RESUMEN
We argue that the mismatch negativity (MMN), elicited by rare auditory events, is generated by the same neural mechanisms as the N100, elicited by any audible stimulus. To date, the MMN has been considered to be a unique index of auditory sensory memory and a component which is functionally and spatially separate from the N100 response because: (i) MMN and N100 appear to have different generator locations; (ii) the MMN occurs too late to be an N100; (iii) the MMN, as opposed to the N100, is elicited by stimulus omissions. By utilizing neural modeling and EEG/MEG results, we show that the above reasoning relies on unwarranted assumptions and propose that the MMN is, essentially, an amplitude- and latency-modulated N100 response. This study offers a physiologically constrained and theoretically plausible framework whereby brain dynamics in terms of stimulus feature maps and their reorganization may be used to describe various memory- and learning-related effects of human auditory cognition.
Asunto(s)
Potenciales Evocados Auditivos/fisiología , Magnetoencefalografía/métodos , Modelos BiológicosRESUMEN
We studied the cortical activation underlying perception of variations in speech fundamental frequency (F0) as indexed by the amplitude, latency and source location of the auditory N100m response registered with magnetoencephalography (MEG). Ten subjects were presented with Finnish vowels with either a constant or an ascending/descending F0. We found that the human auditory cortex is sensitive to these time-varying changes in the F0 of speech: vowels with a constant F0 elicited more prominent N100m responses than did vowels with ascending or descending changes in F0. These results suggest that the speech-related behavior of the N100m arises out of cortical sensitivity to variations in the F0 and its harmonics which underlie the perception of pitch and intonation. The present observations are interpreted in terms of the interrelatedness of speech production and perception.
Asunto(s)
Estimulación Acústica/métodos , Corteza Auditiva/fisiología , Acústica del Lenguaje , Percepción del Habla/fisiología , Adulto , Análisis de Varianza , Femenino , Humanos , MasculinoRESUMEN
Brain processes phase-locked to stimuli can be readily observed with electro- and magnetoencephalography (EEG & MEG, respectively) using stimulus-triggered averaging of the measured signal. The detection of non-phase-locked brain processes depends on the method used for analyzing the unaveraged data. Here we introduce a technique, partition-referenced moment (PRM) power spectrum, which uses established spectral estimation algorithms but yields a power spectrum with sharp, easily distinguishable peaks in an otherwise level spectrum even when the signal (such as EEG & MEG) is of the one-over-frequency-slope type. Employing this method and wavelet transforms, we show that transient auditory brain responses are followed by dispersed small-magnitude power reductions. Power reductions occurred around 400-600 ms and were specific to ongoing 10 Hz-oscillations. The PRM-method also indicated ongoing oscillations in the 15-30 Hz frequency range where power reductions occurred at around 200-400 ms. Thus, the presented methods enable the straightforward detection of ongoing brain oscillations and their association with event-related power changes.
Asunto(s)
Estimulación Acústica/métodos , Percepción Auditiva/fisiología , Potenciales Evocados Auditivos/fisiología , Magnetoencefalografía/métodos , Tiempo de Reacción/fisiología , HumanosRESUMEN
Voiced speech is created by the fluctuating vocal folds generating the glottal pulseform. This excitation signal is the source of the speech fundamental frequency and its harmonic integer multiples. Periodic glottal excitation is required for the elicitation of speech-specific cortical processes indexed by the auditory N100m response. Here, we studied the cortical processing underlying the perception of the vowels /a/ and /u/ produced using normal and aperiodic phonation. The behavior of the N100m, registered with magnetoencephalography (MEG), was studied in 10 subjects. The amplitude and latency of the N100m as well as the center of gravity of the activated cortical areas varied as a function of stimulus periodicity. Further, the presence of glottal excitation had differential effects on the latency of the N100m elicited by the vowels /a/ and /u/. Thus, changes affecting the perceptual quality of speech signals without changing their phonetic content modify the dynamics of human auditory cortex.
Asunto(s)
Estimulación Acústica/métodos , Potenciales Evocados Auditivos/fisiología , Glotis , Magnetoencefalografía/métodos , Percepción del Habla/fisiología , Adulto , Corteza Auditiva/fisiología , Femenino , Humanos , Masculino , Fonética , Tiempo de Reacción/fisiologíaRESUMEN
We studied the cortical processing of spatial stimuli by magnetoencephalographic (MEG) measurements using broadband noise bursts presented from eight sound source directions in the horizontal plane. The stimuli were individually created for each subject by using three-dimensional (3D) sound techniques. The subjects carried out a behavioral task where their accuracy for localizing the 3D stimuli was established. We found that the auditory N100m response was sensitive to the sound source direction, exhibiting contralaterally more preponderant responses in both the left and the right hemisphere. Generally, responses were more prominent in the right hemisphere. The behavioral performance of the subjects correlated positively with N100m amplitude organization, showing that the dynamics of auditory cortex predict behavioral sound detection.
Asunto(s)
Estimulación Acústica/métodos , Corteza Auditiva/fisiología , Magnetoencefalografía/métodos , Desempeño Psicomotor/fisiología , Localización de Sonidos/fisiología , Lateralidad Funcional/fisiología , HumanosRESUMEN
The generation mechanism of stimulus-evoked electro- and magnetoencephalographic (EEG & MEG) responses has remained controversial. One view holds that evoked responses are independent components, additive to ongoing brain activity. The other view holds that evoked responses are generated via stimulus-induced phase reorganization of ongoing brain activity. This issue has been commonly addressed with signal processing techniques that assume a high level of stationarity (i.e., unchanging properties over time) of the measured signal. Here we used signal analysis methods suitable for analyzing non-stationary signals. We found that auditory stimulation leads to a large power increase of the poststimulus signal compared to prestimulus level. Linear superposition of the (time-domain) averaged response and the unaveraged prestimulus signal accounted for 90% of the power increase. Further, we found that auditory stimulation does not lead to a phase-coherent state of ongoing oscillations. Taken together our results show that auditory evoked responses are directly additive to ongoing oscillations and only 10% of the observed power increases are explained by non-phase-locked brain activity. When examining evoked brain activity with methods providing simultaneous frequency and time information, emphasizing temporal accuracy is likely to provide more accurate descriptions of non-stationary processes of the human brain.
Asunto(s)
Estimulación Acústica/métodos , Relojes Biológicos/fisiología , Encéfalo/fisiología , Potenciales Evocados Auditivos/fisiología , Magnetoencefalografía/métodos , Electroencefalografía/métodos , HumanosRESUMEN
We consider the neural dynamics underlying auditory streaming, the perceptual grouping of transient auditory events, by using neural modeling and magnetoencephalographic (MEG) measurements in humans. We demonstrate that spatial variations in the strength of feedback inhibition leads to differential amplitude modulation (AM) tuning resembling that found in animal models. In our model, neurons respond selectively to stimuli presented at different onset-to-onset interstimulus intervals (ISIs), and their summed activity (corresponding to the MEG signal) exhibits both transient and sustained responses (SRs) at fast ISIs. In MEG measurements utilizing 2-s trains of 50-ms stimuli presented at 0-1950 ms ISIs, we observed the transient N100m and SRs predicted by the model, with a prominent SR emerging for discrete stimuli at ISIs below 200 ms. Our results explain why, at fast stimulus rates, the amplitude of the auditory N100m appears to be strongly attenuated even though auditory cortex continues to respond vigorously to the stimuli. The results suggest that the longer and shorter forms of auditory sensory memory may be reflected in the N100m and the SR, respectively. As the emergence of the SR coincides with the stimuli being perceived as auditory streams, our study suggests that auditory sensory memory as indexed by transient and sustained cortical activity might underlie auditory scene analysis.
Asunto(s)
Estimulación Acústica/métodos , Corteza Auditiva/fisiología , Percepción Auditiva/fisiología , Potenciales Evocados Auditivos/fisiología , Memoria/fisiología , Análisis de Varianza , HumanosRESUMEN
Cortical activity underlying speech perception has been studied mostly by using isolated vowels with constant formant frequencies. Speech, however, is characterized by formant transitions whereby formant frequencies change as a function of time. We used magnetoencephalography (MEG) to investigate cortical activity elicited by isolated vowels and diphthongs containing formant transitions. Ten subjects were presented with two isolated vowels /a/ and /u/ and diphthongs /au/ and /ua/. Stimulus duration was 200 ms, and the diphthongs started and ended with a 50-ms constant-formant period and included a 100-ms linear transition period. Apart from studying the auditory N100m response, we examined subsequent brain activity in a 500-ms poststimulus time window, as the transitions were expected to elicit activity also in later stages of cognitive processing. All the stimuli elicited prominent N100m responses. Thereafter, both the isolated vowels and diphthongs elicited sustained brain activity lasting up to 500 ms. The present observations indicate that identification of the speech sounds as well as changes in their identity are reflected in the auditory N100m. Notably, the stimuli appeared to elicit left-hemispheric activity resembling the N400, typically obtained by using more complicated speech stimuli such as words and sentences.