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PubMedDate not supplied

Spatial frequency response functions obtained from cat visual evoked potentials.

Visual evoked potentials (VEPs) were obtained from the surface of the cat visual cortex in response to contrast reversing sinusoidal gratings. Gratings of different spatial frequency were presented either separately, using signal averaging to increase the signal-to-noise ratio, or as a spatial frequency sweep, in which spatial frequency was sequentially increased every 5 sec during a 40 sec trial (3.99 Hz) or every 3 sec during a 24 sec trial (6.65 Hz). The second harmonic amplitude- and phase-spatial frequency functions derived from averaging or from sweep trials were similar, indicating that the swept stimulus method can be used to provide a rapid and reliable measure of the VEP-spatial frequency function. Intravenous administration of physostigmine, an acetylcholinesterase inhibitor, evoked a spatial frequency-dependent change in VEP amplitude. At 3.99 Hz, responses to low spatial frequencies were enhanced to a greater extent than were responses to high spatial frequency stimuli. At 6.65 Hz, responses to mid-range spatial frequencies were enhanced to a greater extent than were responses to low and high spatial frequency stimuli. VEP phase at both 3.99 and 6.65 Hz was advanced to a greater degree at the higher spatial frequencies. These results indicate that the swept spatial frequency method may be useful in studying spatial frequency-dependent pharmacological effects on the VEP and support the possibility that pharmacological disruption of the cholinergic visual system can produce such changes.

Analysis of Variance↗
PubMed2000

Spatial scale of visual analysis for vernier acuity does not vary over time.

The visual system filters spatial pattern through a range of narrowly tuned spatial mechanisms, but the rules by which the outputs of these mechanisms are combined across time to extract precise geometrical information are not yet clear. One hypothesis is that spatial analysis shifts over time from coarse to finer spatial scales, in order to extract fine spatial information. An alternative hypothesis is that thresholds are determined by the signal-to-noise ratio within an optimal spatial scale. In this study, we measured vernier acuity across exposure duration for equally visible long lines and short lines and found no improvement in spatial precision with time. Using a simultaneous spatial-masking paradigm, we determined the active spatial scales at 100 and 1000 ms. The results show no significant changes in spatial scale, or in the size-range of active scales, for the two exposure durations. Furthermore, whereas vernier thresholds vary markedly with line contrast, we find only modest shifts in spatial scale. Taken together, our results suggest that for vernier acuity, spatial scale is selected very early, and that vernier thresholds are predominantly limited by signal strength within that spatial scale.

Humans↗
PubMed2001

Spatial filter approach for comparison of the forward and inverse problems of electroencephalography and magnetoencephalography.

We present an analysis of the relative information content of cortical current source reconstructions from electroencephalogram (EEG) and magnetoencephalogram (MEG) forward calculations by examining the spatial filters that relate the internal sources with the externally measured electric potentials and magnetic fields. The forward spatial filters are seen to be low-pass functions of spatial frequency and spatial resolution degrades in external measurements. Inverse spatial filters may be used to reconstruct cortical sources from external data, but since they are high-pass functions of spatial frequency, they must be regularized to avoid instabilities caused by noise at higher spatial frequencies. The regularization process limits the spatial resolution of source reconstructions. EEG forward spatial filters fall off at lower spatial frequencies than MEG filters; hence, there is less information available in higher spatial frequencies resulting in lower spatial resolution in inverse reconstructions. The tangential component of the magnetic field provides even higher spatial resolution than can be obtained using the radial component. An accompanying article examines the surface Laplacian for both the EEG and the MEG.

Electric Conductivity↗
PubMed2006

On the use of ZIP codes and ZIP code tabulation areas (ZCTAs) for the spatial analysis of epidemiological data.

BACKGROUND: While the use of spatially referenced data for the analysis of epidemiological data is growing, issues associated with selecting the appropriate geographic unit of analysis are also emerging. A particularly problematic unit is the ZIP code. Lacking standardization and highly dynamic in structure, the use of ZIP codes and ZIP code tabulation areas (ZCTA) for the spatial analysis of disease present a unique challenge to researchers. Problems associated with these units for detecting spatial patterns of disease are explored. RESULTS: A brief review of ZIP codes and their spatial representation is conducted. Though frequently represented as polygons to facilitate analysis, ZIP codes are actually defined at a narrower spatial resolution reflecting the street addresses they serve. This research shows that their generalization as continuous regions is an imposed structure that can have serious implications in the interpretation of research results. ZIP codes areas and Census defined ZCTAs, two commonly used polygonal representations of ZIP code address ranges, are examined in an effort to identify the spatial statistical sensitivities that emerge given differences in how these representations are defined. Here, comparative analysis focuses on the detection of patterns of prostate cancer in New York State. Of particular interest for studies utilizing local, spatial statistical tests, is that differences in the topological structures of ZIP code areas and ZCTAs give rise to different spatial patterns of disease. These differences are related to the different methodologies used in the generalization of ZIP code information. Given the difficulty associated with generating ZIP code boundaries, both ZIP code areas and ZCTAs contain numerous representational errors which can have a significant impact on spatial analysis. While the use of ZIP code polygons for spatial analysis is relatively straightforward, ZCTA representations contain additional topological features (e.g. lakes and rivers) and contain fragmented polygons that can hinder spatial analysis. CONCLUSION: Caution must be exercised when using spatially referenced data, particularly that which is attributed to ZIP codes and ZCTAs, for epidemiological analysis. Researchers should be cognizant of representational errors associated with both geographies and their resulting spatial mismatch, especially when comparing the results obtained using different topological representations. While ZCTAs can be problematic, topological corrections are easily implemented in a geographic information system to remedy erroneous aggregation effects.

Bias↗
PubMed2003

[Spatial heterogeneity of vegetation and soil characteristics in oasis-desert ecotone].

The spatial distribution of bio- and abiotic factors in ecosystem has the features of spatial heterogeneity in general, and spatial heterogeneity is important in the function and process of ecosystem at a variety of scales. In this paper, semivariance theory and methods were used to study the degree, composition, scale and pattern of spatial heterogeneity of vegetation coverage and soil environment factors, and their relationships in the Fukang oasis-desert ecotone of Xinjiang. With block kriging in the studied area of sampling, the spatial pattern of each factor was expressed in three dimensions as well. The results indicated that except the little spatial autocorrelation of salt salinity, there existed a spatial autocorrelation above mean of the other factors, and the spatial variation was mainly limited to smaller scales. The vegetation coverage, for which auto-correlated spatial heterogeneity was a main component, had a high degree of spatial heterogeneity with obvious spatial pattern at the scale of 3 km. From the perspective of spatial distributions, there existed a remarkable difference between soil holard and soil pH value in the studied area. Besides this, the higher value areas interleaved with the lower ones, significant positive correlations were observed between herbage coverage and soil factors (surface SH and SPH), and shrub coverage depended on the deep SH. The difference of spatial heterogeneity and pattern between SC and HC indicated the different ecological function and process in the ecosystem.

Desert Climate↗
PubMed2004

[Effect of land use change on spatial distribution pattern of soil total nitrogen in Keerqin sandy land].

By using statistics and geostatistics, this paper studied the content and spatial pattern of soil total nitrogen (TN) in a glassland and an arable land of southeast Keerqin sandy land. The TN content in 0-10 cm soil layer of the arable land reclaimed from a glassland 8 years ago was not different from that of the virgin grassland, but the TN content in 10-20 cm soil layer was higher than that in the grassland (P < 0.05). The soil TN storage in the grassland was 262.36 g x hm(-2) higher than that in the arable land, and its spatial pattern was significantly different between arable land and grassland. The heterogeneity and spatial dependence of TN in each soil layer (0-10 cm and 10-20 cm) of the arable land were larger than those of the grassland, and the ranges and fractal dimensions of TN in each soil layer of the arable land were larger than those of the grassland. There existed a clear horizontal spatial structure heterogeneity and spatial distribution pattern in the arable land, while in the grassland, the horizontal spatial structure heterogeneity was very low, and the horizontal spatial distribution pattern was random and uniform. The TN content in the 0-10 cm and 10-20 cm soil layers of the arable land had no difference (P < 0.05), its spatial structure characters had some difference (P < 0.05), and its spatial distribution pattern was very similar (r = 0.683, P < 0.05). The TN content and its spatial structure characters between two soil layers of the grassland had remarkable difference (P < 0.05), but its spatial distribution pattern in the two layers was relatively similar (r = 0.395, P < 0.05). In conclusion, agricultural activity could not only influence soil TN content, but also influence its spatial structure characteristics and spatial distribution pattern. Therefore, our study might promote the further study on the effect of land use change on soil nitrogen distribution pattern and its cycling in ecosystems.

Crops, Agricultural↗
PubMed2005

[Spatial variance characters of urban synthesis pattern indices at different scales].

Scale holds the key to understand pattern-process interactions, and indeed, becomes one of the corner-stone concepts in landscape ecology. Geographic Information System and remote sensing techniques provide an effective tool to characterize the spatial pattern and spatial heterogeneity at different scales. As an example, these techniques are applied to analyze the urban landscape diversity index, contagion index and fractal dimension on the SPOT remote sensing images at four scales. This paper modeled the semivariogram of these three landscape indices at different scales, and the results indicated that the spatial variance characters of diversity index, contagion index and fractal dimension were similar at different scales, which was spatial dependence. The spatial dependence was showed at each scale, the smaller the scale, the stronger the spatial dependence. With the scale reduced, more details of spatial variance were discovered. The contribution of spatial autocorrelation of these three indices to total spatial variance increased gradually, but when the scale was quite small, spatial variance analysis would destroy the interior structure of landscape system. The semivariogram models of different landscape indices were very different at the same scale, illuminating that these models were incomparable at different scales. According to above analyses and based on the study of urban land use structure, 1 km extent was the more perfect scale for studying the spatial variance of urban landscape pattern in Shanghai. The spatial variance of landscape indices had the character of scale-dependence, and was a function of scale. The results differed at different scales we chose, and thus, the influence of scales on pattern could not be neglected in the research of landscape ecology. The changes of these three landscape indices displayed the regularity of urban spatial structure at different scales, i. e., they were complicated and no regularity at small scale, polycentric at moderate scale, and circle-zoning at big scale.

City Planning↗
PubMed1988

Development of spatial and temporal selectivity in the suprasylvian visual cortex of the cat.

We have studied the development of the spatial and temporal properties of neurons in the medial bank of the suprasylvian visual cortex (PMLS) in kittens aged between 9 d and 8 weeks. Quantitative measurements were made of the responses to drifting high-contrast gratings of optimum orientation and direction of motion, but varying in spatial and temporal frequency. The spatial resolution ("acuity") of cells increased rapidly and was fully mature (over 2 cycles/deg for the best cells) at 3 weeks of age. The optimum spatial frequency also tended to improve and reached adult values (around 0.5 cycles/deg for the best cells) at about the end of the third week. In younger kittens, the spatial resolution of neurons was not obviously correlated with the eccentricity of their receptive fields, but in older animals acuity was clearly elevated for receptive fields in the central visual field. The proportion of "low-pass" cells (showing no obvious attenuation of response for gratings of low spatial frequency) decreased with age and simultaneously there was a slight increase in the mean spatial bandwidth of "bandpass" cells. Responses to drifting sinusoidal gratings were generally dominated by an unmodulated elevation of discharge at all ages. In tests with stationary, contrast-modulated gratings presented at different spatial positions, cells in the youngest kittens behaved nonlinearly and showed mainly an unmodulated increase in discharge, whereas in older kittens, as in adult cats, most neurons responded to contrast-modulated gratings with a small, phase-dependent response at the temporal frequency of modulation and a larger component at twice the fundamental frequency. None of the cells recorded at any age had a true "null position." As in adult PMLS, the widths of receptive fields in kittens were, on average, about twice the size of the preferred spatial period (4 times the preferred bar width). At all ages, therefore, neurons in PMLS resembled striate complex cells with respect to the nonlinearity of their responses and the spatial structure of their receptive fields. The preferred temporal frequency and high-temporal-frequency cutoff also improved, on average, during the first 3 weeks of life, and the range of temporal frequencies over which cells responded continued to increase until at least 8 weeks. Although the low-spatial-frequency inhibition that creates spatial bandpass characteristics probably depends on cortical mechanisms, the postnatal development of both temporal and spatial resolution might well be limited by maturation at the level of the retina.

Animals↗
PubMed1986

Space and spatial frequency: analysis and representation in the macaque striate cortex.

Simple cells in the macaque striate cortex were tested with bars, edges and gratings. Spatial frequency tuning curves could be predicted from the spatial profiles plotted with bars and edges and the bandwidth could be evaluated more accurately by computing the mean from measured and predicted tuning curves. The results suggest that the mean relative spatial frequency bandwidth (delta f/fo) is nearly constant and of a moderate value. But at each optimal spatial frequency, cells with different bandwidths (about a factor of two) were recorded. The shapes of spatial response profiles resemble the corresponding spatial and spatial frequency characteristics of line and edge detectors evaluated psychophysically. Among the remaining cell types, concentric cells tend to be tuned to lower spatial frequencies and have broader bandwidths, whereas periodic cells prefer higher spatial frequencies and have narrower bandwidths. Thus the mean relative bandwidth tends to decrease significantly with spatial frequency (as required by a system of patch-by-patch Fourier analysis) only when cells with poor orientation selectivity and the non-linear silent periodic cells are included along with the simple cells. Simple cells, on their own, seem to form a quasi-linear contrast processing system which is more biased towards spatial accuracy than spatial frequency selectivity.

Animals↗
PubMed1983

Spatial localization in normal and amblyopic vision.

Spatial localization was investigated for each eye of amblyopic observers using a bisection paradigm. The stimuli were comprised of a grating composed of bright lines, and a test line. The test line was either placed above the grating (bisection-no overlap) or within the row of lines comprising the grating (bisection-with overlap) and thresholds for each bisection task were measured as a function of the fundamental spatial frequency of the grating. Vernier thresholds were also measured. For the nonamblyopic eyes at low spatial frequencies, bisection thresholds were a constant fraction ("Weber" fraction) of the space to be bisected, while at high spatial frequencies thresholds were approximately a constant retinal distance (a hyperacuity). However the spatial localization of an amblyopic eye depends upon both the type of amblyopia, and the stimulus configuration. Specifically, for anisometropic amblyopia, spatial localization (bisection-no overlap) and vernier, when scaled to the resolution losses, were normal. However, spatial adjacency (bisection with overlap), while enhancing the spatial localization of nonamblyopic eyes at high spatial frequencies, markedly elevated thresholds in the amblyopic eyes of anisometropic amblyopes. Strabismic amblyopes on the other hand show disturbances in both spatial localization tasks which can not be accounted for on the basis of reduced resolution. Their results are characterized by an absence of a constant Weber fraction at low spatial frequencies and "crowding" effects at high spatial frequencies. For strabismic amblyopes, the optimal localization thresholds were similar to the Snellen threshold, while for anisometropic amblyopes, the optimal localization thresholds were several times better than the Snellen threshold.

Adult↗
PubMed1989

A choice reaction time analysis of spatial frequency discrimination.

Simple reaction time to the onset of sinewave gratings was measured as a function of spatial frequency in two observers. These results are compared to the choice reaction time required for the observer to correctly discriminate the spatial frequency of two gratings flashed sequentially. Grating contrast was either 0.75 or 1.5 logarithmic units above the detection threshold for each spatial frequency tested. The spatial phase and contrast of the reference and test gratings were varied from trial to trial by small random amounts to eliminate fixed cues other than the difference frequency. The spatial frequency difference between the reference and test grating was either 0.125, 0.25 or 0.5 octave. As has been earlier reported, simple reaction time increases with increasing spatial frequency. Contrary to this, choice reaction time first increases (up to 4 c/deg) and then decreases. We derived the time required by the observer to make a spatial frequency judgment by subtracting the simple reaction time from the choice reaction time for a given spatial frequency and contrast. The maximum decision time occurs in the medium spatial frequency range (between 1 and 4 c/deg), at which frequencies we are most sensitive. The time required to make a correct spatial-frequency discrimination decreases with increasing spatial-frequency difference. The decision time is, however, fairly invariant over a large range of suprathreshold contrast levels. The findings suggest that the decision time for spatial frequency discrimination increases with the number of mechanisms involved.

Contrast Sensitivity↗
PubMed1993

Modelling the dependence of contrast sensitivity on grating area and spatial frequency.

We modelled the human foveal visual system in a detection task as a simple image processor comprising (i) low-pass filtering due to the optical transfer function of the eye, (ii) high-pass filtering of neural origin, (iii) addition of internal neural noise, and (iv) detection by a local matched filter. Its detection efficiency for gratings was constant up to a critical area but then decreased with increasing area. To test the model we measured Michelson contrast sensitivity as a function of grating area at spatial frequencies of 0.125-32 c/deg for simple vertical and circular cosine gratings. In circular gratings luminance was sinusoidally modulated as a function of the radius of the grating field. In agreement with the model, contrast sensitivity at all spatial frequencies increased in proportion to the square-root of grating area at small areas. When grating area exceeded critical area, the increase saturated and contrast sensitivity became independent of area at large grating areas. Spatial integration thus obeyed Piper's law at small grating areas. The critical area of spatial integration, marking the cessation of Piper's law, was constant in solid degrees at low spatial frequencies but inversely proportional to spatial frequency squared at medium and high spatial frequencies. At low spatial frequencies the maximum contrast sensitivity obtainable by spatial integration increased in proportion to spatial frequency but at high spatial frequencies it decreased in proportion to the cube of the increasing spatial frequency. The increase was due to high-pass filtering of neural origin (lateral inhibition) and the decrease was mainly due to the optical transfer function of the eye. Our model explained 95% of the total variance of the contrast sensitivity data.

Computer Simulation↗
PubMed1998

Spatial uncertainty and sampling efficiency in amblyopic position acuity.

Spatial uncertainty and undersampling are two of the major hypotheses for the losses of amblyopic spatial vision. To test these two hypotheses, equivalent spatial uncertainty and spatial integration efficiency in spatial position judgments were quantified with a spatial perturbation paradigm. Specifically, three-line bisection thresholds were measured for the amblyopic eyes of two strabismic and two anisometropic amblyopes, and for normal controls. The horizontal stimulus lines comprised discrete dark dots distributed randomly around the mean line position according to a gaussian function. Line separation, the number of dots on each line (N), stimulus contrast (C), and the vertical standard deviation (sigma e) of the dot distribution were varied. An ideal observer analysis quantified the magnitude of equivalent spatial uncertainty (sigma s), the effective number of dots used (k), and spatial integration efficiency (k/N). At the optimal separation, equivalent spatial uncertainty (sigma s) is approximately ten-fold higher in both types of amblyopic visual systems than in control observers, even when stimulus visibility is accounted for. This apparent increase in sigma s is largely due to a shift in spatial scale of analysis in the amblyopic eye. Integration efficiency (k/N) increases in proportion to stimulus contrast or visibility (in units of detection threshold). Unlike sigma s, k/N is different between the two types of amblyopia. For the anisometropic observers, k/N is quantitatively similar to that of control observers. For the strabismic observers, on the other hand, k/N is reduced even after taking stimulus visibility into account. The decreased spatial integration efficiency in the strabismic visual system suggests that spatial undersampling may occur at a secondary stage of visual processing, beyond the detection stage.

Adult↗
PubMed1999

Stress impairs performance in spatial water maze learning tasks.

The water maze task has been developed to test spatial learning abilities in rats or mice, and is widely used. Though it has been reported before that numerous cognitive abilities are of importance for learning this task, poor performance is usually interpreted as an impairment of spatial memory formation. Previous investigations that tried to correlate long-term potentiation (LTP) of synaptic transmission with spatial learning abilities in rats reported that injection of drugs or specific gene deletions which blocked the expression of LTP correlated with learning impairments of spatial tasks in a water maze. Recent studies, however, have shown that pretraining enables these animals to learn such spatial tasks even though LTP was still found to be blocked. I investigated to what degree altered fear condition and stress perception could account for the impaired spatial learning when no pretraining is given. In a fear habituation task, unhandled rats preferred a dark over a well lit chamber more than handled animals did, but unhandled rats favoured the lit chamber more in an active avoidance task. They also performed poorly in a spatial water maze task compared with handled rats. Rats pretrained in a radial arm maze performed better in a water maze than non-pretrained rats. No difference between groups was found in a non-spatial water maze task. On the other hand, when pretrained in a water maze, rats performed only marginally better in a radial arm maze compared to non-pretrained animals. Since animals have to be handled to learn a radial arm maze, the difference in this task was not due to stress but most probably due to getting accustomed to the room dimensions prior to learning the spatial task. The results suggest that impaired learning of spatial tasks in the water maze can be due to increased stress and decreased fear conditioning without actually affecting spatial learning abilities. These results question the interpretations of the results of some previously published results of spatial water maze tasks.

Animals↗
PubMed1992

Binocular interactions in the cat's dorsal lateral geniculate nucleus, II: Effects on dominant-eye spatial-frequency and contrast processing.

The present study tested the hypothesis that nondominant-eye influences on lateral geniculate nucleus (LGN) neurons affect the processing of spatial and contrast information from the dominant eye. To do this, we determined the effects of stimulating the nondominant eye at its optimal spatial frequency on the responses of LGN cells to sine-wave gratings of different spatial frequency and contrast presented to the dominant eye. Detailed testing was carried out on 49 cells that had statistically significant responses to stimulation of the nondominant eye alone. Spatial-frequency response functions to nondominant-eye stimulation indicated that the responses were spatially tuned, as reported previously (Guido et al., 1989). Optimal spatial frequencies through the nondominant eye were significantly correlated with the optimal spatial frequencies through the dominant eye (r = 0.54; P less than 0.0001), and the optimal spatial frequencies were fairly similar for the two eyes. Nondominant-eye stimulation changed the maximal amplitude of the fundamental (F1) response to dominant-eye stimulation for only about 45% (22 of 49) of the cells that responded to nondominant-eye stimulation alone. The response vs. contrast function through the dominant eye was altered for 73% of the cells (51% independent of spatial frequency). Three types of effects were observed: a change in the initial slope of the response vs. contrast function (contrast gain), a change in the response amplitude at which saturation occurred, or an overall change in response at all contrasts. The incidence of these changes was similar for X and Y cells in LGN layers A, A1, and C (only four W cells were tested). Nondominant-eye stimulation had little or no effect on the sizes or sensitivities of the receptive-field centers or surrounds for the dominant eye. In addition, nondominant-eye stimulation had little or no effect on optimal spatial frequency, spatial resolution, or the bandwidth of spatial-frequency contrast sensitivity curves for the dominant eye. Possible functions of binocular interactions in the LGN are considered. The present results suggest a role in interocular contrast-gain control. Interocular contrast differences can occur before the acquisition of binocular fusion, when the two eyes are viewing different aspects of a visual stimulus. Psychophysical and physiological studies suggest that an interocular mechanism exists to maintain relatively constant binocular interactions despite differences in interocular contrast. The present results suggest that at least part of this mechanism occurs in the LGN.

Animals↗
PubMedDate not supplied

Spatial-frequency and orientation tuning in psychophysical end-stopping.

A psychophysical analog to cortical receptive-field end-stopping has been demonstrated previously in spatial filters tuned to a wide range of spatial frequencies (Yu & Levi, 1997a). The current study investigated tuning characteristics in psychophysical spatial filter end-stopping. When a D6 (the sixth derivative of a Gaussian) target is masked by a center mask (placed in the putative spatial filter center), two end-zone masks (placed in the filter end-zones) reduce thresholds. This "end-stopping" effect (the reduction of masking induced by end-zone masks) was measured at various spatial frequencies and orientations of end-zone masks. End-stopping reached its maximal strength when the spatial frequency and/or orientation of the end-zone masks matched the spatial frequency and/or orientation of the target and center mask, showing spatial-frequency tuning and orientation tuning. The bandwidths of spatial-frequency and orientation tuning functions decreased with increasing target spatial frequency. At larger orientation differences, however, end-zone masks induced a secondary facilitation effect, which was maximal when the spatial frequency of end-zone masks equated the target spatial frequency. This facilitation effect might be related to certain types of contour and texture perception, such as perceptual pop-out.

Adult↗
PubMed1999

The role of distinct components of visuo-spatial working memory in the processing of texts.

Three experiments examined whether visuo-spatial working memory is involved in processing spatial descriptions and tested whether different processes within the visuo-spatial working memory are involved in processing different kinds of descriptions. Experiment 1 considered the performance of two groups with low and high visuo-spatial working memory abilities in memorizing the description of a city. The high visuo-spatial working memory group had a better memory performance than the low visuo-spatial working memory group. In Experiment 2, the Brooks' (1967) task was adapted to investigate the selective interference of four different concurrent tasks (a verbal, a visual, a spatial-sequential, and a spatial-simultaneous task) on the recall of short abstract, visual and spatial texts. In Experiment 3, the same distinction was extended to longer and natural descriptions of different environments. Participants listened to three descriptions: a description that mainly stressed the visual properties of an environment, a description from a route perspective, and a description from a survey perspective. They also performed the concurrent visual and the two concurrent spatial tasks proposed in Experiment 2. Results of Experiments 2 and 3 demonstrated that tasks involving different cognitive resources (i.e. the verbal, visual, spatial-simultaneous, and the spatial-sequential tasks) had a differential interference with the free recall of different kinds of descriptions.

Cognition↗
PubMed1999

The spatial tuning of steady state pattern electroretinogram in multiple sclerosis.

In normal subjects, the steady-state electroretinogram in response to contrast reversing gratings (PERG), is spatially band-pass tuned in amplitude, with a maximum at intermediate spatial frequencies and an attenuation at lower and higher ones. The amplitude attenuation at low spatial frequencies is believed to reflect centre-surround antagonistic interactions in the receptive fields of inner retinal neurons. The aim of this study was to evaluate the PERG spatial tuning in multiple sclerosis (MS) patients without a previous optic neuritis history. Steady- state PERGs in response to counterphase-modulated (8 Hz) sinusoidal gratings of variable spatial frequency (0.6, 1.0, 1.4, 2.2 and 4.8 c/deg), were recorded from 18 patients with definite or probable MS and no history of optic neuritis (ON-). Nine of them had no signs of subclinical optic nerve demyelination (asymptomatic) in either eye, while nine had symptoms or signs of optic pathways involvement (symptomatic) in one or both eyes. Results were compared with those obtained from 10 MS patients with a previous history of optic neuritis (ON+) in one or both eyes, as well as from 21 age-matched controls. The amplitudes and phases of the responses' 2nd harmonics were measured. Compared with the controls, asymptomatic ON- patients showed selective losses in mean PERG amplitudes at medium and high (1.0-4.8 c/deg) spatial frequencies. Symptomatic ON- patients and ON+ patients had reductions in mean PERG amplitudes, with respect to controls, involving the whole spatial frequency range, but with greater losses at medium-high (1.0-4.8 c/deg) than at lower spatial frequencies. In all patients' groups, the average PERG spatial tuning function differed significantly from that of the controls, assuming a low-pass instead of the normal band-pass shape. The PERG phase was delayed in ON+ but not in ON- patients, as compared to controls. However, the phase delay was independent of spatial frequency. In both ON- and ON+ patients, losses in PERG amplitude and spatial tuning tended to be associated with corresponding abnormalities in perimetric sensitivity, visual acuity, colour vision and transient visual evoked potential (VEP) latency. The results indicate that abnormalities of the spatial tuning of steady-state PERG can be found in MS patients without either optic neuritis or signs of subclinical optic nerve demyelination. These changes may reflect a retinal dysfunction, developing early in the course of MS, due to a loss of specific subpopulations of inner neurons, changes in lateral interactions of their receptive fields, or both.

Adolescent↗