Biomedical subjects

U Polat

Publications and source records attributed to U Polat.

21 records · Page 2Linked to original sources

PubMed1994

The architecture of perceptual spatial interactions.

Lateral interactions between spatial filters were explored with a lateral masking paradigm. Contrast sensitivity (two-alternative forced-choice) for a Gabor signal in the presence of two flanking high contrast Gabor signals (masks) was measured. When the target to mask distance was less than 2 target wavelengths the contrast sensitivity decreased up to a factor of two relative to a no mask condition. At larger separations, up to eight wavelengths, an increase in contrast sensitivity occurred. This increase was maximal at separation distances of 2-3 wavelengths, where sensitivity increased by a factor of two. However, the enhancement magnitude and range was dependent on the offset between the Gabor signal orientation and the direction defined by the virtual line connecting the two masks (global orientation). Maximal effects occurred when this offset was zero (100% increase in sensitivity) and 90 deg (50% increase). A 45 deg offset yielded only a small enhancement (20%). The enhancement dependence on spatial arrangement was found to be invariant across different global orientations (meridian). This pattern of interactions may be involved in grouping colinear line segments into smooth curves.

Contrast Sensitivity↗

PubMed1993

Lateral interactions between spatial channels: suppression and facilitation revealed by lateral masking experiments.

We measured contrast detection thresholds for a foveal Gabor signal flanked by two high contrast Gabor signals. The spatially localized target and masks enabled investigation of space dependent lateral interactions between foveal and neighboring spatial channels. Our data show a suppressive region extending to a radius of two wavelengths, in which the presence of the masking signals have the effect of increasing target threshold. Beyond this range a much larger facilitatory region (up to a distance of ten wavelengths) is indicated, in which contrast thresholds were found to decrease by up to a factor of two. The interactions between the foveal target and the flanking Gabor signals are spatial-frequency and orientation specific in both regions, but less specific in the suppression region.

Contrast Sensitivity↗

PubMedDate not supplied

Facilitation and suppression of single striate-cell activity by spatially discrete pattern stimuli presented beyond the receptive field.

Visual stimulation of a region outside the receptive field of single cells in visual cortex often results in the modulation of their responses. The modulatory effects are thought to be mediated through lateral connections within visual cortex. Research on lateral interactions commonly shows suppression. There has been no systematic study of the optimal conditions for facilitation. Here we have studied the nature of the modulation using a new type of compound stimulus: contrast reversal of pattern stimuli made of three discrete grating patches. The middle patch, optimally fitted to the receptive field in orientation, size, and spatial as well as temporal frequencies, was flanked by two similar patches presented well outside the receptive field. We found that (1) both facilitation and suppression occurred often in the same cells, when orientations of the target and flankers matched the receptive-field's optimal orientation; (2) facilitation with collinear flankers occurred most frequently at target contrasts just above the cell's firing threshold and suppression prevailed at high contrasts; (3) facilitative or suppressive modulation was obtained with target-flankers separation of up to 12 deg or more; (4) collinear facilitation was lost when flankers' orientation was rotated by 90 deg, while keeping all other parameters the same; and (5) neither the modulation mode nor the proportion of modulated cells was related to the cell types (simple vs. complex cells) and cells' laminar locations. Here we have provided physiological evidence for contrast-dependent, collinear facilitation probably underlying perceptual grouping in humans.

Animals↗