Processing of auditory deviants with changes in one versus two stimulus dimensions

Auditory event-related potentials (ERPs) in response to 50-ms tones were recorded from the human scalp. A standard stimulus ( p = .88) and three different deviants were randomly presented via earphones. There were two one-dimensional deviants (one frequency and one location deviant) and one two-dimensional deviant, with changes in both frequency and location. In one condition, subjects read a book and ignored the auditory stimuli, whereas in another condition they tried to discriminate deviants from standards. In the ignore condition, the two-dimensional deviant elicited an enhanced mismatch negativity (MMN) as compared with the MMNs elicited by the one-dimensional deviants. The temporal and the topographic distributions of the two-dimensional MMNs could be modeled by adding the one-dimensional MMNs. This additivity of the MMNs probably results from the independent activity of separate neural populations generating the frequency and the location MMN. In the attend condition, the deviance-related ERP effects were not additive in the N2b and P3 range, implicating that the neural processes involved in the conscious detection of changes in location and frequency were not independent.     

Responses of the human auditory cortex to vowel onset after fricative consonants

Summary Neuromagnetic responses to different auditory stimuli (noise bursts and short speech stimuli) were mapped over both hemispheres of seven healthy subjects. The results indicate that a particular acoustic feature of speech, vowel onset after voice-less fricative consonants, evokes a prominent response in the human supratemporal auditory cortex. Although the observed response seems to be specific to acoustic rather than phonetic characteristics of the stimuli, it might reflect feature detection essential for further speech processing.     

Characterization of the human auditory cortex by the neuromagnetic method

Summary Neuromagnetic studies show that the location of cortical activity evoked by modulated tones and by click stimuli in the steady state paradigm can be determined non-invasively with a precision of a few millimeters. The progression of locations for tones of increasing frequency establish an orderly tonotopic map in which the distance along the cortex varies as the logarithm of the frequency. The active region responding to clicks lies at a position that is consistent with this map if the stimulus is characterized by the frequency of the peak of its power spectrum. A latency of about 50 ms observed for the response to clicks is in close correspondance with a strong component of the transient response to an isolated click reported in the literature. Monaural stimulation of the ear contralateral to the hemisphere being monitored produces a latency which is about 8 ms shorter than stimulation of the ipsilateral ear, in agreement with previous studies of transient responses. The amplitudes of the responses for binaurally presented clicks for sleeping subjects is substantially diminished for repetition rates above 20 Hz but is enhanced for lower rates.Supported in part by Office of Naval Research Contract N00014-76-C-0568Supported by Consiglio Nazionale delle Ricerche and by Progetto Finalizzato Superconduttivitá — C.N.R.     

Deficits in automatically detecting changes in conjunction of auditory features in patients with schizophrenia

Disturbances in processing simple acoustic changes in a stream of stimuli have been widely reported in patients with schizophrenia, but little is know about auditory feature conjunction in these individuals. This study was designed to examine the extent to which patients with schizophrenia automatically process changes in conjunction of auditory features by using event-related brain potentials. Seventeen patients and 17 age-matched controls were presented with frequent low pitch tones at 45 degrees to the left of center and frequent high pitch tones at 45 degrees to the right of center while performing a continuous visual serial-choice reaction time task. The sequence of auditory stimuli included rare conjunction-deviants comprised of a different combination of features (e.g., low pitch tone at 45 degrees right) and double-deviant tones that differed from the standard tones in both pitch and location (i.e., middle pitch at 0 degrees azimuth). Conjunction-deviant stimuli elicited an MMN wave that was maximum at frontocentral sites. Compared with controls, the MMN to conjunction-deviant was reduced in patients and was more centrally distributed. Double-deviant sounds generated a biphasic MMN followed by a P3a wave at central sites. Both MMN and P3a were reduced in patients compared with controls. These results show that patients with schizophrenia have difficulty in automatically detecting changes in a combination of auditory features as well as orienting to what "normally" would be considered salient by healthy individuals.     

Activation of the human posterior parietal cortex by median nerve stimulation

We recorded somatosensory evoked magnetic fields from ten healthy, right-handed subjects with a 122-channel whole-scalp SQUID magnetometer. The stimuli, exceeding the motor threshold, were delivered alternately to the left and right median nerves at the wrists, with interstimulus intervals of 1, 3, and 5 s. The first responses, peaking around 20 and 35 ms, were explained by activation of the contralateral primary somatosensory cortex (SI) hand area. All subjects showed additional deflections which peaked after 85 ms; the source locations agreed with the sites of the secondary somatosensory cortices (SII) in both hemispheres. The SII responses were typically stronger in the left than the right hemisphere. All subjects had an additional source, not previously reported in human evoked response data, in the contralateral parietal cortex. This source was posterior and medial to the SI hand area, and evidently in the wall of the postcentral sulcus. It was most active at 70–110 ms.     

Contra- and ipsilateral auditory stimuli produce different activation patterns at the human auditory cortex

Auditory evoked magnetic fields were recorded over the right hemisphere of healthy humans The stimuli were noise bursts presented either to the contra- (C) or ipsilateral (I) ear in different combinations. The largest deflection of the responses, N100m (magnetic counterpart of electric N100), showed a field pattern which suggests activation of the supratemporal auditory cortex. In an oddball paradigm, where the standards (90%) were 400-ms noise bursts presented to the contralateral ear, and the deviants (10%) similar stimuli to the ipsilateral ear, the deviants elicited on the average 130% stronger equivalent dipoles for N100m than standards. Contralateral standards did not substantially decrease the response amplitude of ipsilateral deviants as compared with the response amplitude to ipsilateral stimuli alone presented at the interstimulus interval of the deviants. When two 50 ms noise bursts, separated by 310 ms, were presented once every 2 s, N100m evoked by the second stimulus of the pair was smaller when the stimuli were presented monaurally (C-C, or I-I) than to different ears (I-C or C-I). The results suggest that contra- and ipsilateral auditory stimuli are analyzed, at least in part, in different neural networks at the human auditory cortex.     

Processing of complex sounds in the human auditory cortex as revealed by magnetic brain responses

Processing of simple and complex sounds in the human brain was compared by recording extracranial magnetic mismatch responses (MMNm; the magnetic counterpart of the mismatch negativity, or MMN) to frequency changes in these sounds. Generator sources, modeled as equivalent current dipoles (ECDs), of MMNm responses to a change in one frequency element of complex sounds (a chord and a serial tone pattern) were located in supratemporal auditory cortex, on average, 10 mm medially to the source of an MMNm elicited by an identical frequency change in a simple tone. These results suggest that at least partially different supratemporal neuron populations are invovled in processing changes in simple and complex sounds and that sensory-memory representations for these sounds may be located in different fields of the auditory cortex.     

Selective listening modifies activity of the human auditory cortex

Summary We have studied the effect of selective listening on the neuromagnetic evoked activity of the human auditory cortex. In the word categorization experiment the stimuli were 5-letter words, each beginning with /k/. Half of them were targets, i.e., names of animals or plants, and half other meaningful Finnish words. In the duration discrimination experiment equiprobable tones of 425 ms (targets) or 600 ms duration were presented. In both experiments the interstimulus interval (ISI) was 2.3 s and the stimuli of the two classes were presented randomly. Subjects either ignored the stimuli (reading condition) or counted the number of targets (listening condition). The magnetic field over the head was measured with a 7-channel 1st-order SQUID-gradiometer. The stimuli evoked a transient response followed by a sustained field. The transient response did not differ between the two conditions but the sustained field was significantly larger in the listening than reading condition; the increase began 120–200 ms after stimulus onset and continued for several hundred milliseconds. The equivalent source locations of both transient and sustained responses agreed with activation of the supratemporal auditory cortex. In the dichotic listening experiment 25-ms square-wave stimuli were presented randomly and equiprobably either to the left or to the right ear at an ISI of 0.8–1 s, either alone or in presence of a speech masker. Counting the stimuli of either ear resulted in differences between responses to relevant and irrelevant sounds. The difference began 140–150 ms after stimulus onset and peaked at 200–240 ms. During monaural speech masking, N100m was larger for attended than ignored stimuli. The results suggest that neural mechanisms underlying direction of attention include modification of the activity of the auditory cortex and that the mechanisms are similar for words and tones.     

Mismatch negativity to auditory stimulus change recorded directly from the human temporal cortex

Mismatch negativity (MMN) is an event-related potential (ERP) component elicited by any discernible change in a repetitive sound even in the absence of attention. Previous studies have established that MMN is generated by change detection in a process comparing the deviant sensory input with the neural memory trace encoding the physical features of the repetitive sound. In the present study, we recorded MMNs to tonal frequency changes directly from the human temporal cortex of patients with electrodes implanted in the brain for diagnosis and therapy. The intracranially recorded MMN was found to be attention independent and modality specific. It was confined to a rather small area in temporal cortex, which was different from the structures where attention-dependent N2 and P3 responses to the frequency change could be recorded.     

Disrupting human auditory change detection: Chopin is superior to white noise

A deviant sound in a sequence of standard sounds elicits a neuromagnetic mismatch field (MMF) reflecting change detection based on the auditory sensory memory trace. To illuminate the nature of this trace, we investigated the effects of white noise and music maskers on the MMF. The stimuli were delivered to the participant's right ear, and the maskers were delivered to the same or contralateral ear. Only maskers containing transients (music) presented to either car abolished the MMF. In parallel, the ability to discriminate the deviants decreased dramatically, probably because of integration of transient features of the music to the neural representations of standards and deviants. As a result, the similarity of these representations prevents change detection. White noise affected the MMF amplitude only when presented to the same ear to which the stimuli were presented. All maskers decreased the M100 but not the M50 amplitude, suggesting that the neural generators behind these responses are functionally separate.     

Neuromagnetic study of the auditory responses in right and left hemispheres of the human brain evoked by pure tones and speech sounds

Summary Neuromagnetic responses in the human auditory cortex evoked by various burst stimuli of pure tones and monosyllable speech sounds were measured separately from two hemispheres. Both the pure tones and speech sounds elicited clear responses with two main peaks. The field patterns over the scalp at the peak latencies indicated a single current dipole as an equivalent field generator. The depth of the current dipoles computed from the mapped field data was deeper from the scalp for a tone stimulus of higher frequency, which confirms the tonotopic organization in the auditory cortex. A difference was found in the dipole locations in the horizontal plane for the speech stimuli of a vowel /a/ and a consonant-vowel /ka/. It suggests the sensitivity of the magnetic responses to the acoustic structure of the speech sound.     

Independent manipulation of stimulus change and unexpectedness dissociates indices of the orienting response

Results obtained with the standard repetition-change paradigm of orienting research cannot be attributed unambiguously to either stimulus change or to unexpectedness. By adding announcement conditions, in which participants were told about an impending stimulus change, these two factors were disentangled. In Experiment 1, reaction times (RTs) were longer and ratings of surprise were higher with unannounced than with announced stimulus change. In contrast, larger skin conductance response (SCR) magnitudes occurred following change, irrespective of its congruence with participants' expectations. Experiment 2 replicated the results for SCR magnitude and, furthermore, revealed the same pattern of results for the evoked cardiac response. Surprise ratings again reflected the unexpectedness of stimulus presentations. The dissociation between RT and autonomic measures provides difficulties for resource allocation accounts of the orienting response.     

Processing of novel sounds and frequency changes in the human auditory cortex: Magnetoencephalographic recordings

Whole-head magnetoencephalographic (MEG) responses to repeating standard tones and to infrequent slightly higher deviant tones and complex novel sounds were recorded together with event-related brain potentials (ERPs). Deviant tones and novel sounds elicited the mismatch negativity (MMN) component of the ERP and its MEG counterpart (MMNm) both when the auditory stimuli were attended to and when they were ignored. MMNm generators were located bilateral to the superior planes of the temporal lobes where preattentive auditory discrimination appears to occur. A subsequent positive P3a component was elicited by deviant tones and with a larger amplitude by novel sounds even when the sounds were to be ignored. Source localization for the MEG counterpart of P3a (P3am) suggested that the auditory cortex in the superior temporal plane is involved in the neural network of involuntary attention switching to changes in the acoustic environment.     

Automatic change detection: Does the auditory system use representations of individual stimulus features or gestalts?

The effects of global and feature-specific probabilities of auditory stimuli were manipulated to determine their effects on the mismatch negativity (MMN) of the human event-related potential. The question of interest was whether the automatic comparison of stimuli indexed by the MMN was performed on representations of individual stimulus features or on gestalt representations of their combined attributes. The design of the study was such that both feature and gestalt representations could have been available to the comparator mechanism generating the MMN. The data were consistent with the interpretation that the MMN was generated following an analysis of stimulus features.     

Source analysis of magnetic field responses from the human auditory cortex elicited by short speech sounds

We made a detailed source analysis of the magnetic field responses that were elicited in the human brain by different monosyllabic speech sounds, including vowel, plosive, fricative, and nasal speech. Recordings of the magnetic field responses from a lateral area of the left hemisphere of human subjects were made using a multichannel SQUID magnetometer, having 37 field-sensing coils. A single source of the equivalent current dipole of the field was estimated from the spatial distribution of the evoked responses. The estimated sources of an N1m wave occurring at about 100 ms after the stimulus onset of different monosyllables were located close to each other within a 10-mm-sided cube in the three-dimensional space of the brain. Those sources registered on the magnetic resonance images indicated a restricted area in the auditory cortex, including Heschl's gyri in the superior temporal plane. In the spatiotemporal domain the sources exhibited apparent movements, among which anterior shift with latency increase on the anteroposterior axis and inferior shift on the inferosuperior axis were common in the responses to all monosyllables. However, selective movements that depended on the type of consonants were observed on the mediolateral axis; the sources of plosive and fricative responses shifted laterally with latency increase, but the source of the vowel response shifted medially. These spatiotemporal movements of the sources are discussed in terms of dynamic excitation of the cortical neurons in multiple areas of the human auditory cortex.     

Peripheral and central stimulus detection in auditory pathways of monkeys

Monkeys were trained in an auditory signal detection paradigm to detect increments in noise intensity. The task was analogous to the yes-no detection task in that two responses were predicated on the occurrence of the intensity increment, and the behavior was analyzed with respect to sensitivity and response bias. Central processing of peripheral stimulus information was investigated by examining stimulus evoked potentials at inferior colliculus and by conditioning or replacing the peripheral stimulus with central electrical stimulation at the inferior colliculus or auditory cortex. Evoked potentials at the colliculus reflected increment presentation but did not predict responses. Central conditioning stimuli of long duration resulted in sensitivity decrements indicating some interference with information processing at colliculus and cortex as well as producing some change in bias. Discrete, 5 msec conditioning pulses led to pure bias changes under appropriate timing conditions suggesting additive interaction of central and peripheral stimulus information. Receiver operating characteristics were generated with collicular and cortical stimuli serving as the signal indicating that such stimulation affects memory as does peripheral stimulation. Stimulation at parietal and occipital lobes proved ineffective for conditioning or replacing peripheral stimuli. Implications for serial versus parallel processing are discussed.     

The auditory novelty system: An attempt to integrate human and animal research

In this account, we attempt to integrate two parallel, but thus far, separate lines of research on auditory novelty detection: (1) human studies of EEG recordings of the mismatch negativity (MMN), and (2) animal studies of single‐neuron recordings of stimulus‐specific adaptation (SSA). The studies demonstrating the existence of novelty neurons showing SSA at different levels along the auditory pathway's hierarchy, together with the recent results showing human auditory‐evoked potential correlates of deviance detection at very short latencies, that is, at 20–40 ms from change onset, support the view that novelty detection is a key principle that governs the functional organization of the auditory system. Furthermore, the generation of the MMN recorded from the human scalp seems to involve a cascade of neuronal processing that occurs at different successive levels of the auditory system's hierarchy.     

Suppression of the auditory middle-latency response and evoked gamma-band response in a paired-click paradigm

When two clicks are presented within 500 ms and the clicks are separated by several seconds, a typical finding is a suppression of the amplitude of the P50 component of the middle-latency auditory-evoked response. In the present study, we investigated whether only the P50 or also the earlier components Po, Na, Pa and Nb, and the exogenous components N100 and P200 exhibit an amplitude suppression to the second click. In addition, we studied the suppression behaviour of the auditory-evoked 40-Hz gamma-band response in the time and frequency domain. We found a significant amplitude suppression to the second click for all components of the auditory-evoked potential following Po, which was most pronounced at electrode Cz. When testing the successive peaks and troughs of the evoked 40-Hz gamma-band response in the time domain, we found a significant amplitude suppression for peaks and troughs with the same latency and polarity as the middle-latency components following Po, which was most pronounced at electrodes Fz and Cz. Consequently, the amplitude of the 40-Hz evoked gamma-band response in the frequency domain paralleled the findings of the time domain, with a significant amplitude suppression to the second tone, which was most pronounced at electrodes Fz and Cz. Results are discussed with reference to the early sensory-gating hypothesis.     

Responses of the cerebellar cortex to cutaneous and visceral afferents

Summary To investigate the relationship between the surface evoked potentials of the superficial radial nerve and the splanchnic nerve, and the fiber evoked responses of the cerebellum of the cat was the purpose of the present study. The discharges of the mossy fibers, climbing fibers and Purkinje cell axons were identified and their relationship with the surface-potentials established. The mossy fibers fire with train of spikes in different groups, which probably correspond to impulses transmitted through fibers of different conduction velocities and synchronously with the first positivity of the evoked potential. Climbing fibers respond synchronously with the development of the sharp negative deflection of the evoked potentials and were, for the first time, identified. Finally, the Purkinje cell axons exhibited the same firing pattern as the Purkinje cells. Cutaneous and visceral afferents use not only common elements of the cerebellar cortex but also common pathways, in spite of measurable differences between both afferents.