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PubMedDate not supplied

Visual acuity deficits following neonatal lead exposure: cholinergic interactions.

Clinical reports describe long-term alterations in the spatial resolution properties of the visual system following developmental lead (Pb) exposure. To determine if the spatial resolution limit, visual acuity, of the adult rat was altered following low-level, neonatal Pb exposure psychophysical studies were conducted. To examine possible mechanisms of action (and an additional stressor) the effects of scopolamine were studied in the psychophysical procedure. [3H]-QNB binding was analyzed in retina, superior colliculus, lateral geniculate nucleus and visual cortex. Neonatal hooded rats were exposed to Pb (days 0-21) via the milk of dams drinking 0.2% Pb acetate. Visual acuity was 1.8 cycles/degree in controls compared to 1.3 cycles/degree in the Pb group. Scopolamine caused dose-dependent decreases in spatial resolution with decreases at higher spatial frequencies being greater in the Pb group. Positive controls revealed the effects to be centrally mediated. A decrease in the [3H]-QNB binding was found only in the visual cortex. The results suggest that long-term effects of developmental Pb exposure may be directly on the visual cortex.

Animals↗

PubMed1999

Theoretical limits of spatial resolution in elliptical-centric contrast-enhanced 3D-MRA.

The point spread function (PSF) for contrast-enhanced three-dimensional (3D) MR angiography using the elliptical centric view order is derived. This view order has been shown previously to provide high venous suppression thereby enabling long acquisition times capable of high spatial resolution. The dependence of the PSF on TR, field of view (FOV), scan time, and trigger time are shown explicitly. Theoretical predictions are corroborated with experimental results in phantoms and in vivo. The PSF width decreases as the square root of the product of TR and the two phase encoding FOV's for fixed nominal voxel size. The PSF peak amplitude increases as the reciprocal of this product. Theory and experiment demonstrate that acquisition times over 40 sec provide superior resolution compared to shorter acquisitions, despite falling levels of contrast agent concentration. The analysis predicts that an isotropic spatial resolution of 1 mm before zero filling is possible in a FOV large enough to encompass the carotid and vertebral arteries bilaterally. Magn Reson Med 42:1106-1116, 1999.

Carotid Arteries↗

PubMed1995

Hippocampal volume and food-storing behavior are related in parids.

The size of the hippocampus has been previously shown to reflect species differences and sex differences in reliance on spatial memory to locate ecologically important resources, such as food and mates. Black-capped chickadees (Parus atricapillus) cached more food than did either Mexican chickadees (P. sclateri) or bridled titmice (P. wollweberi) in two tests of food storing, one conducted in an aviary and another in smaller home cages. Black-capped chickadees were also found to have a larger hippocampus, relative to the size of the telencephalon, than the other two species. Differences in the frequency of food storing behavior among the three species have probably produced differences in the use of hippocampus-dependent memory and spatial information processing to recover stored food, resulting in graded selection for size of the hippocampus.

Animals↗

PubMed1999

The lens paradigm in experimental myopia: oculomotor, optical and neurophysiological considerations.

The lens-rearing paradigm has developed great importance in the field of experimental myopia. Although an apparently simple paradigm, the results of any experiment can be influenced by a variety of factors including habitual viewing distance, ocular refraction, oculomotor performance and the spatial sensitivity of the retinal elements involved in retinal image assessment. Computer modelling has been used to evaluate the expected impact when lenses are placed in front of a primate eye as a function of the above parameters. Spatial band-pass responses of the mechanisms responsible for emmetropization are predicted to lead to a limited range of retinal defocus over which compensation to lenses could occur. Even assuming an equal ability to detect hyperopic and myopic defocus, it is predicted that primate eyes should be able to compensate for a much larger range of minus lenses than plus lenses. This derives from the ability of the accommodation system to keep retinal defocus within this operating range over a wider range of minus lenses than plus lenses. The range over which compensation can occur will depend on the spatial tuning of the elements responsible for detecting retinal defocus and capabilities of the accommodation system. The observed asymmetry in responses of the primate eye to rearing with plus and minus lenses and observed differences between primates and chickens in lens rearing studies may therefore be partly attributable to optical and neurophysiological considerations.

Accommodation, Ocular↗

PubMed2004

Fully Bayesian spatio-temporal modeling of FMRI data.

We present a fully Bayesian approach to modeling in functional magnetic resonance imaging (FMRI), incorporating spatio-temporal noise modeling and haemodynamic response function (HRF) modeling. A fully Bayesian approach allows for the uncertainties in the noise and signal modeling to be incorporated together to provide full posterior distributions of the HRF parameters. The noise modeling is achieved via a nonseparable space-time vector autoregressive process. Previous FMRI noise models have either been purely temporal, separable or modeling deterministic trends. The specific form of the noise process is determined using model selection techniques. Notably, this results in the need for a spatially nonstationary and temporally stationary spatial component. Within the same full model, we also investigate the variation of the HRF in different areas of the activation, and for different experimental stimuli. We propose a novel HRF model made up of half-cosines, which allows distinct combinations of parameters to represent characteristics of interest. In addition, to adaptively avoid over-fitting we propose the use of automatic relevance determination priors to force certain parameters in the model to zero with high precision if there is no evidence to support them in the data. We apply the model to three datasets and observe matter-type dependence of the spatial and temporal noise, and a negative correlation between activation height and HRF time to main peak (although we suggest that this apparent correlation may be due to a number of different effects).

Bayes Theorem↗

PubMed1989

Binocular interactions in the cat's dorsal lateral geniculate nucleus. I. Spatial-frequency analysis of responses of X, Y, and W cells to nondominant-eye stimulation.

1. X, Y, and W cells in the A and C layers of the cat's dorsal lateral geniculate nucleus (LGN) were tested for responses to stimulation of the nondominant eye. The main purpose was to determine the incidence of nondominant-eye excitation and inhibition among different classes of cells and to examine the spatial-frequency tuning of responses to the nondominant eye. 2. Of 198 cells that were tested with drifting sine-wave gratings presented to the nondominant eye, 109 (55%) showed statistically significant responses. Four types of responses were observed: an increase in the mean discharge rate (F0 excitation), a decrease in the mean discharge rate (F0 inhibition), an increased modulation at the fundamental frequency of the grating (F1 excitation), and a decreased modulation at the fundamental frequency of the grating (F1 inhibition). Overall, 29% of the cells responded with inhibition, 24% responded with excitation, and 2% showed both excitation and inhibition, depending upon the spatial frequency and/or the harmonic response component. The relative incidence of excitation and inhibition was similar for X, Y, and W cells, for cells with on-center and off-center receptive fields, for cells with different receptive-field eccentricities, and for cells in each LGN layer. In addition, within layers A and A1, responses were similar for cells at different distances from the laminar borders. 3. Spatial-frequency response functions indicated that cells could have band-pass or low-pass spatial-frequency tuning through the nondominant eye. Band-pass cells tended to be narrowly tuned (less than or equal to 1 octave), and low-pass cells responded to a broader range of spatial frequencies. These properties were similar for X, Y, and W cells. Spatial resolution tended to be low (less than or equal to 0.8 c/deg for most cells), although a few cells responded to the highest spatial frequency tested (5.4 c/deg). Likewise, optimal spatial frequency was low (less than or equal to 0.2 c/deg) for most cells. These properties were similar for X and Y cells, and there was a weak tendency for X and Y cells to have higher optimal spatial frequencies and spatial resolutions than W cells. 4. In terms of absolute change in activity, responses to drifting gratings were weak. However, cells that were inhibited generally showed 20-60% decreases in activity to the optimal spatial frequency, and cells that were excited generally showed 40-100% increases. Response amplitudes were similar for X, Y, and W cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed2000

Plasticity in the neural coding of auditory space in the mammalian brain.

Sound localization relies on the neural processing of monaural and binaural spatial cues that arise from the way sounds interact with the head and external ears. Neurophysiological studies of animals raised with abnormal sensory inputs show that the map of auditory space in the superior colliculus is shaped during development by both auditory and visual experience. An example of this plasticity is provided by monaural occlusion during infancy, which leads to compensatory changes in auditory spatial tuning that tend to preserve the alignment between the neural representations of visual and auditory space. Adaptive changes also take place in sound localization behavior, as demonstrated by the fact that ferrets raised and tested with one ear plugged learn to localize as accurately as control animals. In both cases, these adjustments may involve greater use of monaural spectral cues provided by the other ear. Although plasticity in the auditory space map seems to be restricted to development, adult ferrets show some recovery of sound localization behavior after long-term monaural occlusion. The capacity for behavioral adaptation is, however, task dependent, because auditory spatial acuity and binaural unmasking (a measure of the spatial contribution to the "cocktail party effect") are permanently impaired by chronically plugging one ear, both in infancy but especially in adulthood. Experience-induced plasticity allows the neural circuitry underlying sound localization to be customized to individual characteristics, such as the size and shape of the head and ears, and to compensate for natural conductive hearing losses, including those associated with middle ear disease in infancy.

Adaptation, Physiological↗

PubMedDate not supplied

A role for ERK MAP kinase in physiologic temporal integration in hippocampal area CA1.

Recent studies demonstrate a requirement for the Extracellular signal Regulated Kinase (ERK) mitogen-activated protein kinase (MAPK) cascade in both the induction of long-lasting forms of hippocampal synaptic plasticity and in hippocampus-dependent associative and spatial learning. In the present studies, we investigated mechanisms by which ERK might contribute to synaptic plasticity at Schaffer collateral synapses in hippocampal slices. We found that long-term potentiation (LTP) induced with a pair of 100-Hz tetani does not require ERK activation in mice whereas it does in rats. However, in mice, inhibition of ERK activation blocked LTP induced by two LTP induction paradigms that mimicked the endogenous theta rhythm. In an additional series of studies, we found that mice specifically deficient in the ERK1 isoform of MAPK showed no impairments in tests of hippocampal physiology. To investigate ERK-dependent mechanisms operating during LTP-inducing stimulation paradigms, we monitored spike production in the cell body layer of the hippocampus during the period of theta-like LTP-inducing stimulation. Theta-burst stimulation (TBS) produced a significant amount of postsynaptic spiking, and the likelihood of spike production increased progressively over the course of the three trains of TBS independent of any apparent increase in Excitatory Post-Synaptic Potential (EPSP) magnitude. Inhibition of ERK activation dampened this TBS-associated increase in spiking. These data indicate that, for specific patterns of stimulation, ERK may function in the regulation of neuronal excitability in hippocampal area CA1. Overall, our data indicate that the progressive increase in spiking observed during TBS represents a form of physiologic temporal integration that is dependent on ERK MAPK activity.

Action Potentials↗

PubMed1980

Peripheral neural representation of spatial dimensions of a textured surface moving across the monkey's finger pad.

1. The responses of single cutaneous mechanoreceptive fibres evoked by a geometrically patterned surface sweeping across the skin were examined. The surfaces used were each a fine diamond-shaped array of "dots' in relief against a flat background. The fibres examined were rapidly adapting, slowly adapting and Pacinian fibres innervating the monkey's finger pad skin. 2. Each fibre's response to the two-dimensioned surface was assessed using a procedure in which the surface was swept across the fibre's receptive field many times, the position of the surface relative to the underlying receptive field being precisely known at any instant in time. Between each successive sweep the stimulus surface was shifted lateral to the direction of movement by a small increment. The response pattern generated by this scanning procedure specified the extent to which a single response of the fibre was dependent on the spatial and temporal dimensions of the stimulus. 3. The invariant spatial features of the moving surface were represented only in the responses of populations of fibres; responses of the constituent fibres always confounded the information relayed about the surface pattern and its rate of movement across the skin. 4. Factors determining the representation of the spatial and temporal characteristics of the moving surface in the responding mechanoreceptive fibre population were examined. These included the response characteristics of the constituent fibres, the innervation density, and the total number of fibres engaged by the moving surface.

Action Potentials↗

PubMed2002

Spatial learning and synaptic hippocampal plasticity in type 2 somatostatin receptor knock-out mice.

Somatostatin is implicated in a number of physiological functions in the CNS. These effects are elicited through the activation of at least five receptor subtypes. Among them, sst2 receptors appear the most widely expressed in the cortex and hippocampal region. However, the specific role of this somatostatin receptor subtype in these regions is largely undetermined. In this study, we investigated the role of the sst2 receptor in the hippocampus using mice invalidated for the sst2 gene (sst2 KO mice). Complementary experimental approaches were used. First, mice were tested in behavioral tests to explore the consequences of the gene deletion on learning and memory. Spatial discrimination learning in the radial maze was facilitated in sst2 KO mice, while operant learning of a bar-pressing task was slightly altered. Mice were then processed for electrophysiological study using the ex vivo hippocampal slice preparation. Extracellular recordings in the CA1 area showed an enhancement in glutamatergic (AMPA and NMDA) responses in sst2 KO mice which displayed an increase in the magnitude of the short-term potentiation and long-term depression. In contrast, long-term potentiation was not significantly altered. Taken together, these data demonstrate that somatostatin, acting via sst2 hippocampal receptors, may contribute to a global decrease in glutamate efficiency and consequently alter glutamate-dependent plasticity and spatial learning.

Animals↗

PubMed2001

Bud8p and Bud9p, proteins that may mark the sites for bipolar budding in yeast.

The bipolar budding pattern of a/alpha Saccharomyces cerevisiae cells appears to depend on persistent spatial markers in the cell cortex at the two poles of the cell. Previous analysis of mutants with specific defects in bipolar budding identified BUD8 and BUD9 as potentially encoding components of the markers at the poles distal and proximal to the birth scar, respectively. Further genetic analysis reported here supports this hypothesis. Mutants deleted for BUD8 or BUD9 grow normally but bud exclusively from the proximal and distal poles, respectively, and the double-mutant phenotype suggests that the bipolar budding pathway has been totally disabled. Moreover, overexpression of these genes can cause either an increased bias for budding at the distal (BUD8) or proximal (BUD9) pole or a randomization of bud position, depending on the level of expression. The structures and localizations of Bud8p and Bud9p are also consistent with their postulated roles as cortical markers. Both proteins appear to be integral membrane proteins of the plasma membrane, and they have very similar overall structures, with long N-terminal domains that are both N- and O-glycosylated followed by a pair of putative transmembrane domains surrounding a short hydrophilic domain that is presumably cytoplasmic. The putative transmembrane and cytoplasmic domains of the two proteins are very similar in sequence. When Bud8p and Bud9p were localized by immunofluorescence and tagging with GFP, each protein was found predominantly in the expected location, with Bud8p at presumptive bud sites, bud tips, and the distal poles of daughter cells and Bud9p at the necks of large-budded cells and the proximal poles of daughter cells. Bud8p localized approximately normally in several mutants in which daughter cells are competent to form their first buds at the distal pole, but it was not detected in a bni1 mutant, in which such distal-pole budding is lost. Surprisingly, Bud8p localization to the presumptive bud site and bud tip also depends on actin but is independent of the septins.

Actins↗

PubMed1992

Asymmetry in visual evoked potentials to gratings registered in the two hemispheres of the human brain.

The study aimed at testing, by a visual evoked potential method, the hypothesis of the hemispheric specialization in processing of high and low-spatial frequencies. Twenty four right-handed subjects (12 males and 12 females) were presented with square-wave vertical gratings of various spatial frequencies (0.67, 0.86, 1.20, 2.00, 2.40, 3.00, 3.30, 6.00 and 7.50 c/deg). Gratings were presented in nine separate blocks each containing 64 exposures. Time of exposure was 30 ms and the interstimulus interval varied from 2 to 3.5 s. VEPs were recorded with electrodes located at O1 and O2 and referred to Cz according to the 10/20 system. Amplitudes and latencies of two VEPs components (N130-150 and P200-240) were analyzed. The results showed larger amplitudes of VEPs registered in the right hemisphere of both males and females. This difference, however, was apparent in the earlier component of VEPs in females and in the later component in males. The observed hemispheric asymmetry did not depend on the spatial frequency of grating. Females demonstrated longer latencies than males for both N and P components. Our data suggest that the right hemisphere predominates in processing grating stimuli, but the dynamics of this process differ in the two sexes. The results do not support Sergent's hypothesis which postulate the right hemisphere specialization for low spatial frequencies and the left hemisphere specialization for high spatial frequencies.

Adult↗

PubMed2003

Addition of organometallic compounds to tin-containing cyclic ketones. Remote stereocontrol induced by the stannyl group.

The addition of organometallic reagents to cyclic ketones bearing stannyl groups at an appropriate distance to the carbonyl group occurs with a high level of stereocontrol, giving alcohols resulting from attack of the nucleophile syn to the tin center. This remarkable remote control is a consequence of the anchoring of the organometallic reagent by the tin and carbonyl groups. The degree of selectivity observed depends on the spatial distance between the carbonyl group and the tin center. (Z)-beta-Stannylvinyl ketones (Sn/CO separation: 5 bonds) react with organolithium reagents, showing a high degree of stereocontrol. On the contrary, the analogous ketones with E stereochemistry do not show selectivity at all. In the case of beta-stannyl ketones (Sn/CO separation: 3 bonds), the long distance between the tin center and the carbonyl group does not favor selective addition except when allyllithium derivatives are used. A chelation-controlled pathway assisted by the three-carbon chain of the allyl anion, which compensates the distance between tin and carbonyl groups, has been proposed. The selectivity found for ketones 34-36 (Sn/CO separation: 4 bonds) depends on their structure and varies with the hybridization of the carbon atom linked to the trialkyltin group. Deuterium labeling experiments as well as ab initio molecular-orbital analysis support the mechanistic hypothesis of an intramolecular delivery. Grignard reagents are less selective than organolithium compounds.

Journal Article↗

PubMed2006

The effects of plant distribution and frugivore density on the scale and shape of dispersal kernels.

For many plant species, seed dispersal is one of the most important spatial demographic processes. We used a diffusion approximation and a spatially explicit simulation model to explore the mechanisms generating seed dispersal kernels for plants dispersed by frugivores. The simulation model combined simple movement and foraging rules with seed gut passage time, plant distribution, and fruit production. A simulation experiment using plant spatial aggregation and frugivore density as factors showed that seed dispersal scale was largely determined by the degree of plant aggregation, whereas kernel shape was mostly dominated by frugivore density. Kernel shapes ranged from fat tailed to thin tailed, but most shapes were between an exponential and that of the solution of a diffusion equation. The proportion of dispersal kernels with fat tails was highest for landscapes with clumped plant distributions and increased with increasing number of dispersers. The diffusion model provides a basis for models including more behavioral details but can also be used to approximate dispersal kernels once a diffusion rate is estimated from animal movement data. Our results suggest that important characteristics of dispersal kernels will depend on the spatial pattern of plant distribution and on disperser density when frugivores mediate seed dispersal.

Animals↗

PubMed2003

Simulation analysis of the ability to estimate motor unit propagation velocity non-invasively by different two-channel methods and types of multi-electrodes.

Ability to estimate motor unit propagation velocity correctly using different two-channel methods for delay estimation and different non-invasive spatial filters was analysed by simulation. It was established that longitudinal double difference electrodes could be not a better choice than simple bipolar parallel electrodes. Spatial filtration with a new multi-electrode (performing difference between signals detected by two transversal double difference electrodes positioned along the muscle fibres) promises to give the best estimate. Delay estimation between reference points is more preferable than that based on the cross-correlation technique, which is considerably sensitive to the fundamental properties of the muscle fibre extracellular fields. Preliminary averaging and approximation of the appropriate parts of the signals around chosen reference points could reduce the larger noise sensitivity and the effects of local tissue inhomogeneities as well as eliminate the sampling problem. A correct estimate of the propagation velocity could be impossible, even in the case of not very deep motor units (15 or 10 mm, depending on the spatial filter used) with relatively long (about 120 mm) muscle fibres. In the case of fibres with asymmetrical location of the end-plates in respect to the fibre ends, the propagation velocity estimates could be additionally biased above the longer semilength of the motor unit fibres.

Action Potentials↗

PubMed2002

Selective attention regulates spatial and intensity information processing in the human primary somatosensory cortex.

Attention-related cognitive processes in the primary somatosensory cortex (SI) were studied by measuring somatosensory evoked magnetic fields (SEFs). Twenty-one normal adult human subjects participated in this study for investigating effects of attention and stimulus intensity on cortical finger representation in the SI cortex. Electric stimuli at low and high intensity were delivered to the index or middle finger in finger discrimination and non-discrimination task. For the low intensity stimulation at 1.25 times sensory threshold, an early component (M50) showed clear segregation of the sources for the two fingers and an increase of the amplitude specific to the finger discrimination task. Such an attentional effect on the SI cortex was masked by the high intensity stimulation (2.5 times sensory threshold); the M50 source separation by the fingers was induced irrespective of the discrimination or non-discrimination task. The results suggest that a conscious regulation of stimulus intensity coding in the SI cortex underlies the attention-dependent enhancement of spatial finger information processing.

Adult↗

PubMedDate not supplied

Long-range interactions modulate the contrast gain in the lateral geniculate nucleus of cats.

In previous work, we have shown that sudden image displacements well outside the classical receptive field modulate the visual sensitivity of LGN relay cells. Here we report the effect of image displacements on the response versus contrast function. The stimuli consisted of a central spot of optimal size and polarity (contrast range: 3-98%), flashed alone or in the presence of a peripheral annulus (radii: 5-15 deg) containing a low spatial-frequency grating displaced at saccade-like velocities (shift). The most consistent effect of the shift on the response to a central spot was to reduce the responsiveness of Y relay cells and, to a lesser extent, of X relay cells. The reduction in responsiveness was primarily a divisive rather than a subtractive effect and could be modelled by assuming that a greater contrast was required to produce a given excitatory response. In the absence of a central spot, remote motion had inhibitory effects on the firing rates of the majority of relay cells, but its effect on retinal ganglion cells was mainly excitatory. When the shifting grating covered the classical receptive field and its periphery, facilitatory effects or suppressive effects, depending on the spatial phase of the pattern, were observed in both retinal and geniculate cells. Remote motion strongly suppresses the responsiveness of relay cells to stimuli within the classical receptive field. This suppressive effect involves intrageniculate processing and is primarily associated with a reduction in contrast gain. It is likely that shift suppression contributes to the loss of visual sensitivity observed in saccadic suppression.

Animals↗

PubMed1998

Contrast-modulation flicker: dynamics and spatial resolution of the light adaptation process.

We report a perceptual phenomenon that originates from a nonlinear operation during the visual process, and we use these observations to study the functional organization of the responsible nonlinearity; the regulation of visual sensitivity to light. When the contrast of a high frequency grating was modulated while its spatial and temporal average luminance was kept constant, observers saw brightness changes or desaturation in the field. If the contrast was modulated periodically between zero and a peak value, observers saw vivid flicker (contrast-modulation flicker), and this flicker could be seen even when the grating was too fine to be visually resolved as a pattern. This uniform-field flicker can be nulled by a modulation of space-average luminance at the contrast-modulation frequency, with appropriate phase and modulation depth. Contrast-modulation flicker is still measurable with gratings at 100 cycles/deg. The dynamics of contrast-modulation flicker suggest that it results from an early sensitivity-controlling mechanism, acting very rapidly (within about 20 msec). Its dependence on stimulus spatial frequency implies a strictly local luminance nonlinearity, one that either resides within individual photoreceptors or operates on signals from individual receptors.

Adaptation, Ocular↗