Lateral masking reveals facilitation and suppression from the same single cells in cat area 17

Purpose. Collinear (i.e., co-axial and co-oriented) stimuli are known to reduce the detection threshold of foveally viewed objects in psychophysics (Polat & Sagi, 1994) and to facilitate the amplitude of human VEP (Polat & Norcia, in press). Here we studied whether single cells in cat area 17 respond in a similar manner. Methods. Single-unit responses were recorded from the anesthetized and paralyzed cat. Classical receptive fields (CRFs) were plotted using a single moving-light slit. A Gabor patch (target) was superimposed on the CRF to approximately cover its spatial extent, in isolation or in the presence of two flanking high-contrast Gabor patches (masks) presented outside the CRF. The target and the masks were presented at the cell's optimal orientation and temporal frequency. The response strength and phase were measured for the target alone, the masks alone and the combination of both. The target contrast was varied between 4% and 80%, while the mask contrast was held constant at 50% or 80%. The responses of the target alone and the masks alone were coherently added to form a linear prediction (linear additivity) of the expected response to the combined stimuli. Linear summation would be predicted if there were no neural interactions between the response elicited by stimulation of the target and that by the masks. Deviations from the prediction were taken as a piece of evidence for the presence of neural interaction ; between them. Results. Response modulation varied depending on target contrast, target-mask separation and relative orientations between them. The majority of recorded cells showed both facilitation and suppression depending on the combination of the above parameters. Sometimes the response modulation involved a phase shift with or without any changes in response strength. Conclusions. Single cells in cat area 17 were able to respond nonlinearly when compound stimuli were presented in a collinear manner. The present finding is consistent with an idea of conceptual grouping of collinear configurations.