Biomedical subjects

P Heil

Publications and source records attributed to P Heil.

At least 19 recordsLinked to original sources

PubMed2006

Multisensory processing via early cortical stages: Connections of the primary auditory cortical field with other sensory systems.

It is still a popular view that primary sensory cortices are unimodal, but recent physiological studies have shown that under certain behavioral conditions primary sensory cortices can also be activated by multiple other modalities. Here, we investigate the anatomical substrate, which may underlie multisensory processes at the level of the primary auditory cortex (field AI), and which may, in turn, enable AI to influence other sensory systems. We approached this issue by means of the axonal transport of the sensitive bidirectional neuronal tracer fluorescein-labeled dextran which was injected into AI of Mongolian gerbils (Meriones unguiculatus). Of the total number of retrogradely labeled cell bodies (i.e. cells of origin of direct projections to AI) found in non-auditory sensory and multisensory brain areas, approximately 40% were in cortical areas and 60% in subcortical structures. Of the cell bodies in the cortical areas about 82% were located in multisensory cortex, viz., the dorsoposterior and ventroposterior, posterior parietal cortex, the claustrum, and the endopiriform nucleus, 10% were located in the primary somatosensory cortex (hindlimb and trunk region), and 8% in secondary visual cortex. The cortical regions with retrogradely labeled cells also contained anterogradely labeled axons and their terminations, i.e. they are also target areas of direct projections from AI. In addition, the primary olfactory cortex was identified as a target area of projections from AI. The laminar pattern of corticocortical connections suggests that AI receives primarily cortical feedback-type inputs and projects in a feedforward manner to its target areas. Of the labeled cell bodies in the subcortical structures, approximately 90% were located in multisensory thalamic, 4% in visual thalamic, and 6% in multisensory lower brainstem structures. At subcortical levels, we observed a similar correspondence of retrogradely labeled cells and anterogradely labeled axons and terminals in visual (posterior limitans thalamic nucleus) and multisensory thalamic nuclei (dorsal and medial division of the medial geniculate body, suprageniculate nucleus, posterior thalamic cell group, zona incerta), and in the multisensory nucleus of the brachium of the inferior colliculus. Retrograde, but not anterograde, labeling was found in the multisensory pontine reticular formation, particularly in the reticulotegmental nucleus of the pons. Conversely, anterograde, but no retrograde, labeling was found in the visual laterodorsal and lateroposterior thalamic nuclei, in the multisensory peripeduncular, posterior intralaminar, and reticular thalamic nuclei, as well as in the multisensory superior and pericentral inferior colliculi (including cuneiform and sagulum nucleus), pontine nuclei, and periaqueductal gray. Our study supports the notion that AI is not merely involved in the analysis of auditory stimulus properties but also in processing of other sensory and multisensory information. Since AI is directly connected to other primary sensory cortices (viz. the somatosensory and olfactory ones) multisensory information is probably also processed in these cortices. This suggests more generally, that primary sensory cortices may not be unimodal.

Afferent Pathways↗

PubMed2001

Temporal integration of sound pressure determines thresholds of auditory-nerve fibers.

Current propositions of the quantity of sound driving the central auditory system, specifically around threshold, are diverse and at variance with one another. They include sound pressure, sound power, or intensity, which are proportional to the square of pressure, and energy, i.e., the integral of sound power over time. Here we show that the relevant sound quantity and the nature of the threshold can be obtained from the timing of the first spike of auditory-nerve (AN) fibers after the onset of a stimulus. We reason that the first spike is triggered when the stimulus reaches threshold and occurs with fixed delay thereafter. By probing cat AN fibers with characteristic frequency tones of different sound pressure levels and rise times, we show that the differences in relative timing of the first spike (including latencies >100 msec of fibers with low spontaneous rates) can be well accounted for by essentially linear integration of pressure over time. The inclusion of a constant pressure loss or gain to the integrator improves the fit of the model and also accounts for most of the variation of spontaneous rates across fibers. In addition, there are tight correlations among delay, threshold, and spontaneous rate. First-spike timing cannot be explained by models based on a fixed pressure threshold, a fixed power or intensity threshold, or an energy threshold. This suggests that AN fiber thresholds are best measured in units of pressure by time. Possible mechanisms of pressure integration by the inner hair cell-AN fiber complex are discussed.

Acoustic Stimulation↗

PubMed2001

Influence of coconut water on hemostasis.

Coconut water (CNW) can be used as short-term intravenous hydration and resuscitation fluid. We investigated the influence of coconut water on plasma coagulation in vitro. Either CNW or physiological saline (PS) was added to citrated plasma of 8 healthy volunteers. Coagulation capability of diluted plasma was evaluated by thrombelastography (TEG). Replacement of up to 50 % of citrated plasma by CNW or PS did not influence initiation of coagulation as indicated by split point and reaction time, respectively. Strength of fibrin clot as expressed by maximum amplitude (MA) of TEG recording dose dependently declined in both groups. Replacing 50 % of citrated plasma by CNW or PS reduced MA by 39% and 32%, respectively. The influence of coconut water on hemostasis as assessed by TEG does not differ from the effect caused by an identical volume of PS.

Blood Coagulation↗

PubMed2000

New insights into the hemodynamic blood oxygenation level-dependent response through combination of functional magnetic resonance imaging and optical recording in gerbil barrel cortex.

Fast, low-angle shoot functional magnetic resonance imaging (fMRI), based on the blood oxygenation level-dependent (BOLD) effect, was combined with optical recording of intrinsic signals (ORIS) and 2-deoxyglucose labeling in gerbil barrel cortex. We observed over the activated barrel a positive BOLD signal and increased levels of deoxyhemoglobin and total hemoglobin during each period of prolonged (30 sec) D2 vibrissal stimulation. These data show that the hemodynamic basis of this fMRI signal is not necessarily a washout of deoxyhemoglobin, as generally assumed. Instead, they suggest that a positive BOLD signal can also be caused by a local increase of blood volume, even if deoxyhemoglobin levels are persistently elevated. We also show that this alternative interpretation is consistent with theoretical models of the BOLD signal. The changes in BOLD signal and blood volume, which are most tightly correlated with the periodic stimulation, peak at the site of neuronal activation. These results contribute to the understanding of the hemodynamic mechanisms underlying the BOLD signal and also suggest analysis methods, which improve the spatial localization of neuronal activation with both fMRI and ORIS.

Animals↗

PubMed2000

Functional organization of auditory cortex in the Mongolian gerbil (Meriones unguiculatus). III. Anatomical subdivisions and corticocortical connections.

The auditory cortex of the Mongolian gerbil comprises several physiologically identified fields, including the primary (AI), anterior (AAF), dorsal (D), ventral (V), dorsoposterior (DP) and ventroposterior (VP) fields, as established previously with electrophysiological [Thomas et al. (1993) Eur. J. Neurosci., 5, 882] and functional metabolic techniques [Scheich et al. (1993) Eur. J. Neurosci., 5, 898]. Here we describe the cyto-, myelo- and chemoarchitecture and the corticocortical connections of the auditory cortex in this species. A central area of temporal cortex corresponding to AI and the rostrally adjacent AAF is distinguished from surrounding cortical areas by its koniocortical cytoarchitecture, by a higher density of myelinated fibres, predominantly in granular and infragranular layers, and by characteristic patterns of immunoreactivity for the calcium-binding protein parvalbumin (most intense staining in layers III/IV and VIa) and for the cytoskeletal neurofilament protein (antibody SMI-32; most intense staining in layers III, V and VI). Concerning the cortical connections, injections of the predominantly anterograde tracer biocytin into the four tonotopically organized fields AI, AAF, DP and VP yielded the following labelling patterns. (i) Labelled axons and terminals were seen within each injected field itself. (ii) Following injections into AI, labelled axons and terminals were also seen in the ipsilateral AAF, DP, VP, D and V, and in a hitherto undescribed possible auditory field, termed the ventromedial field (VM). Similarly, following injections into AAF, DP and VP, labelling was also seen in each of the noninjected fields, except in VM. (iii) Each field projects to its homotopic counterpart in the contralateral hemisphere. In addition, field AI projects to contralateral AAF, DP and VP, field DP to contralateral AI and VP, and field VP to contralateral AI and DP. (iv) Some retrogradely filled pyramidal neurons within the areas of terminal labelling indicate reciprocal connections between most fields, both ipsilateral and contralateral. (v) The labelled fibres within the injected and the target fields, both ipsilateral and contralateral, were arranged in continuous dorsoventral bands parallel to isofrequency contours. The more caudal the injection site in AI the more rostral was the label in AAF. This suggests divergent but frequency-specific connections within and, at least for AI and AAF, also across fields, both ipsilateral and contralateral. (vi) Projections to associative cortices (perirhinal, entorhinal, cingulate) and to other sensory cortices (olfactory, somatosensory, visual) from AAF, DP and VP appeared stronger than those from AI. These data support the differentiation of auditory cortical fields in the gerbil into at least 'core' (AI and AAF) and 'noncore' fields. They further reveal a complex pattern of interconnections within and between auditory cortical fields and other cortical areas, such that each field of auditory cortex has its unique set of connections.

Animals↗

PubMed2000

Functional organization of auditory cortex in the Mongolian gerbil (Meriones unguiculatus). IV. Connections with anatomically characterized subcortical structures.

The subcortical connections of the four tonotopically organized fields of the auditory cortex of the Mongolian gerbil, namely the primary (AI), the anterior (AAF), the dorsoposterior (DP) and the ventroposterior field (VP), were studied predominantly by anterograde transport of biocytin injected into these fields. In order to allow the localization of connections with respect to subdivisions of subcortical auditory structures, their cyto-, fibre- and chemoarchitecture was characterized using staining methods for cell bodies, myelin and the calcium-binding protein parvalbumin. Each injected auditory cortical field has substantial and reciprocal connections with each of the three subdivision of the medial geniculate body (MGB), namely the ventral (MGv), dorsal (MGd) and medial division (MGm). However, the relative strengths of these connections vary: AI is predominantly connected with MGv, AAF with MGm and MGv, and DP and VP with MGd and MGv. The connections of at least AI and MGv are topographic: injections into caudal low-frequency AI label laterorostral portions of MGv, whereas injections into rostral high-frequency AI label mediocaudal portions of MGv. All investigated auditory fields send axons to the suprageniculate, posterior limitans, laterodorsal and lateral posterior thalamic nuclei, with strongest projections from DP and VP, as well as to the reticular and subgeniculate thalamic nuclei. AI, AAF, DP and VP project to all three subdivisions of the inferior colliculus, namely the dorsal cortex, external cortex and central nucleus ipsilaterally and to the dorsal and external cortex contralaterally. They also project to the deep and intermediate layers of the ipsilateral superior colliculus, with strongest projections from DP and VP to the lateral and basolateral amygdaloid nuclei, the caudate putamen, globus pallidus and the pontine nuclei. In addition, AAF and particularly DP and VP project to paralemniscal regions around the dorsal nucleus of the lateral lemniscus (DNLL), to the DNLL itself and to the rostroventral aspect of the superior olivary complex. Moreover, DP and VP send axons to the dorsal lateral geniculate nucleus. The differences with respect to the existence and/or relative strengths of subcortical connections of the examined auditory cortical fields suggest a somewhat different function of each of these fields in auditory processing.

Amygdala↗

PubMed2000

Parallels between timing of onset responses of single neurons in cat and of evoked magnetic fields in human auditory cortex.

Sound onsets constitute particularly salient transients and evoke strong responses from neurons of the auditory system, but in the past, such onset responses have often been analyzed with respect to steady-state features of sounds, like the sound pressure level. Recent electrophysiological studies of single neurons from the auditory cortex of anesthetized cats have revealed that the timing and strength of onset responses are shaped by dynamic stimulus properties at their very onsets. Here we demonstrate with magnetoencephalography that stimulus-response relationships very similar to those of the single neurons are observed in two onset components, N100m and P50m, of auditory evoked magnetic fields (AEFs) from the auditory cortex of awake humans. In response to tones shaped with cosine-squared rise functions, N100m and P50m peak latencies vary systematically with tone level and rise time but form a rather invariant function of the acceleration of the envelope at tone onset. Hence N100m and P50m peak latencies, as well as peak amplitudes, are determined by dynamic properties of the stimuli within the first few milliseconds, though not necessarily by acceleration. The changes of N100m and P50m peak latencies with rise time and level are incompatible with a fixed-amplitude threshold model. The direct comparison of the neuromagnetic and single-neuron data shows that, on average, the variance of the neuromagnetic data is larger by one to two orders of magnitude but that favorable measurements can yield variances as low as those derived from neurons with mediocre precision of response timing. The striking parallels between the response timing of single cortical neurons and of AEFs provides a stronger link between single neuron and population activity.

Acceleration↗

PubMed2000

The impact of polygraphy on admissions of victims and offenses in adult sexual offenders.

Sexual offenders are extremely reluctant to disclose their offending histories for a variety of psychosocial and legal reasons. The polygraph has shown promise as a intervention for eliciting admissions of past sexual offending behaviors. For 60 adult male sexual offender (35 inmates and 25 parolees), the number of victims and offenses were recorded from the Presentence Investigative Report, Sexual History Disclosure form, and 2 consecutive polygraph examination reports. Dramatic increases in the number of admitted victims and offenses were found for inmates, but not for parolees, across each source. However, there was a substantial decline in the number of victim and offense admissions by the second polygraph examination for both groups, even though 80% of the examination results reveled deception about sexual offending behaviors. Standardized use of sanctions and privileges for deceptive and nondeceptive polygraph results, respectively, are proposed as a way of eliciting full disclosure of offending histories for these offenders.

Adult↗

PubMed1998

Further observations on the threshold model of latency for auditory neurons.

This paper examines the recent claim of Phillips, that the threshold model of latency is inadequate to account for the changes of latency of auditory cortical neurons in response to tones of different amplitudes and rise times. I argue that Phillips' analysis was based on an incorrect assumption and that he therefore rejected the model for the wrong reasons, though correctly, as the model is in fact inadequate, as demonstrated here and previously. The failure of the model has significant implications for signal processing in the auditory system.

Algorithms↗

PubMed1998

The posterior field P of cat auditory cortex: coding of envelope transients.

The posterior field (P) of the cat auditory cortex contains a very high proportion of neurons whose responses change non-monotonically with the sound pressure level (SPL) of tonal stimuli, leading to circumscribed frequency-SPL response areas, and it has therefore been suggested that field P may be specialized for processing of sound intensity. We demonstrate here a great diversity of response areas in field P. Furthermore, by varying tone SPL and rise time, we show that, as in primary auditory cortex (AI), the onset response of a field P neuron is better described as a function of the instantaneous peak pressure (envelope) at the time of response generation than of the steady-state SPL of the stimulus. Such responses could be used to track transients or represent envelopes in more general terms, rather than to code SPL. Compared with AI, field P neurons have relatively long minimum latencies along with a large jitter in spike timing. Tracking would therefore be most effective for slowly varying envelopes, and one function of the inhibition that generates non-monotonicity in field P may be to suppress temporally sluggish responses to rapid transients, such as the onsets of high-SPL, short rise time tones. Field P may thus be specialized for coding slowly varying signals.

Acoustic Stimulation↗

PubMed1998

Neuronal coding of interaural transient envelope disparities.

Onsets are salient and important transient (i.e. dynamic) features of acoustic signals, and evoke vigorous responses from most auditory neurons, but paradoxically these onset responses have most often been analysed with respect to steady-state stimulus features, e.g. the sound pressure level (SPL). In nearly all studies concerned with the coding of differences in SPL at the two ears (interaural level differences; ILDs), which provide a major cue for the azimuthal location of high frequency sound sources, interaural onset disparities were covaried with ILD, but the possibly confounding effects of this covariation on neuronal responses have been entirely neglected. Therefore, dichotic stimulus paradigms were designed here in which onset and steady-state features were varied independently. Responses were recorded from single neurons in the inferior colliculus of rats, anaesthetized with pentobarbital and xylazine. It is demonstrated that onset responses, or the onset response components of neurons with more complex temporal response patterns, are dependent on the binaural combination of dynamic envelope features associated with conventional ILD stimulus paradigms, but not on the binaural combination of steady-state SPLs reached after the onset. In contrast, late or sustained response components appear more sensitive to the binaural combination of steady-state SPLs. These data stress the general necessity for a separate analysis of onset and late response components, with respect to different stimulus features, and suggest a need for re-evaluation of existing studies on ILD coding. The sensitivity of onset responses to the binaural combination of envelope transients, rather than to steady-state ILD, is in line with their sensitivity to other interaural envelope disparities, created by stationary or moving sounds.

Acoustic Stimulation↗

PubMed1998

Functional specialization in auditory cortex: responses to frequency-modulated stimuli in the cat's posterior auditory field.

The mammalian auditory cortex contains multiple fields but their functional role is poorly understood. Here we examine the responses of single neurons in the posterior auditory field (P) of barbiturate- and ketamine-anesthetized cats to frequency-modulated (FM) sweeps. FM sweeps traversed the excitatory response area of the neuron under study, and FM direction and the linear rate of change of frequency (RCF) were varied systematically. In some neurons, sweeps of different sound pressure levels (SPLs) also were tested. The response magnitude (number of spikes corrected for spontaneous activity) of nearly all field P neurons varied with RCF. RCF response functions displayed a variety of shapes, but most functions were of low-pass characteristic or peaked at rather low RCFs (<100 kHz/s). Neurons with strong responses to high RCFs (high-pass or nonselective RCF response function characteristics) all displayed spike count-SPL functions to tone burst onsets that were monotonic or weakly nonmonotonic. RCF response functions and best RCFs often changed with SPL. For most neurons, FM directional sensitivity, quantified by a directional sensitivity (DS) index, also varied with RCF and SPL, but the mean and width of the distribution of DS indices across all neurons was independent of RCF. Analysis of response timing revealed that the phasic response of a neuron is triggered when the instantaneous frequency of the sweep reaches a particular value, the effective Fi. For a given neuron, values of effective Fi were independent of RCF, but depended on FM direction and SPL and were associated closely with the boundaries of the neuron's frequency versus amplitude response area. The standard deviation (SD) of the latency of the first spike of the response decreased with RCF. When SD was expressed relative to the rate of change of stimulus frequency, the resulting index of frequency jitter increased with RCF and did so rather uniformly in all neurons and largely independent of SPL. These properties suggest that many FM parameters are represented by, and may be encoded in, orderly temporal patterns across different neurons in addition to the strength of responses. When compared with neurons in primary and anterior auditory fields, field P neurons respond better to relatively slow FMs. Together with previous studies of responses to modulations of amplitude, such as tone onsets, our findings suggest more generally that field P may be best suited for processing signals that vary relatively slowly over time.

Acoustic Stimulation↗

PubMed1997

Field-specific responses in the auditory cortex of the unanaesthetized Mongolian gerbil to tones and slow frequency modulations.

Responses of multi-units in the auditory cortex (AC) of unanaesthetized Mongolian gerbils to pure tones and to linearly frequency modulated (FM) sounds were analysed. Three types of responses to pure tones could be clearly distinguished on the basis of spectral tuning properties, response latencies and overall temporal response pattern. In response to FM sweeps these three types discharged in a temporal pattern similar to tone responses. However, for all type-1 units the latencies of some phasic response components shifted systematically as a function of range and/or speed of modulation. Measurements of response latencies to FMs revealed that such responses were evoked whenever the modulation reached a particular instantaneous frequency (Fi). Effective Fi was: (1) independent of modulation range and speed, (2) always reached before the modulation arrived at a local maximum of the frequency response function (FRF) and consequently differed for downward and upward sweeps, and (3) was correlated with the steepest slope of that FRF maximum. The three different types of units were found in discrete and separate fields or regions of the AC. It is concluded that gross temporal response properties are one of the key features distinguishing auditory cortical regions in the Mongolian gerbil.

Acoustic Stimulation↗

PubMed1997

Frequency and periodicity are represented in orthogonal maps in the human auditory cortex: evidence from magnetoencephalography.

Timbre and pitch are two independent perceptual qualities of sounds closely related to the spectral envelope and to the fundamental frequency of periodic temporal envelope fluctuations, respectively. To a first approximation, the spectral and temporal tuning properties of neurons in the auditory midbrain of various animals are independent, with layouts of these tuning properties in approximately orthogonal tonotopic and periodotopic maps. For the first time we demonstrate by means of magnetoencephalography a periodotopic organization of the human auditory cortex and analyse its spatial relationship to the tonotopic organization by using a range of stimuli with different temporal envelope fluctuations and spectra and a magnetometer providing high spatial resolution. We demonstrate an orthogonal arrangement of tonotopic and periodotopic gradients. Our results are in line with the organization of such maps in animals and closely match the perceptual orthogonality of timbre and pitch in humans.

Acoustic Stimulation↗

PubMed1997

In vivo fluorescence kinetics of porphyrins following intravesical instillation of 5-aminolaevulinic acid in normal and tumour-bearing rat bladders.

The fluorescence kinetics of protoporphyrin IX (PPIX) following intravesical instillation of 5-aminolaevulinic acid (5-ALA) have been studied in vivo in a rat bladder tumour model. 5-ALA dissolved in NaHCO3 was intravesically instilled for 60 min in tumour-bearing and normal bladders of Wistar rats. The fluorescence was excited with the violet lines of a Kr(+)-laser and recorded in vivo by means of a fibre coupled optical multichannel analyser. The fluorescence emission bands of PPIX at lambda = 636 nm and lambda = 708 nm were detected in normal and tumorous urothelium after only 30 min. The maximum fluorescence intensity was obtained in tumorous and normal urothelium 3-4 h after instillation. The ratio of the fluorescence intensity in tumorous to normal urothelium decreased continuously from four to about two during the time range of 6 h. PPIX fluorescence following 5-ALA instillation could also be observed in kidney and liver. Fluorescence from further porphyrin species with emission bands at lambda = 617 nm and lambda = 682 nm was detected in the bladder, indicating an efflux of hydrophilic porphyrins from the hepatic pathway.

Administration, Intravesical↗

PubMed1997

Auditory cortical onset responses revisited. I. First-spike timing.

Sound onsets are salient and behaviorally relevant, and most auditory neurons discharge spikes locked to such transients. The acoustic parameters of sound onsets that shape such onset responses are unknown. In this paper is analyzed the timing of spikes of single neurons in the primary auditory cortex of barbiturate-anesthetized cats to the onsets of tone bursts. By parametric variation of sound pressure level, rise time, and rise function (linear or cosine-squared), the time courses of peak pressure, rate of change of peak pressure, and acceleration of peak pressure during the tones' onsets were systematically varied. For cosine-squared rise function tones of a given frequency and laterality, any neuron's mean first-spike latency was an invariant and inverse function of the maximum acceleration of peak pressure occurring at tone onset. For linear rise function tones, latency was an invariant and inverse function of the rate of change of peak pressure. Thus latency is independent of rise time or sound pressure level per se. Latency-acceleration functions, obtained with cosine-squared rise function tones under different stimulus conditions (frequency, laterality) from any given neuron and across the neuronal pool, were of strikingly similar shape. The same was true for latency-rate of change of peak pressure functions obtained with linear rise function tones. Latency-acceleration/rate of change of peak pressure functions could differ in their extent and in their position within the coordinate system. The positional differences reflect neuronal differences in minimum latency Lmin and in a sensitivity S to acceleration and rate of change of peak pressure (transient sensitivity), a hitherto unrecognized neuronal property that is distinctly different from firing threshold. Estimates of Lmin and S, which were derived by fitting a simple function to the neuronal latency-acceleration/rate of change of peak pressure functions, were independent of rise function. On average, Lmin decreased with increasing characteristic frequency (CF), but varied widely for neurons with the same CF. S varied with CF in a fashion similar to the cat's audiogram and, for a given neuron, varied with frequency. SD of first-spike latency was roughly proportional to the slope of the functions relating latency to acceleration/rate of change of peak pressure. Thus SD increased exponentially, rather than linearly, with mean latency, and did so at about twice the rate for linear than for cosine-squared rise function tones. The proportionality coefficients were quite similar across the neuronal pool and similar for both rise functions. Minimum SD increased nonlinearly with increasing Lmin. These findings suggest a peripheral origin of S and a peripheral establishment of latency-acceleration/rate of change of peak pressure functions. Because of the striking similarity in the shapes of such functions across the neuronal pool, sound onsets will produce orderly and predictable spatiotemporal patterns of first-spike timing, which could be used to instantaneously track rapid transients and to represent transient features by partly scale-invariant temporal codes.

Acoustic Stimulation↗

PubMed1997

Auditory cortical onset responses revisited. II. Response strength.

Most neurons of the auditory pathway discharge spikes locked to the onset of an acoustic stimulus, but it is largely unknown in which way the acoustic parameters of sound onsets shape the neuronal responses. In this paper is analyzed the number of spikes discharged by single neurons in primary auditory cortex of barbiturate-anesthetized cats to the onsets of tones of characteristic frequency. The time course of the peak pressure (i.e., the envelope) was altered by parametrically varying sound pressure level (SPL), rise time, and rise function (linear or cosine-squared). For both rise functions, rise time had manifold, and in some cases dramatic, effects on conventional spike count-level functions. In general, threshold SPL, dynamic range, and the lowest SPL at which monotonic spike count functions saturated increased with prolongation of the rise time. In neurons with mostly nonmonotonic spike count-level functions, "best SPL" increased and the descending high-SPL arms flattened, so that functions obtained with long rise times were often monotonic whereas those obtained with shorter rise times were highly nonmonotonic. Consequently, the "tuning" to SPL was less sharp for longer rise time tones, and spike count versus rise time functions changed from "short-pass" to "long-pass" with an increase in SPL. Systematic effects of rise time persisted when spike counts were plotted against the rate of change of peak pressure or against the maximum acceleration of peak pressure. However, when spike counts were plotted as a function of the instantaneous peak pressure at the time of response initiation, the functions obtained with different rise times, and even with different rise functions, were in close register. This suggests that the stimulus-dependent component of first-spike latency can be viewed as an integration window, during which rate of change of peak pressure is integrated. The window commences with tone onset and its duration is inversely related to the maximum acceleration (or, for linear rise functions, the rate of change) of peak pressure and the neuron's transient sensitivity. The present findings seriously question, for onset responses, the usefulness of the spike count-level function and measures derived from it, such as threshold SPL, dynamic range, best SPL, or degree of nonmonotonicity. They further cast doubt onto the validity of current concepts of intensity coding at cortical levels, because most neurons' onset responses are not indicative of a signal's steady-state SPL. However, they suggest a mechanism by which a neuronal population will sample a given transient in an orderly, sensitivity-dependent, temporal sequence. The sampling rate is automatically adjusted to, and adjusted by, the rapidity of the signal's change. And the instantaneous properties of the transient could be represented by the ratios and spatial distribution of responses across the simultaneously active subpopulation. Such a mechanism could provide the basis for the demonstrated capability of discrimination of rapid transients.

Animals↗

PubMed1997

First-spike timing of auditory-nerve fibers and comparison with auditory cortex.

First-spike timing of auditory-nerve fibers and comparison with auditory cortex. J. Neurophysiol. 78: 2438-2454, 1997. The timing of the first spike of cat auditory-nerve (AN) fibers in response to onsets of characteristic frequency (CF) tone bursts was studied and compared with that of neurons in primary auditory cortex (AI), reported previously. Tones were shaped with cosine-squared rise functions, and rise time and sound pressure level were parametrically varied. Although measurement of first-spike latency of AN fibers was somewhat compromised by effects of spontaneous activity, latency was an invariant and inverse function of the maximum acceleration of peak pressure (i.e., a feature of the 2nd derivative of the stimulus envelope), as previously found in AI, rather than of tone level or rise time. Latency-acceleration functions of all AN fibers were of very similar shape, similar to that observed in AI. As in AI, latency-acceleration functions of different fibers were displaced along the latency axis, reflecting differences in minimum latency, and along the acceleration axis, reflecting differences in sensitivity to acceleration [neuronal transient sensitivity (S)]. S estimates increased with spontaneous rate (SR), but values of high-SR fibers exceeded those in AI. This suggests that S estimates are biased by SR per se, and that unbiased true S values would be less tightly correlated with response properties covarying with SR, such as firing threshold. S estimates varied with CF in a fashion similar to the cat's audiogram and, for low- and medium-SR fibers, matched those for AI neurons. Minimum latency decreased with increasing SR and CF. As in AI, the standard deviation of first-spike timing (SD) in AN was also an inverse function of maximum acceleration of peak pressure. The characteristics of the increase of SD with latency in a given AN fiber/AI neuron and across AN fibers/AI neurons revealed that the precision of first-spike timing to some stimuli can actually be higher in AI than in AN. The data suggest that the basic characteristics of the latency-acceleration functions of transient onset responses seen in cortex are generated at inner hair cell-AN fiber synapses. Implications for signal processing in the auditory system and for first-spike generation and adaptation in AN are discussed.

Acoustic Stimulation↗