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William H Ridder, Kai-Ming Zhang, James A Burke; Comparison of Psychophysical and Electrophysiological Acuity Measurements in Non-Human Primates (NHPs).. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):4310.
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Non-human primates are frequently investigated to understand human diseases of the visual system. A common clinical measurement for many visual diseases is visual acuity. Visual acuity can be measured by employing psychophysical (e.g., the contrast sensitivity function) or electrophysiological (e.g. the sweep visual evoked potential (sVEP)) techniques in NHPs. The purpose of this investigation was to determine the relationship between acuities determined with psychophysical and electrophysiological techniques in the same NHPs.
Four normal, adult NHPs took part in this project. The PowerDiva was used to measure the sVEP on three separate occasions. The NHPs were anesthetized and paralyzed for the measurements (10 mg/kg ketamine and 0.45 mg/kg zemuron). The active electrode (i.e., a stainless steel needle) was placed over the lunate sulcus of each hemisphere. Ten spatial frequencies from 3 to 30 cpd were presented at 80% contrast. The stimulus was focused on the retina with a fundus camera stimulator. A minimum of 10 sweeps (minimum S/N 3.0) were averaged for an acuity estimate. The psychophysical estimate of acuity was determined from the contrast sensitivity function (measured three times on different days). The monkeys were trained to release a lever when they detected a sine wave grating on a monitor. Threshold was determined with a descending method of limits. The contrast sensitivity functions were fit with a double exponential function and the acuity was taken at the 80% contrast level.
The average sVEP acuity was 18.6 ± 0.045 cpd and the average acuity estimated from the contrast sensitivity function was 27.5 ± 7.86 cpd for the four NHPs. A paired t-test indicated that the two test results were not significantly different (p = 0.109).
Even though the visual acuity is higher with the psychophysical technique, there is no statistically significant difference between the two measurements. Both results produced repeatable data for a given NHP. Similar results are seen in human subjects.
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