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PubMed2006

Learning cross-modal spatial transformations through spike timing-dependent plasticity.

A common problem in tasks involving the integration of spatial information from multiple senses, or in sensorimotor coordination, is that different modalities represent space in different frames of reference. Coordinate transformations between different reference frames are therefore required. One way to achieve this relies on the encoding of spatial information with population codes. The set of network responses to stimuli in different locations (tuning curves) constitutes a set of basis functions that can be combined linearly through weighted synaptic connections to approximate nonlinear transformations of the input variables. The question then arises: how is the appropriate synaptic connectivity obtained? Here we show that a network of spiking neurons can learn the coordinate transformation from one frame of reference to another, with connectivity that develops continuously in an unsupervised manner, based only on the correlations available in the environment and with a biologically realistic plasticity mechanism (spike timing-dependent plasticity).

Action Potentials↗

PubMed2002

Dynamic modification of cortical orientation tuning mediated by recurrent connections.

Receptive field properties of visual cortical neurons depend on the spatiotemporal context within which the stimuli are presented. We have examined the temporal context dependence of cortical orientation tuning using dynamic visual stimuli with rapidly changing orientations. We found that tuning to the orientation of the test stimulus depended on a briefly presented preceding stimulus, with the preferred orientation shifting away from the preceding orientation. Analyses of the spatial-phase dependence of the shift showed that the effect cannot be explained by purely feedforward mechanisms, but can be accounted for by activity-dependent changes in the recurrent interactions between different orientation columns. Thus, short-term plasticity of the intracortical circuit can mediate dynamic modification of orientation tuning, which may be important for efficient visual coding.

Animals↗

PubMed1998

Motion detection on flashed, stationary pedestal gratings: evidence for an opponent-motion mechanism.

Contrast thresholds were measured for discriminating left vs right motion of a vertical, 1 c/deg luminance grating lasting for one cycle of motion. This test was presented on a 1 c/deg stationary grating (pedestal) of twice-threshold, flashed for the duration of the test motion. Lu and Sperling [(1995). Vision Research, 35, 2697-2722] argue that the visual system detects the underlying, first-order motion of the test and is immune to the presence of the stationary pedestal (and the 'feature wobble' which it induces). On the contrary, we observe that the stationary pedestal has large effects on motion detection at 7 and 15 Hz, and smaller effects at 0.9-3.7 Hz, evidenced by a spatial phase dependency between the stationary pedestal and moving test. At 15 Hz the motion threshold drops as much as five-fold, with the stationary pedestal in the optimal spatial phase (i.e., pedestal and test spatially in phase at middle of motion), and the perceived direction of the test motion reverses with the pedestal in the opposite phase. Phase dependency was also explored using a very brief (approximately 1 msec) static pedestal presented with the moving test. The pedestal of Lu and Sperling (flashed for the duration of the test) has a broad spectrum of left and right moving components which interact with the moving test. The pedestal effects can be explained by the visual system's much higher sensitivity to the difference of the contrast of right vs left moving components than to either component alone.

Contrast Sensitivity↗

PubMed2006

Sampling and mapping a soil erosion cover factor by integrating stratification, model updating and cokriging with images.

Cost-efficient sample designs for collection of ground data and accurate mapping of variables are required to monitor natural resources and environmental and ecological systems. In this study, a sample design and mapping method was developed by integrating stratification, model updating, and cokriging with Landsat Thematic Mapper (TM) imagery. This method is based on the spatial autocorrelation of variables and the spatial cross-correlation among them. It can lead to sample designs with variable grid spacing, where sampling distances between plots vary depending on spatial variability of the variables from location to location. This has potential cost-efficiencies in terms of sample design and mapping. This method is also applicable for mapping in the case in which no ground data can be collected in some parts of a study area because of the high cost. The method was validated in a case study in which a ground and vegetation cover factor was sampled and mapped for monitoring soil erosion. The results showed that when the sample obtained with three strata using the developed method was used for sampling and mapping the cover factor, the sampling cost was greatly decreased, although the error of the map was slightly increased compared to that without stratification; that is, the sample cost-efficiency quantified by the product of cost and error was greatly increased. The increase of cost-efficiency was more obvious when the cover factor values of the plots within the no-significant-change stratum were updated by a model developed using the previous observations instead of remeasuring them in the field.

Cost-Benefit Analysis↗

PubMed2005

Order parameter segregation in Ce0.7La0.3B6: 4f octopole and 5d dipole magnetic order.

The intriguing thermophysical properties of CeB6 have been subject to investigation for more than 20 years. In particular, an exotic ground state, phase IV, emerges under doping with La. We report resonant x-ray scattering results on the order parameter symmetries in phase IV of Ce0.7La0.3B6, which condenses below T(IV)=1.5 K. The results reveal a degree of mesoscopic 5d dipole antiferromagnetic order, with propagation vector Q0=(1/2 1/2 1/2), both below and above T(IV). Below T(IV), this polarization coexists with long-range 4f antiferro-octupole (AFO) order also at Q0. The marked differences in temperature dependence and spatial correlation suggest a state of order parameter segregation at low temperature. A simple model of AFO order, consistent with the polarization dependent azimuth symmetries, the Bragg angle, and temperature dependence is given.

Journal Article↗

PubMed2006

Power spectrum and fractal dimension of laser backscattering from the ocean.

We flew an airborne lidar perpendicular to the coastline along straight-line transects that varied in length between 230 and 280 km. The sample spacing was approximately 3 m, so we sampled almost five decades of spatial scales. Except for the return from right at the surface, the power spectra of backscattered power had a power-law dependence on spatial frequency, with a slope of approximately 1.49. This corresponds to a fractal dimension of 1.76. This implies that the distribution is not as patchy as that of a purely turbulent process.

Journal Article↗

PubMed1979

Temporal and intensitive factors in comparative judgments of tactile space.

Temporal imbalances between the successive stimulation of pairs of loci on each forearm altered judgments of the equality of the spatial extents between sets of loci. The spatial judgments became increasingly inaccurate as the temporal imbalance was increased, i.e., the Tau effect. Changes in stimulus intensity also influenced the dependency of spatial judgments on temporal factors. The data suggest that concomitant variations of spatial, temporal, and intensitive components of a vibrotactile stimulus "package" can produce systematic perceptual effects of potential significance in the design of vibrotactile sensory substitution systems.

Electric Stimulation↗

PubMedDate not supplied

[Submicroscopic structure of the membrane apparatus of methanotrophic bakteria].

The fine structure of the membrane apparatus in obligate methanotrophic bacteria, Methylomonas methanica, Methylomonas rubrum 15 and Methylobacter bovis 53b was studied. Complex intracytoplasmic membrane systems of the I type were found in all cultures, but they differed by their structure and spatial organization depending on microorganisms. The cytoplasmic membrane was shown to be involved in the formation of complex membrane structures. A scheme of the spatial organization of the intracytoplasmic membrane systems on the basis of studying serial sections of the methanotrophs is suggested. The formation of the membranex packs is supposed to be a continuous process in Methylomonas methanica and Methylomonas rubrum but a discrete process in Methylobacter bovis.

Cell Membrane↗

PubMed2003

Spatially localized states with size-dependent optical bistability.

Experimental results of a new type of localized state generated in an optical system consisting of a liquid-crystal spatial light modulator with optical feedback are demonstrated. The localized state is triggered at a position that depends on the size of a control beam because its formation depends on the fraction of high-spatial-frequency components in the feedback light that passes through a low-cut spatial-frequency filter. This novel size-dependent switching property is useful for erasing individual target localized states.

Journal Article↗

PubMed1995

Spatial structure of two-locus genotypes under isolation by distance.

Extensive Monte Carlo simulations are conducted of spatial distributions of two-locus genotypes in large, continuous populations under isolation by distance models. The results show that substantial patches of double homozygotes are present in the spatial structures, even when loci are unlinked. The stochastic spread of identical two-locus genotypes largely outpowers the tendency for recombination to decouple patterns for separate loci. A spatial patch is a large area containing mostly one double homozygous genotype in a highly contiguous constellation. This patch structure is reflected in high positive spatial autocorrelations and large excesses of pairs, or joins, of identical double homozygotes at short-to-intermediate distances of spatial separation. Although spatial patches of double homozygotes are the dominant spatial feature, and the major contributors to overall high levels of autocorrelations among two-locus genotypes, other substantial features include areas of concentrations of identical genotypes heterozygous at only one locus. One implication of the patch structure is the presence of high levels of linkage disequilibrium, caused by isolation by distance even for unlinked loci, at some spatial scales; yet the disequilibrium in the large total populations is near 0. Thus linkage disequilibrium produced by isolation by distance is highly dependent on spatial scale. Another implication is that high degrees of spatial structuring and autocorrelations are produced for genetic variation controlling quantitative traits, at least when the number of loci is relatively small, under a wide range of situations, even if the trait is selectively neutral. The significance of the results to field studies is also examined.

Diploidy↗

PubMed1994

A laboratory CT scanner for dynamic imaging.

A high-resolution laboratory CT scanner has been developed for imaging objects undergoing periodic motion. The scanner comprises an x-ray image intensifier, optically coupled to a linear photodiode array. Gated time-evolved projections of a single slice of the moving object are acquired, reformatted, and reconstructed. The resulting series of CT images shows the object at different phases of its motion cycle. The scanner has an adjustable field of view (FOV) and the resolution can be as high as 3.2 mm-1 (for the 40-mm FOV). The spatial resolution depends on the inherent resolution of the scanner and on the object's velocity. For objects moving at 1 cm s-1, the spatial resolution is reduced by 9% in the direction of motion. The signal intensity in the reconstructed image is linear for materials with attenuation coefficients as high as 1.5 cm-1 (for a 90-kVp x-ray beam), with an average accuracy of +/- 0.02 cm-1. The average accuracy of circumference measurements made from the CT images is +/- 0.3 mm. Lastly, an application of this dynamic CT scanner to imaging excised human arterial specimens under simulated physiological pressure conditions is presented as an example.

Aged↗

PubMed2002

Immunocharacterization of Vitis vinifera L. ferredoxin-dependent glutamate synthase, and its spatial and temporal changes during leaf development.

The grapevine (Vitis vinifera L.) partial fragment of cDNA clone pGOGAT1 [Loulakakis and Roubelakis-Angelakis (1997) Physiol Plant 101:220-228], encoding the ferredoxin-dependent glutamate synthase (Fd-GOGAT; EC 1.4.7.1), was overexpressed in Escherichia coli cells. A hybrid between the Fd-GOGAT fragment and maltose-binding protein was purified and used to raise a polyclonal antibody in a rabbit. The prepared antibody appeared to be specific towards Fd-GOGAT; it recognized a protein band of approximately 160 kDa on nitrocellulose blots after SDS-PAGE of total proteins from leaves, internodes, roots and calluses, and precipitated most of the enzyme activity present in grapevine protein extracts. The quantity of Fd-GOGAT protein was substantially higher in leaves than in other grapevine tissues tested, coincident with a similar distribution of the enzyme specific activity. Intracellular localization studies revealed that both the enzyme activity and the 160-kDa immunoreactive protein were associated with the chloroplastic fraction. Furthermore, the accumulation of Fd-GOGAT, glutamine synthetase (GS) and glutamate dehydrogenase (GDH), at the activity and protein levels, was monitored during leaf development of field-grown plants, from the stage of the newly expanding leaf to the senescing old leaf. Both the specific activity and quantity of the 160-kDa polypeptide of Fd-GOGAT were higher in the mature, full sized leaves and substantially lower in young and senescing leaves. GS specific activity and immunoreactive protein followed the same trend as Fd-GOGAT, while GDH showed opposite developmental patterns of accumulation. The biological significance of the presence of Fd-GOGAT in the various grapevine tissues and its physiological role during early development and natural senescence of the leaves are discussed.

Amino Acid Oxidoreductases↗

PubMed1997

Scaling effects in the perception of higher-order spatial correlations.

Human texture discrimination depends both on spatial-frequency content and on higher-order or multi-point correlations. Spatial-frequency discrimination exhibits a high degree of scale invariance over a range of several octaves, but the scaling behavior of sensitivity to higher-order correlation structure is unknown. We explored the scale dependence of texture discrimination for image ensembles which shared the same power spectrum, but differed in their higher-order correlations. Literally scaling the ensembles so that they occupy larger retinal regions results in discrimination performance that is largely independent of scale over a 3 octave range. Holding the display size constant and scaling the texture being sampled within the display over the same range produces performance that varies with scale appreciably. The ideal observer performance is computed, and the absolute efficiency is seen to be quite small, on the order of 10(-2)-10(-1). As the texture is scaled down, increasing the number of checks within the fixed display size, performance increases while the efficiency decreases. These dependencies remain when the stimulus onset asynchrony is increased from 50 to 500 msec. We created sets of textures which varied both in check number and correlation strength, for which ideal observer performance was equated. For the human observers, efficiency was significantly higher for textures with higher correlation strength, but fewer checks. These results are consistent with a model in which a fixed number of checks is processed in a scale-invariant manner, while the remainder of the display is processed much less efficiently.

Humans↗

PubMedDate not supplied

Stimulus dependence of orientation and direction sensitivity of cat LGNd relay cells without cortical inputs: a comparison with area 17 cells.

The cortical contribution to the orientation and direction sensitivity of LGNd relay cells was investigated by recording the responses of relay cells to drifting sinusoidal gratings of varying spatial frequencies, moving bars, and moving spots in cats in which the visual cortex (areas 17, 18, 19, and LS) was ablated. For comparison, the spatial-frequency dependence of orientation and direction tuning of striate cortical cells was investigated employing the same quantitative techniques used to test LGNd cells. There are no significant differences in the orientation and direction tuning to relay cells in the LGNd of normal and decorticate cats. The orientation and direction sensitivities of cortical cells are dependent on stimulus parameters in a fashion qualitatively similar to that of LGNd cells. The differences in the spatial-frequency bandwidths of LGNd cells and cortical cells may explain many of their differences in orientation and direction tuning. Although factors beyond narrowness of spatial-frequency tuning must exist to account for the much stronger orientation and direction preferences of cells in area 17 when compared to LGNd cells, the evidence suggests that the orientation and direction biases present in the afferents to the visual cortex may contribute to the orientation and direction selectivities found in cortical cells.

Animals↗

PubMed1984

The dependence of monocular rivalry on spatial frequency: some interaction variables.

The effect of spatial frequency upon the rate of monocular rivalry has been investigated on several occasions, with conflicting results. With increases in spatial frequency the rivalry rate has been variously reported as declining monotonically, increasing slightly and briefly before declining sharply, or showing a sharp increase before a sharp decrease. Experiments are reported which show that all three functions may be obtained depending on whether the number of cycles of the gratings is held constant across spatial frequency, or the field size is held constant (and, if the latter, what field size is used). The point at which any high-frequency decline occurs is also shown to increase with increasing luminance. All functions obtained correspond to visual sensitivity functions obtained under comparable conditions.

Dominance, Cerebral↗

PubMed1994

Spatial integration in position acuity.

To determine whether and how spatial integration takes place in position acuity, bisection and Vernier thresholds were measured in the fovea of four normal observers with spatially "undersampled" dark lines (i.e. lines comprised of discrete samples). The size, contrast, and density of samples, and the separation of the lines were varied. For a given sampling density, sample size (0.17-2.72 min) has negligible effect on position threshold. For all sample sizes, position threshold decreases as sampling density increases, indicating that spatial integration takes place. The form of spatial integration depends on line separation. At the optimal line separation (2 min for bisection and 0 min for Vernier), position threshold decreases as sampling density increases with a slope of about -0.8 on log axes, steeper than a slope of -0.5 as would be expected from statistical position averaging. This effect of sampling density can be completely explained by spatial contrast summation for visibility. At the 16 min line separation, position threshold also decreases as sampling density increases but with a slope shallower than -0.5. However, this effect of sampling density can not be explained by contrast summation. Position thresholds decrease even after discounting the effect of contrast summation on visibility, suggesting a genuine position averaging. These findings are independent of line orientation (horizontal or vertical), and hold for both random and uniform dot distributions, and for both bisection and Vernier. Thus, two separate mechanisms of position acuity are suggested. A spatial filter mechanism operates at the optimal (or narrow) line separation where position threshold is critically dependent on stimulus visibility. A local sign mechanism operates at the relatively wider line separation where position acuity benefits from local sign position averaging. For both mechanisms, spatial integration is not perfect.

Contrast Sensitivity↗

PubMed1994

Evaluation of the relative roles of slowly and rapidly adapting afferent fibers in roughness perception.

Previous studies of the neural mechanisms of roughness perception have provided evidence that the magnitude of perceived roughness depends on spatial variation in the afferent population discharge. This paper reviews those studies and provides new data that appear to show that roughness perception cannot depend on activity in cutaneous rapidly adapting afferent fibers when surface element spacings exceed 1 mm. Finer surfaces have not been studied in combined psychophysical and neurophysiological studies. This paper also reinterprets the data of an earlier study, showing that all within-fiber neural coding mechanisms, which include impulse rate codes and codes based on the temporal structure of the afferent signal, can be rejected as the basis for roughness perception when the finger scans a textured surface.

Adaptation, Physiological↗

PubMed2005

stathmin, a gene enriched in the amygdala, controls both learned and innate fear.

Little is known about the molecular mechanisms of learned and innate fear. We have identified stathmin, an inhibitor of microtubule formation, as highly expressed in the lateral nucleus (LA) of the amygdala as well as in the thalamic and cortical structures that send information to the LA about the conditioned (learned fear) and unconditioned stimuli (innate fear). Whole-cell recordings from amygdala slices that are isolated from stathmin knockout mice show deficits in spike-timing-dependent long-term potentiation (LTP). The knockout mice also exhibit decreased memory in amygdala-dependent fear conditioning and fail to recognize danger in innately aversive environments. By contrast, these mice do not show deficits in the water maze, a spatial task dependent on the hippocampus, where stathmin is not normally expressed. We therefore conclude that stathmin is required for the induction of LTP in afferent inputs to the amygdala and is essential in regulating both innate and learned fear.

Amygdala↗