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A F Dean

Publications and source records attributed to A F Dean.

34 records · Page 2Linked to original sources
PubMed1983

Adaptation-induced alteration of the relation between response amplitude and contrast in cat striate cortical neurones.

The activity of neurones in cat striate cortex in response to laterally moving sinusoidal gratings has been examined. The effect of prolonged visual stimulation with a high contrast moving grating stimulus of optimal orientation and spatial frequency was investigated in terms of the changes wrought in the relationship between response amplitude and stimulus contrast. The adaptation which resulted was characterized by an elevation of contrast threshold by about 0.4 log units and a reduction in the steepness of the relation between response and contrast by about 0.36 log units. Both simple and complex cells exhibited this behaviour. The similarity of these values to the size of contrast threshold elevations demonstrated psychophysically suggests a possible physiological basis for the perceptual phenomenon.

Adaptation, Ocular↗
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

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↗
PubMed1983

Ambulatory surgery.

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Ambulatory Surgical Procedures↗
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

Non-linear temporal summation by simple cells in cat striate cortex demonstrated by failure of superposition.

Simple cells in area 17 of the anaesthetized, paralysed cat were stimulated with stationary sinusoidal gratings whose contrast was temporally modulated in different ways. The response to a temporal waveform which was the sum of two sinusoids (1.25 Hz and 7.75 Hz) was compared with the response elicited by each component when presented alone. The responses to the high temporal frequency in the compound stimulus were relatively enhanced by the addition of the low temporal frequency; those to the low frequency were relatively depressed.

Animals↗
PubMed1981

Preferred direction of movement as an element in the organization of cat visual cortex.

Neurones recorded close together in the cat's striate cortex prefer not only the same orientation of elongated visual stimulus but also the same direction of stimulus movement. The degree of similarity in both preferred orientation and preferred direction is greater in electrode penetrations made perpendicular to the cortical surface than in oblique penetrations. This suggests that preferred direction is organized in columnar fashion, just as is orientation.

Animals↗
PubMed1981

The variability of discharge of simple cells in the cat striate cortex.

The relationship between the variance and mean rate of discharges of simple cells in the cat striate cortex has been examined when mean rate was varied by changing either stimulus spatial frequency or contrast. In both cases, the variance was related to the mean discharge rate by an exponent of about 1.15; the relation was thus roughly linear. The discharge variance was on average 1.7 times the mean rate for data obtained from measurements of the neurones' spatial frequency tuning curves, and 1.48 times the mean for data from the response-contrast determination. However, this difference was not statistically significant.

Animals↗
PubMed1981

The relationship between response amplitude and contrast for cat striate cortical neurones.

1. The activity of forty-three neurones in the cat's striate cortex in response to laterally moving sinusoidal gratings of various contrasts was recorded, in order to examine the relationship between response amplitude and contrast. 2. Neurones seemed to exhibit contrast thresholds: stimuli of very low contrast failed to evoke a change in the response amplitude from the concurrent levels of spontaneous activity. 3. The suprathreshold portion of the response-contrast relation was found to be monotonic. Typically, the relation was adequately described as linear up to contrasts of about 0.3, above which, varying degrees of saturation were evident. 4. The response-contrast relation had a higher threshold and a shallower slope when the spatial frequency was not optimal for the neurone. 5. The slope, or gain, of the response-contrast relation for a stimulus of optimal orientation and spatial frequency varied considerably from neurone to neurone. The gains of special complex cells were significantly greater than those of either standard complex cells or simple cells. 6. The distributions of contrast threshold and contrast gain were examined for their dependence on optimal spatial frequency. Contrast threshold was significantly positively correlated with optimal spatial frequency, while contrast gain was significantly negatively correlated with optimal spatial frequency. This behaviour is consistent with an optical contribution to the measured response properties of striate cortical neurones.

Action Potentials↗
PubMed1980

Non-linearities of temporal summation in neurones in area 17 of the cat.

Sinusoidal temporal modulation of the contrast of sinusoidal gratings has been used to provide one description of the temporal properties of neurones in area 17 of the cat's visual cortex. Concurrently, the waveforms of the neurones' responses to 2 s flashes of identical gratings were examined. In most neurones, the response to the long flash was much more transient than would be expected from the shape of the temporal-frequency tuning curve. Temporal summation in cortical neurones is non-linear.

Animals↗