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, to targets and registers changes in their position, generating a highlevel percept of apparent motion. Nancy G. Kanwisher, Anders M. Dale, and Roger B. H. Tootell. Cortical fMRI activation produced by attentive tracking of moving targets. J. Neurophysiol. 80: 2657Neurophysiol. 80: -2670Neurophysiol. 80: , 1998. Attention can be I N T R O D U C T I O N used to keep track of moving items, particularly when there are multiple targets of interest that cannot all be followed with eye movements. Functional magnetic resonance imaging ( fMRI) was Two strategies can be employed by the visual system to used to investigate cortical regions involved in attentive tracking. enhance processing of important targets. First, eye moveCortical flattening techniques facilitated within-subject compari-ments can direct the high-resolution fovea to the target of sons of activation produced by attentive tracking, visual motion, interest either by discrete jumps to different targets (sacdiscrete attention shifts, and eye movements. In the main task, cades) or by continuous visual tracking of a moving target subjects viewed a display of nine green ''bouncing balls'' and used (smooth pursuit). Second, even in the absence of eye moveattention to mentally track a subset of them while fixating. At the ments, processing can be facilitated when attention is distart of each attentive-tracking condition, several target balls (e.g., rected to the target by either discrete attentional shifts be-3/9) turned red for 2 s and then reverted to green. Subjects then tween targets (''attentional saccades'') or continuous attenused attention to keep track of the previously indicated targets, tive tracking of one or more moving targets (''attentional which were otherwise indistinguishable from the nontargets. Attentive-tracking conditions alternated with passive viewing of the pursuit''). Although eye movements and attentional shifts same display when no targets had been indicated. Subjects were have been widely investigated, little is known about attentive pretested with an eye-movement monitor to ensure they could per-tracking and its relationship to these other mechanisms. To form the task accurately while fixating. For seven subjects, func-our knowledge, this paper provides the first comprehensive tional activation was superimposed on each individual's cortically neuroimaging study of attentive tracking and its relationship unfolded surface. Comparisons between attentive tracking and pas-to these associated processes. areas and weak in the MT complex. However, in parietal and of interest such as a face in a crowd, the attentional focus frontal areas, the signal change produced by the moving stimuli can be maintained on that target even as it moves. At first was more than doubled when items were tracked attentively. Com-thought, attentive tracking may seem unnecessary because parisons between attentive tracking and attention shifting revealed smooth-pursuit eye movements serve essentially the same essentially identical activation patterns that differe...

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D.F., a patient with severe visual form agnosia, has been the subject of extensive research during the past decade. The fact that she could process visual input accurately for the purposes of guiding action despite being unable to perform visual discriminations on the same visual input inspired a novel interpretation of the functions of the two main cortical visual pathways or 'streams'. Within this theoretical context, the authors proposed that D.F. had suffered severe bilateral damage to her occipitotemporal visual system (the 'ventral stream'), while retaining the use of her occipitoparietal visual system (the 'dorsal stream'). The present paper reports a direct test of this idea, which was initially derived from purely behavioural data, before the advent of modern functional neuroimaging. We used functional MRI to examine activation in her ventral and dorsal streams during object recognition and object-directed grasping tasks. We found that D.F. showed no difference in activation when presented with line drawings of common objects compared with scrambled line drawings in the lateral occipital cortex (LO) of the ventral stream, an area that responded differentially to these stimuli in healthy individuals. Moreover, high-resolution anatomical MRI showed that her lesion corresponded bilaterally with the location of LO in healthy participants. The lack of activation with line drawings in D.F. mirrors her poor performance in identifying the objects depicted in the drawings. With coloured and greyscale pictures, stimuli that she can identify more often, D.F. did show some ventral-stream activation. These activations were, however, more widely distributed than those seen in control participants and did not include LO. In contrast to the absent or abnormal activation observed during these perceptual tasks, D.F. showed robust activation in the expected dorsal stream regions during object grasping, despite considerable atrophy in some regions of the parietal lobes. In particular, an area in the anterior intraparietal sulcus was activated more for grasping an object than for just reaching to that object, for both D.F. and controls. In conclusion, we have been able to confirm directly that D.F.'s visual form agnosia is associated with extensive damage to the ventral stream, and that her spared visuomotor skills are associated with visual processing in the dorsal stream.

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Picking up a cup requires transporting the arm to the cup (transport component) and preshaping the hand appropriately to grasp the handle (grip component). Here, we used functional magnetic resonance imaging to examine the human neural substrates of the transport component and its relationship with the grip component. Participants were shown three-dimensional objects placed either at a near location, adjacent to the hand, or at a far location, within reach but not adjacent to the hand. Participants performed three tasks at each location as follows: (1) touching the object with the knuckles of the right hand; (2) grasping the object with the right hand; or (3) passively viewing the object. The transport component was manipulated by positioning the object in the far versus the near location. The grip component was manipulated by asking participants to grasp the object versus touching it. For the first time, we have identified the neural substrates of the transport component, which include the superior parieto-occipital cortex and the rostral superior parietal lobule. Consistent with past studies, we found specialization for the grip component in bilateral anterior intraparietal sulcus and left ventral premotor cortex; now, however, we also find activity for the grasp even when no transport is involved. In addition to finding areas specialized for the transport and grip components in parietal cortex, we found an integration of the two components in dorsal premotor cortex and supplementary motor areas, two regions that may be important for the coordination of reach and grasp.

Although both reaching and grasping require transporting the hand to the object location, only grasping also requires processing of object shape, size and orientation to preshape the hand. Behavioural and neuropsychological evidence suggests that the object processing required for grasping relies on different neural substrates from those mediating object recognition. Specifically, whereas object recognition is believed to rely on structures in the ventral (occipitotemporal) stream, object grasping appears to rely on structures in the dorsal (occipitoparietal) stream. We used functional magnetic resonance imaging (fMRI) to determine whether grasping (compared to reaching) produced activation in dorsal areas, ventral areas, or both. We found greater activity for grasping than reaching in several regions, including anterior intraparietal (AIP) cortex. We also performed a standard object perception localizer (comparing intact vs. scrambled 2D object images) in the same subjects to identify the lateral occipital complex (LOC), a ventral stream area believed to play a critical role in object recognition. Although LOC was activated by the objects presented on both grasping and reaching trials, there was no greater activity for grasping compared to reaching. These results suggest that dorsal areas, including AIP, but not ventral areas such as LOC, play a fundamental role in computing object properties during grasping.

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