Different analysis of frequency and amplitude modulations of a continuous tone in the human auditory cortex: a neuromagnetic study

Task-relevance and change detection in action-effect binding

Features of actions are bound to coincidentally occurring stimuli so that re-encountering a stimulus retrieves a previous action episode. One hallmark of the purported mechanism in binding/retrieval tasks is a reliable reaction time advantage for repeating a previous response if tone stimuli repeat rather than alternate across trials. Other measures than reaction times yielded surprisingly mixed results, however. This is particularly true for continuous response features like force or response duration. We therefore conducted two experiments to resolve this disconnect between different measures. Experiment 1 tested for a potentially inflated effect in reaction time data, whereas Experiment 2 took the converse approach of studying conditions that would elicit similarly strong effects on alternative measures. Our results show that confounds in terms of auditory change detection do not inflate reaction time differences, reinforcing an interpretation of these effects as reflecting binding and retrieval. Moreover, strong effects on alternative measures appeared if these features were rendered task-relevant and came with sufficient variability. These observations provide critical evidence for binding and retrieval accounts, especially by showing that these accounts extend from binary decisions to continuous features of an actual motor response.

Representation of frequency-modulated sounds in the human brain

Frequency-modulation is a ubiquitous sound feature present in communicative sounds of various animal species and humans. Functional imaging of the human auditory system has seen remarkable advances in the last two decades and studies pertaining to frequency-modulation have centered around two major questions: a) are there dedicated feature-detectors encoding frequency-modulation in the brain and b) is there concurrent representation with amplitude-modulation, another temporal sound feature? In this review, we first describe how these two questions are motivated by psychophysical studies and neurophysiology in animal models. We then review how human non-invasive neuroimaging studies have furthered our understanding of the representation of frequency-modulated sounds in the brain. Finally, we conclude with some suggestions on how human neuroimaging could be used in future studies to address currently still open questions on this fundamental sound feature. This article is part of a Special Issue entitled Human Auditory Neuroimaging.

The role of attention in auditory information processing as revealed by event-related potentials and other brain measures of cognitive function

This article examines the role of attention and automaticity in auditory processing as revealed by event-related potential (ERP) research. An ERP component called the mismatch negativity, generated by the brain's automatic response to changes in repetitive auditory input, reveals that physical features of auditory stimuli are fully processed whether or not they are attended. It also suggests that there exist precise neuronal representations of the physical features of recent auditory stimuli, perhaps the traces underlying acoustic sensory (“echoic”) memory. A mechanism of passive attention switching in response to changes in repetitive input is also implicated.

Conscious perception of discrete acoustic stimuli might be mediated by some of the mechanisms underlying another ERP component (NI), one sensitive to stimulus onset and offset. Frequent passive attentional shifts might accountforthe effect cognitive psychologists describe as “the breakthrough of the unattended” (Broadbent 1982), that is, that even unattended stimuli may be semantically processed, without assuming automatic semantic processing or late selection in selective attention.

The processing negativity supports the early-selection theory and may arise from a mechanism for selectively attending to stimuli defined by certain features. This stimulus selection occurs in the form ofa matching process in which each input is compared with the “attentional trace,” a voluntarily maintained representation of the task-relevant features of the stimulus to be attended. The attentional mechanism described might underlie the stimulus-set mode of attention proposed by Broadbent. Finally, a model of automatic and attentional processing in audition is proposed that is based mainly on the aforementioned ERP components and some other physiological measures.

The M100 component of evoked magnetic fields differs by scaling factors: implications for signal averaging

MEG and EEG studies of event-related responses often involve comparisons of grand averages, requiring homogeneity of the variances. Here, we examine the possibility, implied by the nature of neural sources and the measuring principles involved, that the M100 component of auditory-evoked magnetic fields of different subjects, hemispheres, to different stimuli, and at different sensors differs by scaling factors. Such a multiplicative model predicts a linear increase in the standard deviation with the mean, and thus would have important implications for averaging and comparing such data. Our analyses, at the sensor and the source level, clearly show that the multiplicative model applies. We therefore propose geometric, rather than arithmetic, averaging of the M100 component across subjects and suggest a novel and superior normalization procedure. Our results question the justification of the common practice of subtracting arithmetic grand averages.

MEG does not reveal impaired sensory gating in first-episode schizophrenia

OBJECTIVE

The inability to adequately suppress the second of two identical stimuli is called sensory gating deficit and can be studied by recording evoked potentials to auditory stimuli, e.g. the P50 and the N100. It has been considered the physiological correlate of schizophrenia patients' perception of being flooded by sensory impressions. According to the notion that the gating deficit constitutes a genetic trait, we expected to demonstrate the phenomenon in first-episode schizophrenia patients by using Magnetencephalography (MEG).

METHODS

Eighteen inpatients in remission of their first psychotic episode and 24 healthy, age- and sex-matched control subjects participated in the study. Diagnoses, psychopathology, and handedness were assessed with established instruments. Stimulation was performed with the double click paradigm (ISI 500 ms, ITI 9-10 s). MEG recordings of 15 patients and 18 controls entered further analyses with the software BESA for spatio-temporal source analyses and statistical analyses with MATLAB.

RESULTS

Neither P50 nor N100 responses differed statistically between the groups, which means that gating was not impaired in this sample of first-episode schizophrenia patients.

CONCLUSIONS

These results are not in line with the majority of studies on sensory gating in schizophrenia, however, studies on first-episode patients are scarce. The most likely reasons for not observing a gating deficit in our study are patients' first-episode status and atypical antipsychotic medication.

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.

Spatiotemporal reconstruction of the auditory steady-state response to frequency modulation using magnetoencephalography

The aim of this study was to investigate the mechanisms involved in the perception of perceptually salient frequency modulation (FM) using auditory steady-state responses (ASSRs) measured with magnetoencephalography (MEG). Previous MEG studies using frequency-modulated amplitude modulation as stimuli (Luo et al., 2006, 2007) suggested that a phase modulation encoding mechanism exists for low (<5 Hz) FM modulation frequencies but additional amplitude modulation encoding is required for faster FM modulation frequencies. In this study single-cycle sinusoidal FM stimuli were used to generate the ASSR. The stimulus was either an unmodulated 1-kHz sinusoid or a 1-kHz sinusoid that was frequency-modulated with a repetition rate of 4, 8, or 12 Hz. The fast Fourier transform (FFT) of each MEG channel was calculated to obtain the phase and magnitude of the ASSR in sensor-space and multivariate Hotelling's T(2) statistics were used to determine the statistical significance of ASSRs. MEG beamformer analyses were used to localise the ASSR sources. Virtual electrode analyses were used to reconstruct the time series at each source. FFTs of the virtual electrode time series were calculated to obtain the amplitude and phase characteristics of each source identified in the beamforming analyses. Multivariate Hotelling's T(2) statistics were used to determine the statistical significance of these reconstructed ASSRs. The results suggest that the ability of auditory cortex to phase-lock to FM is dependent on the FM pulse rate and that the ASSR to FM is lateralised to the right hemisphere.

Effects of the task of categorizing FM direction on auditory evoked magnetic fields in the human auditory cortex

We examined effects of the task of categorizing linear frequency-modulated (FM) sweeps into rising and falling on auditory evoked magnetic fields (AEFs) from the human auditory cortex, recorded by means of whole-head magnetoencephalography. AEFs in this task condition were compared with those in a passive condition where subjects had been asked to just passively listen to the same stimulus material. We found that the M100-peak latency was significantly shorter for the task condition than for the passive condition in the left but not in the right hemisphere. Furthermore, the M100-peak latency was significantly shorter in the right than in the left hemisphere for the passive and the task conditions. In contrast, the M100-peak amplitude did not differ significantly between conditions, nor between hemispheres. We also analyzed the activation strength derived from the integral of the absolute magnetic field over constant time windows between stimulus onset and 260 ms. We isolated an early, narrow time range between about 60 ms and 80 ms that showed larger values in the task condition, most prominently in the right hemisphere. These results add to other imaging and lesion studies which suggest a specific role of the right auditory cortex in identifying FM sweep direction and thus in categorizing FM sweeps into rising and falling.

The perception of FM sweeps by Chinese and English listeners

Frequency-modulated (FM) signals are an integral acoustic component of ecologically natural sounds and are analyzed effectively in the auditory systems of humans and animals. Linearly frequency-modulated tone sweeps were used here to evaluate two questions. First, how rapid a sweep can listeners accurately perceive? Second, is there an effect of native language insofar as the language (phonology) is differentially associated with processing of FM signals? Speakers of English and Mandarin Chinese were tested to evaluate whether being a speaker of a tone language altered the perceptual identification of non-speech tone sweeps. In two psychophysical studies, we demonstrate that Chinese subjects perform better than English subjects in FM direction identification, but not in an FM discrimination task, in which English and Chinese speakers show similar detection thresholds of approximately 20 ms duration. We suggest that the better FM direction identification in Chinese subjects is related to their experience with FM direction analysis in the tone-language environment, even though supra-segmental tonal variation occurs over a longer time scale. Furthermore, the observed common discrimination temporal threshold across two language groups supports the conjecture that processing auditory signals at durations of approximately 20 ms constitutes a fundamental auditory perceptual threshold.

Relationships between human auditory cortical structure and function

The human auditory cortex comprises multiple areas, largely distributed across the supratemporal plane, but the precise number and configuration of auditory areas and their functional significance have not yet been clearly established. In this paper, we discuss recent research concerning architectonic and functional organisation within the human auditory cortex, as well as architectonic and neurophysiological studies in non-human species, which can provide a broad conceptual framework for interpreting functional specialisation in humans. We review the pattern in human auditory cortex of the functional responses to various acoustic cues, such as frequency, pitch, sound level, temporal variation, motion and spatial location, and we discuss their correspondence to what is known about the organisation of the auditory cortex in other primates. There is some neuroimaging evidence of multiple tonotopically organised fields in humans and of functional specialisations of the fields in the processing of different sound features. It is thought that the primary area, on Heschl's gyrus, may have a larger involvement in processing basic sound features, such as frequency and level, and that posterior non-primary areas on the planum temporale may play a larger role in processing more spectrotemporally complex sounds. Ways in which current knowledge of auditory cortical organisation and different data analysis approaches may benefit future functional neuroimaging studies which seek to link auditory cortical structure and function are discussed.

Conscious and preconscious adaptation to rhythmic auditory stimuli: a magnetoencephalographic study of human brain responses

This study was triggered by the experimental evidence that subjects required to tap in synchrony with a heard rhythm spontaneously time their tapping to variations in rhythm frequency even when these variations are so small that they are not consciously detectable. We performed a series of magnetoencephalographic (MEG) measurements, aimed at investigating whether the response of the auditory cortex discriminates randomly administered series of brief tones differing from each other only by their interstimulus intervals (ISI). Moreover, by combining psychophysical measurements, conscious and preconscious adjustments of tapping to rhythm variations were compared with brain cortical responses. The ISIs were varied by 2% or 20% from a "central" value of 500 ms. Subjects always consciously detected the 20% ISI changes and easily adjusted their tapping accordingly, whereas they never consciously detected the 2% ISI changes, even though they always correctly adjusted their tapping to them. Analysis of the auditory evoked fields (AEFs) showed that the intensity of the M100 component decreased with decreasing ISI both for 20% and 2% variations in a statistically significant manner, despite the fact that the 2% variation was not consciously perceived. The M100 behavior indicated that connections between auditory and motor cortexes may exist that are able to use the information on rhythm variations in the stimuli even when these are not consciously identified by the subject. The ability of the auditory cortex to discriminate different time characteristics of the incoming rhythmic stimuli is discussed in this paper in relation to the theories regarding the physiology of time perception and discrimination.

Spectro-temporal analysis of complex tones: two cortical processes dependent on retention of sounds in the long auditory store

OBJECTIVES

To examine whether two cortical processes concerned with spectro-temporal analysis of complex tones, a 'C-process' generating CN1 and CP2 potentials at cf. 100 and 180 ms after sudden change of pitch or timbre, and an 'M-process' generating MN1 and MP2 potentials of similar latency at the sudden cessation of repeated changes, are dependent on accumulation of a sound image in the long auditory store.

METHODS

The durations of steady (440 Hz) and rapidly oscillating (440-494 Hz, 16 changes/s) pitch of a synthesized 'clarinet' tone were reciprocally varied between 0.5 and 4.5 s within a duty cycle of 5 s. Potentials were recorded at the beginning and end of the period of oscillation in 10 non-attending normal subjects.

RESULTS

The CN1 at the beginning of pitch oscillation and the MN1 at the end were both strongly influenced by the duration of the immediately preceding stimulus pattern, mean amplitudes being 3-4 times larger after 4.5 s as compared with 0.5 s.

CONCLUSIONS

The processes responsible for both CN1 and MN1 are influenced by the duration of the preceding sound pattern over a period comparable to that of the 'echoic memory' or long auditory store. The store therefore appears to occupy a key position in spectro-temporal sound analysis. The C-process is concerned with the spectral structure of complex sounds, and may therefore reflect the 'grouping' of frequency components underlying auditory stream segregation. The M-process (mismatch negativity) is concerned with the temporal sound structure, and may play an important role in the extraction of information from sequential sounds.

Cortical, auditory, evoked potentials in response to changes of spectrum and amplitude

The acoustic change complex (ACC) is a scalp-recorded negative-positive voltage swing elicited by a change during an otherwise steady-state sound. The ACC was obtained from eight adults in response to changes of amplitude and/or spectral envelope at the temporal center of a three-formant synthetic vowel lasting 800 ms. In the absence of spectral change, the group mean waveforms showed a clear ACC to amplitude increments of 2 dB or more and decrements of 3 dB or more. In the presence of a change of second formant frequency (from perceived /u/ to perceived /i/), amplitude increments increased the magnitude of the ACC but amplitude decrements had little or no effect. The fact that the just detectable amplitude change is close to the psychoacoustic limits of the auditory system augurs well for the clinical application of the ACC. The failure to find a condition under which the spectrally elicited ACC is diminished by a small change of amplitude supports the conclusion that the observed ACC to a change of spectral envelope reflects some aspect of cortical frequency coding. Taken together, these findings support the potential value of the ACC as an objective index of auditory discrimination capacity.

Magnetoencephalographic studies of functional organization and plasticity of the human auditory cortex

Magnetoencephalography has proven to be a powerful noninvasive tool for investigating the functional organization of the human auditory cortex and its plastic changes. The first part of this review summarizes some recent experiments on the tonotopic organization, which can be observed not only in the slow auditory evoked fields, but also in the middle-latency and the steady-state fields. In the second part of this review, recent studies on plasticity of the auditory cortex are outlined. These studies showed that the cortical representation of tones may change within hours after a reversible "functional deafferentation" (short-term plasticity) and that early musical training leads to an expansion in the cortical representation of complex harmonic sounds (long-term plasticity).

Hemispheric differences in processing tone frequency and amplitude modulations

Transient frequency and amplitude modulations (FMs, AMs) of sound are requisite to speech recognition. We recorded whole-head magnetoencephalographic signals from seven subjects to binaural 620 ms 667 Hz tones, with 3, 30, or 300 ms FMs or AMs in the beginning or middle of the tone. Responses were significantly larger and earlier for FMs than AMs, for rapid than slow modulations, and for modulations at the beginning (BEG) than in the middle (MID) of the sound. BEG 3 ms FMs elicited strongest signals in the left and MID 3 ms FMs in the right hemisphere. Fast MID modulations produced significantly stronger responses in the right than left hemisphere. These differences may reflect different functions of the left and right hemisphere in speech perception.

A pitch glide activates an intermediate response between auditory N1 and mismatch negativity

Pitch glides of a continuous tone elicit auditory N1-like responses. However, their characteristics have not well been investigated, and it remained unclear whether the response is an auditory true N1 or the mismatch negativity (MMN). We found here that a rapid pitch glide activates almost the same response as a true N1. On the contrary, as the rate of the pitch glide decreases, the response continuously varies the characteristics from true N1 to MMN. This suggests that there would exist intermediate responses between auditory N1 and MMN.

Speech sound perception, neurophysiology, and plasticity

An approach to understanding biological processes underlying speech perception, is to discover how speech sounds are represented in the central auditory system and to relate that representation to the perception of speech as a meaningful acoustic signal. Research from our group that pertains to the neurophysiologic representation of speech in the central pathways is reviewed here. Specifically considered is the relation between the representation of sound in the auditory pathway and the perception/misperception of speech, and neurophysiologic plasticity associated with speech-sound perceptual learning.

Cortical processing of complex tone stimuli: mismatch negativity at the end of a period of rapid pitch modulation

In this study, synthesised instrumental tones were used to examine human auditory cortical processes engaged at the end of a period of rapid pitch modulation. It was previously [S.J. Jones, O. Longe, M. Vaz Pato, Auditory evoked potentials to abrupt pitch and timbre change of complex tones: electrophysiological evidence of 'streaming'?, Electroencephalogr. Clin. Neurophysiol., 108 (1998) 131-142] suggested that the 'change-N1' produced by infrequent changes in pitch or timbre of a continuous complex tone represents the activity of a neuronal population topographically distinct from that responsible for the 'onset-N1' at the beginning of the tone. In the present study a superficially similar negativity was produced when the tone came to rest on a steady pitch after a period of rapid (8-16 changes/s) modulation; its scalp maximum was anterior to that of the two previously identified potentials but similar to that of the mismatch negativity elicited by discontinuous tones. By varying the modulation rate the latency was shown to be relatively constant with respect to the time the next pitch change was expected but failed to occur. The largest responses averaging c. 7 microV were evoked at the end of modulation sequences which were both rhythmic and repetitive, but a potential was still produced when there was no rhythmic pattern or repetition of individual notes. This response to non-occurrence of an expected but not necessarily specified change implies an automatic process for comparing the incoming sound with an extrapolated template of the preceding pattern in which timing as well as pitch information is accurately represented. We suggest this technique offers a robust method for eliciting the mismatch negativity, which may extend the opportunities for electrophysiological investigation of higher auditory processes.

Speech sound representation, perception, and plasticity: a neurophysiologic perceptive

Historically, auditory research has focused predominately upon how relatively simple acoustic signals are represented in the neuronal responses of the auditory periphery. However, in order to understand the neurophysiology underlying speech perception, the ultimate objective is to discover how speech sounds are represented in the central auditory system and to relate that representation to the perception of speech as a meaningful acoustic signal. This paper reviews three areas that pertain to the central auditory representation of speech: (1) the differences in neural representation of speech sounds at different levels of the auditory system; (2) the relation between the representation of sound in the auditory pathway and the perception/misperception of speech, and (3) the training-related plasticity of speech sound neural representation and speech perception.

Compartments within human primary auditory cortex: evidence from cytochrome oxidase and acetylcholinesterase staining

The human primary auditory area (AI) corresponds to granular cortex located on Heschl's gyrus. We studied its pattern of cytochrome oxidase and acetylcholinesterase activity in 10 normal human hemispheres. In cytochrome-oxidase-stained coronal sections layer IV was prominent by its dark staining. The overall staining intensity varied along the medio-lateral extent of Al; a 2.0-2.5-mm-wide antero-posterior dark band was present at mid-AI. In acetylcholinesterase-stained coronal sections a dark antero-posterior band appeared at the same location, corresponding to the highly granular part of Al. In cytochrome-oxidase-stained tangential sections of flattened Al, approximately 500-microm thick alternating dark and light cytochrome oxidase stripes were present in layers III and IV. These stripes were perpendicular to the dark band. Comparison with tonotopic maps of human Al obtained by activation studies suggests that the cytochrome oxidase and acetylcholinesterase dark band is most likely parallel to isofrequency lines and may correspond to the representation of frequencies critical for speech comprehension. The narrow stripes may be related to particular binaural or ampliotopic domains, whose presence is suggested by evidence from electrophysiological recordings in cat Al and from magnetoencephalographic studies in humans.

Steady-state auditory evoked responses to pulsed frequency modulations in children

This study investigated steady-state auditory evoked responses to pulsed frequency modulations (FM) of a continuous tone in normal children ranging in age from 6 to 12 years. We examined variations in response amplitude and phase as a function of age, recording site, and FM pulse duration. The surface topography of these evoked potentials suggested a relatively broad distribution with maximal responses observed at frontal electrode sites, smaller responses from parietal leads and the smallest responses were evident at the temporal lobe placements. Response parameters varied significantly as a function of pulse duration. Fifty milliseconds pulses elicited responses that were on average 20% larger than 100 ms FM pulses. Mean phase differences suggested that responses to the 100 ms pulses also lagged behind responses to the 50 ms pulses by the equivalent of 20 ms. There were no significant age-related variations in response amplitude. Phase varied with age only in response to the 50 ms FM pulses. The findings indicated that steady-state responses are sensitive to temporal parameters of frequency change present in pulsed modulations. The possibility is raised that this paradigm may be clinically useful in detecting dysfunction of specialized auditory mechanisms involved in frequency modulation analysis.

Human long-latency responses to brief interaural disparities of intensity

The electroencephalographic responses to abrupt changes in interaural differences of time (ITD) and intensity (IID) should provide important information on the dynamic characteristics and integrity of the binaural mechanisms detecting the azimuthal shifts of a sound image. However, a change in either or both of these cues to sound lateralization would stimulate not only the binaural mechanisms but also the monaural ones. There are several reports evidencing that in the case of ITD changes this problem can be overcome by using time-shifted noise or repetitive clicks. Any change in IID, however, will inevitably have a stimulating effect also on purely monaural mechanisms. Therefore, the stimulation techniques described in the literature so far for recording the long-latency responses related to IID mechanism cannot be regarded as being specific for binaural mechanisms. We used dichotically presented 100/s click trains which were amplitude modulated with a random sequence of 50 or 100 ms square wave-intervals, so that the sound intensities at the two ears simultaneously alternated between 60 dB and 80 dB levels except during brief periods of time (50 ms) in which the interaural intensity balance was impaired, leading to an IID of 20 dB every 2 s. Owing to the fact that the cortical mechanisms remain unresponsive to repetitive stimuli presented with intervals shorter than a certain recovery period, this stimulus did not evoke any significant potential when it was presented monotically or diotically, yet it could produce lateral sound image shifts and therefore evoke pronounced long-latency responses when presented dichotically. The main components N1 and P2 of these shift responses and those of the pip responses, also recorded from the same subjects, were compared with respect to their midline distributions and hemispheric or bilateral asymmetries. The significant differences found between the shift and pip responses indicated that those evoked by the IID stimulation we designed should not be considered simply as a non-specific vertex potential.

Non-verbal auditory recognition in normal subjects and brain-damaged patients: evidence for parallel processing

Three different aptitudes involved in sound object recognition were tested in 60 normal subjects and 20 brain-damaged patients: (i) capacity to segregate sound objects on different cues (intensity steps, coherent temporal modulations or signal onset synchrony); (ii) asemantic recognition of sounds of real objects by judging whether two different sound samples belonged to the same object; and (iii) semantic identification of sounds of real objects as judged by means of a multiple choice response test. In 12 patients, different aptitudes involved in auditory recognition were disrupted separately and in a way which speaks in favour of parallel rather than hierarchical processing. There was no strong association between deficits in non-verbal auditory recognition and aphasia or the side of lesion.

Pitch change of a continuous tone activates two distinct processes in human auditory cortex: a study with whole-head magnetometer

Previous studies have shown that a frequency change in a continuous tone elicits an NI type of ERP (event-related potential) component. It remained unclear, however, whether this response is a "genuine" N1 (onset detector response) or the mismatch negativity (MMN), a change-detector type of ERP response, elicited in previous studies by an infrequent change in a sequence of homogeneous stimuli. A further possibility is a nearly perfect overlap of the two types of ERP components. The advent of modern, high-resolution magnetometers has opened a new, powerful way to tackle such component-overlap problems. Subjects were presented with a continuous tone of 988 Hz which was occasionally increased to 1108 Hz for a period of 100 msec. The magnetic responses to this change consisted of two partially overlapping components with peaks separated by 30 msec. The earlier component was probably generated by neuronal populations of the auditory cortex corresponding to the supratemporal N1, whereas the later one, generated anteriorly and inferiorly to the first, probably reflects a mismatch process causing the magnetic equivalent of the electrical MMN.

Functional topography of cat primary auditory cortex: responses to frequency-modulated sweeps

The spatial distribution of neuronal responses to frequency-modulated (FM) sweeps was mapped with microelectrodes in the primary auditory cortex (AI) of barbiturate-anesthetized cats. Increasing and decreasing FM sweeps (upward- and downward-directed FM sweeps, respectively) covering a range of 0.25-64.0 kHz were presented at three different rates of frequency change over time (i.e, sweep speed). Using multiunit recordings, the high-frequency domain (between 3.2 and 26.3 kHz) of AI was mapped over most of its dorsoventral extent (as determined by the distribution of the excitatory bandwidth, Q10dB) for all six cases studied. The spatial distributions of the preferred sweep speed and the preferred sweep direction were determined for each case. Neuronal responses for frequency sweeps of different speeds appeared to be systematically distributed along the dorsoventral axis of AI. In the dorsal region, cortical cells typically responded best to fast and/or medium FM sweeps, followed more ventrally by cells that responded best to medium--then slow--, then medium-speed FM sweeps. In the more ventral aspect of AI (which in some cases may also have included cells located in the dorsal region of the second auditory field, AII), neurons generally preferred fast FM sweeps. However, a comparison of maps from different animals showed that there was more variability in the distribution of preferred speed responses in the ventral region of the cortex. The directional preference of units for FM sweeps was determined for the sweep speed producing the strongest response. Direction selectivity appeared to be nonrandomly distributed along the dorsoventral axis of AI. In general, units that responded best to upward-directed FM sweeps were located in the more dorsal and ventral aspects of AI while units that responded best to downward-directed FM sweeps were usually located in the mid-region of AI. Direction selectivity was also determined for multiunit responses at each of the three FM sweep speeds. In general, there was a relatively close agreement between the spatial distributions of direction selectivity determined for the strongest response with those calculated for the fast and medium speeds. The spatial distribution of direction selectivity determined for slow FM sweeps deviated somewhat from that determined for the strongest response. Near the dorsoventral center of the mapped areas, the distribution of units that responded best to downward sweeps tended to overlay the distribution of units that responded best to slow speeds, suggesting some spatial covariance of the two parameters.(ABSTRACT TRUNCATED AT 400 WORDS)