Showing papers on "Summation published in 1993"

Spatiotemporal organization of simple-cell receptive fields in the cat's striate cortex. II. Linearity of temporal and spatial summation

Abstract: 1. We have tested the hypothesis that simple cells in the cat's visual cortex perform a linear spatiotemporal filtering of the visual image. To conduct this study we note that a visual neuron behaves linearly if the responses to small, brief flashes of light are mathematically related, via the Fourier transform, to the responses elicited by sinusoidal grating stimuli. 2. We have evaluated the linearity of temporal and spatial summation for 118 simple cells recorded from the striate cortex (area 17) of adult cats and kittens at ages 4 and 8 wk postnatal. These neurons represent a subset of the population of cells for which we have described the postnatal development of spatiotemporal receptive-field structure in the preceding paper. Spatiotemporal receptive-field profiles are constructed, with the use of a reverse correlation technique, from the responses to random sequences of small bar stimuli that are brighter or darker than the background. Fourier analysis of spatiotemporal receptive-field profiles yields linear predictions of the cells' spatial and temporal frequency tuning. These predicted responses are compared with spatial and temporal frequency tuning curves measured by the use of drifting, sinusoidal-luminance grating stimuli. 3. For most simple cells, there is good agreement between spatial and temporal frequency tuning curves predicted from the receptive-field profile and those measured by the use of sinusoidal gratings. These results suggest that both spatial and temporal summation within simple cells are approximately linear. There is a tendency for predicted tuning curves to be slightly broader than measured tuning curves, a finding that is consistent with the effects of a threshold nonlinearity at the output of these neurons. In some cases, however, predicted tuning curves deviate from measured responses only at low spatial and temporal frequencies. This cannot be explained by a simple threshold nonlinearity. 4. If linearity is assumed, it should be possible to predict the direction selectivity of simple cells from the structure of their spatiotemporal receptive-field profiles. For virtually all cells, linear predictions correctly determine the preferred direction of motion of a visual stimulus. However, the strength of the directional bias is typically underestimated by a factor of about two on the basis of linear predictions. Consideration of the expansive exponential nonlinearity revealed in the contrast-response function permits a reconciliation of the discrepancy between measured and predicted direction selectivity indexes. 5. Overall, these findings show that spatiotemporal receptive-field profiles obtained with the use of reverse correlation may be used to predict a variety of response properties for simple cells.(ABSTRACT TRUNCATED AT 400 WORDS)

The development of synaptic function and integration in the central auditory system

Abstract: The development of inhibitory synaptic transmission is difficult to assess because the afferents usually arise from intrinsic neurons that are difficult to stimulate independently The postnatal maturation of excitatory and inhibitory synaptic function was compared in the gerbil lateral superior olive (LSO), where it is possible to stimulate physically discrete afferent projections Intracellular recordings obtained in a brain slice preparation revealed that transmission was prominent at birth The EPSPs and IPSPs were up to 2 orders of magnitude longer than in more mature animals Brief trains of electrical stimulus pulses led to a temporal summation of postsynaptic potentials (PSPs) in 1-14 d animals resulting in prolonged depolarizations or hyperpolarizations In neonates, the depolarization could exceed 1 sec following a 70 msec stimulus train The IPSPs in neonates were often of sufficient amplitude to evoke a rebound depolarization or action potential The number of converging afferents was estimated from the quantized increases in PSP size There was a significant decrease with age, suggesting that both inhibitory and excitatory afferents were eliminated during the first 3 postnatal weeks The integration of action potentials with IPSPs was examined with conjoint stimuli to the two afferent pathways, and demonstrated that the effective IPSP duration decreased approximately 20-fold during the first 3 postnatal weeks The magnitudes and durations of electrical stimulus-evoked PSPs suggest that spontaneous discharge of afferents to the LSO could have a substantial impact on their development, even prior to the response to airborne sound at 12 d Furthermore, the synaptic responses obtained at 12-14 d postnatal indicated that both amplitude and temporal processing remain compromised These immature synaptic properties would be expected to compound the inadequacies present in the cochlea and cochlear nucleus

Temporal summation--the key to motor evoked potential spinal cord monitoring in humans.

Abstract: Spinal motor evoked potentials (SMEP) were recorded from tibialis anterior muscle after epidural stimulation of the spinal cord at the low cervical or high thoracic level during scoliosis surgery. By using a double stimulus pulse to produce temporal summation within the spinal cord a maximal CMAP response was readily achieved despite good surgical anaesthesia.

Facilitation and depression at different branches of the same motor axon: evidence for presynaptic differences in release

Abstract: This study provides evidence that a neuron can exhibit differences in activity-dependent transmitter release at two synaptic sites due to variations in the properties of its presynaptic terminals. Two muscles in the stomatogastric system of the lobster Homarus americanus are innervated by a single motor neuron but respond differently to that motor neuron's input, resulting in two different movements evoked by one motor neuron. During continued motor neuron stimulation, the gm8 muscle contracts slowly and maintains contraction, while the gm9 muscle contracts rapidly and then relaxes. These different muscle responses can be accounted for, in large part, by the properties of the respective neuromuscular synapses: the excitatory junctional potentials recorded in gm8 are initially small but summate and facilitate with repeated stimulation, while those in gm9 are initially large but depress with repeated stimulation. Presynaptic differences in neurotransmitter release contribute strongly to the divergent responses; reduction of the excitatory junction potential amplitude by partial postsynaptic receptor blockade or by desensitization does not change the amount of depression at gm9. However, reduction of neurotransmitter release with low-Ca2+, high-Mg2+ saline removes gm9 synaptic depression and reveals that both neuromuscular junctions exhibit frequency-dependent homosynaptic facilitation. Postsynaptic differences in muscle input resistance and muscle composition may enhance the effects of the divergent release properties, but are not responsible for the activity-dependent changes. Ultrastructural features of the nerve terminals on the two muscles are consistent with the differential output of the terminals; the synapses on gm9 are larger and have more presynaptic dense bars than their counterparts on gm8. These data suggest that the basis for the differences in transmitter release between the two muscles may be a higher density of release sites in the gm9 synapses that leads to a higher output of neurotransmitter, rapid depletion of transmitter stores, and synaptic depression.

Facilitation of motor evoked potentials from magnetic brain stimulation in man: a comparative study of different target muscles.

Abstract: The influence of tonic muscle contraction and stimulus intensity on compound muscle action potentials (CMAPs) elicited by magnetic brain stimulation was studied in the biceps brachii (34 subjects), the abductor digiti minimi (11 subjects), the anterior tibial muscle (12 subjects), and the soleus muscle (5 subjects). The muscles were examined at rest and with various degrees of background contraction of up to 60% of maximum force. Stimulus intensity was set at threshold (TSI) or 20% above threshold (1.2 TSI), and in one series additionally at 50% above threshold (1.5 TSI). The effect of voluntary background contraction on CMAP onset latency was similar in the four muscles tested: the latencies shortened by approximately 3 ms when the muscle changed from the relaxed to the contracted state of the 10% of maximum force. An additional increase in the background contraction up to 60% of maximum force induced only few, if any, additional decreases in latency. The uniformity of the latency shift in distal and proximal muscles conflicts with the idea of recruitment of larger and rapidly conducting motoneurons being the cause, since this hypothesis would imply a more pronounced latency reduction in distal than in proximal muscles. The shorter latency during voluntary contraction is more likely due to an enhanced synaptic efficacy at spinal level. Since the motoneurons are brought into an increased state of activity during contraction, they require less temporal summation to reach firing threshold and thus discharge earlier. The CMAP amplitudes of the different muscles were more distinctly affected by voluntary background contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Temporal and spatial summation in the human rod visual system.

Abstract: 1. Absolute and increment thresholds were measured in a retinal region 12 deg temporal from the fovea with 520 nm targets of varying size and duration. Measurements were made under rod-isolation conditions in two normal observers and in a typical, complete achromat observer who has no cone-mediated vision. The purpose of these experiments was to determine how the temporal and spatial summation of rod-mediated vision changes with light adaptation. 2. The absolute threshold and the rise in increment threshold with background intensity depend upon target size and duration, but the psychophysically estimated dark light of the eye (the hypothetical light assumed to be equivalent to photoreceptor noise) does not. 3. The rise in increment threshold for tiny (10 min of arc), brief (10 ms) targets approaches the de Vries-Rose square-root law, varying according to the quantal fluctuations of the background light. The slope of the rod increment threshold versus background intensity (TVI) curves in logarithmic co-ordinates is about 0.56 +/- 0.04 (when cones are not influencing rod field adaptation). For large (6 deg) and long (200 ms) targets, a maximum slope of about 0.77 +/- 0.03 is attained. 4. The steeper slopes of the rod-detected TVI curves for large, long targets implies some reduction in temporal or spatial summation. In fact, the change in summation area is much more critical: under conditions where only the rod system is active the TVI curve slope is independent of target duration, suggesting that temporal summation is practically independent of background intensity. 5. The rise in threshold also depends on the wavelength of the background field in the normal observer but not in the achromat, confirming reports that the field adaptation of the rods is not independent of the quantal absorptions in the cones. The cone influence is most conspicuous on long-wavelength backgrounds and is found for all target sizes and durations, but is greater for large and long targets than for the other conditions.

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.

Tonic interocular suppression, binocular summation, and the visual evoked potential.

Abstract: Purpose. Psychophysical studies have shown that a dark-adapted eye exerts a tonic interocular suppression (TIS) upon spatial vision mediated by the contralateral eye. The present study was designed to demonstrate TIS by means of visual evoked potential (VEP) procedures. Methods. Evoked cortical potentials were obtained in response to reversing checkerboard patterns with fundamental Fourier frequencies between 3 and ] 2 cycles per degree. Responses were obtained under monocular viewing conditions when the contralateral "adapting" eye was dark adapted, under monocular viewing conditions when the adapted state of the adapting eye was experimentally manipulated, or under binocular viewing conditions. Data were collected from three healthy young men, two naive regarding purpose of experimentation. Results. Regardless of spatial frequency, monocular responses evoked by stimulating a "test eye" were always smaller in amplitude when the contralateral adapting eye was dark adapted than when adapted to a dim, homogeneous field. The monocular evoked response obtained in the presence of an interocular adapting field was similar in amplitude to the binocular evoked response. During dark adaptation of the contralateral adapting eye, the amplitude of the monocular evoked response decreased: the time course of this decline follows that of psychophysically measured rod thresholds in the directly adapted eye. Conclusions. TIS is easily demonstrated by means of VEP as well as psychophysical procedures. The well-known increase in VEP amplitude resulting from binocular viewing may be attributable to the removal of TIS rather than to "physiologic, binocular summation." Invest Ophthalmol Vis Sci. 1993;34:2443-2448.

Spatial summation in blindsight.

Abstract: Spatial summation curves were determined in the circumscribed visual-field defects of five patients with blindsight. Under light-adapted conditions that favor the color-opponent system, increment thresholds for white and red targets presented on a white background were measured as a function of stimulus size which ranged from 9-110 min arc. In both normal and defective hemifields, summation was linear for the red stimuli. In contrast, the curves measured with the white stimuli showed some nonlinearity in the normal hemifield, and a pronounced eccentricity-dependent notch in the field defect. The results indicate that the neurons mediating sensitivity differ in their summation properties for wavelength and intensity information.

Memory for duration in pigeons: Dissociation of choose-short and temporal-summation effects

Abstract: Five groups of pigeons were trained in a symbolic choice-matching feast involving short (2-sec) and long (10-sec) durations of houselight as samples. Four groups also received training with a second set of samples: line orientations or 2- and 10-sec presentations of keylight. The type of sample-to-comparison mapping varied across groups. Although only two of the five groups demonstrated a choose-short effect (a tendency to choose the comparison associated with a short sample at longer delays), all groups demonstrated temporal summation (a tendency to respond on the basis of the combined duration of two successively presented samples). Moreover, the magnitude of temporal summation was equivalent in groups that did and did not-demonstrate a choose-short effect. The results suggest that the processes underlying the perception of sample duration remain invariant across different sample-to-comparison mapping arrangements, but that the memory code used to retain temporal information varies.

The Temporal Noisy-Leaky Integrator Neuron with Additional Inhibitory Inputs

Abstract: The Temporal Noisy-Leaky Integrator (TNLI) neuron model with additional inhibitory inputs is presented together with its theoretical mathematical basis. The TNLI is a biologically inspired hardware neuron which models temporal features of real neurons like the temporal summation of the dendritic postsynaptic response currents of controlled delay and duration and the decay of the somatic potential due to its membrane leak. In addition, it models the stochastic neurotransmiller release by the synapses of real neurons, as pRAMs are used at each input. Using the TNLI, we investigated the effect of synoptic integration between excitatory and inhibitory inputs on the transfer function of the neuron. We observed that inhibitory inputs increase the fluctuations of the input current and reduce the slope of the sigmoidal transfer function of the neuron, which highlights one of the differences between biological neurons and formal neurons.