Review Visuomotor Origins of Covert Spatial Attention an Event Can Be Biased Either by Virtue of the Character- Tion by the Subject (voluntary). Other Work from the 19th

and oculomotor control mechanisms during scanning eye movements is of particular importance to visual neurobiologists. Just as our eyes are unable to simultaneously fixate all the elements contained within a scene, our perception Princeton, New Jersey 08544 2 Systems Neurobiology Laboratory of those elements is likewise limited. The tremendous volume of information impinging on the retina imposes Covert spatial attention produces biases in perceptual of gaze and the direction of attention are aligned. That performance and neural processing of behaviorally is, we normally fixate objects of interest, overtly at-relevant stimuli in the absence of overt orienting move-tending to them. But attention can be disengaged from ments. The neural mechanism that gives rise to these the point of fixation, and a great deal has been made effects is poorly understood. This paper surveys past of this fact (Sperling and Melchner, 1978; Posner and evidence of a relationship between oculomotor control Cohen 1984). Covert attention has intrigued physiolo-and visual spatial attention and more recent evidence gists and psychologists alike for more than a century. of a causal link between the control of saccadic eye In more recent years, a great number of studies of the movements by frontal cortex and covert visual selec-monkey and human visual systems have revealed a de-tion. Both suggest that the mechanism of covert spa-pendence of the strength of visual cortical signals on tial attention emerges as a consequence of the recip-the locus of covert visual attention (for reviews, see rocal interactions between neural circuits primarily Maunsell, 1995; Desimone and Duncan, 1995; Kastner involved in specifying the visual properties of potential and Ungerleider, 2000). However, the mechanism by targets and those involved in specifying the move-which the processing of relevant visual information is ments needed to fixate them. selectively biased has eluded investigators. A wealth of indirect evidence suggests that the neural mechanisms Introduction of covert attention are largely overlapping with those The primate cerebral cortex is comprised of neurons controlling the programming of saccadic eye move-that are largely involved in processing visual information areas organized more or less hierarchically beyond pri-2000), at least when attention is directed spatially. This mary visual cortex (V1) has proven to be particularly paper surveys evidence of a relationship between oculo-important in that it establishes the relatively modular motor control and spatial attention, including some re-nature of visual feature extraction (Zeki, 1978; Felleman cent tests of a causal link between the control …