Figure 7.

Linking the N2pc and surround attenuation in the ST model of visual attention. A, Illustration of the feedforward activation pattern elicited by two search items (a red target and a blue distractor C) in a four-level hierarchical connectivity model of the retinotopic visual cortex. Feedforward activations are represented by diverging pyramids (fan-out factor of 5:1) of red and blue units, which show increasing overlap toward the top level (n) associated with an increase of competition among units representing the target and the distractor. To find a global winner representing the target at the top level (the dark red unit), top-down attention must decide the competition in favor of the red units—an operation proposed to be indexed by the N2pc (double arrow). This sets the starting point for a top-down propagating WTA process to increase the spatial resolution of discrimination beyond that of the top-level units. B, Downward divergent connectivity associated with the top-level winner (units within dashed pyramid in A) determines the extension of the WTA process at the next lower level (n − 1). At n − 1, a new winner is determined (dark red) and connections from nonwinning units to the next level winner (n) are pruned (black lines and gray units in C). This process propagates downwards until the input level (n − 3) is reached (D), thereby leaving a pass zone matching the spatial resolution of units at the input level surrounded by attenuation (surround attenuation). Note that at level n − 1, the forward projection from blue units still overlaps with units representing the red input, which gives rise to competition (double arrow in B) and therefore modulatory effects producing the N2pc. In contrast, at level n − 2, red-representing units projecting to the winner at level n − 1 do not overlap with blue units. Hence, no competition arises and no N2pc response appears.