Chapter 51 - Nonsensory Signals in Early Visual Cortex

Abstract

Two sets of experiments were performed that revealed non-sensory signals in early visual cortex. The first experiment concerned visual attention. Activity in early visual cortex was measured using functional magnetic resonance imaging (fMRI) while subjects performed a challenging contrast-detection task. It was observed a large, stimulus-independent response in early visual cortex, the base response. The base response showed three attributes that are characteristic of visual attention: it depended on task difficulty; it was spatially selective; and most significant, the trial-to-trial variability in the base response predicted behavioral performance on the task. The second set of experiments concerned perception. The behavioral protocol was modified to control the effects of the base response and thereby reliably observe the smaller perceptual signals. Subjects now attempted to detect the presence of slight contrast increments added to a high-contrast background pattern. Behavioral responses were recorded so that the corresponding cortical activity could be grouped into the usual signal detection categories: hits, false alarms, misses, and correct rejects. The cortical activity ranked as hits ≈ false alarms > correct rejects ≈ misses. Thus, the activity in early visual cortex corresponded to the subjects' percepts, even when that percept was the opposite of what was physically presented in the stimulus. Together, the results suggest that early visual areas do more than encode raw sensory signals: they also participate in processing activities that correspond to visual attention and perception.