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S. Tran, W.K. Stell; Quantification of Chick Spatial Vision Using Optokinetic Following Response . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3331.
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© ARVO (1962-2015); The Authors (2016-present)
Although the chick is a common model for studying myopia and the control of ocular growth, little is known about its visual performance. This study sought to correct uncertainty regarding visual acuity and contrast sensitivity in chicks by determining a standard Contrast Sensitivity Function (CSF), through use of an optokinetic nystagmus (OKN) paradigm.
White Leghorn cockerels were tested 7–9 days after hatching, in a virtual–reality optomotor system (OptoMotryTM) which generated sine–wave gratings of variable spatial frequency (0.011 – 2.0 cy/deg) and contrast (3 – 100%) under constant luminance conditions (min= 4, max =104 cd/m2). Stimuli were rotated horizontally at 12 deg/s. Reflexive following movements of the head (OKN) indicated stimulus visibility. The lowest positive and highest negative contrast thresholds were determined as follows: (1) Contrast Sensitivity Function of untreated chicks, viewing binocularly (n=16); (2) Loss–of–Function Control, after injecting 20µL containing 40 nmoles (n = 5) or 200 nmoles (n = 6) kainic acid into one eye; and (3) Retinal Action–Potential Blockade, after injecting 7 µL of 1 mg/mL tetrodotoxin (TTX) into one eye (n=25). In (2) and (3), left eyes were treated, the contralateral control eye was injected with saline, and treated and control eyes were tested separately. In (2), testing resumed one week after injection, and in (3) repeated testing was done 1.5, 24, 48, and 72 hrs after injection.
(1) Contrast Sensitivity Function: The normal chick CSF is a band–pass function, with peak sensitivity between 5–7% at 0.5 cy/deg and a sharp high–frequency cut–off at 1.0 cy/deg which was observed in all chicks. OKN was observed at 1.3 cy/deg in 10/16 chicks, and no OKN at 1.5 cy/deg. CS declined gradually at low frequencies, with only 3/16 chicks responding at 0.031 cy/deg and none responding below 0.011 cy/deg. (2) Kainic Acid: One week after injection, 40 nmoles had no effect on the normal CSF, but 200 nmoles completely abolished OKN. (3) TTX, at the same dose that abolished the pupillary response for ≥ 48 hrs (McBrien et al, 1995), also abolished OKN; recovery was partial at 48 hrs and complete at 72 hrs.
Rapid and versatile OKN testing by OptoMotryTM is useful for assessing visual function and its alteration by retinal treatments in chicks. OKN acuity agrees with pattern discrimination acuity (Over & Moore, 1981), and the high spatial–frequency tuning of OKN matches that of nBOR cells (Crowder & Wiley, 2001).
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