THE SPATIAL-DISTRIBUTION OF VISUAL-ATTENTION IN HEMINEGLECT AND EXTINCTION PATIENTS

We studied the visual held distribution of speed and accuracy of manua l responses to small brief light flashes, in patients with left hemine glect or extinction resulting from right hemisphere vascular lesions a nd in brain-damaged and healthy control subjects. All patients with ri ght hemisphere lesions showed a greater impairment in both the speed o f response and the detection rate in the contralesional than in the ip silesional hemifield, This interfield difference increased with the ec centricity of stimulus presentation and was especially pronounced in n eglect patients who showed a paradoxical increase in speed of response and detection rate at increasingly larger eccentricities in the ipsil esional hemifield, We hypothesize that both the contralesional slowing down and the ipsilesional speeding up of the response depends upon an exaggerated gradient of attention towards the ipsilesional hemifield, To assess whether these abnormalities concern automatic or controlled attentional processes, in a second experiment, we manipulated the pre dictability of the side of the stimulus presentation by using blocked rather than randomized stimulus presentations. This resulted in a spee ding up of responses in both hemifields thus showing that the patients were able to focus attention to the side of stimulus presentation vol untarily. However, there was no modification of the contra-ipsilesiona l differences which, therefore, are likely to be related to abnormal a utomatic processes rather than controlled attention.