to induce suppression.

With both eyes open and the right eye fixating, no change in the firing rate occurred following the transition from stimulation of the right eye’s receptive field to stimulation of both eyes’ receptive fields (Fig. 5B). The RFII was 0.51, reflecting a lack of facilitation from binocular stimulation. Loss of binocular facilitation is characteristic of strabismus. This is an example of trial type 2.

With the right eye closed, stimulation of the left eye’s receptive field produced no response (Fig. 5C). This was expected: the cell was selected for testing because it was strongly right eye dominant. Uncovering the right eye after 5 s made no difference in the firing rate, because the grating presented to the left eye’s receptive field landed on the “diplopia point” in the right eye. The diplopia point is displaced from the right eye’s receptive field by the magnitude of the ocular deviation. When gratings were presented to both receptive fields after 5 s (Fig. 5D), the discharge rate increased sharply, because the right eye’s receptive field was now being stimulated.

Firing rates under all conditions that involved stimulation of the right eye’s receptive field were similar. It made little difference if the left eye was occluded (67.7 spikes/s, Fig. 5A), if the left eye was fixating (61.4 spikes/s, Fig. 5D), if the right eye was fixating under conditions of monocular stimulation (59.6 spikes/s, Fig. 5B) or binocular stimulation (62.7 spikes/s, Fig. 5B).

Figure 6 highlights a neuron that displayed several features, common among the population of 82 recorded cells, which had a spurious impact on the indices used to quantify suppression. For trial type 1, when the grating appeared in the right eye’s receptive field, there was a transient followed by sustained response (Fig. 6A). When the left eye was uncovered, there was no transient response. Over the entire 10 s period of stimulation there was a slight but steady decline in the spike rate. These two properties, the transient peak and the falling spike rate, meant that the response to stimulation of the right eye’s receptive field during monocular viewing was greater (62.3 spikes/s) than during binocular viewing (55.3 spikes/s). This yielded a PRII of 0.47, suggesting a mildly suppressive effect from uncovering the left eye. However, in truth, it simply reflected the properties of transience and adaptation in the neuron’s response.

Figure 6.

Contrast adaptation and response transience. A: trial type 1, right eye fixating. The neuron responds to grating onset with a transient peak in activity followed by a sustained component that declines slightly over 10 s. Opening the left eye has no impact. The PRII (0.47) indicates mild suppression, but may reflect neuronal adaptation and lack of a transient peak upon uncovering the left eye. B: trial type 2, right eye fixating. The response is similar. The RFII (0.44) suggests suppression, but is actually due to adaptation and response transience. C: trial type 1, left eye fixating. There is no response because the cell is right eye dominant. D: trial type 2, left eye fixating. The cell remains unresponsive until the second grating is added at 5 s, stimulating the right eye’s receptive field. PRII, peripheral retinal interaction index; RFII, receptive field interaction index.

The same issues emerged when comparing trials of stimulation of the right eye’s receptive field versus both receptive fields (Fig. 6B). Trial type 2 began with stimulation of the right eye’s receptive field, which produced a transient response followed by a sustained response that declined slightly over 5 s. Appearance of a second grating, in the left eye’s receptive field, evoked a small (unexplained) transient response followed by continued dwindling of the spike rate. Consequently, the response during stimulation of the right eye’s receptive field (62.4 spikes/s) was greater than during stimulation of both eye’s receptive fields (49.5 spikes/s), yielding an RFII of 0.44. This would seem to suggest an inhibitory effect of the left eye upon the right eye, but again, is due to transience and adaptation of the cell’s response.

This was a monocular cell, typical of those encountered in V1 in strabismus. Stimulation of the left eye’s receptive field evoked no response (Fig. 6, C and D).

An Example of Strong Interocular Suppression

Figure 7 illustrates an exceptional neuron that showed powerful interocular suppression when both eyes’ receptive fields were stimulated. Presentation of a grating to the right eye’s receptive field, while the left eye was occluded, resulted in both a transient and sustained response (Fig. 7A). Subsequently, uncovering the left eye made little difference, although the spike rate continued to taper. The transient response and the gradual decline in spike rate (also observed for the unit in Fig. 6) resulted in a PRII of 0.43. Subsequent presentation of a grating to the left eye’s receptive field shut down completely the cell’s discharge (Fig. 7B). The RFII was 0.04.

Figure 7.

A neuron showing powerful suppression, but only for trial type 2. A: trial type 1, right eye fixating. The neuron responds to a grating in the right eye’s receptive field, with no effect when the left eye is uncovered. As in Fig. 6. the neuron adapts over 10 s and its transient peak is confined to the initial grating presentation. Note that the firing rate from −2.0 to 0 s is relatively high, because of disinhibition from removal of the grating stimulating the left eye’s receptive field in the preceding trial. B: trial type 2, right eye fixating. After 5 s of stimulating the receptive field of the right eye, addition of a grating that stimulates the corresponding point in the left eye induces dramatic suppression. C: trial type 1, left eye fixating. No response occurs because the cell is right eye dominant. D: trial type 2, left eye fixating. Introduction of a grating to the left eye’s receptive field suppresses the cell below its spontaneous firing rate. After 5 s, a transient peak occurs with stimulation of the right eye’s receptive field, following by a low rate of sustained firing, not seen in B. Units displaying these properties were expected to be common in this study, but in fact, this neuron was nearly unique among the entire population of 82 cells. Note that the firing rate from −2.0 to 0 s is relatively high, because of disinhibition from removal of the grating stimulating the left eye’s receptive field in the preceding trial.

When a grating was presented to the left eye’s receptive field, with the right eye occluded, spontaneous discharge was inhibited and the cell became nearly silent (Fig. 7C). Subsequently, uncovering the right eye did not relieve the suppression. Grating presentation to the left eye’s receptive field, with the right eye viewing, inhibited spontaneous discharge (Fig. 7D). Subsequent display of a grating to the right eye’s receptive field resulted in a transient peak in the firing rate, followed by inhibition. However, the inhibition was weaker than during binocular stimulation after right eye receptive field stimulation (compare the 5–10 s period in Fig. 7, B vs. D). This difference revealed a long lasting influence of the preceding stimulation condition.

Stimulation of the Right Eye’s Receptive Field, Left Eye Occluded versus Open (PRII)

The PRII compares stimulation of the right eye’s receptive field, with the left eye occluded versus open (Fig. 8). Once the left eye opens, the monkey is exposed to diplopia, because the grating lands on noncorresponding points in the two eyes. Given the reduction of CO activity in the right eye’s ocular dominance columns, we postulated that the response driven by the grating in the right eye’s receptive field might be suppressed by the left eye. The suppression would presumably be generated by neurons at another site in the cortex with receptive fields located at the noncorresponding point in the left eye. The PRII, the measure of this potential phenomenon, was 0.48 ± 0.05 (n = 82 neurons). This was statistically different (P = 0.018, two-tailed t test) from an ideal population of 82 neurons with equal variance having no facilitation or suppression (PRII = 0.50). However, as mentioned above, the reduction in the PRII might be explained by other factors than suppression.

Figure 8.

Peripheral retina interaction index (PRII). This index compares stimulation of the dominant right eye’s receptive field with the left eye occluded versus open. Each circle represents data from a single cell (green = Monkey 1; orange = Monkey 2). The mean PRII was 0.48 ± 0.05 (n = 82). Although this value suggests a trend toward interocular suppression, the interpretation is potentially confounded by adaptation and transience of neurons’ responses.

Stimulation of the Right Eye’s Receptive Field versus Both Eyes’ Receptive Fields (RFII)

The RFII compares, during right eye fixation, the response to stimulation of the right eye’s receptive field versus the subsequent stimulation of both eye’s receptive fields (Fig. 9). In normal animals, binocular stimulation usually results in an enhanced discharge rate of V1 neurons (33). The mean RFII was 0.46 ± 0.08 (n = 82 neurons). This was statistically different (P = 0.002, two-tailed t-test) from an ideal population of 82 neurons with equal variance having no facilitation or suppression (RFII = 0.50). However, the reduction in RFII was only 0.041 (95% confidence interval 0.016–0.067). For a handful of neurons the decreased RFII was genuinely due to interocular suppression, but for most neurons the reduction in RFII, if present, could be attributed to adaptation of the cell’s response or lack of a transient phase during the second interval from 5–10 s.

Figure 9.

Receptive field interaction index (RFII). This index compares stimulation of the receptive field in the dominant right eye to stimulation of the receptive fields in both eyes, while the right eye was fixating. The mean RFII was 0.46 ± 0.08. The unique cell shown in Fig. 7 is denoted by the arrow.

DISCUSSION

Sengpiel and colleagues (14, 15) reported a remarkable phenomenon: in strabismus a neuron driven by a grating displayed in the receptive field of the dominant eye is suppressed by a grating presented in the receptive field of the nondominant eye. Critically, they found that the suppression depends on the temporal sequence of events. It occurs only when the neuron is already being driven actively by a stimulus in the dominant eye. When grating onset in the nondominant eye is simultaneous, rather than staggered, no suppression occurs. The authors concluded that the primary visual cortex is the site of interocular suppression in strabismus, and moreover, that this sequence-dependent form of suppression could explain how diplopia and visual confusion are prevented.

In a previous study we confirmed that in strabismus, when presentation of gratings to both eyes’ receptive fields is simultaneous, there is no overall suppression or facilitation (1). Macaques raised with exotropia were trained to fixate a spot while identical drifting gratings were presented to both eyes’ receptive fields for 250 ms. The RFII compared the response to stimulation of the receptive field in the dominant eye alone to simultaneous stimulation of the receptive fields in both eyes. The mean RFII was 0.50 ± 0.13 (n = 500), a value exactly half way between full suppression (RFII = 0) and facilitation (RFII = 1).

After failing to uncover physiological evidence of suppression, we performed these additional single cell recordings to test the effect of staggering the grating presentation to each eye. We followed precisely the timing protocol previously reported to evoke vigorous suppression (14). Specifically, a grating was presented to the dominant eye for 5 s, followed by a grating to both eyes for 5 more s (trial type 2). We targeted our recordings to neurons dominated by the right eye, whose receptive fields in the right eye were located in the peripheral nasal visual field (Fig. 1). Neurons meeting these criteria are situated in ocular dominance columns with reduced metabolic activity (Fig. 3), representing visual field that is suppressed during binocular viewing (19, 23). These neurons were the most obvious candidates to exhibit suppression.

Eighty-two neurons were recorded, with a mean RFII = 0.46 ± 0.08. Compared with the mean RFII = 0.50 ± 0.13 from simultaneous grating presentation (1), this result suggests a mildly suppressive effect from modifying the experimental protocol to stagger the grating presentations. However, the difference in RFII might reflect more prolonged stimulus exposure (10 s vs. 250 ms), rather than staggered timing. With protracted stimulation, the response amplitude of some neurons shows a decline (34–36). This property is thought to underlie contrast gain control (37, 38). When trial types 1 and 2 were designed, we failed to anticipate that neuronal contrast gain control could be confounded with interocular suppression. Regrettably, we did not include control trials, which would have consisted of 10 s of continuous dominant eye stimulation. Sengpiel and colleagues (see Fig. 9 of Ref. 15) did perform such control trials, but only in four cells. After observing no dwindling of the response over 10 s of continuous grating stimulation for these four units, they concluded that neuronal “fatigue” could not explain their results.

The transient component of the neuronal response is another feature that biases the RFII during staggered presentation. A burst of firing came at the beginning of most trials. This burst was absent or reduced when the second grating appeared at 5 s. This phenomenon tended to increase the mean spike rate during the epoch when one grating was displayed (0–5 s) relative to the epoch when two gratings were displayed (5–10 s).

The index we employed, the RFII, was chosen because it has a symmetrical distribution for any given value of positive or negative percentage change in firing rate. Sengpiel and Blakemore (14) measured suppression using a different metric: the response during grating presentation to both eyes’ receptive fields divided by the response during grating presentation to only the dominant eye’s receptive field. Their data, plotted in a histogram, showed a median suppression of 40% (Fig. 10A).

Figure 10.

Percent change in firing rate in trial type 2. A: data from Sengpiel and Blakemore (Fig. 1B; 15), dividing the response during display of a grating in each eye’s receptive field by the response during display of a grating only to the dominant eye’s receptive field. The median suppression was −40%. B: data from Dougherty and colleagues (Fig. 2A; 33), showing suppression of monocular neurons from binocular grating stimulation in normal macaques. The median suppression bin was −30%. A single unit showed dramatic suppression (arrow). C: our data, plotted the same way, showing a median suppression of only −10%. Few cells in our recordings, or those conducted by Dougherty, showed ≥ −50% change in firing rate. Arrow marks the cell shown in Fig. 7.

Dougherty et al. (33) have examined binocular interactions in striate cortex of normal, awake monkeys viewing grating stimuli. Analysis of a subpopulation of strictly monocular neurons (n = 33) revealed predominantly suppressive effects from stimulation of the “silent,” nondominant eye. The same metric, binocular response divided by dominant eye response, showed a median suppression of 30% (Fig. 10B). This finding suggests that inhibition of monocular cells by the other eye is a property found in normal animals, and not an aberrant feature induced by strabismus. Such units have also been described by Read and Cumming (39).

To facilitate comparison, we have replotted our data using the same quotient (Fig. 10C). The median suppression for our monocular cell population was only 10%, less than the value of 30% obtained by Dougherty and colleagues. The difference may reflect the fact that in addition to loss of faciliatory interactions, there is also a breakdown of inhibitory interactions between the eyes in strabismus (40, 41).

The discrepancy between our data (10% suppression) and those of Sengpiel and colleagues (40% suppression) is more difficult to explain. Suppose that in both strabismic cats and monkeys, there is reduced inhibition of monocular cells by the nondominant eye because of loss of interocular projections in the cortex. In both species, what appears to represent interocular suppression from strabismus might largely be a reflection of contrast gain adaptation. In cat, contrast gain adaptation is much stronger than in monkey (42–44). Consequently, one would predict a greater decline in spike rate over 10 s of continuous stimulation in cats than in monkeys, producing a greater value for the suppression index.

When two gratings are displayed to a person with strabismus and suppression, the subject reports two gratings just as does a normal person, and localizes them accurately. In the laboratory one can present gratings that are identical and separated precisely by the ocular deviation. In real life, stimuli falling on corresponding retinal points in strabismus are seldom identical. They have the potential to induce visual confusion (“Konfundierung,” Ref. 45) because dissimilar stimuli that overlap spatially cannot be perceived simultaneously. In exotropia this sensory dilemma is resolved through anomalous retinal correspondence. Spatial coordinates for one retina are translocated relative to the other retina to compensate for the ocular deviation. This phenomenon eliminates confusion, maintains coherence in perception of the visual scene, and expands the angular range of the combined visual fields (17, 19, 46, 47). The mechanism is not understood (48), but seems unlikely to involve suppression, given that corresponding points in both retinas are perceived simultaneously, albeit in different locations relative to a body-centered (rather than retinotopic) coordinate system. In this context, our failure to find convincing evidence for interocular suppression from stimulation of corresponding retinal loci (RFII = 0.46) was not unexpected.

Display of two gratings to an exotropic subject results in four gratings on the retinas. Why are only two perceived? Each grating lands in one retina at a location that is perceived, and in the fellow eye at a noncorresponding point (the so-called “diplopia point”) that is suppressed. To probe the neuronal correlate of this suppression we employed trial type 1. A grating was presented to a right eye dominant neuron to drive a response while the left eye was occluded. During monocular viewing there is no potential for diplopia, so suppression is absent (Fig. 1A). After 5 s the left eye was opened. In this receptive field location, dichoptic mapping in humans predicts that the grating is perceived via the left eye and suppressed at the diplopia point in the right eye (Fig. 1B; 19). This should inhibit the firing of the right eye dominant neuron. In our previous study, surprisingly, this prediction was not borne out: the PRII was 0.51 ± 0.12 (1). In this present study, following Sengpiel’s approach, we applied a “conditioning” stimulus to activate the neuron for 5 s via the right eye, and then subsequently opened the left eye to induce suppression. Even this staggered sequence of events failed to evoke suppression of right eye dominant neurons. The PRII was 0.48 ± 0.05. This value was tipped slightly towards suppression (0.50 = no suppression), but could be attributed to contrast adaptation.

In our recordings, we encountered only a handful of neurons that displayed the powerful interocular suppression described by Sengpiel and colleagues. The right eye dominant neuron shown in Fig. 7 was shut down completely when a grating was introduced after a 5 s delay to the left eye, exactly as they described. Unfortunately, we did not test this unit for suppression in the context of simultaneous grating presentation, so it is unknown if staggered presentation was required to suppress it. Data from Dougherty et al. (33) indicate that such units occur occasionally in normal monkeys (Fig. 10B) and don’t require staggered presentation for suppression.

What function might be served by such a neuron in strabismus? We have argued that a role in the avoidance of diplopia would require the neuron to be suppressed simply by uncovering the left eye (Fig. 7A). It did not display this property. Had such suppression occurred, it would presumably have been mediated by neurons whose receptive fields were stimulated by the grating at the noncorresponding point in the left eye. In the absence of suppression under such stimulus conditions, this unit does not seem capable of preventing diplopia. Moreover, cells like this example were rare, raising the question whether they exist in sufficient numbers to mediate any phenomenon as robust as strabismic suppression.

In exotropic subjects, stimuli that impinge on a neuron’s receptive field in each eye are perceived as noncorresponding in their location within the visual scene, owing to anomalous retinal correspondence. A unit like the example shown in Fig. 7 might provide a mechanism for triggering the shift in spatial localization that occurs during the transition from stimulation of one eye’s receptive field to both eye’s receptive fields. The inhibition of the cell could signal that although the two stimuli are at corresponding locations in the retinas, they are not at corresponding locations in the visual scene.

In summary, our recordings in awake strabismic monkeys did not confirm the report by Sengpiel and colleagues (14, 15) that delayed presentation of the stimulus to one eye induces interocular suppression of V1 neurons in anesthetized strabismic animals. Our population data showed little evidence of interocular suppression from staggered presentation, whether a single grating (PRII) or two gratings (RFII) were presented to each eye. Previously, in the same two exotropic macaques, we found no interocular suppression when stimuli were presented simultaneously (1). We have no explanation for these negative results, especially given the strong perceptual and anatomical evidence for suppression in exotropia (16, 19, 21, 23, 49–51). This unsolved problem is left for future investigators.

GRANTS

This work was supported by grants EY029703 (J.C.H.) and EY02162 (Vision Core Grant) from the National Eye Institute and by an unrestricted grant from Research to Prevent Blindness. The California National Primate Research Center is supported by a Base Grant from the National Institutes of Health (NIH) Office of the Director, OD011107.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

J.R.E., M.D.D., D.L.A., and J.C.H. conceived and designed research; J.R.E., M.D.D., D.L.A., and J.C.H. performed experiments; J.R.E., M.D.D., D.L.A., and J.C.H. analyzed data; J.R.E., M.D.D., D.L.A., and J.C.H. interpreted results of experiments; J.R.E., M.D.D., D.L.A., and J.C.H. prepared figures; J.R.E., M.D.D., D.L.A., and J.C.H. drafted manuscript; J.R.E., M.D.D., D.L.A., and J.C.H. edited and revised manuscript; J.R.E., M.D.D., D.L.A., and J.C.H. approved final version of manuscript.