Neural correlates of spatial judgement during object construction in parietal cortex

We recorded the activity of parietal area 7a neurons in monkeys performing an object construction task. In each trial, a model object consisting of a variable arrangement of squares was presented, followed after a delay by a copy of the model object that was missing a single square. Monkeys replaced the missing square to reconstruct the model configuration. Activity of many 7a neurons varied systematically with the position of the missing square and predicted where monkeys were going to add parts to the object they were building. The location of the missing square was a computed spatial datum important to object construction which did not correlate with the retinal location of a visual stimulus or the direction of the required motor response. The population of cells coding this coordinate was generally inactive when the same spatial locations were made relevant by visual targets to which monkeys either planned saccades or directed attention in other behavioral contexts. The data suggest that some parietal neurons participate in neural representations of space that reflect spatial cognitive as opposed to sensorimotor processing, coding the results of spatial computations performed on visual stimuli to meet cognitive objectives.     

The representation of spatial attention in human parietal cortex dynamically modulates with performance

The control and allocation of attention is an essential, ubiquitous neural process that gates our awareness of objects and events in the environment. Neural representations of the locus of spatial attention have been previously demonstrated in parietal cortex. However, the behavioral relevance of these neural representations is not known. While undergoing functional magnetic resonance imaging, subjects performed a covert spatial attention task that yielded a wide range of performance values. Voxels in parietal cortex selective for attended target location also dynamically modulated, becoming more or less responsive as performance levels changed. Surprisingly, this relationship was not linear. Responses peaked at intermediate performance levels and dropped both when performance was very high and when it was very low. Such dynamic modulation may represent a mechanism for organizing neural control signals according to behavioral task demands.     

Representing spatial relationships in posterior parietal cortex: single neurons code object-referenced position

The brain computes spatial relationships as necessary to achieve behavioral goals. Loss of this spatial cognitive ability after damage to posterior parietal cortex may contribute to constructional apraxia, a syndrome in which a patient's ability to reproduce spatial relationships between the parts of an object is disrupted. To explore neural correlates of object-relative spatial representation, we recorded neural activity in parietal area 7a of monkeys performing an object construction task. We found that neurons were activated as a function of the spatial relationship between a task-critical coordinate and a reference object. Individual neurons exhibited an object-relative spatial preference, such that different neural populations were activated when the spatial coordinate was located to the left or right of the reference object. In each case, the representation was robust to translation of the reference object, and neurons maintained their object-relative preference when the position of the object varied relative to the angle of gaze and viewer-centered frames of reference. This provides evidence that the activity of a subpopulation of parietal neurons active in the construction task represented relative position as referenced to an object and not absolute position with respect to the viewer.     

Emergence of connectivity in the embryonic rat parietal cortex.

In order to understand how cortical circuitry is put together, we examined the emergence of corticofugal projection cells and the arrival of subcortical afferents in the presumptive parietal cortex of the embryonic rat cerebrum. Afferent and efferent projections were selectively labeled by applications of the lipophilic tracers DiI and DiA in aldehyde-fixed brains of 12-18-d-old rat embryos (E12-E18; gestation: 21 d). On E12 and E13, the neocortical anlage consists of a ventricular zone and a preplate, with no extracortical connections. By E14, just prior to the appearance of the cortical plate, polymorphic cells located in the ventrolateral preplate of the telencephalic vesicle send out the first group of corticofugal axons toward the ganglionic eminence. Shortly thereafter, the cortical plate emerges as a dense band of radially oriented cells that also contribute to the corticofugal projection. By E15, axons of the early cortical projection cells cascade through the striatal primordium, the future site of the internal capsule. At the time of cortical plate formation and initial corticofugal axon outgrowth, ascending corticopetal axon systems have not yet arrived in the neocortex. Double-labeling experiments in which one dye is placed in the neocortex and the other in the ipsilateral dorsal thalamus reveal that cortical efferents encounter the first ascending wave of thalamofugal axons at the level of the striatum. Collectively, these two axonal systems bridge the necortex and the diencephalon. Upon their arrival in the neocortex on E16, thalamic axons follow a ventrolateral to dorsomedial course within the intermediate zone. Thalamic axons are the first subcortical afferent system to arrive in the neocortex. Other ascending afferent systems arising from the midbrain tegmentum enter the neocortex after E17. Comparison of thalamocortical and tegmentocortical projections in two halves of the same brain and across various embryonic ages clearly reveals that the two projection systems differ in their trajectories as well as in their time of arrival. Present observations challenge the view that the precocious arrival of subcortical axons provides the impetus for cortical maturation, and suggest that cortical plate differentiation and the initial organization of corticofugal projection patterns occur independent of ascending pathways.     

Attentional functions of parietal and frontal cortex

A model of normal attentional function, based on the concept of competitive parallel processing, is used to compare attentional deficits following parietal and frontal lobe lesions. Measurements are obtained for visual processing speed, capacity of visual short-term memory (VSTM), spatial bias (bias to left or right hemifield) and top-down control (selective attention based on task relevance). The results show important differences, but also surprising similarities, in parietal and frontal lobe patients. For processing speed and VSTM, deficits are selectively associated with parietal lesions, in particular lesions of the temporoparietal junction. We discuss explanations based on either grey matter or white matter lesions. In striking contrast, measures of attentional weighting (spatial bias and top-down control) are predicted by simple lesion volume. We suggest that attentional weights reflect competition between broadly distributed object representations. Parietal and frontal mechanisms work together, both in weighting by location and weighting by task context.     

Some temporal and parietal cortical connections converge in CA1 of the primate hippocampus.

Large sectors of polymodal cortex project to the hippocampal formation via convergent input to the entorhinal cortex. The present study reports an additional access route, whereby several cortical areas project directly to CA1. These are parietal areas 7a and 7b, area TF medial to the occipitotemporal sulcus (OTS), and a restricted area in the lateral bank of the OTS that may be part of ventromedial area TE. These particular cortical areas are implicated in visuospatial processes; and their projection to and convergence within CA1 may be significant for the elaboration of 'view fields', for the postulated role of the hippocampal formation in topographic learning and memory, or for the snapshot identification of objects in the setting of complex visuospatial relationships. Convergence of vestibular and visual inputs (from areas 7b and 7a respectively) would support previous physiological findings that hippocampal neurons respond to combinations of whole-body motion and a view of the environment. The direct corticohippocampal connections are widely divergent, especially those from the temporal areas, which extend over much of the anteroposterior axis of the hippocampal main body. Divergent connections potentially influence large populations of CA1 pyramidal neurons, consistent with the suggestion that these neurons are involved in conjunctive coding. The same region of ventromedial TE, besides the direct connections to CA1, also gives rise to direct projections to area V1, and may correspond to a functionally specialized subdivision, perhaps part of VTF.     

The effect of water maze spatial training on posterior parietal cortex transcallosal evoked field potentials in the rat

Long-term potentiation (LTP) is the principal model of synaptic plasticity often used to explain the changes that occur in the brain as a result of learning and memory. In this experiment the relationship between rat posterior parietal cortex (PPC) transcallosal evoked field potentials (TCEPs) and spatial training in the water maze was examined to determine if LTP-like changes (i.e. learning-induced LTP) in PPC TCEPs occur as a result of spatial training. Spatial training consisted of 10 trials per day for 10 consecutive days. The location of the hidden platform was changed over the course of spatial training to ensure the rats' acquisition of several different platform positions. TCEPs were taken 1 and 23 h after each training session. Upon completion of all water maze training, the animals were administered LTP-inducing trains to ensure that the recording arrangement and procedure was capable of detecting LTP. The results showed that the rats quickly acquired the water maze task and that the recording arrangement and procedure were capable of detecting LTP, even after the first session of induction. However, despite robust place learning, the TCEPs taken after water maze training did not differ from those taken before water maze training. Although the present results failed to provide any evidence for a role of neocortical LTP in learning and memory, further studies of this nature are required to determine if the present results generalize to different behavioural tasks and/or cortical areas.      

A pure salience response in posterior parietal cortex

When exploring a visual scene, some objects perceptually popout because of a difference of color, shape, or size. This bottom-up information is an important part of many models describing the allocation of visual attention. It has been hypothesized that the lateral intraparietal area (LIP) acts as a "priority map," integrating bottom-up and top-down information to guide the allocation of attention. Despite a large literature describing top-down influences in LIP, the presence of a pure salience response to a salient stimulus defined by its static features alone has not been reported. We compared LIP responses with colored salient stimuli and distractors in a passive fixation task. Many LIP neurons responded preferentially to 1 of the 2 colored stimuli, yet the mean responses to the salient stimuli were significantly higher than to distractors, independent of the features of the stimuli. These enhanced responses were significant within 75 ms, and the mean responses to salient and distractor stimuli were tightly correlated, suggesting a simple gain control. We propose that a pure salience signal rapidly appears in LIP by collating salience signals from earlier visual areas. This contributes to the creation of a priority map, which is used to guide attention and saccades.     

The role of the dorsolateral prefrontal cortex during sequence learning is specific for spatial information.

Many studies have implicated the dorsolateral prefrontal cortex in the acquisition of skill, including procedural sequence learning. However, the specific role it performs in sequence learning has remained uncertain. This type of skill has been intensively studied using the serial reaction time task. We used three versions of this task: a standard task where the position of the stimulus cued the response; a non-standard task where the color of the stimulus was related to the correct response; and a combined task where both the color and position simultaneously cued the response. We refer to each of these tasks based upon the cues available for guiding learning as position, color and combined tasks. The combined task usually shows an enhancement of skill acquisition, a result of being driven by two simultaneous and congruent cues. Prior to the performance of each of these tasks the function of the dorsolateral prefrontal cortex was disrupted using repetitive transcranial magnetic stimulation. This completely prevented learning within the position task, while sequence learning occurred to a similar extent in both the color and combined tasks. So, following prefrontal stimulation the expected learning enhancement in the combined task was lost, consistent with only a color cue being available to guide sequence learning in the combined task. Neither of these effects was observed following stimulation at the parietal cortex. Hence the critical role played by the dorsolateral prefrontal cortex in sequence learning is related exclusively to spatial cues. We suggest that the dorsolateral prefrontal cortex operates over the short term to retain and manipulate spatial information to allow cortical and subcortical structures to learn a predictable sequence of actions. Such functions may emerge from the broader role the dorsolateral prefrontal cortex has in spatial working memory. These results argue against the dorsolateral prefrontal cortex constituting part of the neuronal substrate responsible for general aspects of implicit or explicit sequence learning.     

Differential contribution of right and left parietal cortex to the control of spatial attention: a simultaneous EEG-rTMS study

We have recently shown that interference with repetitive transcranial magnetic stimulation (rTMS) of right posterior intraparietal sulcus (IPS) cortex during the allocation of spatial attention leads to abnormal desynchronization of anticipatory (pretarget) electroencephalographic alpha rhythms (8-12 Hz) in occipital-parietal cortex and the detection of subsequently presented visual targets (Capotosto et al. 2009). Since lesion data suggest that lesions of the right frontoparietal cortices produce more severe and long-lasting deficits of visual spatial attention than lesions of the left hemisphere, here, we used the mentioned rTMS-electroencephalographic procedure to test if the control of anticipatory alpha rhythms by IPS is asymmetrically organized in the 2 hemispheres. Results showed that interference with either left or right IPS during covert spatial attention equally disrupted the normally lateralized anticipatory modulation of occipital visual cortex, with stronger alpha desynchronization contralaterally to the attended visual field. In contrast, only interference with right IPS induced a paradoxical pretarget synchronization of alpha rhythms and bilateral deficits of target identification. These results suggest that the control of spatial topography of anticipatory alpha rhythms in occipital-parietal cortex is shared between left and right IPS cortex, but that right IPS uniquely contributes to a bilateral prestimulus activation of occipital visual cortex.     

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.     

Role of prefrontal and parietal cortices in associative learning

Two studies were performed that compared a "Paired" condition in which participants studied paired associates with a "Generated" condition in which participants completed word fragments to produce paired associates. In both tasks, participants were responsible for memory of the material either studied or generated. The experiments revealed significant differences between the responses of a predefined prefrontal region and a predefined parietal region. The parietal region responded more in the Generated condition than the Paired condition, whereas there was no difference in the prefrontal region. On the other hand, the prefrontal region responded to the delay between study and test in both the Paired and Generated conditions, whereas the parietal region only responded to delay in the Generated condition. This pattern of results is consistent with the hypothesis that the parietal region is responsive to changes in problem representation and the prefrontal region to retrieval operations. An information-processing model embodying these assumptions was fit to the blood oxygen level-dependent responses in these regions.     

Morphogenesis of callosal arbors in the parietal cortex of hamsters

The morphogenesis of callosal axons originating in the parietal cortex was studied by anterograde labeling with Phaseolus lectin or biocytin injected in postnatal (P) hamsters aged 7-25 days. Some labeled fibers were serially reconstructed. At P7, some callosal fibers extended as far as the contralateral rhinal fissure, with simple arbors located in the homotopic region of the opposite cortical gray matter, and two or three unbranched sprouts along their trajectory. From P7 to P13, the homotopic arbors became more complex, with branches focused predominantly, but not exclusively, in the supra- and infragranular layers of the homotopic region. Simultaneously, the lateral extension of the trunk axon in the white matter became shorter, finally disappearing by P25. Arbors in the gray matter were either bilaminar (layers 2/3 and 5) or supragranular. A heterotopic projection to the lateral cortex was consistently seen at all ages; the heterotopic arbors follow a similar sequence of events to that seen in homotopic regions. These observations document that callosal axons undergo regressive tangential remodeling during the first postnatal month, as the lateral extension of the trunk fiber gets eliminated. Radially, however, significant arborization occurs in layer-specific locations. The protracted period of morphogenesis suggests a correspondingly long plastic period for this system of cortical fibers.     

Structure--function spatial covariance in the human visual cortex.

The value of sulcal landmarks for predicting functional areas was quantitatively examined. Medial occipital sulci were identified using anatomical magnetic resonance images to create individual cortical-surface models. Functional visual areas were identified using retinotopically organized visual stimuli, and positron emission tomography subtraction imaging with intra-subject averaging. Functional areas were assigned labels by placement along the cortical surface from V1. Structure-function spatial covariances between sulci and functional areas, and spatial covariances among functional areas, were determined by projecting sulcal landmarks and functional areas into a standardized stereotaxic space and computing the 'r' statistics. A functional area was considered to spatially covary with a sulcus or another functional area if their geometric centers correlated significantly (P < 0.05) in two or more axes. Statistically significant spatial covariances were found for some, but not all comparisons. The finding of significant spatial covariances within a standardized stereotaxic space indicates that nine-parameter spatial normalization does not account for all the predictive value of structural or functional locations, and may be improved upon by using selected sulcal and functional landmarks. The present findings quantify for the first time the strength of structure--function spatial covariance and comment directly on developmental theories addressing the etiology of structure--function correspondence.     

Local computation of decision-relevant net sensory evidence in parietal cortex

To investigate the contribution of parietal cortex to perceptual decisions, we trained monkeys on a perceptual decision task that allowed simultaneous experimental control over how much sensory evidence was provided for each of 3 possible alternative choices and recorded single unit activity and local field potentials (LFPs) from the lateral intraparietal area (LIP). While both the behavior and the spiking activity were largely determined by the difference between how much supporting sensory evidence was provided for a particular choice (pro evidence) and how much sensory evidence was provided for the other alternatives (anti evidence), the LFP reflected roughly the sum of these 2 components. Furthermore, the firing rates showed an earlier influence of the anti evidence than the pro evidence. These observations indicate that LIP does not simply receive already precomputed decision signals but that it plays an active role in computing the decision-relevant net sensory evidence and that this local computation is reflected in the LFP. The results further demonstrate that the competition between the different alternatives cannot solely be mediated by lateral or feedback inhibition, as proposed by a major class of decision models but that feedforward inhibition makes an important contribution.     

Parcellation in left lateral parietal cortex is similar in adults and children

A key question in developmental neuroscience involves understanding how and when the cerebral cortex is partitioned into distinct functional areas. The present study used functional connectivity MRI mapping and graph theory to identify putative cortical areas and generate a parcellation scheme of left lateral parietal cortex (LLPC) in 7 to 10-year-old children and adults. Results indicated that a majority of putative LLPC areas could be matched across groups (mean distance between matched areas across age: 3.15 mm). Furthermore, the boundaries of children's putative LLPC areas respected the boundaries generated from the adults' parcellation scheme for a majority of children's areas (13/15). Consistent with prior research, matched LLPC areas showed age-related differences in functional connectivity strength with other brain regions. These results suggest that LLPC cortical parcellation and functional connectivity mature along different developmental trajectories, with adult-like boundaries between LLPC areas established in school-age children prior to adult-like functional connectivity.     

Right parietal cortex plays a critical role in change blindness

There is increasing evidence from functional magnetic resonance imaging (fMRI) that visual awareness is not only associated with activity in ventral visual cortex but also with activity in the parietal cortex. However, due to the correlational nature of neuroimaging, it remains unclear whether this parietal activity plays a causal role in awareness. In the experiment presented here we disrupted activity in right or left parietal cortex by applying repetitive transcranial magnetic stimulation (rTMS) over these areas while subjects attempted to detect changes between two images separated by a brief interval (i.e. 1-shot change detection task). We found that rTMS applied over right parietal cortex but not left parietal cortex resulted in longer latencies to detect changes and a greater rate of change blindness compared with no TMS. These results suggest that the right parietal cortex plays a critical role in conscious change detection.     

Connection patterns distinguish 3 regions of human parietal cortex

Three regions of the macaque inferior parietal lobule and adjacent lateral intraparietal sulcus (IPS) are distinguished by the relative strengths of their connections with the superior colliculus, parahippocampal gyrus, and ventral premotor cortex. It was hypothesized that connectivity information could therefore be used to identify similar areas in the human parietal cortex using diffusion-weighted imaging and probabilistic tractography. Unusually, the subcortical routes of the 3 projections have been reported in the macaque, so it was possible to compare not only the terminations of connections but also their course. The medial IPS had the highest probability of connection with the superior colliculus. The projection pathway resembled that connecting parietal cortex and superior colliculus in the macaque. The posterior angular gyrus and the adjacent superior occipital gyrus had a high probability of connection with the parahippocampal gyrus. The projection pathway resembled the macaque inferior longitudinal fascicle, which connects these areas. The ventral premotor cortex had a high probability of connection with the supramarginal gyrus and anterior IPS. The connection was mediated by the third branch of the superior longitudinal fascicle, which interconnects similar regions in the macaque. Human parietal areas have anatomical connections resembling those of functionally related macaque parietal areas.     

Observer-independent cytoarchitectonic mapping of the human superior parietal cortex

The human superior parietal cortex (SPC; Brodmann areas [BA] 5 and 7) comprises the superior parietal lobule and medial wall of the intraparietal sulcus (mIPS) laterally and the posterior paracentral lobule and precuneus medially. Receptor autoradiographic and functional studies indicate more complex segregations in the SPC than suggested by Brodmann (1909). Differences to other historical maps may be due to anatomical variability between brains and different definition criteria for areas. To provide a reliable anatomical reference of the SPC, we performed an observer-independent cytoarchitectonic mapping of this region in 10 human postmortem brains. Cytoarchitecture was analyzed in cell-body-stained brain sections using gray-level index profiles. Multivariate statistical analysis of profile shape allowed the exact localization of cytoarchitectonic borders and quantification of interareal differences. We identified 3 areas in BA 5 (5L, 5M, and 5Ci), 4 in BA 7 (7PC, 7A, 7P, and 7M), and 1 in the anterior mIPS (hIP3). Locations of their borders relative to macroanatomical landmarks varied considerably between brains and hemispheres. Cytoarchitectonic profiles of areas 5Ci and hIP3 differed most from those of the remaining areas, and differences between subareas were stronger in BA 5 than in BA 7. These areas are possible structural correlates of functional segregations within the SPC.     

Cortical connections of functional zones in posterior parietal cortex and frontal cortex motor regions in new world monkeys

We examined the connections of posterior parietal cortex (PPC) with motor/premotor cortex (M1/PM) and other cortical areas. Electrical stimulation (500 ms trains) delivered to microelectrode sites evoked movements of reach, defense, and grasp, from distinct zones in M1/PM and PPC, in squirrel and owl monkeys. Tracer injections into M1/PM reach, defense, and grasp zones showed dense connections with M1/PM hand/forelimb representations. The densest inputs outside of frontal cortex were from PPC zones. M1 zones were additionally connected with somatosensory hand/forelimb representations in areas 3a, 3b, and 1 and the somatosensory areas of the upper bank of the lateral sulcus (S2/PV). Injections into PPC zones showed primarily local connections and the densest inputs outside of PPC originated from M1/PM zones. The PPC reach zone also received dense inputs from cortex caudal to PPC, which likely relayed visual information. In contrast, the PPC grasp zone was densely connected with the hand/forelimb representations of areas 3a, 3b, 1, and S2/PV. Thus, the dorsal parietal-frontal network involved in reaching was preferentially connected to visual cortex, whereas the more ventral network involved in grasping received somatosensory inputs. Additional weak interlinks between dissimilar zones (e.g., PPC reach and PPC grasp) were apparent and may coordinate actions.