Delayed effects of attention in visual cortex as measured with fMRI.

To what extent does attention modulate neural activity in early visual areas? fMRI measurements of attentional modulation in primary visual cortex (V1) show large effects, while single unit recordings show much smaller ones. This discrepancy suggests that fMRI measures of attention may be inflated, perhaps by activity related to other processes. To test whether effects measured with fMRI actually reflect attentional enhancement, we used a rapid acquisition protocol to determine their timing. Subjects were presented with two stimuli on either side of fixation and were cued to attend one and ignore the other. Attended stimuli showed a greater magnitude of response in V1, but this increase was delayed, by roughly one second in time, relative to both unattended responses and response increases due to boosting stimulus contrast. These results suggest that fMRI measurements of attention may primarily depend upon other processes that take a relatively long time to feed back to V1. Our results demonstrate the importance of using the fine timing information available in the fMRI response.