Dynamic effects in the input/output relationship of auditory nerve
Abstract
Cyclic histograms of the responses of single auditory ganglion cells in the guinea pig were recorded during stimulation with amplitude modulated tones. Modulation frequencies ranged from 10 Hz to 800 Hz. The response histograms, phase-locked to the modulation signals, were analysed for mean action potential rate and for the amplitude of the fundamental component at the modulation frequency. Expected values for the amplitude of the modulation responses were calculated using the variation of mean firing rate with intensity. The observed responses differed from the expected responses in several ways. First, the amplitudes of the modulation responses were larger than expected. Second, the stimulus intensities at which the observed modulation responses peaked was greater by about 7–10 dB than the expected intensity for maximum response. Third, both the magnitude of the response at a given intensity and the intensity at which the response peaked increased with modulation frequency. Fourth, the responses extended to higher stimulus intensities than expected. The observed modulation responses were compared with predictions from the Schroeder and Hall model of adaptation and were found to agree with good quantitative precision. These results suggest that the observed modulation responses are another manifestation of the very rapid ( < 20 ms) adaptation seen in the onset responses of nerve fibres [(1985) Hear. Res. 17, 1–12]. It is concluded that the static input-output responses of auditory nerve are not a good predictor of the dynamic responses to fluctuating stimuli.
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Between sound and perception: Reviewing the search for a neural code
2001, Hearing ResearchThis review investigates the roles of representation, transformation and coding as part of a hierarchical process between sound and perception. This is followed by a survey of how speech sounds and elements thereof are represented in the activity patterns along the auditory pathway. Then the evidence for a place representation of texture features of sound, comprising frequency, periodicity pitch, harmonicity in vowels, and direction and speed of frequency modulation, and for a temporal and synchrony representation of sound contours, comprising onsets, offsets, voice onset time, and low rate amplitude modulation, in auditory cortex is reviewed. Contours mark changes and transitions in sound and auditory cortex appears particularly sensitive to these dynamic aspects of sound. Texture determines which neurons, both cortical and subcortical, are activated by the sound whereas the contours modulate the activity of those neurons. Because contours are temporally represented in the majority of neurons activated by the texture aspects of sound, each of these neurons is part of an ensemble formed by the combination of contour and texture sensitivity. A multiplexed coding of complex sound is proposed whereby the contours set up widespread synchrony across those neurons in all auditory cortical areas that are activated by the texture of sound.
Processing of sinusoidally amplitude modulated signals in the nuclei of the lateral lemniscus of the big brown bat, Eptesicus fuscus
1998, Hearing ResearchChanges in amplitude are a characteristic feature of most natural sounds, including the biosonar signals used by bats for echolocation. Previous evidence suggests that the nuclei of the lateral lemniscus play an important role in processing timing information that is essential for target range determination in echolocation. Neurons that respond to unmodulated tones with a sustained discharge are found in the dorsal nucleus (DNLL), intermediate nucleus (INLL) and multipolar cell division of the ventral nucleus (VNLLm). These neurons provide a graded response over a broad dynamic range of intensities, and would be expected to provide information about the amplitude envelope of a modulated signal. Neurons that respond only at the onset of a tone make up a small proportion of cells in DNLL, INLL and VNLLm, but are the only type found in the columnar division of the ventral nucleus (VNLLc). Onset neurons in VNLLc maintain a constant latency across a wide range of stimulus frequencies and intensities, thus providing a precise marker for when a sound begins. To determine how these different functional classes of cells respond to amplitude changes, we presented sinusoidally amplitude modulated (SAM) signals monaurally to awake, restrained bats and recorded the responses of single neurons extracellularly. There were clear differences in the ability of neurons in the different cell groups to respond to SAM. In the VNLLm, INLL and DNLL, 90% of neurons responded to SAM with a synchronous discharge. Neurons in the VNLLc responded poorly or not at all to SAM signals. This finding was unexpected given the precise onset responses of VNLLc neurons to unmodulated tones and their ability to respond synchronously to sinusoidally frequency modulated (SFM) signals. Among neurons that responded synchronously to SAM, synchronization as a function of modulation rate described either a bandpass or a lowpass function, with the majority of bandpass functions in neurons that responded to unmodulated tones with a sustained discharge. The maximal modulation rates that elicited synchronous responses were similar for the different cell groups, ranging from 320 Hz in VNLLm to 230 Hz in DNLL. The range of best modulation rates was greater for SAM than for SFM; this was also true of the range of maximal modulation rates at which synchronous discharge occurred. There was little correlation between a neuron's best modulation rate or maximal modulation rate for SAM signals and those for SFM signals, suggesting that responsiveness to amplitude and frequency modulations depends on different neural processing mechanisms.
Temporal modulation transfer functions in the European starling (Sturnus vulgaris): II. Responses of auditory-nerve fibres
1995, Hearing ResearchThe temporal resolution of cochlear-nerve fibres in the European starling was determined with sinusoidally amplitude-modulated noise stimuli similar to those previously used in a psychoacoustic study in this species (Klump and Okanoya, 1991). Temporal modulation transfer curves (TMTFs) were constructed for cochlear afferents allowing a direct comparison with the starling's behavioural performance. On average, the neuron's detection of modulation was less sensitive than that obtained in the behavioural experiments, although the most sensitive cells approached the values determined psychophysically. The shapes of the neural TMTFs generally resembled low-pass or band-pass filter functions, and the shapes of the averaged neural functions were very similar to those obtained in the behavioural study for two different types of stimuli (gated and continuous carrier). Minimum integration times calculated from the upper cut-off frequency of the neural TMTFs had a median of 0.97 ms with a range of 0.25 to 15.9 ms. The relations between the minimum integration times and the tuning characteristics of the cells (tuning curve bandwidth, Q10 dB-value, high- and low-frequency slopes of the tuning curves) are discussed. Finally, we compare the TMTF data recorded in the starling auditory nerve with data from neurophysiological and behavioural observations on temporal resolution using other experimental paradigms in this and other vertebrate species.
Encoding of amplitude-modulated tones by neurons of the inferior colliculus of the kitten
1993, Brain ResearchResponses of single neurons of the central nucleus of the inferior colliculus (ICC) of kittens 4–43 days of age were studied using sinusoidally amplitude-modulated (AM) tones delivered monaurally or binaurally via sealed and calibrated earphones. The carrier frequency of the AM signal was set to the CF of the neuron. CFs ranged from 2–26 kHz. During the about first 2 weeks of postnatal life, ICC neurons responded to sound with periodic bursts of activity. In response to AM tones, discharges of ICC neurons at all ages studied were phase-locked to the envelope of the modulation waveform over a wide range of stimulus level and modulation depth. A linear relationship, independent of SPL, was found between the average phase of discharge on the modulation cycle and modulation frequency. The slope of the line represents a time delay, which was highly correlated with the first-spike latency to tone onset, and hence with the age of the animal. The mean effective phase of the discharge remained relatively constant with age. There was little systematic change in average phase of discharge with changing stimulus level or modulation depth, although the number of spikes evoked and the temporal pattern of the spikes within a modulation cycle could vary. The sensitivity function relating spike synchrony or spike count to modulation was typically band-pass in nature. The most effective modulation frequency (MEMF) was, on average, 15 Hz, far below that reported for adult cat ICC cells. When AM tones were delivered binaurally, the discharge was a periodic function of the interaural phase difference of the stimulus envelopes. The results indicate that prior to the time the cochlea is able to respond to most environmental sounds, monaural and binaural circuits involving the ICC faithfully transmit information pertaining to amplitude-modulated signals in the rate and timing of their discharges. During the next several weeks, when neural thresholds fall to adult levels, ICC circuits are activated by amplitude modulated sounds at levels encountered in the normal acoustic environment even though they are restricted to modulation frequencies below those encoded by the adult.
Periodicity coding in the auditory system
1992, Hearing ResearchPeriodic envelope fluctuations are a common feature of acoustic communication signals, and as a result of physical constraints, many natural, nonliving sound sources also produce periodic waveforms. In human speech and music, for example, periodic sounds are abundant and reach a high degree of complexity. Under noisy conditions these amplitude fluctuations may be reliable indicators of a common sound source responsible for the activation of different frequency channels of the basilar membrane. To make use of this information, a central periodicity analysis is necessary in addition to the peripheral frequency analysis.
The present review summarizes our present knowledge about representation and processing of periodic signals, from the cochlea to the cortex in mammals, and in homologous or analogous anatomical structures as far as these exist and have been investigated in other animals. The first sections describe important physical and perceptual attributes of periodic signals, and the last sections address some theoretical issues.
Effects of level on nonspectral frequency difference limens for electrical and acoustic stimuli
1990, Hearing ResearchThe purpose of this experiment was to study the effects of stimulus level on discrimination of frequency as represented in the temporal waveforms of acoustic and electrical signals. The subjects were four nonhuman primates in which one ear had been deafened and implanted with an electrode array and the other ear was untreated. Frequency difference limens for 100 Hz electrical sinusoidal stimulation via a cochlear implant in the deafened ear were compared to those for 100 Hz sinusoidally amplitude-modulated white noise (SAM noise) acoustic stimuli to the normal-hearing contralateral ear. To correct for loudness cues, levels of the test stimuli were varied relative to the reference-stimulus level. The test-stimulus levels at which the percent responses were minimum were determined. These levels were used to measure the frequency difference limens. Frequency difference limens for the electrical stimuli decreased as a function of reference-stimulus level through most of the dynamic range, while those for the acoustic stimuli reached a minimum at 20 dB to 40 dB above threshold. For the electrical stimuli the slopes and relative positions of the frequency difference limen vs. level functions varied from subject to subject and with changes in electrode configuration within a subject. These differences were related to threshold level and dynamic range. At higher levels of stimulation, frequency difference limens for acoustic and electrical stimuli fell in the same range. The slopes and relative positions of the frequency difference limen vs. level functions for electrical stimuli did not parallel those of level difference limen vs. level functions collected simultaneously from the same ears. The data suggest that nonspectral frequency discrimination may depend on the number of nerve fibers stimulated. With prostheses in cochleas with less than a full complement of auditory nerve fibers, the data suggest that stimulation level is an important variable influencing discriminability.