Frequency-following responses in primary auditory and reticular formation structures

Binaural unmasking of the accuracy of envelope-signal representation in rat auditory cortex but not auditory midbrain

Accurate neural representations of acoustic signals under noisy conditions are critical for animals' survival. Detecting signal against background noise can be improved by binaural hearing particularly when an interaural-time-difference (ITD) disparity is introduced between the signal and the noise, a phenomenon known as binaural unmasking. Previous studies have mainly focused on the binaural unmasking effect on response magnitudes, and it is not clear whether binaural unmasking affects the accuracy of central representations of target acoustic signals and the relative contributions of different central auditory structures to this accuracy. Frequency following responses (FFRs), which are sustained phase-locked neural activities, can be used for measuring the accuracy of the representation of signals. Using intracranial recordings of local field potentials, this study aimed to assess whether the binaural unmasking effects include an improvement of the accuracy of neural representations of sound-envelope signals in the rat IC and/or auditory cortex (AC). The results showed that (1) when a narrow-band noise was presented binaurally, the stimulus-response (S-R) coherence of the FFRs to the envelope (FFR_envelope) of the narrow-band noise recorded in the IC was higher than that recorded in the AC. (2) Presenting a broad-band masking noise caused a larger reduction of the S-R coherence for FFR_envelope in the IC than that in the AC. (3) Introducing an ITD disparity between the narrow-band signal noise and the broad-band masking noise did not affect the IC S-R coherence, but enhanced both the AC S-R coherence and the coherence between the IC FFR_envelope and AC FFR_envelope. Thus, although the accuracy of representing envelope signals in the AC is lower than that in the IC, it can be binaurally unmasked, indicating a binaural-unmasking mechanism that is formed during the signal transmission from the IC to the AC.

Auditory frequency-following response: a neurophysiological measure for studying the "cocktail-party problem"

How do we recognize what one person is saying when others are speaking at the same time? The "cocktail-party problem" proposed by Cherry (1953) has puzzled scientific societies for half a century. This puzzle will not be solved without using appropriate neurophysiological investigation that should satisfy the following four essential requirements: (1) certain critical speech characteristics related to speech intelligibility are recorded; (2) neural responses to different speech sources are differentiated; (3) neural correlates of bottom-up binaural unmasking of responses to target speech are measurable; (4) neural correlates of attentional top-down unmasking of target speech are measurable. Before speech signals reach the cerebral cortex, some critical acoustic features are represented in subcortical structures by the frequency-following responses (FFRs), which are sustained evoked potentials based on precisely phase-locked responses of neuron populations to low-to-middle-frequency periodical acoustical stimuli. This review summarizes previous studies on FFRs associated with each of the four requirements and suggests that FFRs are useful for studying the "cocktail-party problem".

Speech auditory brainstem response (speech ABR) characteristics depending on recording conditions, and hearing status: an experimental parametric study

Speech elicited auditory brainstem responses (Speech ABR) have been shown to be an objective measurement of speech processing in the brainstem. Given the simultaneous stimulation and recording, and the similarities between the recording and the speech stimulus envelope, there is a great risk of artefactual recordings. This study sought to systematically investigate the source of artefactual contamination in Speech ABR response. In a first part, we measured the sound level thresholds over which artefactual responses were obtained, for different types of transducers and experimental setup parameters. A watermelon model was used to model the human head susceptibility to electromagnetic artefact. It was found that impedances between the electrodes had a great effect on electromagnetic susceptibility and that the most prominent artefact is due to the transducer's electromagnetic leakage. The only artefact-free condition was obtained with insert-earphones shielded in a Faraday cage linked to common ground. In a second part of the study, using the previously defined artefact-free condition, we recorded speech ABR in unilateral deaf subjects and bilateral normal hearing subjects. In an additional control condition, Speech ABR was recorded with the insert-earphones used to deliver the stimulation, unplugged from the ears, so that the subjects did not perceive the stimulus. No responses were obtained from the deaf ear of unilaterally hearing impaired subjects, nor in the insert-out-of-the-ear condition in all the subjects, showing that Speech ABR reflects the functioning of the auditory pathways.

The temporal relationship between speech auditory brainstem responses and the acoustic pattern of the phoneme /ba/ in normal-hearing adults

OBJECTIVE

To investigate the temporal relationship between speech auditory brainstem responses and acoustic pattern of the phoneme /ba/.

METHODS

Speech elicited auditory brainstem responses (Speech ABR) to /ba/ were recorded in 23 normal-hearing subjects. Effect of stimulus intensity was assessed on Speech ABR components latencies in 11 subjects. The effect of different transducers on electromagnetic leakage was also measured.

RESULTS

Speech ABR showed a reproducible onset response (OR) 6ms after stimulus onset. The frequency following response (FFR) waveform mimicked the 500Hz low pass filtered temporal waveform of phoneme /ba/ with a latency shift of 14.6ms. In addition, the OR and FFR latencies decreased with increasing stimulus intensity, with a greater rate for FFR (-1.4ms/10dB) than for OR (-0.6ms/10dB).

CONCLUSIONS

A close relationship was found between the pattern of the acoustic stimulus and the FFR temporal structure. Furthermore, differences in latency behaviour suggest different generation mechanisms for FFR and OR.

SIGNIFICANCE

The results provided further insight into the temporal encoding of basic speech stimulus at the brainstem level in humans.

Brainstem origins for cortical ‘what’ and ‘where’ pathways in the auditory system

We have developed a data-driven conceptual framework that links two areas of science: the source-filter model of acoustics and cortical sensory processing streams. The source-filter model describes the mechanics behind speech production: the identity of the speaker is carried largely in the vocal cord source and the message is shaped by the ever-changing filters of the vocal tract. Sensory processing streams, popularly called 'what' and 'where' pathways, are well established in the visual system as a neural scheme for separately carrying different facets of visual objects, namely their identity and their position/motion, to the cortex. A similar functional organization has been postulated in the auditory system. Both speaker identity and the spoken message, which are simultaneously conveyed in the acoustic structure of speech, can be disentangled into discrete brainstem response components. We argue that these two response classes are early manifestations of auditory 'what' and 'where' streams in the cortex. This brainstem link forges a new understanding of the relationship between the acoustics of speech and cortical processing streams, unites two hitherto separate areas in science, and provides a model for future investigations of auditory function.

Atypical brainstem representation of onset and formant structure of speech sounds in children with language-based learning problems

This study investigated how the human auditory brainstem represents constituent elements of speech sounds differently in children with language-based learning problems (LP, n = 9) compared to normal children (NL, n = 11), especially under stress of rapid stimulation. Children were chosen for this study based on performance on measures of reading and spelling and measures of syllable discrimination. In response to the onset of the speech sound /da/, wave V-V(n) of the auditory brainstem response (ABR) had a significantly shallower slope in LP children, suggesting longer duration and/or smaller amplitude. The amplitude of the frequency following response (FFR) was diminished in LP subjects over the 229-686 Hz range, which corresponds to the first formant of the/da/ stimulus, while activity at 114 Hz, representing the fundamental frequency of /da/, was no different between groups. Normal indicators of auditory peripheral integrity suggest a central, neural origin of these differences. These data suggest that poor representation of crucial components of speech sounds could contribute to difficulties with higher-level language processes.

Frequency-following responses in the cat

The frequency-following response was studied in five cats aged 1 week to 3 years. Animals studied had either monaural or binaural hearing. Responses were recorded differentially from vertex and circumaural electrodes to tonebursts at octave intervals from 0.5 to 4 kHz. Thresholds and dynamics of the responses with and without white-noise masking were investigated. Evidence was found to support the view that the cochlear microphonic makes a major contribution to the frequency-following response, while the double frequency-following response to the 0.5 kHz stimulus is neural.

Electrical stimulation of the superior olivary complex can produce cortical evoked potential and behavioral discrimination correlates of pitch perception in the rat

Patterned electrical stimulation of the superior olivary complex (SOC) which simulated the neural frequency following response (FFR) extracellular potential was used as a stimulus in behavioral frequency discrimination and cortical evoked potential studies. Behavioral judgments of SOC stimulation frequency were found to be as accurate as those obtained for 80 dB acoustic stimuli within the spectral band of the FFR (200 to 3800 Hz). Cortical evoked potentials elicited by acoustic and electrical stimulation of the SOC were then compared for preservation of waveform similarity. Frequency dependent similarity was observed in slow wave events elicited by stimuli with frequencies in the FFR band. A 3 msec time lag was found between acoustic and SOC stimulation produced waveforms which can be accounted for by forward stimulation of the auditory pathway. Our study supports the idea that integrated extracellular waveforms of the FFR index low frequency representations in the auditory brainstem, perhaps by selecting patches of SOC cell transmembrane potential changes. Because bilateral cochlear damage did not prevent behavioral discrimination of SOC electrical stimulation, feedback to the ear is not necessary for perceptual significance of simulated FFR extracellular field potentials in the SOC.

Periodic appearance of frontal midline theta activity during performance of a sensory-motor task

The present experiment was designed to demonstrate the presence of a constant rhythm or periodicity in the appearance of Fm theta using the Mirror Drawing Test (MDT), a task with a changing degree of difficulty. Thirty male university students were recorded on the EEGs during performance of the MDT for 5 min. Fm theta appeared most frequently in the block requiring the longest time for passage, or the most difficult block. The present study further revealed that Fm theta tended to appear mainly from four points at about 40, 90, 190 and 270 sec after beginning the MDT. From these results, the appearance of Fm theta was shown to be more distinctly related to the concentration of attention. In the process of concentration and attention distraction, the presence of a rhythm with a periodicity of approximately 45 sec was suggested.

Frequency-dependence of early auditory evoked responses in the guinea pig

Neural auditory responses in the guinea pig, monitored by surface electrodes (brain stem potentials and frequency-following responses) and by electrodes at the round window, were analyzed for evidence of frequency dependence in the range from 500 Hz to 15 kHz. The characteristics of the brain stem potentials and frequency-following responses with stimuli near threshold intensity for frequencies below 2 kHz indicate that this activity derives from the excitation of apical regions of the basilar membrane. The same interpretation applies to the potentials recorded at the round window. Comparison of the responses seen with surface electrodes and those appearing at the round window reveals that the broad potential PI of the brain stem response to low-frequency stimuli corresponds to the compound action potential, while the frequency-following responses correspond to phase-locked responses in the acoustic nerve.

Different patterns of harmonics in frequency-following responses

Preliminary studies (Krogh et al., 1977) indicated pronounced intersubject differences with respect to harmonic distortion in the frequency-following responses (FFRs) elicited by high intensity, low frequency stimuli and recorded by means of surface electrodes from man. In the present investigation it was confirmed that the responses from 10 test subjects could be divided into three significantly different patterns on the basis of the relative amplitudes of the fundamental (first harmonic) and the second harmonic response component as measured by spectral analyses. In three of the subjects the ratio of the fundamental to the second harmonic was greater than 10 dB (which were termed a type I response). In five, the ratio was between 0 and 10 dB (type II response) and in two the second harmonic was equal to or greater than the fundamental (type III response). The possible causes of harmonic distortion if FFRs are discussed, but not definite explanation of the interindividual differences can be given. A psychoacoustical evaluation of the second harmonic distortion using a tone-on-tone octave masking technique (TOM) showed no correlation between TOM results and FFR type in 10 subjects.

Dynamics of spontaneous and evoked activities of the substructures within the cat inferior colliculus

The spontaneous EEG and evoked electrical activities (EP) recorded simultaneously and bilaterally from different depths below the dorsal surface of the inferior colliculus (IC), have been systematically analyzed in freely moving cats. Power spectral analysis techniques were employed for evaluating the frequency characteristics of the EEG and EP which were recorded from substructures of the IC. The changes in the electrical activities following local lesions further helped us to understand whether these frequencies represent volume conducted field potentials or local neural activity. It was found that in the IC, high frequency electrical activities up to 1000 Hz are generated locally and recorded with maximal amplitudes in the ventral and ventralmost parts. Following the stimulation, strong amplitude enhancement and frequency stabilization were obtained in 70-80 Hz, 350-400 Hz, and 850-950 Hz frequency ranges.