Attention does more than modulate suppressive interactions: attending to multiple items

Abstract

Directing attention to a visual item enhances its representations, making it more likely to guide behavior (Corbetta et al. 1991). Attention is thought to produce this enhancement by biasing suppressive interactions among multiple items in visual cortex in favor of the attended item (e.g., Desimone and Duncan 1995; Reynolds and Heeger 2009). We ask whether target enhancement and modulation of suppressive interactions are in fact inextricably linked or whether they can be decoupled. In particular, we ask whether simultaneously directing attention to multiple items may be one means of dissociating the influence of attention-related enhancement from the effects of inter-item suppression. When multiple items are attended, suppressive interactions in visual cortex limit the effectiveness with which attention may act on their representations, presumably because "biasing" the interactions in favor of a single item is no longer possible (Scalf and Beck 2010). In this experiment, we directly investigate whether applying attention to multiple competing stimulus items has any influence on either their evoked signal or their suppressive interactions. Both BOLD signal evoked by the items in V4 and behavioral responses to those items were significantly compromised by simultaneous presentation relative to simultaneous presentation, indicating that when the items appeared at the same time, they interacted in a mutually suppressive manner that compromised their ability to guide behavior. Attention significantly enhanced signal in V4. The attentional status of the items, however, had no influence on the suppressive effects of simultaneous presentation. To our knowledge, these data are the first to explicitly decouple the effects of top-down attention from those of inter-item suppression.

Neural evidence for competition-mediated suppression in the perception of a single object

“…The resulting d' scores provide a more precise measure of performance than accuracy percentages alone, with higher scores indicating better performance. This analysis revealed a mean d' of 2.36, which falls within the range of normal d' scores that others have observed on this task (e.g., Scalf et al, 2011;Scalf & Beck, 2010). Therefore, although accuracy was at floor, the d' analysis…”

“…The use of lateralized presentations uncovered a previously unobserved hemispheric asymmetry: competition-mediated ground suppression was only evident in the LH with RVF presentation, not in the RH with LVF presentation. Hemispheric differences were not observed previously because stimuli were either presented centrally (e.g., Likova & Tyler, 2008;Peterson & Kim, 2001;Peterson & Skow, 2008;Salvagio et al, 2012;Strother et al, 2012) or were presented in the RVF only and only LH activation was assessed (Kaster & Beck, 2005;Scalf et al, 2011). Although laterality effects are not uncommon in cognitive neuroscience, this result still raises the question of why our effect was only observed the LH.…”

“…To assay competition in the perception of a single object, we adapted an fMRI design similar to that used by others to measure competitive suppression between objects (e.g., Beck & Kastner, 2005;Scalf, Basak & Beck, 2011). Participants performed a demanding rapid serial visual presentation (RSVP) task at fixation while the silhouettes appeared individually in either the upper right visual field (RVF) or the upper left visual field (LVF).…”

“…Using lateralized presentations allowed us to accurately locate representations in visual cortex of the contralateral hemisphere. Previously published fMRI studies have presented competing stimuli to the RVF only and assessed left hemisphere (LH) activation only (e.g., Beck & Kastner, 2005;Scalf et al, 2011). Thus, we expected to observe evidence of competition-mediated ground suppression in the LH when we presented our silhouettes in the RVF, but we were uncertain whether we would observe evidence of competition-mediated ground suppression in the right hemisphere (RH) when we presented our silhouettes in the LVF, especially given that our stimuli are so different from those previously used.…”

“…11 attention is focused on fixation (cf. Beck & Kastner, 2005;Scalf, Basak & Beck, 2011). 1 We used it for the same purpose.…”