1 Dipartimento di Fisiologia umana e Farmacologia, Università di Roma La Sapienza, Piazzale A. Moro 5, 00185 Rome, Italy and 2 Dottorato di Ricerca in Neurofisiologia, Università di Roma La Sapienza, Piazzale A. Moro 5, 00185 Rome, Italy
Address correspondence to: Alexandra Battaglia-Mayer, Dipartimento di Fisiologia umana e Farmacologia, Università di Roma La Sapienza, Piazzale Aldo Moro 5, I-00185 Rome, Italy. Email:alexandra.battagliamayer{at}uniroma1.it.
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Abstract |
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Key Words: directional hypokinesia eye movement hand movement hemispatial neglect inferior parietal cortex
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Introduction |
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The disordered representation of contralesional part of space impairs many of neglect patients' routinely activities, such as shaving, dressing, reading and drawing, so that, for example, they tend to shave only half of their face, read only the right part of a word or sentence, and copy or draw only the right part of a figure or self-portrait. Neglect is a complex syndrome composed by a constellation of symptoms. For instance, neglect patients with normal perceptual awareness can be impaired in encoding space for action. When presented with hierarchical figures (Navon, 1977), consisting of global letters or figures formed by smaller components, they recognize and name the global form, but are unable to cancel out with a pencil in their right hand the local components on the left side of each figure (Marshall and Halligan, 1995
). The hallmark of output-related disorders of neglect is directional hypokinesia (Heilman et al., 1985
; Mattingley et al., 1992
, 1998
), which consists of an elongated reaction time and inaccuracy of reaching to visual targets in the contralesional part of space, regardless of the limb used. Neglect patients also suffer from similar disorders in the oculomotor domain (Girotti et al., 1983
; Pierrot-Deseilligny et al., 1991
; Niemeier and Karnath, 2003
; for reviews, see De Renzi, 1982
; Husain and Rorden, 2003
). Although the nature of eyes deficit is still disputed, lengthening of saccadic reaction time to visual targets in the contralesional space has been reported by several authors (Girotti et al., 1983
; Nagel-Leiby et al., 1990
; Karnath et al., 1991
; Pierrot-Deseilligny et al., 1991
). All together, these hand and eye motor disorders reveal a profound impairment in the treatment of directional motor information. Therefore, understanding the representation of visuomotor space in parietal cortex is a necessary step to investigate the mechanisms underlying the motor aspects of neglect from a neurophysiological perspective. To this end, we have studied the dynamic properties of neurons in area 7a of the inferior parietal lobule of monkeys while these performed six different tasks, aimed at assessing the relationships between neural activity and the direction of different forms of visually- and memory-guided eye/hand movements. The behavioural tasks were intended to reproduce items of behaviour that are compromised by inferior parietal lesions in neglect patients. They were aimed at identifying and dissociating the relative influence of eye and hand directional signals on neural activity, at evaluating if and to what extent they were combined at single cell level, at describing the nature of their representation at the population level.
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Materials and Methods |
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Two rhesus monkeys (Macaca mulatta; body weights 4.3 and 6.1 kg) were used in this study. The monkeys sat on a primate chair with head fixed, and the eyes 17 cm in front of a 21'' touch-sensitive (MicroTouch Systems, Wilmington) computer monitor used to display the tasks and control the animals' hand position.
Monkeys performed six different tasks in total darkness. All arm movements were made with the hand contralateral to the hemisphere of recording. Arm and/or eye movements originated from a central position (Fig. 1A) and were directed toward eight peripheral targets (subtending 1.5° visual angle) located on a circle of 7.5 cm radius (23.8° visual angle).
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Behavioral Control
Hand position was monitored using the touch screen, with 0.28 x 0.3 mm (1 screen pixel) resolution. Hand accuracy was controlled through 3cm diameter circular windows (10° visual angle). Eye position signals were recorded by using implanted scleral search coils (1° resolution) and sampled at 100 Hz (Remmel Labs, Ashland, MA). Fixation accuracy was controlled through circular windows (3.5° diameter) around the targets. Eye velocity was calculated in off-line analysis. The onset time of the saccade was defined as the time when eye velocity exceeded 50°/s, and 180°/s, respectively, in the two adjacent 10 ms intervals beginning at the onset time of change of eye velocity. The end of the saccade was defined as the time when eye velocity fell below 50°/s.
Neural Recording
The activity of single neurons was recorded extracellularly. A seven-channel multielectrode recording system (System-Echkorn, Thomas Recording, Marburg) was used. In combination with seven dual timeamplitude window discriminators (Bak Electronics Inc, Mt. Airy, MD), recording could be obtained from up to 14 cells simultaneously. Electrodes were glass-coated tungsten-platinum fibres (12 MOhm impedance at 1 kHz). The eye-coil, recording chamber and head-holder were implanted aseptically under general anaesthesia (sodium pentobarbital, 25 mg/kg i.v.).
Data Analysis
Analysis of neuronal activity. In each task, the average firing rate during different epochs was computed trial by trial. Significant modulation of neural activity relative to the CT and to target direction was assessed by a two-way ANOVA (factor 1: epoch; factor 2: direction). Directional modulation was defined by the significance of either factor 2 or the interaction factor 1 x 2 (P < 0.05). At single cell level, coexistence of eye and hand directional signals was assessed through significant directional modulation of neural activity occurring at once in at least one hand-related (RT, MT, THT of Memory ReachFix task) and one eye-related (RT, MT, THT of Memory Eye task) epoch. The temporal span of analysis of some epochs was adjusted: the first 500 ms of CT were excluded from analysis to prevent potential effects of previous eye and/or hand movement; the first 500 ms of memory delay activity after cue presentation were excluded to prevent carry-over effects of visual responses; finally, the first 300 ms and the last 500 ms of THT were not considered, to avoid influence of previous movements or planning of return movements to the centre during inter-trial interval.
Directional Relationships
The relationship between cell activity and direction of movement was described through a modified cosine tuning function (Amirikian and Georgopoulos, 2000; Battaglia-Mayer et al., 2000
). In our 2-D experimental set-up, the angular variable of interest is the location of the target, univocally determined by the angle
, varying from 0 to 360°.
The standard cosine function (Georgopoulos et al., 1982) has the general form
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An additional analysis was prompted by the observation that several PDs were obtained for each neuron, depending on the number of epochs in which cell activity was (i) directionally modulated (ANOVA, P < 0.05; see above for criteria), and (ii) directionally tuned (R2 0.7) across different tasks. Therefore, for each individual cell the Rayleigh test of randomness (P < 0.05) was performed to assess whether the distribution of their PDs had a significant mean vector, i.e. whether the distribution was unimodal. The unimodality of the distribution of PDs defines the existence of a global tuning field (GTF; Battaglia-Mayer et al., 2000
) whose mean orientation is given by the direction of a significant mean vector. Furthermore, the uniformity of the distribution of the mean vectors of the population of cells with GTF was assessed through the Rayleigh test.
The polar distributions of PDs reported in Figures 57 were obtained by taking into consideration one epoch of a given task or similar behavioural epochs from different tasks, as follows:
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Results |
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We recorded the activity of 559 individual neurons in area 7a of two left hemispheres of two monkeys, while these performed the six different tasks illustrated in Figure 1. In monkey 1 penetrations were made in a region (Fig. 2A) of the inferior parietal lobule (IPL) which has been identified as area 7a, on the basis of two main criteria (i) the histological reconstructions of the microelectrode tracks relative to gross-anatomical landmarks, such as the position of the intraparietal (IPS) and superior temporal (STS) sulci; and (ii) the architectonic features of the area of recording on the assumption that area 7a of Vogt and Vogt (1919) is coextensive with areas Opt and PG, according to Pandya and Seltzer (1982)
and Gregoriou et al. (2003)
. Monkey 2 has not yet been sacrificed. In both animals, the location of IPS and STS were reconstructed from the readings of the top and bottom of neural activity during recording sessions, also thanks to a computer-aided reconstruction (Fig. 2B,D). This was made by interpolating the available values of the top of neural activity across the area of recording, adjusted for dura mater thickening over time. In monkey 1 (Fig. 2B) this procedure allowed a definition of the location of the IPS that perfectly matched that verified on the animal's brain (Fig. 2A), thus supporting the prediction of the reconstruction made in monkey 2 (Fig. 2D), indicating that microelectrode penetrations were performed in the desired area (Fig. 2C). Furthermore, in monkey 2 neurons attributed to area 7a had functional properties virtually identical to those found in monkey 1. In both animals, penetrations were perpendicular to the cortical surface and the extent of recording was usually confined within 2 mm from the top of neural activity, with an average depth across all penetrations of 853 ± 25 (SE) µm. This confirms that the data obtained in this study come from the flat exposed part of IPL.
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A summary of the basic results obtained from cells analyzed in a quantitative way (ANOVA, P < 0.05; see Materials and Methods) across the six different tasks is shown in Table 1. Only data from directional modulation are reported.
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In most cells, neural activity was maximal (Fig. 4) when the eye and/or the hand prepared to move, or moved in a given PD, or stayed immobile on peripheral targets. Cell activity decreased in an orderly fashion for directions further and further away from the preferred one. This directional tuning was observed across different behavioural epochs and tasks conditions, suggesting that encoding direction of eye and hand motor behaviour is a basic property of neurons in area 7a. In most instances, the PD of individual cells pointed toward the contralateral space (Fig. 4), and this was true for many of the activity types analyzed. As an example, Figure 4 shows the neural activity of three different cells in the form of rasters and spike-density function, as studied in three typical task conditions. In Figure 4A, the activity recorded during the Reaching task is aligned to the RTh epoch. This cell is directionally modulated during preparation of hand movement to foveated target. However, activity in this epoch could be dependent from an eye position signal, as in fact emerged from its significant (P < 0.01) modulation during THT of the Memory Eye task (not shown in Fig. 4). Since neural firing during the latter epoch was significantly (P < 0.01) different from that shown in Figure 4A, one can conclude that this cell's activity is influenced not only by eye position on the peripheral target, but also by preparation of hand movement towards it, i.e. to the fixation point. In Figure 4B, neural activity was recorded during the ReachFixation task, and it is aligned to the RTh epoch, that in this case accounts for signals related to hand movement preparation without the influence of directional information about eye position and/or movement. Thus, in this cell neural activity reflects a genuine directional hand reaction- and movement-time signal. Finally, Figure 4C shows the case of another cell with neural activity directionally modulated during eye holding time (THT) in the Memory Eye task, thus encoding an eye position signal. Common to these cells is the significant increase of firing rate for planning or execution of eye and/or hand actions in the contralateral space.
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Concerning positional signals (Fig. 7), a distribution of PDs skewed toward contralateral space was found during eye holding on memorized targets (Fig. 7a), an epoch that reflects encoding of an eye position signal, with the hand at the centre of the workspace. A different picture emerged for hand position signals (Fig. 7b), since their distribution showed a marked anisotropy that favoured the representation of ipsilateral space. In such instance, the eyes were at the centre of the workspace. When the eye and the hand were both on the same peripheral target (Fig. 7c), these opposite anisotropies tended to cancel each other out, and the resulting distribution of PDs was not significantly different from uniformity.
The data presented above raise the question of a possible evolution in time of the directional properties of 7a cells across the different behavioural epochs of each task. In other words, does cell activity remain directionally modulated during the time course of each task? To answer this question, Figure 8 shows, in form of Venn's diagrams, the number of cells directionally modulated in the different epochs across the task used. The overlap regions, indicating the number of cells which are, at once, directional in two consecutive behavioural epochs, are scarce, since the subpopulation of cells directional in a given epoch is generally different from that recruited in the subsequent one. These results indicate that only 20% of the cells remain directionally modulated in two contiguous epochs. This proportion increases to
35% during preparation and execution of movement, as well as static holding of arm in space. It is worth noticing that this analysis takes into account only the comparison of two consecutive epochs, and does not compare those that are non adjacent in time.
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Discussion |
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This study focuses on the directional tuning properties of cells in area 7a of the monkey. There are three main results to be discussed. First, in area 7a most neurons combine eye and hand signals in their neural activity. Secondly, the overwhelming majority of cells are directionally tuned, and in a consistent population of them the PDs computed across epochs and task conditions cluster within a restricted part of the workspace (GTF). Thirdly, the distribution of PDs during epochs that reflect different eye and/or hand signals is highly skewed toward the contralateral part of space.
The first conclusion of our study is that eye and hand motor signals influence neural activity in area 7a. Early neurophysiological studies (Hyvärinen and Poranen, 1974; Mountcastle et al., 1975
; MacKay, 1992
) had described the properties of cells in area 7. In these experiments, the influence of eye and visual signals on neural activity was not dissociated from that of hand signals. By adopting memory tasks, Snyder et al. (1997)
described the relative preponderance of activity-types reflecting the intention to perform eye versus hand movements in the IPL. This last study did not explicitly refer to properties of cells in 7a, and the proportion of non-specific cells combining eyehand signals across the reach zone studied in posterior parietal cortex (PPC) was observed to be minimal. In contrast, the results of our study show that the majority of cells in 7a are influenced by combined eyehand information. However, in Snyder et al. (1997)
, only the memory epochs across two conditions were analyzed, while in our experiment the co-occurrence of eyehand influences was assessed across a multiplicity of behavioural epochs, thanks to the multi-task approach adopted. Thus, the first main conclusion of our study is that in area 7a most cells encode eye and hand directional visuomotor signals. In our experimental set-up, the origin of eye and/or hand movement was not varied in space. Therefore, each target was attained through movements along a single direction. This implies that we cannot decide whether neural activity is correlated to abstract direction of movement, final target location or a combination of both. However, studies of cell activity in motor, premotor and superior parietal cortex have consistently shown that neural activity is highly correlated to the direction of hand movement within a high-order reference frame, although in a way that depends on a variety of factors, including the starting hand position (Caminiti et al., 1990
, 1991
; Lacquaniti et al., 1995
).
A second result of this study concerns the question of the relationships among different directional signals at single cell level. In fact, in about 25% of directionally tuned cells, the many preferred directions computed during different task epochs clustered within a restricted part of space, the GTF. This visuomotor domain can be considered as the neural substrate whereby parietal area 7a contributes to the distributed system subserving eyehand coordination. The relative invariance of PDs in front of varying eyehand behaviour suggests a coding mechanism for coordinated eyehand movements in head or allocentric reference frames, since cells are tuned to eyehand actions toward particular spatial locations independently of the specific spatial and temporal combination of effectors involved. The GTF was originally found in the superior parietal lobule (Battaglia-Mayer et al., 2000, 2001
), where, however, it accounts for the activity of a majority (
70%) of the cells studied.
The third, crucial observation obtained of this study concerns the analysis of the tuning properties of parietal neurons at the population level. This analysis revealed a dynamic over-representation of contralateral motor space. In fact, when the animal held in memory the target location for a future eye, hand or coordinated eyehand movement, as well as in the No-Go task, the corresponding preferred directions were represented in a uniform fashion across the workspace. When these plans evolved into real movement, a dynamic update of the visuomotor representation occurred, characterized by a magnification of contralateral space. This anisotropy was also evident for some special properties of oculo-manual cells, such as the GTF, whose mean vectors pointed to contralateral space.
The dynamic change of configuration of these distributions, when evolving from uniform to anisotropic, could result from at least two different mechanisms: each cell might change its preferred direction during two consecutive epochs, or different populations of cells might be recruited during the temporal evolution of the task. Our results favour the latter hypothesis, for two different reasons. First, the cells that remain directional across adjacent temporal epochs are a small proportion, that can hardly contribute to the change in the configuration of the overall distribution; secondly, the orientation of the PDs of these cells does not change dramatically over time, as also indicated by the existence of a GTF in many of them.
Our results are in line with previous qualitative studies reporting that in area 7 reach-neurons are active preferentially during movements toward contralateral space (Hyvärinen and Poranen, 1974; MacKay, 1992
), and also with parietal lesion studies in monkeys. Although the literature on this matter is exuberant (for a review, see Rizzolatti et al., 2000
), only selected studies will be discussed here those that may be relevant to the issue of directional motor components of neglect. Inactivation (Stein, 1978
) and unilateral lesions (Faugier-Grimaud et al., 1985
) of area 7 leads to inaccuracy of reaching and to elongation of reaction time for movements toward contralateral visual targets. In the former study, this impairment was reported for both arms. In the latter, all monkeys showed a significant increase of reaction time when using the contralesional arm (more severe for movements toward contra- than ipsilateral space). In one animal, an increase of hand RT was observed also for the ipsilesional arm, especially for movements towards contralateral space. However, it is worth noting that in this study the two arms were tested each in only one direction of movement, i.e. the right hand for leftward movements and the left hand for rightward ones. Therefore, it is difficult to conclude whether the elongation of reaction time was related to the arm used (ipsi- or contraletaral) or to the direction of movement (toward or away from the side of the lesion). In any case, the movements impaired were mainly those directed to target locations in the contralesional space. An additional parietal lesion study (LaMotte and Acuña, 1978
) shows a directional impairment of reaches to visual targets performed with the contralesional arm, in either the presence or absence of visual guidance of movement. In fact, reaches towards targets in contralesional space were consistently hypometric, since they were systematically misdirected toward the midline, as if the contralateral space was somehow compressed or under-represented. In this experiment, the lesion included both superior and inferior parietal lobules. Finally, monkeys with unilateral lesions confined to area 7a (Deuel and Farrar, 1993
) are reluctant, slow and inaccurate when reaching to moving targets only in the contralesional space, although able to detect and glance at them.
In humans performing reaches to visual targets in the ispi- and contralateral visual spaces (Kertzman et al., 1997), PET activation of the inferior parietal lobule was predominantly observed in the hemisphere contralateral to the visual space where targets were located, contrary to what observed in the superior parietal lobule, where activation was mainly bilateral.
In our study, over-representation of contralateral space in area 7a was found also for eye position signals, as well as for information concerning eye movement to visual targets. Contralateral anisotropy of gaze fields in monkey's area 7 has been described by Lynch et al. (1977). Concerning eye movement, our results conform to observations made in area 7 (Lynch et al., 1977
) and in the lateral intraparietal area (LIP; Barash et al., 1991)
, where saccadic activity is mostly tuned toward contralateral space. Interestingly, after unilateral IPL lesions, latencies of saccadic eye movements toward contralesional visual targets are significantly increased (Lynch and McLaren, 1989
). Moreover, inactivation of LIP results in impairments occurring in the contralateral visual space, although according to one study (Li et al., 1999
) the deficit consists of hypometric memory-guided saccades, while other studies report elongation in eye search time during visual target selection (Wardak et al., 2002)
, and deficits in covert attention (Wardak et al., 2004)
.
In humans the position of the eye modulates neural activity during pointing movements in parietal cortex. An fMRI study (DeSouza et al., 2000) reported a significant increase of activation in the right and left hemispheres when subjects fixate leftward or rightward to the pointing location, respectively, as would be predicted by the over-representation of contralateral eye position space observed in this study.
It is interesting that anisotropic distributions of directional properties of parietal neurons have also been described during higher-order mental processes. In the case of covert maze solving, the distribution of preferred path directions in area 7a is skewed toward the contralateral workspace (Crowe et al., 2004).
Neural Mechanisms Underlying the Directional Motor Components of Neglect
Although, many single-unit recording studies of PPC have shown the existence of different types of cells with a rich variety of functional properties, it often remains difficult to understand how the loss of these populations contribute to the emergence of the symptoms of neglect. Studies of IPL have either stressed the relationships between neural activity and attention orienting processes (for a review, see Colby and Goldberg, 1999) or the role of parietal cortex in intentional mechanisms leading to movement initiation (for a review, see Andersen and Buneo, 2002
). So far, no neurophysiological study has directly addressed the problem of the neural basis of directional motor components of neglect. Neglect might be associated to a predominant, although not exclusive, representation of the contralateral space (Pouget and Driver, 2000
; Rizzolatti et al., 2000
; Behrmann and Geng, 2002
), so that the loss of this representation could lead to an asymmetry in space coding, the contralesional sites being less well represented then the ipsilesional ones. This hypothesis is based on scrutiny of the functional properties of parietal neurons. When attempting to explain the motor deficit of neglect, this idea rests on the observation that parietal neural activity relates to the animal's intention to make purposeful eye or hand movement (Snyder et al., 1997
). The failure of these intentional mechanisms would lead to a reluctance to initiate movement in the contralateral space. This hypothesis, although fully acceptable, does not capture the main feature of the motor disorder of neglect: its directional nature. Our results, while supporting the contention of neglect as consequence of under-representation of contralateral space due to cortical damage, in addition might explain a variety of directional-selective alterations associated to parietal neglect, concerning both eye and arm movements. In other words, the results of this study might help understanding directional motor components of neglect from a physiological perspective.
In fact, parietal patients with neglect suffer from hand and eye motor disorders. Among the former ones, the most commonly reported is directional hypokinesia (Heilman et al., 1985; Mattingley et al., 1992
, 1998
). This deficit is often described as characterized by a constellation of direction-specific disorders for hand movement toward contralateral space, including bradikinesia and hypometria. However, there is wide consensus that the main feature of directional hypokinesia consists of a significant elongation of hand reaction time to visual targets in the contralesional space, a disturbance reflecting difficulties in movement planning. Although directional hypokinesia has been described in patients with both frontal and parietal lesions (for a review, see Vallar, 2001
), the latter seem to play a crucial role in the genesis of this disorder. The behaviour of neglect patients with inferior frontal and parietal lesions in the right hemisphere (Mattingley et al., 1992
, 1998
) has been contrasted during hand reaches within a task where perceptual components remained invariant. Hand reaches were performed to left visual targets from right and left starting positions relative to the movement endpoint. Both frontal and parietal patients displayed an elongation of reaction time for reaches toward contralesional targets. However, only parietal patients had a specific directional motor impairment, since they were much slower in initiating leftwards, as opposed to rightward, hand movements to targets in the left side of space.
Concerning disturbances of eye movements in parietal patients, the literature offers conflicting results. Lengthening of eye reaction time to targets in the contralesional space has been described in many studies (Girotti et al., 1983; Nagel-Leiby et al., 1990
; Karnath et al., 1991
; Pierrot-Deseilligny et al., 1991
) based on visual reflexive saccades. In such studies, errors in amplitude, i.e. hypometric staircase saccades, in contralesional space have been reported only by Girotti et al. (1983)
. On the contrary, during free exploration (Niemeier and Karnath, 2000
) and visual search (Karnath et al., 1996
), only non-direction-specific hypometric impairments of eye movements were found. Right parietal patients with neglect, when tested in the intact hemifield, display a direction specific hypometric impairment of leftward saccades only when these are stimulus-driven and not when they occur within a visual search task, where subjects make voluntary saccades to potential targets (Niemeier and Karnath, 2003
). Task-dependent emergence of temporal disorders of eye movements toward contralateral space in parietal patients had been described by Braun et al. (1992)
. Our observation of a magnification of contralateral directional space during reaction time of visually- but not memory-guided saccades is in line with this view, since it implies the existence of context-dependent representations of visuomotor space in parietal cortex.
The present study also revealed anisotropy in the representation of hand position information. However, contrary to what seen for other hand and eye signals, this representation favoured the ipsilateral, rather than the contralateral space. We have no easy explanation for this result, which can hardly be due to sampling bias, since it was observed in both monkeys from the analysis of the same populations of cells from which we derived preferred directions with marked contralateral anisotropy of their orientation during other epochs. Furthermore, this ipsilateral anisotropy of hand position signals was observed while the eyes were at the centre of the workspace. On the contrary, contralateral anisotropy was observed for eye-position signals, while the hand was at the centre. When eye and hand were both on peripheral targets, their opposite anisotropies tended to cancel out, and the resulting representation of the eyehand directional space was not significantly different from uniformity. Thus, the apparent conflict brought about by the ipsilateral skew of hand position signals could be reconciled by noticing that, in both anisotropic distributions, the most represented situation is the one where the eye is on the right (i.e. contralateral to the hemisphere of recording) of the hand. This result therefore becomes a further expression of contralateral anisotropy, under the assumption that the observed signals refer to eye position relative to a hand-centred reference frame.
The disruption of a directional visuomotor representation favouring the contralateral space might explain the difficulties in planning and performing directional eye and hand movements observed in neglect patients. This representation seems unique to inferior parietal cortex, since in motor cortex the distribution of PDs of reach-neurons is uniform (Schwartz et al., 1988; Caminiti et al., 1990
). Similarly, the areas of the parieto-occipital junction (Battaglia-Mayer et al., 2000
, 2001
) and dorsal premotor cortex (Caminiti et al., 1991
), to which they are linked by association connections (Marconi et al., 2001
), display a uniform distribution of directional eye- and hand-movement signals. This is coherent to what observed by PET studies in humans performing reaches to ispi- and contralateral visual targets (Kertzman et al., 1997
). Interestingly, unilateral superior parietal lesions result in optic ataxia, which is characterized by a reaching disorder to visual targets (Perenin and Vighetto, 1988)
that, when damage is on the right or left hemisphere, affects reaches of either hand in the contralateral visual field (visual field effect), while in left-damaged patients, in addition, reaching with the right hand is inaccurate in the ispilesional field as well (hand effect). Thus, in optic ataxia the reaching disorder does not show the strict directional polarity described for directional hypokinesia.
Finally, it is worth stressing that network models of neglect rely heavily on anisotropy of spatial representations (Pouget and Sejnowski, 1997; Pouget and Driver, 2000
). The assumption is that a gradient of spatial representation is embedded in the activity of neurons in inferior parietal cortex, such that the lesion in one hemisphere results in lack of information concerning the contralesional space. This impairs the performance of the neural net model across different tasks, which might include those referring to motor plans toward contralesional space. The results of our study can offer neurophysiological underpinnings and new material to theoretical models of neglect.
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Acknowledgments |
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References |
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