ATTENTION TO ONE OR 2 FEATURES IN LEFT OR RIGHT VISUAL-FIELD - A POSITRON EMISSION TOMOGRAPHY STUDY

In human vision, two features of the same object can be identified con currently without loss of accuracy. Performance declines, however, whe n the features belong to different objects in opposite visual fields. We hypothesized that different positron emission tomography activation patterns would reflect these behavioral results. We first delineated an attention network for single discriminations in left or right visua l field and then compared this with the activation pattern when subjec ts divided attention over two features of a single object or over two objects in opposite hemifields. When subjects attended to a single fea ture, parietal, premotor, and anterior cingulate cortex were activated . These effects were strongest in the right hemisphere and were, remar kably, unaffected by the direction of attention. In contrast, directio n of attention affected occipital and frontal activity: right occipita l and left lateral frontal activity were higher with attention to the left, whereas right lateral frontal activity was higher with attention to the right. When subjects identified two features of the same objec t, parietal, premotor, and anterior cingulate activity was enhanced fu rther, predominantly this time in the left hemisphere. Again, there wa s no direction sensitivity. Direction-sensitive activation of lateral frontal cortex also was increased. Finally, when subjects divided thei r attention over opposite hemifields, activity in the direction-sensit ive occipital and frontal regions fell to a level midway between those seen during exclusively leftward or rightward attention. Thus, the be havioral efficiency with which we attend to multiple features of a sin gle peripheral object is paralleled by enhanced activity in structures generally active during peripheral selective attention as well as in structures that depend on the specific direction of attention, most no tably lateral frontal cortex. In addition, in the direction-sensitive regions, dividing attention over hemifields causes a compromise patter n between the extreme levels obtained during unilateral attention.