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PubMed1986

Aged rats need a preserved complement of perforated axospinous synapses per hippocampal neuron to maintain good spatial memory.

Spatial working memory, which crucially depends on the structural integrity of the hippocampal formation and its afferent connections, is impaired in the most, but not all, of aged rats. This study was designed to verify whether aged animals that do not exhibit the spatial memory deficit are the ones in which the hippocampal synaptic connectivity remains preserved with advancing chronological age. Young adult rats with good spatial memory, aged rats with impaired spatial memory and equally aged rats with intact spatial memory were compared. The number of synapses per neuron was estimated in the hippocampal dentate gyrus. The most important results were obtained when axospinous synapses were divided into perforated and non-perforated ones according to the appearance of their postsynaptic density. A significant decrease in the number of perforated synapses was found in memory-impaired aged rats as compared to either young adults or aged animals without memory deficits. The number of non-perforated synapses per neuron was diminished in memory-deficient aged rats relative to young adults, but not to memory-intact aged rats. However, it was only the loss of perforated synapses which correlated with the degree of spatial memory impairment. Thus, aged rats need a preserved complement of hippocampal perforated synapses to maintain good spatial memory.

Aging↗
PubMed1988

Auditory spatial sensitivity of inferior collicular neurons of echolocating bats.

The sensitivity of 94 inferior collicular (IC) neurons of Eptesicus fuscus and Myotis lucifugus to spatial location of the acoustic stimulus were studied under free-field stimulus conditions. The best frequency (BF) and minimum threshold (MT) of each neuron were determined with sound delivered in front of the bat. Then the variation in discharge rate of the neuron was measured with a BF sound broadcast from a moving loudspeaker at different angular positions along the horizontal, vertical or diagonal plane of the frontal auditory space. A wide range of stimulus intensities above the MT of the neuron was used. Neurons were classified into 3 classes on the basis of their spatial sensitivity: (1) omnisensitive neurons (15%) were broadly tuned to sound delivered in the frontal auditory space and their responses did not show any correlation with sound location; (2) stimulus intensity-dependent neurons (28%) varied their discharge rates with sound location and intensity so that the peak of their spatial response profiles also varied with stimulus intensity; and (3) stimulus intensity-independent neurons (57%) varied their discharge rates only with sound location over a wide range of stimulus intensities so that their peak discharge always appeared at the same or a small range of angle. In most cases, the medial limbs of the spatial sensitivity curve for these neurons were extremely sharp and congruent. By moving the loudspeaker along the horizontal, vertical and diagonal planes, it was possible to approximate the boundary of the spatial response area of a neuron. Most IC neurons responded to sound delivered within 20 degrees ipsilateral, 60 degrees contralateral, 45 degrees up and 40 degrees down of the frontal auditory space, confirming previous similar studies. In general, an increasing stimulus repetition rate appeared to sharpen the spatial sensitivity curve of a neuron. Conversely, an increasing moving velocity of the stimulus decreased its response. The possible role of these 3 classes of neurons in echolocation and neural mechanisms underlying the spatial sensitivity of these neurons is discussed.

Acoustic Stimulation↗
PubMed1990

Hippocampal protein kinase C activity is reduced in poor spatial learners.

Activation of protein kinase C (PKC) via neurotransmitter coupling processes has been associated with long-term potentiation (LTP) or classical conditioning, but whether natural variation in PKC activity affects learning performance remains to be determined. Inbred strains of mice differ in their ability to exhibit spatial reference memory as measured by the Morris water task. C57BL/6Ibg (C57) mice perform the task better than DBA/2Ibg (DBA) mice, which show relatively little spatial preference. Hippocampal PKC activity extracted from the particulate fraction was lower in DBA mice than in C57 mice. To examine the potential relationship of PKC activity with spatial learning performance, 11 C57BL/6J x DBA/2J recombinant inbred strains (BXD RIs) were trained in the place learning version of the Morris water task. Cortical and hippocampal PKC activities were measured. Variation in spatial learning performance and PKC activity from cortex and hippocampus was observed. A positive significant correlation was observed between measures of spatial learning accuracy and hippocampal PKC in these strains. No correlation was observed between spatial learning accuracy and cortical PKC activity. These data suggest that animals with lower hippocampal PKC activity may have problems performing spatial reference memory tasks with the same degree of accuracy as those with higher hippocampal PKC activity.

Animals↗
PubMed1993

Intrahippocampal transplants of septal cholinergic neurons: choline acetyltransferase activity, muscarinic receptor binding, and spatial memory function.

Recent studies have demonstrated that intrahippocampal cholinergic septal grafts can ameliorate deficits in spatial memory function and hippocampal cholinergic neurochemical activity in animals with disruptions of the septohippocampal pathway. Further studies have revealed that hippocampal cholinergic activity, as measured by high affinity choline uptake, correlates significantly with performance on tests of spatial memory function. The present study was designed to examine the effect of cholinergic septal grafts on reversing deficits in hippocampal choline acetyltransferase activity and on normalizing muscarinic receptor binding in animals with lesions of the septohippocampal system, and to examine the correlations between these cholinergic parameters and performance of spatial memory tasks. The results of this study indicated that in animals with lesions plus septal grafts, hippocampal ChAT activity was restored significantly and muscarinic receptor binding was normalized to a level not different from the control animals. Regression analyses indicated that ChAT activity was significantly correlated with performance on spatial reference memory, spatial navigation and spatial working memory, while muscarinic receptor binding correlated significantly with spatial reference memory performance.

Animals↗
PubMed1990

Hemispheric processing of spatial frequencies in two commissurotomy patients.

To test the hypothesis that in humans the left brain hemisphere is specialized for processing high spatial frequencies while the right hemisphere is specialized for processing low spatial frequencies, pairs of Gaussian windowed sinusoidal gratings were presented for 167 msec within the left and right visual fields of two commissurotomy patients. The gratings employed had spatial frequencies ranging from 1 to 8 cycles per degree, and horizontal or vertical orientations. The two gratings in each pair were identical in spatial frequency but could differ in orientation. Subjects reported if their orientations were the same or different. Twelve normal controls were also run. Accuracy data provides no indication of a relative advantage for high frequencies in the RVF or low frequencies in the LVF. One commissurotomy subject showed a trend in the reverse direction; the other was better with LVF presentations for all spatial frequencies. Control subjects failed to show a spatial frequency x visual field interaction. These outcomes suggest that at the processing stages required by the task, the hemispheres are not specialized for particular ranges of spatial frequencies.

Adult↗
PubMed1983

Disparity range for local stereopsis as a function of luminance spatial frequency.

The disparity range for stereo sensitivity was investigated with spatially filtered bars, tuned narrowly over a broad range of spatial frequencies. When measured with narrow (high spatial frequency) bars the disparity range for stereopsis exceeded two orders of magnitude. The range was reduced with broad (low spatial frequency) bars by an elevation of stereothreshold that increased according to a constant 6 deg phase disparity. The upper disparity limit also increased at broad spatial periods but at a lower rate. These size disparity correlations illustrate quantitative stereopsis along a continuum from fine to coarse disparities (+/- 2 deg). A disparity matching task revealed that greater amounts of uncrossed than crossed disparity were required to match suprathreshold disparities. The ratio of standard/matched disparity was lower in both directions for small suprathreshold disparities subtended by broad than by narrow spatial periods. This selective reduction of stereo-efficiency ratio illustrates tuning to coarse disparities subtended by broad (low spatial frequency) stimuli and accounts for the marked reduction of stereoacuity caused by spectacle blur.

Depth Perception↗
PubMed1984

Spatial visual channels in the Fourier plane.

Properties of human spatial visual channels were studied in two-dimensional form by a signal detection masking paradigm. Tuning surfaces of contrast threshold elevation induced by a sinusoidal mask were generated for four Subjects, interpolated from an 11 X 11 Cartesian grid over the Fourier plane, and numerically Fourier transformed in two dimensions to infer putative filter profiles in the 2D space domain. Among the main findings in the 2D frequency domain were: (1) Threshold elevation surfaces are highly polar nonseparable--they cannot be described as the product of a spatial frequency tuning curve times an orientation tuning curve. (2) Iso-half-amplitude contours of the spectral tuning surfaces have a length/width elongation ratio of about 2:1. (3) Necessarily, resolution for spatial frequency and for orientation are in fundamental competition with 2D spatial resolution. By calculating the occupied area of the inferred filters both in the 2D space domain and in the 2D frequency domain, it was estimated that these mechanisms approach within a factor of 2.5 of the theoretical limit of joint resolution in the two 2D domains that can be derived by 2D generalization of Gabor's famous Theory of Communication (1946). Other classes of 2D filters, such as an ideal 2D bandpass filter, have joint 2D entropies which are suboptimal by a factor of 13 or more. Subject to the inherent constraints on inference from these 2D masking experiments, the evidence suggests that 2D spatial frequency channels can be described as elongated 2D spatial wave-packets which crudely resemble optimal forms for joint information resolution in the 2D spatial and 2D frequency domains.

Adult↗
PubMed1984

Spatial tuning of static and dynamic local stereopsis.

The range of spatial tuning for channels that process static and dynamic disparities was investigated in the central visual field by measuring stereoscopic thresholds as a function of the difference in size of spatially filtered bar-like patterns presented to the two eyes. Spatial tuning functions were revealed by an elevation of stereothreshold as the difference between the widths of bar patterns increased. Functions tuned to low spatial frequencies (0.075-2 c/deg) were classified as transient since their stereosensitivity was greater for dynamic (1 Hz) than static disparities. Functions tuned to high spatial frequencies (2.4-19 c/deg) were classified as sustained since their stereosensitivity was equal for dynamic and static disparities. When equal width patterns were presented to the two eyes, stereothreshold increased with spatial periods greater than 0.4 deg according to a constant 6 deg phase disparity. This size-disparity correlation suggests that large disparities are processed by spatial filters tuned to disparities proportional to their receptive field dimensions.

Humans↗
PubMed1984

Spatial and velocity tuning of processes underlying induced motion.

A nulling procedure was used to quantify the velocity and spatial frequency tuning of induced motion for sinusoidal gratings. For each spatial frequency of test and inducing gratings, there was a range of low velocities which resulted in strong induction, with a gain of close to 1. For low spatial frequencies induction occurred at higher velocities than was the case for high spatial frequencies. Induced motion shows bandpass spatial frequency tuning, with a bandwidth of about two octaves at half-height. Induced motion appears to be mediated by spatial channels with a low pass temporal characteristic. To a first approximation, induced motion appears to be a product of velocity and spatial frequency.

Form Perception↗
PubMed1985

Determinants of the spatial properties of cone-rod interaction.

Photopic increment thresholds can be elevated by 0.2-1.9 log units, depending on the diameter of a concentric scotopic background. This cone-rod interaction displays spatial properties that resemble the spatial sensitization (Westheimer effect) observed in the isolated scotopic and photopic systems. This raises the possibility that the spatial properties of the interaction are determined by the same mechanisms or pathways that determine the spatial properties of either scotopic or photopic vision. When annulus backgrounds are used, the spatial properties of interaction match those of the scotopic system but not the photopic system. When disk backgrounds are used, the spatial properties of interaction match those of neither photopic nor scotopic systems. Thus, under some conditions, the scotopic visual system alone is sufficient to determine the spatial properties of cone-rod interaction. Under other conditions, additional complications arise. The results are discussed in terms of the center-surround model that has previously been applied to cone-rod interaction.

Adaptation, Ocular↗
PubMed1988

Large-scale relative localization across spatial frequency channels.

Large-scale relative localization accuracy is measured with objects that stimulate different ranges of spatial frequencies. The author has previously made measurements using objects that stimulate only high-spatial-frequency channels or only low-spatial-frequency channels and found no effect of spatial frequency. In the present study, relative localization accuracy, i.e. interval discrimination, is measured with an object pair consisting of a low-spatial-frequency object and a high-spatial-frequency object. Relative localization accuracy for this cross-channel stimulus is as high as for the same-channel stimuli used previously, showing that the relative localization mechanism operates effectively across spatial frequency channels.

Humans↗
PubMed1989

Interocular differences in contrast and spatial frequency: effects on stereopsis and fusion.

Anisometropia produces interocular differences in contrast and spatial frequency. The influence of these two parameters on Panum's fusional limit (PFL) and stereoscopic depth thresholds was investigated with sinusoidal gratings and one-dimensional band-pass-limited targets. Vertical fusion limits were unaffected by large interocular differences in contrast (40-10%) at two spatial frequencies (0.8 and 1.6 cpd). However, when tested with a low spatial frequency (0.8 cpd), stereothresholds increased 150% with an interocular difference in contrast as small as 50-25%. Stereoacuity was reduced less by differential contrast when tested with higher spatial frequencies (3.2 cpd). When tested with low spatial frequencies the stereothreshold was elevated more by reducing the contrast of one image than by equal contrast reductions of both ocular images. Stereothresholds appear to be elevated by binocular suppression evoked by interocular differences in contrast. Vertical as well as horizontal fusion limits decreased with increasing interocular size difference. Horizontal fusion limits fell off more gradually with increasing size difference than did vertical fusion limits, particularly at higher spatial frequencies (2.4 cpd). Similarly, stereothresholds increased with increasing interocular size differences. Changes in the fusion limit and stereothreshold that occur with interocular size differences are predicted from positional disparities between edge features rather than from differences in spatial frequency.

Contrast Sensitivity↗
PubMed1989

Receptive field properties of human motion detector units inferred from spatial frequency masking.

This study was designed to investigate the spatial frequency selectivity and spatial structure of receptive fields of motion sensitive mechanisms in human vision. Spatial frequency selectivity was inferred from masking measurements, using dynamic test and mask stimuli. For test frequencies between 0.025 and 15.0 c/deg, maximal masking occurred when the mask frequency matched that of the test, suggesting that the test was detected by mechanisms tuned to (or near to) that frequency. For tests below 0.025 c/deg or above 15.0 c/deg, maximal masking occurred at 0.025 and 15.0 c/deg, respectively, suggesting that there exist no mechanisms selective to frequencies outside these limits. A masking model, suitable for interpreting results obtained with drifting test stimuli, was developed and used to calculate spatial frequency selectivity functions from masking data. Assuming small signal linearity, and a constant phase spectrum, the selectivity functions were inverse-Fourier transformed to yield estimates of the extent and structure of receptive fields. Field width was found to vary with test spatial frequency from 5.8 deg at 0.03 c/deg to 0.05 deg at 10.0 c/deg. These estimates were compared with width estimates previously obtained by a summation technique (Anderson & Burr, 1987), and found to be similar over a wide range of spatial frequencies (2.5 log units). Gabor functions provided a reasonable fit to the calculated field profiles at high spatial frequencies (above 1.0 c/deg), but not at low frequencies.

Fourier Analysis↗
PubMed1990

The influence of spatial frequency on perceived temporal frequency and perceived speed.

Speed matching experiments were conducted using drifting gratings of different spatial frequencies in order to assess the influence of spatial frequency on perceived speed. It was found that gratings of high spatial frequency appear to drift more slowly than low spatial frequency gratings of the same actual velocity. The perceived temporal frequency of a counterphase grating similarly declines as spatial frequency increases. The previously reported effect of temporal frequency on perceived spatial frequency probably does not contribute to these phenomena. Our results suggest that the motion sensors thought to operate within different spatial frequency ranges have different velocity transfer functions, a fact not incorporated in existing computational models of motion perception.

Contrast Sensitivity↗
PubMed1991

Optimal spatial displacement for direction selectivity in cat visual cortex neurons.

Responses of single neurons in cat visual cortex were measured in response to sinewave grating stimuli. Firstly, a neuron's spatial frequency tuning was determined, and subsequent stimuli were set at the optimal spatial frequency for that neuron. Then a "jumping grating" stimulus was used: a sinewave grating subjected to a series of abrupt spatial displacements, while remaining stationary for a fixed exposure time between displacements. The amount of direction selectivity elicited by this stimulus was measured as a function of the amount of spatial displacement. Visual cortex neurons generally showed an optimal spatial displacement, corresponding to somewhat less than one quarter of a spatial period of the neuron's optimal spatial frequency (close to, but systematically less than, "quadrature phase"). In a majority of neurons tested, this optimal displacement was not affected by increasing the exposure time between displacements, indicating that the measurements were not a simple consequence of temporal frequency tuning. These results closely parallel recent human psychophysical data obtained from measurements of motion aftereffect or direction discrimination elicited by jumping grating stimuli.

Animals↗
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

Motion discrimination in two-frame sequences with differing spatial frequency content.

We measured the upper threshold for directional motion discrimination (Dmax) in two-frame random binary luminance patterns (random dot kinematograms) in which either one or both frames was spatially low-pass filtered by convolution with a Gaussian filter. When both frames were low-pass filtered, Dmax increased as a function of the standard deviation of the Gaussian blurring function, in agreement with previous findings. However, when only one of the two frames was blurred, Dmax showed little change with blurring space constants below about 20 min arc, and at larger space constants motion discrimination became impossible. We take this as evidence against the proposal that Dmax is preferentially determined by motion signals from high spatial frequencies; and as evidence for the alternative that Dmax depends upon the mean spatial interval between features in the pattern after a single stage of spatial frequency pre-filtering. The breakdown in motion discrimination for space constants above about 20 min arc can be predicted from the computed effects of blurring upon the correlation between features (zero-bounded regions) in the broad-band and spatially filtered patterns. At values of blur where motion discrimination began to collapse there was a temporal order asymmetry; discrimination was easier when the low-pass pattern preceded the broadband pattern than when the broadband pattern appeared first. We propose that the temporally sustained high spatial frequency signal in the broadband pattern is delayed relative to the more transient low frequency signal; or alternatively, that the inhibitory surround of the spatial prefilter is switched in after a delay relative to the excitatory centre. The processing-delay interpretation was tested and confirmed in a second experiment by manipulating the frame duration.

Discrimination, Psychological↗
PubMed1995

Spatial frequency channels in experimentally strabismic monkeys revealed by oblique masking.

Although the spatial vision deficits of human strabismic amblyopes have been well documented, surprisingly little is known about the mechanisms underlying their visual performance. In an effort to reveal the structure underlying the spatial vision deficits associated with strabismic amblyopia, we measured the performance of monkeys (Macaca nemestrina) with experimental strabismus in a contrast detection task with oblique masks. The masks were two adjacent identical oblique sine-wave gratings modulated in space by a Gaussian envelope. The target stimulus was a vertically oriented Gabor patch that appeared superimposed on the center of either the left or the right mask. The animals were trained by operant methods to indicate the location of the target. We measured detection thresholds in each eye independently for a large number of test and mask spatial frequencies. For each test spatial frequency, detection thresholds were elevated in the presence of the mask. The threshold evaluations showed a peak for a particular spatial frequency that was typically similar to the test spatial frequency. This pattern of results is consistent with the idea that the tests are detected by a discrete number of channels tuned to a narrow range of spatial frequencies. The data from the deviated eyes did not appear qualitatively different from those of the fellow eyes, and could be accounted by the same number of channels in both eyes. Quantitative estimates of the channels' characteristics revealed that the channels derived from the deviated eyes' data were similar to those yielded by the fellow eyes, but showed a reduction in their sensitivity to contrast.

Amblyopia↗