Slow potentials preceding self-paced hand movements in the parietal cortex of monkeys.

In our previous paper, surface negative-deep positive slow potentials were reported to appear prior to self-paced hand movements in the premotor and the forelimb motor cortices of unanesthetized, freely moving monkeys, as recorded with the elctrodes implanted chronically in the cortices. The present study revealed that similar slow potentials were recorded also in the somatosensory cortex in most cases, but infrequently and unmarkedly in area 7, never in area 19.     

Thalamocortical and the dual pattern of corticothalamic projections of the posterior parietal cortex in macaque monkeys

The corticothalamic projection includes a main, modulatory projection from cortical layer VI terminating with small endings whereas a less numerous, driving projection from layer V forms giant endings. Such dual pattern of corticothalamic projections is well established in rodents and cats for many cortical areas. In non-human primates (monkeys), it has been reported for the primary sensory cortices (A1, V1, S1), the motor and premotor cortical areas and, in the parietal lobe, also for area 7. The present study aimed first at refining the cytoarchitecture parcellation of area 5 into the sub-areas PE and PEa and, second, establishing whether area 5 also exhibits this dual pattern of corticothalamic projection and what is its precise topography. To this aim, the tracer biotinylated dextran amine (BDA) was injected in area PE in one monkey and in area PEa in a second monkey. Area PE sends a major projection terminating with small endings to the thalamic lateral posterior nucleus (LP), ventral posterior lateral nucleus (VPL), medial pulvinar (PuM) and, but fewer, to ventral lateral posterior nucleus, dorsal division (VLpd), central lateral nucleus (CL) and center median nucleus (CM), whereas giant endings formed restricted terminal fields in LP, VPL and PuM. For area PEa, the corticothalamic projection formed by small endings was found mainly in LP, VPL, anterior pulvinar (PuA), lateral pulvinar (PuL), PuM and, to a lesser extent, in ventral posterior inferior nucleus (VPI), CL, mediodorsal nucleus (MD) and CM. Giant endings originating from area PEa formed restricted terminal fields in LP, VPL, PuA, PuM, MD and PuL. Furthermore, the origin of the thalamocortical projections to areas PE and PEa was established, exhibiting clusters of neurons in the same thalamic nuclei as above, in other words predominantly in the caudal thalamus. Via the giant endings CT projection, areas PE and PEa may send feedforward, transthalamic projections to remote cortical areas in the parietal, temporal and frontal lobes contributing to polysensory and sensorimotor integration, relevant for visual guidance of reaching movements for instance.     

Topographic organization for delayed saccades in human posterior parietal cortex

Posterior parietal cortex (PPC) is thought to play a critical role in decision making, sensory attention, motor intention, and/or working memory. Research on the PPC in non-human primates has focused on the lateral intraparietal area (LIP) in the intraparietal sulcus (IPS). Neurons in LIP respond after the onset of visual targets, just before saccades to those targets, and during the delay period in between. To study the function of posterior parietal cortex in humans, it will be crucial to have a routine and reliable method for localizing specific parietal areas in individual subjects. Here, we show that human PPC contains at least two topographically organized regions, which are candidates for the human homologue of LIP. We mapped the topographic organization of human PPC for delayed (memory guided) saccades using fMRI. Subjects were instructed to fixate centrally while a peripheral target was briefly presented. After a further 3-s delay, subjects made a saccade to the remembered target location followed by a saccade back to fixation and a 1-s inter-trial interval. Targets appeared at successive locations "around the clock" (same eccentricity, approximately 30 degrees angular steps), to produce a traveling wave of activity in areas that are topographically organized. PPC exhibited topographic organization for delayed saccades. We defined two areas in each hemisphere that contained topographic maps of the contra-lateral visual field. These two areas were immediately rostral to V7 as defined by standard retinotopic mapping. The two areas were separated from each other and from V7 by reversals in visual field orientation. However, we leave open the possibility that these two areas will be further subdivided in future studies. Our results demonstrate that topographic maps tile the cortex continuously from V1 well into PPC.     

Non-spatial,motor-specific activation in posterior parietal cortex

A localized cluster of neurons in macaque posterior parietal cortex, termed the parietal reach region (PRR), is activated when a reach is planned to a visible or remembered target. To explore the role of PRR in sensorimotor transformations, we tested whether cells would be activated when a reach is planned to an as-yet unspecified goal. Over one-third of PRR cells increased their firing after an instruction to prepare a reach, but not after an instruction to prepare a saccade, when the target of the movement remained unknown. A partially overlapping population (two-thirds of cells) was activated when the monkey was informed of the target location but not the type of movement to be made. Thus a subset of PRR neurons separately code spatial and effector-specific information, consistent with a role in specifying potential motor responses to particular targets.     

Specialization of reach function in human posterior parietal cortex

Posterior parietal cortex (PPC) plays an important role in the planning and control of goal-directed action. Single-unit studies in monkeys have identified reach-specific areas in the PPC, but the degree of effector and computational specificity for reach in the corresponding human regions is still under debate. Here, we review converging evidence spanning functional neuroimaging, parietal patient and transcranial magnetic stimulation studies in humans that suggests a functional topography for reach within human PPC. We contrast reach to saccade and grasp regions to distinguish functional specificity and also to understand how these different goal-directed actions might be coordinated at the cortical level. First, we present the current evidence for reach specificity in distinct modules in PPC, namely superior parietal occipital cortex, midposterior intraparietal cortex and angular gyrus, compared to saccade and grasp. Second, we review the evidence for hemispheric lateralization (both for hand and visual hemifield) in these reach representations. Third, we review evidence for computational reach specificity in these regions and finally propose a functional framework for these human PPC reach modules that includes (1) a distinction between the encoding of reach goals in posterior–medial PPC as opposed to reach movement vectors in more anterior–lateral PPC regions, and (2) their integration within a broader cortical framework for reach, grasp and eye–hand coordination. These findings represent both a confirmation and extension of findings that were previously reported for the monkey.     

Thalamocortical connections of rat posterior parietal cortex.

The neuronal connections of rat posterior parietal cortex (PPC) have been examined using retrograde fluorescent axonal tracers. We have found that PPC receives thalamic input predominantly from the lateral posterior and lateral dorsal nuclei, and not from the ventrobasal nucleus, which projects to the rostrally adjacent hindlimb cortex, or from the dorsal lateral geniculate nucleus, which projects to the caudally adjacent visual association area. PPC has reciprocal corticocortical connections with medial agranular cortex and orbital cortex; together, these three cortical areas may function as a network for directed attention in rats.     

Neuronal activities in the ventral premotor cortex during a visually guided jaw movement in monkeys

Neuronal activities in the ventral part of the premotor cortex (PMv) and the primary motor cortex (MI) were analyzed during a visually guided jaw movement task. Based on the type of neuronal activity observed, when monkeys closed or opened their mouths in response to a visual stimulus, PMv neurons could be classified into three categories: (1) signal-related neurons, which transiently responded to visual stimuli, (2) movement-related neurons which were time-locked to jaw opening and/or jaw closing movements, and (3) set related neurons which exhibited gradually increasing activities while jaw position was maintained. However, all MI neurons exhibited movement-related activities and responded differently between the closing and opening dynamic phases. These results suggest that PMv neurons may be involved in motor preparation, initiation and control of jaw movements and task behavior based on visual information, and that MI neurons may be involved in controlling jaw movements, especially contraction of the masticatory muscles.     

Remapping the remembered target location for anti-saccades in human posterior parietal cortex

We used functional magnetic resonance imaging (fMRI) to investigate the role of the human posterior parietal cortex (PPC) in anti-saccades. To do so, we exploited the laterality of a subregion of the PPC for remembered target location. Using an event-related design, we tracked fMRI signal changes in this region while subjects remembered the location of a flashed target, then were instructed to plan either an anti- or pro-saccade to that location, and finally were instructed to execute the movement. At first, the region responded preferentially to the memory of a target location presented in the contralateral visual field. However, when an anti-cue specified a saccadic response into the opposite visual field, we observed a dynamic shift in cortical activity from one hemisphere to the other. This shows that this region within the human posterior parietal cortex codes the target location for an upcoming saccade, rather than the location of the remembered visual stimulus in an anti-saccade task.     

Effects of posterior parietal lesions on visually guided movements in monkeys

Summary Four monkeys were trained to position, with either hand, a vertical rod in front of one of 5 target lights spaced 20° apart on a semicircular screen. After the monkeys had reached the preoperative criterion (80% trials correct per session) they received a 1- or 2-stage bilateral lesion of posterior parietal cortex restricted to area 7. The lesion produced in all the monkeys considerable but temporary changes in movement latency, accuracy, velocity and duration. Latency increase appeared to be independent of changes in the other parameters. After the first lesion, movement latency increased for the contralateral arm in both left and right working spaces, from 100 ms up to 400 ms depending on the animal. A second lesion symmetrical to the first one increased movement latency of the arm contralateral or ipsilateral to the last lesion, depending on the time interval between the two lesions. In addition, unilateral lesions of area 7 induced a gross inaccuracy in movements of the arm contralateral to the lesion, more marked in the contralateral working space. These lesions also increased movement peak velocity and simultaneously decreased movement duration for the arm contralateral to the lesion. The increase in velocity appeared to be related to the decrease in duration. A second lesion of area 7 in the opposite hemisphere similarly affected accuracy, velocity and duration but for the arm contralateral to the second lesion.     

Prefrontal cortical modulation of acetylcholine release in posterior parietal cortex

Attentional processing is a crucial early stage in cognition and is subject to "top-down" regulation by prefrontal cortex (PFC). Top-down regulation involves modification of input processing in cortical and subcortical areas, including the posterior parietal cortex (PPC). Cortical cholinergic inputs, originating from the basal forebrain cholinergic system, have been demonstrated to mediate important aspects of attentional processing. The present study investigated the ability of cholinergic and glutamatergic transmission within PFC to regulate acetylcholine (ACh) release in PPC. The first set of experiments demonstrated increases in ACh efflux in PPC following AMPA administration into the PFC. These increases were antagonized by co-administration of the AMPA receptor antagonist DNQX into the PFC. The second set of experiments demonstrated that administration of carbachol, but not nicotine, into the PFC also increased ACh efflux in PPC. The effects of carbachol were attenuated by co-administration (into PFC) of a muscarinic antagonist (atropine) and partially attenuated by the nicotine antagonist mecamylamine and DNQX. Perfusion of carbachol, nicotine, or AMPA into the PPC did not affect PFC ACh efflux, suggesting that these cortical interactions are not bi-directional. These studies demonstrate the capacity of the PFC to regulate ACh release in the PPC via glutamatergic and cholinergic prefrontal mechanisms. Prefrontal regulation of ACh release elsewhere in the cortex is hypothesized to contribute to the cognitive optimization of input processing.     

Thalamo-cortical projections to the posterior parietal cortex in the monkey

Thalamo-cortical projections to the posterior parietal cortex (PPC) were investigated electrophysiologically in the monkey. Cortical field potentials evoked by the thalamic stimulation were recorded with electrodes chronically implanted on the cortical surface and at a 2.0-3.0 mm cortical depth in the PPC. The stimulation of the nucleus lateralis posterior (LP), nucleus ventralis posterior lateralis pars caudalis (VPLc), and nucleus pulvinaris lateralis (Pul.l) and medialis (Pul.m) induced surface-negative, depth-positive potentials in the PPC. The LP and VPLc projected mainly to the superior parietal lobule (SPL) and the anterior bank of the intraparietal sulcus (IPS), and the Pul.m mainly to the inferior parietal lobule (IPL) and the posterior bank of the IPS. The Pul.l had projections to all of the SPL, the IPL and both the banks. The significance of the projections is discussed in connection with motor functions.     

Opposite biases in salience-based selection for the left and right posterior parietal cortex

Visual selection is determined in part by the saliency of stimuli. We assessed the brain mechanisms determining attentional responses to saliency. Repetitive transcranial magnetic stimulation (rTMS) was applied to the left and right posterior parietal cortices (PPC) immediately before participants were asked to identify a compound letter. rTMS to the right PPC disrupted the guidance of attention toward salient stimuli, whereas rTMS to the left PPC affected the ability to bias selection away from salient stimuli. We conclude that right and left PPC have opposite roles in biasing selection to and from salient stimuli in the environment.     

Transcranial magnetic stimulation disrupts eye-hand interactions in the posterior parietal cortex

Recent neurophysiological studies have started to shed some light on the cortical areas that contribute to eye-hand coordination. In the present study we investigated the role of the posterior parietal cortex (PPC) in this process in normal, healthy subjects. This was accomplished by delivering single pulses of transcranial magnetic stimulation (TMS) over the PPC to transiently disrupt the putative contribution of this area to the processing of information related to eye-hand coordination. Subjects made open-loop pointing movements accompanied by saccades of the same required amplitude or by saccades that were substantially larger. Without TMS the hand movement amplitude was influenced by the amplitude of the corresponding saccade; hand movements accompanied by larger saccades were larger than those accompanied by smaller saccades. When TMS was applied over the left PPC just prior to the onset of the saccade, a marked reduction in the saccadic influence on manual motor output was observed. TMS delivered at earlier or later periods during the response had no effect. Taken together, these data suggest that the PPC integrates signals related to saccade amplitude with limb movement information just prior to the onset of the saccade.     

Memory related motor planning activity in posterior parietal cortex of macaque

Summary Unit recording studies in the lateral bank of the intraparietal cortex (area LIP) have demonstrated a response property not previously reported in posterior cortex. Studies were performed in the Rhesus monkey during tasks which required saccadic eye movements to remembered target locations in the dark. Neurons were found which remained active during the time period for which the monkey had to withhold eye movements while remembering desired target locations. The activity of the cells was tuned for eye movements of specific direction and amplitude, and it was not necessary for a visual stimulus to fall within the response field. The responses appeared to represent a memory-related motor-planning signal encoding motor error. The relation of the activity to the behavior of the animal suggests that the response represents the intent to make eye movements of specific direction and amplitude.     

Cerebello-thalamo-cortical projections to the posterior parietal cortex in the macaque monkey

This study reinvestigated the functional neuroanatomy of phonological and visual working memory in humans. Articulatory suppression was used to deprive the human subjects of species-specific verbal strategies in order to make the functional magnetic resonance imaging results more comparable to findings in non-human primates. Both phonological and visual working memory processes activated similar prefronto-parietal networks but were found to be differentially distributed along several cortical structures, in particular along the anterior and posterior parts of the intermediate frontal sulcus. These results suggest that a domain-specific topographical organization of neural working memory mechanisms in the primate brain is conserved in evolution. However, the findings also underline the critical dynamic influence that the additional availability of language may have on working memory processes and their functional implementation in the human brain.     

Auditory spatial discriminatory and mnemonic neurons in rat posterior parietal cortex

The present study was designed to investigate whether the rat posterior parietal cortex is involved in the perception and the representation of the auditory space. We recorded single neural activity in the posterior parietal cortex of rats that performed a directional delayed nonmatching-to-sample task. In the task, cue tones were presented in one of six speakers that were placed symmetrically around the rats. "Familiar tones" were those repeatedly used in the course of behavioral training. Novel tones were presented only during the unit recording time and less frequently used (e.g., only once in alternate weeks). The responses of the posterior parietal cortex neurons were typically tested with familiar cue tones while the rats were situated in a particular geomagnetic orientation. The same cells were further tested while the rats were reoriented by 180 degrees, or by novel cue tones. As the task included a delay period, in which the cue tone was removed, the rats had to maintain the directional information of the cue tones during this period to maximize the reward rates. A well-trained rat could perform the task with 85% success. We found two major types of neurons intermixed in the rat posterior parietal cortex. One type (n = 14) mainly discriminated the direction of the cue tones, whereas the other (n = 36) carried a mnemonic value of the cue tones while the tones were removed. The former responded only during the cue tone period (discriminatory neurons), whereas the latter responded during the cue tone period and the delay period (mnemonic neurons). These cells also exhibited broad directional tuning. The results agreed with previous studies, suggesting that a population coding scheme exists in the posterior parietal cortex. When the cells were tested with novel tones or when the rats were rotated through 180 degrees, the vast majority of the cells exhibited a directional tuning similar to those under the control conditions. Three quarters (18/24) of the cells that exhibited a mnemonic characteristic persisted in their directional preference when the rat's orientation was changed (12/17 neurons) or when an unfamiliar auditory stimulus was used (6/7 neurons). Half of the discriminatory neurons (4/8 neurons) persisted in their directional preference. These results, consistent with previous behavioral studies, suggest an allocentric representation of the auditory processing in this area. Furthermore, when the rat was reoriented or an unfamiliar cue tone was used, both the average and peak directional responses were enhanced in more than half of the mnemonic or discriminatory neurons. These results support the frequency-dependent neocortical gating hypothesis of the entorhinal hippocampal loop.     

Directional selectivity of BOLD activity in human posterior parietal cortex for memory-guided double-step saccades

We used functional magnetic resonance imaging (fMRI) to investigate the role of the human posterior parietal cortex (PPC) in storing target locations for delayed double-step saccades. To do so, we exploited the laterality of a subregion of PPC that preferentially responds to the memory of a target location presented in the contralateral visual field. Using an event-related design, we tracked fMRI signal changes in this region while subjects remembered the locations of two sequentially flashed targets, presented in either the same or different visual hemifields, and then saccaded to them in sequence. After presentation of the first target, the fMRI signal was always related to the side of the visual field in which it had been presented. When the second target was added, the cortical activity depended on the respective locations of both targets but was still significantly selective for the target of the first saccade. We conclude that this region within the human posterior parietal cortex not only acts as spatial storage center by retaining target locations for subsequent saccades but is also involved in selecting the target for the first intended saccade.