Temporal dynamics of selective attention during dichotic listening.

The ability to selectively attend to one sound and ignore other competing sounds is essential for auditory communication. Subjects in our study detected occasional changes in the frequency of amplitude modulation in sounds presented to one ear while ignoring sounds in the other ear. Neuromagnetic source analysis revealed attention-related activity in a cortical network including primary auditory cortices, posterior superior temporal gyri, inferior parietal lobules (IPLs), inferior frontal gyri (IFG), and medial frontal gyri. Time courses of event-related magnetoencephalography responses were analyzed during the interval between stimulus presentation and behavioral response. Enhanced neural responses to targets and standards in the attended ear indicated early modulation of sensitivity in the attended sensory channel. A subsequent process of discriminative stimulus selection was indexed by a response increase over time for targets and decreasing activity for standards. Enhanced responses to deviants in the unattended ear indicated discriminative processing of unattended inputs as well, though to a lesser extent than for attended stimuli. Superior temporal gyrus, planum temporale, and the IPL were prominently involved in stimulus selection, whereas medial frontal regions were linked to initiation of behavioral responses and sustained activity in IFG suggested a role in attentional control.

A method for removing cochlear implant artifact.

When cortical auditory evoked potentials (CAEPs) are recorded in individuals with a cochlear implant (CI), electrical artifact can make the CAEP difficult or impossible to measure. Since increasing the interstimulus interval (ISI) increases the amplitude of physiological responses without changing the artifact, subtracting CAEPs recorded with a short ISI from those recorded with a longer ISI should show the physiological response without any artifact. In the first experiment, N1-P2 responses were recorded using a speech syllable and tone, paired with ISIs that changed randomly between 0.5 and 4s. In the second experiment, the same stimuli, at ISIs of either 500 or 3000ms, were presented in blocks that were homogeneous or random with respect to the ISI or stimulus. In the third experiment, N1-P2 responses were recorded using pulse trains with 500 and 3000ms ISIs in 4 CI listeners. The results demonstrated: (1) N1-P2 response amplitudes generally increased with increasing ISI. (2) Difference waveforms were largest for the homogeneous and random-stimulus blocks than for the random-ISI block. (3) The subtraction technique almost completely eliminated the electrical artifact in individuals with cochlear implants. Therefore, the subtraction technique is a feasible method of removing from the N1-P2 response the electrical artifact generated by the cochlear implant.

Cognitive association formation in episodic memory: evidence from event-related potentials.

The present study focused on the processes underlying cognitive association formation by investigating subsequent memory effects. Event-related potentials were recorded as participants studied pairs of words, presented one word at a time, for later recall. The findings showed that a frontal-positive late wave (LW), which occurred 1-1.6s after the presentation of the second word of a pair during study, was associated with later paired associate recall. The observed LW likely reflected cognitive association formation processing. Paired associate recall was also associated with a larger P555 to each word of a pair, likely reflecting the encoding of each individual word of a pair, which necessarily precedes association formation between the two words. Moreover a larger N425 was elicited by pairs that were encoded in a low context-similarity condition compared to that of a high context-similarity condition, likely reflecting semantic integration. Minimum norm source analyses showed that the likely sources of these ERP effects changed dynamically in time: a widespread fronto-temporo-parietal activation during the N425 was followed by a fronto-temporal activation during the P555, and finally by a left prefrontal activation during the LW.

Brain oscillations during semantic evaluation of speech.

Changes in oscillatory brain activity have been related to perceptual and cognitive processes such as selective attention and memory matching. Here we examined brain oscillations, measured with electroencephalography (EEG), during a semantic speech processing task that required both lexically mediated memory matching and selective attention. Participants listened to nouns spoken in male and female voices, and detected an animate target (p=20%) in a train of inanimate standards or vice versa. For a control task, subjects listened to the same words and detected a target male voice in standards of a female voice or vice versa. The standard trials of the semantic task showed enhanced upper beta (25-30 Hz) and gamma band (GBA, 30-60 Hz) activity compared to the voice task. Upper beta and GBA enhancement was accompanied by a suppression of alpha (8-12 Hz) and lower to mid beta (13-20 Hz) activity mainly localized to posterior electrodes. Enhancement of phase-locked theta activity peaking near 275 ms also occurred over the midline electrodes. Theta, upper beta, and gamma band enhancement may reflect lexically mediated template matching in auditory memory, whereas the alpha and beta suppression likely indicate increased attentional processes and memory demands.

Age-related changes in transient and oscillatory brain responses to auditory stimulation during early adolescence.

Maturational changes in the capacity to process quickly the temporal envelope of sound have been linked to language abilities in typically developing individuals. As part of a longitudinal study of brain maturation and cognitive development during adolescence, we employed dense-array EEG and spatiotemporal source analysis to characterize maturational changes in the timing of brain responses to temporal variations in sound. We found significant changes in the brain responses compared longitudinally at two time points in early adolescence, namely 10 years (65 subjects) and 11.5 years (60 of the 65 subjects), as well as large differences between adults, studied with the same protocol (Poulsen, Picton & Paus, 2007), and the children at 10 and 11.5 years of age. The transient auditory evoked potential to tone onset showed decreases in the latency of vertex and T-complex components, and a highly significant increase in the amplitude of the N1 wave with increasing age. The auditory steady state response to a 40-Hz frequency-modulated tone increased in amplitude with increasing age. The peak frequency of the envelope-following response to sweeps of amplitude-modulated white noise also increased significantly with increasing age. These results indicate persistent maturation of the cortical mechanisms for auditory processing from childhood into middle adulthood.

Multiple auditory steady state responses (80-101 Hz): effects of ear, gender, handedness, intensity and modulation rate.

OBJECTIVE: To evaluate how the amplitudes and latencies of auditory steady state responses (ASSRs) to multiple stimuli presented at rates between 80 and 101 Hz vary with the ear of stimulation, the handedness or gender of a subject, and the rate and intensity of the stimuli. DESIGN: ASSRs were recorded in a group of 56 young adults (27 females, 13 left handed) using several stimulus conditions. In the two main conditions, four sinusoidally amplitude-modulated tones (each uniquely modulated using rates between 80 and 105 Hz) with carrier frequencies of 500, 1000, 2000, and 4000 Hz, were presented concurrently to each ear (eight total). In the first condition the modulation rates for the left ear were slower than those for the right and in the second condition this relationship was reversed. Other conditions evaluated the responses to single stimuli, to multiple stimuli presented in one ear only and to multiple stimuli presented dichotically (four in each ear) with rates that decreased rather than increased with increasing carrier frequency. Stimuli were presented at an intensity of 73 dB SPL except in two conditions wherein the intensity was 53 dB SPL. RESULTS: At 73 dB SPL, multiple-stimulus ASSRs were significantly reduced (monotic or dichotic) compared with single-stimulus ASSRs, especially at 1000 and 2000 Hz. There were significant differences between monotic and dichotic stimulation. When the stimuli were presented dichotically, the amplitude of the response varied with the relative rates of modulation for the stimuli presented in each ear. ASSRs were larger in the ear with the higher rate when the carrier frequencies were 500 and 1000 Hz and when the modulation rates were <90 Hz. There were no consistent effects of gender or ear of stimulation. There were also no significant effects of handedness. CONCLUSIONS: Presenting multiple stimuli at 73 dB SPL in the same ear decreases the amplitude of the ASSR compared with when the stimuli are presented singly. This is caused by the masking effect of low on higher carrier frequencies and some other inhibitory effect of high on lower frequencies. Dichotic stimulation can increase the amplitude of the response to stimuli modulated more rapidly (and concomitantly decrease the responses to the stimuli modulated more slowly). This effect occurs only for carrier frequencies <2000 Hz and for modulation frequencies <90 Hz. Dichotic stimulation also causes a small but highly significant decrease in the latency of the response compared with monotic stimulation.

Age-related differences in processing irrelevant information: evidence from event-related potentials.

Ignoring irrelevant information becomes more difficult with increasing age. The present cross-sectional study addressed this issue by investigating age-related differences in the ability to withhold a response to non-target stimuli. Fourteen young (20-34 years) and 14 elderly (60-80 years) participants performed two go/nogo tasks (simple vs. complex). In the simple task the subjects responded to red O and blue X (target go stimuli) while withholding responses to the blue O and red X (conflict nogo stimuli) and to numbers of either color (irrelevant nogo stimuli). In the complex version, 4 vowels and 4 consonants were used instead of O and X. Accuracy, response times (RTs) and event-related potentials (ERPs) were recorded. Both young and elderly groups made more commission errors to conflict nogo stimuli (mean 5% and 8% in the simple and complex tasks, respectively, age differences not significant) than to irrelevant nogo stimuli (mean<1%), indicating difficulty in withholding a response when a pertinent stimulus feature (letter identity) was shared with the go stimuli. In addition to later RTs to go stimuli and later P3 waves for the conflicting stimuli than the young group, elderly participants showed a very prominent left posterior P2 and a large pre-central P3 to the irrelevant nogo stimuli. These findings suggest that elderly have difficulty in ignoring irrelevant nogo stimuli even when they are easily distinguishable from the go stimuli.

Mapping task switching in frontal cortex through neuropsychological group studies.

This paper considers evidence provided by large neuropsychological group studies and meta-analyses of functional imaging experiments on the location in frontal cortex of the subprocesses involved in the carrying out of task-switching paradigms. The function of the individual subprocesses is also considered in the light of analyses of the performance of normal subjects.

Envelope and spectral frequency-following responses to vowel sounds.

Frequency-following responses (FFRs) were recorded to two naturally produced vowels (/a/ and /i/) in normal hearing subjects. A digitally implemented Fourier analyzer was used to measure response amplitude at the fundamental frequency and at 23 higher harmonics. Response components related to the stimulus envelope ("envelope FFR") were distinguished from components related to the stimulus spectrum ("spectral FFR") by adding or subtracting responses to opposite polarity stimuli. Significant envelope FFRs were detected at the fundamental frequency of both vowels, for all of the subjects. Significant spectral FFRs were detected at harmonics close to formant peaks, and at harmonics corresponding to cochlear intermodulation distortion products, but these were not significant in all subjects, and were not detected above 1500 Hz. These findings indicate that speech-evoked FFRs follow both the glottal pitch envelope as well as spectral stimulus components.

Human cortical responses to the speech envelope.

OBJECTIVE: To evaluate the response of the human auditory cortex to the temporal amplitude-envelope of speech. Responses to the speech envelope could be useful for validating the neural encoding of intelligible speech, particularly during hearing aid fittings--because hearing aid gain and compression characteristics for ongoing speech should more closely resemble real world performance than for isolated brief syllables. DESIGN: The speech envelope comprises energy changes corresponding to phonemic and syllabic transitions. Envelope frequencies between 2 and 20 Hz are important for speech intelligibility. Human event-related potentials were recorded to six different sentences and the sources of these potentials in the auditory cortex were determined. To improve the signal to noise ratio over ongoing electroencephalographic recordings, we averaged the responses over multiple presentations, and derived source waveforms from multichannel scalp recordings. Source analysis led to bilateral, symmetrical, vertical, and horizontal dipoles in the posterior auditory cortices. The source waveforms were then cross-correlated with the low frequency log-envelopes of the sentences. The significance and latency of the maximum correlation for each sentence demonstrated the presence and latency of the brain's response. The source waveforms were also cross-correlated with a simple model based on a series of overlapping transient responses to stimulus change (the derivative of the log-envelope). RESULTS: Correlations between the log-envelope and vertical dipole source waveforms were significant for all sentences and for all but one of the participants (mean r = 0.35), at an average delay of 175 (left) to 180 (right) msec. Correlations between the transient response model (P1 at 68 msec, N1 at 124 msec, and P2 at 208 msec) and the vertical dipole source waveforms were detected for all sentences and all participants (mean r = 0.30), at an average delay of 6 (right) to 10 (left) msec. CONCLUSIONS: These results show that the human auditory cortex either directly follows the speech envelope or consistently reacts to changes in this envelope. The delay between the envelope and the response is approximately 180 msec.

The multiple dimensions of sustained attention.

Sustained counting (or temporal numerosity judgements) has been one of the key means of investigating anterior attentional processes. Forty-three patients with localised lesions to the frontal lobes were assessed on two tests of the ability to count the number (8-22) of stimuli presented at either a slow (roughly one per 3 sec) or fast (roughly three per sec) rate. Patients with lesions to the Superior Medial (SM) region (particularly Brodmann areas 24, 32, and 9) were impaired both in the Slow condition and also in the Fast condition, where they underestimated the number of stimuli. Patients with Right Lateral (RL) lesions (8, 45, and 46) also had difficulties in the Fast condition, especially when the number of targets was greater than 15. The results are considered from the perspectives of alternative positions on anterior attentional processes developed by Posner and Petersen (1990) and by Stuss et al. (1995). The most plausible interpretation is in terms of energising processes which involve the SM frontal cortex and monitoring processes which involve the RL frontal cortex.

Attenuation of the 40-hertz auditory steady state response by propofol involves the cortical and subcortical generators.

BACKGROUND: The 40-Hz auditory steady state response (40-Hz ASSR) provides a reliable marker of anesthetic-induced unconsciousness. Brain electric source analysis indicates that the 40-Hz ASSR arises from cortical and subcortical generators. The authors used source analysis to assess the effect of propofol anesthesia on the cerebral generators of the 40-Hz ASSR. They also examined the effect of propofol on two auditory evoked potentials of cortical origin: the N1 and the sustained potential. METHODS: Eleven healthy human volunteers were anesthetized with propofol given in target-concentration mode at the minimal concentration causing unconsciousness. The 40-Hz ASSR was recorded before, during, and after anesthesia. The source model consisted of five concurrently active generator dipoles: two in the contralateral auditory cortex (one tangentially oriented, one radially oriented), two in the ipsilateral auditory cortex (same orientations), and one in the midline brainstem. RESULTS: During anesthesia, the strength of the cortical and brainstem dipoles was reduced to the same extent (to 54% of baseline for the four cortical dipoles pooled vs. 53% for the brainstem dipole). Dipole strength during anesthesia was significantly less (P < 0.01) than during baseline and recovery for both cortical and brainstem dipoles. The N1 and sustained potential were no longer recordable during anesthesia. CONCLUSIONS: The attenuation of the 40-Hz ASSR during propofol anesthesia results from a reduction of similar magnitude of the activity of the cortical and brainstem generators. The N1 and sustained potential are so profoundly attenuated during propofol anesthesia that they are no longer recordable from the scalp.

Aging in binaural hearing begins in mid-life: evidence from cortical auditory-evoked responses to changes in interaural phase.

Older adults often have difficulty understanding speech in a noisy environment or with multiple speakers. In such situations, binaural hearing improves the signal-to-noise ratio. How does this binaural advantage change with increasing age? Using magnetoencephalography, we recorded cortical activity evoked by changes in interaural phase differences of amplitude-modulated tones. These responses occurred for frequencies up to 1225 Hz in young subjects but only up to 940 Hz in middle-aged and 760 Hz in older adults. Behavioral thresholds also decreased with increasing age but were more variable, likely because some older adults make effective use of compensatory mechanisms. The reduced frequency range for binaural hearing became significant in middle age, before decline in hearing sensation and the morphology of cortical responses, which became apparent only in the older subjects. This study provides evidence from human physiological data for the early onset of biological aging in binaural hearing.

Cortical responses to the 2f1-f2 combination tone measured indirectly using magnetoencephalography.

The simultaneous presentation of two tones with frequencies f(1) and f(2) causes the perception of several combination tones in addition to the original tones. The most prominent of these are at frequencies f(2)-f(1) and 2f(1)-f(2). This study measured human physiological responses to the 2f(1)-f(2) combination tone at 500 Hz caused by tones of 750 and 1000 Hz with intensities of 65 and 55 dB SPL, respectively. Responses were measured from the cochlea using the distortion product otoacoustic emission (DPOAE), and from the auditory cortex using the 40-Hz steady-state magnetoencephalographic (MEG) response. The perceptual response was assessed by having the participant adjust a probe tone to cause maximal beating ("best-beats") with the perceived combination tone. The cortical response to the combination tone was evaluated in two ways: first by presenting a probe tone with a frequency of 460 Hz at the perceptual best-beats level, resulting in a 40-Hz response because of interaction with the combination tone at 500 Hz, and second by simultaneously presenting two f(1) and f(2) pairs that caused combination tones that would themselves beat at 40 Hz. The 2f(1)-f(2) DPOAE in the external auditory canal had a level of 2.6 (s.d. 12.1) dB SPL. The 40-Hz MEG response in the contralateral cortex had a magnitude of 0.39 (s.d. 0.1) nA m. The perceived level of the combination tone was 44.8 (s.d. 11.3) dB SPL. There were no significant correlations between these measurements. These results indicate that physiological responses to the 2f(1)-f(2) combination tone occur in the human auditory system all the way from the cochlea to the primary auditory cortex. The perceived magnitude of the combination tone is not determined by the measured physiological response at either the cochlea or the cortex.

Human auditory steady-state responses during sweeps of intensity.

OBJECTIVE: To record steady-state responses to amplitude-modulated tones that change their intensity over time and to see how well behavioral thresholds can be estimated from such responses. DESIGN: The intensity of the stimuli used in this experiment increased from 25 to 75 dB SPL for 8 sec and then decreased back to 25 dB HL during the subsequent 8 sec. Responses to this intensity sweep were averaged and then analyzed using a short-time Fast-Fourier Transform to measure how the amplitude and phase of the responses changed with intensity. One experimental condition presented single 2-kHz tones to the left ear; a second condition examined the use of simultaneously presented multiple tones (0.5, 1, 2, and 4 kHz) to the left ear; a third condition used multiple tones presented dichotically; and a fourth condition presented the multiple dichotic tones in masking noise to simulate either low-frequency (less than 1400 Hz) or high-frequency (greater than 1400 Hz) hearing loss. Physiological thresholds were determined using six different algorithms and the relations between physiological and behavioral thresholds were evaluated to see how well behavioral thresholds could be estimated. RESULTS: The amplitude-intensity functions for the 1 and 2 kHz responses both demonstrated a plateau at higher intensities in the multiple-stimulus conditions but not in the single-stimulus condition. The slope of the amplitude-intensity functions varied significantly with the carrier frequency of the stimulus: 1.30 at 500 Hz, 0.87 at 1000 Hz, 0.75 at 2000 Hz, and 1.40 at 4000 Hz. The slope of the phase-intensity function averaged 1.16 degrees per dB and did not vary with carrier frequency. Estimates of latency, however, indicated that latency increased with decreasing carrier frequency and with decreasing intensity. The performance of the threshold estimating algorithms differed between normal hearing and simulated hearing loss, since the amplitude- and phase-intensity functions in the latter condition were not linear. Physiological-behavioral threshold differences were generally greater for normal hearing than for simulated hearing loss. Linear regression provided the least physiological-behavioral difference but was quite variable during simulated hearing loss. Simply defining threshold as the lowest intensity above which all responses were significantly different from residual EEG noise was the most accurate method in terms of yielding the least standard deviation of the physiological-behavioral difference with an average standard deviation of 10 dB, provided EEG noise levels were low enough in the normal hearing condition. CONCLUSIONS: Thresholds can be estimated using intensity sweeps with about the same accuracy as recording separate responses to discrete intensities. Sweep recordings provide additional information about the responses at suprathreshold intensities by clearly determining amplitude- and phase- intensity functions at these intensities.

Electrophysiological evaluation of human brain development.

The complex development of the human brain during infancy can only be understood by convergent structural, functional, and behavioral measurements. The evaluation of event-related potentials (ERPs) is the most effective current way to look at infant brain function. ERP paradigms can be used to examine the simple transmission of sensory information to the cortex and the discrimination of this information within the cortex. The main developmental changes involve localization of function as the brain becomes tuned to the experienced world (related to synaptic pruning) and a speeding up of transmission as pathways become efficient (related to myelination). ERPs that occur in relation to different temporal aspects of a stimulus (onset-responses, offset-responses, sustained potentials and steady-state responses) and ERPs recorded at different stimulus rates may help track perceptual development from a temporal perspective. Particularly important in human development are the ERP changes that occur in the processing of speech sounds and human faces. At present, ERP studies can show differences between groups of subjects that can demonstrate developmental disorders or elucidate mechanisms of development. However, because of their variability, ERPs are less helpful in determining whether an individual infant is developing abnormally. Where possible, ERP measurements should be used in conjunction with behavioral tests so as to relate performance to mechanism, and with anatomical brain measurements to relate mechanism to structure.

Physiological detection of interaural phase differences.

Auditory evoked cortical responses to changes in the interaural phase difference (IPD) were recorded using magnetoencephalography (MEG). Twelve normal-hearing young adults were tested with amplitude-modulated tones with carrier frequencies of 500, 1000, 1250, and 1500 Hz. The onset of the stimuli evoked P1m-N1m-P2m cortical responses, as did the changes in the interaural phase. Significant responses to IPD changes were identified at 500 and 1000 Hz in all subjects and at 1250 Hz in nine subjects, whereas responses were absent in all subjects at 1500 Hz, indicating a group mean threshold for detecting IPDs of 1250 Hz. Behavioral thresholds were found at 1200 Hz using an adaptive two alternative forced choice procedure. Because the physiological responses require phase information, through synchronous bilateral inputs at the level of the auditory brainstem, physiological "change" detection thresholds likely reflect the upper limit of phase synchronous activity in the brainstem. The procedure has potential applications in investigating impaired binaural processing because phase statistic applied to single epoch MEG data allowed individual thresholds to be obtained.

Age-related changes in transient and oscillatory brain responses to auditory stimulation in healthy adults 19-45 years old.

The capacity of the human cerebral cortex to track fast temporal changes in auditory stimuli is related to the development of language in children and to deficits in speech perception in the elderly. Although maturation of temporal processing in children and its deterioration in the elderly has been investigated previously, little is known about naturally occurring changes in auditory temporal processing between these limits. The present study examined age-related (19-45 years) changes in 3 electrophysiological measures of auditory processing: 1) the late transient auditory evoked potentials to tone onset, 2) the auditory steady-state response (ASSR) to a 40-Hz frequency-modulated tone, and 3) the envelope following response (EFR) to sweeps of amplitude-modulated white noise from 10 to 100 Hz. With increasing age, the latency of the auditory P1-N1 complex decreased, the oscillatory (ASSR) response became larger and more stable, and the resonant peak of the EFR increased from 38 Hz at 19 years to 46 Hz at 45 years. Source analysis localized these changes to the auditory regions of the temporal lobe. These results indicate persistent adaptation of cortical auditory processes into middle adulthood. We speculate that experience-driven myelination and/or refinement of inhibitory circuits may underlie these changes.

Effects of focal frontal lesions on response inhibition.

This study examined the performance of 38 normal subjects and 43 patients with focal lesions of the frontal lobes on a simple go-nogo task where the probability of the nogo stimulus was either 75% or 25%. Patients with lesions to the superior medial parts of the frontal lobes, in particular to the left superior portion of Brodmann area 6 (which includes the supplementary motor areas and the premotor areas for the right hand) had an increased number of false alarms (incorrect responses to the nogo stimulus). These results indicate that area 6 is specifically involved in the inhibition of response. Patients with lesions to the right anterior cingulate (areas 24 and 32) were slower and more variable in their reaction time. These findings could be explained by an inability to sustain stimulus-response contingencies. Lesions to the right ventrolateral prefrontal cortex (Brodmann areas 44, 45, 47) also increased the variability of response, perhaps by disrupting monitoring performance.

Enhanced anterior-temporal processing for complex tones in musicians.

OBJECTIVE: To examine how auditory brain responses change with increased spectral complexity of sounds in musicians and non-musicians. METHODS: Event-related potentials (ERPs) and fields (ERFs) to binaural piano tones were measured in musicians and non-musicians. The stimuli were C4 piano tones and a pure sine tone of the C4 fundamental frequency (f0). The first piano tone contained f0 and the first eight harmonics, the second piano tone consisted of f0 and the first two harmonics and the third piano tone consisted of f0. RESULTS: Subtraction of ERPs of the piano tone with only the fundamental from ERPs of the harmonically rich piano tones yielded positive difference waves peaking at 130 ms (DP130) and 300 ms (DP300). The DP130 was larger in musicians than non-musicians and both waves were maximally recorded over the right anterior scalp. ERP source analysis indicated anterior temporal sources with greater strength in the right hemisphere for both waves. Arbitrarily using these anterior sources to analyze the MEG signals showed a DP130m in musicians but not in non-musicians. CONCLUSIONS: Auditory responses in the anterior temporal cortex to complex musical tones are larger in musicians than non-musicians. SIGNIFICANCE: Neural networks in the anterior temporal cortex are activated during the processing of complex sounds. Their greater activation in musicians may index either underlying cortical differences related to musical aptitude or cortical modification by acoustical training.