Unconscious Familiarity-based Color-Form Binding: Evidence from Visual Extinction.

There is good evidence that early visual processing involves the coding of different features in independent brain regions. A major question, then, is how we see the world in an integrated manner, in which the different features are "bound" together. A standard account of this has been that feature binding depends on attention to the stimulus, which enables only the relevant features to be linked together [Treisman, A., & Gelade, G. A feature-integration theory of attention. Cognitive Psychology, 12, 97-136, 1980]. Here we test this influential idea by examining whether, in patients showing visual extinction, the processing of otherwise unconscious (extinguished) stimuli is modulated by presenting objects in their correct (familiar) color. Correctly colored objects showed reduced extinction when they had a learned color, and this color matched across the ipsi- and contralesional items (red strawberry + red tomato). In contrast, there was no reduction in extinction under the same conditions when the stimuli were colored incorrectly (blue strawberry + blue tomato; Experiment 1). The result was not due to the speeded identification of a correctly colored ipsilesional item, as there was no benefit from having correctly colored objects in different colors (red strawberry + yellow lemon; Experiment 2). There was also no benefit to extinction from presenting the correct colors in the background of each item (Experiment 3). The data suggest that learned color-form binding can reduce extinction even when color is irrelevant for the task. The result is consistent with preattentive binding of color and shape for familiar stimuli.

The attraction of yellow corn: reduced attentional constraints on coding learned conjunctive relations.

Physiological evidence indicates that different visual features are computed quasi-independently. The subsequent step of binding features, to generate coherent perception, is typically considered a major rate-limiting process, confined to one location at a time and taking 25 ms per item or longer (A. Treisman & S. Gormican, 1988, Feature analysis in early vision: Evidence from search asymmetries, Psychological Review, Vol. 95, pp. 15-48). We examined whether these processing limitations remain once bindings are learned for familiar objects. Participants searched for objects that could appear either in familiar or unfamiliar colors. Objects in familiar colors were detected efficiently at rates consistent with simultaneous binding across multiple stimuli. Processing limitations were evident for objects in unfamiliar colors. The advantage for the learned color for known targets was eliminated when participants searched for geometric shapes carrying the object colors and when the colors fell in local background areas around the shapes. The effect occurred irrespective of whether the nontargets had familiar colors, but was largest when nontargets had incorrect colors. The efficient search for targets in familiar colors held, even when the search was biased to favor objects in unfamiliar colors. The data indicate that learned bindings can be computed with minimal attentional limitations, consistent with the direct activation of learned conjunctive representations in vision.

The grouping benefit in extinction: overcoming the temporal order bias.

Grouping between contra- and ipsilesional stimuli can alleviate the lateralised bias in spatial extinction (Gilchrist, Humphreys, & Riddoch, 1996; Ward, Goodrich, & Driver, 1994). In the current study we demonstrate for the first time that perceptual grouping can also modulate the spatio/temporal biases in temporal order judgements affecting the temporal as well as the spatial coding of stimuli. Perceived temporal order was assessed by presenting two coloured letter stimuli in either hemi-field temporally segregated by a range of onset-intervals. Items were either identical (grouping condition) or differed in both shape and colour (non-grouping condition). Observers were required to indicate which item appeared second. Patients with visual extinction had a bias against the contralesional item appearing first, but this was modulated by perceptual grouping. When both items were identical in shape and colour the temporal bias against reporting the contralesional item was reduced. The results suggest that grouping can alter the coding of temporal relations between stimuli.

Extinction: a window into attentional competition.

Extinction is an example of how stimulus selection may be affected by an imbalance in competition for attentional selection. Patients with extinction are able to process stimuli in either hemispace, but only when presented in isolation. Following brain injury, stimuli will not be processed as efficiently in the damaged hemisphere and so may fail to be detected when other stimuli are competing for selection. In this review we discuss some of the factors that contribute to the recovery from extinction, and consider their implications for functional and neural theories of selection. Work shows that extinction can be modulated by multiple bottom-up factors including: low-level visual grouping (e.g., reflecting Gestalt properties in an array) and grouping based on higher level factors (such as the lexical identity of a stimulus or action relations between objects). Top-down factors (such as holding items in working memory) can also facilitate recovery from extinction. Furthermore, the competition for selection may also be modulated by the programming of action to a given location, consistent with pre-motor feedback to perceptual processes. While often discussed in terms of spatial biases, non-spatial extinction can also be demonstrated (dictated by the coherence of stimuli). In contrast to extinction, a phenomenon of anti-extinction has also been documented where patients are better at report when two items rather than single items are presented. Although superficially distinct, evidence indicates that grouping may be important in both cases, with temporal grouping being important in generating the anti-extinction effect. Overall, the work indicates that the disorder of extinction plays an important role in the understanding of attentional selection.