Spatial attention does not strongly modulate neuronal responses in early human visual cortex

Attention can dramatically enhance behavioral performance based on a visual stimulus, but the degree to which attention modulates activity in early visual cortex is unclear. Whereas single-unit studies of spatial attention in monkeys have repeatedly revealed relatively modest attentional modulations in V1, human functional magnetic resonance imaging studies demonstrate a large attentional enhancement of the blood oxygen level-dependent (BOLD) signal in V1. To explore this discrepancy, we used intracranial electrodes to directly measure the effect of spatial attention on the responses of neurons near the human occipital pole. We found that spatial attention does not robustly modulate stimulus-driven local field potentials in early human visual cortex, but instead produces modest modulations that are consistent with those seen in monkey neurophysiology experiments. This finding suggests that the neuronal activity that underlies visual attention in humans is similar to that found in other primates and that behavioral state may alter the linear relationship between neuronal activity and BOLD.