Ventral Premotor Area Research Articles

Callosal connections of dorsal versus ventral premotor areas in the macaque monkey: a multiple retrograde tracing study

BackgroundThe lateral premotor cortex plays a crucial role in visually guided limb movements. It is divided into two main regions, the dorsal (PMd) and ventral (PMv) areas, which are in turn subdivided into functionally and anatomically distinct rostral (PMd-r and PMv-r) and caudal (PMd-c and PMv-c) sub-regions. We analyzed the callosal inputs to these premotor subdivisions following 23 injections of retrograde tracers in eight macaque monkeys. In each monkey, 2–4 distinct tracers were injected in different areas allowing direct comparisons of callosal connectivity in the same brain.ResultsBased on large injections covering the entire extent of the corresponding PM area, we found that each area is strongly connected with its counterpart in the opposite hemisphere. Callosal connectivity with the other premotor areas, the primary motor cortex, prefrontal cortex and somatosensory cortex varied from one area to another. The most extensive callosal inputs terminate in PMd-r and PMd-c, with PMd-r strongly connected with prefrontal cortex. Callosal inputs to PMv-c are more extensive than those to PMv-r, whose connections are restricted to its counterpart area. Quantitative analysis of labelled cells confirms these general findings, and allows an assessment of the relative strength of callosal inputs.ConclusionPMd-r and PMv-r receive their strongest callosal inputs from their respective counterpart areas, whereas PMd-c and PMv-c receive strong inputs from heterotopic areas as well (namely from PMd-r and PMv-r, respectively). Finally, PMd-r stands out as the lateral premotor area with the strongest inputs from the prefrontal cortex, and only the PMd-c and PMv-c receive weak callosal inputs from M1.

The selection of intended actions and the observation of others' actions: A time-resolved fMRI study

Whenever we plan, imagine, or observe an action, the motor systems that would be involved in preparing and executing that action are similarly engaged. The way in which such common motor activation is formed, however, is likely to differ depending on whether it arises from our own intentional selection of action or from the observation of another's action. In this study, we use time-resolved event-related functional MRI to tease apart neural processes specifically related to the processing of observed actions, the selection of our own intended actions, the preparation for movement, and motor response execution. Participants observed a finger gesture movement or a cue indicating they should select their own finger gesture to perform, followed by a 5-s delay period; participants then performed the observed or self-selected action. During the preparation and readiness for action, prior to initiation, we found activation in a common network of higher motor areas, including dorsal and ventral premotor areas and the pre-supplementary motor area (pre-SMA); the more caudal SMA showed greater activation during movement execution. Importantly, the route to this common motor activation differed depending on whether participants freely selected the actions to perform or whether they observed the actions performed by another person. Observation of action specifically involved activation of inferior and superior parietal regions, reflecting involvement of the dorsal visual pathway in visuomotor processing required for planning the action. In contrast, the selection of action specifically involved the dorsal lateral prefrontal and anterior cingulate cortex, reflecting the role of these prefrontal areas in attentional selection and guiding the selection of responses.

Input–output organization of jaw movement‐related areas in monkey frontal cortex

The brain mechanisms underlying mastication are not fully understood. To address this issue, we analyzed the distribution patterns of cortico-striatal and cortico-brainstem axon terminals and the origin of thalamocortical and intracortical fibers by injecting anterograde/retrograde tracers into physiologically and morphologically defined jaw movement-related cortical areas. Four areas were identified in the macaque monkey: the primary and supplementary orofacial motor areas (MIoro and SMAoro) and the principal and deep parts of the cortical masticatory area (CMaAp and CMaAd), where intracortical microstimulation produced single twitch-like or rhythmic jaw movements, respectively. Tracer injections into these areas labeled terminals in the ipsilateral putamen in a topographic fashion (MIoro vs. SMAoro and CMaAp vs. CMaAd), in the lateral reticular formation and trigeminal sensory nuclei contralaterally (MIoro and CMaAp) or bilaterally (SMAoro) in a complex manner of segregation vs. overlap, and in the medial parabranchial and Kölliker-Fuse nuclei contralaterally (CMaAd). The MIoro and CMaAp received thalamic projections from the ventrolateral and ventroposterolateral nuclei, the SMAoro from the ventroanterior and ventrolateral nuclei, and the CMaAd from the ventroposteromedial nucleus. The MIoro, SMAoro, CMaAp, and CMaAd received intracortical projections from the ventral premotor cortex and primary somatosensory cortex, the ventral premotor cortex and rostral cingulate motor area, the ventral premotor cortex and area 7b, and various sensory areas. In addition, the MIoro and CMaAp received projections from the three other jaw movement-related areas. Our results suggest that the four jaw movement-related cortical areas may play important roles in the formation of distinctive masticatory patterns.

Quantitative Investigation of Connections of the Prefrontal Cortex in the Human and Macaque using Probabilistic Diffusion Tractography

The functions of prefrontal cortex (PFC) areas are constrained by their anatomical connections. There is little quantitative information about human PFC connections, and, instead, our knowledge of primate PFC connections is derived from tracing studies in macaques. The connections of subcortical areas, in which white matter penetration and hence diffusion anisotropy are greatest, can be studied with diffusion-weighted imaging (DWI) tractography. We therefore used DWI tractography in four macaque and 10 human hemispheres to compare the connections of PFC regions with nine subcortical regions, including several fascicles and several subcortical nuclei. A distinct connection pattern was identified for each PFC and each subcortical region. Because some of the fascicles contained connections with posterior cortical areas, it was also possible to draw inferences about PFC connection patterns with posterior cortical areas. Notably, it was possible to identify similar circuits centered on comparable PFC regions in both species; PFC regions probably engage in similar patterns of regionally specific functional interaction with other brain areas in both species. In the case of one area traditionally assigned to the human PFC, the pars opercularis, the distribution of connections was not reminiscent of any macaque PFC region but, instead, resembled the pattern for macaque ventral premotor area. Some limitations to the DWI approach were apparent; the high diffusion anisotropy in the corpus callosum made it difficult to compare connection probability values in the adjacent cingulate region.

Neuronal correlates of movement dynamics in the dorsal and ventral premotor area in the monkey

We investigated how neurons in the different motor areas of the frontal lobe reflect the movement dynamics, and how their neuronal activity undergoes plastic changes when monkeys adapt to perturbing forces (they learn new dynamics). Here we describe the results obtained in the dorsal premotor area (PMd) and ventral premotor area (PMv). Monkeys performed visually instructed, delayed reaching movements before, during and after exposure and adaptation to a viscous, curl force field. During movement planning (i.e., during an instructed delay that followed the cue and preceded the go signal), we found dynamics-related activity in PMd but not in PMv. A closer analysis revealed that the population of PMd reflected the dynamics of the upcoming movement increasingly over the course of the delay, starting from a kinematics-related signal. During movement execution, dynamics-related activity was present in both PMd and PMv. In this respect, the results for PMd were similar to that previously found for the supplementary motor area (SMA) whereas the results for PMv were more similar to that previously found for the primary motor cortex (M1). Plastic changes associated with the acquisition of new dynamics found in PMd and PMv were qualitatively similar to those previously observed in M1 and SMA. The ensemble of our experiments suggest a broader picture of the cortical control of movements, whereby multiple areas all contribute to the various sensorimotor processes, including "low" computations such as the movement dynamics, but also express a degree of specialization.

Passive somatosensory discrimination tasks in healthy volunteers: Differential networks involved in familiar versus unfamiliar shape and length discrimination

Somatosensory discrimination of unseen objects relies on processing of proprioceptive and tactile information to detect spatial features, such as shape or length, as acquired by exploratory finger movements. This ability can be impaired after stroke, because of somatosensory-motor deficits. Passive somatosensory discrimination tasks are therefore used in therapy to improve motor function. Whereas the neural correlates of active discrimination have been addressed repeatedly, little is known about the neural networks activated during passive discrimination of somatosensory information. In the present study, we applied functional magnetic resonance imaging (fMRI) while the right index finger of ten healthy subjects was passively moved along various shapes and lengths by an fMRI compatible robot. Comparing discriminating versus non-discriminating passive movements, we identified a bilateral parieto-frontal network, including the precuneus, superior parietal gyrus, rostral intraparietal sulcus, and supramarginal gyrus as well as the supplementary motor area (SMA), dorsal premotor (PMd), and ventral premotor (PMv) areas. Additionally, we compared the discrimination of different spatial features, i.e., discrimination of length versus familiar (rectangles or triangles) and unfamiliar geometric shapes (arbitrary quadrilaterals). Length discrimination activated mainly medially located superior parietal and PMd circuits whereas discrimination of familiar geometric shapes activated more laterally located inferior parietal and PMv regions. These differential parieto-frontal circuits provide new insights into the neural basis of extracting spatial features from somatosensory input and suggest that different passive discrimination tasks could be used for lesion-specific training following stroke.

Frontal Lobe Inputs to the Digit Representations of the Motor Areas on the Lateral Surface of the Hemisphere

We examined the frontal lobe connections of the digit representations in the primary motor cortex (M1), the dorsal premotor area (PMd), and the ventral premotor area (PMv) of cebus monkeys. All of these digit representations lie on the lateral surface of the hemisphere. We used intracortical stimulation to identify the digit representations physiologically, and then we injected different tracers into two of the three cortical areas. This approach enabled us to compare the inputs to two digit representations in the same animal. We found that the densest inputs from the premotor areas to the digit representation in M1 originate from the PMd and the PMv. Both of these premotor areas contain a distinct digit representation, and the two digit representations are densely interconnected. Surprisingly, the projections from the digit representation in the supplementary motor area (SMA) to the PMd and PMv are stronger than the SMA projections to M1. The projections from other premotor areas to M1, the PMd, and the PMv are more modest. Of the three digit areas on the lateral surface, only the PMv receives dense input from the prefrontal cortex. Based on these results, we believe that M1, the PMd, and the PMv form a densely interconnected network of cortical areas that is concerned with the generation and control of hand movements. Overall, the laminar origins of neurons that interconnect the three cortical areas are typical of "lateral" interactions. Thus, from an anatomical perspective, this cortical network lacks a clear hierarchical organization.

Mirror Neurons Responding to Observation of Actions Made with Tools in Monkey Ventral Premotor Cortex

In the present study, we describe a new type of visuomotor neurons, named tool-responding mirror neurons, which are found in the lateral sector of monkey ventral premotor area F5. Tool-responding mirror neurons discharge when the monkey observes actions performed by an experimenter with a tool (a stick or a pair of pliers). This response is stronger than that obtained when the monkey observes a similar action made with a biological effector (the hand or the mouth). These neurons respond also when the monkey executes actions with both the hand and the mouth. The visual and the motor responses of each neuron are congruent in that they share the same general goal, that is, taking possession of an object and modifying its state. It is hypothesized that after a relatively long visual exposure to tool actions, a visual association between the hand and the tool is created, so that the tool becomes as a kind of prolongation of the hand. We propose that tool-responding mirror neurons enable the observing monkey to extend action-understanding capacity to actions that do not strictly correspond to its motor representations. Our findings support the notion that the motor cortex plays a crucial role in understanding action goals.

ミラーニューロンの明らかにしたもの：運動制御から認知機能へ

Many studies have emphasized simulation theory as a core concept for the mirror neurons that are found in the ventral premotor area (F5) and inferior parietal cortex (PF) of the macaque. Imitation, social cognition, empathy and language are linked to mirror neurons. However, function of mirror neurons in the macaque is still missing, because such cognitive functions are limited or lacking in these monkeys. It is important to discuss these neurons in the aspect of motor function. I would suggest that mirror neurons in area PF might have a roll of monitoring of self generated action. This idea may provide neural correlate of self other distinction.

Ipsilateral cortical connections of motor, premotor, frontal eye, and posterior parietal fields in a prosimian primate,Otolemur garnetti

The ipsilateral connections of motor areas of galagos were determined by injecting tracers into primary motor cortex (M1), dorsal premotor area (PMD), ventral premotor area (PMV), supplementary motor area (SMA), and frontal eye field (FEF). Other injections were placed in frontal cortex and in posterior parietal cortex to define the connections of motor areas further. Intracortical microstimulation was used to identify injection sites and map motor areas in the same cases. The major connections of M1 were with premotor cortex, SMA, cingulate motor cortex, somatosensory areas 3a and 1, and the rostral half of posterior parietal cortex. Less dense connections were with the second (S2) and parietal ventral (PV) somatosensory areas. Injections in PMD labeled neurons across a mediolateral belt of posterior parietal cortex extending from the medial wall to lateral to the intraparietal sulcus. Other inputs came from SMA, M1, PMV, and adjoining frontal cortex. PMV injections labeled neurons across a large zone of posterior parietal cortex, overlapping the region projecting to PMD but centered more laterally. Other connections were with M1, PMD, and frontal cortex and sparsely with somatosensory areas 3a, 1-2, S2, and PV. SMA connections were with medial posterior parietal cortex, cingulate motor cortex, PMD, and PMV. An FEF injection labeled neurons in the intraparietal sulcus. Injections in posterior parietal cortex revealed that the rostral half receives somatosensory inputs, whereas the caudal half receives visual inputs. Thus, posterior parietal cortex links visual and somatosensory areas with motor fields of frontal cortex.

Evolution of somatosensory and motor cortex in primates.

Inferences about how the complex somatosensory systems of anthropoid primates evolved are based on comparative studies of such systems in extant mammals. Experimental studies of members of the major clades of extant mammals suggest that somatosensory cortex of early mammals consisted of only a few areas, including a primary area, S1, bordered by strip-like rostral and caudal somatosensory fields, SR and SC. In addition, the second somatosensory area, S2, and the parietal ventral area, PV, were probably present. S1, S2, and PV were activated independently via parallel projections from the ventroposterior nucleus, VP. Little posterior parietal cortex existed, and it was unlikely that a separate primary motor area, M1, existed until placental mammals evolved. Early primates retained this basic organization and also had a larger posterior parietal region that mediated sensorimotor functions via connections with motor and premotor areas. The frontal cortex included M1, dorsal and ventral premotor areas, supplementary motor area, and cingulate motor fields. Ventroposterior superior and ventroposterior inferior nuclei were distinct from the ventroposterior nucleus in the thalamus. In early anthropoid primates, areas S1, SR, and SC had differentiated into the fields now recognized as areas 3b, 3a, and 1. Areas 3b and 1 contained parallel mirror-image representations of cutaneous receptors and a parallel representation in area 2 was probable. Serial processing became dominant, so that neurons in areas 1, S2, and PV became dependent on area 3b for activation. Posterior parietal cortex expanded into more areas that related to frontal cortex. Less is known about changes that might have occurred with the emergence of apes and humans, but their brains were larger and posed scaling problems most likely solved by increasing the number of cortical areas and reducing the proportion of long connections.

Functional Properties of Grasping-Related Neurons in the Ventral Premotor Area F5 of the Macaque Monkey

We investigated the motor and visual properties of F5 grasping neurons, using a controlled paradigm that allows the study of the neuronal discharge during both observation and grasping of many different three-dimensional objects with and without visual guidance. All neurons displayed a preference for grasping of an object or a set of objects. The same preference was maintained when grasping was performed in the dark without visual feedback. In addition to the motor-related discharge, about half of the neurons also responded to the presentation of an object or a set of objects, even when a grasping movement was not required. Often the object evoking the strongest activity during grasping also evoked optimal activity during its visual presentation. Hierarchical cluster analysis indicated that the selectivity of both the motor and the visual discharge of the F5 neurons is determined not by the object shape but by the grip posture used to grasp the object. Because the same paradigm has been used to study the properties of hand-grasping neurons in the dorsal premotor area F2, and in the anterior intraparietal area (AIP), a comparison of the functional properties of grasping-related neurons in the three cortical areas (F5, F2, AIP) is addressed for the first time.

Macaque ventral premotor cortex exerts powerful facilitation of motor cortex outputs to upper limb motoneurons.

The ventral premotor area (F5) is part of the cortical circuit controlling visuomotor grasp. F5 could influence hand motor function through at least two pathways: corticospinal projections and corticocortical projections to primary motor cortex (M1). We found that stimulation of macaque F5, which by itself evoked little or no detectable corticospinal output, could produce a robust modulation of motor outputs from M1. Arrays of fine microwires were implanted in F5 and M1. During terminal experiments under chloralose anesthesia, single stimuli delivered to M1 electrodes evoked direct (D) and indirect (I1,I2, and I3) corticospinal volleys. In contrast, single F5 shocks were ineffective; double shocks (3 msec separation) evoked small I waves but no D wave. However, when the test (T) M1 shock was conditioned (C) by single or double F5 shocks, there was strong facilitation of I2 and I3 waves from M1, with C-T intervals of <1 msec. Intracellular recordings from 79 arm and hand motoneurons (MNs) revealed no postsynaptic effects from single F5 shocks. In contrast, these stimuli produced a robust facilitation of I2 and I3 EPSPs evoked from M1 (60% of MNs); this was particularly marked in hand muscle MNs (92%). Muscimol injection in M1 reduced I waves from F5 and abolished the F5-induced facilitation of late I waves from M1, and of EPSPs associated with them. Thus, some motor effects evoked from F5 may be mediated by corticocortical inputs to M1 impinging on interneurons generating late corticospinal I waves. Similar mechanisms may allow F5 to modulate grasp-related outputs from M1.

Functional imaging of face and hand imitation: towards a motor theory of empathy

Empathy requires the ability to map the feelings of others onto our own nervous system. Until recently, there was no plausible mechanism to explain how such a mapping might occur. The discovery of mirror neurons, however, suggests that the nervous system is capable of mapping the observed actions of others onto the premotor cortex of the self, at least for reaching and grasping movements. Is there a mirroring system for emotive actions, such as facial expression? Subjects ( N = 15; all right-handed; eight men, seven women) watched movies of facial expressions (smile or frown) and hand movements (move index or middle finger) while brain activity was imaged using functional magnetic resonance imaging (fMRI). Subjects watched the movies under three different conditions: passive viewing, active imitation, and an active motor control. Subjects also performed a verb generation task to functionally identify language-processing areas. We found evidence for a common cortical imitation circuit for both face and hand imitation, consisting of Broca's area, bilateral dorsal and ventral premotor areas, right superior temporal gyrus (STG), supplementary motor area, posterior temporo-occipital cortex, and cerebellar areas. For faces, passive viewing led to significant activation in the right ventral premotor area, whereas imitation produced bilateral activation. This result is consistent with evidence for right hemisphere (RH) dominance for emotional processing, and suggests that there may be a right hemisphere mirroring system that could provide a neural substrate for empathy.

Effector-independent representations of simple and complex imagined finger movements: a combined fMRI and TMS study.

Kinesthetic motor imagery and actual execution of movements share a common neural circuitry. Functional magnetic resonance imaging was used in 12 right-handed volunteers to study brain activity during motor imagery and execution of simple and complex unimanual finger movements of the dominant and the nondominant hand. In the simple task, a flexible object was rhythmically compressed between thumb, index and middle finger. The complex task was a sequential finger-to-thumb opposition movement. Premotor, posterior parietal and cerebellar regions were significantly more active during motor imagery of complex movements than during mental rehearsal of the simple task. In 10 of the subjects, we also used transcranial magnetic brain stimulation to examine corticospinal excitability during the same motor imagery tasks. Motor-evoked potentials increased significantly over values obtained in a reference condition (visual imagery) during imagery of the complex, but not of the simple movement. Imagery of finger movements of either hand activated left dorsal and ventral premotor areas and the supplementary motor cortex regardless of task complexity. The effector-independent activation of left premotor areas was particularly evident in the simple motor imagery task and suggests a left hemispherical dominance for kinesthetic movement representations in right-handed subjects.

Callosal connections of dorso-lateral premotor cortex.

This study investigated the organization of the callosal connections of the two subdivisions of the monkey dorsal premotor cortex (PMd), dorso-rostral (F7) and dorso-caudal (F2). In one animal, Fast blue and Diamidino yellow were injected in F7 and F2, respectively; in a second animal, the pattern of injections was reversed. F7 and F2 receive a major callosal input from their homotopic counterpart. The heterotopic connections of F7 originate mainly from F2, with smaller contingent from pre-supplementary motor area (pre-SMA, F6), area 8 (frontal eye fields), and prefrontal cortex (area 46), while those of F2 originate from F7, with smaller contributions from ventral premotor areas (F5, F4), SMA-proper (F3), and primary motor cortex (M1). Callosal cells projecting homotopically are mostly located in layers II-III, those projecting heterotopically occupy layers II-III and V-VI. A spectral analysis was used to characterize the spatial fluctuations of the distribution of callosal neurons, in both F7 and F2, as well as in adjacent cortical areas. The results revealed two main periodic components. The first, in the domain of the low spatial frequencies, corresponds to periodicities of cell density with peak-to-peak distances of approximately 10 mm, and suggests an arrangement of callosal cells in the form of 5-mm wide bands. The second corresponds to periodicities of approximately 2 mm, and probably reflects a 1-mm columnar-like arrangement. Coherency and phase analyses showed that, although similar in their spatial arrangements, callosal cells projecting to dorsal premotor areas are segregated in the tangential cortical domain.

When vision guides movement: a functional imaging study of the monkey brain

Goal-directed reaching requires a precise neural representation of the arm position and the target location. Parietal and frontal cortical areas rely on visual, somatosensory, and motor signals to guide the reaching arm to the desired position in space. To dissociate the regions processing these signals, we applied the quantitative [ 14C]-deoxyglucose method on monkeys reaching either in the light or in the dark. Nonvisual (somatosensory and memory-related) guidance of the arm, during reaching in the dark, induced activation of discrete regions in the parietal, premotor, and motor cortices. These included the dorsal part of the medial bank of the intraparietal sulcus, the ventral premotor area F4, the dorsal premotor area F2 below the superior precentral dimple, and the primary somatosensory and motor cortices. Additional parietal and premotor regions comprising the ventral intraparietal cortex, ventral premotor area F5, and the ventral part of dorsal premotor area F2 were activated by visual guidance of the arm during reaching in the light. This study provides evidence that different regions of the parieto–premotor circuit process the visual, somatosensory, and motor-memory-related signals which guide the moving arm.

Reorganization of remote cortical regions after ischemic brain injury: a potential substrate for stroke recovery.

Although recent neurological research has shed light on the brain's mechanisms of self-repair after stroke, the role that intact tissue plays in recovery is still obscure. To explore these mechanisms further, we used microelectrode stimulation techniques to examine functional remodeling in cerebral cortex after an ischemic infarct in the hand representation of primary motor cortex in five adult squirrel monkeys. Hand preference and the motor skill of both hands were assessed periodically on a pellet retrieval task for 3 mo postinfarct. Initial postinfarct motor impairment of the contralateral hand was evident in each animal, followed by a gradual improvement in performance over 1-3 mo. Intracortical microstimulation mapping at 12 wk after infarct revealed substantial enlargements of the hand representation in a remote cortical area, the ventral premotor cortex. Increases ranged from 7.2 to 53.8% relative to the preinfarct ventral premotor hand area, with a mean increase of 36.0 +/- 20.8%. This enlargement was proportional to the amount of hand representation destroyed in primary motor cortex. That is, greater sparing of the M1 hand area resulted in less expansion of the ventral premotor cortex hand area. These results suggest that neurophysiologic reorganization of remote cortical areas occurs in response to cortical injury and that the greater the damage to reciprocal intracortical pathways, the greater the plasticity in intact areas. Reorganization in intact tissue may provide a neural substrate for adaptive motor behavior and play a critical role in postinjury recovery of function.

Functional connectivity revealed by single-photon emission computed tomography (SPECT) during repetitive transcranial magnetic stimulation (rTMS) of the motor cortex.

In the present study, we studied effects of 1 Hz repetitive transcranial magnetic stimulation (rTMS) over the left primary motor cortex (M1) on regional cerebral blood flow (rCBF) using single-photon emission computed tomography (SPECT). SPECT measurements were carried out under two experimental conditions: real and sham stimulation. In sham stimulation, to exclude other components besides currents in the brain in rTMS, we applied sound and electrical stimulation to the skin of the head. 99mTc-ethyl cysteinate dimer was injected during the real or sham stimulation. Images were analyzed with the statistical parametric mapping software (SPM99). Relative differences in adjusted rCBF between two conditions were determined by a voxel-by-voxel paired t test. 1 Hz rTMS at an intensity of 1.1 x active motor threshold evoked increase of rCBF in the contralateral (right) cerebellar hemisphere. Reduction of rCBF was observed in the contralateral M1, superior parietal lobule (most probably corresponding to PE area in the monkey) (Rizzolatti G, Luppino G, Matelli M. Electroenceph clin Neurophysiol 1998;106:283-296), inferior parietal lobule (PF area in the monkey (Rizzolatti et al., 1998)), dorsal and ventral premotor areas (dPM, vPM) and supplementary motor area (SMA). Increase of rCBF in the contralateral cerebellum must reflect facilitatory connection between the motor cortex and contralateral cerebellum. Reduced rCBF in the contralateral M1 may be produced by transcallosal inhibitory effect of the left motor cortical activation. CBF decrease in the right PM, SMA and parietal cortex may reflect some secondary effects. Low frequency rTMS at an intensity of around threshold for active muscles can evoke rCBF changes. We demonstrated that rCBF changes could be elicited even by low frequency rTMS at such a low intensity as the threshold for an active muscle. Combination of rTMS and SPECT is one of powerful tools to study interareal connection within the human brain.

Both parietal lobes are involved in drawing: a functional MRI study and implications for constructional apraxia

In clinical studies, many researchers have reported that drawing can be disturbed by left or right unilateral parietal lobe damage (constructional apraxia). There seem to be two possible predictions about the cerebral laterality for drawing. The first is that drawing requires both parietal lobes, therefore, a lesion to either side can disrupt drawing. The second is that individuals can differ in laterality: some have only right or left activations, and some have bilateral. To test these predictions, we investigated with functional magnetic resonance imaging (fMRI) the cerebral activation whilst 17 right-handed healthy subjects performed a drawing task. The experiment consisted of two conditions: (1) naming an object in a presented picture and drawing it by using right index finger (DRAWING&NAMING); (2) naming an object in a presented picture (NAMING). We considered the brain regions that had greater activity in the DRAWING&NAMING condition than in the NAMING condition were the neural substrates of drawing. Individual analysis revealed that all subjects showed parietal activation bilaterally. We interpret that the results support the first prediction that both parietal lobes are required for drawing. By calculating the laterality indices of the individual parietal activations, it was found that there were more left dominant subjects than right dominant subjects (left, 12; right, 5). The results are inconsistent with previous studies on the incidence of constructional apraxia. In addition, we found activation in regions that were not previously reported in the literature of constructional apraxia: they are the ventral premotor area and posterior part of inferior temporal sulcus.