Lateral interactions
Selective contour integration
It has already been found that the visibility of an oriented target stimulus improves when presented in the physical context of a contour (i.e. target and contextual stimuli are collinear). This effect is thought to reflect the fundamental function of integrating contours, and most likely takes place via modulation of integrative ‘horizontal’ connections between cells in early visual cortex (e.g. V1).
I found new evidence that the specific task of judging global properties of a contour robustly gated the lateral interactions, such that the influence of collinear context was effectively switched off when task-irrelevant. (e.g. Nature Neuroscience, 2001
). This result demonstrates that attention may affect the fundamental processes involved in the integration of visual elements into the perception of continuous contours.
Further experiments established that this effect was attributable to attentional gating of the contextual interactions (rather than to contrast-gain of the contextual elements themselves; Current Biology, 2003
).
We then confirmed that this attentional modulation of facilitatory lateral interactions depends specifically on the flankers being lined up to form a contour (i.e. collinear), with no such modulation when this configuration is disrupted (Perception, 2004
).
A further psychophysics study (Perception & Psychophysics, 2005
) established that such attentional modulation of lateral interactions arises only when observers must specifically judge global properties of the virtual contour formed between the target and any collinear flankers. Strikingly, the attentional modulation of contrast thresholds for detecting the target Gabor, as a function of which surrounding flankers were attended for a secondary task, was completely abolished when observers had to judge only the local properties of these flankers (such as their individual orientation, contrast or colour), rather than their global relation to the target. This provides a striking demonstration of top-down task-dependence in lateral interactions between Gabor stimuli. As such, it shows that a form of grouping thought to be among the very earliest in the visual system (putatively attributed to horizontal connections in early visual cortex) is in fact highly task-dependent, being subject to top-down influences.
In collaboration with Dr Wayne Khoe at UC San Diego, event-related EEG was used to measure, firstly, the electrophysiological correlates of the basic collinear-facilitation phenomenon (Vision Research, 2004
), and secondly, the attentional modulation of such collinear interactions of the kind described above (Brain Research, 2006 
). In the latter study, EEG results confirmed that such attentional modulation involves brain mechanisms in early visual cortex, but this modulation has a relatively long latency indicating the involvement of top-down feedback processes.
Taken together, these results help to define very specifically the function served by attentional modulation of lateral interactions: to facilitate perception of global contours when they become task-relevant. Thus the act of selecting one task-relevant contour from among a scene composed of other possible contours actively modulates the contour-integration mechanisms in early visual cortex that can group the selected local oriented segments into a unique continuous contour (see chapter in Neurobiology of Attention for a summary
).
Thursday, 29 March 2001
