Different Contribution of the Monkey Prefrontal and Premotor Dorsal Cortex in Decision Making During a Transitive Inference Task.

Several studies have reported similar neural modulations between brain areas of the frontal cortex, such as the dorsolateral prefrontal (DLPFC) and the premotor dorsal (PMd) cortex, in tasks requiring encoding of the abstract rules for selecting the proper action. Here we compared the neuronal modulation of the DLPFC and PMd of monkeys trained to choose the higher rank from a pair of abstract images (target item), selected from an arbitrarily rank-ordered set (A > B > C > D > E > F) in the context of a transitive inference task. Once acquired by trial-and-error, the ordinal relationship between pairs of adjacent images (i.e., A > B; B > C; C > D; D > E; E > F), monkeys were tested in indicating the ordinal relation between items of the list not paired during learning. During these decisions, we observed that the choice accuracy increased and the reaction time decreased as the rank difference between the compared items enhanced. This result is in line with the hypothesis that after learning, the monkeys built an abstract mental representation of the ranked items, where rank comparisons correspond to the items' position comparison on this representation. In both brain areas, we observed higher neuronal activity when the target item appeared in a specific location on the screen with respect to the opposite position and that this difference was particularly enhanced at lower degrees of difficulty. By comparing the time evolution of the activity of the two areas, we observed that the neural encoding of target item spatial position occurred earlier in the DLPFC than in the PMd.

Controlled movement processing: evidence for a common inhibitory control of finger, wrist, and arm movements.

We used the behavioral task and theoretical construct of the countermanding paradigm to test whether there is any difference between the inhibitory control of the finger, wrist, and arm. Participants were instructed (primary task) to respond to a directional go signal presented at the fovea by pressing a button with either their index or middle fingers, moving a joystick with their wrists, or reaching to a stimulus on a touch screen with their arms. They were also instructed (secondary task) to withhold their responses when a stop signal was presented on 25% of trials. The participants' ability to inhibit each of the commanded movements was captured by their inhibition probability function, which describes how withholding is increasingly difficult as the delay between the go and stop signals increased. By modeling each participant's inhibition function, we estimated that the time needed to inhibit a commanded movement was about 240 ms, a variable that did not differ significantly between the three limb segments. Moreover, we found that the best-fit model of each segment's inhibition function could fit equally well the inhibition functions obtained with the other two segments. These results provide evidence that the upper limb segments share a common inhibitory control, which may facilitate the regulation of neuronal activity within the distributed motor cortical representations and thus simplify the voluntary control of multi-segmental movements.

Neural correlates of cognitive control of reaching movements in the dorsal premotor cortex of rhesus monkeys.

Canceling a pending movement is a hallmark of voluntary behavioral control because it allows us to quickly adapt to unattended changes either in the external environment or in our thoughts. The countermanding paradigm allows the study of inhibitory processes of motor acts by requiring the subject to withhold planned movements in response to an infrequent stop-signal. At present the neural processes underlying the inhibitory control of arm movements are mostly unknown. We recorded the activity of single units in the rostral and caudal portion of the dorsal premotor cortex (PMd) of monkeys trained in a countermanding reaching task. We found that among neurons with a movement-preparatory activity, about one-third exhibit a modulation before the behavioral estimate of the time it takes to cancel a planned movement. Hence these neurons exhibit a pattern of activity suggesting that PMd plays a critical role in the brain networks involved in the control of arm movement initiation and suppression.

Gaze modulates non-propositional reasoning: further evidence for spatial representation of reasoning premises.

Human and animals are able to decide that A>C after having learnt that A>B and B>C. This basic property of logical thinking has been studied by transitive inference (TI) tasks. It has been hypothesized that subjects displace the premises of the inference on a mental line to solve the task. An evidence in favor of this interpretation is the observation of the symbolic distance effect, that is the improvement of the performance as the distance between items increases. This effect has been interpreted as support to the hypothesis that ability to perform TI tasks follows the same rules and is mediated by the same brain circuits involved in the performance of spatial tasks. We tested ten subjects performing a TI on an ordered list of Japanese characters while they were fixating either leftwards or rightwards, to evaluate whether the eye position modulated the performance in making TI as it does in spatial tasks. Our results show a significant linear decrease of the reaction time with the increase of the symbolic distance and a shift of this trend towards lower reaction times when subjects were fixating to the left. We interpret this eye position effect as a further evidence that spatial and reasoning tasks share the same underlying mechanisms and neural substrates. The eye position effect also points to a parietal cortex involvement in the neural circuit involved in transitive reasoning.

Signal transformations from cerebral cortex to superior colliculus for the generation of saccades.

The ability of primates to make rapid and accurate saccadic eye movements for exploring the natural world is based on a neuronal system in the brain that has been studied extensively and is known to include multiple brain regions extending throughout the neuraxis. We examined the characteristics of signal flow in this system by recording from identified output neurons of two cortical regions, the lateral intraparietal area (LIP) and the frontal eye field (FEF), and from neurons in a brainstem structure targeted by these output neurons, the superior colliculus (SC). We compared the activity of neurons in these three populations while monkeys performed a delayed saccade task that allowed us to quantify visual responses, motor activity, and intervening delay activity. We examined whether delay activity was related to visual stimulation by comparing the activity during interleaved trials when a target was either present or absent during the delay period. We examined whether delay activity was related to movement by using a Go/Nogo task and comparing the activity during interleaved trials in which a saccade was either made (Go) or not (Nogo). We found that LIP output neurons, FEF output neurons, and SC neurons can all have visual responses, delay activity, and presaccadic bursts; hence in this way they are all quite similar. However, the delay activity tended to be more related to visual stimulation in the cortical output neurons than in the SC neurons. Complementing this, the delay activity tended to be more related to movement in the SC neurons than in the cortical output neurons. We conclude, first, that the signal flow leaving the cortex represents activity at nearly every stage of visuomotor transformation, and second, that there is a gradual evolution of signal processing as one proceeds from cortex to colliculus.

Eye-hand coordination during reaching. I. Anatomical relationships between parietal and frontal cortex.

The anatomical and physiological substrata of eye-hand coordination during reaching were studied through combined anatomical and physiological techniques. The association connections of parietal areas V6A and PEc, and those of dorso-rostral (F7) and dorso-caudal (F2) premotor cortex were studied in monkeys, after physiological characterization of the parietal regions where retrograde tracers were injected. The results show that parieto-occipital area V6A is reciprocally connected with F7, and receives a smaller projection from F2. Local parietal projections to V6A arise from areas MIP and, to a lesser extent, 7m, PEa and PEC: On the contrary, parietal area PEc is strongly and reciprocally connected with the part of F2 located close to the pre-central dimple (pre-CD). Local parietal projections to PEc come from a distributed network, including PEa, MIP, PEci and, to a lesser extent, 7m, V6A, 7a and MST. Premotor area F7 receives parietal projections mainly from 7m and V6A, and local frontal projections mainly from F2. On the contrary, premotor area F2 in the pre-CD zone receives parietal inputs from PEc and, to a lesser extent, PEci, while in the peri-arcuate zone F2 receives parietal projections from PEa and MIP. Local frontal projections to F2 pre-CD mostly stem from F4, and, to a lesser extent, from F7 and F3, and CMAd; those addressed to peri-arcuate zone of F2 arise mainly from F5 and, to a lesser extent, from F7, F4, dorsal (CMAd) and ventral (CMAv) cingulate motor areas, pre-supplementary (F6) and supplementary (F3) motor areas. The distribution of association cells in both frontal and parietal cortex was characterized through a spectral analysis that revealed an arrangement of these cells in the form of bands, composed of cell clusters, or 'columns'. The reciprocal connections linking parietal and frontal cortex might explain the presence of visually related and eye-position signals in premotor cortex, as well as the influence of information about arm position and movement direction in V6A and PEC: The association connections identified in this study might carry sensory as well motor information that presumably provides a basis for a re-entrant signaling. This might be necessary to match retinal-, eye- and hand-related information underlying eye-hand coordination during reaching.

Eye-hand coordination during reaching. II. An analysis of the relationships between visuomanual signals in parietal cortex and parieto-frontal association projections.

The relationships between the distribution of visuomanual signals in parietal cortex and that of parieto-frontal projections are the subject of the present study. Single cell recording was performed in areas PEc and V6A, where different anatomical tracers were also injected. The monkeys performed a variety of behavioral tasks, aimed at studying the visual and motor properties of parietal cells, as well as the potential combination of retinal-, eye- and hand-related signals on cell activity. The activity of most cells was related to the direction of movement and the active position of the hand. Many of these reach-related cells were influenced by eye position information. Fewer cells displayed relationships to saccadic eye movements. The activity of most neurons related to a combination of both hand and eye signals. Many cells were also modulated during preparation for hand movement. Light-dark differences of activity were common and interpreted as related to the sight and monitoring of hand motion and/or position in the visual field. Most cells studied were very sensitive to moving visual stimuli and also responded to optic flow stimulation. Visual receptive fields were generally large and extended to the periphery of the visual field. For most neurons, the orientation of the preferred directions computed across different epochs and tasks conditions clustered within a limited sector of space, the field of global tuning. This can be regarded as an ideal frame to combine spatially congruent eye- and hand-related information for different forms of visuomanual behavior. All these properties were common to both PEc and V6A. Retinal, eye- and hand-related activity types, as well as parieto-frontal association cells, were distributed in a periodic fashion across the tangential domain of areas PEc and V6A. These functional and anatomical distributions were characterized and compared through a spectral and coherency analysis, which revealed the existence of a selective 'match' between activity types and parieto-frontal connections. This match depended on where each individual efferent projection was addressed. The results of the present and of the companion study can be relevant for a re-interpretation of optic ataxia as the consequence of the breakdown of the combination of retinal-, eye- and hand-related directional signals within the global tuning fields of parietal neurons.

Early coding of visuomanual coordination during reaching in parietal area PEc.

The parietal mechanisms of eye-hand coordination during reaching were studied by recording neural activity in area PEc while monkeys performed different tasks, aimed at assessing the influence of retinal, hand-, and eye-related signals on neural activity. The tasks used consisted of 1) reaching to foveated and 2) to extra-foveal targets, with constant eye position; and 3) saccadic eye movement toward, and holding of eye position on peripheral targets, the same as those of the reaching tasks. In all tasks, hand and/or eye movements were made from a central position to eight peripheral targets. A conventional visual fixation paradigm was used as a control task, to assess location and extent of visual receptive field of neurons. A large proportion of cells in area PEc displayed significant relationships to hand movement direction and position. Many of them were also related to the eye's position. Relationships to saccadic eye movements were found for a smaller proportion of cells. Most neurons were tuned to different combination of hand- and eye-related signals; some of them were also influenced by visual information. This combination of signals can be an expression of the early stages of the composition of motor commands for different forms of visuomotor coordination that depend on the integration of hand- and eye-related information. These results assign to area PEc, classically considered as a somatosensory association cortex, a new visuomotor role.

Early coding of reaching in the parietooccipital cortex.

Neural activity was recorded in the parietooccipital cortex while monkeys performed different tasks aimed at investigating visuomotor interactions of retinal, eye, and arm-related signals on neural activity. The tasks were arm reaching 1) to foveated targets; 2) to extrafoveal targets, with constant eye position; 3) within an instructed-delayed paradigm, under both light and darkness; 4) saccadic eye movements toward, and static eye holding on peripheral targets; and 5) visual fixation and stimulation. The activity of many cells was modulated during arm reaction (68%) and movement time (58%), and during static holding of the arm in space (64%), when eye position was kept constant. Eye position influenced the activity of many cells during hand reaction (45%) and movement time (51%) and holding of hand static position (69%). Many cells (56%) were also modulated during preparation for hand movement, in the delayed reach task. Modulation was present also in the dark in 59% of cells during this epoch, 51% during reaction and movement time, and 48% during eye/hand holding on the target. Cells (50%) displaying light-dark differences of activity were considered as related to the sight and monitoring of hand motion and/or position in the visual field. Saccadic eye movements modulated a smaller percentage (25%) of cells than eye position (68%). Visual receptive fields were mapped in 44% of the cells studied. They were generally large and extended to the periphery of the tested (30 degrees ) visual field. Sixty-six percent of cells were motion sensitive. Therefore the activity of many neurons in this area reflects the combined influence of visual, eye, and arm movement-related signals. For most neurons, the orientation of the preferred directions computed across different epochs and tasks, therefore expression of all different eye- and hand-related activity types, clustered within a limited sector of space, the field of global tuning. These spatial fields might be an ideal frame to combine eye and hand signals, thanks to the congruence of their tuning properties. The relationships between cell activity and oculomotor and visuomanual behavior were task dependent. During saccades, most cells were recruited when the eye moved to a spatial location that was also target for hand movement, whereas during hand movement most cells fired depending on whether or not the animal had prior knowledge about the location of the visual targets.

Disparity sensitivity of frontal eye field neurons.

Information about depth is necessary to generate saccades to visual stimuli located in three-dimensional space. To determine whether monkey frontal eye field (FEF) neurons play a role in the visuo-motor processes underlying this behavior, we studied their visual responses to stimuli at different disparities. Disparity sensitivity was tested from 3 degrees of crossed disparity (near) to 3 degrees degrees of uncrossed disparity (far). The responses of about two thirds of FEF visual and visuo-movement neurons were sensitive to disparity and showed a broad tuning in depth for near or far disparities. Early phasic and late tonic visual responses often displayed different disparity sensitivity. These findings provide evidence of depth-related signals in FEF and suggest a role for FEF in the control of disconjugate as well as conjugate eye movements.

Parieto-frontal coding of reaching: an integrated framework.

In the last few years, anatomical and physiological studies have provided new insights into the organization of the parieto-frontal network underlying visually guided arm-reaching movements in at least three domains. (1) Network architecture. It has been shown that the different classes of neurons encoding information relevant to reaching are not confined within individual cortical areas, but are common to different areas, which are generally linked by reciprocal association connections. (2) Representation of information. There is evidence suggesting that reach-related populations of neurons do not encode relevant parameters within pure sensory or motor "reference frames", but rather combine them within hybrid dimensions. (3) Visuomotor transformation. It has been proposed that the computation of motor commands for reaching occurs as a simultaneous recruitment of discrete populations of neurons sharing similar properties in different cortical areas, rather than as a serial process from vision to movement, engaging different areas at different times. The goal of this paper was to link experimental (neurophysiological and neuroanatomical) and computational aspects within an integrated framework to illustrate how different neuronal populations in the parieto-frontal network operate a collective and distributed computation for reaching. In this framework, all dynamic (tuning, combinatorial, computational) properties of units are determined by their location relative to three main functional axes of the network, the visual-to-somatic, position-direction, and sensory-motor axis. The visual-to-somatic axis is defined by gradients of activity symmetrical to the central sulcus and distributed over both frontal and parietal cortices. At least four sets of reach-related signals (retinal, gaze, arm position/movement direction, muscle output) are represented along this axis. This architecture defines informational domains where neurons combine different inputs. The position-direction axis is identified by the regular distribution of information over large populations of neurons processing both positional and directional signals (concerning the arm, gaze, visual stimuli, etc.) Therefore, the activity of gaze- and arm-related neurons can represent virtual three-dimensional (3D) pathways for gaze shifts or hand movement. Virtual 3D pathways are thus defined by a combination of directional and positional information. The sensory-motor axis is defined by neurons displaying different temporal relationships with the different reach-related signals, such as target presentation, preparation for intended arm movement, onset of movements, etc. These properties reflect the computation performed by local networks, which are formed by two types of processing units: matching and condition units. Matching units relate different neural representations of virtual 3D pathways for gaze or hand, and can predict motor commands and their sensory consequences. Depending on the units involved, different matching operations can be learned in the network, resulting in the acquisition of different visuo-motor transformations, such as those underlying reaching to foveated targets, reaching to extrafoveal targets, and visual tracking of hand movement trajectory. Condition units link these matching operations to reinforcement contingencies and therefore can shape the collective neural recruitment along the three axes of the network. This will result in a progressive match of retinal, gaze, arm, and muscle signals suitable for moving the hand toward the target.

Early coding of reaching: frontal and parietal association connections of parieto-occipital cortex.

The ipsilateral association connections of the cortex of the dorsal part of the rostral bank of the parieto-occipital sulcus and of the adjoining posterior part of the superior parietal lobule were studied by using different retrograde fluorescent tracers. Fluoro-Ruby, Fast blue and Diamidino yellow were injected into visual area V6A, and dorso-caudal (PMdc, F2) and dorso-rostral (PMdr, F7) premotor cortex, respectively. The parietal area of injection had been previously characterized physiologically in behaving monkeys, through a variety of oculomotor and visuomanual tasks. Area V6A is mainly linked by reciprocal projections to parietal areas 7m, MIP (medial intraparietal) and PEa, and, to a lesser extent, to frontal areas PMdr (rostral dorsal premotor cortex, F7) and PMdc (F2). All these areas project to that part of the dorsocaudal premotor cortex that has a direct access to primary motor cortex. V6A is also connected to area F5 and, to a lesser extent, to 7a, ventral (VIP) and lateral (LIP) intraparietal areas. This pattern of association connections may explain the presence of visually-related and eye-position signals in premotor cortex, as well as the influence of information concerning arm position and movement direction on V6A neural activity. Area V6A emerges as a potential 'early' node of the distributed network underlying visually-guided reaching. In this network, reciprocal association connections probably impose, through re-entrant signalling, a recursive property to the operations leading to the composition of eye and hand motor commands.

Early motor influences on visuomotor transformations for reaching: a positive image of optic ataxia.

Coding of reaching in the cerebral cortex is based on the operation of distributed populations of parietal and frontal neurons, whose main functional characteristics reside in their combinatorial power, i.e., in the capacity for combining different information related to the spatial aspects of reaching. The tangential distribution of reach-related neurons endowed with different functional properties changes gradually in the cortex and defines, in the parieto-frontal network, trends of functional properties. These visual-to-somatic gradients imply the existence of cortical regions of functional overlaps, i.e., of combinatorial domains, where the integration of different reach-related signals occurs. Studies of early coding of reaching in the mesial parietal areas show how somatomotor information, such as that related to arm posture and movement, influences neuronal activity in the very early stages of the visuomotor transformation underlying the composition of the motor command and is not added "downstream" in the frontal cortex. This influence is probably due to re-entrant signals traveling through fronto-parietal-association connections. Together with the gradient architecture of the network and the reciprocity of cortico-cortical connections, this implies that coding of reaching cannot be regarded as a top-down, serial sequence of coordinate transformation, each performed by a given cortical area, but as a recursive process, where different signals are progressively matched and further elaborated locally, due to intrinsic cortical connections. This model of reaching is also supported by psychophysical studies stressing the parallel processing of the different relevant parameters and the "hybrid" nature of the reference frame where they are combined. The theoretical frame presented here can also offer a background for a new interpretation of a well-known visuomotor disorder, due to superior parietal lesions, i.e., optic ataxia. More than a disconnection syndrome, this can now be interpreted as the consequence of the breakdown of the operations occurring in the combinatorial domains of the superior parietal segment of the parieto-frontal network.

Visuomotor transformations: early cortical mechanisms of reaching.

Recent studies of visually guided reaching in monkeys support the hypothesis that the visuomotor transformations underlying arm movements to spatial targets involve a parallel mechanism that simultaneously engages functionally related frontal and parietal areas linked by reciprocal cortico-cortical connections. The neurons in these areas possess similar combinations of response properties. The multimodal combinatorial properties of these neurons and the gradient architecture of the parietofrontal network emerge as a potential substrate to link the different sensory and motor signals that arise during reaching behavior into common hybrid reference frames. This convergent combinatorial process is evident at early stages of visual information processing in the occipito-parietal cortex, suggesting the existence of re-entrant motor influences on cortical areas once believed to have only visual functions.

Visual control of hand-reaching movement: activity in parietal area 7m.

The activity of single neurons was studied in parietal area 7m while monkeys performed an instructed-delay reaching task to visual targets under normal light conditions and in darkness. The task was aimed at assessing the influence of vision of hand position on the neural activity of 7m related either to static posture and movement of the hand or to eye position in the orbit. The results show the existence of preparatory, movement-related and postural activity for the control of reaching, all of which are strongly modulated by vision. The activity of many 7m neurons, otherwise insensitive to pure visual stimuli, seems to reflect complex interactions between gaze angle and hand position in the visual field.

Combination of hand and gaze signals during reaching: activity in parietal area 7 m of the monkey.

The role of area 7 m has been studied by recording the activity of single neurons of monkeys trained to fixate and reach toward peripheral targets. The target was randomly selected from eight possible locations on a virtual circle, of radius 30 degrees visual angle from a central target. Three tasks were employed to dissociate hand- from eye-related contributions. In the first task, animals looked and reached to the peripheral target. In a second task, the animal reached to the peripheral target while maintaining fixation on the central target. In the third task, the monkey maintained fixation on peripheral targets that were spatially coincident with those of the reaching tasks. The results show that cell activity in area 7 m relates, for some cells to eye position, for others to hand position and movement, and for the majority of cells to a combination of visuomanual and oculomotor information. This area, therefore, seems to perform an early combination of information in the processing leading from target localization to movement generation.

Cortical networks for visual reaching: intrinsic frontal lobe connectivity.

The anatomical substrates of reaching to visual targets were studied in monkeys (Macaca nemestrina) by combining behavioural neurophysiology and neuroanatomy. An instructed-delay reaching task was used to characterize the arm-related regions of the dorsolateral frontal cortex. This task revealed gradients of signal-, set- movement- and position-related activity along the rostrocaudal extent of the frontal lobe. The frontal mesial projections to these physiologically defined gradients were studied through anatomical methods based on the retrograde transport of distinguishable tracers. The tangential distribution of the cells of origin of these projections displayed a gradient-like arrangement similar to that defined physiologically in their terminal territory. These mesial projections to the dorsolateral frontal cortex may therefore be considered part of a cortical network wherein connections make only a limited contribution to the integration of different sources of information for the control of reaching movements. Further combination of such information must occur within each given cortical region by intrinsic local connections.

The sources of visual information to the primate frontal lobe: a novel role for the superior parietal lobule.

Reaching movements are performed in order to bring the hand to targets of interest. It is widely believed that the distributed cortical network underlying visual reaching transforms the information concerning the spatial location of the target into an appropriate motor command. Modern views decompose this process into sequences of coordinate transformations between informational domains. The set of cortical areas and pathways by which the information on target location is relayed from the visual areas of the occipital lobe to the motor areas of the frontal lobe have, so far, been poorly understood. Recent data from different fields of neuroscience offer the basis for a new definition of the cortical system subserving reaching and, at the same time, for a reconsideration of the nature of the underlying visuo-to-motor transformation.

Cortical networks for visual reaching: physiological and anatomical organization of frontal and parietal lobe arm regions.

The functional and structural properties of the dorsolateral frontal lobe and posterior parietal proximal arm representations were studied in macaque monkeys. Physiological mapping of primary motor (MI), dorsal premotor (PMd), and posterior parietal (area 5) cortices was performed in behaving monkeys trained in an instructed-delay reaching task. The parietofrontal corticocortical connectivities of these same areas were subsequently examined anatomically by means of retrograde tracing techniques. Signal-, set-, movement-, and position-related directional neuronal activities were distributed nonuniformly within the task-related areas in both frontal and parietal cortices. Within the frontal lobe, moving caudally from PMd to the MI, the activity that signals for the visuo-spatial events leading to target localization decreased, while the activity more directly linked to movement generation increased. Physiological recordings in the superior parietal lobule revealed a gradient-like distribution of functional properties similar to that observed in the frontal lobe. Signal- and set-related activities were encountered more frequently in the intermediate and ventral part of the medial bank of the intraparietal sulcus (IPS), in area MIP. Movement-and position-related activities were distributed more uniformly within the superior parietal lobule (SPL), in both dorsal area 5 and in MIP. Frontal and parietal regions sharing similar functional properties were preferentially connected through their association pathways. As a result of this study, area MIP, and possibly areas MDP and 7m as well, emerge as the parietal nodes by which visual information may be relayed to the frontal lobe arm region. These parietal and frontal areas, along with their association connections, represent a potential cortical network for visual reaching. The architecture of this network is ideal for coding reaching as the result of a combination between visual and somatic information.

Representing spatial information for limb movement: role of area 5 in the monkey.

How is spatial information for limb movement encoded in the brain? Computational and psychophysical studies suggest that beginning hand position, via-points, and target are specified relative to the body to afford a comparison between the sensory (e.g., kinesthetic) reafferences and the commands that generate limb movement. Here we propose that the superior parietal lobule (Brodmann area 5) might represent a substrate for a body-centered positional code. Monkeys made arm movements in different parts of 3D space in a reaction-time task. We found that the activity of area 5 neurons can be related to either the starting point, or the final point, or combinations of the two. Neural activity is monotonically tuned in a body-centered frame of reference, whose coordinates define the azimuth, elevation, and distance of the hand. Each spatial coordinate tends to be encoded in a different subpopulation of neurons. This parcellation could be a neural correlate of the psychophysical observation that these spatial parameters are processed in parallel and largely independent of each other in man.