Using functional magnetic resonance imaging (fMRI) and cortical unfolding techniques, we analyzed the retinotopy, motion sensitivity, and functional organization of human area V3A. These data were compared with data from additional human cortical visual areas, including V1, V2, V3/VP, V4v, and MT (V5). Human V3A has a retinotopy that is similar to that reported previously in macaque: (1) it has a distinctive, continuous map of the contralateral hemifield immediately anterior to area V3, including a unique retinotopic representation of the upper visual field in superior occipital cortex; (2) in some cases the V3A foveal representation is displaced from and superior to the confluent foveal representations of V1, V2, V3, and VP; and (3) inferred receptive fields are significantly larger in human V3A, compared with those in more posterior areas such as V1. However, in other aspects human V3A appears quite different from its macaque counterpart: human V3A is relatively motionselective, whereas human V3 is less so. In macaque, the situation is qualitatively reversed: V3 is reported to be prominently motion-selective, whereas V3A is less so. As in human and macaque MT, the contrast sensitivity appears quite high in human areas V3 and V3A. Key words: fMRI; V3A; retinotopy; motion selectivity; visual cortex; MT/V5; human; primate After cortical visual areas V3 and V4 were identified and named in macaque monkeys, another region was discovered between them and named “V3 accessory ” (V3A) (Van Essen and Zeki, 1978; Zeki, 1978a,b). V3A is now regarded as a cortical area that is entirely independent and distinct from its similarly named neighbor, V3, in terms of its retinotopy (Van Essen and Zeki, 1978; Zeki, 1978a,b; Gattass et al., 1988), its histology (Burkhalter et al., 1986; Felleman and Van Essen, 1987; DeYoe et al.,

Brain metabolic mapping techniques, such as positron emis-sion tomography (PET), can provide information about the functional interactions within entire neural systems. With the large quantity of data that can accumulate from a mapping study, a network analysis, which makes sense of the com-plex interactions among neural elements, is necessary. A network analysis was performed on data obtained from a PET study that examined both the changes in regional ce-rebral blood flow (rCBF) and interregional correlations among human cortical areas during performance of an object vision (face matching) and spatial vision (dot-location matching) task. Brain areas for the network were selected based on regions showing significant rCBF or interregional correla-tions between tasks. Anterior temporal and frontal lobe regions were added to the network using a principal com-

Delay-related sustained activity in the prefrontal cortex of primates, a neurological analogue

We investigated how the brain selects the targets for eye movements, a process in which the outcome of visual pro-cessing is converted into guided action. Macaque monkeys were trained to make a saccade to fixate a salient target presented either alone or with multiple distracters during visual search. Neural activity was recorded in the frontal eye field, a cortical area at the interface of visual process-ing and eye movement production. Neurons discharging after stimulus presentation and before saccade initiation were analyzed. The initial visual response of frontal eye field neurons was modulated by the presence of multiple stimuli and by whether a saccade was going to be pro-duced, but the initial visual response did not discriminate the target of the search array from the distracters. In the latent period before saccade initiation, the activity of most

organization for delayed saccades in human posterior parietal

Electrophysiological studies in monkeys have identified effector-related regions in the posterior parietal cortex (PPC). The lateral intraparietal area, for example, responds preferentially for saccades, whereas the parietal reach region responds preferentially for arm movements. However, the degree of effector selectivity actually observed is limited; each area contains neurons selective for the nonpreferred effector, and many neurons in both areas respond for both effectors. We used functional magnetic resonance imaging to assess the degree of effector preference at the population level, focusing on topographically organized regions in the human PPC [visual area V7, intraparietal sulcus 1 (IPS1), and IPS2]. An event-related design adapted from monkey experiments was used. In each trial, an effector cue preceded the appearance of a spatial target, after which a Go signal instructed subjects to produce the specified movement with the specified effector. Our results show that the degree of effector specificity is limited in many cortical areas and transitions gradually from saccade to reach preference as one moves through the hierarchy of areas in the occipital, parietal, and frontal cortices. Saccade preference was observed in visual cortex, including early areas and V7. IPS1 exhibited balanced activation to saccades and reaches, whereas IPS2 showed a weak but significant preference for reaches. In frontal cortex, areas near the central sulcus showed a clear and absolute preference for reaches, whereas the frontal eye field showed little or no effector selectivity. Although these results contradict many theoretical conclusions about effector specificity, they are compatible with the complex picture arising from electrophysiological studies and also with previous imaging studies that reported mostly overlapping saccade- and arm-related activation. The results are also compatible with theories of efficient coding in cortex. Key words: effector specificity; eye movements; arm movements; LIP; PRR; fMRI

Functional imaging studies identified a motion-sensitive area (V5/ MT1) in the vicinity of the posterior branch of the inferior temporal sulcus that has no correlate in any classical cytoarchitectonic map. The aim of the present study was to identify a cytoarchitectonic correlate of this region in 10 human postmortem brains and to provide a probability map of this area. Observer-independent mapping revealed an area, hOc5 (h for human, Oc for occipital lobe), that has a broad layer III, a high cell density in layer II/III, and a low one in layer V. Most of area hOc5 is found in the depths of the anterior occipital sulcus and the anterior parts of either the inferior lateral occipital or the inferior occipital sulcus. After 3-dimensional reconstruction and registration to a standard reference space, a probability map of the area measured the individual variability of its size and location. The mean spatial locations of area hOc5 are

Caudal area PE (PEc) of the macaque posterior parietal cortex has been shown to be a crucial node in visuomotor coordination during reaching. The present study was aimed at studying visual and somatosensory organization of this cortical area. Visual stimulations activated 53 % of PEc neurons. The overwhelming majority (89%) of these visual cells were best activated by a dark stimulus on a lighter background. Somatosensory stimulations activated 56% of PEc neurons: most were joint neurons (73%); a minority (24%) showed tactile receptive fields, most of them located on the arms. Area PEc has not a clear retinotopy or somatotopy. Among the cells tested for both somatosensory and visual sensitivity, 22 % were bimodal, 25 % unimodal somatosensory, 34 % unimodal visual, and 19 % were insensitive to either stimulation. No clear clustering of the different classes of sensory neurons was observed. Visual and somatosensory receptive fields of bimodal cells were not in register. The damage in the human brain of the likely homologous of macaque PEc produces deficits in locomotion and in whole-body interaction with the visual environment. Present data show that macaque PEc has sensory properties and a functional organization in line with the view of an involvement of this area in those processes.

Discrimination between active and passive head movements by macaque intraparietal (VIP, MIP) cortex neurons

In a previous study, we identified three cortical areas in human posterior parietal cortex that exhibited topographic responses during memory-guided saccades [visual area 7 (V7), intraparietal sulcus 1 (IPS1), and IPS2], which are candidate homologs of macaque parietal areas such as the lateral intraparietal area and parietal reach region. Here, we show that these areas exhibit sustained delay-period activity, a critical physiological signature of areas in macaque parietal cortex. By varying delay duration, we disambiguated delay-period activity from sensory and motor responses. Mean time courses in the parietal areas were well fit by a linear model comprising three components representing responses to (1) the visual target, (2) the delay period, and (3) the eye movement interval. We estimated the contributions of each component: the response amplitude during the delay period was substantially smaller (�30%) than that elicited by the transient visual target. All three parietal regions showed comparable delay-period response amplitudes, with a trend toward larger responses from V7 to IPS1 and IPS2. Responses to the cue and during the delay period showed clear lateralization with larger responses to trials in which the target was placed in the contralateral visual field, suggesting that both of these components contributed to the topography we measured.