A. F. Dean

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 neurons 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.

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.

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

SummaryWe 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 sub-region (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°. 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°. The phase of response to both temporally modulated bars and laterally moving gratings advanced by 20–35° as the stimulus contrast was raised by a factor of 5.

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

SummaryNon-linearities of spatial summation were examined in simple cells in the cats 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.

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.

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

SummarySpatial summation has been studied in simple cells of the cats visual cortex by examining the responses to 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 cells 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 non-linear spatial summation.

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

SummaryNeurones recorded close together in the cats 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.

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

SummarySimple 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 responses 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.

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.