The neurophysiology of backward visual masking: Information analysis

Backward masking can potentially provide evidence of the time needed for vi sual processing, a fundamental constraint that must be incorporated into co mputational models of vision. Although backward masking has been extensivel y used psychophysically, there is little direct evidence for the effects of visual masking on neuronal responses. To investigate the effects of a back ward masking paradigm on the responses of neurons in the temporal visual co rtex, we have shown that the response of the neurons is interrupted by the mask. Under conditions when humans can just identify the stimulus, with sti mulus onset asynchronies (SOA) of 20 msec, neurons in macaques respond to t heir best stimulus for approximately 30 msec. We now quantify the informati on that is available from the responses of single neurons under backward ma sking conditions when two to six faces were shown. We show that the informa tion available is greatly decreased as the mask is brought closer to the st imulus. The decrease is more marked than the decrease in firing rate becaus e it is the selective part of the firing that is especially attenuated by t he mask, not the spontaneous firing, and also because the neuronal response is more variable at short SOAs. However, even at the shortest SOA of 20 ms ec, the information available is on average 0.1 bits. This compares to 0.3 bits with only the 16-msec target stimulus shown and a typical value for su ch neurons of 0.4 to 0.5 bits with a 500-msec stimulus. The results thus sh ow that considerable information is available from neuronal responses even under backward masking conditions that allow the neurons to have their main response in 30 msec. This provides evidence for how rapid the processing o f visual information is in a cortical area and provides a fundamental const raint for understanding how cortical information processing operates.