Top-down modulation of lateral interactions in early vision: Does attention affect integration of the whole or just perception of the parts?

Attention can modulate sensitivity to local stimuli in early vision [1-6]. But, can attention also modulate integration of local stimuli into global visual patterns? We recently measured effects of attention [7] on the phenomenon of lateral interactions between collinear elements [8, 9], commonly thought to reflect long-range mechanisms in early visual cortex [10-15] underlying contour integration [16]. We showed improved detection of low-contrast central Gabor targets in the context of collinear flankers, but only when the collinear flankers were attended for a secondary task rather than ignored in favor of an orthogonal flanker pair. Here, we contrast two hypotheses for how attention might modulate flanker influences on the target: by changing just local sensitivity to the flankers themselves (flanker-modulation-only hypothesis), or by weighting integrative connections between flanker and target (connection-weighting hypothesis). Modeled on the known nonlinear dependence of target visibility on collinear flanker contrast [17-21], the first hypothesis predicts that an increase in physical flanker contrast should readily offset any reduction in their effective contrast when ignored, thus eliminating attentional modulation. Conversely, the second hypothesis predicts that attentional modulation should persist even for the highest flanker contrasts. Our results showed the latter outcome and indicated that attention modulates flanker-target integration, rather than just processing of local flanker elements.