Academic journal article Cognitive, Affective and Behavioral Neuroscience

On the Relations between Implicit and Explicit Spatial Binding: Evidence from Balint's Syndrome

Academic journal article Cognitive, Affective and Behavioral Neuroscience

On the Relations between Implicit and Explicit Spatial Binding: Evidence from Balint's Syndrome

Article excerpt

We report two experiments examining whether factors that affect binding in explicit report tasks also affect implicit binding in a patient with Balint's syndrome, G.K. Replicating prior studies, we showed that there could be implicit binding of visual features in a patient with Balint's syndrome who was at chance at explicitly discriminating the relations between the features. Nevertheless, when color-form relationships were coded, we showed that G.K.'s implicit binding, as well as his explicit report of binding relationships, was affected by the spatial distance between the stimuli. Similarly, we demonstrated effects of grouping on both implicit and explicit binding of the spatial relations between shapes. We interpret the qualitative similarities between implicit and explicit binding as indicating that they reflect a single process in which binding relations, initially established preattentively, are later consolidated by attention. This later process, of attentional consolidation, is disrupted in Balint's syndrome.

There is good evidence that many basic properties of visual stimuli-their color, direction of motion, and shape-are processed independently and in parallel (see Heywood & Cowey, 1999, and Heywood & Zihl, 1999, for neuropsychological evidence). Hence, to see a coherent world, in which color, motion, and shape are linked correctly, the brain may need to bind together the independently processed features (see Robertson, 2003, for a recent review). There are several accounts of how this binding process may take place. For example, binding may depend on temporal synchrony of firing between neurons responding to the properties of a common object (e.g., Singer & Gray, 1995). Alternatively, it may depend on some common registration of features within some master map of locations and/or on the application of serial attention to that map (as in Treisman's feature integration theory; Treisman, 1998).

Neuropsychological evidence has linked the binding process to regions of the parietal lobe (e.g., Friedman-Hill, Robertson, & Treisman, 1995) and the pulvinar (Ward, Danziger, Owen, & Rafal, 2002). For example, following bilateral parietal damage, there can be marked problems with feature binding, so that patients frequently misattribute the features of one object to another (e.g., reporting a red X and blue O as a red O and a blue X; Friedman-Hill etal., 1995; Humphreys, Cinel, Wolfe, Olson, & Klempen, 2000). These illusory conjunctions (ICs) can also be found in patients with unilateral damage when the stimuli are presented within the contralesional field (A. Cohen & Rafal, 1991 ). The ICs may reflect the coarse spatial coding available to these patients, so that (say) color information may overlap the spatial location recorded for a different shape, leading to an incorrect binding of color and shape (Humphreys et al., 2000). It may also reflect the difficulty such patients have in attending to spatial location. Attention may normally serve to filter out irrelevant stimuli (cf. Moran & Desimone, 1985). For patients with difficulties in visual attention, the irrelevant information may remain available to be bound incorrectly across stimuli.

Despite the fact that patients with parietal damage can make binding errors (ICs) when asked to explicitly report stimuli, there is also evidence that the same attributes can be bound implicitly, when the patients do not have to report the relations between the attributes. Evidence on implicit binding in a Stroop task was reported by Wojciulik and Kanwisher (1998) for R.M., a patient with Balint's syndrome following bilateral parietal damage. FriedmanHill et al. (1995) found that R.M. made abnormal numbers of ICs between colors and shapes when asked to identify multiple colored letters. Despite this, Wojciulik and Kanwisher demonstrated that R.M. was sensitive to Stroop interference, suggesting that color and form information was bound together, at least at some stage. …

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