Different spatial tunings for ON and OFF pathway stimulation.
We compared the spatial tuning of sensitivity to luminance increments and decrements in three types of localized stimulus presentation with a smooth spatiotemporal envelope. The first type consisted of spatiotemporal Gabor grating functions with either a positive or a negative bias in luminance. The spatial tuning showed a substantially narrower bandwidth and greater peak sensitivity for the positive Gabors. A similar description could be applied to the results for detection of spots with a 2-D difference-of-Gaussian profile, although the shape of the tuning function differed in several respects. We also used the biased Gabor modulation in a contrast self-masking paradigm, where the increment (or decrement) was presented against a steady background with the same spatial configuration, over a range of base contrasts. At medium spatial frequencies the biased masking functions were similar to those typically found for unbiased gratings, in showing a threshold facilitation (dipper) at low contrast, and sub-Weber masking behaviour at higher contrasts. At low spatial frequencies, however, a pronounced asymmetry appeared. Stimuli with a positive bias again showed typical masking behaviour, but a virtually flat masking function was obtained for negative bias stimuli. We conclude that stimuli without abrupt luminance transients reveal pronounced differences in the spatial tuning of responses to positive and negative stimuli, which probably reflect differences in the neural connectivity of the ON and OFF processing systems.
Tyler
C W
CW
Smith-Kettlewell Eye Research Institute, San Francisco, CA 94115.
Chan
H
H
Liu
L
L
D015350:Contrast Sensitivity / Q000502:physiology*; D006801:Humans; D010775:Photic Stimulation; D012165:Retinal Ganglion Cells / Q000502:physiology*; D012684:Sensory Thresholds; D013028:Space Perception; D014795:Visual Pathways / Q000502:physiology*