Showing papers on "Summation published in 1970"

Invariant features of spatial summation with changing locus in the visual field.

Abstract: 1. Spatial summation curves have been determined under photopic conditions for loci between 5 and 55° from fixation. At each locus the area and Weber fraction for the largest stimulus showing complete summation (Sc) has been estimated. While the area of Sc increases progressively towards the periphery, its Weber fraction remains constant. 2. The invariance of the Weber fraction for Sc, coupled with changes in its area, have been compared with similar findings observed during changes in adaptation level, in impaired fields, and during accommodative and convergence changes. It is suggested that the Weber fraction for Sc may be an important parameter of visual function which is maintained under changing conditions by alterations in the area over which background energy is summated. 3. It has been shown that, for stimulus areas up to at least 1 log unit greater than Sc, all the summation curves from 5 to 55° from fixation can be superimposed by simple displacement along the log area axis. For loci closer together, the superimposition has been shown for larger ranges of areas beyond Sc. This suggests that, so far as spatial summation is concerned, changes in locus serve only to change the spatial scale of the visual system. 4. Comparisons have been made between the area of Sc at various loci and the sizes of the central regions of the receptive fields of retinal ganglion cells determined electrophysiologically in the primate.

Temporal Summation Phenomena at Threshold: Their Relation to Visual Mechanisms

Abstract: Threshold energies were determined for brief flashes as a function of their duration in order to determine the maximum duration for which the flash intensity and duration could be varied reciprocally without affecting detectability (the Bunsen-Roscoe effect). A pair of threshold-level flashes for which reciprocity obtained in the determination of threshold were shown to be discriminable from each other at several imperfectly detectable energy levels. Thus equal detectability of flashes of equal energy does not imply identical neural responses to such stimuli. It is suggested that the summation reflects primarily the operation of the detection mechanism rather than of the peripheral visual mechanism. Some general implications for the interpretation of threshold measures are also discussed.

A quantitative analysis of the spatial summation of excitation within the receptive field centers of retinal neurons.

Abstract: In light-adapted cats action potentials of single optic tract fibers were recorded. Four light-stimuli of 0.09 degree diameter and 0.18 degree distance were projected to the receptive field center (sinusoidal stimulation 0.5 to 25 cps, degrce of modulation>0.9, maximal luminance 54 to 122 asb). The position of the 4 spot stimulus pattern was adjusted within the RF-center, as each light-spot elicited approximately the same neuronal activation. The spatial summation of the response of on-center and off-center neurons to one, two, three and four-spot illumination does not support a hypothesis of linear spatial summation and lateral interaction within the receptive field. The summing properties of on-center neurons in the RF-center are best described bymodels with multiplicative, lateral forward inhibition within the receptive field center [eq. (5–7)]. The inhibition coefficientk i in these equations was dependent on the stimulus frequency. It decreased with stimulus frequencies between 3 and 10 cps and increased above 10 cps. The experimental results indicate that the non-linear spatial summation of excitation within the RF-center is dependent on the temporal properties of the stimulus pattern.

Temporal summation and refractoriness in hypothalamic reward neurons as measured by self-stimulation behavior.

Abstract: A neurophysiological technique of double-pulse stimulation has been applied to freely moving rats with chronic indwelling electrodes in the hypothalamic reward area. Self-stimulation thresholds, measured as a function of the interpulse interval, generated curves with time constants characteristic of refractory periods and temporal synaptic summation. The results indicate a way of studying central neuronal processes for which the overt behavior of the animal is the dependent variable.

Behavioral measurement of neural poststimulation excitability cycle: pain cells in the brain of the rat.

Abstract: A new technique in which elicited behavior of the freely moving rat is used to measure the poststimulation excitability cycle of the central neurons mediating that behavior has been adapted from accepted methods of neurophysiology. A continuous train of pairs of brief pulses was delivered to pain systems in the midbrain. Rate of lever pressing to achieve 3-second rests from this stimulation was measured as a function of the interval separating the pulses within pairs. Evidence for latent addition, absolute refractory period, temporal summation, and adaptation was demonstrated. Obtained relationships suggested that three sets of fibers may carry the aversive signal and that synaptic integration of pain in the brain may be related to Stevens' power law functions.

A behavioral measure of homosynaptic and heterosynaptic temporal summation in the self-stimulation system of rats

Abstract: Bilateral stimulation of the medial forebrain bundle with pulses of varying interpulse intervals elicited a pattern of self-stimulation behavior in the rat indicative of temporal synaptic summation: the shorter the interval the greater the response. In contrast, the effectiveness of unilateral stimulation at very short intervals was limited by neuronal refractory periods. The results support the notion that there is convergence of the medial forebrain bundle self-stimulation system from the two sides of the brain and suggest ways of studying the degree of convergence. They also suggest a technique for behaviorally comparing heterosynaptic and homosynaptic mechanisms of summation.

Spatially-limited attention to vibrotactile stimulation

Abstract: Weak vibrotactile signals were presented to human Os, in some instances to the index finger alone, in other instances to the middle finger alone, and in still other instances to the two fingers simultaneously. When the 0 knew in advance of a trial which finger(s) would be presented a signal, no spatial summation resulted, that is, the sensitivity of two fingers to two signals was no greater than the sensitivity of one finger to one signal. When the 0 was left uncertain about which finger(s) would be presented a signal on any given trial, a result having the appearance of spatial summation was obtained. Theappearance is misleading;the difference between two-finger and one-finger sensitivity in this case reflects, instead, a decrement in the performance of the single fingers. Both results are consistent with a single-channel model of attention.

Functional classification of neurons of the cat inferior colliculus on the basis of temporal characteristics

Abstract: We investigated the impulse activity of 103 neurons in cats anesthetized with a mixture of chloralose and urethane. The following response characteristics were studied in detail: 1) the latent period of the first impulse as a function of tone intensity; 2) threshold as a function of duration; 3) the frequency-threshold curves under the action of short (1–2 msec) and long (100–200 msec) tones; 4) the discharge "pattern" and the number of impulses under the action of signals with different durations and intensities. We demonstrated that the temporal characteristics of different neurons exhibit substantial differences. The high positive correlation among the temporal characteristics investigated enabled us to distinguish three groups of neurons. The first group was characterized by the following properties: a) short latent periods with a threshold tone intensity; 2) a short summation time (or none at all); 3) similarity of the frequency-threshold curves for short and long signals; 4) an initial on-discharge of 1–3 impulses. The second group was characterized by: 1) long latent periods with a threshold tone intensity; 2) pronounced temporal summation; 3) steeper frequency-threshold curves when the tone duration was increased; 4) prolonged discharge.

Neural Activity in the Somatic Primary Receiving Area of the Human Brain and Its Relation to Perceptual Estimates

Abstract: Experiments were performed to establish the relation between scales of perceived intensity and primary cortical potentials obtained under identical stimulation conditions. When the maximal positive-negative deflection of the primary component of the averaged evoked potential was plotted against stimulus displacement, all data were well represented by power functions with exponents of nearly the same size as those of the psychophysical functions. The results indicate also that spatial summation takes place when two neighboring locations are stimulated with an increase in apparent intensity of only about ?2 of that corresponding to monodigital stimulation.