A New Optokinetic Testing Method to Measure Rat Vision

When the eye is exposed to sustained full-field visual motion, a distinct pattern of fast and smooth tracking eye movements and low-acceleration head movements emerge in the direction of visual motion, called optokinetic nystagmus (OKN)^1,2. The neurological pathway of OKN passes from the retina to the lateral geniculate body, occipital lobe, and cerebellar flocculus and connects to the ocular motor neurons³. Neural damage anywhere along these neural pathways may lead to changes in the OKN responses. The OKN response is used as a tool to assess cerebral symmetry, psychogenic blindness, and visual acuity in human patients^4,5. Visual acuity is assessed by quantifying functional responses, which can be integral in determining the success of treatments and experiments centered around restoring vision lost due to neurodegenerative diseases^3,6,7. In animals, OKN responses can be used to accurately assess visual acuity, which provides researchers with the ability to collect both quantitative and qualitative data regarding visual function. In rodents, it is possible to measure the visual acuity of the left and right eyes independently based on the direction of rotation of the stripes both in the clockwise and counter-clockwise directions⁸. This counter-clockwise and clockwise motion exposes each eye to either naso-temporal (N-T) or tempo-nasal (T-N) motion⁹, respectively. The T-N stimulus results in significantly higher response compared to the N-T stimulus since rodents are more sensitive to the dangers coming from behind or from the side.

Previously, visual function in normal laboratory rats and retinal degenerate rats was tested using different OKN testing methods^{6,10,11,12,13}. However, certain variabilities in the visual acuity scores are observed between different studies, including the data which is shown in the present investigation. This variability can be mostly attributed to the differences in the testing setup used. Differences in the size of the testing arena and the type of the OKN visual stimuli used^6,10 can be the major factors. The stimuli used in these experiments include sinewave gratings for the appearance of a virtual cylinder¹⁴, interchangeable rotating cylinders¹⁵, and high contrast (black and white) stripes displayed on four computer monitors¹⁰. Major limitations associated with these OKN testing apparatus and methods include the large size of the equipment, movement of the animals in the testing arena, and frequent incidence of the animal falling from the testing platform^7,11,12.

To minimize the above limitations, a new apparatus for OKN testing in rats has been developed. This apparatus is comparatively inexpensive, proven to be efficient, easy to operate, and enables assessment of visual function (Figure 1). The apparatus uses two tablet screens to display the OKN visual stimuli (visualization software) at different spatial frequencies. A micro-camera is used to record the animal's activities during testing for later analysis of the data. With the objective of making an easy-to-set up OKN apparatus in research laboratories, this new setup outlines critical modifications to the existing OKN testing apparatus. The OKN stimuli used here consists of black and white stripes at different spatial frequencies and different directions of rotation (left to right or right to left). The major component of the OKN testing apparatus includes two touchscreen tablet screens (7.9 inches) used to display the OKN stimuli (Figure 2). Two adjustable holders are used to hold the tablet screens in the desired position. The holders are securely attached to the edge of a procedure table that allows adjustment of its height and angle. The rats are placed in a rat holder that faces the display screens. The rat holders are made of transparent plastic (polymethyl methacrylate) tube. The holder is attached to a pedestal and a metal stand to ensure stable placement on the procedure table. The size of the holding tubes varies from 4 to 6 inches in length and 2.5 to 3 inches in diameter, depending on the size of the rats used. The distance between the rat and the display screen is adjusted by changing the position of the rat holder. The rat holder helps to maintain continuous exposure of the rat's head toward the display screens and reduces its movements during testing. A micro-camera is used to record the head tracking responses. The shortcomings of this new setup include different refresh rates for the screen and the possibility of optical illusions when using narrow stripes. However, these can be considered as common issues associated with a computer-based OKN setup. In addition to the above issues, in the current setup, rats are not tested using a virtual cylinder¹⁴ that influences the optimum OKN response. The novelty of this method lies within the technique and apparatus in which the method is employed. This technique can be easily set up in research laboratories for reliable visual acuity measurement in rodents.