Sensitivity of human auditory evoked potentials to the harmonicity of complex tones: evidence for dissociated cortical processes of spectral and periodicity analysis

Emotional prosody Stroop effect in Hindi: An event related potential study

Prosody processing is an important aspect of language comprehension. Previous research on emotional word-prosody conflict has shown that participants are worse when emotional prosody and word meaning are incongruent. Studies with event-related potentials have shown a congruency effect in N400 component. There has been no study on emotional processing in Hindi language in the context of conflict between emotional word meaning and prosody. We used happy and angry words spoken using happy and angry prosody. Participants had to identify whether the word had a happy or angry word meaning. The results showed a congruency effect with worse performance in incongruent trials indicating an emotional Stroop effect in Hindi. The ERP results showed that prosody information is detected very early, which can be seen in the N1 component. In addition, there was a congruency effect in N400. The results show that prosody is processed very early and emotional meaning-prosody congruency effect is obtained with Hindi. Further studies would be needed to investigate similarities and differences in cognitive control associated with language processing.

Temporal integration of vowel periodicity in the auditory cortex

Cortical sensitivity to the periodicity of speech sounds has been evidenced by larger, more anterior responses to periodic than to aperiodic vowels in several non-invasive studies of the human brain. The current study investigated the temporal integration underlying the cortical sensitivity to speech periodicity by studying the increase in periodicity-specific cortical activation with growing stimulus duration. Periodicity-specific activation was estimated from magnetoencephalography as the differences between the N1m responses elicited by periodic and aperiodic vowel stimuli. The duration of the vowel stimuli with a fundamental frequency (F0=106 Hz) representative of typical male speech was varied in units corresponding to the vowel fundamental period (9.4 ms) and ranged from one to ten units. Cortical sensitivity to speech periodicity, as reflected by larger and more anterior responses to periodic than to aperiodic stimuli, was observed when stimulus duration was 3 cycles or more. Further, for stimulus durations of 5 cycles and above, response latency was shorter for the periodic than for the aperiodic stimuli. Together the current results define a temporal window of integration for the periodicity of speech sounds in the F0 range of typical male speech. The length of this window is 3-5 cycles, or 30-50 ms.

Cortical evoked potentials to an auditory illusion: binaural beats

OBJECTIVE

To define brain activity corresponding to an auditory illusion of 3 and 6Hz binaural beats in 250Hz or 1000Hz base frequencies, and compare it to the sound onset response.

METHODS

Event-Related Potentials (ERPs) were recorded in response to unmodulated tones of 250 or 1000Hz to one ear and 3 or 6Hz higher to the other, creating an illusion of amplitude modulations (beats) of 3Hz and 6Hz, in base frequencies of 250Hz and 1000Hz. Tones were 2000ms in duration and presented with approximately 1s intervals. Latency, amplitude and source current density estimates of ERP components to tone onset and subsequent beats-evoked oscillations were determined and compared across beat frequencies with both base frequencies.

RESULTS

All stimuli evoked tone-onset P(50), N(100) and P(200) components followed by oscillations corresponding to the beat frequency, and a subsequent tone-offset complex. Beats-evoked oscillations were higher in amplitude with the low base frequency and to the low beat frequency. Sources of the beats-evoked oscillations across all stimulus conditions located mostly to left lateral and inferior temporal lobe areas in all stimulus conditions. Onset-evoked components were not different across stimulus conditions; P(50) had significantly different sources than the beats-evoked oscillations; and N(100) and P(200) sources located to the same temporal lobe regions as beats-evoked oscillations, but were bilateral and also included frontal and parietal contributions.

CONCLUSIONS

Neural activity with slightly different volley frequencies from left and right ear converges and interacts in the central auditory brainstem pathways to generate beats of neural activity to modulate activities in the left temporal lobe, giving rise to the illusion of binaural beats. Cortical potentials recorded to binaural beats are distinct from onset responses.

SIGNIFICANCE

Brain activity corresponding to an auditory illusion of low frequency beats can be recorded from the scalp.

Comparator and non-comparator mechanisms of change detection in the context of speech--an ERP study

Automatic change detection reflects a cognitive memory-based comparison mechanism as well as a sensorial non-comparator mechanism based on differential states of refractoriness. The purpose of this study was to examine whether the comparator mechanism of the mismatch negativity component (MMN) is differentially affected by the lexical status of the deviant. Event-related potential (ERP) data was collected during an "oddball" paradigm designed to elicit the MMN from 15 healthy subjects that were involved in a counting task. Topography pattern analysis and source estimation were utilized to examine the deviance (deviants vs. standards), cognitive (deviants vs. control counterparts) and refractoriness (standards vs. control counterparts) effects elicited by standard-deviant pairs ("deh-day"; "day-deh"; "teh-tay") embedded within "oddball" blocks. Our results showed that when the change was salient regardless of lexical status (i.e., the /e:/ to /eI/ transition) the response tapped the comparator based-mechanism of the MMN which was located in the cuneus/posterior cingulate, reflected sensitivity to the novelty of the auditory object, appeared in the P2 latency range and mainly involved topography modulations. In contrast, when the novelty was low (i.e., the /eI/ to /e:/ transition) an acoustic change complex was elicited which involved strength modulations over the P1/N1 range and implicated the middle temporal gyrus. This result pattern also resembled the one displayed by the non-comparator mechanism. These findings suggest spatially and temporally distinct brain activities of comparator mechanisms of change detection in the context of speech.

Cortical processing of quasi-periodic versus random noise sounds

The first objective was to confirm using auditory evoked potentials (AEPs) the findings of magnetoencephalographic studies, that quasi-periodic iterated rippled noise (IRN) elicits a population response in the human auditory cortex which is topographically distinct from that elicited by random noise with a similar overall frequency spectrum. AEPs were recorded at the onset of random noise from silence, at the transition from random noise to IRN with a period of 5 ms, and in the two complementary conditions, IRN onset from silence and the transition from IRN to random noise. An N1/P2 complex was recorded to all four stimuli, that to the transition to IRN being significantly the most anteriorly distributed on the scalp. The second objective was to determine whether the response to the transition to IRN was due to detection of its quasi-periodicity, rather than its spectral "ripples". Virtually no effect was found of applying a 2 kHz low- or high-pass filter, above which it is unlikely that the spectral ripples at intervals of 200 Hz would have been resolved on the cochlear partition. It is concluded that a substantial neuronal population in the auditory cortex is influenced by temporal regularity in sounds, and that this population is equally responsive to spectral frequencies below and above 2 kHz.

The ‘F-complex’ and MMN tap different aspects of deviance

OBJECTIVE

To compare the 'F(fusion)-complex' with the Mismatch negativity (MMN), both components associated with automatic detection of changes in the acoustic stimulus flow.

METHODS

Ten right-handed adult native Hebrew speakers discriminated vowel-consonant-vowel (V-C-V) sequences /ada/ (deviant) and /aga/ (standard) in an active auditory 'Oddball' task, and the brain potentials associated with performance of the task were recorded from 21 electrodes. Stimuli were generated by fusing the acoustic elements of the V-C-V sequences as follows: base was always presented in front of the subject, and formant transitions were presented to the front, left or right in a virtual reality room. An illusion of a lateralized echo (duplex sensation) accompanied base fusion with the lateralized formant locations. Source current density estimates were derived for the net response to the fusion of the speech elements (F-complex) and for the MMN, using low-resolution electromagnetic tomography (LORETA). Statistical non-parametric mapping was used to estimate the current density differences between the brain sources of the F-complex and the MMN.

RESULTS

Occipito-parietal regions and prefrontal regions were associated with the F-complex in all formant locations, whereas the vicinity of the supratemporal plane was bilaterally associated with the MMN, but only in case of front-fusion (no duplex effect).

CONCLUSIONS

MMN is sensitive to the novelty of the auditory object in relation to other stimuli in a sequence, whereas the F-complex is sensitive to the acoustic features of the auditory object and reflects a process of matching them with target categories.

SIGNIFICANCE

The F-complex and MMN reflect different aspects of auditory processing in a stimulus-rich and changing environment: content analysis of the stimulus and novelty detection, respectively.

Spectral processing and sound source determination

Humans normally listen in mixed environments, in which sounds originating from more than one source overlap in time and in frequency. The auditory system is able to extract information specific to the individual sources that contribute to the composite signal and process the information for each source separately; this is called “auditory scene analysis” or “sound-source determination.” Sounds that are simultaneously present but generated independently tend to differ along relatively simple acoustic dimensions. These dimensions may be temporal, as when sounds begin or end asynchronously, or spectral, as when the sounds have different fundamental frequencies. Psychophysical experiments have identified some of the ways in which human listeners use these dimensions to isolate sources of sound. A simple but useful stimulus, a harmonic complex tone with or without a mistuned component, can be used for parametric investigation of the processing of spectral structure. This “mistuned tone” stimulus has been used in several psychophysical experiments, and more recently in studies that specifically address the neural mechanisms that underlie segregation based on harmonicity. Studies of the responses of single neurons in the chinchilla auditory system to mistuned tones are reviewed here in detail. The results of those experiments support the view that neurons in the inferior colliculus (IC) exhibit responses to mistuned tones that are larger and temporally more complex than the same neurons’ responses to harmonic tones. Mistuning does not produce comparable changes in the discharge patterns of auditory nerve (AN) fibers, indicating that a major transformation in the neural representation of harmonic structure occurs in the auditory brainstem. The brainstem processing that accomplishes this transformation may contribute to the segregation of competing sounds and ultimately to the identification of sound sources.