The auditory C-process of spectral profile analysis

Auditory discrimination: the relationship between psychophysical and electrophysiological measures

OBJECTIVES

This study aimed to (1) investigate the relationship between the acoustic change complex (ACC) and perceptual measures of frequency and intensity discrimination, and gap detection; and (2) examine the effects of acoustic change on the amplitudes and latencies of the ACC.

DESIGN

Psychophysical thresholds for frequency and intensity discrimination and gap detection, as well as ACCs elicited by stimuli containing increments in frequency, or intensity or gaps, were recorded from the same group of subjects. The magnitude of the acoustic change was systematically varied for the ACC recording.

STUDY SAMPLE

Twenty-six adults with normal hearing, ranging in age between 19 and 39 years.

RESULTS

Electrophysiological and psychophysical measures for frequency and intensity discrimination were significantly correlated. Electrophysiological thresholds were comparable to psychophysical thresholds for intensity discrimination but were higher than psychophysical thresholds for gap detection and frequency discrimination. Increasing the magnitude of acoustic change increased the ACC amplitude but did not show consistent effects across acoustic dimensions for ACC latency.

CONCLUSIONS

The ACC can be used as an objective index of auditory discrimination in frequency and intensity. The ACC amplitude is a better indicator for auditory processing than the ACC latency.

MMN evidence for asymmetry in detection of IOI shortening and lengthening at behavioral indifference tempo

Most behavioral investigations indicated an indifference interval of 500-700 ms, at which the detection of inter-onset interval (IOI) shortening and lengthening are equally difficult and no perceptual bias would be expected. However, some event-related potential (ERPs) studies showed that even at this behavioral indifference time, the detection of shortening and lengthening might rely on different aspects of information processing. This work was aimed to investigate whether the pre-attentive processing of local tempo perturbations, i.e., IOI shortening and lengthening, are different when the basic tempo is at the behavioral indifference area. Tempo perturbations were introduced by shortening or lengthening the third IOI by 10% of the base IOI of the 5-beat isochronous sequence. ERPs recorded in a passive experiment showed that both tempo perturbations elicited a distinct frontal mismatch negativity (MMN). The low resolution electromagnetic tomography (LORETA) source estimation indicated a left prefrontal predominance activity around the MMN peak, implicating an important role of the frontal lobe in the processing of local tempo perturbations. Statistical analysis revealed that the MMN to IOI shortening had an earlier and greater peak than that to IOI lengthening, implying that IOI shortening might be more easily to be detected than IOI lengthening even at indifference tempo. Our results suggested that local IOI perturbations at behavioral indifference area have an asymmetric effect on the pre-attentive processing of temporal variation detection.

Auditory cortical N100 in pre- and post-synaptic auditory neuropathy to frequency or intensity changes of continuous tones

OBJECTIVES

Auditory cortical N100s were examined in ten auditory neuropathy (AN) subjects as objective measures of impaired hearing.

METHODS

Latencies and amplitudes of N100 in AN to increases of frequency (4-50%) or intensity (4-8 dB) of low (250 Hz) or high (4000 Hz) frequency tones were compared with results from normal-hearing controls. The sites of auditory nerve dysfunction were pre-synaptic (n=3) due to otoferlin mutations causing temperature sensitive deafness, post-synaptic (n=4) affecting other cranial and/or peripheral neuropathies, and undefined (n=3).

RESULTS

AN consistently had N100s only to the largest changes of frequency or intensity whereas controls consistently had N100s to all but the smallest frequency and intensity changes. N100 latency in AN was significantly delayed compared to controls, more so for 250 than for 4000 Hz and more so for changes of intensity compared to frequency. N100 amplitudes to frequency change were significantly reduced in ANs compared to controls, except for pre-synaptic AN in whom amplitudes were greater than controls. N100 latency to frequency change of 250 but not of 4000 Hz was significantly related to speech perception scores.

CONCLUSIONS

As a group, AN subjects' N100 potentials were abnormally delayed and smaller, particularly for low frequency. The extent of these abnormalities differed between pre- and post-synaptic forms of the disorder.

SIGNIFICANCE

Abnormalities of auditory cortical N100 in AN reflect disorders of both temporal processing (low frequency) and neural adaptation (high frequency). Auditory N100 latency to the low frequency provides an objective measure of the degree of impaired speech perception in AN.

Discrimination of speech stimuli based on neuronal response phase patterns depends on acoustics but not comprehension

Speech stimuli give rise to neural activity in the listener that can be observed as waveforms using magnetoencephalography. Although waveforms vary greatly from trial to trial due to activity unrelated to the stimulus, it has been demonstrated that spoken sentences can be discriminated based on theta-band (3-7 Hz) phase patterns in single-trial response waveforms. Furthermore, manipulations of the speech signal envelope and fine structure that reduced intelligibility were found to produce correlated reductions in discrimination performance, suggesting a relationship between theta-band phase patterns and speech comprehension. This study investigates the nature of this relationship, hypothesizing that theta-band phase patterns primarily reflect cortical processing of low-frequency (<40 Hz) modulations present in the acoustic signal and required for intelligibility, rather than processing exclusively related to comprehension (e.g., lexical, syntactic, semantic). Using stimuli that are quite similar to normal spoken sentences in terms of low-frequency modulation characteristics but are unintelligible (i.e., their time-inverted counterparts), we find that discrimination performance based on theta-band phase patterns is equal for both types of stimuli. Consistent with earlier findings, we also observe that whereas theta-band phase patterns differ across stimuli, power patterns do not. We use a simulation model of the single-trial response to spoken sentence stimuli to demonstrate that phase-locked responses to low-frequency modulations of the acoustic signal can account not only for the phase but also for the power results. The simulation offers insight into the interpretation of the empirical results with respect to phase-resetting and power-enhancement models of the evoked response.

Comparator and non-comparator mechanisms of change detection in the context of speech--an ERP study

Automatic change detection reflects a cognitive memory-based comparison mechanism as well as a sensorial non-comparator mechanism based on differential states of refractoriness. The purpose of this study was to examine whether the comparator mechanism of the mismatch negativity component (MMN) is differentially affected by the lexical status of the deviant. Event-related potential (ERP) data was collected during an "oddball" paradigm designed to elicit the MMN from 15 healthy subjects that were involved in a counting task. Topography pattern analysis and source estimation were utilized to examine the deviance (deviants vs. standards), cognitive (deviants vs. control counterparts) and refractoriness (standards vs. control counterparts) effects elicited by standard-deviant pairs ("deh-day"; "day-deh"; "teh-tay") embedded within "oddball" blocks. Our results showed that when the change was salient regardless of lexical status (i.e., the /e:/ to /eI/ transition) the response tapped the comparator based-mechanism of the MMN which was located in the cuneus/posterior cingulate, reflected sensitivity to the novelty of the auditory object, appeared in the P2 latency range and mainly involved topography modulations. In contrast, when the novelty was low (i.e., the /eI/ to /e:/ transition) an acoustic change complex was elicited which involved strength modulations over the P1/N1 range and implicated the middle temporal gyrus. This result pattern also resembled the one displayed by the non-comparator mechanism. These findings suggest spatially and temporally distinct brain activities of comparator mechanisms of change detection in the context of speech.

Frequency changes in a continuous tone: auditory cortical potentials

OBJECTIVE

We examined auditory cortical potentials in normal hearing subjects to spectral changes in continuous low and high frequency pure tones.

METHODS

Cortical potentials were recorded to increments of frequency from continuous 250 or 4000Hz tones. The magnitude of change was random and varied from 0% to 50% above the base frequency.

RESULTS

Potentials consisted of N100, P200 and a slow negative wave (SN). N100 amplitude, latency and dipole magnitude with frequency increments were significantly greater for low compared to high frequencies. Dipole amplitudes were greater in the right than left hemisphere for both base frequencies. The SN amplitude to frequency changes between 4% and 50% was not significantly related to the magnitude of spectral change.

CONCLUSIONS

Modulation of N100 amplitude and latency elicited by spectral change is more pronounced with low compared to high frequencies.

SIGNIFICANCE

These data provide electrophysiological evidence that central processing of spectral changes in the cortex differs for low and high frequencies. Some of these differences may be related to both temporal- and spectral-based coding at the auditory periphery. Central representation of frequency change may be related to the different temporal windows of integration across frequencies.

Auditory temporal edge detection in human auditory cortex

Auditory objects are detected if they differ acoustically from the ongoing background. In simple cases, the appearance or disappearance of an object involves a transition in power, or frequency content, of the ongoing sound. However, it is more realistic that the background and object possess substantial non-stationary statistics, and the task is then to detect a transition in the pattern of ongoing statistics. How does the system detect and process such transitions? We use magnetoencephalography (MEG) to measure early auditory cortical responses to transitions between constant tones, regularly alternating, and randomly alternating tone-pip sequences. Such transitions embody key characteristics of natural auditory temporal edges. Our data demonstrate that the temporal dynamics and response polarity of the neural temporal-edge-detection processes depend in specific ways on the generalized nature of the edge (the context preceding and following the transition) and suggest that distinct neural substrates in core and non-core auditory cortex are recruited depending on the kind of computation (discovery of a violation of regularity, vs. the detection of a new regularity) required to extract the edge from the ongoing fluctuating input entering a listener's ears.

The mismatch negativity (MMN) in basic research of central auditory processing: a review

In the present article, the basic research using the mismatch negativity (MMN) and analogous results obtained by using the magnetoencephalography (MEG) and other brain-imaging technologies is reviewed. This response is elicited by any discriminable change in auditory stimulation but recent studies extended the notion of the MMN even to higher-order cognitive processes such as those involving grammar and semantic meaning. Moreover, MMN data also show the presence of automatic intelligent processes such as stimulus anticipation at the level of auditory cortex. In addition, the MMN enables one to establish the brain processes underlying the initiation of attention switch to, conscious perception of, sound change in an unattended stimulus stream.

Processing asymmetry of transitions between order and disorder in human auditory cortex

Auditory environments vary as a result of the appearance and disappearance of acoustic sources, as well as fluctuations characteristic of the sources themselves. The appearance of an object is often manifest as a transition in the pattern of ongoing fluctuation, rather than an onset or offset of acoustic power. How does the system detect and process such transitions? Based on magnetoencephalography data, we show that the temporal dynamics and response morphology of the neural temporal-edge detection processes depend in precise ways on the nature of the change. We measure auditory cortical responses to transitions between "disorder," modeled as a sequence of random frequency tone pips, and "order," modeled as a constant tone. Such transitions embody key characteristics of natural auditory edges. Early cortical responses (from approximately 50 ms post-transition) reveal that order-disorder transitions, and vice versa, are processed by different neural mechanisms. Their dynamics suggest that the auditory cortex optimally adjusts to stimulus statistics, even when this is not required for overt behavior. Furthermore, this response profile bears a striking similarity to that measured from another order-disorder transition, between interaurally correlated and uncorrelated noise, a radically different stimulus. This parallelism suggests the existence of a general mechanism that operates early in the processing stream on the abstract statistics of the auditory input, and is putatively related to the processes of constructing a new representation or detecting a deviation from a previously acquired model of the auditory scene. Together, the data reveal information about the mechanisms with which the brain samples, represents, and detects changes in the environment.

The ‘F-complex’ and MMN tap different aspects of deviance

OBJECTIVE

To compare the 'F(fusion)-complex' with the Mismatch negativity (MMN), both components associated with automatic detection of changes in the acoustic stimulus flow.

METHODS

Ten right-handed adult native Hebrew speakers discriminated vowel-consonant-vowel (V-C-V) sequences /ada/ (deviant) and /aga/ (standard) in an active auditory 'Oddball' task, and the brain potentials associated with performance of the task were recorded from 21 electrodes. Stimuli were generated by fusing the acoustic elements of the V-C-V sequences as follows: base was always presented in front of the subject, and formant transitions were presented to the front, left or right in a virtual reality room. An illusion of a lateralized echo (duplex sensation) accompanied base fusion with the lateralized formant locations. Source current density estimates were derived for the net response to the fusion of the speech elements (F-complex) and for the MMN, using low-resolution electromagnetic tomography (LORETA). Statistical non-parametric mapping was used to estimate the current density differences between the brain sources of the F-complex and the MMN.

RESULTS

Occipito-parietal regions and prefrontal regions were associated with the F-complex in all formant locations, whereas the vicinity of the supratemporal plane was bilaterally associated with the MMN, but only in case of front-fusion (no duplex effect).

CONCLUSIONS

MMN is sensitive to the novelty of the auditory object in relation to other stimuli in a sequence, whereas the F-complex is sensitive to the acoustic features of the auditory object and reflects a process of matching them with target categories.

SIGNIFICANCE

The F-complex and MMN reflect different aspects of auditory processing in a stimulus-rich and changing environment: content analysis of the stimulus and novelty detection, respectively.

Auditory temporal processes in normal-hearing individuals and in patients with auditory neuropathy

OBJECTIVE

To study objectively auditory temporal processing in a group of normal hearing subjects and in a group of hearing-impaired individuals with auditory neuropathy (AN) using electrophysiological and psychoacoustic methods.

METHODS

Scalp recorded evoked potentials were measured to brief silent intervals (gaps) varying between 2 and 50ms embedded in continuous noise. Latencies and amplitudes of N100 and P200 were measured and analyzed in two conditions: (1) active, when using a button in response to gaps; (2) passive, listening, but not responding.

RESULTS

In normal subjects evoked potentials (N100/P200 components) were recorded in response to gaps as short as 5ms in both active and passive conditions. Gap evoked potentials in AN subjects appeared only with prolonged gap durations (10-50ms). There was a close association between gap detection thresholds measured psychoacoustically and electrophysiologically in both normals and in AN subjects.

CONCLUSIONS

Auditory cortical potentials can provide objective measures of auditory temporal processes.

SIGNIFICANCE

The combination of electrophysiological and psychoacoustic methods converged to provide useful objective measures for studying auditory cortical temporal processing in normals and hearing-impaired individuals. The procedure used may also provide objective measures of temporal processing for evaluating special populations such as children who may not be able to provide subjective responses.

Sensitivity of human auditory evoked potentials to the harmonicity of complex tones: evidence for dissociated cortical processes of spectral and periodicity analysis

A strong subjective tendency exists for simultaneous sound frequencies forming an harmonic series (integer multiples of the fundamental) to "group" together into a unified auditory percept whose pitch is similar to that of the fundamental. The aim of the study was to determine whether cortical auditory evoked potentials (AEPs) to complex tones differ according to whether the component frequencies of the stimuli are harmonically related or not. AEPs were recorded to continuous complex tones comprising four or more sinusoids. The vertex-maximal "change-potentials" (CP1, CN1, CP2), recorded to a stimulus cycle comprising one harmonic and five inharmonic complexes changing every second, showed no sensitivity to harmonicity, although an additional mismatch negativity was possibly present to the harmonic complex. In a second study the CP2 was significantly attenuated when an harmonic complex changed to a new one in the presence of an unchanging sinusoidal background tone, harmonically related to the first complex but not to the second, and thus becoming perceptually distinct. This, however, might be caused by lateral inhibitory effects not related to harmonicity. In a third experiment, when four concurrent sinusoidal tones came to rest on steady frequencies after a 5-s period of 16/s pseudo-random frequency changes, fronto-centrally maximal "mismatch-potentials" (MN1, MP2), were recorded. Both the MN1 and the MP2 were significantly shorter in latency when the steady frequencies formed an harmonic complex. Since the harmonic complex had a short overall periodicity, equal to that of the fundamental, while that of the inharmonic complex was much longer, the effect might be explained if the latencies of the mismatch-potential are related to periodicity. The perceptual grouping of harmonically related frequencies appears not to be a function of spectral domain analysis, reflected in the change-potentials, but of periodicity analysis, reflected in the mismatch-potentials