A New Optokinetic Testing Method to Measure Rat Vision

Abstract

Optokinetic nystagmus (OKN) is a reflexive eye movement initiated by the motion of visual stimuli in the field of vision. The head-tracking movement associated with OKN is commonly used as a measure of visual function in rodents. To record OKN responses in normal and experimental rats, a simple and inexpensive apparatus has been developed. This setup uses two tablet screens to display the OKN visual stimulus consisting of high contrast black and white stripes generated using the OKN Stripes Visualization Web Application, a freely available software. The rat is placed inside a clear Plexiglass holder that limits movement so that the rat's head continuously faces the OKN display screen. The position of the rat holder can be changed to adjust the distance between the rat and the display screen. A micro-camera positioned above the rat holder is used to record the rat's visual activities. These recordings can be used for quantitative assessments. Based on the presence or absence of clear head-tracking, the OKN responses at different spatial frequencies can be determined. The collected data demonstrates a novel technique for reliable measurement of visual acuity in normal and retinal degenerate rats.

Figure 1:. Diagrammatic sketch and representative image of the new OKN set-up.

(A) diagrammatic sketch of the rat holder attached to a pedestal and a metal stand placed over a flat surface, (B) dimensions of rat holder and stand that are used for a 6-month-old rat, (C) an image of the new OKN setup. The tablet screens facing each other are placed at a 155° angle.

Figure 2:. New OKN testing apparatus.

Rat is held inside the transparent plastic tube for testing. (A-C) image taken from different angles showing the rat viewing the OKN stimuli.

Figure 3:. Schematic illustration showing different stages of OKN testing and analysis.

Step-by-step instructions describing different stages of OKN testing procedure and data analysis using the new OKN apparatus.

Figure 4:. Visual acuity (±SD) measurements in normal LE rats and retinal degenerate (RD) Royal College of Surgeons (RCS) rats at different stages of the retinal degeneration disease.

Normal LE rats showed the ability to track the stripes at all spatial frequencies (0.15 c/d to 0.33 c/d). RCS rats ages between P35 and P95 (n = 9) were subjected to OKN testing at five different time points. At P35, majority of the rats tracked at all the above spatial frequencies. A drastic decline in visual acuity was noticed in RCS rats between the age of 2 and 3 months. After the age of P100, none of the rats were able to show strong head tracking behavior.