Abstract
It has been known from animal neurophysiology that (i) receptive field sizes increase, and (ii) effects of stimulus contrast decrease (response saturation) along the visual pathway. The purpose of our study was two-fold. First, we wanted to confirm these properties of visual neurons for human brain. Second, we exploited these properties to reveal signatures of feedback interactions among visual areas. High-density VEPs evoked by a 16 deg diameter checkerboard contrast reversing at 0.5 Hz were collected from 22 normal subjects using 128-channel EGI Inc. system. In a given trial the stimulus appeared either as the full-field checkerboard (FF) or as a one-quadrant checkerboard (Q1, Q2, Q3, Q4). We then compared the sum of the 4 quadrants VEPs with the full-field VEPs: D = Q1 + Q2 + Q3 + Q4 − FF. The tested hypothesis was that the visual areas further down the visual processing stream with larger receptive fields and stronger response saturation will show larger differences D. Individual VEPs were averaged across epochs, interpolated and averaged across subjects, and then the VEP sources were localized on the average (FreeSurfer) cortex. We quantified the effect of the stimulus configuration with the area summation index ASI = D/(Q1 + Q2 + Q3 + Q4 + FF). Distinct bilateral loci of surprisingly high ASI were found in the occipital (V1/V2/V3), temporoparietal junction (MT/LOC), temporal pole, and frontal pole areas. ASI increased from the occipital to temporal and frontal areas; the strongest ASIs were observed for temporal and frontal poles. These results indicate, that receptive field sizes increase along the visual processing stream. ASI also grew along the course of the VEP epoch in the occipital and temporoparietal areas. We suggest, that this increase indicates feedback from downstream visual areas of high ASI.