The timing of face selectivity and attentional modulation in visual processing.

Despite the complete imprint of a visual scene on the retina, the brain selects particular items for further processing. However, there is considerable debate about when and where the first attentional effects take hold in the cortex. We examined the timing of face specificity and attentional influences in the primary/secondary visual cortex (V1/V2) and in the fusiform gyrus (FG) in two experiments using magnetoencephalography (MEG). In experiment 1, using a passive viewing task, we identified three components in response to "Face," "Hand," and "Shoe" stimuli bilaterally in the FG: M(FG)100, M(FG)170, and M(FG)200-all showing a stronger preference for faces. The timing of these three activations of the FG is consistent with earlier studies claiming distinct stages of processing of visual stimuli in the first 300 ms. In experiment 2, subjects performed a gender-discrimination task on either faces or hands, drawing attention to only one of the two object categories. In addition to the previously identified three components in FG, here we found object-selective attentional enhancement first appearing in V1/V2 at around 170 ms, and then in FG at around 200 ms, i.e. concurrent with the third component. No attentional effects were evident on the first or second magnetoencephalography components. These findings may indicate that the visual input for an object is first encoded and matched to an attended "cue" object held in mind. When the attended and encoded objects match, a third stage involving attentive processing is enhanced.