SEEING ONLY THE RIGHT HALF OF THE FOREST BUT CUTTING DOWN ALL THE TREES

Unilateral neglect following damage to the right hemisphere of the bra in can be characterized by failure of the global attentional mechanism s of the right hemisphere to direct the local detail processors of the left hemisphere towards the contralesional left hemispace, This is su ggested by patients who recognize the global form of the left side of shapes (the forest) but fail to cancel out its local details (the tree s)(1). Here we report the opposite behavioural dissociation in a patie nt (Q,M,) with damage to the right hemisphere of the brain. Q.M, detec ted local details (such as the tail of a dog) on the left or right sid e of visual shapes, regardless of whether these details belonged to pr edefined target shapes (a dog in this case) or to distracter shapes di ffering on the opposite side (a dog with a swan's neck and head, for e xample). Psychological testing showed an abnormal tendency of this pat ient to respond to local features, but perfect accuracy in interpretin g global features when the local features could not interfere in globa l processing. The results indicate that the left hemisphere can integr ate multiple local features simultaneously but loses global awareness as soon as local features individually compete for response selection. However, awareness of the whole is not necessary for the sequential p rocessing of the parts.