Showing papers on "Summation published in 1973"

Spatial summation of warmth: influence of duration and configuration of the stimulus.

Abstract: It was found that the spatial summation of warmth takes place primarily in the central nervous system and that the rules by which area and intensity trade to preserve a constant liminal or supraliminal sensation of warmth are basically the same for brief (.5 sec) and longer (3 sec) exposures as well as for unitary (on one dermatome) and split (on two dermatomes) fields. Warmth approximated a power function of thermal intensity, but the size of the function's exponent depended systematically on the areal extent and therefore, like spatial summation, on central neural factors. A striking feature of the warmth sense is its capacity to sum the neural effects of thermal stimulation over large areas of skin-up to at least several hundred square centimeters. This spatial summation reveals itself in measurements of absolute threshold. Intensity of stimulation and its areal extent can be traded for each other to preserve a threshold sensation. The rule of trading approximates a reciprocity so that, for example, the effect of halving thermal irradiance can be offset by approximately doubling the stimulated area (Hardy and Oppel, 1937; Kenshalo, Decker, and Hamilton, 1967). Spatial summation reveals itself also in measurements of supraliminal warmth. Irradiance and area can be traded to preserve any constant level of warmth above the absolute threshold. (Often, two combinations of area and irradiance that have equal warmth appear virtually indistinguishable even in their subjective areal dimensions.) The rules for trading area and irradiance vary, however, with level of warmth. With increasing warmth, area makes an ever smaller proportional contribution to the warmth sensation; accordingly, it takes an ever larger proportional change in area to compensate for a given change in irradiance (Stevens and Marks, 1971). In other words, the degree of spatial summation declines with increasing warmth and may fail altogether at a high enough level. Spatial summation reveals itself also in simple reaction time to thermal

A study of the H-reflex by single fibre EMG.

Abstract: The latency of consecutive H-reflex responses of single human triceps surae motoneurones varies up to 2,500 μs. A large part of this variation was shown to occur at the synaptic transmission. A moderate increase in stimulus strength from the threshold value shortened the mean latency and reduced the latency variation, presumably as a result of spatial summation of excitatory inputs. Further increase to maximum strength lengthened the mean latency, increased the variation, and resulted in a dropping out of some responses which was not produced by collision by antidromic impulses. These effects are believed to be due to an active inhibition. Changes of the latency were also obtained by Jendrassik9s manoeuvre and facilitatory and inhibitory conditioning stimuli.

Temporal summation at the warmth threshold

Abstract: Threshold levels for warmth aroused by infrared irradiation were measured in sLx Ss at durations between 0.05 and 10 sec. Beyond a critical duration of about a second, the threshold does not depend on duration. Below critical duration time t trades for irradiance ϕ by the formula ϕ = kt−0.82. That these properties do not depend much on areal extent of stimulation was demonstrated by a study that compared temporal summation for two different areas of the same S’s skin. Individual differences in apparent absolute sensitivity were explored under the rubric of the theory of signal detection.

Modification of the Motor Reflex Responses Due to Repetition of the Peripheral Stimulus in the Cockroach: II. Conditions of Activation of the Motoneurones

Abstract: 1. The synaptic transfer properties within the 3rd thoracic ganglion (T.G.) from the abdominal cord axons to the motoneurones has been studied in the cockroach. This ganglion was completely de-afferented except for motor nerve 4, whose links with the muscles of the posterior legs were left intact. 2. The response of one of these nerves to electrical stimulation of the abdominal cord involves activation of 2 types of units: (a) Slow excitatory fibres which have a tonic discharge and respond to each lowthreshold abdominal cord stimulation by a transient increase of this firing rate; these units are responsible for muscular tonus and for low amplitude movements (startle reactions). (b) Fast excitatory fibres, which have no tonic discharge and require for their activation higher intensity and frequency of stimulation, i.e. an important temporal and spatial summation. They are responsible for larger and more rapid movements. They fire without any precise chronological relation with the stimuli, often in bursts which continue after the end of the stimulations and cause sudden rhythmic movements. 3. During repetition of the stimuli, the two types of synaptic pathways show both habituation and facilitation through temporal summation and post-tetanic potentiation. These two phenomena persist after the end of the stimulations and have long (minutes) but different time courses. Moreover, habituation always prevails over facilitation if stimulations are continued during a sufficient time. These antagonistic properties existing at the same time might explain the complex way in which the motor responses develop with the application of repetitive trains of stimuli to the cord. 4. The role of these properties in the changes of the reflex motor responses to successive air puffs applied to the cerci has been studied. These properties appear to be responsible for the sensitization of the responses which can be sometimes observed. They lead, in conjunction with the habituation properties of the 6th abdominal ganglion (A.G.), to the disappearance of the escape reflex involving firing of both fast and slow fibres. Finally, they seem to have a minor role in habituation of the startle reactions (involving firing of only the slow fibres) which would be mainly due to the 6th A.G. habituability.

Temporal summation related to the nature of the proximal stimulus for the warmth sense

Abstract: Magnitude estimations of the warmth aroused by radiant stimulation of the forehead showed that warmth obeys the psychophysical power law when the’ duration of the stimulus is relatively long (3 or 6 sec). When duration is short (0.25, 0.5. or 1 sec), however, warmth grows as a more complex function of irradiation. The family of psychophysical functions measured for the various durations can be used to generate the rules by which radiant intensity and duration trade to preserve constant warmth. These rules vary systematically from complete temporal summation (i.e., complete reciprocity) near threshold to less and less complete summation as warmth level increases. When, however, the stimulus is expressed as an equivalent change of temperature in the skin, there is no summation, only adaptation. It can be shown that temporal summation observed psychophysically must result from the heat-transfer properties of skin tissue.

Temporal summation in the acoustic stapedius reflex mechanism.

Abstract: Temporal summation in the acoustic stapedius reflex mechanism was studied by monitoring acoustic impedance changes at the ear-drum. The results show that the duration of short acoustic stimuli (“white noise” and 1/1 octave bands of noise) has a pronounced effect on the stapedius reflex response. For a ten-fold increase of stimulus duration the intensity level had to be lowered by about 22 dB in order to elicit the criterion response (50% of maximal obtainable impedance change). For stimulus durations shorter than 5&80 ms, the relationship between sound intensity and stimulus duration was about the same for all types of stimuli, indicating that for such short stimuli temporal summation in the acoustic stapedius reflex is independent of the type of stimulus. For longer durations, several time constants of summation were found. The time constants were largely dependent on the type of stimulus. Possible explanations of these findings are discussed.

Spatial Summation of Retinal Excitation as Obtained by the Scotopic VECP and the Sensory Threshold

Abstract: The illumination required for a criterion latency of the scotopic VECP was compared with the sensory threshold in a number of situations where the area and the spatial distribution of the test stimulus was altered. Plots of the log area versus log illumination derived from both VECP (criterion latency) and psychophysical (threshold) measurements allowed the determination of slopes indicating summation indices. Areal summation was shown to be present within the whole area stimulated (up to 18° in diameter). It was found that the slopes in both sets of measurements probably reflect lack of retinal homogeneity rather than areal summation as such. Qualitatively, the slopes of both types of measurements are similar; quantitatively there are small systematic discrepancies.

Temporal Summation for Tones at Threshold

Abstract: The threshold for tones as a function of their duration (500, 200, 100, 50, 20, and 10 msec) was investigated with the tracking method in a population of 50 normal‐hearing listeners (25 male, 25 female; 25 right ears, 25 left ears) at nine frequencies from 125 through 8000 Hz under test‐retest conditions. The results indicate that temporal auditory summation, at least as measured with the tracking method, conforms with theoretical expectations. There was no variation in the function with frequency, sex, or ear. When the thresholds for individual listeners are normalized to the threshold for 20‐msec tones, the variability both among and within subjects is about the same at all frequencies for all duration tones (standard deviation of 2 dB). These results permit further investigation on the effect of other variables (e.g., hearing impairment) on the threshold for tones as a function of their duration. [Supported by a PHS Research Grant from NINDS.]

Psychometric Functions in Quiet for Long‐ and Short‐Duration Tones at 1000 Hz

Abstract: The psychometric function for 500‐ and 20‐msec tones was measured in quiet at 1000 Hz in three normal hearing listeners in an effort to determine whether differences in method have contributed to the variability among the results obtained by different investigators of temporal auditory summation at threshold. The obtained P (C) for the methods of adjustment, tracking, and PEST were related to their relative location on each psychometric function. Although the slopes of the psychometric functions, both among and within observers, were consistent (6%/dB), there was no consistency among observers in their respective P (C) positions on the psychometric functions, indicating that methodological considerations in the measurement of temporal summation are not as simplistic as they might first appear. [Supported by PHS Research Grant NS 9663 from NINDS.]