Central auditory processing is complex and can not be completely evaluated by a single method. This thesis focuses on assessment of some aspects of central auditory functions by the use of dichotic speech tests, cognitive tests that tax functions important for speech processing and electrophysiological tests.
Paper A deals with the cognitive effects in dichotic speech testing in elderly hearing impaired subjects. Dichotic speech tests and cognitive tests were performed on 30 hearing impaired subjects ranging in age from 42 to 84 years. The dichotic test material consisted of digits, short sentences and consonant-vowel-syllables. The cognitive test material comprised tests for short-term memory, verbal information processing and phonological processing. Effects of age in dichotic speech tests in elderly were verified. The degree of effect was dependent upon focusing condition and test material. The different listening tasks in the dichotic tests put different demands on cognitive ability, shown by a varying degree of correlation between cognitive functions and dichotic test parameters. Age-related cognitive decline in the elderly was strongly connected with problems to perceive stimuli presented to the left ear.
Paper B presents a new cognitive test battery, performed in text, auditory and audiovisual modalities. The tests are sensitive for functions important for speech processing and understanding. The test battery was evaluated in four groups, differing in age and hearing status. Each group comprised twelve subjects. The effect of low-level noise on cognitive functions was investigated. The only significant result in the text modality was an age effect in speed of performance, which was also seen in the auditory and audiovisual modalities. In the auditory and audiovisual modalities effects of hearing status and modality were seen in accuracy parameters. Interaction between hearing status and modality, both in accuracy and speed of performance, shows that hearing-impaired subjects benefit more from visual cues. In noise generally poorer results were obtained, especially in the auditory modality and for the elderly, affecting both accuracy and speed. The test battery has proven to be useful in assessing the relative contribution of different input-modalities and the effect of age, hearing-impairment and visual contribution on functions important for speech processing.
Paper C describes the development of a system for recording of auditory evoked potentials. The system is built around a personal computer with a data-acquisition board with analogue input and output channels. Acoustic stimuli are presented to the test subject and responses to the acoustic stimuli are recorded in an EEG-recording system. The program is written in the graphical programming environment LabView (National Instruments). With the system it is possible to perform all clinical standard measurements. In addition, test-procedures not available in commercial auditory evoked potential systems have been developed. For the assessment of auditory discrimination a toolkit for evaluation of the mismatch negativity potential was developed and evaluated in six normal-hearing subjects.
Linköping: Linköpings University Electronic Press , 2001. , 22 p.