[Sound correlated d-c changes from the intact head of human subjects]

Neural Signatures of the Processing of Temporal Patterns in Sound

The ability to detect regularities in sound (i.e., recurring structure) is critical for effective perception, enabling, for example, change detection and prediction. Two seemingly unconnected lines of research concern the neural operations involved in processing regularities: one investigates how neural activity synchronizes with temporal regularities (e.g., frequency modulation; FM) in sounds, whereas the other focuses on increases in sustained activity during stimulation with repeating tone-frequency patterns. In three electroencephalography studies with male and female human participants, we investigated whether neural synchronization and sustained neural activity are dissociable, or whether they are functionally interdependent. Experiment I demonstrated that neural activity synchronizes with temporal regularity (FM) in sounds, and that sustained activity increases concomitantly. In Experiment II, phase coherence of FM in sounds was parametrically varied. Although neural synchronization was more sensitive to changes in FM coherence, such changes led to a systematic modulation of both neural synchronization and sustained activity, with magnitude increasing as coherence increased. In Experiment III, participants either performed a duration categorization task on the sounds, or a visual object tracking task to distract attention. Neural synchronization was observed regardless of task, whereas the sustained response was observed only when attention was on the auditory task, not under (visual) distraction. The results suggest that neural synchronization and sustained activity levels are functionally linked: both are sensitive to regularities in sounds. However, neural synchronization might reflect a more sensory-driven response to regularity, compared with sustained activity which may be influenced by attentional, contextual, or other experiential factors.SIGNIFICANCE STATEMENT Optimal perception requires that the auditory system detects regularities in sounds. Synchronized neural activity and increases in sustained neural activity both appear to index the detection of a regularity, but the functional interrelation of these two neural signatures is unknown. In three electroencephalography experiments, we measured both signatures concomitantly while listeners were presented with sounds containing frequency modulations that differed in their regularity. We observed that both neural signatures are sensitive to temporal regularity in sounds, although they functionally decouple when a listener is distracted by a demanding visual task. Our data suggest that neural synchronization reflects a more automatic response to regularity compared with sustained activity, which may be influenced by attentional, contextual, or other experiential factors.

Sound-induced changes of infraslow brain potential fluctuations in the medial geniculate nucleus and primary auditory cortex in anaesthetized rats

Recent publications indicate the presence of infraslow activity (<0.5 Hz) in subcortical and cortical sites of the auditory system of the brain. It has been reported that this activity might be sensitive to acoustic stimuli. Yet the dynamics of infraslow brain potential (ISBP) fluctuations in these structures and their potential sensitivity to auditory stimuli are unknown. The present study was performed in order to test the hypothesis that extracellular ISBP activity in the medial geniculate nucleus (MGN) and the primary auditory cortex (A1) responds concurrently to acoustic stimuli. The experimental subjects were 5 adult rats with chronic stereotaxic electrodes implanted in MGN and A1. The animals were anesthetized and recordings were made in both sites during both silence and rhythmical acoustic stimulation. Our results support the hypothesis that these fluctuations are sensitive to acoustic stimuli. There were similar changes in ISBP activity in the MGN and A1 in response to rhythmic acoustic stimulation. Specifically, there were significant increases in the frequency range of seconds. Based on these findings, we suggest that sound-correlated changes in infraslow activity in the range of seconds in the MGN and A1 reflect specific mechanisms of neural processing of acoustic information in the auditory system of the brain.

Sustained magnetic fields reveal separate sites for sound level and temporal regularity in human auditory cortex

Magnetoencephalography was used to investigate the relationship between the sustained magnetic field in auditory cortex and the perception of periodic sounds. The response to regular and irregular click trains was measured at three sound intensities. Two separate sources were isolated adjacent to primary auditory cortex: One, located in lateral Heschl's gyrus, was particularly sensitive to regularity and largely insensitive to sound level. The second, located just posterior to the first in planum temporale, was particularly sensitive to sound level and largely insensitive to regularity. This double dissociation to the same stimuli indicates that the two sources represent separate mechanisms; the first would appear to be involved with pitch perception and the second with loudness. The delay of the offset of the sustained field was found to increase with interclick interval up to 200 ms at least, which suggests that the sustained field offset represents a sophisticated offset-monitoring mechanism rather than simply the cessation of stimulation.

Near-DC magnetic fields following a periodic presentation of long-duration tonebursts

OBJECTIVES

The purpose of this study was to determine the time course of low-frequency (<0.1 Hz) magnetic field components evoked by long-duration tonebursts. The following two questions were of central interest. Does the sustained field (SF) show adaptation as described before for the sustained potential (SP)? How does the field amplitude return to the pre-stimulus baseline after stimulus offset?

METHODS

Neuromagnetic measurements were done with a 37-channel first-order gradiometer system. The stimulus was a 1 kHz toneburst of 10 s duration presented at fixed 20 s intervals. The averaged data (high-pass filtered, 0.03 Hz cut-off) were analyzed using the model of an equivalent current dipole with time-invariant location and orientation (fixed dipole).

RESULTS

In the grand average of the subjects with the best signal-to-noise ratio, the SF exhibited adaptation with a time constant of 3.6 s. After stimulus offset, the amplitude of the dipole moment dropped to a lower level within 300 ms and decayed exponentially to the baseline thereafter (time constant 2.7 s).

CONCLUSIONS

A two-component model is proposed: One component roughly follows the envelope of the stimulus, the other behaves like a leaky integrator. A better understanding of near-DC fields appears to be crucial for the understanding of the relationship between magnetoencephalography and other functional imaging techniques like functional magnetic resonance imaging and positron emission tomography.

Perception of species-specific vocalizations in rats: role of the cholinergic septo-hippocampal pathway and aging

The effect of a chemical lesion of the cholinergic septo-hippocampal pathway induced by ethylcholine aziridinium (AF64A) on brain potentials evoked by species-specific vocalization containing informations of high biological relevance was studied in young adult (10 months) and aged (24 months) rats by means of neocortical electroencephalographic recordings. In control rats, the perception of a rat's vocalization in a life endangering situation (lasting 0.8 s) initiated an evoked potential followed by a late positive slow wave (LPSW)-complex and a direct current shift with a duration of up to 16 s. Four months after treatment with AF64A (2 nmol into each of the lateral ventricles), the mean negative component of the initial acoustic evoked potential (peak latency of about 60 ms after stimulus onset) was reduced (P = 0.04) both in young adult and aged rats. Further changes included a decrease of the late positive wave amplitude in young adult rats (P = 0.001) and a shorter duration of the LPSW-complex in aged rats (P = 0.03). AF64A induced also changes in specific components revealed by Principle Component Analysis, but only in the group of young rats. A decrease in the slow wave component (factor 1, 3000-4000 ms after stimulus onset; P = 0.02) was observed. Age per se affected the late positive potential shifts as indicated by a shorter latency of the late positive wave (P = 0.03). A detailed analysis of the major neurotransmitter markers proved an almost exclusive reduction of the activity of choline acetyltransferase in the ventral and dorsal hippocampus (up to 60%), which was equal in young adult and aged rats. The irreversible loss in choline acetyltransferase activity, which was restricted to the hippocampus, was associated by a slight reduction in serotonergic function. The present data suggest that the complex cognitive and emotional processes initiated by species-specific vocalization appear to be affected by aging. Furthermore, as a consequence of a cholinergic deficit in the hippocampus, the integration processes essential for the recognition of the biological meaning of a species-specific vocalization are considerably disturbed. These findings provide an experimental basis for studying disturbances in the perceptual response to stimuli of high emotional value in patients with hypocholinergic function as in Alzheimer's disease.

The auditory evoked sustained field: origin and frequency dependence

A sound lasting for several seconds is known to elicit a baseline shift in electrical and magnetic records. We have studied the dependence of the magnetic field distribution of this "per-stimulatory" sustained field (SF) on tone frequency. Tone bursts of 2 sec duration and 60 dB nHL intensity were presented to 11 subjects at varying interstimulus intervals between 5 and 7 sec. The carrier frequencies of 250, 1000 and 4000 Hz varied randomly from trial to trial. The field distributions obtained are consistent with the view that the auditory evoked sustained field activity originates in the supratemporal cortex. Differences in the locations of equivalent current dipoles of the SF from those of the M100 wave of the slow auditory evoked field are consistent across subjects. The SF source locations corresponding to stimulus frequencies over an extended frequency range are arranged in a tonotopic manner and support the idea that the sources of the M100 and the SF are current dipole sheets located on the superior surface of the primary auditory cortex.

Human DC scalp potentials during vestibular and optokinetic stimulation: non-specific responses?

In normal subjects transient horizontal body rotation in the dark (vestibular stimulation) elicited a DC potential change with the maximum at Cz. The response appeared to covary with stimulus velocity, which is the most relevant stimulus parameter transferred by the horizontal semicircular canal system. A similar response was obtained during optokinetic stimulation. However, the following findings suggested that the responses are related to 'high' perceptual functions rather than representing a visuo-vestibular evoked cortical potential: (a) the visual response was found independent of whether the subjects perceived, as a task, self-motion in a stationary environment or 'object' motion about the stationary body; its amplitude depended on the subjects' subjective compliance to the task; (b) when presenting various combinations of optokinetic and vestibular stimuli, the response amplitude depended on the subjective and objective intersensory conflict in the combinations; (c) sinusoidal stimulation yielded a negative shift of the DC potential, but the potential was not modulated along with the waxing and waning of stimulus velocity or of the self-motion sensation evoked; (d) patients with loss of vestibular functions showed a similar Cz response during body rotation in the dark.

Neurophysiological assessment of sensory gating in psychiatric inpatients: comparison between schizophrenia and other diagnoses

Gating of auditory sensory responsiveness was examined in 75 psychiatric inpatients using a conditioning-testing paradigm with the P50 wave of the auditory evoked response, in which pairs of stimuli are presented to the subject. In previous studies, most schizophrenics did not decrement the second response to the extent seen in normals. Acutely ill patients, who were representative of patients admitted to a public university teaching service and a proprietary hospital, were used to examine the extent to which diminished sensory gating is found in diagnoses other than schizophrenia. About half of these patients showed diminished sensory gating that correlated with measures of severity of illness. The data, taken together with that from other studies using this paradigm, suggest that diminished sensory gating, like several other psychophysiological abnormalities, is a trait deficit in schizophrenia, but a state deficit in many other mental illnesses.

Visual persistence of stereoscopic stimuli: electric brain activity without perceptual correlate

Dynamic random-dot stereograms (RDS) were used to study cortical neuronal mechanisms related to visual persistence in nine subjects. Electric brain potential components evoked by stereoscopic stimuli were compared to those evoked by conventional checkerboard stimuli with contrast borders. In a pattern-onset/offset presentation visual persistence thresholds were significantly lower for stereoscopic than for contrast stimuli: with temporally modulated patterns stereoscopic stimuli appeared to visually persist at much shorter interstimulus intervals than contrast stimuli. With stereoscopic stimuli all subjects reported not seeing changes of the RDS pattern, while the corresponding evoked potentials showed components related to the stimulus changes indicating a consistent discrepancy between psychophysical and electrophysiological data. The electrical brain activity was not caused by vergence eye movements elicited by the stereoscopic stimulus. In addition, for dynamic RDS stimuli a significant inverse linear relationship between temporal modulation frequency and evoked potential amplitude was found which was not observed with comparable contrast stimuli.

Premenstrual mood changes and gating of the auditory evoked potential

To assess the effect of the menstrual cycle on a measure of brain physiology known to be affected in psychiatric illness, auditory-evoked electroencephalographic potentials were recorded from 12 women on the day prior to menstruation and nine days after the initiation of their cycle. Eight were normal college students, and four were patients in a premenstrual syndrome clinic. The women showed significant changes in self-ratings of mood between the two recordings. The P50 wave of the auditory-evoked response was evaluated in a conditioning-testing paradigm, in which stimuli were presented in pairs, allowing assessment of putative excitatory and inhibitory processes involved in the gating of central nervous system sensory responsiveness to auditory stimuli. The auditory-evoked potentials were unchanged between the two recording periods. There was also no difference between the women and age-matched male controls. The data suggest that these central nervous system functions are not responsive to hormonal fluxes in menstruation. Since inhibitory gating of the P50 wave is lost in the manic phase of manic-depressive illness, the data also suggest that premenstrual mood changes in normal women do not share electrophysiological properties of mania.

Gating of auditory evoked responses in normal college students

Several neurophysiological indicators of abnormal sensory processing found in psychosis are also found in lower incidence in normal populations, particularly those at the age of risk for illness. One such parameter, gating of the auditory evoked potential, was evaluated in college students. Auditory stimuli were presented in pairs to 21 normal subjects, who generally showed gating or suppression of the vertex P50 wave to the second stimulus. Since suppression in this conditioning-testing paradigm is often absent in psychotic patients, we attempted to see if particular parameters, perhaps related to the psychotic state, that would interfere with suppression could be identified in these normal subjects. Increase in motor activity by either active or passive movement did not alter suppression. Distraction of attention by a serial subtraction test also did not interfere. Suppression was not related to mood in most subjects. Extreme self-ratings of tension-anxiety and anger-hostility were closely associated with absence of suppression, however. The results suggest that changes in concentration or motor activity do not affect suppression, but absence of suppression is associated with significant disturbance of mood in a fraction of a normal population of college students.

Neurophysiologic studies of sensory gating in schizophrenia: comparison of auditory and visual responses

Gating of visual and auditory evoked responses was assessed in chronic schizophrenic patients treated with neuroleptic drugs. Middle latency components of the visual evoked response (N90-P130) were recorded at the occiput after flash stimulus. Possible inhibitory mechanisms of sensory gating were assessed in a conditioning-testing paradigm by measuring the change in amplitude of response to a second stimulus, relative to the response to the first stimulus. Simultaneous electrooculograms were recorded to detect contamination of recordings by eye movement. Neither schizophrenic patients nor normal control subjects demonstrated significant suppression of visual evoked responses in the conditioning-testing paradigm. These results differed markedly from similar measurements of a middle latency component of the auditory evoked response (P50) recorded using the same conditioning-testing paradigm in these subjects. Normal controls showed significant decrements of the P50 response to the second auditory stimulus (mean decrement over 80%), whereas schizophrenic patients failed to show a significant decrement (mean less than 40%). This finding for auditory evoked responses replicated previous studies of normal and schizophrenic subjects. Multiple conditioning stimuli were substituted for the single conditioning stimulus used previously in an attempt to enhance gating of auditory responses, but suppression of the P50 test response did not increase in either normal or schizophrenics.

Evoked potentials elicited by long vibrotactile stimuli in the human EEG

Evoked potentials elicited by long vibrotactile stimuli were studied in six subjects. Stimuli were vibratory bursts (60 HZ, 600 ms, 680 micrometers) delivered to the back of the hand trains of four stimuli with an interstimulus-interval of 2 s. Stimulus trains were repeated once every minute. EEG was recorded from vertex and from ipsi- and contralateral parietal areas. First stimuli of the trains elicited a transient response followed by a large negative sustained potential which reached its peak amplitude about 700 ms after the stimulus onset and which was not restricted to the stimulus duration. The negative component of the transient on-response as well as the sustained potential was at the 1st stimulus of the train maximal in amplitude at the vertex, and no significant difference between the ipsi- and contralateral parietal areas was detected. Stimulus repetition in the train resulted in a marked decrease of the transient response, and in an almost complete disappearance of the sustained potential, which was only occasionally observed during repeated stimuli.

Auditory evoked transient and sustained potentials in the human EEG: I. Effects of expectation of stimuli

The characteristics of auditory evoked transient and sustained potentials were recorded using trains of four-tone stimuli of 1-second duration (interstimulus interval = 1 second) presented once every minute. The subject either attentively expected the stimuli or ignored them while reading. The electroencephalogram was recorded from derivations Cz-Al and Fz-Al. Expectation of the stimuli was associated with increased amplitudes of the transient responses both at the first stimulus of the train and during stimulus repetition. In contrast, the sustained potential at the first stimulus of the train was unchanged or smaller when the subject expected the stimuli. During stimulus repetition, however, the amplitude of the potential was enhanced by expectation of the stimuli. The results support the hypothesis of two sustained potential components and stress the importance of stimulus repetition rate when sustained potentials are studied.

Effect of stimulus repetition on negative sustained potentials elicited by auditory and visual stimuli in the human EEG

Effect of repetition of auditory and visual stimuli on sensory-evoked sustained potentials was studied. The stimuli were tones and flashes of 1 sec duration presented in trains of six stimuli with an inter-stimulus-interval of 1 sec. The repetition rate of the trains was 1 train/min. The EEG was recorded from electrodes located at Cz, Pz and Oz. Both auditory and visual stimuli elicited negative sustained potentials which during the first stimuli of the trains were maximal in amplitude and shortest in latency at Cz. Repetition of the stimulus resulted in almost complete disappearance of the sustained potentials; for auditory stimulation a small negative shift was recorded only at Cz, whereas for visual stimulation a small sustained potential was seen only at Pz and Oz. The results indicate that the slow potentials evoked by short tones or lights are composed of at least two components. One of these components may reflect neural processing associated with orienting reflex. The other component seems to be related to specific neural processing of the stimulus.

Human auditory sustained potentials. II. Stimulus relationships

The auditory sustained potential recorded from the human scalp increases in amplitude with increasing stimulus intensity. At rapid rates of stimulus presentation its amplitude decreases but proportionately less so than the amplitude of the transient onset auditory evoked potential. The frequency specificity of this rate effect is complex, suggesting that there may be two underlying components of the scalp-recorded auditory sustained potential. The amplitude of the auditory sustained potential is smaller when the tonal frequency of the stimulus is higher. With prolonged stimulus durations there is some adaptation of the amplitude of the auditory sustained potential. This potential is larger in amplitude when sounds are presented binaurally than monaurally, and has a symmetrical coronal scalp distribution that is unaffected by the ear of stimulation.

Human auditory sustained potentials. I. The nature of the response

In response to a sustained toneburst a negative baseline shift can be recorded from the human fronto-central scalp regions with an onset latency of approximately 150 msec. This auditory sustained potential is distinct both in its scalp distribution and in its stimulus relationships from the transient response occurring at the onset or offset of the toneburst. It differs from the contingent negative variation in that it can occur in the absence of attention or during sleep. Attention to the auditory stimulus can increase the amplitude of the sustained potential, possibly through the addition of an extra negative potential related to auditory expectancy or uncertainty.

[The importance of the "contingent negative variation" (CNV) for audiometry (author's transl)]

Previous proofs of "central processes" in hearing were based on the determination of the late "auditory evoked potentials" (AEP's: 50--300 ms) in EEG. The appearance of AEP's in different sleep stages raises the question, if the AEP actually conscious hearing. Since the discovery of the "Contingent Negative Variation" (CNV), as a potential depending on conscious and cognitive factors, an appropriate "instrument" is available to solve this problem. Using the CNV-technique three proofs of central auditory perception are described and discussed: a) "The objective proof of the perception of pure-tones", useful for threshold determination, test in the case of simulation and central hearing disorders. b) "The objective proof of the perception of stimulus-changes", useful for exact determination of differential thresholds for frequencies or intensities. c) "The objective speech audiometry", proof of speech comprehension, useful in case of aphasia and simulation.