Showing papers on "Summation published in 1979"

Absolute timing of mental activities

Abstract: A new tool that may measure certain absolute temporal properties of information processing in intact organisms is suggested by. investigations of temporal summation in single nerve cells. Two findings have led to this suggestion: One, the form of the temporal summation function (relating the intensity and duration required to evoke a criterion neural signal) depends on the analysis used by the investigator. Corresponding form variations occur in behavioral studies when the observer's task is varied. Two, the critical durations of fixed neural signals depend on the latency of the feature of the signal chosen as criterion; early features yield short critical durations and vice versa. The critical duration also varies in behavioral studies if one varies the observer's task, keeping the stimulus ensemble fixed.These data lead to two inferences: One, the form of a behavioral temporal-summation function expresses the kind of hidden mental analysis mediating that behavior. Two, a behavioral critical duration is an indicator of the absolute timing of the hidden mental analysis mediating that behavior.

Empirical examination of the threshold model of neuron firing

Abstract: An elementary model of neuronal activity involves temporal and spatial summation of postsynaptic currents that are elicited by presynaptic spikes and that, in turn, elicit postsynaptic potentials at a trigger zone; when the potential at the trigger zone exceeds a "threshold" level, a postsynaptic spike is generated. This paper describes three methods of estimating the "summation function", that is, the function of time that converts the synaptic current into potential at the trigger zone: namely, maximum likelihood, cross-correlation analysis and cross-spectral analysis. All three methods, when applied to input-output data collected on various neurons of Aplysia californica, give comparable results. As estimated, the summation function involved in the explored cells has an early positive-going swing that is large and brief. In the cell L5, but not in R2, there was also a late negative-going swing of longer duration.

Temporal summation of pulsate brain stimulation in normal and deafened cats

Abstract: Behaviorally measured, electrical‐stimulation thresholds were obtained from 11 electrodes permanently positioned in the auditory system and other brain loci. Number of pulses and interpulse intervals were varied to determine how detection thresholds were affected by stimulation parameters. Detection thresholds generally decreased with increased number of pulses and with shorter interpulse intervals. A method is presented to describe the parametric threshold data for each electrode in terms of three constants: a single‐pulse threshold which characterizes the sensitivity of the placement; a time constant of temporal summation; and a compression factor which describes the range of threshold variation. For three placements in the vicinity of cochlear nucleus, bilateral cochlear destruction permanently altered parametric thresholds. In particular, single‐pulse threshold was lowered by 9.2 dB; time constant of temporal summation was reduced by a factor of 100; and the compression factor was increased. Classic strength‐duration time constants were determined using behavioral methods and were shown to be equal in magnitude to the greatly reduced time constants for temporal summation in the deafened animals. This implies that capacity for temporal integration may be substantially reduced or lost in at least the lower level of the auditory system following deafness.

Non-linear spatial summation in cat retinal ganglion cells at different background levels.

Abstract: Cat retinal ganglion cells were identified as X-cells (linear) or Y-cells (non-linear) on the basis of the spatial summation properties of their receptive fields. For each cell, the degree of non-linearity in spatial summation was assessed at a number of different mean luminance levels in order to determine how spatial linearity depended on mean luminance. The stimuli were counterphase sinusoidal gratings whose contrast was sinusoidally modulated in time. A grating with one bar centered on the receptive field was used to measure the contrast sensitivity of the mechanisms which produced responses at the stimulus frequency. A grating with a zero crossing centered on the receptive field was used to measure the contrast sensitivity of mechanisms responsible for the non-linear frequency doubled responses of Y-cells. As the mean luminance was reduced from low photopic to scotopic, the contrast sensitivity decreased for both the linear and non-linear responses. The ratio of non-linear to linear sensitivity in Y-cells changed less with background than did either contrast sensitivity. In some Y-cells this ratio decreased slightly at low luminance levels, but in others it did not. X-cells appeared to sum signals linearly at all levels of illumination. X-cells and Y-cells could still be distinguished on the basis of their spatial summation properties in the scotopic range.

Spatial summation in human vision: simple reaction time measurements.

Abstract: The reaction-time technique was applied to examine spatial summation or area-intensity reciprocity at suprathreshold levels in the fovea. A family of reaction time vs luminance curves was measured in two experiments, and the luminance required to produce a criterion reaction time was computed from these curves to estimate the extent of summation. The first experiment showed that the upper level of spatial summation, the Ricco area, defined for constant reaction time increases with decreasing luminance level and that the upper limit of temporal summation is independent of the change in target size. The second indicated that the Ricco area decreases with increasing pulse duration up to 20–30 ms and then remains constant.