Attentive and pre-attentive aspects of figural processing.

Here we use the steady-state visual evoked potential (SSVEP) to study attentive versus non-attentive processing of simple texture-defined shapes. By "tagging" the figure and background regions with different temporal frequencies, the method isolates response components associated with the figure region, the background region, and with non-linear spatio-temporal interactions between regions. Each of these response classes has a distinct scalp topography that is preserved under differing attentional task demands. In one task, attention was directed to discrimination of shape changes in the figure region. In the other task, a difficult letter discrimination was used to divert attentive processing resources away from the texture-defined form. Larger task-dependent effects were observed for figure responses and for the figure/background interaction than for the background responses. The figure region responses were delayed in occipital areas in the shape versus letter task conditions, while the region interactions were enhanced, especially in frontal areas. While a basic differentiation of figure from background processing occurred independent of task, attentive processing of elementary shapes recruited later occipital activity for figure processing and sustained non-linear figure/background interaction in frontal areas. Collectively, these results indicate that basic aspects of scene segmentation proceed pre-attentively, but that directed attention to the object shape engages a widely distributed network of brain areas including frontal and occipital regions.