Coding Of Spatial Information Research Articles

Prediction of behavioral performance by alpha-band phase synchronization in working memory

Working memory (WM) is a cognitive system with limited capacity that can temporarily store and process information. The purpose of this study was to investigate functional connectivity based on phase synchronization during WM and its relationship with the behavioral response. In this regard, we recorded EEG/Eye tracking data of seventeen healthy subjects while performing a memory-guided saccade (MGS) task with two different positions (near eccentricity and far eccentricity). We computed saccade error as memory performance and measured functional connectivity using Phase Locking Value (PLV) in the alpha frequency band (8–12 Hz). The results showed that PLV is negatively correlated with saccade error. Our finding indicated that during the maintenance period, PLV between the frontal and visual area in trials with low saccade error increased significantly compared to trials with high saccade error. Furthermore, we observed a significant difference between PLV for near and far conditions in the delay period. The results suggest that PLV in memory maintenance, in addition to predicting saccade error as behavioral performance, can be related to the coding of spatial information in WM.

Hippocampal place cells exhibit impairments in spatial information coding in an APP‐knock in model of Alzheimer’s Disease

AbstractBackgroundThe hippocampus is one of the first cortical regions to exhibit Alzheimer’s Disease (AD) pathology. The spatially‐related firing of hippocampal place cells provides the cellular basis for spatial memory and this is impaired relatively early in AD, yet few studies examine place cell activity in AD mouse models. We undertook an in‐depth characterisation of hippocampal pyramidal cell activity in an APP knock‐in model of AD.MethodMicrodrives carrying 8 tetrodes were implanted into the left CA1 hippocampal subfield of 14‐month female mice (four APPNL‐G‐F, four wild‐type littermates). Mice undertook daily foraging sessions in an open field and ran on a linear track as CA1 activity was recorded. On completion of experiments, 40µm coronal sections of fixed brain tissue were stained using Cresyl Violet to verify tetrode placement and Thioflavin‐S to quantify amyloid β plaque burden.Result270 cells were recorded (119 wild‐type, 151 APPNL‐G‐F). CA1 pyramidal cells were classified as “place cells” or “non‐place cells” based on their spatial information content (bits/spike). Pyramidal cells recorded in wild‐type mice had significantly greater spatial information content than APPNL‐G‐F pyramidal cells (Figure 1), and a higher proportion were classified as place cells. Wild‐type place cells had significantly greater spatial information content than APPNL‐G‐F place cells. Corresponding differences were identified in place field size (Figures 2, 3). A positive correlation was identified between Aβ plaque burden and pyramidal cell spatial information content in the four APPNL‐G‐F mice (Figure 4).Coordination of place cell firing by the hippocampal theta rhythm was disrupted in APPNL‐G‐F mice; phase‐locking of pyramidal cells and place cells was significantly reduced (Figure 5), there was less coherence in the preferred theta phase across the pyramidal cell population, and a reduction in theta phase precession was observed.ConclusionAPPNL‐G‐F CA1 place cells exhibited deficits in both rate coding and temporal coding of spatial information indicating that amyloid β pathology, in the absence tau tangles, was associated with a disruption of hippocampal place cell function. These results provide preliminary support for the hypothesis that disruption of hippocampal function by AD pathology manifests as altered place cell activity and spatial behaviour.

Distortions in Spatial Mental Representation Affect Sketch Maps in Young Adults.

Humans tend to misrepresent spatial information which leads to systematic errors due to distorted organizational processes regarding metric and positional judgments. This study is aimed at analyzing metric and positional distortions in cognitive maps by using external representations, namely sketch maps, in two experiments with young participants. In the first experiment, we use the sketching area of Northern Europe. In the second experiment, the University campus area was used. The first aim was to test the hypothesis that the presence of the sea between the triplet of landmarks generates an overestimation of the distances between them in the case of Northern Europe; and to test the hypothesis that the number of turns in a route influences the overestimation of the distance between landmarks in the case of the campus area. The second aim was to investigate alignment and rotation errors using the same maps. Concerning metric errors, the results showed the overestimation of distances with a geographical gap between the cities (the sea in the Northern European Area), and those with more turns between landmarks (the campus area). The results concerning rotations and alignments were in line with the previous research about positional errors. The present study extended findings on distortions in spatial mental representation as emerging from verbal judgments, to sketch maps: direct visuospatial external representations eluding the conversion into verbal coding of spatial information. The presence of distortions in cognitive maps could be considered the consequence of the encoding of spatial information.

Rhythmic Activity in the Hippocampus and Entorhinal Cortex is Impaired in a Model of Kainate Neurotoxicity in Rats in Free Behavior

The hippocampus and medial entorhinal cortex (MEC) interact by means of bidirectional connections and play an important role in the processing, memorization, and reproduction of information. Data obtained in healthy animals show that the θ and γ oscillations are critical activities necessary for the interaction of the hippocampus and the MEC in signal processing. At the same time, these structures are among the most vulnerable parts of the brain to hyperactivation leading to excitotoxic damage and neuron death. In the present study toxicity was provoked by systemic administration of kainic acid (KA), inducing the development of status epilepticus. In control rats given physiological saline and rats given injections of KA, local field potentials were recorded simultaneously in hippocampal field CA1 and the MEC during exploratory behavior in an open field. A clearly apparent θ rhythm (4–10 Hz) was observed, along with a slow γ rhythm (25–50 Hz) and a fast γ rhythm (55–100 Hz) in the hippocampus and MEC of animals of both groups. Movement of control animals to the center of the open field was accompanied by an increase in the frequency of the θ rhythm and a decrease in the frequency of the fast γ rhythm in the hippocampus; the MEC showed a decrease in the power of the slow γ rhythm. This was not seen in rats given KA. This group also showed impairment to the phase-amplitude modulation of MEC activity by the hippocampal θ rhythm: changes in this modulation on movement of animals from the peripheral zones to the center of the open field were significantly less marked than in controls. There was also a significant increase in θ coherence between the hippocampus and MEC for all locations of the animal in the open field. Changes in the characteristics of rhythms in hippocampus-entorhinal interactions are potential biomarkers for impairments to the coding of spatial information and its retrieval from memory due to status epilepticus and often leading to the development of a convulsive focus in the temporal structures of the brain.

Neuroscience highlights: Main cell types underlying memory and spatial navigation.

Interest in the hippocampal formation and its role in navigation and memory arose in the second part of the 20th century, at least in part due to the curious case of Henry G. Molaison, who underwent brain surgery for intractable epilepsy. The temporal association observed between the removal of his entorhinal cortex along with a significant part of hippocampus and the developing severe memory deficit inspired scientists to focus on these regions. The subsequent discovery of the so-called place cells in the hippocampus launched the description of many other functional cell types and neuronal networks throughout the Papez-circuit that has a key role in memory processes and spatial information coding (speed, head direction, border, grid, object-vector etc). Each of these cell types has its own unique characteristics, and together they form the so-called "Brain GPS". The aim of this short survey is to highlight for practicing neurologists the types of cells and neuronal networks that represent the anatomical substrates and physiological correlates of pathological entities affecting the limbic system, especially in the temporal lobe. For that purpose, we survey early discoveries along with the most relevant neuroscience observations from the recent literature. By this brief survey, we highlight main cell types in the hippocampal formation, and describe their roles in spatial navigation and memory processes. In recent decades, an array of new and functionally unique neuron types has been recognized in the hippocampal formation, but likely more remain to be discovered. For a better understanding of the heterogeneous presentations of neurological disorders affecting this anatomical region, insights into the constantly evolving neuroscience behind may be helpful. The public health consequences of diseases that affect memory and spatial navigation are high, and grow as the population ages, prompting scientist to focus on further exploring this brain region.

Alpha-band oscillations reflect external spatial coding for tactile stimuli in sighted, but not in congenitally blind humans

We investigated the function of oscillatory alpha-band activity in the neural coding of spatial information during tactile processing. Sighted humans concurrently encode tactile location in skin-based and, after integration with posture, external spatial reference frames, whereas congenitally blind humans preferably use skin-based coding. Accordingly, lateralization of alpha-band activity in parietal regions during attentional orienting in expectance of tactile stimulation reflected external spatial coding in sighted, but skin-based coding in blind humans. Here, we asked whether alpha-band activity plays a similar role in spatial coding for tactile processing, that is, after the stimulus has been received. Sighted and congenitally blind participants were cued to attend to one hand in order to detect rare tactile deviant stimuli at this hand while ignoring tactile deviants at the other hand and tactile standard stimuli at both hands. The reference frames encoded by oscillatory activity during tactile processing were probed by adopting either an uncrossed or crossed hand posture. In sighted participants, attended relative to unattended standard stimuli suppressed the power in the alpha-band over ipsilateral centro-parietal and occipital cortex. Hand crossing attenuated this attentional modulation predominantly over ipsilateral posterior-parietal cortex. In contrast, although contralateral alpha-activity was enhanced for attended versus unattended stimuli in blind participants, no crossing effects were evident in the oscillatory activity of this group. These findings suggest that oscillatory alpha-band activity plays a pivotal role in the neural coding of external spatial information for touch.

Presynaptic NMDARs and astrocytes ally to control circuit-specific information flow.

The entorhinal cortex (EC) conveys spatial, limbic, and sensory information to the hippocampus, which performs critical brain functions, including learning and memory processes and spatial information coding. Axons from superficial [layer (L)2] EC neurons make excitatory synapses onto granule cells (GCs) of the hippocampal dentate gyrus (DG), which prepare the information for further processing in other hippocampal regions (1). Afferents from the lateral and medial perforant path (LPP and MPP, respectively) convey different aspects of information to the DG, with the former related more to sensory information, and the latter to spatial location and limbic signals related to attention and motivation. They also have distinct patterns of input, contacting the outer (LPP) or middle (MPP) third of the molecular layer of the DG, and exhibit different functional properties (1, 2). The mechanistic bases of these differences are unknown and represent a challenge to understand circuit-specific biological computations as well as susceptibility to pathological insults. In PNAS, Savtchouk et al. (3) identify presynaptic N -methyl-d-aspartate receptors (pre-NMDARs) as a source of the differences in information processing between MPP and LPP fibers. Previous work by the group (4) demonstrated that glutamate released by astrocytes enhances the strength of PP-GC synapses. The effect seemed to be mediated by pre-NMDARs, which increased the probability of transmitter release and contained the glutamate receptor subunit GluN2b. However, the work made no distinction between LPP and MPP contributions and left unsolved an apparent incongruence: GluN2b subunits confer high voltage-dependent Mg2+ block to NMDAR channels, but pre-NMDARs at PP-GC synapses could be activated without previous depolarization or in the absence of action potential firing in the axons. The current work (3) demonstrates that this is possible because of the additional presence in the receptor channel of the atypical GluN3a subunit, which largely relieves NMDAR … [↵][1]1To whom correspondence may be addressed. Email: arodmor{at}upo.es. [1]: #xref-corresp-1-1

Coherent Coding of Spatial Position Mediated by Theta Oscillations in the Hippocampus and Prefrontal Cortex.

Interactions between the hippocampus (area CA1) and prefrontal cortex (PFC) are crucial for memory-guided behavior. Theta oscillations (∼8 Hz) underlie a key physiological mechanism for mediating these coordinated interactions, and theta oscillatory coherence and phase-locked spiking in the two regions have been shown to be important for spatial memory. Hippocampal place-cell activity associated with theta oscillations encodes spatial position during behavior, and theta phase-associated spiking is known to further mediate a temporal code for space within CA1 place fields. Although prefrontal neurons are prominently phase-locked to hippocampal theta oscillations in spatial memory tasks, whether and how theta oscillations mediate processing of spatial information across these networks remains unclear. Here, we addressed these questions using simultaneous recordings of dorsal CA1-PFC ensembles and population decoding analyses in male rats performing a continuous spatial working memory task known to require hippocampal-prefrontal interactions. We found that in addition to CA1, population activity in PFC can also encode the animal's current spatial position on a theta-cycle timescale during memory-guided behavior. Coding of spatial position was coherent for CA1 and PFC ensembles, exhibiting correlated position representations within theta cycles. In addition, incorporating theta-phase information during decoding to account for theta-phase associated spiking resulted in a significant improvement in the accuracy of prefrontal spatial representations, similar to concurrent CA1 representations. These findings indicate a theta-oscillation-mediated mechanism of temporal coordination for shared processing and communication of spatial information across the two networks during spatial memory-guided behavior.SIGNIFICANCE STATEMENT Theta oscillation- (∼8 Hz) mediated interactions between the hippocampus and prefrontal cortex are known to be important for spatial memory. Hippocampal place-cell activity associated with theta oscillations underlies a rate and temporal code for spatial position, but it is not known whether these oscillations mediate simultaneous coding of spatial information in hippocampal-prefrontal networks. Here, we found that population activity in prefrontal cortex encodes animals' current position coherently with hippocampal populations on a theta-cycle timescale. Further we found that theta phase-associated spiking significantly improves prefrontal coding of spatial position, in parallel with hippocampal coding. Our findings establish that theta oscillations mediate a temporal coordination mechanism for coherent coding of spatial position in hippocampal-prefrontal networks during memory-guided behavior.

The impact of pathological high-frequency oscillations on hippocampal network activity in rats with chronic epilepsy.

In epilepsy, brain networks generate pathological high-frequency oscillations (pHFOs) during interictal periods. To understand how pHFOs differ from normal oscillations in overlapping frequency bands and potentially perturb hippocampal processing, we performed high-density single unit and local field potential recordings from hippocampi of behaving rats with and without chronic epilepsy. In epileptic animals, we observed two types of co-occurring fast oscillations, which by comparison to control animals we could classify as 'ripple-like' or 'pHFO'. We compared their spectral characteristics, brain state dependence, and cellular participants. Strikingly, pHFO occurred irrespective of brain state, were associated with interictal spikes, engaged distinct subnetworks of principal neurons compared to ripple-like events, increased the sparsity of network activity, and initiated both general and immediate disruptions in spatial information coding. Taken together, our findings suggest that events that result in pHFOs have an immediate impact on memory processes, corroborating the need for proper classification of pHFOs to facilitate therapeutic interventions that selectively target pathological activity.

Abnormal wiring of CCK+ basket cells disrupts spatial information coding.

The function of cortical GABAergic interneurons is largely determined by their integration into specific neural circuits, but the mechanisms controlling the wiring of these cells remain largely unknown. This is particularly true for a major population of basket cells that express the neuropeptide cholecystokinin (CCK). Here we found that the tyrosine kinase receptor ErbB4 is required for the normal integration of CCK+/VGlut3+ basket cells into cortical circuits. The number of inhibitory synapses made by CCK+/VGlut3+ basket cells and the inhibitory drive they exert on pyramidal cells are severely reduced in conditional mice lacking ErbB4. Developmental disruption of the connectivity of these cells diminishes the power of theta oscillations during exploratory behavior, disrupts spatial coding by place cells, and causes selective alterations in spatial learning and memory in adult mice. These results suggest that normal integration of CCK+ basket cells in cortical networks is key to support spatial coding information in hippocampus.

Efficient Coding of Spatial Information in the Primate Retina

Sensory neurons have been hypothesized to efficiently encode signals from the natural environment subject to resource constraints. The predictions of this efficient coding hypothesis regarding the spatial filtering properties of the visual system have been found consistent with human perception, but they have not been compared directly with neural responses. Here, we analyze the information that retinal ganglion cells transmit to the brain about the spatial information in natural images subject to three resource constraints: the number of retinal ganglion cells, their total response variances, and their total synaptic strengths. We derive a model that optimizes the transmitted information and compare it directly with measurements of complete functional connectivity between cone photoreceptors and the four major types of ganglion cells in the primate retina, obtained at single-cell resolution. We find that the ganglion cell population exhibited 80% efficiency in transmitting spatial information relative to the model. Both the retina and the model exhibited high redundancy (~30%) among ganglion cells of the same cell type. A novel and unique prediction of efficient coding, the relationships between projection patterns of individual cones to all ganglion cells, was consistent with the observed projection patterns in the retina. These results indicate a high level of efficiency with near-optimal redundancy in visual signaling by the retina.

Coding, stability, and non-spatial inputs in a modular grid-to-place cell model

Grid cells in the medial entorhinal cortex (mEC) and place cells in the hippocampus are paradigms for population coding of spatial information [1]. Both the spatiallyperiodic firing fields of grid cells and the spatially localized firing fields of place cells show systematic increases in spatial scale along the dorso-ventral axes of the mEC and hippocampus, respectively [2,3], which has led to speculation that place field size is determined simply by the spatial scale of a place cell’s grid cell inputs. However, in addition to receiving spatially-modulated entorhinal inputs, place cells receive contextual, non-spatial inputs from sources such as the amygdala and hypothalamus [4], which may be important in determining place cell firing properties. These non-spatial inputs are particularly prominent toward the ventral pole of the hippocampus [4], and thus could also play a role in producing dorso-ventral place cell differences. In order to understand the relative contributions of grid cells and non-spatial inputs in determining place field size and stability, we propose a computational model of the hippocampal-entorhinal network that includes a modular organization of grid cell inputs arranged in order of increasing spatial scale, as is seen experimentally in the mEC. Our underlying place cell model is a firing-rate based model inspired by previous work [5], in which place fields are formed via competition between place cells. We also introduce a dorsoventral gradient in the amount of non-spatial input to place cells, with ventral cells receiving more input from non-spatial sources. Finally, we introduce heterogeneity into the firing rates of grid vertices within individual grid fields. This heterogeneity is observed in experimental recordings [6] but has received relatively little attention in experimental or theoretical work, despite its potential impact on place field stability. Our main findings suggest that: 1.) For a wide range of parameters, the relative amounts of spatial and non-spatial inputs to place cells plays a more important role in determining place field size and stability than the spatial scale of grid cell inputs. This implies that the dorso-ventral gradient in place field size may reflect a dorso-ventral gradient in non-spatial inputs, rather than grid scale, and is agreement with prior suggestions of a functional distinction between the dorsal and ventral regions of the hippocampus [7]. 2.) In our model, place fields are sensitive to changes in the firing rates of the grid vertices of individual grid cells, emphasizing the potential implications of this grid field heterogeneity for place field formation and stability.

Stability and variability of place cell activity during behavior: Functional implications for dynamic coding of spatial information

In addition to their discharge strongly related to a rat’s location in the environment, hippocampal place cells have recently been discovered to carry other more subtle signals. For instance, place cells exhibit overdispersion, i.e., a tendency to have highly variable firing rates across successive passes in the firing field, which may reflect the processing of different classes of cues. In addition, the place cell population tends to fire synchronously during specific phases of place navigation, presumably signaling the animal’s arrival at the goal location, or to be reactivated during either sleep or wakefulness following exposure to a new environment, a process thought to be important for memory consolidation. Although these various phenomena are expressed at different timescales, it is very likely that they can occur at the same time during an animal’s exposure to a spatial environment. The advantage of such simultaneous processing is that it permits the organism both to be aware of its own location in the environment, and to attend to other environmental features and to store multiple experiences. However its pitfall is that it may result in noisy signals that are difficult to decipher by output structures. Therefore the question is asked of how the information carried by each process can be disentangled. We provide some examples from recent research work showing that this problem is far from being trivial and we propose an explanatory framework in which place cell activity at different timescales could be viewed as a series of dynamic attractors nested within each other.

L’hippocampe et le code neural de la mémoire spatiale

Recent work on the neural substrate of spatial memory strongly suggests the existence of a neuronal network dedicated to the coding of spatial information and allowing the subject to orient in space. This network includes place cells of the hippocampus, head direction cells which are found in several brain regions and particularly the post-subiculum, and grid cells in the entorhinal cortex. Several recently discovered features of place cell activity shed light on how the hippocampus contributes to memory construction. For instance, learning a novel environment relies on a dynamic population code in which place cell activity suddenly changes towards stable states built during previous experience. We briefly review these properties to show how they constrain the hippocampal memory code.

Covert auditory spatial orienting: An evaluation of the spatial relevance hypothesis.

The spatial relevance hypothesis (J. J. McDonald & L. M. Ward, 1999) proposes that covert auditory spatial orienting can only be beneficial to auditory processing when task stimuli are encoded spatially. We present a series of experiments that evaluate 2 key aspects of the hypothesis: (a) that "reflexive activation of location-sensitive neurons is not sufficient to produce attentional facilitation" and (b) that "any task constraint that makes space important for the listener will produce auditory spatial cue effects" (p. 1236). Experiment 1 showed significant reflexive-orienting benefits on a nonspatial task, refuting the first claim. However, Experiments 2 to 4 reveal that informative spatial cues can improve performance on a nonspatial task, consistent with the second claim. Auditory spatial-cue benefits found with nonspatial tasks appear smaller and less reliable than those found in visual spatial-orienting studies, possibly due to differences in the coding of spatial information in vision and audition. The final experiments consider the mechanisms by which auditory spatial orienting might facilitate auditory processing and provide tentative evidence that attention enhances processing at one ear rather than influencing neurons tuned to the attended location.

Coding of spatial information by soma and dendrite of pyramidal cells in the hippocampal CA1 of behaving rats

The soma and dendrite of a single neuron differ markedly in their anatomical and chemical organization. However, the difference between the neuronal codes by the soma and dendrite in the brain of behaving animals remains unknown. Here, we show that in the hippocampal CA1 of behaving rats, the soma and dendrite of pyramidal cells code distinct spatial information. To detect these neuronal codes, we used a unique extracellular multiunit recording technique with special electrodes (dodecatrodes) and a novel spike-sorting system with an independent component analysis (ICSort). First, we examined whether ICSort could separate extracellular signals from the soma and those from the dendrite of a single cell, in comparison with the separation obtained by a conventional spike-sorting technique. The results suggest that ICSort could distinguish extracellular signals originating from the soma and dendrite. Second, we examined spatial information coded by signals from the soma and dendrite of hippocampal pyramidal cells when the rats were moving in a familiar open environment. The results indicate that the somatic units had single place fields, and showed higher spatial specificity, lower sparsity and lower firing rates than the dendritic units. Therefore, we conclude that a hippocampal pyramidal cell has the ability to transform redundant spatial information received from upstream neurons via the dendrite into more place-specific information along the dendrosomatic axis and transmit this information to downstream neurons via the soma.

Neurodevelopmental Aspects of Spatial Navigation: A Virtual Reality fMRI Study

Navigation in spatial contexts has been studied in diverse species, yielding insights into underlying neural mechanisms and their phylogenetic progression. Spatial navigation in humans is marked by age-related changes that may carry important implications for understanding cortical development. The emergence of “allocentric” processing, reflecting that ability to use viewer-independent spatial abstractions, represents an important developmental change. We used fMRI to map brain regions engaged during memory-guided navigation in a virtual reality environment in adolescents and adults. Blood oxygen level-dependent (BOLD) signal was monitored in eight adolescents and eight adults in a 1.5-T MRI scanner during three conditions: (1) memory-guided navigation (NAV); (2) arrow-guided navigation (ARROW); and (3) fixation (FIX). We quantified navigation ability during scanning and allocentric memory after scanning, based on subjects' ability to label a previously unseen, aerial view of the town. Adolescents and adults exhibited similar memory-guided navigation ability, but adults exhibited superior allocentric memory ability. Memory-guided navigation ability during scanning correlated with BOLD change between NAV/ARROWS in various regions, including a right frontal and right-anterior medial temporal lobe region. Age group and allocentric memory together explained significant variance in BOLD change in temporoparietal association cortex and the cerebellum, particularly in the left hemisphere. Consistent with developmental models, these findings relate maturation in the coding of spatial information to functional changes in a distributed, left-lateralized neural network.

Holophony versus ambisonic: Deriving a hybrid method for 3-D sound reproduction in videoconferencing

Current research in videoconferencing intends to introduce 3-D reproduction methods both for image and sound. This paper will deal only with the problem of 3-D sound reproduction. First, one specific requirement must be borne in mind: in a videoconference, the listening area has to be wide, since it is addressed to several listeners. Two approaches have been considered: Holophony—acoustical equivalent to holography—and ambisonic. In a previous paper, it has been shown that these two methods are based on the same principle, i.e., physical reconstruction of the acoustical 3-D field, which appears as the most relevant solution to obtain an extensive listening area. However, holophony and ambisonic achieve opposite compromises between system complexity (number of signals recorded, number of microphones and loudspeakers) and reconstruction performance (accuracy, size of listening area, implementation opportunities). The two approaches are compared in order to point out a hybrid method, which optimizes the result. First studies focus on sound pick-up and show that ambisonic is more efficient concerning the coding of spatial information. In this paper, this analysis will be completed, first by examining the sound reproduction step, and second by studying the method robustness toward the actual behavior of transducers.

Double anterograde tracing of outputs from adjacent “barrel columns” of rat somatosensory cortex. Neostriatal projection patterns and terminal ultrastructure

The sensory input to the neostriatum from groups of cortical cells related to individual facial vibrissae has been investigated at both light- and electron-microscopic resolution. The purpose of the study was to establish the extent to which corticostriatal input maintains the anatomical coding of spatial information that is present in cortex. A double anterograde tracing method was used to identify the output projections from groups of adjacent neurons in different barrel columns, so that the anatomical relationships between two groups could be studied throughout their length. Adjacent whiskers are represented in adjoining cortical barrels and an examination of corticostriatal projections from these reveals two patterns of projection. In one, the anatomical topography is partially preserved; the barrels are represented in adjoining, discrete, areas of the somatosensory neostriatum. In the second projection pattern, the neostriatal innervation is diffuse and adjacent barrels are represented in overlapping regions of the neostriatum. Moreover, the fibres are thinner, have smaller boutons, and are present in both the ipsilateral and contralateral neostriatum. The two systems also enter the neostriatal neuropile separately. The discrete topographic system enters the adjacent neostriatum as collaterals which leave the descending corticofugal fibres at right angles, while the diffuse system enters directly from the corpus callosum at an acute angle. Examination of the neostriatal terminal fields by correlated light and electron microscopy, shows that characteristic axospinous terminals on spiny neurons are made by both groups of cortical fibres, although they differ in their size and morphology. It is concluded that at least two corticostriatal pathways arise from the barrel cortex. One connection maintains some of the anatomical code implicit in the barrel pattern of primary somatosensory cortex, but another, more diffuse, system is overlaid upon it which may carry different information from this complex area of cortex.

Homing in virtual environments: effects of field of view and path layout.

Triangle completion (ie homing to the starting point after completing two legs of a triangle) is a widely used method for examining path-integration abilities in animals and humans. Two experiments are reported in which homing was used to examine the efficiency of purely visual mechanisms (eg optical flow) for spatial-information coding and integration. Adult observers had to complete triangles in an interactively simulated three-dimensional environment which consisted of two critical objects and a homogeneous set of white cylinders serving as background. Each participant completed twenty-seven triangles corresponding to a factorial combination of three geometrical fields of view (40 degrees, 60 degrees, or 80 degrees) and nine triangle layouts (with variations of the first turning angle and the second leg). Homing performances revealed strong effects of triangle layout, but no effect of geometrical fields of view: variations in the amount of simultaneous visible spatial information did not influence the acquisition of spatial knowledge in the environments used. Applying the encoding-error model to the data revealed severe systematic errors of picking up directional information while moving through visually simulated environments. These results are discussed with respect to informational differences between situations of purely visual and nonvisual navigations in space.