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.

Temporal Summation during Dark Adaptation

Abstract: The change of critical duration and of the form of the function relating threshold energy to stimulus duration was determined during the first 22 s of dark adaptation following light adaptation to several luminances. For all adapting luminances tested, the critical duration increased during dark adaptation from the value obtained for the increment threshold. As adapting luminance decreased or time in the dark increased, the form of the summation function changed, so that the transition from complete temporal summation to no summation became less abrupt. The data show departures from the equivalent-background analysis of dark adaptation. A filter model relating threshold energy and time constant under varying adaptation conditions provides a general, but not precise, description of the data. The change of temporal summation during dark adaptation is similar to that found for decreased size of test field, decreased wavelength of test field, and decreased background luminance.

Supraspinal characterization of the thermal grill illusion with fMRI

Abstract: Simultaneous presentation of non-noxious warm (40°C) and cold (20°C) stimuli in an interlacing fashion results in a transient hot burning noxious sensation (matched at 46°C) known as the thermal grill (TG) illusion. Functional magnetic resonance imaging and psychophysical assessments were utilized to compare the supraspinal events related to the spatial summation effect of three TG presentations: 20°C/20°C (G2020), 20°C/40°C (G2040) and 40°C/40°C (G4040) with corresponding matched thermode stimuli: 20°C (P20), 46°C (P46) and 40°C (P40) and hot pain (HP) stimuli. For G2040, the hot burning sensation was only noted during the initial off-line assessment. In comparison to P40, G4040 resulted in an equally enhanced response from all supraspinal regions associated with both pain sensory/discriminatory and noxious modulatory response. In comparison to P20, G2020 presentation resulted in a much earlier diminished/sedative response leading to a statistically significantly (P < 0.01) higher degree of deactivation in modulatory supraspinal areas activated by G4040. Granger Causality Analysis showed that while thalamic activation in HP may cast activation inference in all hot pain related somatosensory, affective and modulatory areas, similar activation in G2040 and G2020 resulted in deactivation inference in the corresponding areas. In short, the transient TG sensation is caused by a dissociated state derived from non-noxious warm and cold spatial summation interaction. The observed central dissociated state may share some parallels in certain chronic neuropathic pain states.

Spatial summation in the human fovea: Do normal optical aberrations and fixational eye movements have an effect?

Abstract: Psychophysical inferences about the neural mechanisms supporting spatial vision can be undermined by uncertainties introduced by optical aberrations and fixational eye movements, particularly in fovea where the neuronal grain of the visual system is fine. We examined the effect of these preneural factors on photopic spatial summation in the human fovea using a custom adaptive optics scanning light ophthalmoscope that provided control over optical aberrations and retinal stimulus motion. Consistent with previous results, Ricco's area of complete summation encompassed multiple photoreceptors when measured with ordinary amounts of ocular aberrations and retinal stimulus motion. When both factors were minimized experimentally, summation areas were essentially unchanged, suggesting that foveal spatial summation is limited by postreceptoral neural pooling. We compared our behavioral data to predictions generated with a physiologically-inspired front-end model of the visual system, and were able to capture the shape of the summation curves obtained with and without pre-retinal factors using a single postreceptoral summing filter of fixed spatial extent. Given our data and modeling, neurons in the magnocellular visual pathway, such as parasol ganglion cells, provide a candidate neural correlate of Ricco's area in the central fovea.

Distinct properties of two major excitatory inputs to hippocampal pyramidal cells: a computational study.

Abstract: The two main sources of excitatory input to CA1 pyramidal cells, the Schaffer collaterals (SC) and the perforant path (PP), target different regions of the dendritic tree. This spatial segregation may have important consequences for the way in which different inputs affect the activity of principal neurons. We constructed detailed biophysical models of CA1 pyramidal cells, incorporating a variety of active conductances, and investigated the ability of synapses located in different dendritic segments to elicit a somatic voltage response. Synaptic efficacy as seen by the soma was strongly dependent on the site of the synapse, with PP inputs being more severely attenuated than SC inputs. Variability within SC inputs, but not between SC inputs and PP inputs, could be eliminated by appropriate scaling of synaptic efficacy. The spatial and temporal summation of multiple synaptic inputs was also investigated. While summation of SC inputs was linear up to the somatic spike threshold, PP inputs summed in a strongly sublinear fashion, with the somatic response remaining subthreshold even following the simultaneous activation of a large number of synapses and during stimulation with high-frequency trains. Finally, the relative impact of different pathways on somatic activity could be effectively altered by modulating the kinetic properties of dendritic transient K+ channels, corresponding to the activation of ascending modulatory neurotransmitter systems. In this case, the efficacy of the PP was enhanced by the dendritic generation and limited spread of action potentials. Strong PP activation could also evoke dendritic Ca++ spikes, which often triggered a somatic burst.