Neurons in area V1 respond similarly to pop-out and non-popout stimuli

Purpose. A blue vertical bar pops out from among yellow vertical bars, but usually not from among yellow vertical bars and blue horizontal bars. Psychophysical studies suggest that pop-out is an early visual process; it is believed to be a result of analysis of local contrasts, such as those take place in the primary visual cortex (area V1). Method. In two fixating macaque monkeys, we tested whether 103 V1 neurons responded preferentially to pop-out stimuli consisting of colored oriented bars. A bar of optimal orientation and color was positioned within the classical receptive field and additional bars were placed in the surround. We then examined a range of center-surround contrasts on the cell's response. We used 36 different stimuli which included all combinations of center-surround contrast, ranging from no contrast (i.e., center and the surround identical in color and orientation) to high contrast (i.e., color and orientation pop-out). Results. Stimuli falling outside the classical receptive field often modulate the response to a central stimulus. Most V1 cells are suppressed by surround stimuli and only a small minority show response facilitation. In general, V1 cells do not differentiate between popout and non-popout stimulus contrasts in that they show equal surround suppression to homogeneous fields or color and/or orientation contrast. Conclusion. Our results suggest that cells in area V1 signal the presence of surrounds, but they do not fully extract center-surround contrast. If individual neurons respond preferentially to popout stimuli, they must be located in extrastriate cortex.