Single Unit Level Research Articles

Electrophysiological investigations of the somatosensory thalamus of the pigeon.

Somatosensory areas in the thalamus of the pigeon were studied electrophysiologically at the single-unit level. Of 459 units, 429 responded to somatosensory stimuli. Of these, 394 units responded specifically to somatosensory stimuli, whereas 8 responded in addition to visual stimuli and 27 to auditory stimuli also (bimodal neurons). Seven units were exclusively driven by visual stimuli and 23 units by auditory stimuli. Recording sites included nucleus dorsalis intermedius ventralis anterior (DIVA) and nucleus dorsolateralis posterior (DLP). The number of bimodal or visual and auditory neurons was much larger in DLP (40%) than in DIVA (7%). This points to a specific somatosensory function of DIVA. There was only a poor somatotopic organization in both nuclei. However, in vertical penetrations, wing representation was usually dorsal to leg representation. Distal parts of the limbs had smaller receptive fields (RFs) than proximal parts, and the smallest RFs were found on the toes, which seem to be represented in most detail.

A chronic microelectrode investigation of the tonotopic organization of human auditory cortex

We investigated the functional organization of human auditory cortex using a new chronic microelectrode technique. Tonotopic mapping data was obtained at the single unit level for the first time in humans. All sound-driven units were noted to have frequency-dependent response patterns. The majority of units (73%) demonstrated sharply tuned excitatory best-frequency responses. Twenty seven percent of units showed wide receptive fields, representing excitatory responses to almost the entire range of frequencies presented. A tonotopic pattern was observed with best frequencies systematically increasing as more medial-caudal recording sites were sampled.

The hypothesis of the uniqueness of the oculomotor neural integrator: direct experimental evidence in the cat.

1. As far as horizontal eye movements are concerned, the well-known hypothesis, not yet experimentally proved, of the common neural integrator states that the eye-position signal is generated by a common network, regardless of the type of versional movement. The aim of this study was to evaluate the validity of this hypothesis by checking whether the sensitivity to eye position of the neurones of the nucleus prepositus hypoglossi (NPH) (the main component of the system integrating the different incoming velocity signals) would be the same regardless of the type of versional movement. 2. The discharge of sixty-five NPH neurones was recorded in the alert cat during spontaneous eye movements made in the light and in response to sinusoidal rotations of the head in complete darkness. 3. For each NPH neurone, the sensitivity to eye position was determined from measurements carried out during intersaccadic fixation. The discharge rate of the studied neurone was plotted against eye position. The slope of the resulting regression line gave the sensitivity (measured during intersaccadic fixation in the light) of the neurone to eye position, which was termed K(f). 4. A new method was developed to measure the sensitivity to eye position (K(v)) of neurones during vestibular slow phases. The difficulty came from the fact that, during slow phases, eye velocity and eye position changed simultaneously and that each of those two variables could influence neuronal activity. For each neurone, the instantaneous firing rate was measured each time the eye passed through a given position during any slow phase generated during any vestibulo-ocular reflex. At a given position, the discharge rate of the neurone under study was plotted against the eye velocity. From the resulting linear regression line, two interesting values were obtained: its slope, corresponding to the sensitivity of the neurone to eye velocity, R(v), (at that given eye position) and its 'y'-intercept, F(0), the interpolated firing rate when the eye velocity was zero. This procedure was repeated for different eye positions. The values of F(0) were then plotted against the eye positions. The slope of the resulting regression line gave the sensitivity (measured during vestibular stimulation) of the neurone to eye position, which was termed K(v). 5. The errors on the individual values of K(f) and K(v) were assessed in order to allow a statistical comparison at the single unit level. 6. We found that, for each of our sixty-five neurones, the sensitivity to eye position measured during intersaccadic fixation in the light was equal to the sensitivity to eye position measured during the vestibulo-ocular reflex (VOR) elicited in complete darkness. We conclude that our results favour the hypothesis of a unique horizontal oculomotor integrator for all versional movements.

Reduced Representation by Neural Networks with Restricted Receptive Fields

Model neural networks can perform dimensional reductions of input data sets using correlation-based learning rules to adjust their weights. Simple Hebbian learning rules lead to an optimal reduction at the single unit level but result in highly redundant network representations. More complex rules designed to reduce or remove this redundancy can develop optimal principal component representations, but they are not very compelling from a biological perspective. Neurons in biological networks have restricted receptive fields limiting their access to the input data space. We find that, within this restricted receptive field architecture, simple correlation-based learning rules can produce surprisingly efficient reduced representations. When noise is present, the size of the receptive fields can be optimally tuned to maximize the accuracy of reconstructions of input data from a reduced representation.

Decrements in auditory responses to a repeated conspecific song are long-lasting and require two periods of protein synthesis in the songbird forebrain.

Earlier work showed that playbacks of conspecific song induce expression of the immediate early gene ZENK in the caudo-medial neostriatum (NCM) of awake male zebra finches and that this response disappears with repeated presentations of the same stimulus. In the present study, we investigated whether repetitions of a song stimulus also elicited a decrement in the electrophysiological responses in the NCM neurons of these birds. Multiunit auditory responses in NCM were initially vigorous, but their amplitude decreased (habituated) rapidly to repeated stimulation, declining to about 40% of the initial response during the first 50 iterations. A similar time course of change was seen at the single unit level. This habituation occurred specifically for each song presented but did not occur when pure tones were used as a stimulus. Habituation to conspecific, but not heterospecific, song was retained for 20 h or longer. Injections of inhibitors of protein or RNA synthesis at the recording site did not affect the initial habituation to a novel stimulus, but these drugs blocked the long-term habituation when injected at 0.5-3 h and at 5.5-7 h after the first exposure to the stimulus. Thus, at least two waves of gene induction appear to be necessary for long-lasting habituation to a particular song.

An anatomically constrained neural network model of fear conditioning.

Conditioning of fear reactions to an auditory conditioned stimulus (CS) paired with a footshock unconditioned stimulus (US) involves CS transmission to the amygdala from the auditory thalamus, the auditory cortex, or both. This article presents a simple neural network model of this neural system. The model consists of modules of mutually inhibitory nonlinear units representing the different relevant anatomical structures of the thalamo-amygdala and thalamo-corticoamygdala circuitry. Frequency-specific changes produced by fear conditioning were studied at the behavioral level (stimulus generalization) and the single-unit level (receptive fields). The findings mirror effects observed in conditioning studies of animals. This computational model provides an initial grounding for explorations of how emotional information and behavior are related to anatomical and physiological observations.

Changes in oxidative capacity and fatigue resistance in skeletal muscle

In conclusion, it appears that in general an increase in the fatigue resistance of a muscle is accompanied by an increase in its oxidative capacity. Fatigue resistance of a muscle seems to be partly determined by its oxidative capacity. On the single motor unit (Burke et al, 1973; Hamm et al, 1988; Kugelberg and Lindegren 1979; Larsson et al, 1991) and single fibre level (Nemeth et al, 1981) the relation between fatigue resistance and oxidative capacity seems to be valid. However, this does not appear necessarily to be the case on the level of the whole muscle. Kugelberg and Lindegren (1979) suggested, that the endurance of each link in the chain of events leading to contraction is under aerobic conditions matched to the contractile capacity of the fibre expressed by its oxidative enzyme activity. Therefore, it might be that several tests for endurance capacity are more strenuous than the aerobic capacity of the muscle. Indeed, several studies suggest that the Burke test (Burke et al, 1973) or other fatiguing protocols might primarily test for other endurance-related properties as the excitation-contraction coupling (Kernell et al, 1987; Mayne et al, 1991b). Another explanation for the discrepancy in changes in oxidative capacity and fatigue resistance might be, that the mechanical responses of the motor units (which have different biochemical and contractile properties) during the fatigue test do not summate linearly during whole muscle contraction as was found by Gardiner and Olha (1987).(ABSTRACT TRUNCATED AT 250 WORDS)

A Nonstochastic String Model of Sympathetic Neuromuscular Transmission

Several apparently contradictory findings in studies of the release of norepinephrine and ATP and the resulting neurogenic contraction call for a radical revision of the current model of sympathetic neuromuscular transmission at the single unit level, i.e., at the junction between the individual varicosity and smooth muscle cell.

Task dependence of responses in first dorsal interosseous muscle to magnetic brain stimulation in man.

1. The response of the first dorsal interosseous (1DI) muscle to non-invasive magnetic and scalp electrical stimulation of the brain have been investigated during performance of different manual tasks. 2. The six tasks tested required activation of the 1DI muscle, either in isolation (during abduction of the index finger) or as part of a more complex pattern of muscle synergies (e.g. during power grip). The level of 1DI EMG activity across tasks was kept constant by providing subjects with visual feedback of their muscle activity. 3. In every subject (n = 14) magnetic stimulation produced larger responses during performance of complex tasks than during the simple index abduction task. The pooled results from all subjects showed that four of the five complex tasks were associated with significantly larger 1DI responses (paired t test, P < 0.05). 4. These results were confirmed at the single motor unit level for nine motor units recorded from six subjects. Subjects were requested to produce a steady discharge of the same motor unit during performance of different tasks. The probability of motor unit discharge in response to magnetic stimulation was significantly greater during complex tasks (rotation or pincer grips) than during abduction. 5. Scalp electrical stimulation was performed in three subjects with the cathode at the vertex and the anode over the contralateral motor cortex. The pattern of response amplitudes in the different tasks tended to parallel that obtained for magnetic stimulation, but the task-related differences were smaller. 6. These results suggest that during performance of the different tasks, the corticospinal volleys evoked by magnetic stimulation may vary in amplitude. The task-related cortical mechanisms that may contribute to this variability are discussed.

Distribution and activity of glutamine synthase and carbamoylphosphate synthase upon enlargement of the liver lobule by repeated partial hepatectomies

Glutamine synthase and carbamoylphosphate synthase show a strikingly heterogeneous and fully complementary distribution in the rat liver. In the human liver, however, there is a midlobular zone where both enzymes are absent. The diameter of the human liver lobule is approximately twice the size of the rat lobule. To investigate whether lobule size is a major determinant for the expression patterns of glutamine synthase and carbamoylphosphate synthase, Wistar strain rats were partially hepatectomized 3 times, at weekly or monthly intervals. Due to hepatic regeneration the cross-sectional area of the liver lobules increased twofold. However, a midlobular zone which lacked expression of both glutamine synthase and carbamoylphosphate synthase did not develop in these livers, thus showing that lobular size is not a major determinant for the distribution patterns of glutamine and carbamoylphosphate synthase. The twofold increase in the cross-sectional area of the liver lobule was associated with a similar reduction in the relative number of glutamine synthase-positive cells and in the enzyme content of the liver, indicating that the regeneration process does not affect the pericentral pattern of glutamine synthase expression. After regeneration was complete, the glutamine synthase content in the liver was restored to its original value, demonstrating a twofold increase in the cellular concentration of glutamine synthase-positive hepatocytes. An increase in the diameter of the liver lobule was only seen after the first partial hepatectomy. Liver growth following subsequent partial hepatectomies can be explained by an increase in the length of the liver lobule and/or by splitting of liver lobules. The zonal distribution of DNA replication, which is characteristic of the first partial hepatectomy, is lost after repeated partial hepatectomies. Furthermore, evidence was obtained that the signal for inducing DNA synthesis may originate at the level of single liver units.

Two-tone suppression in inner hair cell responses: Correlates of rate suppression in the auditory nerve

Inner hair cell (IHC) recordings were made from second turn of the guinea pig cochlea where characteristic frequencies are approximately 4000 Hz. In order to compare IHC responses with rate suppression measured in the auditory nerve, suppressors were introduced that produced little or no response in the hair cell. The effects of a variable-frequency suppressor on a constant-frequency probe, placed near characteristic frequency, were also investigated since this paradigm is commonly used in single unit experiments. Resulting magnitude changes were measured in the fundamental component of the ac receptor potential and/or in the total de produced in the region of temporal overlap between the two stimulus inputs. This latter component is escecially important when considering how changes in IHC responses relate to decreases in discharge rate in single auditorynnerve fibers. Since the ac receptor potential is filtered by the hair cell's basolateral membrane, the dc component probably controls transmitter release at the characteristic frequency of these second-turn IHCs. Based on results from these and previous experiments, a proposal is advanced to explain the evolution of two-tone suppression in the peripheral auditory system. The paper also discusses the use of excitatory versus non-excitatory suppressors and includes a description of two-tone suppression areas at the mechanical, IHC and single unit levels. The explanation of low-side suppression areas is of special interest since hitherto they have been difficult to model (Kim, 1985).

A Model of Primate Visual-Motor Conditional Learning

Observations of behavior and neural activity in premotor cortex of monkeys learning to pair an arbitrary visual stimulus with one of a set of previously learned behaviors are modeled with a network comprising a large number of motor selection columns. Reinforcement learning is used to recognize new visual patterns and acquire the appropriate visual-motor conditions. The architecture employs a distributed representation in which a single pattern is coded by a small subset of columns. A column is initially able to respond to many different inputs; as it learns to trigger a motor program, its responses become more narrowly defined. Each column's output is a set of votes for the various motor programs. The votes for each program are collected by selection units, which drive a winner-take-all circuit to determine whether a particular motor program is executed. The model is successful in reproducing the sequence of behavioral responses given by the subjects, as well as a number of phenomena that have been observed at the single-unit level. Finally, we offer a comparison to the backpropagation learning algorithm that demonstrates key principles which have been designed into our algorithm.

Action potential tuning curves recorded with a derived response technique

A number of laboratories use tonal masking methods in conjunction with the cochlear action potential (AP) to generate AP tuning curves. In these studies, the amplitude of AP elicited by a probe stimulus is monitored as the level of masker is varied. An amplitude reduction (AR) by a specified amount (25%–100%) is used to indicate masking of the response. An alternative method was investigated in which interaction of the probe and masker is monitored by digital subtraction of the unmasked and masked responses, generating a tone‐derived (TD) response. An amplitude criterion of 10‐μV TD response is used to indicate masker/probe interaction. Tuning curves were recorded from the round window of anesthetized guinea pigs using AR and TD methods. With near threshold probe stimuli, similar tuning curves were obtained. With higher levels however, markedly different results were observed. AR tuning curves became elevated and sharper while TD curves became lower and broader. The TD method appears to better represent the frequency spread of responses with increasing stimulus level as demonstrated at the single unit level. In addition, TD tuning curves were able to detect second harmonic distortions of the probe stimulus, while AR tuning curves did not. [Work supported by Program project grant P01 NS24372 and Training Grant NS 07278.]

Making the Transition From Single-Unit To Multi-Unit Fast-Service Management: What Are the Requisite Skills and Educational Needs?

In this research the requisite functional skills and training needs of multi-unit fast service managers are examined. Responses from 164 multi-unit managers are analyzed, and the results indicate that there are additional skills, beyond those required at the single-unit level, necessary to operate effectively at the multi-unit level. Implications for managers and educators are discussed.

Ontogenesis of tonotopy in inferior colliculus of a hipposiderid bat reveals postnatal shift in frequency-place code.

The postnatal development of midbrain tonotopy was investigated in the inferior colliculus (IC) of the south Indian CF-FM bat Hipposideros speoris. The developmental progress of the three-dimensional frequency representation was determined by systematic stereotaxic recordings of multiunit clusters from the 1st up to the 7th postnatal week. Additional developmental measures included the tuning characteristics of single units (Figs. 3f; 4f; 5f), the analysis of the vocalised pulse repertoire (Figs. 3e, 4e, 5e), and morphometric reconstructions of the brains of all experimental animals (Fig. 1). The maturation of auditory processing could be divided into two distinct, possibly overlapping developmental periods: First, up to the 5th week, the orderly tonotopy in the IC developed, beginning with the low frequency representation and progressively adding the high frequency representation. With regard to the topology of isofrequency sheets within the IC, maturation progresses from dorsolateral to ventromedial (Figs. 3c, 4c). At the end of this phase the entire IC becomes specialised for narrowly tuned and sensitive frequency processing. This includes the establishment of the 'auditory fovea', i.e. the extensive spatial representation of a narrow band of behaviorally relevant frequencies in the ventromedial part of the IC. In the 5th postnatal week the auditory fovea is concerned with frequencies from 100-118 kHz (Fig. 4c, d). During subsequent development, the frequency tuning of the auditory fovea increases by 20-25 kHz and finally attains the adult range of ca. 125-140 kHz. During this process, neither the bandwidth of the auditory fovea (15-20 kHz) nor the absolute sensitivity of its units (ca. 50 dB SPL) were changed. Further maturation occurred at the single unit level: the sharpness of frequency tuning increased from the 5th to the 7th postnatal weeks (Q-10-dB-values up to 30-60), and upper thresholds emerged (Figs. 4f, 5f). Although in the adult the frequency of the auditory fovea matches that of the vocalised pulses, none of the juvenile bats tested from the 5th to the 7th weeks showed such a frequency match between vocalisation and audition (Figs. 4e, 5e). The results show that postnatal maturation of audition in hipposiderid bats cannot be described by a model based on a single developmental parameter.

Comments on Quanta in the Analysis of Stimulus Control

According to the quantal interpretation of stimulus control, stimulus-response relations are integral “quantal” units that either occur or do not occur—no middle ground is possible. This paper analyzes the theoretical implications of this interpretation and concludes that, at the level of single response units, stimulus-response relations can always be viewed as quantal. The interpretation is, thus, not falsifiable. Empirical work, then, is necessarily interpretable so as to support the interpretation, and research considered relevant to the quantal interpretation is examined in this light. The theory has heuristic value, however, especially for the determination of behavioral units. Several problems common to all theories of stimulus control concerning the identification of stimulus and response classes are also discussed.

Sensitivity of neurons in the auditory midbrain of the grassfrog to temporal characteristics of sound. I. Stimulation with acoustic clicks

The coding of fine-temporal structure of sound, especially pulse repetition rate, was investigated on the single-unit level in the auditory midbrain of the grassfrog. As stimuli periodic click trains and Poisson distributed click ensembles have been used. The response to periodic click trains was studied in two aspects, focussing on two types of possible codes: a rate code and a synchrony code. From the iso-intensity rate histogram five basic average response rate characteristics as function of pulse repetition rate have been established: low-pass, band-pass, high-pass, bimodal and non-selective unit types. The synchronization capability, expressed in a synchronization index, was for a small majority of units non-significant and a low-pass function of pulse repetition rate for most of the other units. The rate code showed the largest diversity of response types and an enhanced selectivity to pulse repetition rate. The stimulus-response relation to Poisson distributed click ensembles was investigated by a non-linear system theoretical approach. On the basis of first- and second-order Poisson kernels possible neural mechanisms accounting for temporal selectivity were determined. A considerable fraction of units exhibited response characteristics that were invariant to changes in sound pressure level and average click rate. These units may function as feature detectors of fine-temporal structure of sound. The spectro-temporal sensitivity range of the auditory midbrain of the grassfrog appeared to be broad and not particularly tuned to the ensemble of conspecific cells.

Sensitivity of neurons in the auditory midbrain of the grassfrog to temporal characteristics of sound. II. Stimulation with amplitude modulated sound

The coding of fine-temporal structure of sound, especially of frequency of amplitude modulation, was investigated on the single-unit level in the auditory midbrain of the grassfrog. As stimuli sinusoidally amplitude modulated sound bursts and continuous sound with low-pass Gaussian noise amplitude modulation have been used. Both tonal and wideband noise carriers have been applied. The response to sinusoidally amplitude modulated sound bursts was studied in two aspects focussing on two types of possible codes: a rate code and a synchrony code. From the iso-intensity rate histogram five basic average response characteristics as function of modulation frequency have been observed: low-pass, band-pass, high-pass, bimodal and non-selective types. The synchronization capability, expressed in a synchronization index, was non-significant for 38% of the units and a low-pass function of modulation frequency for most of the other units. The stimulus-response relation to noise amplitude modulated sound was investigated by a non-linear system theoretical approach. On the basis of first- and second-order Wiener-Volterra kernels possible neural mechanisms accounting for temporal selectivity were obtained. About one quarter of the units had response characteristics that were invariant to changes in sound pressure level and spectral content of the carrier. These units may function as feature detectors of fine-temporal structure of sound. The spectro-temporal sensitivity range of the auditory midbrain of the grassfrog appeared not to be restricted to and showed no preference for the spectro-temporal characteristics of the ensemble of conspecific calls. Comparison of response characteristics to periodic click trains as studied in the companion paper (Epping and Eggermont, 1986) and sinusoidally amplitude modulated sound bursts revealed that the observed temporal sensitivity is due to a combination of sensitivities to sound periodicity and pulse duration. It was found that for most units the first-order kernels for Gaussian amplitude modulated stimuli and Poisson distributed click stimuli were alike. In contrast second-order kernels for the Gaussian amplitude modulated stimuli often represented only static non-linearities, while second-order kernels for Poisson distributed clicks (Epping and Eggermont, 1986) mostly revealed dynamic non-linearities.

Room descriptions∗

Verbal descriptions of rooms were collected and analyzed. The analyses are based on the initial selection of an optimal factorization of rooms and are further conducted on the level of single description units, sequencing of such units, and the global structure of the description. On all levels of analysis, correspondences are noted between the underlying cognitive structures and the particular linguistic forms in which they are expressed. It is noted that linguistic constructions may serve as markers for peculiar patterns in the structure of the description. Together, the different analyses indicate that the act of describing is not based on a naive‐realistic mapping of things and words. Rather, descriptions presuppose a cognitive structuring of the world which may not be expressed by simple recursive algorithms.

Visual (optokinetic) and somesthetic inputs to the cerebellum of bilaterally labyrinthectomized frogs

Unitary responses to visual (optokinetic) and somesthetic (cutaneous and propioceptive) stimulation were recorded from the Purkinje cell layer of the cerebellum of acute (up to 30 h) and chronic (30–90 days) bilaterially labyrinthectomized frogs. Simple spikes from Purkinje cells as well as activity from cells without complex spikes were considered in this paper. The properties of the response and the distribution (restricted to the dorsal rim and auricular lobes) of the units sensitive to optokinetic stimulation of labyrinthectomized frogs, both acute and chronic, were similar to those previously reported for normal animals. The properties of the responses to somesthetic neck and limb stimulation remained similar to those of normal animals. However, there was an increase in the number of units responsive to somesthetic stimulation within the dorsal half of the corpus cerebelli (including the dorsal rim), a region experimentally deprived of vestibular afferents, neck responsive units were 33% of the total in acutely and 61% in chronically labyrinthectomized animals (compared to 5% in normal). Limb responsive units were 49% in acute and 65% in chronic animals (compared to 12% in normal). A consequence of the increase in somesthetic input was convergence of optokinetic and somesthetic inputs at the level of single units within the dorsal rim, totally absent before the lesion. The results suggest that the somesthetic spinal input might substitute for at least some features of the vestibular input to the cerebellum in bilaterally labyrinthectomized frogs.