Abstract
Early visual cortex exhibits widespread hemodynamic responses during task performance even in the absence of a visual stimulus. Unlike the effects of spatial attention, these “task-related responses” rise and fall around trial onsets, are spatially diffuse, and even occur in complete darkness. In visual cortex, task-related and stimulus-evoked responses are similar in amplitude and sum together. Therefore, to interpret BOLD fMRI signals, it is critical to characterize task-related responses and understand how they change with task parameters. We measured fMRI responses in early visual cortex (V1/2/3) while human observers judged the orientation of a small peripheral grating in the right visual field. We measured task-related responses by only analyzing voxels in the ipsilateral hemisphere, i.e., far from the stimulus representation. Task-related responses were present in all observers. Response amplitude and timing precision were modulated by task difficulty, reward, and behavioral performance, variables that are frequently manipulated in cognitive neuroscience experiments. Surprising events, e.g., responding incorrectly when the task was easy, produced the largest modulations. Response amplitude also covaried with peripheral signatures of arousal, including pupil dilation and changes in heart rate. Our findings demonstrate that activity in early visual cortex reflects internal state — to such a large extent that behavioral performance can have a greater impact on BOLD activity than a potent visual stimulus. We discuss the possible physiological origins of task-related responses, what information about internal state can be gleaned from them, and analytic approaches for modelling them.