REVIEWS and neurons in a more anterior area of the IP with atendingtolocatk in the for arn oto aining attentional sets in memory What psy h I and neural m begun to is which defined as the ability g human areas e of in sory or mot ns for directing a ntoa it is pos ed by the premo nt in the sion of the attention ithovetoculonmotor& ng atte a lo king he bilit Too-down nulu 2 imuli at w th un tote is re ed to the tal n sin LIP that are r H d in bo be th dine for th source of a n al fire pp evs ure po d.red reg ese regions mple ring t he IPs (FG 4ergosd (specif of the p byhomologous areas LIP and oe for the d in task that a person performsn one c of n aratory activity that i he ng the digits.I area LIP code for hat th )impending ven)or letter RE REVIEWS UME 3|MARCH 2002 20NATURE REVIEWS | NEUROSCIENCE VOLUME 3 | MARCH 2002 | 205 REVIEWS (FIG. 4a), and neurons in a more anterior area of the IPs code for impending grasping movements and three￾dimensional objects44. Preparatory activity has been observed for eye movements in the FEF45, and for arm movements in the premotor cortex46. Brain-imaging studies have reported both eye- and arm-related activity in the frontal cortex and IPs47–49, although these studies did not separate preparatory signals from those involved in executing the response. More recent event-related studies have begun to isolate preparatory signals that are related to response selection in what are presumed to be corresponding human areas50. Because eye movements are important in stimulus selection, mechanisms for directing attention to a loca￾tion might be similar to mechanisms for preparing an eye movement, as proposed by the premotor theory of attention51. Imaging studies that have compared covert spatial attention with overt oculomotor shifts have found strong overlap in activations of both the FEF and IPs18,52,53 (FIG. 4b). Top-down signals for task sets. Although stimulus and response selection can be separated in the laboratory, stimuli and responses are inextricably linked in real life. While looking at a canvas, the brain not only selects the stimuli at which to look, but also programs the eye movements with which to look at them. This close functional linkage is related to the convergence of stimulus- and response-selection signals in areas of the frontoparietal network. Neurons in LIP and FEF show signals that are related to attention, memory and eye movements26,38,45. However, in many circumstances, the association between stimulus and response must be learned. For instance, although the natural response to the museum alarm is flight, a firefighter will respond by actively looking for the source of a potential fire. So, a crucial aspect of attentional selection is not just the iso￾lated selection of a stimulus or a response, but the assembly and coordination of stimulus–response asso￾ciations or mappings. A recent study54 found that online changes in the appropriate stimulus–response mapping for hand responses activated two clusters in posterior parietal cortex: one more posterior and medial, extend￾ing from the IPs into the SPL, and one more lateral, along the IPs (FIG. 4d, red regions). These regions were distinct from those involved in stimulus selection, more anteriorly in the IPs (FIG. 4d, green regions, and FIG. 2a,b). The spatial distribution of activity for stimulus selection and stimulus–(hand)-response mapping (specifically, the medial cluster) might correspond to the spatial dis￾tribution of the possibly homologous areas LIP and PRR in the macaque54. A change in the task that a person performs on a stimulus often changes the appropriate response (the stimulus–response mapping). Subtracting one digit from another results in a different response from that seen when adding the two digits. Performance analyses indi￾cate that switching between task sets, particularly when they involve the same stimuli, is effortful and expensive in terms of resources and time55,56. For example, switching between categorizing numbers (as odd or even) or letters The dorsal frontoparietal network that is recruited when subjects expect to see object features other than location clearly overlaps with regions that are recruited by attending to location (FIG. 2a,b), but the exact overlap between regions recruited by different kinds of advance information is unclear, and many visual features have yet to be tested. Maintaining attentional sets in memory. What psycho￾logical and neural mechanisms are responsible for the maintenance of attentional sets? One likely candidate is working memory, which is defined as the ability to maintain and manipulate information online in the absence of incoming sensory or motor stimulation. Attentional set and working memory mechanisms over￾lap functionally. For example, it is possible to maintain a memory of a visual attribute for many seconds without any apparent decrement in the precision of the visual information34,35. A more direct link is provided by reports that directing attention away from a location during a delay disrupts working memory for that loca￾tion36 (FIG. 3a). Therefore, spatial rehearsal, or the ability to maintain spatial information online in memory, depends crucially on spatial attention. The parietal and frontal regions that are recruited by an attentional set (FIG. 2a–c) show sustained activation during a memory delay in which the set is maintained online for up to ten seconds13,37 (FIG. 3b). The localization of working memory signals in dorsal frontoparietal areas is consistent with the presence of memory activity in neurons in macaque FEF and IPs38,39. In macaques, strong memory-related activity has also been found more anteriorly in the lateral prefrontal cortex39, which has been considered in both species to be the main source of top-down control signals to the visual cor￾tex40,41. However, in humans, the current neuroimaging evidence does not support the involvement of dorso￾lateral prefrontal cortex during the encoding and main￾tenance of a simple visual cue in a well practised task. This discrepancy might represent a species-specific dif￾ference in the neural systems that are involved in atten￾tional control. More anterior prefrontal areas might be recruited in monkeys because of their low memory span42. These regions are probably recruited in humans when the task is initially learned or as the selected items become more complex or increase in number. For example, human prefrontal regions are active during the delay period of match-to-sample tasks in which a sam￾ple face is maintained over a delay period and then matched to a test face37. Top-down signals for attending to effectors. Our discus￾sion so far has emphasized a role for the dorsal fronto￾parietal network in preparatory aspects of stimulus selection, but other results indicate that these areas are also important for response or action selection. Different regions of macaque IPs show preparatory activity that is selective for different effectors. For example, neurons in area LIP code for impending SACCADIC EYE MOVEMENTS, whereas neurons in a more medial area (parietal reach region, PRR) code for impending reaching movements43 SACCADIC EYE MOVEMENT A rapid eye movement that brings the point of maximal visual acuity — the fovea — to the image of interest