Emily Chapman and Dr. David McPherson, Audiology and Speech Language Pathology
Discussion of Results Our objective in this research was to study the categorization of speech sounds (phonemes) in normal subjects and subjects with disorders of auditory discrimination. The hypothesis we set forward was that there would be an evident difference in the MMN elicited from the normal subjects and the subjects with disorders in auditory discrimination.
Thirty-two electrodes were placed on the scalp of each subject using an electrode cap and the ongoing EEG was recorded. The acoustic stimuli consisted of the phonemes /ba/, /da/, and /pa/. Each phoneme was contrasted against a pair (/ba/- /da/, and /ba/- /pa/). The stimuli were presented at a comfortable level to both ears under earphones. The sample time consisted of 1500 ms with an interstimulus interval of 800 ms. the auditory stimuli were presented in random order and the EEG streamed onto computer for off-line analysis. The series was repeated twice.
The tables following the report summary include the results of this study after the statistical analysis.
Table 1 shows the mean and standard deviation ranges for normal listeners and the value obtained under the same conditions for the hearing impaired (H.I.) individual. It should be noted that due to the large difference in the RMS value of the N400, it is evident that the hearing impaired individual does not distinguish the /pa/-/ba/ contrast as well as the normal group.
However Table 2, depicting the /ba/ – /da/ contrast, does not show any difference between the two groups. Thus suggesting that the /ba/ – /da/ contrast is easier to process. This is probably due to the fact that the phonation of /pa/ and /ba/ have less similar voice onset times than the /ba/ – /da/ contrast.
The results of this experiment are particularly interesting as they result to speech reading where previously recognizing the /pa/ – /ba/ contrast in hearing impaired individuals was thought to be due to the lack of distinguishing the visual representation for the articulation of /pa/ and /ba/ versus the visual difference in articulation for the /ba/ – /da/ contrast. Therefore, it is important that a larger study focuses on both the visual, auditory, as well as the visual-auditory of the N400 in the categorization of phonemes.