Abstract
Abstract: :
Purpose: Multifocal methods typically modulate the contrast of checkerboard stimuli with binary psuedo-random sequences. The present work compares binary modulation sequences with two types of temporally sparse ternary stimuli. We also compared monocular and dichoptic stimulation. Methods: In the binary case the contrast of each checkerboard takes the values {-1,1}, the probability of being in either state being ½. In our ternary stimuli the contrasts are {-1, 0, 1}, where 0 indicates a blank field at the mean luminance (48 cd/m²). The two ternary stimuli differed in the probability of in the -1 or 1 state, these being ¼ or 1/16th. Thus in these sparse stimuli the monitor was blank for either ¾ or 14/16 of the time. Stimuli were presented simultaneously to 8 cortically scaled regions containing checkerboards presented around fixation. A liquid-crystal shutter provided dichoptic stimulation of the two eyes in interleaved frames at 50.5 Hz. We obtained 8 repeats of each 40 s stimulus condition. Results: A multiple regression fit ( F=114.6, p < 0.0000) showed that kernel amplitude was greatest for Sparse16 (5.40 ± 0.30 µV) and least for Binary (1.33 ± 0.27 µV). More than 85% of Sparse16 kernels were significant (peak ≷ 2.5 SE), compared with < 15% for the Binary peaks. Dichoptic presentation reduced Sparse16 kernels by 13% (-0.69 ± 0.22 µV) and Binary kernels by 37% (-0.49 ± 0.22 µV) Conclusion: Sparse stimuli appear to appeal to contrast gain control mechanisms[1] that enhance responses to briefly presented, low spatial frequency, stimuli producing larger and more reliable responses. Sparse stimulation provides increased accuracy, or reduced recording time, arising from larger kernel amplitudes. That combined with resistance to binocular rivalry effects appears to make dichoptic multifocal recording feasible. Dichoptic recording affords a better statistical basis for comparison of the responses from the two eyes. [1]Benardete EA, Kaplan E. Visual Neurosci. (1999): 16: 344-368.
Keywords: 394 electrophysiology: non-clinical • 395 electroretinography: clinical • 487 neuro-ophthalmology: optic nerve