Selective dorsal and ventral processing: Evidence for a common attentional mechanism in reaching and perception

The primate visual system can be divided into a ventral stream for perception and recognition and a dorsal stream for computing spatial information for motor action. How are selection mechanisms in both processing streams coordinated? We recently demonstrated that selection-for-perception in the ventral stream (usually termed "visual attention") and saccade target selection in the dorsal stream are tightly coupled (Deubel & Schneider, 1996). Here we investigate whether such coupling also holds for the preparation of manual reaching movements. A dual-task paradigm required the preparation of a reaching movement to a cued item in a letter string. Simultaneously, the ability to discriminate between the symbols "E" and "There Exists" presented tachistoscopically within the surrounding distracters was taken as a measure of perceptual performance. The data demonstrate that discrimination performance is superior when the discrimination stimulus is also the target for manual aiming; when the discrimination stimulus and pointing target refer to different objects, performance deteriorates. Therefore, it is not possible to maintain attention on a stimulus for the purpose of discrimination while directing a movement to a spatially separate object. The results argue for an obligatory coupling of (ventral) selection-for-perception and (dorsal) selection-for-action.