Setting complex tasks to single units in the avian auditory forebrain. II. Do we really need natural stimuli to describe neuronal response characteristics?

The response characteristic of auditory forebrain neurons in the European starling was established both with artificial stimuli (AS) and a conspecific territorial song as a natural stimulus (NS1). Applying experimenter-centred statistical methods for response detection and for scaling response strength, and spike-triggered analyses for the delimitation of the key sound parameters (spectrotemporal receptive field STRF, Aertsen et al. 1980) the study aimed at disclosing differences in the processing of the two stimulus classes, AS and NS. With the STRF as reference, we find congruence (1) in the best frequency with those determined under sweep and bandpass noise stimulation, (2) in response latency, and (3) in response-intensity dependence, further similarity in the overall frequency characteristic. Partitioning the song into 42 acoustically defined segments allowed to further delimit the response criteria under natural stimulation. They are easily understood from the AS response characteristics: (1) In the neuronal sample as a whole, long segments are more effective than short and, among the short, loud segments are more effective than faint; (2) Units showing their best excitatory response to AS in a certain frequency band are most probably excited by segments with a high proportion of their power concentrated upon or near this band; (3) Units with a slow (build-up) AS response react to a lower number of song segments than those dynamically following AS transients. Our data give no hint towards adaptive, feature detection properties of single neurons in field L. Instead, these neurons appear to base their response solely on the short-time spectrotemporal structure of the stimulus, irrespective of its natural or artificial origin.     

The neurophysiology of auditory perception: from single units to evoked potentials

Evoked electric potential and magnetic field studies have the immense benefit that they can be conducted in awake, behaving humans and can be directly correlated with aspects of perception. As such, they are powerful objective indicators of perceptual properties. However, given a set of evoked potential and/or evoked field waveforms and their source locations, obtained for an exhaustive set of stimuli and stimulus contrasts, is it possible to determine blindly, i.e. predict, what the stimuli or stimulus contrasts were? If this can be done with some success, then a useful amount of information resides in scalp-recorded activity for, e.g., the study of auditory speech processing. In this review, we compare neural representations based on single-unit and evoked response activity for vowels and consonant-vowel phonemes with distinctions in formant glides and voice onset time. We conclude that temporal aspects of evoked responses can track some of the dominant response features present in single-unit activity. However, N1 morphology does not reliably predict phonetic identification of stimuli varying in voice onset time, and the reported appearance of a double-peak onset response in aggregate recordings from the auditory cortex does not indicate a cortical correlate of the perception of voicelessness. This suggests that temporal aspects of single-unit population activity are likely not inclusive enough for representation of categorical perception boundaries. In contrast to population activity based on single-unit recording, the ability to accurately localize the sources of scalp-evoked activity is one of the bottlenecks in obtaining an accessible neurophysiological substrate of perception. Attaining this is one of the requisites to arrive at the prospect of blind determination of stimuli on the basis of evoked responses. At the current sophistication level of recording and analysis, evoked responses remain in the realm of extremely sensitive objective indicators of stimulus change or stimulus differences. As such, they are signs of perceptual activity, but not comprehensive representations thereof.     

Trials with the auditory response cradle. 1--Neonatal responses to auditory stimuli

The Auditory Response Cradle enables physiological response measures to be recorded from the neonate. Auditory responses are detected in the form of head rotation, startle, body activity and respiration changes. This paper reports the results of trials with 203 neonates using 250 and 1000 Hz pure tone and broad band noise stimuli. Response criteria are determined and spontaneous control 'responses' compared with those resulting from stimulus presentation. The response rate is correlated with the measured intra-meatal sound pressure level. Clear response thresholds are determined for all three stimuli. The sharpness of these motor thresholds is discussed and found to have significant implications for cost-effective neonatal auditory screening.     

Predicting the temporal responses of non-phase-locking bullfrog auditory units to complex acoustic waveforms

Axons from the basilar papilla of the American bullfrog (Rana catesbeiana) do not phase lock to stimuli within an octave of their best frequencies. Nevertheless, they show consistent temporal patterns of instantaneous spike rate (as reflected in peristimulus time histograms) in response to repeated stimuli in that frequency range. We show that the second-order Wiener kernels for these axons, derived from the cross-correlation of continuous (non-repeating), broad-band noise stimulus with the spike train produced in response to that stimulus, can predict with considerable precision the temporal pattern of instantaneous spike rate in response to a novel, complex acoustic waveform (a repeated, 100-ms segment of noise, band-limited to cover the single octaves above and below best frequency). Furthermore, we show that most of this predictive power is retained when the second-order Wiener kernel is reduced to the highest-ranking pair of singular vectors derived from singular-value decomposition, that the retained pair of vectors corresponds to a single auditory filter followed by an envelope-detection process, and that the auditory filter itself predicts the characteristic frequency (CF) of the axon and the shape of the frequency-threshold tuning curve in the vicinity of CF.     

Processing of pure-tone and FM stimuli in the auditory cortex of the FM bat, Myotis lucifugus.

FM bats perceive their surroundings during echolocation by analyzing frequency-modulated (FM) acoustic signals. Results from this study indicate a cortical organization in Myotis lucifugus which is largely made up of neurons sensitive to FM sounds (FM-sensitive neurons). Three types of neurons were distinguished by their responses to pure-tone and FM stimuli: (1) Type I FM-sensitive units (83%), Type II FM-sensitive units (13%) and pure-tone sensitive units (4%). Type I FM-sensitive units responded to pure tones, but exhibited greater response magnitudes to FM stimuli when the best FM swept through the BF. An orderly frequency representation was found when the frequencies of pure tones essential for response (EPTs) in Type I units were mapped along the cortical surface. The EPTs for Type I neurons were usually found within the last millisecond of a downward FM sweep. As outlined by two neuronal network models, both the responses of Type I and II units could likely result from the convergence of excitatory and inhibitory lower level neurons with slightly differing BFs. Type II units were selective for an FM sweep and showed negligible to no response to pure-tone stimuli. Pure-tone sensitive units exhibited weak or no responses to FM stimuli. These neurons were clustered in a small area located rostrodorsal to the tonotopic zone and had significantly lower best frequencies than adjacent EPT frequencies of Type I FM-sensitive neurons.     

Stimulus induced and spontaneous rhythmic firing of single units in cat primary auditory cortex.

Recordings were made under ketamine anesthesia from 385 neurons in primary auditory cortex in adult cat and from 265 neurons in 10-55 day old kittens. The temporal Modulation Transfer Function for the response to repetitive click stimuli peaked at 8 Hz. After a click a suppression period of 130- 155 ms in duration, depending on click-rate, was observed. This suppression period limited the response to high click rates and thereby determined the 'resonance' in the click response. The suppression duration in kittens decreased in exponential fashion toward the adult value with a time constant of about 1 month. After the one second duration click-trains an oscillatory rebound with a mean period of 113 ms was observed in about 60% of the recordings in the adult cat. Spontaneous activity showed in about 30% of the neurons an oscillatory autocorrelogram with an average period of 126 ms in the adult cats and 170 ms in kittens.     

Sensitivity of neurons in cat primary auditory cortex to tones and frequency-modulated stimuli. I: Effects of variation of stimulus parameters.

In the primary auditory cortex (AI) of barbiturate-anesthetized cats multi-unit responses to tones and to frequency-modulated (FM) tonal stimuli were analyzed. Characteristic frequency (CF), sharpness of tuning, minimum threshold, and dynamic range of spike count--intensity functions were determined. Minimum threshold and dynamic range were positively correlated. The response functions to unidirectional FM sweeps of varying linear rate of change of frequency (RCF) that traversed the excitatory frequency response areas (FRAs) displayed a variety of shapes. Preferences for fast RCFs (> 1000 kHz/s) were most common. Best RCF was not correlated with measures of sharpness of tuning. Directional preference and sensitivity were quantified by a DS index which varied with RCF. About two-thirds of the multi-unit responses showed a preference for downward sweeps. Directional sensitivity was independent of CF and independent of best RCF. Measurements of latencies of phasic responses to unidirectional FM sweeps of different RCF demonstrated that the discharges of a given multi-unit over its effective RCF range were initiated at the same instantaneous frequency (effective Fi), independent of RCF. Effective Fis fell within the excitatory FRA of a given multi-unit. The relationships of effective Fis to CF show that responses were evoked only when the frequency of the signal was modulated towards CF and not when modulated away from it, and that responses were initiated before the modulation reached CF. Changes in the range and depth of modulation had only minor, if any, effects on RCF response characteristics, FM directional sensitivity, and effective Fis, as long as the beginning and ending frequencies of FM sweeps fell outside a multi-unit's FRA. Stimulus intensity also had only moderate effects on RCF response characteristics and DS. However, effective Fis were influenced in systematic fashions; with increases in intensity, effective Fis to upward and downward sweeps decreased and increased, respectively. Thus, for higher intensities FM responses were initiated at instantaneous frequencies occurring earlier in the signal. The results are compared with previous data on tone and FM sensitivity of auditory neurons in cortical and subcortical structures, and mechanisms of FM rate and directional sensitivity are discussed. The topographic representations of these neuronal properties in AI are reported in the companion report.     

Spectro-temporal characteristics of single units in the auditory midbrain of the lightly anaesthetised grass frog (Rana temporaria L.) investigated with tonal stimuli

Responses were obtained from 112 auditory neurons in the midbrain of the grass frog in response to sequences of tones. Their spectre-temporal sensitivities (STS) were determined by a second-order cross-correlation technique. For the majority of units the shape of their action potentials, the degree of timelock to the stimulus and the recording sites were obtained. Two stages of information processing could be distinguished. One was characterized by short latencies (<30 ms), strong timelock to the stimulus and many of these units had axon-like action potential waveforms. They were localised in the ventral part of the principal nucleus from the torus semicircularis and in the transition region between laminar and principal nucleus. The other stage comprised units, found all over the torus, with longer latencies, and a weaker timelock to the stimulus. Several units which were predominantly found in the central part of the torus, especially the magnocellular nucleus, showed a broad or multiple STS. Within the principal nucleus a weak tonotopy was found, the dorsoposterior part being sensitive to lower frequencies, the ventroanterior part to the higher frequencies. Binaural-interaction properties are discussed with respect to the eardrum coupling through the mouth cavity. An organisational plan for the torus semicircularis is proposed.     

Coding of free field intensity in the auditory midbrain of the leopard frog. I. Results for tonal stimuli

Single unit recordings in the auditory midbrain of the leopard frog were obtained in response to free field, random frequency, tone pip stimulation. Five sequences of 81 logarithmically distributed frequencies from 100 Hz to 3200 Hz inclusive at stimulus levels between 94 dB SPL and the unit's threshold were presented. Sigmoidal response-intensity functions were fit through the data points for monotonic and nearly monotonic units (53 in total) at all frequencies for which the units were responsive. From the fitcurves a generalized slope and inflection point were obtained and converted into a dynamic range and a threshold value. Single unit dynamic ranges at CF frequencies were between 5 and 80 dB, and thresholds were between 20 dB and 90 dB. For the population of units the dynamic ranges were positively correlated with the threshold values. A population response was computed by adding the response at each intensity-frequency combination, and again curve fitting with the sigmoidal function. The population dynamic range was about 70 dB for the basilar papilla range (around 1200 Hz) and somewhat less, about 45 dB, for the amphibian papilla neurons. The amount of synchronization of the firings of the individual units with the stimulus was calculated on basis of the shifted auto-coincidence function. The stimulus synchronization appeared to be largely independent of stimulus intensity. Inter-neuron synchronization was computed on basis of the cross-coincidence function of two neuronal spike trains. Also this synchronization was stimulus independent. It was concluded that synchronization or other forms of multiplicative action cannot on its own code for stimulus intensity. From the application of signal detection theory it is concluded that a rate coding on basis of a modest population of units has an acceptable dynamic range, roughly covering 30-100 dB SPL, produces a more than adequate sensitivity for intensity differences and may be used as a code for intensity.     

An approach to the study of attentional components in auditory tasks

Some investigators have proposed a number of strategies or components of attention based on the analysis of the demands or requirements of certain laboratory tasks. The present investigation approached the analysis of auditory and visual attention from a different perspective. Volunteers (14 F, 27 M) from the health professions were given two widely-used auditory attentional tasks (remembering taped word lists, and shadowing, in monaural, binaural, and dichotic modes) and three visual tests (Stroop color-word tests, embedded figures, and anagrams) that appear to tap important aspects of attentional functioning; 18 indices of aspects of performance were taken from or derived from scores on these tests. Strong, significant r's emerged for a number of paired variables, suggesting that performance in these tasks share common elements. A principal factor analysis performed on the correlation matrix yielded a remarkably clean solution that uncovered four common sources for the variation observed in performance: Factor I identifies an aspect of attention tapped whenever distractions must be overcome; II is involved in continuously tracking or monitoring semantic aspects of connected material; III is an "executive" function that maintains or controls other aspects of attention; IV can be labeled breadth of attention, dealing with opposite aspects of the same underlying mechanism. These appear to be attentional components that underlie performance even in tasks that impose dissimilar demands or that have been interpreted as engaging different aspects of the attentional system. The factor analysis also provided some tentative answers for questions related to the comparability of different auditory tasks and of different aspects of performance for the same task.     

Stimulus-dependent central processing of auditory stimuli: a PET study.

Positron emission tomography (PET) was used to investigate the neural systems involved in the central processing of different auditory stimuli. Noise, pure tone and pure-tone pulses, music and speech were presented monaurally. O-15-water PET scans were obtained in relation to these stimulations presented to five normal hearing and healthy subjects. All stimuli were related to a basic scan in silence. Processing of simple auditory stimuli, such as pure tones and noise, predominantly activate the left transverse temporal gyrus (Brodmann area [BA] 41), whereas sounds with discontinued acoustic patterns, such as pure-tone pulse trains, activated parts of the auditory association area in the superior temporal gyri (BA 42) in both hemispheres. Moreover, sounds with complex spectral, intensity, and temporal structures (words, speech, music) activated spatially even more extensive associative auditory areas in both hemispheres (BA 21, 22). PET has revealed a remarkable potential to investigate early central auditory processing, and has provided evidence of the coexistence of functionally linked, but individually active parallel and serial auditory networks.     

Peripheral auditory processing,the precedence effect and responses of single units in the inferior colliculus

The purpose of this paper is to illustrate how interactions occurring within the auditory periphery are relevant to the interpretation of neurophysiological data obtained in recent studies seeking to find physiological correlates of 'the precedence effect'. Similar information was presented orally at the recent International Symposium on the Central Auditory System held in Salamanca, Spain. The physiological data of interest are responses from single neural units in the inferior colliculus recorded following stimulation by successive pairs of binaural clicks. We show how peripheral, monaural, within-filter interactions of such successive clicks can produce internal values of interaural temporal differences (ITDs) and interaural intensitive differences (IIDs) that can differ greatly from those present in the external stimulus. These interactions can produce unintended internal ITDs and IIDs that are well 'outside the tuning range' of the single unit being studied. When this occurs, the responses of the single units to the pairs of clicks would be expected to diminish. We also discuss how hair cell-related adaptation and compression can also lead to diminished responses. It is suggested that effects resulting from peripheral interactions should be taken into account or evaluated quantitatively before other factors, including central inhibition, are invoked.     

The role of auditory stimuli in crying inhibition in the neonate.

An experiment on the latency and duration of the cessation of spontaneous crying supported evidence that a low frequency intense auditory stimulus inhibits crying in neonates. There was a slight increase in the length of the first pause in crying during exposure to a 200-c/s tone, but no difference in the latency of this first pause. Since crying intensity reaches 80 db SPL at the infant ear, it is hypothesized that auditory stimuli juxtaposed during crying is in fact masked. Hence to be effective, an auditory stimulus must be presented at the beginning of a natural pause in crying. An experiment on rhythm in cry patterns did not support the hypothesis that crying may be a naturally rhythmic pattern of behavior analogous to sucking and susceptible to psychophysical investigation. There was a large variance within and between Ss for the mean latency and duration of cry bursts and pauses. Exper. III asked mothers with infants of 10 days, 7 weeks, and 13 weeks of age to keep a crying activity schedule in their own homes. This study revealed a discontinuity between neonates and older infants in the conditions and patterns of crying. Interview data did indicate that reliable positive orienting responses did occur to natural auditory stimuli in a naturalistic setting.     

Response to broadband repetitive stimuli in auditory cortex of the unanesthetized rat

This study examines the ability of multi-unit clusters (MUCs) in layer IV/V of primary auditory cortex of the awake rat to respond to a series of broadband click trains. The data from 113 multi-unit clusters were analyzed for synchronous and nonsynchronized responses using several methods. Synchronous responses were measured using window analysis, circular statistics and spectral analysis. Nonsynchronous responses were measured during different time intervals during the click train (first 50 ms, 50-450 ms, and the entire click train). The results demonstrate that multi-unit clusters are capable of synchronizing to clicks at rates up to 166 Hz. The mean synchronization boundary (limiting rate) for the group was found to be 72 Hz. Mean peak response rate, mean response duration, and mean time-to-peak response decreased as the stimulus presentation rate (SPR) increased, resulting in a temporal sharpening of the population response. For fast SPRs (>50 Hz), 50% of MUCs exhibited nonsynchronous responses in which the firing rate increased with SPR, although this activity was most prevalent during the first 50 ms of the response. Sustained increases in firing rate with SPR were seen in 8% of the MUCs, while another 38% of MUCs exhibited sustained decreases during the click train.     

Activity of single medial geniculate units in response to single and double clicks.

The discharge pattern to clicks and the time course and pattern of the click-induced inhibition in the medial geniculate body have been studied by single unit recording technique. The medial geniculate neurones showed a tendency to fire at preferred latencies after a click. A conditioning click caused a reduced number and increased latency of the discharges to a subsequent test click. Provided conditioning click of a certain strength was employed, the inhibition was often cyclic, each period lasting for about 100-150 ms. Clicks could also trigger barbiturate spindles with synchrony between the activity of the medial geniculate body and the primary auditory cortex. Interaction experiments suggest that the triggered and spontaneous spindle activity operate with the same neurones.     

Time course of rate responses to two-tone stimuli in auditory nerve fibres in the guinea pig.

Peri-stimulus time histograms (PSTHs) were constructed from responses of auditory nerve fibres in anaesthetized guinea pigs. Acoustic stimuli consisted of pure tones, presented either as tone bursts, or in two-tone combinations in which a gated test tone was superimposed on a continuous excitatory tone at characteristic frequency (CF). The majority of the sample of fibres displayed two-tone rate suppression (2TRS). The suppression was either a monotonic or a non-monotonic function of the level of the superimposed test tone. Monotonic suppression of CF-driven rate occurred only for test tones at frequencies higher than CF, presented at levels up to the maximum available (approx. 100 dB SPL). For test tones below CF, 2TRS initially increased, then reverted towards excitation for higher levels of the test tone. Three levels were identified in non-monotonic, two-tone rate functions; (1) the threshold for rate suppression, (2) the maximally suppressing level and (3) the level (referred to as the balance point) at which average firing rate was restored to the background, CF-driven rate. PSTHs for two-tone responses obtained for test tone levels between the maximally-suppressing level and the balance point typically showed brief decrements (notches) in spike rate, at the onset and following the offset of the test tone. The latency, depth and duration of notches, however, depended on the level of the test tone, in a different manner for onset and offset. In some cases, without overt rate excitation above the probe-driven rate, the offset notch became more pronounced and of extended duration with increased level of the test tone, suggestive of adaptation to the test tone. Two-tone responses, in which rate exceeded the background, CF-driven rate, in general were preceded by a reduced onset notch and were followed by a longer-lasting depression of the background spike rate, typical of post-excitatory depression. Relative to responses obtained to the test tones presented alone, excitatory two-tone responses were of lower rate and were delayed by the onset notch. Onset notches sometimes preceded rate excitation in responses to single tones. Some features of the time course of rate suppression and excitation displayed in PSTHs for responses to one and two-tone stimuli seem inconsistent with current models of 2TRS.     

Response patterns and their relationship to frequency analysis in auditory forebrain centers of a songbird.

In the field L complex, the auditory part of the caudal telencephalon, multi-unit recordings were performed in seven awake, adult male starlings (Sturnus vulgaris, L). Pure tones in a frequency range between 0.5 and 6.0 kHz were used as stimuli. The field L complex of starlings consists of at least 11 functionally separated, tonotopically organized subcenters. The auditory processing of frequency information was investigated in eight of these areas. Two kinds of response patterns could be distinguished. The centers NA-L, NA2a, NA3 and NA4 showed phasic and sustained excitation, the other areas responded with phasic excitation only. All these auditory areas show strong tonotopic gradients, each of them representing the complete hearing range. The sharpest picture of the stimulus frequency is represented in the functional area NA-L. In relation to its total size, NA-L shows the smallest active area if stimulated by pure tones. In addition, in NA-L, only the excited neurons are surrounded by inhibited neurons during the response to a pure tone. This leads to an additional sharpening of frequency representation. In comparison with the other auditory areas, NA-L shows the greatest spatial extension of the tonotopic gradient. This, in combination with the smallest active size in NA-L, leads to the conclusion that in the primary projection field NA-L, the most neural space is available for the processing of a given frequency range.     

Test-retest reliability of auditory brainstem responses to chirp stimuli in newborns

Abstract

Objective: The purpose of this study was to examine the test-retest reliability of auditory brainstem responses (ABRs) to air- and bone-conducted chirp stimuli in newborns as a function of intensity. Design: A repeated measures quasi-experimental design was employed. Study sample: Thirty healthy newborns participated. ABRs were evoked using 60, 45, and 30 dB nHL air-conducted CE-Chirps and 45, 30, and 15 dB nHL bone-conducted CE-Chirps at a rate of 57.7/s. Measures were repeated by a second tester. Results: Statistically significant correlations (p <.0001) and predictive linear relations (p <.0001) were found between testers for wave V latencies and amplitudes to air- and bone-conducted CE-Chirps. There were also no statistically significant differences between testers with wave V latencies and amplitudes to air- and bone-conducted CE-Chirps (p >.05). As expected, significant differences in wave V latencies and amplitudes were seen as a function of stimulus intensity for air- and bone-conducted CE-Chirps (p <.0001). Conclusions: These results suggest that ABRs to air- and bone-conducted CE-Chirps can be reliably repeated in newborns with different testers. The CE-Chirp may be valuable for both screening and diagnostic audiologic assessments of newborns.     

钛人工听骨在I期乳突切开+鼓室成形术中应用的短期疗效观察

探讨钛人工听骨用于I期乳突切开和鼓室成形术中的短期疗效。     

Induced suppression in spike responses to tone-on-noise stimuli in the auditory nerve of the pigeon

Spike potentials were recorded from single, afferent fibres in the pigeon auditory nerve. Pure-tone stimuli were presented in quiet and in combination with wide band noise. Presented alone, tones produced tuned response areas; noise generally drove spike rate to well above the spontaneous rate measured in quiet. When presented in combination with noise, tones up to 75 dB SPL at frequencies far from the fibre's response area had no effect on the noise-driven spike rate. As the tone frequency was shifted towards the response area, from above or below CF, suppression of the noise-driven spike rate became stronger until the tone reached the edge of the response area. Suppression of the noise-driven rate was directly proportional to the level of the tone. Within the area of response to the tone, tone-driven spike rates generally were unchanged or variably decreased (occasionally slightly increased) by tone-on-noise stimulation, depending on the relation of the tone frequency to CF and the level of the tone relative to that of the noise. Tuning properties were unaffected. It is suggested that in the pigeon, the suppression of driven spike rate during presentation of combination stimuli, which is common to all fibres, depends on the same mechanism as the suppression of spontaneous firing by tones that is observed in a proportion of fibres (Temchin, A.N. (1988), J. Comp. Physiol. A 163, 99-115; Hill et al., (1989) Hear. Res. 39, 37-48).