Topic
Summation
About: Summation is a research topic. Over the lifetime, 954 publications have been published within this topic receiving 45593 citations. The topic is also known as: summation & sum of a sequence.
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TL;DR: Recurrent excitatory postsynaptic potentials evoked by stimulation of the pyramidal tract were found to be sensitive to N-methyl-D-aspartate receptor antagonists and/or non-N- methyl- D- aspartate receptors in some neurons, and that the synaptic pharmacology of the thalamic input may differ from that of the local recurrent pathways.
37 citations
TL;DR: The convergence of rod signals through increasing spatial summation serves to decrease the variation of responses to dim flashes, thereby increasing the signal-to-noise ratio and boosting small amplitude signals to the level required for initiation of action potentials in the ganglion cells.
Abstract: Responses to light were recorded from rods, horizontal cells, and ganglion cells in dark-adapted toad eyecups. Sensitivity was defined as response amplitude per isomerization per rod for dim flashes covering the excitatory receptive field centers. Both sensitivity and spatial summation were found to increase by one order of magnitude between rods and horizontal cells, and by two orders of magnitude between rods and ganglion cells. Recordings from two hyperpolarizing bipolar cells showed a 20 times response increase between rods and bipolars. At absolute threshold for ganglion cells (Copenhagen, D.R., K. Donner, and T. Reuter. 1987. J. Physiol. 393:667-680) the dim flashes produce 10-50-microV responses in the rods. The cumulative gain exhibited at each subsequent synaptic transfer from the rods to the ganglion cells serves to boost these small amplitude signals to the level required for initiation of action potentials in the ganglion cells. The convergence of rod signals through increasing spatial summation serves to decrease the variation of responses to dim flashes, thereby increasing the signal-to-noise ratio. Thus, at absolute threshold for ganglion cells, the convergence typically increases the maximal signal-to-noise ratio from 0.6 in rods to 4.6 in ganglion cells.
37 citations
TL;DR: It is suggested that γ-hydroxybutyrate can activate presynaptic as well as postsynaptic GABAB receptors in rat hippocampal neurones in vitro, and was reversibly inhibited by a novel and selective GABAB antagonists, CGP 36742.
37 citations
TL;DR: A simple model for the spatial receptive-field organization of nonlagged X-cells is presented, that is consistent with the findings, and suggests that both excitatory and inhibitory transmission in dLGN are close to linear.
Abstract: Spatial summation and degree of center-surround antagonism were examined in the receptive field of nonlagged cells in the dorsal lateral geniculate nucleus (dLGN). We recorded responses to stationary light or dark circular spots that were stepwise varied in width. The spots were centered on the receptive field. For a sample of nonlagged X-cells, we made simultaneous recordings of action potentials and S-potentials, and could thereby compare spatial summation in the dLGN cell and in the retinal input to the cell. Plots of response versus spot diameter showed that the response for a dLGN cell was consistently below the response in the retinal input at all spot sizes. There was a marked increase of antagonism at the retinogeniculate relay. The difference between the retinal input and dLGN cell response suggested that the direct retinal input to a relay cell is counteracted in dLGN by an inhibitory field that has an antagonistic center-surround organization. The inhibitory field seems to have the same center sign (ON- or OFF-center), but a wider receptive-field center than the direct retinal input to the relay cell. The broader center of the inhibitory field can explain the increased center-surround antagonism at the retinogeniculate relay. The ratio between the response of a dLGN cell and its retinal input (transfer ratio) varied with spot width. This variation did not necessarily reflect a nonlinearity at the retinogeniculate relay. Plots of dLGN cell response against retinal input were piecewise linear, suggesting that both excitatory and inhibitory transmission in dLGN are close to linear. The variation in transfer ratio could be explained by sustained suppression evoked by the background stimulation, because such suppression has relatively stronger effect on the response to a spot evoking weak response than to a spot evoking a strong response. A simple model for the spatial receptive-field organization of nonlagged X-cells, that is consistent with our findings, is presented.
37 citations
TL;DR: A model of 'physiological' integration that is based on excitatory lateral interactions in the visual cortex is explained, which explains several phenomena which are confirmed by the experimental data, such as the absence of spatial and temporal uncertainty effects, temporal summation curves, and facilitation by a pedestal in 2AFC tasks.
Abstract: Recent studies of visual detection show a configuration dependent weak improvement of thresholds with the number of targets, which corresponds to a fourth-root power law. We find this result to be inconsistent with probability summation models, and account for it by a model of 'physiological' integration that is based on excitatory lateral interactions in the visual cortex. The model explains several phenomena which are confirmed by the experimental data, such as the absence of spatial and temporal uncertainty effects, temporal summation curves, and facilitation by a pedestal in 2AFC tasks. The summation exponents are dependent on the strength of the lateral interactions, and on the distance and orientation relationship between the elements.
36 citations