Covariations in ERP and PET measures of spatial selective attention in human extrastriate visual cortex

In a previous study using positron emission tomography (PET), we demonstrated that focused attention to a location in the visual field produced increased regional cerebral blood flow in the fusiform gyrus contralateral to the attended hemifield (Heinze et al. [1994]: Nature 372:543). We related these effects to modulations in the amplitude of the P1 component (80-130 msec latency) of the visual event-related brain potentials (ERPs) recorded from the same subjects, under the identical stimulus and task conditions. Here, we replicate and extend these findings by showing that attention effects in the fusiform gyrus and the P1 component were similarly modulated by the perceptual load of the task. When subjects performed a perceptually demanding symbol-matching task within the focus of spatial attention, the fusiform activity and P1 component of the ERP were of greater magnitude than when the subjects performed a less perceptually demanding task that required only luminance detection at the attended location. In the latter condition, both the PET and ERP attention effects were reduced. In addition, in the present data significant activations were also obtained in the middle occipital gyrus contralateral to the attended hemifield, thereby demonstrating that multiple regions of extrastriate visual cortex are modulated by spatial attention. The findings of covariations between the P1 attention effect and activity in the posterior fusiform gyrus reinforce our hypothesis that common neural sources exist for these complementary, but very different measures of human brain activity.