INCREASED ‘NOISE’ IN V2 NEURONS OF AMBLYOPIC MONKEYS

Jonas Auditorium

Objective. To investigate the possible origin of increased internal noise in non-human primates with anisometropic amblyopia. Background. Experiencing early strabismus or chronic monocular defocus due to anisometropia often causes amblyopia unless treated early. Besides reduced acuity and contrast sensitivity, a broad range of more complex spatial and temporal vision deficits have been reported in amblyopes. Increased neural noise in their visual brain has been proposed as one of the sources for reduced visual functions. Methods. We recorded from multiple nearby neurons in Visual Area V2 of adult monkeys reared with monocular defocus between 3 weeks and 3 months of age and developed moderate ansiometropic amblyopia. Stimuli were drifting gratings (3.1 Hz) that were optimized for orientation, spatial frequency, and size for each neuron and were presented at low (25%) and high (80%) contrast for 100 times. For individual neurons, the variance of interspike intervals and variance to mean ratio (VMR) of spike counts were calculated. For multiple neurons recorded at the same site, noise correlation was computed. Results. 1) There was no difference in variance of inter-spike interval among neurons driven by the amblyopic eye, those driven by the fellow eye, and normal V2 neurons. 2) With low contrast stimuli, the VMR (trial-to-trial fluctuation) was significantly elevated in V2 neurons driven by both the amblyopic and fellow eyes compared to that in normal monkeys. With high contrast gratings, there was no difference. 3) Noise correlation in amblyopic monkeys was significantly higher than that in normal monkeys at both low and high contrasts. Conclusion. Our results suggest that the more variable (noisy) responses of individual neurons and the elevated noise correlation in V2 of amblyopic monkeys, combined with abnormal response dynamics and disorganized subfield structures of V2 neurons, may affect signal processing down stream, which in turn may limit their visual performance. Grants. R01-EY0812