Psychoacoustical and Electrophysiological Aspects of Central Auditory Disorder. Summary of the Doctoral Thesis

Abstract

In this thesis work central auditory processing is analyzed by psychoacoustic method — speech audiometry and electrophysiological method — cortical auditory evoked potentials. Implication of electrophysiological measures as objective investigation method and correlating with psychoacoustic method in diagnostics of central auditory processing is evaluated. In this thesis work central auditory processing is analyzed by psychoacoustic method — speech audiometry and electrophysiological method — cortical auditory evoked potentials. Implication of electrophysiological measures as objective investigation method and correlating with psychoacoustic method in diagnostics of central auditory processing is evaluated. In the case of auditory processing disorder people with normal tonal audiometry could complain about hearing disorders. Until now diagnostics of central auditory function in Latvia was restricted — there are no speech tests for speech audiometry in Latvian, moreover, cortical auditory evoked potentials hadn’t investigated. Speech audiometry is recommended to perform in native language. Speech audiometry has some restrictions. It requires active collaboration with patient and the result is influenced by cognitive function. Therefore to evaluate central auditory processing cortical auditory evoked potentials were recorded to investigate the speed of analyses of acoustical signal in the brain cortex. The aim of present thesis was to characterize central auditory function, determine the psychoacoustic and electrophysiological criteria for central auditory disorder, and evaluate effect of medical therapy on the central auditory function in patients with central auditory disorders. One of this thesis work tasks was to create speech material for speech audiometry. Speech tests were approbated in the control group participants with normal hearing. Then created speech tests were used for recognizing speech reception threshold and dichotic speech recognition for patents with auditory disorder. For all of participants were performed cortical auditory evoked potential recordings. Patients received medical therapy, after that cortical auditory evoked potential measures were performed again. In this thesis results of measures of 60 patients with auditory disorders and 30 control group participant were analyzed. In the first stage of the thesis speech tests were created in Latvian — 15 sentence tests, 13 sentences contain every of them, and two types of dichotic speech recognition tests — 13 pairs of dichotic digits and 16 pairs of dichotic words. Speech recognition threshold in masking noise and without noise was determined by sentence tests. The difference between them in controls was under 5 decibels, in patients groups above 10 decibels. Dichotic speech recognition in patient groups was statistically significant lower 47–69 % and right ear advantage 19–25 % in comparison with control group respective 90–97 % and 8 %. Dichotic digit recognition results were higher in all groups that give evidence about the role of cognitive factors in the central auditory processing. Cortical auditory evoked potential’s latencies reflect speed generation excitability by cortex neurons. Cortical auditory evoked potential’s component P1, N1, P2, N2, P3 recordings were statistically significant different. They were dependent from intensity and frequencies of stimuli, presence of masking noise, listening manner — active or passive and position of electrodes on the scalp. While listening signal in noise potential component latencies were prolonged for all subjects, but significantly — in patients groups and for component P3. Results of psychoacoustic and electro-physiological measurements revealed that correlation between latency of component P3 and dichotic speech recognition exists in all participant groups: when P3 latency grows down dichotic digit recognition improves. It confirms that P3 latency as a diagnostic criterion in investigation of central auditory disorders. Based on analyses of ROC curves of the results from control and patient subjects, border values, sensitivity and specificity of cortical auditory evoked potential component latencies were established. The highest sensitivity showed component P3 and N2 latencies. Their border values respectively 304.77 ms (at 95% sensitivity and 99% specifity) and N2 — 220.23 ms (at 90 % sensitivity and 98 % specifity). There are still no known international guidelines for medical treatment of central auditory disorders. Based on previous research that GABA^A, cholinergic and glutamacid receptor dependent neuronal synapses are involved in central auditory processing, medical therapy with nicergolin and pramiracetam, which improves neurometabol processes and speed of impulses in these synapses, was described for patient group participants. After medical therapy significantly were shortened component P3 latencies and improved dichotic speech recognition. Our results suggest that medications beneficially influenced central auditory processing. Our results suggest that as the criteria of central auditory disorders could be considered speech recognition threshold in noise than without noise more higher than 5 decibels, dichotic speech recognition lower than 90 %, right ear advantage more than 8 % and prolonged cortical auditory evoked potential component P3 latency in noise in comparison with recording without noise and prolonged border values of cortical auditory evoked potentials P1, N1, P2, N2. During thesis work created and patented method for diagnosing central auditory processing disorders, using original speech recognition sentence and dichotic speech tests and cortical auditory evoked potentials. The thesis work is written in Latvian. Common value of work is 156 pages, illustrative analytical material in 18 tables, 37 figures and 3 attachments. In the list of references there are 290 titles.