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PubMed1983

Spatio-temporal interactions in cat retinal ganglion cells showing linear spatial summation.

The spatio-temporal characteristics of cat retinal ganglion cells showing linear summation have been studied by measuring both magnitude and phase of the responses of these cells to drifting or sinusoidally contrast-modulated sinusoidal grating patterns. It has been demonstrated not only that X cells behave approximately linearly when responding with amplitudes of less than about 10 impulses/sec to stimuli of low contrast but also that cells of another type with larger receptive field centres (Q cells) behave approximately linearly under the same conditions. These Q cells appear to form a homogeneous group which is probably a subset of the tonic W cells (Stone & Fukuda, 1974) or sluggish centre-surround cells (Cleland & Levick, 1974). The over-all spatio-temporal frequency characteristics of cells showing linear spatial summation are not separable in space and time. The form of the spatial frequency responsivity function of these cells depends upon the temporal frequency at which it is measured while the temporal phase of their resonse measured at any constant temporal frequency depends upon the spatial frequency of the stimulus. The behaviour of X and Q cells is quite well explained by an extension of the model in which signals from centre and surround mechanisms with radially Gaussian weighting functions are summed to provide the drive to the retinal ganglion cell. While the general form of the temporal frequency response characteristics of these ganglion cells are probably provided by the characteristics of elements common to the centre and surround pathways, the spatio-temporal interactions can be explained by assuming that the surround signal is delayed relative to the centre signal by a few milliseconds.

Animals↗

PubMed2006

Lightness, filling-in, and the fundamental role of context in visual perception.

Visual perception is defined by the unique spatial interactions that distinguish it from the point-to-point precision of a photometer. Over several decades, Lothar Spillmann has made key observations about the nature of these interactions and the role of context in perception. Our lab has explored the perceptual properties of spatial interactions and more generally the importance of visual context for neuronal responses and perception. Our investigations into the spatiotemporal dynamics of lightness provide insight into underlying mechanisms. For example, backward masking and luminance modulation experiments suggest that the representation of a uniformly luminous object develops first at the borders and, in some manner, the center fills in. The temporal dynamics of lightness induction are also consistent with a filling-in process. There is a slow cutoff temporal frequency above which surround luminance modulation will not elicit perceptual induction of a central area. The larger the central area, the lower the cutoff frequency for induction, perhaps indicating that an edge-based process requires more time to "complete" the larger area. In recordings from primary visual cortex we find that neurons respond in a manner surprisingly consistent with lightness perception and the spatial and temporal properties of induction. For example, the activity of V1 neurons can be modulated by light outside the receptive field and as the modulation rate is increased response modulation falls off more rapidly for large uniform areas than smaller areas. The conclusion we draw from these experiments is that lightness appears to be computed slowly on the basis of edge and context information. A possible role for the spatial interactions is lightness constancy, which is thought to depend on extensive spatial integration. We find not only that V1 responses are strongly context dependent, but that this dependence makes V1 lightness constant on average. The dependence of constancy on surround interactions underscores the fundamental role that context plays in perception. In more recent studies, further support has been found for the importance of context in experiments using natural scene stimuli.

Animals↗

PubMed1979

A colour-dependent abnormality in human visual detection of stimulus motion and spatial structure.

We have studied the wavelength dependence of visual responses in a single human subject, M.W., who has an unusual visual defect. It is shown that for red stimuli, both spatial resolution for grating patterns and detection of target movement are grossly abnormal. On the basis of these and other [8] experimental data, we argue that M.W.'s response pattern implies interdependent central visual processing of colour, movement and spatial structure. We examine this conclusion in relation to recent anatomical and electrophysiological findings.

Color Perception↗

PubMed1991

Spatial learning in two inbred strains of mice: genotype-dependent effect of amygdaloid and hippocampal lesions.

Spatial learning performance and maze-running strategies were estimated in two inbred strains of mice, C57BL/6 and DBA/2, submitted to an 8-arm radial maze task. Subsequently the genotype-dependent effect of hippocampus and amygdala on the mastering of this task was examined as a function of the different acquisition model provided by each strain. The results firstly show that unoperated C57BL/6 mice reach a higher level of performance and develop a stronger preference for adjacent arms - 45 degrees angle - turns than unoperated DBA/2 mice. In the high learner C57BL/6 strain, both hippocampal and amygdaloid lesions impair performance and modify maze-running strategies. With practice, however, the difference between amygdala-lesioned mice and controls disappears while that between hippocampus-lesioned mice and controls persists. Conversely, in the low learner DBA/2 strain, hippocampal lesions have a negative effect on a single parameter of performance, while amygdaloid lesions only affect maze-running strategies. Taken together, these results confirm the specific control exerted by the hippocampus on spatial learning. Moreover, they suggest that the amygdala can parallel the role of the hippocampus as far as the baseline level of performance of the strain considered is high.

Amygdala↗

PubMed2001

Contrast dependency of VEPs as a function of spatial frequency: the parvocellular and magnocellular contributions to human VEPs.

The present study investigated the contrast dependency of visual evoked potentials (VEPs) elicited by phase reversing sine wave gratings of varying spatial frequency. Sixty-five trials were recorded for each of 54 conditions: 6 spatial frequencies (0.8, 1.7, 2.8, 4.0, 8.0 and 16.0 c deg(-1)) each presented at 9 contrast levels (2, 4, 8, 11, 16, 23, 32, 64 and 90%). At the lowest spatial frequency, the waveform contained mainly one peak (P1). For spatial frequencies up to 8 c deg(-1), P1 had a characteristic magnocellular contrast response: it appeared at low contrasts, increased rapidly in amplitude with increasing contrast, and saturated at medium contrasts. With increasing spatial frequency, an additional peak (N1) gradually became the more dominant component of the waveform. N1 had a characteristic parvocellular contrast response: it appeared at medium to high contrasts, increased linearly in amplitude with increasing contrast, and did not appear to saturate. The data suggest the contribution of both magnocellular and parvocellular responses at intermediate spatial frequencies. Only at the lowest and highest spatial frequencies tested did magnocellular and parvocellular responses, respectively, appear to dominate.

Adult↗

PubMed2005

Spatial mosaic formation through frequency-dependent selection in Müllerian mimicry complexes.

Although contemporary models of Müllerian mimicry have considered the movement of interfacial boundaries between two distinct mimetic forms, and even the possibility of polymorphisms in two patch systems, no model has considered how multiple forms of Müllerian mimics might evolve and be maintained over large geographical areas. A spatially explicit individual-based model for the evolution of Müllerian mimicry is presented, in which two unpalatable species are distributed over discrete cells within a regular lattice. Populations in each cell are capable of genetic drift and experience localized dispersal as well as frequency-dependent selection by predators. When each unpalatable prey species was introduced into a random cell and allowed to spread, then mimicry evolved throughout the system in the form of a spatial mosaic of phenotypes, separated by narrow "hybrid zones". The primary mechanism generating phenotypic diversity was the occasional establishment of new mutant forms in unoccupied cells and their subsequent maintenance (and spread) through frequency-dependent selection. The mean number of discrete clusters of the same morph that formed in the lattice was higher the higher the intensity of predation, and higher the lower the dispersal rate of unpalatable prey. Under certain conditions the hybrid zones moved, in a direction dependent on the curvature of their interfacial boundaries. However, the mimetic mosaics were highly stable when the intensity of predation was high and the rate of prey dispersal was low. Overall, this model highlights how a stable mosaic of different mimetic forms can evolve from a range of starting conditions through a combination of chance effects and localized frequency-dependent selection.

Adaptation, Biological↗

PubMed2001

Spatiotemporal dynamics of Floerkea proserpinacoides (Limnanthaceae), an annual plant of the deciduous forest of eastern North America.

Because environmental filters are temporally and spatially heterogeneous, there often is a lack of significant relationship between the spatial patterns of successive life stages in plant populations. In this study, we determined the spatiotemporal relationships between different life stages in two populations of an annual plant of the deciduous forests of eastern North America, Floerkea proserpinacoides. Demographic surveys were done over a 4-yr period, and experiments were performed in the field and under controlled conditions to test for the effects of various environmental factors on population dynamics. There was a general lack of relationship between the spatial patterns of seed bank and seedling density, and a lack of similarity between their spatial correlograms. This was related mostly to the effects of spatially variable environmental filters operating on germination and emergence. However, environmental filters acting on plant survival were stable through time and contributed to stabilize the density and spatial patterns of the populations. Despite density-dependent presenescence mortality, spatial patterns of seedlings and mature individuals were similar and their correlograms were alike, suggesting that mortality did not fully compensate for density. Estimated fecundity was negatively correlated with population density over the study period. Although flower production started only 2-3 wk after emergence, seed maturation mostly occurred at the end of the life cycle, just before the onset of plant senescence. Yet, individual fecundity was low for an annual plant, i.e., 3.0 ± 0.5 mature seeds/plant (mean ± 1 SE). Seed predation by vertebrates was not significant. Low soil moisture had little effect on the total number of seeds germinating, although it slowed down the germination process. In quadrats where leaf litter was experimentally doubled, seedling emergence was lower than in control quadrats; in quadrats where leaf litter was completely removed, emergence did not differ from that in control quadrats. Susceptibility to drought stress was higher for seedlings than for mature plants. Although the species does not maintain a long-term persistent soil seed bank, other factors, such as density-dependent fecundity and autogamy, may temper population fluctuations through time and reduce the probability of local extinction.

Journal Article↗

PubMed1994

Motion sensitivity in the nucleus of the basal optic root of the pigeon.

1. Single-unit responses to large-field movement (angular velocity, w = 0.25-42 degrees/s) of sine-wave gratings of different spatial wavelength (lambda = 5.2-41 degrees) and contrast have been recorded in the nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) of the pigeon. 2. The steady-state response to moving sine-wave gratings increases with increasing contrast to reach a saturation level at 25%. 3. Generally the steady-state responses of the cells passed through a maximum when stimulated at various velocities. In 12 of the 15 cells tested with six different velocities and four different spatial wavelengths, the location of the response maximum on the velocity scale depended on the spatial wavelength (lambda) used. That is, in these cells the response depends on the temporal frequency (tf = w/lambda) of the stimulus and not on its velocity alone. This is in agreement with the prediction of the theory of motion detection according to the basic version of the correlation scheme. 4. The temporal frequency for maximal response of individual cells shifts to higher values when the contrast of the sine-wave gratings is reduced to 5%. 5. The steady-state response of 16 of the recorded directional selective cells (53) is modulated with the temporal frequency of the stimulus, regardless of the phase of the grating at the beginning of its movement. 6. In phasic-tonically responding cells, the phasic response peak decays to the steady-state level with a time constant that becomes shorter as the temporal frequency of the stimulus increases. 7. The basic version of the correlation scheme includes only the time constant of one low-pass filter. Therefore the phasic response is expected to decay to the steady-state level with one and the same time constant, and the position of the maximal response on the temporal frequency scale should not be influenced by a change of pattern contrast. According to the model, phase-dependent modulations of the steady-state response should occur only when the spatial wavelength of the stimulus pattern is large compared with the sampling base of the underlying detector. Consequently the results given in points 4-6 cannot be described by a basic version of the correlation scheme.

Acceleration↗

PubMed1985

Interneuronal relations within and between cortical areas during conditioning in cats.

Multiunit activity was chronically recorded in auditory, visual and motor cortex during classical conditioning and extinction. A statistical method for cross-correlation analysis of interneuronal relations was used. Plastic changes in cortical integrative function had spatial features and depended upon types of conditioning. Results suggest the spatial features in cortical cell integration in the trained-brain consisting of 3 categories of neuronal system organizations: microset, local and inter-regional. The first two are localized in a cortical projection region, whereas the third occupies cortical regions activated during conditioning. An evaluation of systemic activity in cortical neurons suggests specific organizations of cortical integrative functions in learning.

Animals↗

PubMed2000

Outbreeding depression varies among cohorts of Ipomopsis aggregata planted in nature.

Outbreeding depression in progeny fitness may arise from disruption of local adaptation, disruption of allelic coadaptation, or a combination of these "environmental" and "physiological" mechanisms. Thus the minimum spatial scale over which outbreeding depression arises should depend on the spatial scale of gene dispersal and (with an environmental mechanism) of change in selection regimes. We previously reported substantial outbreeding depression in lifetime fitness of progeny resulting from crosses among parents separated by 100 m in natural populations of the herbaceous plant Ipomopsis aggregata. In this paper we explore the effect of crossing distance on fitness in two additional experiments begun in 1987 and 1990. We planted seed progeny derived from partial diallel crossing designs in randomized blocks in maternal environments and scored emergence of seedlings, survival, and eventual flowering of individuals over the subsequent six to eight years. Nested within each diallel design were crossing distances of 1 m, 10 m, and 100 m. Compared to 1-m and 10-m progeny, 100-m progeny of the 1987 diallel suffered a significant reduction in seedling emergence, and both 1-m and 100-m progeny that survived to flower achieved lower lambda-values on average than 10-m progeny. Total outbreeding depression suffered by 100-m relative to 10-m progeny was approximately 10%, compared to approximately 30% in our earlier study of I. aggregata. Progeny of 10-m crosses also outperformed 1-m and 100-m progeny of the 1990 diallel by approximately 5%, but no difference among crossing distance treatments was significant. Thus, the magnitude of outbreeding depression in 100-m crosses varied among experiments. This is not surprising given likely spatial and temporal variation in gene flow and selection regimes, different population histories, and different parental and progeny environments. Characterizing outbreeding depression on the shortest spatial scales over which it is expressed, as well as its variation and causes, is worthwhile because it promises to shed light on the earliest stages of angiosperm speciation.

Breeding↗

PubMed1998

The precision of velocity discrimination across spatial frequency.

The precision of velocity coding for moving stimuli of different spatial frequencies was assessed by measuring velocity discrimination thresholds for a 1-c/deg grating paired with a grating whose spatial frequency ranged from 0.25 to 4 c/deg and for grating pairs of the same spatial frequency (0.25, 1, and 4 c/deg). The gratings always moved upward, with velocities ranging from 0.5 to 16 deg/sec. Velocity discrimination was as precise for stimuli that varied in spatial frequency by +/- 2 octaves (0.25 vs. 1 c/deg and 4 vs. 1 c/deg) as for stimuli of the same spatial frequency, for specific ranges of velocity that depended on the spatial and, therefore, the temporal frequencies of the stimuli. Compared with a 1-c/deg grating, the perceived velocity of 4-c/deg gratings was about 1.3 times faster and that of 0.25-c/deg gratings was about 1.3 times slower. Although these perceived velocity biases imply variation of velocity-signal processing among spatial frequency channels, the discrimination results indicate that the motion-sensing system can compare signals across different spatial frequency channels to make fine velocity discrimination within appropriate temporal frequency limits.

Acceleration↗

PubMed2002

Task-dependent differences in the exploratory behaviour of patients with spatial neglect.

The present study analysed task-dependent effects on the exploratory behaviour of neglect patients during their spontaneous search of the surroundings. We were asking whether different tasks would be associated with different structuring of the visual display and, therefore, would result in different forms of neglect in one and the same brain-damaged subjects. Neglect patients' eye and head movements were recorded when they searched for a target within a homogeneous stimulus array surrounding the subjects. Subsequently, they explored the same array which was now segmented into different areas. When the patients' attention was allocated to the whole surrounding space, all patients completely neglected the left hemispace and spontaneously attended to the right hemispace. No significant left-right asymmetry was detected in a selected segment located in the periphery of the attended, right hemispace. However, all patients completely ignored the left part of this segment when they had to concentrate visual search on this segment alone. The results suggest an important influence of task-dependent effects on the exploratory behaviour of neglect patients. They show that one and the same physical stimulus at one and the same location in a scene might be attended or, in another situation, neglected, just depending on the behavioural goal of the subject. The findings support the idea that the brain organises and reorganises continuously the representation of the same physical input according to the changing task requirements.

Aged↗

PubMed2004

An experimental characterization of elastographic spatial resolution: analysis of the trade-offs between spatial resolution and contrast-to-noise ratio.

An experimental study of the spatial resolution in elastography was conducted. Models that involved two cylindrical inclusions arranged as a wedge were used to characterize the axial and lateral resolution of the axial strain elastograms. A study of the dependence of the spatial resolution on several factors such as the algorithmic parameters, the applied strain and the modulus contrast was performed. The axial resolution was found to show a linear dependence with respect to the algorithmic parameters, namely the window length and the window shift used for strain estimation. The lateral resolution showed a weak dependence on the algorithmic parameters. A weak dependence of the spatial resolution on factors such as the modulus contrast and the applied strain was found. The trade-offs between the spatial resolution and the elastographic contrast-to-noise ratio (CNR(e)) were then analyzed. A nonlinear trade-off between the CNR(e) and the axial and lateral resolution was shown for conventional strain estimation techniques, with the CNR(e) improving at a more than linear rate with respect to a linear degradation in the resolution. This study provided an experimental framework for characterizing the spatial resolution in elastography and facilitating a comparison of the CNR(e) with spatial resolution.

Elasticity↗

PubMed2001

Experience-dependent gene expression in the rat hippocampus after spatial learning: a comparison of the immediate-early genes Arc, c-fos, and zif268.

Neuronal immediate-early gene (IEG) expression is regulated by synaptic activity and plays an important role in the neuroplastic mechanisms critical to memory consolidation. IEGs can be divided into two functional classes: (1) regulatory transcription factors (RTFs), which can broadly influence cell function depending on the "downstream" genes they regulate, and (2) "effector" proteins, which may directly modulate specific cellular functions. The objective of the current study was to determine whether the expression of an effector IEG (Arc) was similar to, or different from, that of two well characterized RTF IEGs (c-fos and zif268) after learning. IEG RNA levels from rats trained in spatial and nonspatial water tasks were determined using RNase protection assays and in situ hybridization. Overall, the regulation of the three IEGs was similar in the hippocampus and the entorhinal and primary visual cortices. Consequently, IEG RNA levels were positively correlated within a structure. By contrast, Arc and zif268 RNA levels were not correlated or only weakly correlated across structures, although c-fos RNA levels were moderately correlated across structures. Arc RNA expression differed from that of zif268 and c-fos in two regards: (1) hippocampal Arc RNA levels were correlated with learning of the hippocampal-dependent spatial, but not hippocampal-independent cued response, water task, and (2) Arc RNA levels in the hippocampus and entorhinal cortex increased after spatial reversal learning relative to an asymptotic performance group. Thus, although the expression of Arc, zif268, and c-fos exhibited many similarities, Arc was most responsive to differences in behavioral task demands.

Animals↗

PubMed2003

Anaphor resolution as a function of spatial distance and priming: exploring the spatial distance effect in situation models.

Anaphor resolution has been found to depend on the spatial distance between the reader's focus of attention and the location of the anaphor referent in a spatially organized situation model (spatial distance effect; Rinck & Bower, 1995). This effect implies that a) the situation model is spatially organized and b) spatial distance has a stronger effect on the resolution of anaphoric reference than the text priming the anaphor referent. In three experiments, adult participants read 12 short narratives about protagonists moving around a building. Mentionning the location of the anaphor referent in text prior to the anaphoric sentence facilitated anaphor resolution. Decreased spatial distance consistently facilitated anaphor resolution, even when priming the anaphor referent affected anaphor resolution more strongly than spatial distance. Results are discussed with regard to the interpretation and reliability of the spatial distance effect and the interaction of different representational levels in the context of multi-level theories of text comprehension.

Adult↗

PubMed1993

The role of attention for the Simon effect.

It has been claimed that spatial attention plays a decisive role in the effect of irrelevant spatial stimulus-response correspondence (i.e., the Simon effect), especially the way the attentional focus is moved onto the stimulus (lateral shifting rather than zooming). This attentional-movement hypothesis is contrasted with a referential-coding hypothesis, according to which spatial stimulus coding depends on the availability of frames or objects of reference rather than on certain attentional movements. In six experiments, reference objects were made available to aid spatial coding, which either appeared simultaneously with the stimulus (Experiments 1-3), or were continuously visible (Experiments 4-6). In contrast to previous experiments and to the attentional predictions, the Simon effect occurred even though the stimuli were precued by large frames surrounding both possible stimulus positions (Experiment 1), even when the reference object's salience was markedly reduced (Experiment 2), or when the precueing frames were made more informative (Experiment 3). Furthermore, it was found that the Simon effect is not reduced by spatial correspondence between an uninformative spatial precue and the stimulus (Experiment 4), and it does not depend on the location of spatial precues appearing to the left or right of both possible stimulus locations (Experiment 5). This was true even when the precue was made task-relevant in order to ensure attentional focusing (Experiment 6). In sum, it is shown that the Simon effect does not depend on the kind of attentional operation presumably performed to focus onto the stimulus. It is argued that the available data are consistent with a coding approach to the Simon effect which, however, needs to be developed to be more precise as to the conditions for spatial stimulus coding.

Adult↗

PubMed1999

Concomitant gradient field effects in spiral scans.

Maxwell's equations imply that imaging gradients are accompanied by higher order spatially varying fields (concomitant fields) that can cause artifacts in MR imaging. The lowest order concomitant fields depend quadratically on the imaging gradient amplitude and inversely on the static field strength. Time-varying concomitant fields that accompany the readout gradients of spiral scans cause unwanted phase accumulation during the readout, resulting in spatially dependent blurring. Concomitant field phase errors are independent of echo time and, therefore, cannot be detected using Dixon-type field map measurements that are normally used to deblur spiral scan images. Data acquisition methods that reduce concomitant field blurring increase off-resonant spin blurring, and vice versa. Blurring caused by concomitant fields can be removed by variations of image reconstruction methods developed to correct for spatially dependent resonance offsets with nonrectangular k-space trajectories.

Artifacts↗

PubMed2004

Spatial conditional associative learning: effects of thalamo-hippocampal disconnection in rats.

Unilateral lesions to the anterior thalamic nuclei (ATN) and the hippocampus (H) were made in opposite hemispheres in the rat to examine whether these brain structures form part of a functional neural pathway underlying spatial learning and memory. In the first experiment, rats were tested on a spatial-visual conditional associative task in which they had to learn to approach one of two stimuli depending on the spatial context in which the stimuli were embedded. The rats were subsequently trained on delayed forced alternation, a spatial working memory task known to be sensitive to the effects of ATNxH damage. Rats with ATNxH lesions were impaired in the acquisition of both tasks in comparison with normal control animals. The findings support the idea that the anterior thalamic nuclei and the hippocampus are critical components of an anatomical system subserving spatial memory and suggest that these brain regions work in a dependent fashion during the performance of certain spatial learning tasks.

Analysis of Variance↗