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Biomedical subjects

B C Skottun

Publications and source records attributed to B C Skottun.

At least 19 recordsLinked to original sources
PubMed2001

The ability of inferior colliculus neurons to signal differences in interaural delay.

Sound localization in humans depends largely on interaural time delay (ITD). The ability to discriminate differences in ITD is highly accurate. ITD discrimination (Delta ITD) thresholds, under some circumstances, are as low as 10-20 micros. It has been assumed that thresholds this low could only be obtained if the outputs from many neurons were combined. Here we use Receiver Operating Characteristic analysis to compute neuronal Delta ITD thresholds from 53 cells in the inferior colliculus in guinea pigs. The Delta ITD thresholds of single neurons range from several hundreds of micros down to 20-30 micros. The lowest single-cell thresholds are comparable to human thresholds determined with similar stimuli. This finding suggests that the highly accurate sound localization of human observers is consistent with the resolution of single cells and need not reflect the combined activity of many neurons.

Animals↗
PubMed2001

On the use of the Ternus test to assess magnocellular function.

Ternus stimuli give rise to two mutually exclusive visual experiences: with long interstimulus intervals (ISIs) the elements in the stimulus are perceived as moving together as a group ('group movement'), while at shorter ISIs only a single element appears to be moving ('element movement' or 'end-to-end movement'). It has been hypothesized that group and element movements, respectively, reflect magnocellular and parvocellular activity. On this basis, Ternus tests have been used to assess magnocellular function in dyslexic individuals. This use of Ternus stimuli is examined in the present report. On the basis of amplitude spectra of the stimuli and of a review of previous studies it is concluded that to use Ternus tests to assess magnocellular function is problematic.

Dyslexia↗
PubMed2001

On the use of metacontrast to assess magnocellular function in dyslexic readers.

It has been proposed that dyslexia is the result of a deficit in the magnocellular system. Reduced metacontrast masking in dyslexic readers has been taken as support for this view. In metacontrast, a masking stimulus reduces the visibility of a spatially adjacent target stimulus when the target stimulus precedes the masking stimulus by about 30-100 msec. Recent evidence indicates that the latency difference between the magnocellular and parvocellular subcortical pathways is at most 20 msec and may be as small as only 5 msec, or even less. This makes it difficult to attribute the latency in metacontrast to the latency differences between the magnocellular and parvocellular systems. It is therefore problematic to attribute reduced metacontrast masking to a deficit in the magnocellular system.

Animals↗
PubMed2000

Hyperacuity and the estimated positional accuracy of a theoretical simple cell.

Receiver operating characteristic (ROC) analysis was used to estimate positional discrimination thresholds for a theoretical simple cell by assuming Poisson distribution of the response, average response strength, and an optimal spatial frequency of 16.0 cyc/deg. These thresholds were compared to the positional difference required to generate a response change of one action potential. This comparison indicated that the inability to alter the response by less than one spike may be limiting positional accuracy. Taking account of this limitation, displacement thresholds were, depending on parameters, estimated to be as small as, or smaller than, the lowest psychophysical thresholds of about 2 sec of arc. This suggests that it may be possible to account for even the lowest human hyperacuity thresholds in terms of single cortical neurons.

Animals↗
PubMed2000

The magnocellular deficit theory of dyslexia: the evidence from contrast sensitivity.

A number of authors have made the claim that dyslexia is the result of a deficit in the magnocellular part of the visual system. Most of the evidence cited in support of this claim is from contrast sensitivity studies. The present review surveys this evidence. The result of this survey shows that the support for the magnocellular deficit theory is equivocal. In the case of spatial contrast sensitivity there clearly are results that are consistent with the magnocellular deficit theory; however, these results are outnumbered both by studies that have found no loss of sensitivity and by studies that have found contrast sensitivity reductions that are inconsistent with a magnocellular deficit. Many of the studies of temporal contrast sensitivity are also difficult to reconcile with a magnocellular deficit. The evidence from studies of contrast sensitivity is therefore highly conflicting with regard to the magnocellular system deficit theory of dyslexia.

Contrast Sensitivity↗
PubMed2000

Amplitude and phase in the Müller-Lyer illusion.

It has previously been claimed that the Müller-Lyer illusion is the result of low-pass spatial filtering. One way to understand this would be that the distribution of amplitudes is what generates this illusion. This possibility was investigated by computing the 2-D Fourier transforms of the two Müller-Lyer stimuli and extracting their phase and amplitude spectra. These spectra were combined to create hybrid spectra having the phase of one Müller-Lyer figure and the amplitudes of the other. Images were then created by computing the inverse Fourier transform of the hybrid spectra. Except in cases where the analysis was performed patchwise on very small patches, the figures generated with the phase spectrum of the stimuli having outward-pointing fins appear the longer. This was also the case when stimuli were generated with flat amplitude spectra. Because they show that the Müller-Lyer illusion does not depend on any particular distribution of amplitudes, these demonstrations do not support the theory that the Müller-Lyer illusion is the result of low-frequency filtering.

Computer Graphics↗
PubMed1999

Neuronal responses to plaids.

The majority of neurons in the visual cortex are orientation selective. When presented with a plaid, i.e. a stimulus generated by adding two gratings of different orientations, these neurons respond to the individual gratings making up the plaid. However, there are some pattern selective neurons in Area MT of the monkey visual cortex which respond in accordance with the combined plaid. The present study used computer simulation to investigate the response properties of simulated MT neurons to orthogonal plaids. The MT neurons were simulated by first multiplying the outputs of conventional orientation selective V1 neurons and then normalizing the product. It was discovered that pattern selective responses may emerge when the outputs from two orientation selective neurons, which differ in optimal orientation by more than about 50 degrees, are combined in this manner. This demonstrates that pattern selectivity may be the result of a very simple although nonlinear mechanism.

Animals↗
PubMed1998

A note on the possibility of explaining why a color cannot be both red and green.

To provide a neurophysiological basis for the opponent nature of color vision it has been previously argued that a color cannot be both red and green because color-opponent neurons cannot respond to both red and green at the same time. The present analysis shows that such arguments hinge on the possibility of excluding statements of the kind "a color can be both red and green." For an empirical fact to exclude such statements, these statements would have to be meaningful. However, statements like "a color is both red and green" are not meaningful and are not allowed in our language. Thus, the properties of neurons are not in a position to exclude the possibility of "a color that is both red and green." This means that this attempt to establish a neurophysiological basis for opponent colors is flawed.

Color Perception↗
PubMed1998

A model for end-stopping in the visual cortex.

A model for end-stopping that requires only excitatory inputs is presented. This model is based on multiplication of the outputs from two orientation tuned and spatial-frequency selective neurons. Computer simulations show that, provided the optimal orientations of the two neurons are sufficiently different, the resulting product will display orientation-independent end-stopping. Neurons simulated in this manner display the main characteristics of actual hypercomplex cells.

Fourier Analysis↗
PubMed1994

Illusory contours and linear filters.

The border between two abutting gratings is sometimes referred to as an illusory contour. In the Fourier spectrum of such a pattern there is no energy corresponding to the precise orientation of this contour. Neuronal responses elicited by such stimuli are therefore thought to be incompatible with linear filtering. The present investigation tested this notion. Abutting gratings were Fourier analyzed and passed through spatial and temporal filters with properties comparable with those of cortical neurons. It was found that the distribution of amplitudes in the Fourier spectrum is such that these stimuli may indeed stimulate neurons based on linear filtering. In the case of drifting stimuli the amount of stimulation may be maximal when the illusory contour is at the optimal orientation for bars with less stimulation on either side of this orientation.

Cerebral Cortex↗
PubMed1991

Classifying simple and complex cells on the basis of response modulation.

Hubel and Wiesel (1962; Journal of Physiology, London, 160, 106-154) introduced the classification of cortical neurons as simple and complex on the basis of four tests of their receptive field structure. These tests are partly subjective and no one of them unequivocally places neurons into distinct classes. A simple, objective classification criterion based on the form of the response to drifting sinusoidal gratings has been used by several laboratories, although it has been criticized by others. We review published and unpublished evidence which indicates that this simple and objective criterion reliability divides neurons of the striate cortex in both cats and monkeys into two groups that correspond closely to the classically-described simple and complex classes.

Animals↗
PubMed1988

Responses of simple and complex cells to random dot patterns: a quantitative comparison.

1. There are several reports that random dot patterns are potent stimuli for cortical complex cells but not for simple cells. This finding is regarded as evidence against Hubel and Wiesel's hierarchical model of cortical circuitry, in which simple cells are the principal input to complex cells. We have reinvestigated the question quantitatively by recording responses to dot patterns from 106 cells in area 17 and the 17/18 border region of normal adult cats. 2. The cells were classified as simple (n = 62) or complex (n = 40) (4 were end stopped or hypercomplex) on the basis of whether they gave modulated (AC) or unmodulated (DC) responses to drifting sine gratings. 3. Although there are large within-group differences, we found both simple and complex cells that respond to bright random dots on a dark background, drifted across the receptive field at 3 degrees/s. The responses at the optimal direction averaged 6.2 and 18.1 spikes/s (spontaneous activity subtracted) for simple and complex cells, respectively. 4. We also recorded responses to drifting sine gratings. Complex cells were also found to respond more than simple cells to these stimuli. For each cell, we calculated a dot index expressing the dot response relative to grating response. The dot index averaged 0.43 for simple cells and 0.55 for complex cells. It therefore appears that much of the difference in response to dot patterns reflects a difference in general responsivity. 5. In subsamples of cells, we examined the effects of varying dot density, dot size, and drift velocity. These variables affect different cells in a manner largely independent of cell class. Most simple cells in our sample responded well to random dot patterns at several velocities, at two different dot sizes and at both 3 and 50% dot densities. 6. Our results agree with previous studies in showing that complex cells respond more vigorously than simple cells to dot patterns, but the fact that many simple cells also respond to these stimuli makes our results consistent with a hierarchical model of cortical circuitry.

Animals↗