ERP evaluation of auditory sensory memory systems in adults with intellectual disability.

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.

Distinctive MMN relative to sound types in adults with intellectual disability.

The effect of stimulus characteristics (vowel vs pure tone) upon mismatch negativity (MMN) was compared between adults with intellectual disability and healthy controls. Either vowels (synthesized vowels /e/ and /o/) or pure tones (1940 and 851 Hz corresponding to the F2 frequencies of /e/ and /o/, respectively) were presented using an oddball procedure. Both groups showed identical results in latency (vowel MMN>pure tone MMN) and less amplitudes for vowels. However, the disabled group demonstrated amplitude attenuation regardless of the stimulus type, although the vowel MMN amplitude showed a descending trend with age in both groups. These results suggest that auditory sensory memory in intellectual disability might have an insufficient capacity yet share a property common to controls.

Asymmetrical mismatch negativity in humans as determined by phonetic but not physical difference.

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.