Biomedical subjects

I Nelken

Publications and source records attributed to I Nelken.

9 recordsLinked to original sources

PubMed1995

Somatosensory effects on neurons in dorsal cochlear nucleus.

1. Single units and evoked potentials were recorded in dorsal cochlear nucleus (DCN) in response to electrical stimulation of the somatosensory dorsal column and spinal trigeminal nuclei (together called MSN for medullary somatosensory nuclei) and for tactile somatosensory stimuli. Recordings were from paralyzed decerebrate cats. 2. DCN principal cells (type IV units) were strongly inhibited by electrical stimulation (single 50-microA bipolar pulse) in MSN or by somatosensory stimulation. Units recorded in the fusiform cell and deep layers of DCN were inhibited, suggesting that the inhibition affects both types of principal cells (i.e., both fusiform and giant cells). 3. Interneurons (type II units) that inhibit principal cells were only weakly inhibited by electrical stimulation and were never excited, demonstrating that the inhibitory effect on principal cells does not pass through the type II circuit. In the vicinity of the DCN/PVCN (posteroventral cochlear nucleus) boundary, units were encountered that were excited by electrical stimulation in MSN; some of these neurons responded to sound, and some did not. Their response properties are consistent with the hypothesis that they are deep-layer inhibitory interneurons conveying somatosensory information to the DCN. 4. Analysis of the evoked potentials produced by electrical stimulation in MSN suggests that the somatosensory inputs activate the granule cell system of the DCN molecular layer. A model based on previous work by Klee and Rall was used to show that the distribution of evoked potentials in DCN can be explained as resulting from radial currents produced in the DCN molecular and fusiform-cell layers by synchronous activation of granule cells inputs to fusiform and cartwheel cells. Current-source density analysis of the evoked potentials is consistent with this model. Thus molecular layer interneurons (cartwheel and stellate cells) are a second possible source of inhibition to principal cells. 5. With lower stimulus levels (20 microA) and pulse-pair stimuli (50- to 100-ms interstimulus interval), three components of the inhibitory response can be recognized in both fusiform cell layer and deep layer type IV units: a short-latency inhibition that begins before the start of the evoked potential; a longer-latency inhibition whose timing corresponds to the evoked potential; and an excitatory component that occurs on the rising phase of the evoked potential. The excitatory component is usually overwhelmed by the inhibitory components and could be derived from granule cell inputs; the long-latency inhibitory component could be derived from cartwheel cells or the hypothesized deep-layer inhibitory interneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1994

Population responses to multifrequency sounds in the cat auditory cortex: one- and two-parameter families of sounds.

Population responses to multi-frequency sounds were recorded in primary auditory cortex of anesthetized cats. The sounds consisted of single-tone stimuli; two-tone stimuli; and nine-tone stimuli, with the tones evenly spaced on a linear frequency scale. The stimuli were presented through a sealed, calibrated sound delivery system. Single units, cluster activity (CA) and the short-time mean absolute value of the envelope of the neural signal (MABS) were recorded extracellularly from six microelectrodes simultaneously. The CA and MABS were interpreted as measures of the activity of large populations of neurons, in contrast with the single unit activity which is presumably recorded from single neurons. The responses of the MABS signal to simple stimuli were generally similar to those of the CA, but were more stable statistically. Thus, the MABS is better suited for studying the activity of populations of neurons. The responses to tones near the best frequency were strongly influenced by a second tone, even when the second tone was outside the single-tone response area. These influences could be both facilitatory and suppressory. They could not be predicted from the responses to single tones. The responses to the nine-tone stimuli could be explained qualitatively by the responses to the two-tone stimuli. It is concluded that the population responses in primary auditory cortex are shaped by the contributions of the individual frequencies appearing in the stimulus and by the interactions between pairs of frequencies. Interactions between stimulus components are therefore a necessary component of any attempt to explain the processing of complex sounds in the auditory cortex. They may play a role in a global representation of the stimulus spectrum in the primary auditory cortex. The presence of higher-order interactions cannot be excluded by the results presented here.

Acoustic Stimulation↗

PubMed1994

Population responses to multifrequency sounds in the cat auditory cortex: four-tone complexes.

Population responses to two-tone and four-tone sounds were recorded in primary auditory cortex of anesthetized cats. The stimuli were delivered through a sealed, calibrated sound delivery system. The envelope of the neural signal (short time mean absolute value, MABS) was recorded extracellularly from six microelectrodes simultaneously. A new method was developed to describe the responses to the four-tone complexes. The responses were represented as sums of contributions of different orders. The first order contributions described the effect of the single frequencies appearing in the stimulus. The second order contributions described the modulatory effect of the pairs of frequencies. Higher order contributions could in principle be computed. This paper concentrates on the mean onset responses. The extent to which the first and second order contributions described the onset responses was assessed in two ways. First, the actual responses to two-tone stimuli were compared with those predicted using the contributions computed from the four-tone stimuli. Second, the residual variance in the responses, after the subtraction of the first and second order contributions, was computed and compared with the variability in the responses to repetitions of the same stimulus. The first type of analysis showed good quantitative agreement between the predicted and the measured two-tone responses. The second type of analysis showed that the first and second order contributions were often sufficient to predict the responses to four-tone stimuli up to the level of the variability in the responses to repetitions of a single stimulus. In conjunction with the results of the companion paper (Nelken et al., 1994a) it is concluded that the onset responses to multifrequency sounds are shaped mainly by the single frequency content of the sound and by two-tone interactions, and that higher order interactions contribute much less to the responses. It follows that single-tone effects and two-tone interactions are necessary and sufficient to explain the mean population onset responses to the four-tone stimuli. More information can be coded in the temporal evolution of the responses.

Acoustic Stimulation↗

PubMed1994

In search of the best stimulus: an optimization procedure for finding efficient stimuli in the cat auditory cortex.

Units in the auditory cortex of cats respond to a large variety of stimuli: pure tones, AM- and FM-modulated signals, clicks, wideband noise, natural sounds, and more. However, no single family of sounds was found to be optimal (in the sense that oriented lines are optimal in the visual cortex). The search for optimal complex sounds is hard because of the high dimensionality of the space of interesting sounds. In an effort to overcome this problem, an automatic search procedure for finding efficient stimuli in high-dimensional sound spaces was developed. This procedure chooses the stimuli to be presented according to the responses to past stimuli, trying to increase the strength of the response. The results of applying this method to recordings of population activity in the primary auditory cortex of cats are described. The search was applied to single tones, two-tone stimuli, four-tone stimuli and to a two-dimensional subset of nine-tone stimuli, parametrized by the center frequency and the fixed difference between adjacent frequencies. The method was able to find efficient stimuli, and its performance improved with the dimension of the sound spaces. Efficient stimuli, found in different optimization runs using population activity recorded from the same electrode, often shared similar frequencies and pairs of frequencies, and tended to evoke similar levels of activity. This result indicates that a global analysis of the location of spectral peaks is performed at the level of the auditory cortex.

Acoustic Stimulation↗

PubMed1994

Two separate inhibitory mechanisms shape the responses of dorsal cochlear nucleus type IV units to narrowband and wideband stimuli.

1. The principal cells of the dorsal cochlear nucleus (DCN) are mostly inhibited by best frequency (BF) tones but are mostly excited by broadband noise (BBN), producing the so-called type IV response characteristic. The narrowband inhibitory responses can be explained by the inhibitory influence of interneurons with type II response characteristics. However, it is not clear that all the details of the type IV responses can be accounted for by this neural circuit. In particular, many type IV units are inhibited by band-reject noise (notch noise); type II units tend to be only weakly excited by these stimuli, if at all. In this paper we study the relationships between the narrowband, inhibitory and the wideband, excitatory regimens of the type IV responses and present the case for the existence of a second inhibitory source in DCN, called the wideband inhibitor (WBI) below. 2. Type IV units were studied using pure tones, noise bands arithmetically centered on BF, notch noise centered on BF, and BBN. We measured the rate-level function (response rate as function of stimulus level) for each stimulus. This paper is based on the responses of 28 type IV units. 3. Evidence for low-threshold inhibitory input to type IV units is derived from analysis of rate-level functions at sound levels just above threshold. Notch noise stimuli of the appropriate notch width produce inhibition at threshold in this regime. When BBN is presented, this inhibition appears to summate with excitation produced by energy in the band of noise centered on BF, resulting in BBN rate-level functions with decreased slope and maximum firing rate. A range of slopes and maximal firing rates is observed, but these variables are strongly correlated and they are negatively correlated with the strength of the inhibition produced by notch noise; this result supports the conclusion that a single inhibitory source is responsible for these effects. 4. By contrast, there is a weak (nonsignificant) positive correlation between the strength of the inhibitory effect of notch noise and the slope/maximal firing rate in response to narrowband stimuli, including BF tones. The contrast between this positive nonsignificant correlation and the significant negative correlation mentioned above suggests that more than one inhibitory effect operates: specifically, the type II input is responsible for inhibition by narrowband stimuli and a different inhibitory source, the WBI, produces inhibition by notch stimuli. 5. Several lines of evidence are given to show that type II units cannot produce the inhibition seen with notch noise stimuli.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1991

Neural interactions in the frontal cortex of a behaving monkey: signs of dependence on stimulus context and behavioral state.

In order to gain an understanding of the processes taking place within and between neuronal assemblies, we made simultaneous recordings of spike trains from groups of up to 11 neurons in the frontal cortex of a rhesus monkey, that was trained to perform a sensorimotor behavioral task. We report here on preliminary results from correlation analysis of these neuronal activities, with special emphasis on signs of behaviorally induced modifications of neural interaction, possibly due to rapid modulations of discharge synchronization among the neurons. Our findings suggest that different functional groups of neurons may co-exist within each small volume of cortex, and that neurons may be dynamically recruited into such a group to fulfil a specific function.

Animals↗

PubMed1990

A sensitive estimator for crosscorrelograms.

The best established method for finding interactions between extracellularly recorded neurons is the crosscorrelation technique. The method is simple and useful, but it has some drawbacks. One of them is its limited sensitivity to weak interactions, which are common in the mammalian cerebral cortex. In the present paper a new method for the estimation of interaction strength is presented. This method is based on the intensity representation of point processes, and provides an optimal estimator for the intensity of the postsynaptic spike train. The estimator is complicated to use, but it can be approximated by a simple estimator, similar to ordinary measures of synaptic efficacy like the area under the crosscorrelogram peak. Simulation results, showing the advantage of the new estimator over the commonly used efficacy estimators and some measure of its robustness to deviations from model assumptions, are presented. Finally, application of the estimator to the analysis of simultaneous recordings of physiological single units is demonstrated.

Animals↗

PubMed1988

Analysis of the activity of single neurons in stochastic settings.

This paper presents a new way of modeling the activity of single neurons in stochastic settings. It incorporates in a natural way many physiological mechanisms not usually found in stochastic models, such as spatial integration, non-linear membrane characteristics and non-linear interactions between excitation and inhibition. The model is based on the fact that most of the neuronal inputs have a finite lifetime. Thus, the stochastic input can be modeled as a simple finite markov chain, and the membrane potential becomes a function of the state of this chain. Firing occurs at states whose membrane potential is above threshold. The main mathematical results of the model are: (i) the input-output firing rate curve is convex at low firing rates and is saturated at high firing rates, and (ii) at low firing rates, firing usually occurs when there is synchronous convergence of many excitatory events.

Action Potentials↗

PubMedDate not supplied

The role of interstitial potassium in the generation of low-calcium hippocampal seizures.

The explosive nature of a focal cortical seizure suggests the operation of strong positive feedback in the neuronal network. It was previously proposed that in the hippocampus this may be provided by the regenerative accumulation of potassium in the interstitium. This hypothesis was severely criticized in the past. However, it gained new impetus with the recent discovery that focal seizures can arise in mammalian hippocampal slices perfused with low calcium solutions despite the block of chemical synaptic transmission. Here, we examine the relationship of interstitial potassium concentration to the electrogenesis of these low-calcium seizures. Both the experimental data and the behavior of a simplified mathematical model describing neuronal discharge in low calcium, support the contention that interstitial potassium accumulation may play an important role in the buildup of hippocampal seizures.

Animals↗