Abstract
Abstract: :
Purpose: We compared multifocal responses obtained to conventional contrast reversing (CR) and 3 grades of temporally sparse ternary stimuli obtained at 5 contrasts. Methods: Stimuli containing 8 cortically scaled checkerboards/eye were presented dichoptically at 50.5 frames/eye/s. Five contrasts, C= 0.2, 0.4, 0.6, 0.8, and 1.0 were examined. For CR stimuli each region had contrast ±C with probability ½. For the sparse stimuli transiently presented stimuli were present at C or –C interleaved between blank, mean luminance, stimuli presented at 25.4, 6.3 and 1.3 stimuli/eye/s (ses), the last having interleaved binocular and monocular conditions. There were 20 normal subjects (9 men and 11 women, aged 33.1 yr (± 15.2SD). Results: The responses sizes differed little for the different degrees of temporal sparseness when measured at C=0.4. At contrast 1.0 responses to the 1 ses stimulus were 20.3 ± 0.32 dB (10.4×) larger than for the CR stimulus. The concurrently determined responses to the binocular versions of these stimuli were little smaller. The best signal to noise ratios (SNR) were obtained for the 6.3 ses stimuli which were 4.18 ± 0.55 dB (1.62×) larger than for the CR stimuli. Contrast response functions were fit by power functions and exponents increased with increasing sparseness, rising from 0.15 ± 0.06, to 0.46 ± 0.1. Conclusions: The more rapidly rising contrast response, and contrast vs. SNR, functions for sparser stimuli indicate that a contrast gain–control mechanism like that governing responses of M–cells to transiently presented coarse stimuli [Benardete & Kaplan Visual Neurosci. 16, 344–368, 1999] may be driving the larger and more reliable responses to sparser stimuli. The 4.18 dB larger SNRs of the 6.3 ses stimulus translate into 2.6 times shorter recording time than CR stimuli to achieve the same SNR.
Keywords: electrophysiology: non–clinical • temporal vision • contrast sensitivity