Showing papers on "Summation published in 1984"

Responses of human cutaneous afferents to CO2 laser stimuli causing pain.

Abstract: Microelectroneurographic studies in man allow the comparison of stimulus induced activity in the single peripheral nerve unit with the subject's ratings of sensation. Relationships between stimulus intensity, single unit discharges, and pain ratings were investigated using a CO2 laser stimulator which delivers radiant heat pulses of 50 ms duration. Recordings were performed percutaneously from the radial nerve at the wrist. Receptor types were identified by their response to different stimulus modalities and by their reaction delay to electrical test stimuli within the receptive field. Receptive fields of identified units were stimulated with randomised series of different radiant heat intensities between half and double the individual pain threshold (5 to 20 W; stimulation area 64 mm2). The largest receptor class observed to be activated by CO2 laser stimuli were polymodal C-nociceptors. None of them was spontaneously active. High discharge rates up to 75/s were not necessarily associated with pain but, if pain was felt, the impulse trains usually lasted for more than 60 ms. Inter-spike intervals were distributed over a wide range between 8 and 145 ms with a peak at about 25 ms. This peak was only slightly shifted by increasing the stimulus intensity. Higher correlations were found between the number of spikes and stimulus intensity. Measures of Signal Detection Theory indicated that the single unit discharges discriminated stimulus intensities better than the subjects' ratings. These findings underline the importance of temporal summation in the processing of C-fibre input with a considerable loss of information in the nociceptive system.

Individual Differences in Loudness Processing and Loudness Scales

Abstract: Parameters of the psychophysical function for loudness (a 1000-Hz tone) were assessed for individual subjects in three experiments: (a) binaural loudness summation, (b) temporal loudness summation, and (c) judgments of loudness intervals. The loudness scales that underlay the additive binaural summation closely approximated S. S. Stevens's (1956) sone scale but were nonlinearly related to the scales that underlay the subtractive interval judgments, the latter approximating Garner's (1954) lambda scale. Interindividual differences in temporal summation were unrelated to differences in scaling performance or in binaural summation. Although the exponents of magnitude-estimation functions and the exponents underlying interval judgments varied considerably from subject to subject, exponents computed on the basis of underlying binaural summation varied less. The results suggest that interindividual variation in the exponent of magnitude-estimation functions largely reflects differences in the ways that subjects use numbers to describe loudnesses and that the sensory representations of loudness are fairly uniform, though probably not wholly uniform, among people with normal hearing. The magnitude of individual variation in at least one measure of auditory intensity processing, namely, temporal summation, seems at least as great as the magnitude of the variation in the underlying loudness scale.

Temporal integration of pungency

Abstract: Four experiments explored possible temporal summation in olfaction and the common chemical sense. In one experiment, participants judged the perceived magnitude of various concentrations and durations (1.25 - 3.75 s) of the pungent odorant ammonia and the nonpungent odorant isoamyl butyrate. The perceived magnitude of ammonia increased during an inhalation whereas the magnitude of isoamyl butyrate did not. Time-intensity trading relations for ammonia indicated nearly perfect temporal summation. In another experiment, modulation of the concentration of ammonia during an inhalation led to assessments of perceived magnitude that confirmed the high degree of temporal summation seen in the first experiment. That is, approximately equal time-integrated mass of inhaled ammonia led to approximately equal perceived intensity. A third experiment indicated that temporal summation for ammonia arose from its pungency rather than from its odor, a fourth that trigeminally-mediated reflex apnea in response to ammonia also exhibits temporal summation. The degree of temporal summation measured with the reflex came very close to that assessed psychophysically. When stimulated with ammonia, the common chemical sense behaves more like a total mass detector than a concentration detector. The investigation raises the possibility that the short-term sensory reaction to most pungent stimuli may follow this simple rule.

Nonlinear characteristics in the Frog's visual system

Abstract: The receptive fields of retinal fibers in the visual tectum of the frog are mapped with different techniques and the spatial summation characteristics are examined, by presenting stimuli of various shapes and sizes in the center of the receptive field. When the size is increased gradually from the center of the stimulus, for constant stimulus intensity, the maximum response is obtained for stimuli of approximately the size of the most responsive part of the RF. Using a clustering technique to obtain stimuli that are part of the RF and combinations of these parts, it is evident that the spatial summation characteristics are not linear. A model is developed that describes the nonlinear form of these results, based on a power law.

Auditory Temporal Integration at Threshold: Theories and Some Implications of Current Research

Abstract: Publisher Summary This chapter presents diverse theories and implications of current research on the auditory temporal integration at threshold. The different theoretical explanations of auditory temporal summation have been concerned mainly with the following issues: the basic nature of the underlying mechanism (probabilistic or deterministic), the locus of integration (peripheral or central), the type of energy integrated (acoustic or neural), the effect of the power spectrum, and the effects of the temporal pattern of stimuli (tones versus periodical pulses). The most frequent stimulus parameters considered in the context of auditory temporal integration are frequency and bandwidth of stimuli, the effects of masking, the mode of stimulation, the phasic characteristics of the stimuli, and the level of stimulation. At threshold, the ear integrates the acoustic energy of a sinusoid signal linearly up to about 250 msec, that is, in this range, a 10-fold increase in duration decreases the signal intensity necessary for threshold by 10 decibel. The influence of different stimulus conditions on auditory temporal summation has proven to be complex. The probabilistic approach may be considered as the converse of the complete summation hypothesis to the extent that it assumes the absence of any physical or physiological summation. The results of recent investigations calls for the design of experiments aimed at a systematic mapping of the relations among different integration and acuity measures utilizing a variety of behavioral tasks, all within a common framework.

Retinal inhomogeneity. II. Spatial summation

Abstract: Effects of spatial probability summation were measured with concentric cosine patterns forming a set of contiguous annular zones having mean eccentricities of 0°, 2.8°, and 6.1°. To detect these small effects reliably we measured contrast thresholds for all eccentricities at one spatial frequency in each experimental session by an interleaved-staircase method. Sharp edges were eliminated by truncating each zone at a zero crossing of the pattern, and fixation was controlled by stabilizing the retinal image. Under these conditions, each local region of the retina contributes to the sensitivity of larger regions by a standard, fourth-power summation rule, regardless of the number or contiguity of the component regions involved. This was experimentally confirmed with three nested zones for spatial frequencies from 0.5 to 5 cycles/degree. Together with the results of Part I of this series [ D. H. Kelly , J. Opt. Soc. Am. A1, 107 ( 1984)], the summation rule predicts that, over a large range, target size should have no effect on contrast sensitivity at high spatial frequencies, and this was also confirmed. Similar predictions were extrapolated to other contrast-sensitivity data for which empirical tests are not available.

Flicker and suprathreshold spatial summation: evidence for a two-channel model of achromatic brightness.

Abstract: Spatial summation of brightness was measured in the light-adapted periphery for targets that either flickered at 20 Hz or remained steady during a 1-sec exposure. For each target condition, stimuli of different diameters were adjusted in intensity to match a constant foveal reference target in subjective brightness. For comparison, increment thresholds were also measured. Large differences were found among the resulting equal-brightness functions of three normal observers. The data are consistent with a two-channel model of achromatic brightness. The spatial and temporal properties of the two brightness channels resemble those of the tonic and phasic systems of electrophysiology.

Temporal integration in self-stimulation: a paradox.

Abstract: Temporal summation of the rewarding effects of medial forebrain stimulation in the rat was investigated by varying the interval separating the two short bursts of stimulation given as a reward in a runway. One finding--that the reinforcing effect of the two bursts is independent of interburst interval--supports a model in which there is perfect summation of the portion of the reward signal that exceeds some threshold. However, the constant-threshold form of this model is not reconcilable with the results of a second experiment, which shows that charge-duration functions obtained with different levels of performance differ by a multiplicative (scalar) factor, that is, the ratio between the values of the two functions is everywhere the same. (The charge-duration function gives the charge required as a function of the train duration.) None of the models of postsynaptic integration so far suggested is capable of explaining simultaneously the fact that the strength-duration function is a perfect hyperbola that has nearly reached its rheobase at a train duration no greater than 2 s, that there is no statistically detectable effect of interburst interval on summation between bursts separated by intervals up to 2 s and longer, and that the strength-duration functions (or, equivalently, the charge-duration functions) derived by using different performance criteria differ by a multiplicative factor.

Spatial summation in subregions of simple-cell receptive fields in cat striate cortex as a function of slit length.

Abstract: Spatial summation along the optimum stimulus orientation in subregions of simple-cell receptive fields in cat striate cortex was studied quantitatively by measuring the response to stationary light slits of variable length. Before summation analysis, the cell's discharge field was mapped by flashing a test slit on and off in a sequence of positions through the receptive field. A static activation procedure was used to determine the extension of subregions of the receptive field where light stimulation increased (enhancement) or decreased (suppression) the firing rate. An activation slit in the optimum orientation was positioned in the most responsive position of the discharge field and the effects of a parallel test slit, in a series of broadside positions, were assessed from the changes induced in the discharge elicited by the activation slit. Length-response curves for on and off responses were made by positioning a test slit in the respective subregions of the discharge field. The activation procedure was used to make length-response curves for suppression. A test slit of variable length was positioned in a suppression region defined by the activation profiles and an activation slit of fixed length was centred in the most responsive discharge field position. Length summation was found for all cells, both with respect to on and off responses, and suppression. The curves for on and off responses had a maximum value beyond which the response declined or levelled off, but some cells had a secondary, more shallow increase beyond an initial, steeply rising part. Similar properties were found for summation of suppression except that the effects were opposite in sign. Curves made for both on and off regions in the same cell often differed in shape. Such differences were also found when length-response curves made in different suppression regions of the same cell were compared. The various length-summation functions were explained by a model presuming that simple-cell receptive fields consist of partially overlapping non-concentric excitatory and inhibitory fields. This arrangement would also explain why length-response curves in various subregions often had different shapes.

Acoustic reflex temporal summation measured at threshold.

Abstract: This study measured the threshold of the acoustic reflex as a function of reflex-activating stimulus duration. Acoustic reflex thresholds (ARTs) were measured for tonal stimuli at frequencies of 1000 and 3000 Hz and broadband noise at six durations between 20 and 500 msec for both normal and hearing-impaired subjects. Traditional techniques and response averaging were used. Results suggest that these methods have a significant influence on ART and reflex temporal summation at threshold. Lowest thresholds and almost no temporal summation were found with a response averaging technique and a bidirectional shift criterion, whereas clinical procedures yielded highest thresholds and greater summation. These results suggest a need to examine the methodology used for measurement of the acoustic reflex and the interpretation of clinical pathology based on measurements of reflex temporal summation.

Spatial Summation in Dichromats

Abstract: Threshold vs. stimulus area relationships of eight protanopes, eight deuter- anopes and eight normal subjects were investigated. The data was interpreted to provide an integration diameter as a measure of spatial integration. It was shown that the dichromats behave as if they lack the red-green opponent system of the normal. The dichromats demonstrated less spatial summation even though the retinal ganglion cells most probably mediating the response have larger receptive fields.