VISUAL SIZE PROCESSING IN SPATIAL NEGLECT

Evidence from the use of the landmark task and from two size-matching tasks shows that many patients with left-sided neglect systematically under-perceive visual extent in leftward parts of space. This perceptu al distortion of size serves to explain the occurrence of rightward li ne-bisection errors in most neglect patients. One possibility is that attentional biases of a chronic nature might underlie these perceptual changes seen in neglect patients. But contrary to this idea, we have found that attentional cueing in the landmark task causes changes in n eglect patients exactly opposite to those seen in healthy subjects. Th e distortions of size perception seen in neglect could instead be caus ed by a high-level alteration of visual processing rather than by an a ttentional bias. In order to explore which visual stream of cortical p rocessing might be compromised in such a disorder, we have begun to ex amine neglect patients on visuomotor as well as perceptual tasks. We h ave found clear evidence in one patient for intact grip scaling for ob ject size in the neglected half of space, despite gross perceptual und erestimations of the same objects. This result suggests that neglect c an occur without major disruption of the dorsal stream, and may result instead from damage to a cortical system whose predominant visual inp ut comes from the ventral stream.