Showing papers on "Summation published in 1983"

Spatio-temporal interactions in cat retinal ganglion cells showing linear spatial summation.

Abstract: The spatio-temporal characteristics of cat retinal ganglion cells showing linear summation have been studied by measuring both magnitude and phase of the responses of these cells to drifting or sinusoidally contrast-modulated sinusoidal grating patterns. It has been demonstrated not only that X cells behave approximately linearly when responding with amplitudes of less than about 10 impulses/sec to stimuli of low contrast but also that cells of another type with larger receptive field centres (Q cells) behave approximately linearly under the same conditions. These Q cells appear to form a homogeneous group which is probably a subset of the tonic W cells (Stone & Fukuda, 1974) or sluggish centre-surround cells (Cleland & Levick, 1974). The over-all spatio-temporal frequency characteristics of cells showing linear spatial summation are not separable in space and time. The form of the spatial frequency responsivity function of these cells depends upon the temporal frequency at which it is measured while the temporal phase of their resonse measured at any constant temporal frequency depends upon the spatial frequency of the stimulus. The behaviour of X and Q cells is quite well explained by an extension of the model in which signals from centre and surround mechanisms with radially Gaussian weighting functions are summed to provide the drive to the retinal ganglion cell. While the general form of the temporal frequency response characteristics of these ganglion cells are probably provided by the characteristics of elements common to the centre and surround pathways, the spatio-temporal interactions can be explained by assuming that the surround signal is delayed relative to the centre signal by a few milliseconds.

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

Abstract: 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.

Non-pain and pain sensations evoked by tooth pulp stimulation.

Abstract: This study investigated the quality and magnitude of sensations evoked by electrical tooth pulp stimulation. Detection threshold (the minimum current intensity that evoked a sensation) and pain threshold were determined for tooth pulp stimuli varying in frequency from 5 to 500 Hz. The effect of frequency and intensity of tooth pulp stimulation on the magnitude of sensations was assessed using visual analog scales and verbal descriptor scales. Detection thresholds were stable over experimental sessions and independent of the frequency of the stimulating current. Pain threshold varied as a function of frequency with a minimum value at 100 Hz. Stimuli that evoked non-pain sensations at low frequencies evoked pain sensations when frequency was increased from 5 to 100 Hz. Subjects were able to scale non-pain sensations over a range of stimulus intensities and frequencies. The lowest currents evoked sensations that were nonpainful and were of constant magnitude despite changes in the frequency of stimulation. Higher stimulus currents evoked sensations that were non-painful at low stimulus frequencies and painful at high stimulus frequencies. Sensation magnitude at each stimulus intensity increased as a function of frequency. Temporal summation occurred in proportion to stimulus intensity. These findings suggest that the non-pain sensations evoked in tooth pulp are mediated by a distinct population of afferents that are not involved in the coding of pain. High frequency stimulation that increased the discharge rate of the lowest thershold pulpal afferents resulted in no summation of non-pain sensation and never produced pain. However, high frequency stimulation evocked greater magnitude sensations at higher stimulas currents, indicating that central summation mechanisms were critical for higher threshold afferents signaling more intense non-pain and pain sensations.

Vibrotactile temporal summation for threshold and suprathreshold levels of stimulation

Abstract: Threshold measurement and matching procedures were used to determine the amount of temporal summation at threshold and suprathreshold levels of vibrotactile stimulation on the thenar eminence of the hand. The frequency of the stimulus was 25, 40, 80, or 200 Hz. At 25 Hz, temporal summation was absent at all intensity levels. Considerable amounts of temporal summation were observed for 80- and 200-Hz stimuli, although the effects decreased as a function of intensity. At 40 Hz, no temporal summation was observed at threshold, but above threshold, a small amount was observed at all intensity levels. The results support a duplex model of mechanoreception in which one of two receptor systems exhibits temporal summation.

Hyperpolarization following activation of K+ channels by excitatory postsynaptic potentials

Abstract: We have postulated that an excitatory postsynaptic potential (e.p.s.p.) may open voltage-sensitive K+ (‘M’) channels1, in an appropriate depolarizing range, and that this could alter the e.p.s.p. waveform. Consequently, the fast e.p.s.p. in neurones of sympathetic ganglia, elicited by a nicotinic action of acetylcholine (ACh)2, could be followed by a hyperpolarization, produced by the opening of M channels during the depolarizing e.p.s.p. and their subsequent slow closure (time constant∼150 ms)1. This introduces the concept that transmitter-induced p.s.ps may trigger voltage-sensitive conductances other than those initiating action potentials, and that in the present case this could produce a true post-e.p.s.p. hyperpolarization. (Some hyperpolarizations other than inhibitory postsynaptic potentials (i.p.s.ps) have been reported to follow e.p.s.ps3,4.) We show here that this is so.

Spatial summation in the receptive field of simple cells in the cat striate cortex

Abstract: Spatial summation was studied quantitatively through width response curves made with an optimally oriented test slit of variable width, and by comparing the response to combined presentation of several parallel slits with the response to each slit alone. Prior to summation analysis, the cell's discharge field (DF) was mapped by presenting a test slit ON and OFF across the receptive field. Activation profiles, showing the extension of subregions where light stimulation increased (enhancement) or decreased the firing rate (suppression), were made by presenting an optimally oriented activation slit in the most responsive DF-position. Against this activity the effects of a parallel test slit were determined in a series of broadside positions. Width response curves were made over the subregions of the DF and the activation profiles. Spatial summation was found in all cells, but the width of the summation region was smaller than the width of the subregions in the respective profiles. The width of the summation region was related to the degree of activation rather than to specific locations within the receptive field. The effect produced by several slits presented together deviated from the algebraic sum of the effects produced by each slit alone. Linear summation was rarely found. Accumulated response curves obtained by integration of DF or activation profiles were compared with width response curves to test linearity of summation. Linear summation throughout the whole receptive field was never found. A satisfactory fit was found only over a narrow region showing that summation was linear within a small part of the summation region. Linearity ended near response maxima or minima in the response profiles. The results indicate that the receptive field of simple cells consists of overlapping excitatory and inhibitory fields, and that the exact location and width of enhancement and suppression zones are determined by an activity-dependent balance between excitatory and inhibitory inputs.

Frequency Selectivity and Temporal Processing in Friedreich's Ataxia: Clinical Aspects in Two Patients

Abstract: We performed psychoacoustical tuning curves and measured temporal summation in two patients with Friedreich's ataxia to study aspects of frequency selectivity and temporal processing in the auditory system. In the majority of frequencies tested, normal psychoacoustical tuning curves and unimpaired temporal summation functions were obtained. These findings suggest that the processing mechanism responsible for these phenomena is intact.

Three Models of Temporal Summation Evaluated Using Normal-Hearing and Hearing-Impaired Subjects

Abstract: Temporal summation effects were measured in normal-hearing and hearing impaired subjects using stimuli of different durations and temporal patterns. Threshold decreased with increasing stimulus dur...

Temporal Resolution in the Peripheral Visual Field

Abstract: Despite considerable interest in the development of flicker perimetry, little work has been reported on the basic temporal response characteristics of vision across the normal visual field. Those few studies that have addressed this problem have been restricted to the determination of the critical duration for Bloch’s law of complete temporal summation or to the determination of critical flicker fusion frequency. In the present study, the entire temporal summation and temporal contrast sensitivity (TCS) functions were determined at the fovea and at 10, 20, 40 and 50 degrees in the temporal field. A single-channel Maxwellian-view optical system was used which allowed exact superposition of stimulus and adaptation fields, each of which subtended 5.15° of visual angle. Temporal summation functions (Experiment I) were obtained by pulsing square-wave stimuli upon the adaptation field of 500 trolands, for durations from 1 to 999 msec. TCS functions (Experiment II) were obtained by sinusoidal modulation about a mean luminance of 500 trolands, at frequencies from 0.5 to 50 Hz. In both experiments an ascending method of limits was used, with stimulus luminance (Experiment I) or sine-wave amplitude (Experiment II) increased until a threshold response was obtained. Data were obtained from 3 normal observers. The results indicated that neither temporal summation nor TCS functions changed with eccentricity.

Study of the functional characteristics of visual cortex neurons in the cat upon local photic stimulation of their receptive fields

Abstract: Functional characteristics of 103 neurons in field 17 of the visual cortex were studied in immobilized and unanesthetized cats. Unit responses to modification of local light stimuli wer recorded under conditions of light adaptation. Mean photic threshold of investigated neurons was -32 dB, mean critical temporal summation 57 ms, duration of reactivity recovery 190 ms and sharpness of orientation tuning 37 degrees. As compared to dark adaptation, under light adaptation reduction of light sensitivity, sharpening of orientation tuning as well as a lengthening of temporal summation and a shortening in time of reactivity recovery were observed. A number of neuronal characteristics depended on excentricity of their receptive fields: cells with centrally localized receptive fields had a lower threshold, a short summation and rapid recovery of reactivity; their activity was of higher frequency, and they generated brief transient responses more often than cells with receptive fields in the periphery of a visual field. Mechanisms responsible for change of neuronal properties in the central and peripheral visual channels under light and dark adaptation are discussed. Several inhibitory subsystems are suggested by means of which neuronal activity of the visual cortex is regulated.

Functional characteristics of visual cortical neurons during local photic stimulation of their receptive fields in cats

Abstract: A group of functional characteristics of 103 neurons in visual cortical area 17 was investigated in acute experiments on curarized, light-adapted cats during a change in various parameters of the local photic stimuli. The average threshold sensitivity of the neuron population was 32 dB (0.052 nit), the sharpness of orientation tuning was 37°, the critical summation time was 57 msec, and the reactivity recovery time 190 msec. Photic sensitivity was lower during light adaptation than during dark adaptation, orientation selectivity of the neurons was increased, temporal summation was lengthened, and the time required by the neuron to recovery from after-inhibition was shortened. Several properties of the cortical neurons depended on the accentricity of their receptive fields: Cells with centrally localized receptive fields on average had lower thresholds and shorter summation time and they recovered their reactivity more quickly; their activity was of a higher frequency and they more often generated short phasic discharges than neurons with receptive fields in the peripheral part of the visual field. The mechanisms responsible for changes in the properties of neurons in the central and peripheral visual channels during dark and light adaptation are discussed. The presence of several inhibitory subsystems in the cortex regulating unit activity in the primary visual projection area is postulated.

The effects of stimulus pulse rise-decay time and duration on lingual vibrotactile thresholds

Abstract: The present study consisted of two experimental phases. In Experiment 1, the effects on lingual vibrotactile threshold of four different stimulus pulse rise-decay times were investigated for 10 normal adult subjects. The results showed no statistically significant differences between thresholds obtained with the use of the four rise-decay times. In Experiment 2, the effects on lingual vibrotactile threshold of four different stimulus pulse durations were investigated for 10 normal adult subjects. A statistically significant difference was noted between thresholds obtained with stimulus pulse durations of land 50 msec.