Similarity based grouping in right temporal-parietal cortex: Implications for deficits in hierarchical selection

NeuroImage

13, Number

6, 2001,

Part 2 of 2 Parts 1 D E bl@

ATTENTION

Similarity based grouping in right temporal-parietal cortex: Implications for deficits in hierarchical selection David Wilkinson*, *Department

Peter HalliganJ-, Richard HensonS, Raymond J. DolanSQ of Experimental

tSchoo1 $Wellcome

Department $Royal

Psychology,

of Psychology,

Cardiff

of Cognitive Neurology University College Free Hospital

School

University University,

and Institute London, UK of Medicine,

of Oxford,

UK

UK of Cognitive

London,

Neuroscience,

UK

Introduction: Neuropsychological research has established that the right temporal-parietal junction (TPJ) is specialised for inspecting the global properties of visual displays. However, the means by which this global advantage arises remain unclear. One clue is provided by the compound stimuli that are typically used to examine hierarchical processing. These consist of a single, large letter composed of many smaller, identical letters. Inter-item similarity could therefore offer one way of grouping local items into a global description. To assess the role of right TPJ in perceptual grouping, we acquired event-related fMRI data while subjects performed a visual search task in which the identity of distracters was manipulated. If right TPJ mediates similarity-based grouping, then it should be activated during search for a target in displays containing homogeneous distracters. By contrast, search for targets in displays containing heterogeneous distracters should activate other parietal-motor structures associated with a spatially serial mode of inspection, as grouping can no longer be used to aid target detection. Method: 12 right handed, healthy volunteers searched for a conjunction target (an upright T) set amongst or heterogeneous (randomly oriented Ts) distracters. The number of distracters varied between roughly columnar fashion, within which the target randomly appeared for one half of all trials. with an SOA of 25, and present/absent judgments were indicated by pressing one of two buttons. T2*-weighted EPI images were simulta, -- I. __ neously acquired using a 2T Magnetom VISION system (TR 3.2s, 32 ’ .* .p . axial slices, TE 4Os, 3mm*3mm*3mm voxels, 490 scans per run). I Images were analysed using SPM99 and statistical significance was assessed using a random effects model.

either homogeneous (inverted Ts) 1 and 4, and were organised in a Displays were presented for 200ms l.:’ <’ I

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Results:

y. _ “-.d, x The reaction time data conformed to the pattern of search typically 1..Q x associated with displays containing either homogeneous or heteroge: neous distracters (i.e. flat search functions with absent/present ratios of ‘%_.” 1: 1, or linearly increasing functions with absent/present ratios of 2: 1, respectively). As predicted, searching displays containing homogeFigs la (upper panel) and lb (lower panel) neous, relative to heterogeneous, distracters activated an area within right TPJ (pO.Ol, uncorrected) (see Fig la). By contrast, search for an upright T amongst heterogeneous, relative to homogeneous, distracters activated regions normally associated with serial shifts of attention including bilateral superior parietal lobe, bilateral motor cortex, bilateral supplementary motor area, and bilateral cerebellum (p~0.001, uncorrected) (see Fig lb).

The data indicate that the right TPJ is sensitive to inter-item similarity. Previous research has indicated that this region is implicated in the identification of global form. Since the local items of compound stimuli share the same identity, it follows that similarity based grouping, as observed in the current visual search task, provides one means by which the right TPJ constructs global form in compound letter stimuli. Our findings provide one explanation of why patients with damage to this region experience deficits in global processing, and suggest that the cerebral hemispheres are differentially sensitive to certain forms of gestalt property.

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