If a representation of an auditory attention channel was present in the auditory cortices but not in the subcortical structures, it would be predicted that the early event-related brain potential (ERP) would disagree with the late ERP in selective attention effects. To examine this idea, the present study recorded the auditory brain stem response (ABR) as an early ERP and also the negative difference, the processing negativity and the irrelevant positive difference waves as late ERPs during dichotic listening. Each participant experienced two dichotic conditions: (i) 500-Hz standard tones to the left ear and 1000-Hz ones to the right ear (L500/R1000), (ii) 1000-Hz standard tones to the left ear and 500-Hz ones to the right ear (L1000/R500). In a control task, participants performed visual detection and ignored auditory stimuli. Although the negative difference and processing negativity were found to be identical between the two dichotic conditions, the ABR demonstrated a significant difference between relevant and irrelevant tasks only for the L500/R1000 condition. A response preference to lower-frequency tones was found for behavioural measures and late ERPs but not for the ABR. These results suggest difficulty in representing attention channels in the auditory brain stem. In addition, a weak effect of dichotic sound combination in behaviours corresponded only with earlier ERPs.
A two-tone oddball procedure was employed to examine the effect of a phonemic category on the mismatch negativity (MMN). One of the stimuli was a phoneme prototype of Japanese /e/, and the other, [e/ö], which was perceived by Japanese participants as showing deviance from typicality but is nonetheless included in the category /e/. As control stimuli, a pair of pure tones (1940 and 1794 Hz), corresponding to the F2 frequencies of /e/ and [e/ö], respectively, was presented within the same oddball procedure. The MMN for deviant [e/ö] revealed greater amplitude than that of deviant /e/, although there was no significant difference in amplitude between the pure tones. The results suggest that a phonemic category determines the auditory sensory memory.
Auditory sensory memory stage can be functionally divided into two subsystems; transient-detector system and permanent feature-detector system (Naatanen, 1992). We assessed these systems in persons with intellectual disability by measuring event-related potentials (ERPs) N1 and mismatch negativity (MMN), which reflect the two auditory subsystems, respectively. Added to these, P3a (an ERP reflecting stage after sensory memory) was evaluated. Either synthesized vowels or simple tones were delivered during a passive oddball paradigm to adults with and without intellectual disability. ERPs were recorded from midline scalp sites (Fz, Cz, and Pz). Relative to control group, participants with the disability exhibited greater N1 latency and less MMN amplitude. The results for N1 amplitude and MMN latency were basically comparable between both groups. IQ scores in participants with the disability revealed no significant relation with N1 and MMN measures, whereas the IQ scores tended to increase significantly as P3a latency reduced. These outcomes suggest that persons with intellectual disability might own discrete malfunctions for the two detector systems in auditory sensory-memory stage. Moreover, the processes following sensory memory might be partly related to a determinant of mental development.
As determinants facilitating attention-related modulation of the auditory brainstem response (ABR), two experimental factors were examined: (i) auditory discrimination; and (ii) contralateral masking intensity. Tone pips at 80 dB sound pressure level were presented to the left ear via either single-tone exposures or oddball exposures, whereas white noise was delivered continuously to the right ear at variable intensities (none--80 dB sound pressure level). Participants each conducted two tasks during stimulation, either reading a book (ignoring task) or detecting target tones (attentive task). Task-related modulation within the ABR range was found only during oddball exposures at contralateral masking intensities greater than or equal to 60 dB. Attention-related modulation of ABR can thus be detected reliably during auditory discrimination under contralateral masking of sufficient intensity.
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