SEARCH · Search PubMed

Results for “spatial”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 343 records · Page 19Linked to original sources

PubMed1995

Spatial and non-spatial learning in the rat following lesions to the nucleus locus coeruleus.

The present experiment examined the effects of unilateral or bilateral locus coeruleus lesions on general activity, acquisition of a non-monotonic serial pattern (14-0-3-7 food pellets, respectively), and response learning acquisition in a Greek cross version of the Morris water maze. Sham-operated control rats were capable of tracking the elements of the serial pattern while rats with a unilateral locus coeruleus lesion were moderately impaired, and those with bilateral locus coeruleus lesions were severely impaired. A similar pattern of working memory deficits emerged in an analysis of the response-learning data in the Greek cross. The results are discussed in terms of the current understanding of norepinephrine and the locus coeruleus in learning and memory processes.

Analysis of Variance↗

PubMed1996

Spatial properties of envelope-responsive cells in area 17 and 18 neurons of the cat.

1. Many neurons in areas 17 and 18 respond to spatial contrast envelope stimuli whose Fourier components fall outside the cell's spatial-frequency-selective range. The spatial properties of such envelope responses are investigated here and compared with responses to conventional luminance-defined gratings to explore the underlying receptive-field mechanism. 2. Three spatial properties of envelope responses are reported more extensively in this paper. First, the envelope responses were selective to the carrier spatial frequency in a narrow range of frequencies higher than a given cell's luminance spatial frequency selective range (luminance passband). Second, a given cell's dependence on envelope spatial frequency often differed from its luminance passband. Last, the optimal carrier spatial frequency did not shift systematically with the envelope spatial frequency, supporting the hypothesis that the carrier and envelope spatial-frequency dependencies were mediated by distinct mechanisms. 3. In contrast to the direction selectivity to the envelope motion in many envelope-responsive cells, no direction preference to carrier motion was found for envelope responses. The direction of carrier motion did not alter the direction selectivity for envelope motion, further supporting the hypothesis that the carrier and envelope temporal properties were mediated by separate mechanisms. 4. The distributions of the optimal carrier and luminance spatial frequencies among envelope-responsive cells were analyzed. The optimal carrier spatial frequencies were randomly distributed from five times the cell's optimal luminance spatial frequency to the upper resolution limit of the X-retinal ganglion cells at the same retinal eccentricity, suggesting that the selective ranges of envelope responses and luminance responses are not strongly correlated over the population of envelope-responsive cells. 5. Our data support a "two-stream" receptive-field model for envelope-responsive cells. One stream is a conventional, spatially linear receptive-field mechanism, mediating luminance responses for the cell; the other mediates envelope responses and consists of a two-stage processing: a set of spatially small and distributed nonlinear neural subunits whose outputs are spatially pooled at the second stage. 6. In conclusion, this study indicates that envelope responses in area 17 and 18 neurons cannot be due to a nonlinearity that is common to all visual stimuli before narrowband spatial-frequency-selective filtering; instead, a specialized processing stream, parallel to the conventional luminance response stream, is needed to supplement the traditional luminance processing stream in these cells. This specialized stream responds to the envelope stimuli and is selective to their carrier and envelope spatial frequencies. The distributions of the optimal luminance and carrier spatial frequencies indicate a rich variety of possible integration between luminance and envelope information.

Animals↗

PubMed1994

Cat striate cortex: monocular and interocular comparisons of spatial-frequency selectivity.

Spatial frequency and bandwidth characteristics were determined for neurones in cat striate cortex. Responses to drifting sine-wave gratings, optimized for orientation, direction and velocity, were determined over a range of spatial frequencies. Comparative measurements of spatial frequency tuning at constant velocity and at constant temporal drift frequency revealed that, overall, tuning derived by either method was similar. Results were evaluated in relation to neuronal class (simple or complex); complex cell subclass (standard, intermediate or special), defined by length summation; directionality; and velocity selectivity. Distributions of optimal spatial frequency for simple and complex neurones were comparable. By contrast, bandwidths of simple neurones were markedly narrower than for complex neurones. Standard complex neurones, in turn, had narrower bandwidths than special or intermediate complex neurones. Optimal spatial frequency correlated inversely with optimal velocity, directly with orientation selectivity. Thus, neurones tuned to high spatial frequencies tended to respond optimally to low velocities, and were more sharply orientation selective, than neurones tuned to low spatial frequencies. In binocular neurones, spatial frequency tuning characteristics of the two monocular inputs were compared. For either eye, spatial frequency tuning curves were reproducible over time. In a minority of neurones, spatial frequency characteristics were matched for the two eyes. A majority showed mismatch in spatial frequency characteristics between the eyes. Individual neurones were tuned to different bands of spatial frequencies through either eye; more sharply spatial-frequency selective through one eye than the other; or had both dissimilar bandwidth and spatial frequency. Changing input spatial-frequency resulted in profound, systematic shifts in ocular dominance. These were progressive in the case of spatial-frequency mismatch. In cases of bandwidth, or bandwidth and spatial-frequency mismatch, the eye associated with more sharply-tuned input exerted relatively greater influence at centre frequencies, the other eye relatively greater influence at extreme frequencies. There was a marginal tendency for the dominant (or contralateral) eye to be tuned to higher spatial frequencies than the more weakly driving (or ipsilateral) eye. By contrast, interocular differences in bandwidth were pronounced: in a majority of neurones the dominant eye was more broadly tuned than the more weakly driving eye. Related to the established preponderance of contralaterally dominated cortical neurones, the input from the contralateral eye was markedly more broadly tuned than that from the ipsilateral eye, consistent with the notion that stronger drive is associated with greater pooling of inputs. These differences have important implications for binocular vision and, potentially, for coding of visual perspective.

Animals↗

PubMed1982

Spatial frequency discrimination in normal vision and in patients with multiple sclerosis.

This article extends our previous reports that multiple sclerosis can cause a visual dysfunction better described as a distortion than as a blurring of vision. An earlier paper reported that multiple sclerosis spares visual acuity in some patients while reducing visual sensitivity for less fine detail. Specifically, these patients experience a loss of contrast sensitivity for low and/or intermediate spatial frequencies, while contrast sensitivity for high spatial frequencies is unimpaired. We report here that some patients also lose spatial frequency discrimination, so that these patients cannot tell which of two clearly visible gratings has the higher spatial frequency even though control subjects accurately report which grating has the higher spatial frequency. One way of regarding this discrimination loss is in terms of a deterioration of the ability to discriminate size. Contrast sensitivity was measured over the spatial frequency range 1 to 20 cycles/deg using the von Békésy tracking method for 10 patients. (20 eyes) and 16 control subjects (32 eyes). The limit of normality was taken as 2.5 standard deviations from the control mean (99 per cent confidence). Spatial frequency discrimination was measured using the criterion-free method of temporal two-alternative forced choice over the spatial frequency range 2 to 16 cycles/deg for 10 patients (20 eyes), and for 14 to 26 control eyes at each spatial frequency. Three control subjects were studied more extensively over the range 1 to 20 cycles/deg. Control subjects could discriminate two spatial frequencies that differed by more than about 5 per cent. This held for all spatial frequencies tested. Grating contrast had little effect on discrimination, provided that all test gratings were clearly visible. The normal limit for discrimination threshold was set at 2.5 standard deviations from the control mean. Seven of 10 patients have abnormal contrast sensitivity at one or more spatial frequencies. Six of 10 patients had abnormal discrimination at one or more spatial frequencies. At any given spatial frequency the correlation between the magnitudes of sensitivity loss and discrimination loss was weak, though an eye that was less sensitive than its fellow also tended to have poorer discrimination. A more subtle relationship between sensitivity loss and discrimination loss was clearly shown by one patient. Sensitivity loss was restricted to spatial frequencies below 8 cycles/deg, while discrimination loss in the same eye was restricted to spatial frequencies above 8 cycles/deg. We propose that this finding can be straightforwardly understood if discrimination is determined by the relative activities of different spatial frequency channels analogously, to the way opponent-colour mechanisms determine colour discrimination.

Adolescent↗

PubMed1999

Multimodal spatial orientation deficits in left-sided visual neglect.

Patients with right-sided temporo-parietal lesions often show contralesional neglect. However, neglect patients may also show spatial-perceptual deficits beyond the bisection and space exploration deficits frequently assessed in the horizontal plane, that is, deficits in the judgment of the subjective visual vertical or horizontal. In a recent study (Kerkhoff, G. & Zoelch, C.. Disorders of visuo-spatial orientation in the frontal plane in patients with visual neglect following right or left parietal lesions. Exp. Brain Res., 1998;122:108-120) we found significant perturbations in the perception of these three visual spatial axes in patients with contralesional neglect from right or left parietal lesions. To examine if this finding extends also to another modality we investigated how neglect patients perform tasks of visual- and tactile-spatial judgments of axis-orientation in the frontal plane. Visual-spatial and tactile-spatial judgments of the subjective vertical, horizontal and a right oblique orientation were obtained from patients with and without neglect as well as from normal subjects. Patients with left neglect showed a significant, contraversive tilt of all three visual-spatial axes (+5.6 degrees to +9.5 degrees, counterclockwise), and of the three tactile-spatial axes as well (+5.2 degrees to +10.5 degrees, counterclockwise). In contrast, right and left hemisphere lesioned control patients without neglect and normal control subjects showed unimpaired visual and tactile-spatial judgments (constant errors: < 1.0 degree). Difference thresholds in the visual-spatial tasks and unsigned errors in the tactile-spatial tasks were selectively elevated in the neglect group in contrast to all other subject groups. Spatial orientation deficits were significantly associated with the severity of clinical neglect (r = 0.55-0.88), and with the patients' ambulation performance (r = 0.45-0.70). Furthermore, crossmodal axis orientation tests in two neglect patients showed a similar counterclockwise tilt of +5 degrees to +15 degrees, suggesting a similar spatial deficit in both modalities. Orientation judgments were significantly aggravated by a 25 degree-tilt of the head to the left, as tested in one neglect patient, while a comparable rightward head-tilt improved spatial judgments in both modalities. This suggests that spatial orientation judgments are significantly modulated by gravitational input in neglect patients. Together these results are interpreted as evidence for multisensory spatial orientation deficits in neglect patients which are modulated by head-position and are related to their accompanying postural impairment.

Adult↗

PubMed1985

Spatial learning disabilities and underachievement among university anatomy students.

In this study the relationship between underachievement in anatomy and spatial ability (both geometrical and anatomical) is investigated. Subjects were second-year medical students at the University of Cape Town from 1980 to 1983. Geometrical spatial ability was measured using a battery of three-dimensional exercises involving the sectioning, joining, translation, rotation and visualization of simple solid objects. Anatomical spatial achievement was measured using university practical examination scores of April, June and October, as well as students' scores on those items in the April, June and November MCQ anatomy examinations which were classified as spatially three-dimensional by a panel of lecturers in anatomy. Non-spatial anatomical achievement was measured using the university essay examination scores of April, June and November, together with students' scores on the non-spatial items in the MCQ anatomy examinations. From 1980 to 1983 it was found that students who failed the battery of geometrical spatial exercises and/or recorded large, persistent deficits on spatial MCQs relative to non-spatial MCQs, scored significantly lower marks in practical anatomy examinations throughout the year than those students who scored well in the battery of geometrical spatial exercises. Spatially competent and spatially inept students performed equally well on the non-spatial MCQs and the non-spatial essay examinations. Borderline and failing students recorded the greatest deficits in anatomical spatial scores (for whom losses of approximately 18% occurred in practical examinations in anatomy). Since potential failures with persistent spatial handicaps can be readily identified by mid-year, a programme of differentiated teaching methods is recommended for these students.

Achievement↗

PubMed1999

A model of human pattern perception: association fields for adaptive spatial filters.

Visual neurons in the primary visual cortex 'look' at the retinal image through a four-dimensional array of spatial receptive fields (filter-elements): two spatial dimensions and, at each spatial location, two Fourier dimensions of spatial frequency and orientation. In general, visual objects activate filter-elements along each of these dimensions, suggesting a need for some kind of linking mechanism that determines whether two or more filter-elements are responding to the same or different contours or objects. In the spatial domain, a (spatial) association field between filter-elements, arranged to form first-order curves, has been inferred as a flexible method by which different parts of extended (luminance) contours become associated (Field et al., 1993). Linking has also been explored between filters selective for different regions in Fourier space (e.g. Georgeson and Meese, 1997). Perceived structure of stationary plaids suggests that spatial filtering is adaptive: synthetic filters can be created by the linear summation of basis-filters across orientation or spatial frequency in a stimulus-dependent way. For example, a plaid with a pair of sine-wave components at +/-45 deg looks like a blurred checkerboard; a structure that can be understood if features are derived after linear summation of spatial filters at different orientations. However, the addition of an oblique third-harmonic component causes the plaid to perceptually segment into overlapping oblique contours. This result can be understood if filters are summed across spatial frequency, but, in this case, treated independently across orientation. In the present paper, the architecture of an association field is proposed to permit linking and segmentation of filter-elements across spatial frequency and orientation. Three types of link are proposed: (1) A chain of constructive links around sites of common spatial frequency but different orientation, to promote binding of filters across orientation; (2) Constructive links between sites with common orientation but different spatial frequency, to promote binding of filters across spatial frequency; (3) Long-range links between sites of common spatial frequency but different orientation, whose activation and role are determined by activity in a higher spatial frequency band. A model employing the proposed network of links is consistent with at least six previously reported effects on the perception of briefly presented stationary plaids.

Computer Simulation↗

PubMed2002

Cortical control of spatial memory in humans: the visuooculomotor model.

We review current knowledge of the cortical control of spatial memory, studied using visuooculomotor paradigms. Spatial memory is an essential cognitive process that can be involved in preparing motor responses. Our knowledge of spatial memory in humans recently has progressed thanks to the use of ocular saccades as a convenient model of motor behavior. Accuracy of memory-guided saccades, made to the remembered locations of visual targets, is a reflection of spatial memory. For the performance of memory-guided saccades with brief delays (up to 15-20 seconds), that is, involving short-term spatial memory, lesion studies have shown that the posterior parietal cortex, the dorsolateral prefrontal cortex, and the frontal eye field play significant roles. Studies of memory-guided saccades using transcranial magnetic stimulation have suggested that the right posterior parietal cortex is involved at the initial stage (<300 milliseconds) of visuospatial integration, whereas the dorsolateral prefrontal cortex in both hemispheres controls the following phase of short-term memorization, the frontal eye field mainly serving to trigger saccades. The new concept of a medium-term spatial memory has emerged from a behavioral study of memory-guided saccades in normal subjects, showing a paradoxical spontaneous improvement of spatial memory after delays of approximately 20 seconds. Lesion studies have shown that the parahippocampal cortex could specifically control this medium-term spatial memory. Last, different experimental and clinical arguments suggest that, after a few minutes, the hippocampal formation finally takes over the control of spatial memory for long-term spatial memorization. Therefore, spatial memory involved in the memorization of visual items could be successively controlled by the dorsolateral prefrontal cortex (short-term spatial memory), the parahippocampal cortex (medium-term spatial memory), and the hippocampal formation (long-term spatial memory), depending on specific periods of times. The applicability of this simple visuooculomotor model of spatial memory to other types of stimuli and general motoricity has yet to be confirmed.

Animals↗

PubMed1998

Spatial frequency discrimination and detection characteristics for gratings defined by orientation texture.

We describe evidence consistent with the proposal that the visual system contains a parallel array of size-tuned mechanisms sensitive to orientation texture-defined (OTD) form, and propose that the relative activity of these mechanisms determines spatial frequency discrimination threshold for OTD gratings. Using a pattern of short lines we measured spatial frequency discrimination thresholds for OTD gratings and luminance-defined (LD) gratings. For OTD gratings, the orientation of texture lines varied sinusoidally across the bars of the gratings, but line luminance was constant. For LD gratings, line orientation was constant, but line luminance varied sinusoidally across the bars of the grating. When the number of texture lines (i.e. spatial samples) per grating cycle was below about six, spatial sampling strongly affected both the spatial frequency discrimination and grating detection thresholds for OTD and LD gratings. However, when the number of spatial samples per grating cycle exceeded about six, plots of both discrimination threshold and detection threshold were different for OTD and LD gratings. For an OTD grating of any given spatial frequency, spatial frequency discrimination threshold fell as the number of samples per grating cycle was increased while holding texture line length constant: the lower limit was reached at six to ten samples per cycle. When we progressively increased the viewing distance (keeping the cycles per degree (cpd) constant), spatial frequency discrimination threshold reached a lower limit and increased thereafter. We propose that this minimum threshold represents a balance between opposing effects of the number of samples per grating cycle and the length of texture lines, and approaches the absolute physiological lower limit for OTD gratings. Spatial frequency discrimination was possible up to at least 7 cpd. Grating acuity for an OTD grating was considerably lower than the physiological limit for LD gratings, presumably because detectors of OTD form include a spatial integration stage following the processing of individual lines. For an LD grating, discrimination threshold fell as the number of samples per grating cycle was increased and asymptoted at six to ten samples per cycle. Spatial frequency discrimination thresholds for OTD and LD gratings were similar at low spatial frequencies (up to 3-4 cpd), but increased more steeply for OTD gratings at high spatial frequencies. For both OTD and LD gratings, discrimination threshold fell steeply as the number of grating cycles was increased from 0.5 to ca. 2.5 cycles, and thereafter decreased more slowly or not at all suggesting that, for both OTD and LD gratings, spatial frequency discrimination can be regarded as a special case of line interval or bar width discrimination. As orientation contrast was progressively increased, discrimination threshold for an OTD grating fell steeply up to about four to five times grating detection threshold, then saturated. This parallels the effect of luminance contrast on discrimination threshold for an LD grating.

Adult↗

PubMed2004

Antipsychotic and anticholinergic effects on two types of spatial memory in schizophrenia.

Spatial memory is of interest in schizophrenia because of widespread impairments in adaptive functioning, including independent living skills. Short-term spatial memory is impaired in this disease, whereas spatial reference memory, a longer-term spatial memory, has not been evaluated. Animal studies have demonstrated that anticholinergics impair short-term spatial memory but not spatial reference memory. The effects of haloperidol and risperidone on these two types of spatial memory were evaluated in a double-blind randomized comparison in inpatients with schizophrenia. It was predicted that risperidone would have a greater beneficial effect on spatial working memory than haloperidol. Computerized measures of spatial working memory and spatial reference memory were developed based on animal assessment of these functions. Subjects with schizophrenia were assessed during a medication-free period and again following 4 weeks of fixed-dose treatment. Risperidone, compared to haloperidol, improved spatial working memory performance, an effect that became nonsignificant when benztropine co-treatment was controlled. There were no treatment effects on spatial reference memory performance. Consistent with animal studies, benztropine impaired spatial working memory but not spatial reference memory. The relative benefits of risperidone on spatial working memory performance were largely explained by differential benztropine treatment for the haloperidol-treated subjects.

Adult↗

PubMed1998

Spatial structure, environmental heterogeneity, and population dynamics: analysis of the coupled logistic map.

Spatial extent can have two important consequences for population dynamics: It can generate spatial structure, in which individuals interact more intensely with neighbors than with more distant conspecifics, and it allows for environmental heterogeneity, in which habitat quality varies spatially. Studies of these features are difficult to interpret because the models are complex and sometimes idiosyncratic. Here we analyze one of the simplest possible spatial population models, to understand the mathematical basis for the observed patterns: two patches coupled by dispersal, with dynamics in each patch governed by the logistic map. With suitable choices of parameters, this model can represent spatial structure, environmental heterogeneity, or both in combination. We synthesize previous work and new analyses on this model, with two goals: to provide a comprehensive baseline to aid our understanding of more complex spatial models, and to generate predictions about the effects of spatial structure and environmental heterogeneity on population dynamics. Spatial structure alone can generate positive, negative, or zero spatial correlations between patches when dispersal rates are high, medium, or low relative to the complexity of the local dynamics. It can also lead to quasiperiodicity and hyperchaos, which are not present in the nonspatial model. With density-independent dispersal, spatial structure cannot destabilize equilibria or periodic orbits that would be stable in the absence of space. When densities in the two patches are uncorrelated, the probability that the population in a patch reaches extreme low densities is reduced relative to the same patch in isolation; this "rescue effect" would reduce the probability of metapopulation extinction beyond the simple effect of spreading of risk. Pure environmental heterogeneity always produces positive spatial correlations. The dynamics of the entire population is approximated by a nonspatial model with mean patch characteristics. This approximation worsens as the difference between the patches increases and the dispersal rate decreases: Under extreme conditions, destabilization of equilibria and periodic orbits occurs at mean parameter values lower than those predicted by the mean parameters. Apparent within-patch dynamics are distorted: The local population appears to have the wrong growth parameter and a constant number of immigrants (or emigrants) per generation. Adding environmental heterogeneity to spatial structure increases the occurrence of spatially correlated population dynamics, but the resulting temporal dynamics are more complex than would be predicted by the mean parameter values. The three classes of spatial pattern (positive, negative, and zero correlation), while still mathematically distinct, become increasingly similar phenomenologically.

Animals↗

PubMed1994

Spatial scale shifts in peripheral vernier acuity.

Abutting line vernier acuity thresholds are markedly degraded in peripheral vision, while line detection thresholds are elevated to a much lesser extent. To study the spatial and orientation tuning properties of the mechanisms underlying peripheral line vernier acuity, abutting vernier thresholds were measured in the presence of one-dimensional band-limited spatial noise masks varying in orientation and spatial frequency. To examine the effects of these masks on target visibility, line detection thresholds were also measured. We find that in both the fovea and the periphery, noise masking produces marked elevations of vernier thresholds, which are tuned to both spatial frequency and orientation. (i) Spatial frequency tuning: in the fovea, the spatial frequency tuning is bandpass, with a bandwidth of approximately 2.5 octaves, and a peak spatial frequency of about 10 c/deg. In the periphery the spatial tuning is similar in bandwidth, however the peak shifts systematically to lower spatial frequencies with increasing eccentricity, implying that thresholds are mediated by spatial mechanisms tuned to progressively larger spatial scales with eccentricity. (ii) Orientation tuning: at all eccentricities there is a bimodal orientation tuning function for vernier acuity, consistent with the hypothesis that the responses of at least two filters, whose orientations straddle the target lines, are combined to extract vernier offset information. In contrast, at all eccentricities, line detection is most strongly masked when the mask and line target have the same orientation. For both the line detection and line vernier tasks, the scale of the most sensitive spatial mechanisms shifts systematically with eccentricity. The change in line detection threshold with eccentricity is approximately proportional to the variation in spatial scale; however this shift in spatial scale is not sufficient to account for the degraded peripheral vernier acuity. The extra increase in peripheral vernier thresholds may be a consequence of a high degree of positional uncertainty which adds noise at a stage following the combination of filter responses.

Contrast Sensitivity↗

PubMed2005

The role of spatial mixing in the spread of foot-and-mouth disease.

A model of epidemic dispersal (based on the assumption that susceptible cattle were homogeneously mixed over space, or non-spatial model) was compared to a partially spatially explicit and discrete model (the spatial model), which was composed of differential equations and used geo-coded data (Euclidean distances between county centroids). While the spatial model accounted for intra- and inter-county epidemic spread, the non-spatial model did not assess regional differences. A geo-coded dataset that resembled conditions favouring homogeneous mixing assumptions (based on the 2001 Uruguayan foot-and-mouth disease epidemic), was used for testing. Significant differences between models were observed in the average transmission rate between farms, both before and after a control policy (animal movement ban) was imposed. They also differed in terms of daily number of infected farms: the non-spatial model revealed a single epidemic peak (at, approximately, 25 epidemic days); while the spatial model revealed two epidemic peaks (at, approximately, 12 and 28 days, respectively). While the spatial model fitted well with the observed cumulative number of infected farms, the non-spatial model did not (P<0.01). In addition, the spatial model: (a) indicated an early intra-county reproductive number R of approximately 87 (falling to <1 within 25 days), and an inter-county R<1; (b) predicted that, if animal movement restrictions had begun 3 days before/after the estimated initiation of such policy, cases would have decreased/increased by 23 or 26%, respectively. Spatial factors (such as inter-farm distance and coverage of vaccination campaigns, absent in non-spatial models) may explain why partially explicit spatial models describe epidemic spread more accurately than non-spatial models even at early epidemic phases. Integration of geo-coded data into mathematical models is recommended.

Animals↗

PubMed1998

Spatial phase and the temporal structure of the response to gratings in V1.

We recorded single-unit activity of 25 units in the parafoveal representation of macaque V1 to transient appearance of sinusoidal gratings. Gratings were systematically varied in spatial phase and in one or two of the following: contrast, spatial frequency, and orientation. Individual responses were compared based on spike counts, and also according to metrics sensitive to spike timing. For each metric, the extent of stimulus-dependent clustering of individual responses was assessed via the transmitted information, H. In nearly all data sets, stimulus-dependent clustering was maximal for metrics sensitive to the temporal pattern of spikes, typically with a precision of 25-50 ms. To focus on the interaction of spatial phase with other stimulus attributes, each data set was analyzed in two ways. In the "pooled phases" approach, the phase of the stimulus was ignored in the assessment of clustering, to yield an index Hpooled. In the "individual phases" approach, clustering was calculated separately for each spatial phase and then averaged across spatial phases to yield an index Hindiv. Hpooled expresses the extent to which a spike train represents contrast, spatial frequency, or orientation in a manner which is not confounded by spatial phase (phase-independent representation), whereas Hindiv expresses the extent to which a spike train represents one of these attributes, provided spatial phase is fixed (phase-dependent representation). Here, representation means that a stimulus attribute has a reproducible and systematic influence on individual responses, not a neural mechanism for decoding this influence. During the initial 100 ms of the response, contrast was represented in a phase-dependent manner by simple cells but primarily in a phase-independent manner by complex cells. As the response evolved, simple cell responses acquired phase-independent contrast information, whereas complex cells acquired phase-dependent contrast information. Simple cells represented orientation and spatial frequency in a primarily phase-dependent manner, but also they contained some phase-independent information in their initial response segment. Complex cells showed primarily phase-independent representation of orientation but primarily phase-dependent representation of spatial frequency. Joint representation of two attributes (contrast and spatial frequency, contrast and orientation, spatial frequency and orientation) was primarily phase dependent for simple cells, and primarily phase independent for complex cells. In simple and complex cells, the variability in the number of spikes elicited on each response was substantially greater than the expectations of a Poisson process. Although some of this variation could be attributed to the dependence of the response on the spatial phase of the grating, variability was still markedly greater than Poisson when the contribution of spatial phase to response variance was removed.

Action Potentials↗

PubMed2003

Aging effects on spatial tuning of hippocampal place cells in mice.

One reason the electrophysiological correlates of hippocampal neurons are of interest is the possibility that they reflect their representational properties, presumably spatial/relational ones. Stable spatial representations, based on activity of ensembles of hippocampal place cells, initially develop through a series of short-episodic spatial tunings. Hence these short-episodic spatial tunings are important for understanding the establishment of stable place fields. Studies of age-related changes in place cell activities traditionally focus on place fields. In the present study, we characterized the short-episodic spatial tunings (1-min bins) of hippocampal CA1 place cells of freely moving mice in a familiar cylinder arena, and compared these functions in young and old mice. Spatial tuning was expressed by spatial selectivity, which we found fluctuated across a 16-min recording session in both young and old mice. High spatial selectivity, which is mainly due to the low firing of a place cell out of the place field in young mice, was significantly higher in old mice. The high firing rate out of the place field was the main factor contributing to significantly lower spatial selectivity in old mice. In addition, young mice showed a broad peak in the spatial selectivity between 4 and 10 min. In contrast old mice showed no peak in the spatial selectivity during this time period. The stability of place fields after a 24-h interval was also lower in old mice than in young mice. The low spatial tuning and unstable place fields suggest that a hippocampal-based spatial representation was impaired in the old mice. Furthermore, we speculate that the age-related impairment in hippocampal inhibition system may be involved in the impaired spatial representation of hippocampal CA1 place cells in old mice.

Action Potentials↗

PubMed1991

Determinants of spatial priming in environmental memory.

Spatial priming in recognizing objects in experimentally learned environments has been proposed as strong evidence for spatial organization of environmental memory. However, in all studies showing recognition priming effects, encoding and rehearsal contiguity may have coincided with spatial proximity, and thus priming may have been due to temporal associations formed during rehearsal, not encoded spatial relations per se. We investigated this question in four experiments, using a trip trial learning method in which temporal contiguity and spatial relations were independent. In Experiment 1, no spatial priming in recognition was found, even though indirect evidence suggested that subjects had encoded spatial relations. In Experiment 2, the trip trial method was compared with the free study procedure commonly used in previous priming studies. Spatial priming occurred only for free study subjects, even though the two groups were equivalent on direct measures of encoding accuracy. In Experiment 3, spatial priming in recognition was obtained with a modification of the trip trial method in which temporal and spatial contiguity were deliberately confounded. In Experiment 4, the unmodified trip trial method produced spatial priming in a location-decision task. Taken together, our results suggest that environmental memory may be spatially organized, but retrieval of object identities does not necessarily activate encoded spatial relations.

Adult↗

PubMed2000

General theory of competitive coexistence in spatially-varying environments.

A general model of competitive and apparent competitive interactions in a spatially-variable environment is developed and analyzed to extend findings on coexistence in a temporally-variable environment to the spatial case and to elucidate new principles. In particular, coexistence mechanisms are divided into variation-dependent and variation-independent mechanisms with variation-dependent mechanisms including spatial generalizations of relative nonlinearity and the storage effect. Although directly analogous to the corresponding temporal mechanisms, these spatial mechanisms involve different life history traits which suggest that the spatial storage effect should arise more commonly than the temporal storage effect and spatial relative nonlinearity should arise less commonly than temporal relative nonlinearity. Additional mechanisms occur in the spatial case due to spatial covariance between the finite rate of increase of a local population and its local abundance, which has no clear temporal analogue. A limited analysis of these additional mechanisms shows that they have similar properties to the storage effect and relative nonlinearity and potentially may be considered as enlargements of the earlier mechanisms. The rate of increase of a species perturbed to low density is used to quantify coexistence. A general quadratic approximation, which is exact in some important cases, divides this rate of increase into contributions from the various mechanisms above and admits no other mechanisms, suggesting that opportunities for coexistence in a spatially-variable environment are fully characterized by these mechanisms within this general model. Three spatially-implicit models are analyzed as illustrations of the general findings and of techniques using small variance approximations. The contributions to coexistence of the various mechanisms are expressed in terms of simple interpretable formulae. These spatially-implicit models include a model of an annual plant community, a spatial multispecies version of the lottery model, and a multispecies model of an insect community competing for spatially-patchy and ephemeral food.

Animals↗

PubMed1990

Influence of spatial frequency on tuning and bias for orientation and direction in the cat's striate cortex.

Directionality, orientation and spatial frequency tuning were determined for 108 neurones recorded extracellularly from the striate cortex of anaesthetized cats. Significant sharpening of orientation selectivity with increasing spatial frequency was seen in all simple neurones and the overwhelming majority of complex neurones. Orientation selectivity sharpened in 90 and broadened in only 10 of 100 fully characterized neurones. At least four distinct classes of neurone could be characterized on the basis of their directionality at optimal spatial frequency, and the presence or absence of changes in directionality over a range of spatial frequencies: in two classes, directionality was spatial-frequency dependent; in the remaining two it was invariant. With two exceptions Type A neurones (23 cells) were direction-selective; they were narrowly tuned for orientation and spatial frequency, and their directionality was invariant with spatial-frequency. The majority of neurones (52 cells) were Type B, most of which were direction-biased; their bias for direction varied systematically with spatial frequency. Type C were direction-biased and spatial-frequency selective (5 cells), but showed a clear reversal of bias with change in spatial frequency. Type D, a subset of direction-biased cells, were bidirectional and spatial-frequency invariant (8 cells), with comparable response strengths to motion in two opposing directions at all spatial frequencies. These response types crossed traditional boundaries between categories of simple and complex neurones, assigned on the basis of spatial summation, presence or absence of end-inhibition, and receptive field size.

Animals↗