Assessment of visual acuity in Python regius using optokinetic response

Abstract

Snakes are known for their unique abilities including infrared reception and their heavy reliance on heat sensors and vibrations. Infrared reception of snakes has gone under immense investigation; however, there have been very few studies that elaborate on their capacity to see. The goal of this study is to determine visual acuity of ball pythons (Python regius) by observing their optokinetic response (OKR). The OKR is a series of rapid saccadic and smooth pursuit movements of the eyes. It has been used for decades to determine visual acuity in multiple species such as humans, rats, and other nonmammalian species such as zebrafish and box turtles. Past studies have discovered that birds, reptiles, and amphibians achieve gaze stabilization by head and body movements, whereas in mammals and fish, gaze stabilization is conducted by eye movements. In this study, ball pythons were placed in a clear tube in a dark room, and a spinning black and white grating was projected in front of them. The size, direction, and velocity of the grating was manipulated which allowed their visual acuity to be determined. Our hypothesis is that P. regius would have a poor OKR response with low visual acuity due to their heavy reliance on other senses. Results show that P. regius does respond to visual stimuli, shows ocular saccadic movement in the direction of their stimuli, and has a relatively poor visual acuity when compared to other previously studied reptiles.

1. INTRODUCTION

On average, 50 000 ball pythons (Python regius) are imported yearly and are considered one of the most popular pet snake breeds in the United States. ¹ They are known for their protective mechanism of “balling,” in which they coil into a small ball when threatened, hence their common name being ball python. ² In their natural habitat they are found in grassland or open forest in West and Central Africa, spend a majority of their time on or underground in burrows, and are most active at dawn and dusk. ³ Overall, snakes are known for their unique sensory abilities including infrared reception and their reliance on heat sensors and vibrations. ⁴ Infrared reception of snakes has gone under immense investigation, and it has been shown that they have the ability to create a thermal image through infrared radiation detection. ⁵ However, there have been very few studies that elaborate on their capacity to see despite their popularity; therefore, this is an unprecedented investigation.

The optokinetic response (OKR) in P. regius was studied to assess the visual capacity of this species. The OKR refers to a refixation behavior of the eye, often measured by viewing a sequence of vertical stripes moving horizontally in front of their eyes. ⁶ OKR consists of two distinct phases: a slow phase and a fast phase. The slow phase is a smooth pursuit, which stabilizes the image on the retina, and the fast phase is rapid saccadic movement of the eyes. ⁷ The reflex does not take place if the subject is unable to resolve the moving stimulus pattern. ⁶ The visual system has evolved this reflex as an attempt to maintain a fixed image on the retina for as long as possible.

Optokinetic response has been used for decades to determine the visual acuity in species such as humans and rats; however, only one previously documented study has been done to assess the visual acuity of snakes. ⁸ Previous studies have been conducted on reptiles such as Three‐toed box turtle (Terrapene carolina triunguis), Midland water snake (Nerodia sipedon pleuralis), and the Paradise flying snake (Chrysopelea paradisi) which revealed that reptiles often achieved gaze stabilization by head and body movements. ⁸ , ⁹ , ¹⁰ On the other hand, in mammals and fish, gaze stabilization is conducted by eye movements. ⁷ Due to previous studies that showcased the heavy reliance snakes have on other senses, our hypothesis is that the snakes would have a lower visual acuity compared to previous documented studies on other species. Along with a previous lack of understanding about the visual acuity in this species there has been no scientific studies on the pupillary light reflex (PLR) in this species. The PLR is controlled by various factors and can provide insight on visual system function. ¹¹ Through this pilot investigation into visual function, along with the PLR, we aim to improve care practices for the popular pet, P. regius.

2. METHODS AND MATERIALS

2.1. Animals and housing

Four privately owned ball pythons were obtained for this experiment with verbal consent from the owners. Owners confirmed that none of the snakes were currently undergoing ecdysis and that they had not been fed within the last 5 days. All snakes were transported to the research lab located at Western University of Health Sciences in a heated travel carrier. Upon arrival, the animals were maintained within a room that was temperature‐controlled at 21.1°C [70°F] (+/−1°) and humidity‐controlled (between 40% and 60%). Their travel carriers were placed on a heating pad to ensure continued warmth. Prior to commencing the experiment, a physical exam was conducted by a veterinarian to ensure that all of the snakes were clinically healthy. The snakes were kept in a well‐lit (14.2 μW/cm² 275 lux) room for a maximum of 2 h total. The photometer used to measure light intensity in μW is the model IF PM by Industrial Fiber Optics in Tempe, Arizona. The lux meter is the l × 1330 b by Dr. Meter, Shenzhen Thousandshores Technology Co., Newark, California. It is not recommended to make direct comparisons between the 2 meters. However, both meters provide relative data on light intensity which researchers may find useful when furthering the work presented here.

2.2. Pupillary light reflex

The laboratory was darkened to 1.84 μW/cm² 31.2 lux at the beginning of the experiment, and the snakes were allowed to acclimate for 15 min. After this acclimation period, pupils were checked to ensure they were dilated using an infrared light and camera. A handheld light (28.5 μW/cm² 250 lux) was quickly shined in their eyes at a distance of 30.5 cm, and constriction was observed in the eyes.

2.3. Optokinetic experiment

Like the conditions for the PLR, the laboratory was darkened to 1.84 μW/cm² 31.2 lux and the snakes were allowed to acclimate for 15 min. After the acclimation period, pupils were checked to ensure they were dilated and not constricted using an infrared video camera (Arducam Kowloon, Hong Kong). The ball pythons were restrained by hand and placed in a clear acrylic tube (snake tube) with the diameter being just large enough such that there was minimal head movement allowed. The cranial third of the snakes was placed in the clear 30.5 × 2.5 cm acrylic tube while the caudal two thirds of the snake was manually supported and restrained. A white piece of paper, which acted as a screen, was placed in front of the acrylic tube in which a black and white grating was projected from the rear using an LCD projector (model L21 by DBPower Camas, Washington). The projector was placed 118 cm away from the paper. The eyes of the snake were positioned 6–17 cm away from the projected grating, with this distance varying due to where the snakes became comfortable and rested. An infrared video camera was placed on the side of the clear tube to visualize and record any movement of the eye. The camera had to be focused accordingly each time due to the placement of the head and the acrylic tube. The OKR setup is further explained below and shown in Figure 1.

FIGURE 1.

OKR set up. Consisted of a projector illuminating black and white grating. The projector was connected to a computer that controlled the grating. The grating could be manipulated by the computer in thickness, speed, and direction. The cranial third 1/3 of the snakes were placed in a clear 30.5 × 2.5‐cm acrylic tube, and an infrared camera was placed on the side of the clear tube to visualize eye movements.

Once situated, an alternating black and white grating image was projected onto the screen. The mean luminance of the black and white stripes was 0.15 μW/cm² 10 lux and1.11 μW/cm² 54 lux respectively, as presented to the snakes. The measured Michelson contrast was 72% and was kept constant for all experiments. The grating size, speed, and direction were manipulated via computer. We started each testing round with a grating size of 27 mm at a velocity of 6.4 cm/s; the grating was in continuous motion throughout the entire process. Once the OKR was elicited, the grating was decreased by 4 mm increments until an OKR response was no longer noted, indicating that the eyes had stopped tracking the stripes. The threshold at which a response was no longer visualized was noted. At this threshold, the stripe size was then increased and/or decreased by 0.25 mm increments to ensure an accurate threshold was determined. The grating size was adjusted a minimum of 3 times to assure that OKR stopped at the same level each time.

2.4. Calculating visual acuity

Visual acuity was determined by calculating the cycles per degree (cpd):

$$\frac{1}{2{\tan }^{-1}\left(\frac{h}{2a}\right)}$$

A is the distance from the center of the snake eye to the grating, and h is the length of one cycle of the smallest grating at which OKR was observed.

All procedures were approved by Western University of Health Sciences, Institutional Animal Care and Use Committee (IACUC) Protocol #1721500.

3. RESULTS

The authors documented that P. regius responded to the OKR and showed saccadic movement in the direction of the grating. The average visual acuity among the 4 snakes was 0.13 cpd with a standard deviation of 0.1 cpd testing both eyes together for each snake (oculi uterque [OU]) (Figure 2). The best visual acuity recorded among the snakes was 0.23 cpd in snake #2. Gaze stabilization was achieved through ocular movements and not head movements. Tracking was not observed in a well‐lit room and was only observed in a dark room. Under these dark conditions the eye was fully dilated. The snakes exhibited a robust pupillary light reflex (PLR) when a point source of light momentarily shone in their eyes (Figure 3).

FIGURE 2.

Visual acuity in Python regius. The visual acuity was measured from both eyes. The cycles per degree was calculated in order to determine the visual acuity of P. regius and represents the threshold response. The average visual acuity for the 4 snakes was 0.13 (± 0.1 standard deviation) cpd OU.

FIGURE 3.

Pupillary light reflex. Eye fully dilated in a completely dark room (left). Constricted pupil when exposed to moderately bright light (right).

4. DISCUSSION

This pilot study's primary goal was to determine if the visual acuity of P. regius could be determined. In pursuit of this, the authors also discovered some interesting findings concerning the vision of this species. While determining the optimal environment in which to conduct this study, the authors found out that this species is more light sensitive than expected. The study had to be conducted in a room with 1.84 μW/cm² 31.2 lux of light in order to dilate the pupils, which assists in recording the OKR response.

Because this was a pilot study, there was a substantial learning curve in determining the optimal setup of the OKR device and room environment in order to obtain useful results. From too much head movement due to the innate curiosity of the snakes, to inability to focus the camera on the eyes, to initially not conducting this in a dark enough setting, all of these were challenges in the design of this study. Pupillary Light Reflex (PLR) had a great impact on being able to determine P. regius' visual abilities. The PLR affected the OKR greatly, and many adjustments were made to accommodate their sensitivity to light. Initially this experimental procedure was attempted in a room with normal brightness (275 lux); however, P. regius' highly constricted pupils would not track the black and white grating to any noticeable degree in this setting. Because of the highly sensitive PLR, the snakes needed to be placed in a completely dark room (31.2 lux) for at least 15 min for their eyes to properly adjust and dilate (Figure 3). Once the snake was acclimated and the pupils dilated, an infrared light was then used to track the dilated eye and movement so as not to constrict the pupils. Additionally, because of this sensitivity, dark gray and black stripes were initially utilized to minimize contrast of the grating, and then gradually the dark gray stripes lightened until white and black contrast was reached. This gradual progression to white and black allowed the eyes of the snakes to adjust slowly without full constriction of the pupils.

The OKR was only elicited with very low luminosity (31.2 lux), which mimics a night setting; this most imitates their native habitat where they visualize and hunt prey. In other words, sensitivity to light could be attributed to crepuscular behaviors. This sensitivity to light can impact the housing and care that is necessary for these animals. Perhaps it would be more appropriate for P. regius to be kept in darker‐lit rooms to accommodate for their sensitive PLR. Understanding light sensitivity within the species allows for better observation in the future. Due to our significant findings, future research involving ocular tracking and sensitivity to light is warranted.

One previous documented study in snakes observed OKR occurring through oscillation of the head referred to as head wagging. This head wagging was seen in the direction of the stimuli and was elicited in arboreal flying snakes, Chrysopelea paradisi. ⁸ Initially, we attempted the OKR with no acrylic tube to determine if the head wagging would occur in this species. No tracking head movements or head wagging was found within this population of P. regius. While C. paradisi and P. regius fall under the same Class of Reptilia, they exist in vastly different environments and exhibit contrasting behaviors. The authors suggest that differences in lifestyle impacts changes in ocular tracking and head wagging. A stealth‐type hunting strategy that P. regius utilizes requires the head to remain motionless; meanwhile, C. paradisi often performs arial locomotion. C. paradisi may have adapted head wagging as a mechanism to stabilize the images as they are in flight, and the movement of their heads would be perceived similarly to a leaf blowing in the breeze. ⁸ Further investigation is warranted to determine head oscillation activity in species with a lifestyle similar to that of P. regius, such as Boa constrictor or Eublepharis macularius.

For these four snakes in our study, the highest visual acuity found was 0.23 cpd and the lowest was 0.06 cpd. A human with 20/20 vision is equivalent to 30 cpd; therefore, our snakes with an average of 0.13 cpd have a visual acuity of roughly 20/5000. Studies have shown that other reptiles, such as Terrapene carolina triunguis and Nerodia sipedon pleuralis, have an average of 0.26 cpd. ⁹ and 4.25 cpd respectively. ¹⁰ Compared to other reptilian studies, this pilot study indicates that P. regius have rather poor visual acuity. Previous studies have shown that the retinas of P. regius are comprised of 90% rods with a maximum wavelength absorbance of 494 nm. ¹² A rod‐dominated retina is best suited for movement and hunting in dim lighted environment, which was observed in P. regius. This limits their visual capabilities; however, studies have shown that P. regius is utilizing a combination of photoreception and thermoreception and that both heat sensing and visual systems signal the optic tectum. ¹² These are rather unique creatures utilizing multimodal senses to adapt to their natural environments. Additional research is needed to examine the retina cytoarchitecture and physiology in P. regius to better understand their impact on visual acuity in these snakes. Additionally, because we did not intentionally use UV stimulation as part of the OKR examination, future studies may want to incorporate UV as part of the visual stimulus given that UV‐sensitive cones are one of two cone photoreceptors in P. regius. ¹²

The ball python that was found to have the lowest visual acuity of 0.06 cpd was a “bumblebee” mutation compared to the other three with normal skin colorations. This lighter‐colored morph is a result of inbreeding two other lighter colored inbred variations (spider × pastel). ¹³ Beginning in 1992, ball python breeders have been breeding their snakes to achieve over 4000 variations in pigmentation, patterns, and much more. In humans, the median visual acuity for individuals with albinism is 20/80; however, individuals have been found to have a visual acuity of 20/800 when affected with albinism due to foveal hypoplasia. ¹⁴ Further research needs to be done to determine if the genes causing the lighter coloration mutation of these snakes could be affecting the eyesight of this species as well.

There were numerous limitations of the study including a very small sample size, and identification of the OKR response was limited to the author's ability to see the minimal movement of the eye. We also did not explore the effect distance, light wavelength, and other variables may have on the visual response in P. regius. While the authors do not consider the visual acuity numbers to be conclusive by any means, this pilot study does confirm that the OKR of ball pythons can be measured.

5. CONCLUSIONS

In conclusion, this study determined for the first time ever that Python regius does demonstrate an OKR. Based on our findings, the visual acuity is estimated to be 0.13 ± 0.1 cpd which is indicative of poor visual acuity, which the authors hypothesized. P. regius' OKR is demonstrated by saccadic tracking with just the eyes as opposed to head wagging as seen in other species. Lastly, pupillary constriction was noted after brief illumination at 250 lux, which impacts their visual acuity. Due to our novel findings and the questions they posed, additional research is needed to determine the optimal stimulus and conditions for maximal acuity in the P. regius.

AUTHOR CONTRIBUTIONS

Zaira Gomez: Data curation; investigation; methodology; writing – original draft; writing – review and editing. D. Joshua Cameron: Conceptualization; data curation; formal analysis; investigation; methodology; project administration; resources; supervision; validation; writing – review and editing. Curtis Eng: Conceptualization; data curation; formal analysis; funding acquisition; investigation; methodology; project administration; resources; supervision; validation; writing – review and editing.

ETHICS STATEMENT

This study complies with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research and was approved by the WesternU IACUC. Animal owners or owners' representatives provided informed consent for enrollment in the study.

Gomez Z, Cameron DJ, Eng C. Assessment of visual acuity in Python regius using optokinetic response. Vet Ophthalmol. 2025;28:619‐624. doi: 10.1111/vop.13259

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.