Abstract
Previously we presented results of systematic measurements of neural activity in early visual cortex in response to stimuli with skewed luminance histogram. We found that V1 responses to noise images with positively and negatively skewed luminance histogram were significantly larger than responses to images with zero luminance histogram skew.
Moreover BOLD response to images with negative contrast was larger than the response to those with positive luminance histogram contrast. In the current study we investigate the receptive field properties of skew sensitive V1 neurons further.
Stimuli were arrays of randomly oriented 2 cpd Gabor elements. While keeping mean and standard deviation constant, images' luminance histograms had either positive, negative, or zero skew. Images were vignetted by a circular aperture (r=6.35 deg visual angle) and presented dynamically at either a rate of 10Hz or at rate of 4Hz in a 50% duty cycle (alternating blank and stimulus) in blocks of 18s. In each block only one type of luminance histogram skewness was used. Blocks of stimuli were interleaved with ‘blanks’, which consisted of 18s presentation of a static image with uniform mean luminance. The order of presentation during each scan (TR=2s) was randomized. Each stimulus block was repeated twice during each scan, and there were a total of six scans per session. Observers performed a demanding fixation task during each scan. The BOLD signal was analyzed within an independently determined ROI corresponding to an annulus located within the stimulus area.
We find that with these new stimuli the previously observed preferential activity in V1 to negative skew diminished dramatically. Furthermore, our results show a stronger V1 BOLD response to negative skew noise images than to Gabors. However, we do not find such a difference between stimulus type (Gabor vs noise) for positive or zero skew stimuli.
This work was supported by NIH grant EY015261, and CMRR/Mayo NCC grant P30 NS057091. The 3T scanner at the University of Minnesota, Center for Magnetic Resonance Research is supported by BTRR P41 008079 and by the MIND Institute. Partial support has been provided by the Center for Cognitive Sciences, University of Minnesota.