Fractionating the binding process: neuropsychological evidence distinguishing binding of form from binding of surface features

We present neuropsychological evidence demonstrating that the binding of fo rm elements into shapes dissociates from the binding of surface detail to s hape. Data are reported from a patient with bilateral parietal lesions, GK, who manifests left-side visual extinction along with many illusory conjunc tions when asked to discriminate both surface and form information about st imuli. We show that there are effects of grouping on both extinction and il lusory conjunctions when the tasks require report of object shape. In contr ast, illusory conjunctions involving surface and form information were unaf fected by grouping based on shape. In addition, grouping was stronger when forms were presented within the same hemifield than when they appeared in d ifferent hemifields, whilst illusory conjunctions of form and colour occurr ed equally often within and across hemifields. These results support a two- stage account of visual binding: form elements are first bound together loc ally into shapes, and this is followed by a second stage of binding in whic h shapes are integrated with surface details. The second but not the first stage of binding is impaired in this patient. (C) 2000 Elsevier Science Ltd . All rights reserved.