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.

Excitatory and inhibitory synaptic action.

Abstract: Electron microscopy has fully confirmed the neuron theory (Cajal, 1934) in that this technique has revealed the continuity of the surface membrane over the nerve cell, each nerve cell being structurally an independent unit. It has further shown that the functional connections (synapses) between nerve cells and between nerve and muscle occur across regions of very intimate contact, there being a gap (the synaptic cleft) of 150 to 400 A between the respective presynaptic and postsynaptic membranes (De Robertis, 1956; Palay, 1956, 1958; Robertson, 1956, and personal communication). The existence of two types of synaptic action, excitatory and inhibitory, was recognized long before intracellular recording gave information relative to the ways in which excitatory synaptic action evoked the discharge of impulses and in which inhibitory synaptic action was able to prevent this discharge. By degeneration experiments Szentagothai (1958) has recently produced the first evidence distinguishing between the actual synaptic structures responsible for these two opposed functional actions. When, by degeneration procedures, motoneurons were left with only the inhibitory connections from Renshaw cells, they were devoid of all synaptic knobs, but there still remained fine fibrils lacing over their surface. Complementarily, destruction of the inhibitory pathway to oculomotor neurons caused a degeneration only of a similar fine fibrillar structure. Hence it seems possible that inhibitory synapses are formed by the functional contacts made by these fine fibers, and that all of the conventional synaptic knobs are excitatory. Synaptic excitatory action was first studied intracellularly by recording activity a t the neuromuscular junction (Fatt and Katz, 1951), but this transmission lies outside the scope of the present treatment, which is especially concerned with those synaptic transmissions where there is antagonism between excitatory and inhibitory actions. Since the first intracellular studies on the motoneuron (Brock et al., 1952), this dual synaptic action has been investigated in several other types of nerve-cell and nerve-muscle junction. It has therefore been thought appropriate to adopt a comparative approach to the problems of synaptic excitation and inhibition. Because of the much more extensive investigations on motoneurons, their responses will necessarily be treated more fully, but a t each stage the relevant data for other synaptic mechanisms will be discussed in relation to the motoneuronal responses. There have been two rival hypotheses, the electric and the chemical, for the manner in which an excitatory influence is transmitted across the synaptic contact between one nerve cell and the next. With the great majority of excitatory synapses it may now be taken as established that a nerve impulse causes a specific chemical transmitter to be released from the presynaptic terminals and thence to traverse the synaptic cleft and exert a depolarizing influence on the postsynaptic membrane; thus far, most of these transmitters in the central nervous system have not been identified. However, Furshpan and

Vibrotactile temporal summation: Effect of frequency.

Abstract: Temporal summation of vibrotactile stimuli was measured at four frequencies (25, 40, 80, and 160 Hz) using a large contactor. Stimulation at 160 Hz gave temporal summation comparable in amount to that reported in previous studies. Stimulation at 25, 40, and 80 Hz gave less summation. The presence of summation at low frequencies is unexpected in view of existing data obtained with small contactors' those data indicate that the afferent system primarily sensitive to low frequencies may not summate stimulus energy over time. The present data suggest either that the low-frequency afferents do summate energy over time or that, under some conditions, the perception of low-frequency signals presented through a large contactor may be mediated by more than one afferent system.

In vivo tracing of pathways and spatio-temporal activity patterns in rat visual cortex using voltage sensitive dyes.

Abstract: We monitored optical signals from cortex stained with a voltage sensitive dye to study activity evoked by intracortical electrical stimulation. The objectives were to study the spatial and temporal spread of activity from intrinsic connections near the stimulating electrode and to develop a new technique to study extrinsic projections from striate cortex to extrastriate target areas. Various measures were made of the time course of the optical signal (latency, rise time, decay time, temporal summation, facilitation versus depression, and presence or absence of a slow undershoot); in general, these measures were found to vary significantly across different response positions, different experiments, and even different runs within the same experiment. The spatial distribution of responses near the stimulating electrode in striate cortex was usually elliptical and was most often elongated along the anterior-posterior axis, with a typical size (full width at 75% max) of 1.3 mm (anterior-posterior axis) by 0.75 mm (medio-lateral axis). In some cases, complex spatio-temporal patterns were observed, in which the position of the maximum optical signal shifted with time or split into multiple peaks. In eight experiments, a response focus was found in extrastriate cortex at an expected location within the lateromedial area (LM). The response focus in LM was typically about half the size of that in striate cortex. In some experiments we observed additional focal responses in the anterolateral visual area (AL). The extrastriate responses showed a significant delay (3–10 ms) in onset and time to peak relative to the striate response. The validity of this technique for determining extrinsic projections was tested in two types of experiments. In the first, stimulation from two electrodes in striate cortex generated response foci consistent with the known topographic organization of area LM. In the second, the optically measured response focus was shown to correlate with the histologically reconstructed projection of a chemical tracer injected near the site of stimulation. We discuss the chain of neurophysiological events that occur during and after focal electrical stimulation and how they relate to the observed optical signal. We conclude that direct passive responses were a small component of our signal, that the component due to action potentials in directly stimulated neurons should have occurred in the first 1–2 ms post stimulus and is small compared to the peak signal, and that overall our signals were probably dominated by a combination of asynchronously occurring action potentials and excitatory and inhibitory synaptic potentials. These results, together with the prospect of being able to trace many connections in a single experiment, indicate that the combination of optical recording and focal electrical stimulation provides a valuable means for analyzing structural and physiological aspects of cortical circuitry.

Spatial summation in the receptive field of simple cells in the cat striate cortex

Abstract: Spatial summation was studied quantitatively through width response curves made with an optimally oriented test slit of variable width, and by comparing the response to combined presentation of several parallel slits with the response to each slit alone. Prior to summation analysis, the cell's discharge field (DF) was mapped by presenting a test slit ON and OFF across the receptive field. Activation profiles, showing the extension of subregions where light stimulation increased (enhancement) or decreased the firing rate (suppression), were made by presenting an optimally oriented activation slit in the most responsive DF-position. Against this activity the effects of a parallel test slit were determined in a series of broadside positions. Width response curves were made over the subregions of the DF and the activation profiles. Spatial summation was found in all cells, but the width of the summation region was smaller than the width of the subregions in the respective profiles. The width of the summation region was related to the degree of activation rather than to specific locations within the receptive field. The effect produced by several slits presented together deviated from the algebraic sum of the effects produced by each slit alone. Linear summation was rarely found. Accumulated response curves obtained by integration of DF or activation profiles were compared with width response curves to test linearity of summation. Linear summation throughout the whole receptive field was never found. A satisfactory fit was found only over a narrow region showing that summation was linear within a small part of the summation region. Linearity ended near response maxima or minima in the response profiles. The results indicate that the receptive field of simple cells consists of overlapping excitatory and inhibitory fields, and that the exact location and width of enhancement and suppression zones are determined by an activity-dependent balance between excitatory and inhibitory inputs.

Spatial summation in macaque parietal area 7a follows a winner-take-all rule

Abstract: While neurons in posterior parietal cortex have been found to signal the presence of a salient stimulus among multiple items in a display, spatial summation within their receptive field in the absence of an attentional bias has never been investigated. This information, however, is indispensable when one investigates the mechanisms of spatial attention and competition between multiple visual objects. To examine the spatial summation rule in parietal area 7a neurons, we trained rhesus monkeys to fixate on a central cross while two identical stimuli were briefly displayed in a neuron's receptive field. The response to a pair of dots was compared with the responses to the same dots when they were presented individually. The scaling and power parameters of a generalized summation algorithm varied greatly, both across neurons and across combinations of stimulus locations. However, the averaged response of the recorded population of 7a neurons was consistent with a winner-take-all rule for spatial summation. A control experiment where a monkey covertly attended to both stimuli simultaneously suggests that attention introduces additional competition by facilitating the less optimal stimulus. Thus an averaging stage is introduced between ∼200 and 300 ms of the response to a pair of stimuli. In short, the summation algorithm over the population of area 7a neurons carries the signature of a winner-take-all operation, with spatial attention possibly influencing the temporal dynamics of stimulus competition, that is the moment that the “winner” takes “victory” over the “loser” stimulus.