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At least 811 records · Page 45Linked to original sources
PubMed2006

Bayesian fMRI data analysis with sparse spatial basis function priors.

In previous work we have described a spatially regularised General Linear Model (GLM) for the analysis of brain functional Magnetic Resonance Imaging (fMRI) data where Posterior Probability Maps (PPMs) are used to characterise regionally specific effects. The spatial regularisation is defined over regression coefficients via a Laplacian kernel matrix and embodies prior knowledge that evoked responses are spatially contiguous and locally homogeneous. In this paper we propose to finesse this Bayesian framework by specifying spatial priors using Sparse Spatial Basis Functions (SSBFs). These are defined via a hierarchical probabilistic model which, when inverted, automatically selects an appropriate subset of basis functions. The method includes non-linear wavelet shrinkage as a special case. As compared to Laplacian spatial priors, SSBFs allow for spatial variations in signal smoothness, are more computationally efficient and are robust to heteroscedastic noise. Results are shown on synthetic data and on data from an event-related fMRI experiment.

Algorithms↗
PubMed2005

The spatial and temporal meanings of English prepositions can be independently impaired.

English uses the same prepositions to describe both spatial and temporal relationships (e.g., at the corner, at 1:30), and other languages worldwide exhibit similar patterns. These space-time parallelisms have been explained by the Metaphoric Mapping Theory, which maintains that humans have a cognitive predisposition to structure temporal concepts in terms of spatial schemas through the application of a TIME IS SPACE metaphor. Evidence comes from (among other sources) historical investigations showing that languages consistently develop in such a way that expressions that originally have only spatial meanings are gradually extended to take on analogous temporal meanings. It is not clear, however, if the metaphor actively influences the way that modern adults process prepositional meanings during language use. To explore this question, a series of experiments was conducted with four brain-damaged subjects with left perisylvian lesions. Two subjects exhibited the following dissociation: they failed a test that assesses knowledge of the spatial meanings of prepositions, but passed a test that assesses knowledge of the corresponding temporal meanings of the same prepositions. This result suggests that understanding the temporal meanings of prepositions does not necessarily require establishing structural alignments with their spatial correlates. Two other subjects exhibited the opposite dissociation: they performed better on the spatial test than on the temporal test. Overall, these findings support the view that although the spatial and temporal meanings of prepositions are historically linked by virtue of the TIME IS SPACE metaphor, they can be (and may normally be) represented and processed independently of each other in the brains of modern adults.

Aged↗
PubMed2006

Prefrontal cortex hemispheric specialization for categorical and coordinate visual spatial memory.

During visual spatial perception of multiple items, the left hemisphere has been shown to preferentially process categorical spatial relationships while the right hemisphere has been shown to preferentially process coordinate spatial relationships. We hypothesized that this hemispheric processing distinction would be reflected in the prefrontal cortex during categorical and coordinate visual spatial memory, and tested this hypothesis using functional magnetic resonance imaging (fMRI). During encoding, abstract shapes were presented in the left or right hemifield in addition to a dot at a variable distance from the shape (with some dots on the shape); participants were instructed to remember the position of each dot relative to the shape. During categorical memory retrieval, each shape was presented centrally and participants responded whether the previously corresponding dot was 'on' or 'off' of the shape. During coordinate memory retrieval, each shape was presented centrally and participants responded whether the previously corresponding dot was 'near' or 'far' from the shape (relative to a reference distance). Consistent with our hypothesis, a region in the left prefrontal cortex (BA10) was preferentially associated with categorical visual spatial memory and a region in the right prefrontal cortex (BA9/10) was preferentially associated with coordinate visual spatial memory. These results have direct implications for interpreting previous findings that the left prefrontal cortex is associated with source memory, as this cognitive process is categorical in nature, and the right prefrontal cortex is associated with item memory, as this process depends on the precise spatial relations among item features or components.

Adult↗
PubMed2006

Spatial learning and goldfish telencephalon NMDA receptors.

Recent results have demonstrated that the mammalian hippocampus and the dorso-lateral telencephalon of ray-finned fishes share functional similarities in relation to spatial memory systems. In the present study, we investigated whether the physiological mechanisms of this hippocampus-dependent spatial memory system were also similar in mammals and ray-finned fishes, and therefore possibly conserved through evolution in vertebrates. In Experiment 1, we studied the effects of the intracranial administration of the noncompetitive NMDA receptor antagonist MK-801 during the acquisition of a spatial task. The results indicated dose-dependent drug-induced impairment of spatial memory. Experiment 2 evaluated if the MK-801 produced disruption of retrieval of a learned spatial response. Data showed that the administration of MK-801 did not impair the retrieval of the information previously stored. The last experiment analyzed the involvement of the telencephalic NMDA receptors in a spatial and in a cue task. Results showed a clear impairment in spatial learning but not in cue learning when NMDA receptors were blocked. As a whole, these results indicate that physiological mechanisms of this hippocampus-dependent system could be a general feature in vertebrate, and therefore phylogenetically conserved.

Animals↗
PubMed2005

The accessibility of spatial channels for stereo and motion.

Using fractal noise images, we measured the dependence of D(min) on the spatial passband (spatial frequency and orientation) over which information was correlated either between the eyes for stereo or between sequential frames for motion. Without affecting the amplitude spectrum of the noise stimulus we used idealized filters to scramble the phase of components outside a pre-defined passband. Using a simple Gaussian model in which performance depends on the signal/noise within a restricted spatial region, we obtained estimates of the bandwidth of the narrowest underlying spatial frequency and orientation spectral region subserving these two comparable tasks. Spatial bandwidths varied with peak spatial frequency but were very broad approximating the spectrum of the stimulus itself. Orientation properties of the underlying mechanisms were isotropic. These results suggest that the independent activity of individual narrowband spatial channels is not perceptually accessible for these tasks.

Depth Perception↗
PubMed2006

Second-order spatial frequency and orientation channels in human vision.

We compared the number of spatial frequency and orientation mechanisms underlying first- versus second-order processing by measuring discrimination at detection threshold for first- and second-order Gabors to determine the smallest difference in spatial frequency and orientation that permits accurate discrimination at threshold. For second-order gratings, the number of channels is the same as for first-order gratings for spatial frequencies up to about 2 cpd; however, there are fewer second-order channels at higher spatial frequencies. In contrast, the number of labeled channels for orientation is the same for first- and second-order gratings. In conclusion, our findings provide evidence for distinct spatial frequency and orientation labeled detectors in second-order visual processing. We also show that, relative to first-order, there are fewer second-order channels processing higher spatial frequencies. This is consistent with a filter-rectify-filter scheme for second-order in which the second stage of filtering is at lower spatial frequencies.

Contrast Sensitivity↗
PubMed2005

Spatial working memory and planning ability: contrasts between schizophrenia and bipolar I disorder.

Working memory may be conceptualized as a multi-component system involving the active maintenance and manipulation of stored information in the service of planning/guiding behaviour. Impaired spatial working memory is a robust finding in schizophrenia patients which has been related to an impairment in frontostriatal connectivity. The purpose of this study was to examine the specificity of this impairment by comparing the mnemonic and executive aspects of working memory performance in schizophrenia and bipolar disorder with psychotic features, focusing particularly on the functional dynamics between task components. Twenty-four patients with schizophrenia, 14 patients with bipolar I disorder (manic phase) and 33 healthy control subjects were assessed using the Cambridge Neuropsychological Test Automated Battery (CANTAB): including the spatial working memory (between search errors and strategy scores) spatial span (storage capacity) and spatial planning (Stockings of Cambridge: accuracy and latency) tasks. Both patient groups were impaired on the spatial span task, which requires the maintenance and retrieval of stored information. In contrast, only schizophrenia patients showed a significant deficit in between search errors, which requires both maintenance and manipulation of information in working memory. That is, they exhibited both a mnemonic and an executive dysfunction. Spatial span was particularly important to accurate planning ability in bipolar patients. In contrast, in patients with schizophrenia poor spatial working memory was a significant predictor of planning impairments, consistent with failures in goal selection, evaluation and/or execution. Furthermore, initial planning time was positively correlated with the latency to complete a planning sequence. This pattern of slow cognitive processing in schizophrenia patients only, resembled that reported previously in patients with basal ganglia disorders. These findings are discussed in terms of a possible common disturbance in fronto-parietal circuitry in the two disorders together with a specific disturbance of fronto-striatal circuitry in schizophrenia, that is not present in bipolar disorder.

Adult↗
PubMed1982

Handedness and spatial ability.

Data from a large-scale study of cognitive abilities were used to test the hypothesis that there are handedness-related differences in spatial visualization. The spatial factor score of the cognitive test battery provided a measure of spatial visualization. Analyses revealed a sex X handedness X ethnicity interaction on the spatial factor, and no significant handedness-related differences on verbal factor. In all ethnic groups strongly left-handed males had higher spatial scores than strongly right-handed males, whereas strongly left-handed females had lower spatial scores than strongly right-handed females. Among subjects of Japanese or Chinese ancestry, strongly left-handed subjects differed from ambidextrous subjects as well as from right-handed subjects, and these differences, too, were of opposite sign for males and females. It is suggested that the use of spatial ability measures which are relatively insensitive to differences in right hemisphere ability, as well as the failure to attend to degree of handedness, ethnicity, and particularly gender, may be responsible for the negative findings in this area of research.

Adolescent↗
PubMed1996

Spatial working memory and strategy formation in patients with frontal lobe excisions.

Spatial working memory was investigated in 20 patients with unilateral neurosurgical excisions of the frontal cortex (UFL), nine with right (RFL) and eleven with left lesions (LFL), comparing their performance to a matched control group. Spatial memory was tested using the Executive Golf Task, a test that also measures spatial strategy formation. Overall the UFL were significantly impaired, the greatest impairment being found in the RFL group. The difference between the RFL and LFL groups was abolished when a measure of strategy formation was used as a covariate in the analysis. A further test of spatial working memory, the Owl Spatial Working Memory Task, which prevents the use of a spatial strategy, showed a significant and equivalent impairment in both the RFL and LFL patients. The data are consistent with neuropsychological and functional neuroimaging investigations supporting the role of the pre-frontal cortex in spatial working memory.

Adult↗
PubMed2003

Control of spatially heterogeneous and time-varying cellular reaction networks: a new summation law.

A hallmark of a plethora of intracellular signaling pathways is the spatial separation of activation and deactivation processes that potentially results in precipitous gradients of activated proteins. The classical metabolic control analysis (MCA), which quantifies the influence of an individual process on a system variable as the control coefficient, cannot be applied to spatially separated protein networks. The present paper unravels the principles that govern the control over the fluxes and intermediate concentrations in spatially heterogeneous reaction networks. Our main results are two types of control summation theorems. The first type is a non-trivial generalization of the classical theorems to systems with spatially and temporally varying concentrations. In this generalization, the process of diffusion, which enters as the result of spatial concentration gradients, plays a role similar to other processes such as chemical reactions and membrane transport. The second summation theorem is completely novel. It states that the control by the membrane transport, the diffusion control coefficient multiplied by two, and a newly introduced control coefficient associated with changes in the spatial size of a system (e.g., cell), all add up to one and zero for the control over flux and concentration. Using a simple example of a kinase/phosphatase system in a spherical cell, we speculate that unless active mechanisms of intracellular transport are involved, the threshold cell size is limited by the diffusion control, when it is beginning to exceed the spatial control coefficient significantly.

Animals↗
PubMed1998

On the modularity of recognition memory for object form and spatial location: a topographic ERP analysis.

Event-related potentials from 61 scalp sites were used to examine the brain processes subserving recognition memory for object forms and spatial locations. Subjects memorized line drawings of highly familiar objects and their spatial locations within a two-dimensional matrix. Prior to the test phases a cue indicated whether object-based or spatially-based recognition judgements were required. Recognition judgements were faster and more accurate for spatially-based than for object-based judgements. A variety of topographical differences in the ERP waveforms as a function of recognition task emerged: First, when the cue indicated that object-based judgements were required, negative slow wave activity extending for several hundred ms with a maximum at frontal recording sites was obtained. Conversely when spatially-based judgements were required, slow wave activity developed over parieto-occipital areas. Second, early portions of the old/new effects evoked by the test items (i.e. 300-600 ms after stimulus onset) showed a similar anterior-posterior dissociation as a function of recognition task. Third, for object-based, but not for spatially-based, judgements, late old/new effects (i.e. 700-1600 ms) were found with a clear maximum at right frontal recordings. The results are consistent with the view that functionally and anatomically different brain systems are involved in recognition memory for object form and spatial location. They further suggest that the retrieval of object forms involves conceptual semantic integration processes.

Adult↗
PubMed1998

The nature of the spatial deficit in young females with Fragile-X syndrome: a neuropsychological and molecular perspective.

Spatial performance in a group of young Fragile-X syndrome females with FMR-1 full mutation was compared to two control groups of mainstream schoolchildren. Performance was assessed across a wide range of spatial tasks including visuo-spatial, visuo-construction, visuo-motor, visuo-perception and spatial-memory. A spatial deficit emerged only on those tasks which comprised a visuo-constructive component, with the Fragile-X group performing worse overall. All other tasks were performed at a comparable level across the three groups. Molecular analysis of the lymphocyte DNA found minimal evidence for a correlation between expansion size and spatial performance. In addition, there was no evidence for a correlation between the proportion of active to inactive unmethylated FMR-1 genes (activation ratio) and spatial performance. These results conflict with recent reports of a correlation between activation ratio and intellectual functioning.

Adolescent↗
PubMed1999

Verbs, events and spatial representations.

Are concepts expressed in language also represented spatially? To pursue this question we investigated the structure of events. Events are defined as actions with spatial trajectories that can be perceived by our senses and described in language. Events are expressed linguistically in sentences containing verbs which determine the thematic roles of the arguments (e.g., who is doing what to whom, where). Because of previous observations we focused on whether events are represented spatially by location of thematic roles and direction of actions. Location and direction were dissociated by contrasting different kinds of verbs: 'push' vs 'pull' in which actions move toward or away from the agent. To control for spatial effects produced by the surface structure of a left to right written language, we kept the structure of sentences constant and sought for spatial biases produced by differences in the meaning of these sentences. From three experiments using drawing and sentence-picture matching reaction time tasks, we found that normal subjects located agents to the left of patients and represented actions with a left to right directionality. These results are not easily explained by features of the surface structure of language or properties of propositional representations. We suggest that events have spatial representations in addition to their propositional counterparts of verbs and thematic roles. The specific spatial properties observed may relate to functional properties of the left hemisphere.

Adolescent↗
PubMed2003

Age-related sex differences in spatial learning and basal forebrain cholinergic neurons in F344 rats.

Basal forebrain cholinergic neurons are important for spatial learning in rodents. Spatial learning ability is reportedly better in males than females, and declines with age. To examine the role of cholinergic function in sex- or age-related differences in spatial learning, we compared the size of basal forebrain cholinergic neurons (BFCN) of young and aged male and female Fischer 344 (F344) rats that had been trained in the Morris water maze. Young male and female rats were equally proficient in finding the platform during training trials, but probe tests revealed that young male rats had better knowledge of the platform's precise location. Impairments in spatial learning were observed in aged rats, and the advantage of males over females was lost. BFCN were significantly larger in young male than young female rats, and were correlated with spatial memory performance for both groups. BFCN were smaller in aged than young males; no change was seen between young and aged females. In the groups of aged rats the correlation between neuron size and spatial memory was lost. The present findings provide further evidence of a role for the basal forebrain cholinergic system in spatial learning, but reveal a complex interaction between sex, age and behavioral performance.

Aging↗
PubMed1996

Caloric restriction and spatial learning in old mice.

Spatial learning in old mice (19 or 24 months old), some of which had been calorically restricted beginning at 14 weeks of age, was compared to that of young mice, in two separate experiments using a Morris water maze. In the first experiment, only old mice reaching criterion performance on a cued learning task were tested in a subsequent spatial task. Thus, all old mice tested for spatial learning had achieved escape latencies equivalent to those of young controls. Despite equivalent swimming speeds, only about half the old mice in each diet group achieved criterion performance in the spatial task. In the second experiment, old and young mice all received the same number of training trials in a cued task and then in a spatial task. Immediately following spatial training, they were given a 60-s probe trial, with no platform in the pool. Both groups of old mice spent significantly less time in the quadrant where the platform had been and made significantly fewer direct crosses over the previous platform location than did the young control group. As in Experiment 1, calorie restriction failed to provide protection against aging-related deficits. However, in both experiments, some individual old mice evidenced performance in spatial learning indistinguishable from that of young controls. Separate comparisons of "age-impaired" and "age-unimpaired" old mice with young controls may facilitate the identification of neurobiological mechanisms underlying age-related cognitive decline.

Aging↗
PubMed1998

Attentional control of spatial scale: effects on self-organized motion patterns.

Prior to the presentation of a test stimulus, subjects' attentional state was either narrowly focused on a particular location or broadly spread over a large spatial region. In previous studies, it was found that broadly spread attention enhances the sensitivity of relatively large spatial filters (increasing the perceiver's spatial scale), thereby diminishing spatial resolution and enhancing sensitivity to global stimulus structure. In this study it is shown that attentional spread also affects the self-organization of unidirectional versus oscillatory motion patterns for the directionally ambiguous, counterphase presentation of rows of evenly-spaced visual elements (lines segments; dots); i.e. qualitatively different motion patterns can be formed for the same stimulus at different spatial scales. Although the degree to which attention is spread along a spatial axis can be controlled by the perceiver, the effects of spread attention are not limited to a single axis. These results, as well as previously observed effects of attentional spread on spatial resolution, are accounted for by a neural model involving large, foveally-centered receptive fields with co-operatively interacting subunits (probably at the level of MST or higher).

Attention↗
PubMed1998

The push-pull action of dopamine on spatial tuning of the monkey retina: the effects of dopaminergic deficiency and selective D1 and D2 receptor ligands on the pattern electroretinogram.

Retinal dopamine depletion in monkeys using either systemic MPTP or 6-OHDA results in attenuated electroretinographic (ERG) responses to peak spatial frequency stimuli. Diverse dopamine receptors have been identified in the primate retina. ERG studies performed using Haloperidol (a mixed antagonist), L-Sulpiride (D2 antagonist) and CY 208-243 (a D1 agonist) cause spatial frequency dependent diverse effects. 'Tuning' of the normal spatial contrast response PERG, was quantified by dividing the amplitude of the response at the peak spatial frequency with the amplitude to the low spatial frequency response yielding a number greater than one. Tuning for the pharmacological experiments was defined by dividing the actual amplitude obtained at the normal peak response with the actual amplitude at the low spatial frequency response. The PERG spatial contrast response function is discussed as the envelope output of retinal ganglion cells or the average or 'equivalent' retinal ganglion cell. However, we postulate the existence of two dopamine sensitive pathways with different weights for two classes of ganglion cells. It is inferred that D1 receptors are primarily affecting the 'surround' organization of ganglion cells with large centers, while D2 post-synaptic receptors contribute to 'center' response amplification of ganglion cells with smaller centers. These inferences are consistent with some lower vertebrate data. It is also inferred that low affinity D2 autoreceptors may be involved in the D1 'surround' pathway. An understanding of the logic performed by retinal D1 and D2 receptors may be useful to discern the functional role of diverse dopamine receptors in DA circuits elsewhere in the CNS.

Animals↗
PubMed2000

Spatial frequency tuned covariance channels for red-green and luminance-modulated gratings: psychophysical data from human adults.

Both chromatic and luminance-modulated stimuli are served by multiple spatial-frequency-tuned channels. This experiment investigated the independence versus interdependence of spatial frequency channels that serve the detection of red-green chromatic versus yellow-black luminance-modulated stimuli at low spatial frequencies. Contrast thresholds for both chromatic and luminance-modulated gratings were measured within 12 individual subjects using a repeated-measures design. Spatial frequencies ranged from 0.27 to 2.16 c/deg. A covariance structure analysis of individual differences was applied to the data. We computed statistical sources of individual variability, used them to define covariance channels, and determined the number and frequency tuning of these channels. For luminance-modulated gratings, two covariance channels were found, including one above and one below 1 c/deg [cf. Peterzell, & Teller (1996). Individual differences in contrast sensitivity functions: the coarsest spatial pattern analyzer. Vision Research, 36, 3077-3085]. For chromatic gratings, correlations between thresholds for most spatial frequencies were uniformly high, yielding a single covariance channel covering all but the highest spatial frequency tested. A combined analysis of both data sets recovered the same three covariance channels, and showed that detection thresholds for low-frequency red-green chromatic and luminance-modulated stimuli are served by separate, statistically independent processes.

Adult↗