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D J Tolhurst

Publications and source records attributed to D J Tolhurst.

At least 19 recordsLinked to original sources
PubMed1994

Discrimination of changes in the second-order statistics of natural and synthetic images.

It has been suggested that the second-order statistics of different natural images are all remarkably similar and that neurones and channels in the visual system may exploit this similarity. We have measured the ability of human observers to discriminate changes in these statistics using different natural and synthetic stimulus images and have found that the dependence of their discrimination thresholds upon the reference second-order statistics is similar in form, for both kinds of stimuli. However, there is some variety in the magnitudes of the thresholds for the natural stimulus images; in fact, the second-order statistics of different natural images are more diverse than previously suggested. The discrimination task can be modelled as the discrimination of changes in local contrast within restricted spatial frequency bands and is similar to the discrimination of blur.

Contrast Sensitivity↗
PubMed1993

Both the phase and the amplitude spectrum may determine the appearance of natural images.

Several studies have demonstrated that the appearance of a natural image is determined mostly by its global phase spectrum; the amplitude spectrum is regarded as providing little specific information. In this paper, we show several cases where the global amplitude spectrum is also essential for specifying the particular content of natural images. Thus, both the amplitude and the phase spectrum may be important for reliable specification of such images. The results suggest that, although the average amplitude spectra of different natural images may be similar in their overall form, a realistic description of the amplitude spectra must also incorporate the particular way in which the energy is distributed across different orientations.

Humans↗
PubMed1992

Amplitude spectra of natural images.

Several studies have suggested that the amplitude spectra of photographs of natural scenes are remarkably similar and have the form: amplitude varies; is directly proportional to spatial frequency-1.0. This is, of course, a straight line with slope of -1.0 when plotted on double logarithmic coordinates. We have examined the amplitude spectra of 135 digitized photographs of natural scenes and have found that relatively few images conform exactly to the suggestion. About 25% of the images in our sample have spectra which show significant curvature when plotted on log-log coordinates. The best-fitting regression lines have slopes that range from -0.8 to -1.5; the average slope is -1.2, rather steeper than previously suggested.

Animals↗
PubMed1992

Effect of myelination on the conduction velocity of optic nerve fibres.

It was proposed by Rushton in 1951, from theoretical considerations, that myelinated fibres less than 1 micron in diameter would conduct more slowly than unmyelinated fibres of the same size and that myelinated fibres below about 0.7 micron would not conduct at all. The experimental data on which he based his theory are all from the peripheral nervous system where small myelinated fibres are rare, and no experimental verification of Rushton's hypothesis has been attempted. In mammalian optic nerve, nearly all the fibres are myelinated; yet half have diameters below 1 micron, with many below 0.7 micron. The many studies of conduction velocities in the visual system enable a test of Rushton's hypothesis to be made. We have examined the correlations between conduction velocity and fibre diameter from a wide range of published studies of the mammalian visual system. The results of our analysis suggest that the small myelinated fibres of the optic nerve and optic tract conduct action potentials more rapidly than is predicted by Rushton's hypothesis, while the unmyelinated axons within the retina actually conduct more slowly than predicted. There is no reason to believe, in this case, that myelination of a small axon will reduce its conduction velocity.

Action Potentials↗
PubMed1991

Evaluation of a linear model of directional selectivity in simple cells of the cat's striate cortex.

We have compared the responses of simple cells to laterally moving sinusoidal gratings and to stationary temporally-modulated gratings. From the amplitudes and temporal phases of the responses to stationary gratings of different spatial phases, it should be possible to predict the preferred direction of movement, the amplitudes of the responses to gratings moving in the preferred and nonpreferred directions and, thence, the degree of directional preference (Reid et al., 1987). The preferred direction can be predicted reliably. However, the magnitude of the directional preference cannot be predicted, since the measured amplitude of the response in the nonpreferred direction of movement is very much less than that predicted by a linear theory. Nonlinearities in the relationship between response amplitude and contrast may contribute to the failure of the predictions, but this contribution is small. We conclude that the magnitude of the directional preference seems to be determined predominantly by nonlinear suppression of the response in the nonpreferred direction of movement.

Animals↗
PubMed1990

The effects of contrast on the linearity of spatial summation of simple cells in the cat's striate cortex.

Non-linearities of spatial summation were examined in simple cells in the cat's striate cortex. The degree of non-linearity was assessed from an examination of the waveforms of the responses to moving sinusoidal gratings and was quantified by a measure called relative modulation. Relative modulation was affected little by changes in contrast at either optimal or non-optimal spatial frequencies. The non-linearities of spatial summation exhibited by some simple cells are, therefore, essential. Those simple cells which exhibit linear spatial summation are no less linear at high stimulus contrasts. These results support a 'push-pull' model of simple cell receptive field organization in which ON and OFF centre l.g.n. input is combined both additively and subtractively.

Animals↗
PubMed1989

The amount of information transmitted about contrast by neurones in the cat's visual cortex.

The responses of neurones in the cat's visual cortex are very variable in amplitude. Thus, although the average response amplitude of a single neurone depends closely upon the contrast of a sinusoidal grating, the instantaneous amplitude of the response can convey little information about the grating's contrast. This paper shows that a typical cortical neurone can convey less than one bit of information about contrast in 0.5 s. The amount of information that a neurone can convey is closely correlated with the neurone's responsivity.

Animals↗
PubMed1989

The effect of threshold on the relationship between the receptive-field profile and the spatial-frequency tuning curve in simple cells of the cat's striate cortex.

It is believed that spatial summation in most simple cells is a linear process. If this were so, then the Fourier transform of a simple cell's line weighting function should predict the cell's spatial frequency tuning curve. We have compared such predictions with experimental measurements and have found a consistent discrepancy: the predicted tuning curve is much too broad. We show qualitatively that this kind of discrepancy is consistent with the well-known threshold nonlinearity shown by most cortical cells. We have tested quantitatively whether a response threshold could explain the observed disagreements between predictions and measurements: a least-squares minimization routine was used to fit the inverse Fourier Transform of the measured frequency tuning curve to the measured line weighting function. The fitting procedure permitted us to introduce a threshold to the reconstructed line weighting function. The results of the analysis show that, for all of the cells tested, the Fourier transforms produced better predictions when a response threshold was included in the model. For some cells, the actual magnitude of the response threshold was measured independently and found to be compatible with that suggested by the model. The effects of nonlinearities of spatial summation are considered.

Animals↗
PubMed1988

Magnification factors and the organization of the human striate cortex.

We review the evidence relating to the organization of the human striate cortex to determine whether the latter is organized in a way similar to that of the macaque monkey. There are only limited data available to allow direct determination of the magnification factors in human striate cortex. These suggest that the linear magnification factor is about 1.6 times greater than that in the macaque. This is consistent with the observations that the human striate cortex, compared to that of the macaque, has over 2 times the area, has neurones with longer dendrites, wider ocular dominance columns and more widely separated cytochrome oxidase "blobs". The striate cortices in both species have the same total number of neurones. The foveal magnification factor in the macaque is presently believed to be about 15 mm.deg-1; it seems reasonable, therefore, to believe that the human foveal value will be about 20-25 mm.deg-1 rather than the value of 8-11 mm.deg-1 which is currently accepted. Magnification factor falls more rapidly with eccentricity than do current estimates.

Animals↗
PubMed1987

Spatial summation by simple cells in the striate cortex of the cat.

Spatial summation has been studied in simple cells of the cat's visual cortex by examining the responses of pairs of lines. One line was placed in an ON region of the receptive field; the other was placed in an OFF region. When the luminances of the lines were modulated in anti-phase, the excitatory responses to the individual lines were almost synchronous. A simple cell's overt response to the composite stimulus was usually greater than the sum of the overt responses to the two components. The result could be explained by supposing that the underlying response was the linear sum of the excitatory signals but that an overt response occurred only when the underlying response exceeded a fixed threshold value. This was true even of simple cells which exhibited non-linearities of spatial summation, as judged from the waveforms of their responses to moving sinusoidal gratings. When the two lines were modulated in phase, the excitatory responses occurred in different halves of the temporal cycle. Some cells summed antagonistic signals linearly. The waveforms of their responses to moving sinusoidal gratings also implied linear spatial summation. However, other cells whose responses to moving gratings implied linearity of summation did not, in fact, sum antagonistic signals linearly. The excitatory responses evoked in a receptive field region were weaker than the inhibitory responses that could be evoked in the same region. The remaining cells did not sum antagonistic signals linearly. There was imperfect cancellation, resulting in the generation of ON-OFF response components. The excitatory responses evoked in a receptive field region were stronger than the inhibitory responses that could be evoked in the same region. These cells gave responses to sinusoidal gratings that did imply nonlinear spatial summation.

Animals↗
PubMed1986

The structure and symmetry of simple-cell receptive-field profiles in the cat's visual cortex.

Receptive fields of simple cells in the cat visual cortex have recently been discussed in relation to the 'theory of communication' proposed by Gabor (1946). A number of investigators have suggested that the line-weighting functions, as measured orthogonal to the preferred orientation, may be best described as the product of a Gaussian envelope and a sinusoid (i.e. a Gabor function). Following Gabor's theory of 'basis' functions, it has also been suggested that simple cells can be categorized into even- and odd-symmetric categories. Based on the receptive field profiles of 46 simple cells recorded from cat visual cortex, our analysis provides a quantitative description of both the receptive-field envelope and the receptive-field 'symmetry' of each of the 46 cells. The results support the notion that, to a first approximation, Gabor functions with three free parameters (envelope width, carrier frequency and carrier phase) provide a good description of the receptive-field profiles. However, our analysis does not support the notion that simple cells generally fit into even- and odd-symmetric categories.

Animals↗
PubMed1986

Factors influencing the temporal phase of response to bar and grating stimuli for simple cells in the cat striate cortex.

We have characterized the speed of response of simple cells in cat striate cortex by the temporal phase of the response to bar and grating stimuli. Stimulation of the most responsive subregion (either ON or OFF) in the receptive field with a 1 Hz temporally modulated bar elicited responses whose phase led the excitatory phase of the stimulus by about 25 degrees. The response to stationary gratings whose contrast was sinusoidally modulated at 2 Hz also showed a phase lead. The differences in the phase of response of ON and OFF sub-regions exhibited a marked scatter about the expected value of 180 degrees. The phase of response to both temporally modulated bars and laterally moving gratings advanced by 20-35 degrees as the stimulus contrast was raised by a factor of 5.

Animals↗
PubMed1983

The statistical reliability of signals in single neurons in cat and monkey visual cortex.

The variability of the discharge of visual cortical neurons in cats and macaque monkeys limits the reliability with which such neurons can relay signals about weak visual stimuli. In general, the variance of a neuron's firing rate is directly proportional to its mean firing rate. The probability that a neuron will fire a criterion number of impulses on a stimulus trial grows monotonically with the contrast of a sinusoidal grating stimulus. Neural probability functions prepared either by computing the probability of criterion response or by integrating receiver operating characteristics to yield the probability of correct choice in a two-alternative forced-choice situation resemble psychometric functions obtained in psychophysical and behavioral experiments on humans and animals, but are shallower in slope. The slopes of neuronal probability functions are slightly higher when they are estimated over short time periods, but even so do not equal the slopes measured psychophysically in human and monkey observers. This discrepancy in slope could be explained if the whole observer responded only when about four neurons were active together.

Animals↗
PubMed1983

Recovering the parameters of finite mixtures of normal distributions from a noisy record: an empirical comparison of different estimating procedures.

Three procedures were compared for their ability to estimate the known parameters of mixtures of normal distributions: maximum likelihood estimators (MLE), X2 minimization method and the least-square error procedure. For this purpose a Monte-Carlo study was undertaken to evaluate empirically the performance of the estimators. We chose to investigate their behaviour using closely-positioned mixtures of two or 3 univariate Gaussians. The Monte-Carlo simulations clearly demonstrate the superiority of the MLE and X2 minimum methods. Other reasons why the MLE is to be preferred are discussed. The effect of sample size was also examined. All 3 estimators have also been applied to data derived from different physiological experiments, and the use of the estimators is considered in practical terms. The formulae for all 3 procedures are given in the Appendix.

Animals↗
PubMed1983

On the distinctness of simple and complex cells in the visual cortex of the cat.

The behaviour of neurones in cat striate cortex was examined in response to moving sinusoidal gratings and flashed bright and dark lines. The responses were summarized by three indices: discreteness was a measure of the degree of separation of inhibitory and excitatory regions in the receptive field; spatial summation ratio showed the degree of spatial summation within each region; relative modulation was a measure of the degree of modulation in the response to a moving grating. Some neurones had receptive fields with completely discrete excitatory and inhibitory regions; others responded equally to stimulus onset and offset throughout their receptive fields; however, some had overlapping excitatory and inhibitory regions. The degree of overlap varied continuously from complete separation to complete overlap. For neurones with discrete receptive fields, the widths of the regions were compared with the width of the bars in a grating of optimum spatial frequency to assess the degree of spatial summation within the regions. Most neurones with discrete receptive fields showed roughly predictable spatial summation, in that the two width measures agreed; but about 10% of them had receptive field regions that were too large by a factor of over two. The neurones which showed incomplete spatial summation also had considerable overlap of their excitatory and inhibitory regions. The waveforms of the responses to moving gratings of optimal spatial frequency were examined. The degree of modulation in the response was continuously distributed between low values typical of complex cells and high values typical of simple cells; the distribution was not bimodal. The degree of response modulation was closely correlated with the degree to which the excitatory and inhibitory regions in the receptive field were discrete. Both the degree of spatial summation and the degree of response modulation have been previously proposed as means for distinguishing simple and complex cells. In the present study, the continuity of the distributions of both indices ensured that neither index alone could be used to class all neurones unequivocally. However, a criterion based on two indices did allow classification. Simple and complex cells showed distinctive behaviour. However, complex cells with distinguishable excitatory and inhibitory regions in their receptive fields were not distinctly different from other complex cells.

Animals↗
PubMed1982

The distribution of acetylcholinesterase in the lateral geniculate nucleus of the cat and monkey.

The distribution of acetylcholinesterase (AChe) has been examined histochemically in the lateral geniculate nucleus (LGN) of the cat and the monkey, and in the cat visual cortex. It was found that in the cat, AChE is most concentrated in laminae A and A1. Lamina C-proper possessed a weak band of AChE in its ventral part. Only restricted patches of activity were observed in the medial interlaminar nucleus. Laminae C1-3 and the central interlaminar nucleus possessed very little AChE. This pattern of enzyme distribution suggests that in the cat LGN, AChe activity coincides with the sites of neurophysiologically recorded X-cells, which are predominantly found in laminae A and A1 and are scarce in the C laminae and the medial interlaminar nucleus. The presence of AChE over neurones in layer VI of both areas 17 and 18 of the cerebral cortex in the cat suggests the corticothalamic pathway as one possible source of geniculate AChE activity. In the monkey LGN, AChE activity was observed in the parvocellular and magnocellular layers. The activity was greatest in the magnocellular layers, which are believed to contain neurones driven predominantly by retinal Y-cells. Thus, for this species the correlation between AChE activity and X-cells does not seem to hold.

Acetylcholinesterase↗
PubMed1982

Organization of neurones preferring similar spatial frequencies in cat striate cortex.

The optimal spatial frequencies were determined for over 300 neurones in cat striate cortex. Neurones recorded within about 100 microns were more likely to have similar optimal spatial frequencies than were neurones recorded at greater separations. But, even neurones recorded close together sometimes differed markedly in their optima; these differences could not be attributed to differences in receptive field eccentricity. When one lamina was sampled more than once, on different electrode penetrations, the different samples of neurones did not often have similar optimal spatial frequencies. To investigate whether neurones in laminae or in columns prefer the same spatial frequencies, data from normal and oblique penetrations were compared. Little difference in the degree of organization was seen in the two kinds of penetrations.

Animals↗