What is spatial summation and temporal summation?

Although not as widely studied as temporal summation for vibratory stimuli, impulse stimuli also demonstrate temporal summation. Increasing the duration of a pulse from 0.35 ms out to 10 ms results in a more than threefold drop in threshold (amplitude of the pulse). Beyond 5 ms, the threshold remains nearly constant.

View chapterPurchase book

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780080450469019033

Synaptic physiology II

Mordecai P. Blaustein MD, in Cellular Physiology and Neurophysiology, 2020

Temporal and spatial summation of postsynaptic potentials determine the outcome of synaptic transmission

An average CNS neuron can receive as many as 200,000 synaptic inputs, some excitatory and some inhibitory. Activation of a single excitatory synapse initiates an EPSP that is insufficient to driveVm to the AP threshold. For the postsynaptic neuron to reach threshold, several EPSPs must occur at about the same time so that their individual depolarizations can sum to produce a larger response. If, however, an IPSP coincides with an EPSP in the postsynaptic cell, then the EPSP will be reduced in amplitude. A neuron continuously sums its excitatory and inhibitory inputs; the result of this process—the net depolarization at the axon hillock (Fig. 5.1)—determines whether or not an AP is fired.

The neuronal membrane sums, or integrates, synaptic inputs occurring over space and time. This summation is affected by two passive membrane properties, the length constant (λ), and the membrane time constant (τm) (Chapter 6). Recall that λ determines the distance over which a subthreshold change ofVm can spread passively (Chapter 6). If two excitatory synapses at different locations on the postsynaptic neuron are active at the same time, the net effect of their depolarizations at any location (including the axon hillock) will be determined by a process calledspatial summation. Spatial summation is affected by λ: a large λ means that inputs from distant synapses can sumeffectively, whereas a small λ means that distant inputs will be attenuated and thus will sum poorly. If a single excitatory synapse is activated in rapid succession, the consecutive postsynaptic depolarizations sum in a process calledtemporal summation. Temporal summation is affected by τm, which determines the temporal persistence of a subthreshold change inVm (Fig. 13.9). A larger τm means that a synaptic response will last longer and thus be able to sum with a later synaptic input, to generate a larger resultant response.

Postsynaptic Potentials and Synaptic Integration

John H. Byrne, in From Molecules to Networks (Third Edition), 2014

Temporal Summation Allows Integration of Successive PSPs

Temporal summation can be illustrated by firing action potentials in a presynaptic neuron and monitoring the resultant EPSPs. For example, in Figs. 16.14A and 16.14B, a single action potential in sensory neuron 1 produces a 1-mV EPSP in the motor neuron. Two action potentials in quick succession produce two EPSPs, but note that the second EPSP occurs during the falling phase of the first, and the depolarization associated with the second EPSP adds to the depolarization produced by the first. Thus, two action potentials produce a summated potential that is about 2 mV in amplitude. Three action potentials in quick succession would produce a summated potential of about 3 mV. In principle, 30 action potentials in quick succession would produce a potential of about 30 mV and easily drive the cell to threshold. This summation is strictly a passive property of the cell. No special ionic conductance mechanisms are necessary. Specifically, the postsynaptic conductance change [gsyn in Eq. 16.13] produced by the second of two successive action potentials adds to that produced by the first. In addition, the postsynaptic membrane has a capacitance and can store charge. Thus, the membrane temporarily stores the charge of the first EPSP, and the charge from the second EPSP is added to that of the first. However, the “time window” for this process of temporal summation very much depends on the duration of the postsynaptic potential, and temporal summation is possible only if the presynaptic action potentials (and hence postsynaptic potentials) are close in time to each other. The time frame depends on the duration of changes in the synaptic conductance and the time constant (Chapter 17). Temporal summation, however, is rarely observed to be linear as in the preceding examples, even when the postsynaptic conductance change (gsyn in Eq. 16.13) produced by the second of two successive action potentials is identical to that produced by the first (i.e., no presynaptic facilitation or depression), the synaptic current is slightly less because the first PSP reduces the driving force (Vm – Er) for the second. Interested readers should try some numerical examples.

What is spatial summation and temporal summation?

Figure 16.14. Temporal and spatial summation.

(A) Intracellular recordings are made from two idealized sensory neurons (SN1 and SN2) and a motor neuron (MN). (B) Temporal summation: A single action potential in SN1 produces a 1-mV EPSP in the MN. Two action potentials in quick succession produce a dual-component EPSP, the amplitude of which is approximately 2 mV. (C) Spatial summation: Alternative firing of single action potentials in SN1 and SN2 produce 1-mV EPSPs in the MN. Simultaneous action potentials in SN1 and SN2 produce a summated EPSP, the amplitude of which is about 2 mV.

View chapterPurchase book

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780123971791000166

Temporal Properties of Vision

Leonard A. Levin MD PhD, in Adler's Physiology of the Eye, 2011

Temporal summation and the critical duration

To detect the presence of something in the visual world, it must be present for a finite period of time. Although a single quantum of light may be sufficient to generate a neural response, multiple quanta are generally required during a short period before the light is reliably seen, a property known astemporal summation. In the human visual system, temporal summation occurs for durations of approximately 40 to 100 milliseconds, depending on the spatial and temporal properties of the object and its background, the adaptation level, and the eccentricity of the stimulus.1–5 The maximum time over which temporal summation can occur is thecritical duration.

Let's say we wish to determine how long a light needs to be presented on a dark background to be visible. In general terms, a more intense light does not need to be presented for as long as a less intense one to reach threshold visibility. The relationship between the luminance of the light and the duration of its exposure to reach visibility is linear over a limited range. Provided that the light pulse is shorter than the critical duration, it will be at threshold when the product of its duration and its intensity equals a constant. The formula that describes this time–intensity reciprocity isBloch's law:6

Bt=K

where:B = Luminance of the light,t = Duration,K = A constant value.

Bloch's law is shown schematically inFigure 37.1A. When stimulus intensity and duration are plotted on log-log coordinates, as inFigure 37.1A, Bloch's law describes a line with a slope of −1. When the critical duration is reached, the threshold intensity versus duration function is described by a horizontal line; that is, a constant intensity is required to reach threshold. Bloch's law has been shown to be generally valid for a wide range of stimulus and background conditions, including both foveal and peripheral viewing. Once the duration of the stimulus exceeds the critical duration, the luminance required for it to reach visibility is classically considered to be constant.

The preceding discussion assumes that whenever the observer's threshold is exceeded, he or she will respond accurately to the stimulus. This predicts an abrupt and idealized transition between the two curves, as depicted inFigure 37.1A. In the real world, both visual stimuli and the physiologic mechanisms that we use to detect them are subject to random fluctuations in response. We may consider the length of the stimulus presentation to be divided into a number of discrete time intervals. The signal is detected when the response exceeds threshold in at least one interval and the probability of detection in each interval is considered independent. This description of the probabilistic nature of visual detection is known asprobability summation over time.7 The concept of probability summation is included in many models of temporal visual processing and is thought to be at least partly responsible for the less-than-abrupt transition between the region of temporal summation and constant intensity that occurs under some experimental conditions.3 One experimental situation in which this arises is when threshold contrast is measured as a function of signal duration for sinusoidal grating stimuli. This is illustrated inFigure 37.1B.3 InFigure 37.1B the upper curve shows threshold duration data for a 0.8 cycle per degree grating, and the lower curve shows those for an 8 cycle per degree grating. The upper curve conforms well to the schematic diagram that is illustrated inFigure 37.1A. For the lower curve depicting results for the 8 cycle per degree grating, however, a gradual transition is observed. In this latter case the actual critical duration (traditionally the point of intercept of the two slopes inFig. 37.1A) is somewhat difficult to define. Data similar to those shown inFigure 37.1B, have been interpreted to indicate that the critical duration increases with increasing spatial frequency.1,4 Gorea & Tyler3 use an alternative form of analysis that includes the effects of probability summation; they conclude that the critical duration is minimally affected by spatial frequency.

Postsynaptic Potentials and Synaptic Integration

John H. Byrne, in Fundamental Neuroscience (Fourth Edition), 2013

Temporal Summation Allows Integration of Successive PSPs

Temporal summation can be illustrated by firing action potentials in a presynaptic neuron and monitoring the resultant EPSPs. For example, in Figures 10.14A and 10.14B, a single action potential in sensory neuron 1 produces a 1-mV EPSP in the motor neuron. Two action potentials in quick succession produce two EPSPs, but note that the second EPSP occurs during the falling phase of the first, and the depolarization associated with the second EPSP adds to the depolarization produced by the first. Thus, two action potentials produce a summated potential that is about 2 mV in amplitude. Three action potentials in quick succession would produce a summated potential of about 3 mV. In principle, 30 action potentials in quick succession would produce a potential of about 30 mV and easily drive the cell to threshold. This summation is strictly a passive property of the cell. No special ionic conductance mechanisms are necessary. Specifically, the postsynaptic conductance change (gsyn in Eq. 10.3) produced by the second of two successive action potentials adds to that produced by the first. In addition, the postsynaptic membrane has a capacitance and can store charge. Thus, the membrane temporarily stores the charge of the first EPSP, and the charge from the second EPSP is added to that of the first.

What is spatial summation and temporal summation?

Figure 10.14. Temporal and spatial summation. (A) Intracellular recordings are made from two idealized sensory neurons (SN1 and SN2) and a motor neuron (MN). (B) Temporal summation. A single action potential in SN1 produces a 1-mV EPSP in the MN. Two action potentials in quick succession produce a dual-component EPSP, the amplitude of which is approximately 2 mV. (C) Spatial summation. Alternative firing of single action potentials in SN1 and SN2 produce 1-mV EPSPs in the MN. Simultaneous action potentials in SN1 and SN2 produce a summated EPSP, the amplitude of which is about 2 mV.

However, the “time window” for this process of temporal summation very much depends on the duration of the postsynaptic potential, and temporal summation is possible only if the presynaptic action potentials (and hence postsynaptic potentials) are close in time to each other. The time frame depends on the duration of changes in the synaptic conductance and the time constant (Chapter 5). Temporal summation, however, rarely is observed to be linear as in the preceding examples, even when the postsynaptic conductance change (gsyn in Eq. 10.3) produced by the second of two successive action potentials is identical with that produced by the first (i.e., no presynaptic facilitation or depression) and the synaptic current is slightly less because the first PSP reduces the driving force (Vm − Er) for the second. Interested readers should try some numerical examples.

View chapterPurchase book

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B978012385870200010X

Seismic and Vibrational Signals in Animals

P.M. Narins, ... C.E. O’Connell-Rodwell, in Encyclopedia of Neuroscience, 2009

Enhancing Signal Detection Range

Temporal summation can be responsible for significant auditory threshold reduction; thus repeated signals can facilitate signal detection. Moreover, longer signals can increase the signal-to-noise ratio, improving signal detection as well. Examples of this can be found in animals communicating in both the acoustic and the seismic realms.

Weddell seals (Leptonychotes weddelli) repeat calls to enhance signal detection over long distances and during times of high background noise, as do some whales. Killer whales increase call duration in high background noise from boats. It follows, therefore, that species that communicate seismically would also employ similar mechanisms to enhance the propagation and detection of signals in noisy environments or over long distances.

To this end, African elephant family groups vocalize within interactive bouts more often than while calling individually. These bouts create an enhanced signal that is effectively three times longer than one produced by a single individual, serving to increase the signal-to-noise ratio and facilitating signal detection and processing at great distances. Furthermore, during departure from a resource, calling bouts are repeated at a greater rate. Elephants listening at a distance might use multiple bouts of longer signals to adjust their physical position to optimize signal resolution. Observations suggest they do just this by freezing for long periods and shifting positions, aligning themselves in the direction of the acoustic or seismic signal source.

View chapterPurchase book

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B978008045046901826X

Clinical Neurophysiology: Basis and Technical Aspects

Jean-Pascal Lefaucheur, in Handbook of Clinical Neurology, 2019

Plasticity and Priming

Synaptic plasticity depends on firing rate, spike timing, and temporal and spatial summations of the inputs arriving at presynaptic level. However, whether a synapse is strengthened or weakened by presynaptic activity also depends upon the level of activity in the postsynaptic neuron. The processes leading to depression of synaptic transmission are more effective when postsynaptic activity is high. Conversely, potentiation of synaptic transmission is more likely when postsynaptic activity is low. This is known as the Bienenstock–Cooper–Munro (BCM) model (Bienenstock et al., 1982). Generally speaking, previous neuronal activity modulates the capacity for subsequent plastic changes. This has been termed metaplasticity (Abraham and Tate, 1997). All these phenomena could concur in stabilizing neuronal networks and therefore contribute to “homeostatic plasticity” (Turrigiano and Nelson, 2004).

A study showed that rTMS effects on intracortical inhibition depended more on baseline individual values than on stimulation frequency (Daskalakis et al., 2006). Subjects with less inhibition before rTMS tended to have an increased inhibition post-rTMS (and vice-versa). A similar observation was made in patients with chronic pain who showed defective intracortical inhibition at baseline and increased inhibition following rTMS delivered at 10 Hz over the motor cortex (Lefaucheur et al., 2006).

Accordingly, priming cortical stimulation aimed at modulating the initial state of cortical excitability could influence subsequent rTMS-induced changes in cortical excitability. The priming stimulation can have no detectable effects per se on synaptic transmission. There are several reports of efficacious priming protocols in the literature: subthreshold 6-Hz rTMS was found to reinforce the depression of motor responses induced by suprathreshold 1-Hz rTMS subsequently applied to the motor cortex (Iyer et al., 2003); the priming effect of iTBS was assessed on a subsequent 1-Hz rTMS session delivered to temporoparietal language areas during an auditory word-detection task (Andoh et al., 2008); the analgesic effects of “conventional” 10-Hz rTMS delivered to M1 was found to be enhanced by TBS priming, at least using iTBS (Lefaucheur et al., 2012); a PAS session was found to affect the changes in motor cortex excitability induced by a subsequent PAS session (Muller et al., 2007); tDCS was found to enhance or reverse the effects of 1-Hz or 5-Hz rTMS depending on stimulation polarity (Lang et al., 2004; Siebner et al., 2004), and so on. Thus, priming cortical stimulation surely is a potent way of improving rTMS efficacy in clinical practice.

Lesions and diseases can also be at the origin of preexisting homeostatic changes in the activity of a given cortical region. Therefore, the effects of cortical stimulation and priming strategies may differ between patients and healthy subjects. For example, PAS-induced potentiation of synaptic transmission is lacking in Parkinson's disease (Ueki et al., 2006) but enhanced in patients with focal dystonia (writer's cramp) (Quartarone et al., 2003) compared to normal controls. Differential effects of dorsolateral prefrontal cortical stimulation were observed between normal subjects and depressive patients regarding the type of mood changes with respect to the side of stimulation (George et al., 1996; Pascual-Leone et al., 1996). Actually, abnormal plastic responses and altered excitability changes to cortical stimulation have been found in numerous neuropsychiatric diseases. Various mechanisms other than preexisting homeostatic changes may also explain the differences in the responsiveness to cortical stimulation between healthy subjects and patients. These mechanisms include genetic factors (Edwards et al., 2006; Kleim et al., 2006), hormonal factors (Inghilleri et al., 2004), attentional capacities (Stefan et al., 2004), interindividual differences in brain anatomy, and possible shift of cortical areas of interest. The latter can now be corrected by determining the location of cortical stimulation targets with functional neuroimaging data integrated in a navigated approach.

View chapterPurchase book

Read full chapter

URL: https://www.sciencedirect.com/science/article/pii/B9780444640321000370

Contributions to Sensory Physiology

Daniel Algom, Harvey Babkoff, in Contributions to Sensory Physiology, 1984

B Other Nonsummation Interpretations

From time to time, attempts have been made to explain acoustic temporal summation by means of other, somewhat more simple neural phenomena. Thus, according to Miller (1948), the ear is more of a delaying device than an integration device. He assumes that different neural elements have different latencies, especially at higher centers. Loudness is correlated with the instantaneous level of neural activity in these centers. The level of activity is a function of time due to the different latencies of the neural elements in these higher centers, rather than an expression of acoustic summation; “perception of noise is the integral of the distribution of transmission times of the various pathways from the cochlea to the higher center, and not the integral of sound intensity” (Miller, 1948). Miller was also able to predict other auditory phenomena by using this theory, as well as by determining a value of the neural pathway transmission time.

A somewhat similar explanation, involving the assumption of a short temporal integration as part of a neural process, was advanced by Gersuni (1965) and Watson and Gengel (1969). Gersuni also postulated that the more sensitive receptors had much longer time constants (100 msec) than the less sensitive receptors (10 msec). The differential arrival times thus produced could explain the observed, quite long integration data. The assumption of a wide range of receptor thresholds, implicit in Gersuni's hypothesis, requires a stronger empirical base than is presently available. According to an alternative formulation, brief and more intense inputs give rise to a neural process of spatial summation due to the synchronization of impulses, whereas longer and weaker stimuli give rise to synaptic transmission by temporal summation of asynchronous impulses. If such processes are executed in successively higher neural stages, the total effect might account for quite large time constants of integration (Watson and Gengel, 1969).

Although the final theoretical framework for temporal summation data has not been formulated, most authors prefer the hypothesis of an authentic power integration in the auditory system. In this context, the probabilistic formulations may at least serve as useful null hypotheses. In the following section we attempt to review some of the theories that assume actual power summation.

What is temporal and spatial summation?

Spatial summation occurs when several weak signals from different locations are converted into a single larger one, while temporal summation converts a rapid series of weak pulses from a single source into one large signal [Note from Ferguson: summation interval ~ 5-100 msec.)

What is meant by spatial summation?

Spatial summation occurs when stimuli are applied at the same time, but in different areas, with a cumulative effect upon membrane potential. Spatial summation uses multiple synapses acting simultaneously.

What is meant by temporal summation?

Temporal summation is a phenomenon in which repeated and equal-intensity noxious stimuli at a specific frequency cause an increase in the pain experienced. Results are usually obtained by comparing pain ratings of the first to the last equal intensity stimulus.

What is an example of temporal summation?

Temporal summation can be illustrated by firing action potentials in a presynaptic neuron and monitoring the resultant EPSPs. For example, in Figures 10.14A and 10.14B, a single action potential in sensory neuron 1 produces a 1-mV EPSP in the motor neuron.