P62 Simulated effects of deafferentation, deefferentation and their interaction.
MAPsim is a new simulator of sensorineural hearing loss that can be used to help differentially diagnose sensorineural pathologies because it is based on the MATLAB auditory periphery (MAP) model. The MAP-predicted sound encoding at the auditory-nerve is decoded back into an acoustic signal that is presented to participants in psychophysical tasks. MAP could predict dynamic range adaptation at the auditory nerve and reveal that efferent reflexes are the primary contributors to such adaptation, the acoustic reflex causing a shift of rate-level functions towards a context level and the medial olivo-cochlear reflex sharpening auditory nerve sensitivity around that level. The simulated effect of general deafferentation on speech intelligibility in noise (SpIN) showed that simple stochastic under-sampling due to deafferentation underestimates the effect of deafferentation. Indeed, under-sampling alone suggested that over 90% of fibers could be knocked out without any appreciable effect on speech reception threshold (SRT). However, deafferentation also causes a reduction of efferent reflexes. The combined under-sampling and efferent signal reduction suggests that SpIN is sensitive to lower levels of deafferentation than previously thought. When testing for the effect of deafferentation on ITD discrimination, threshold elevation appeared more sensitive to under-sampling alone, the added effect of efferent signal reduction being more limited as a result. The simulated effect of deefferentation on SpIN was operationalized with both separate and combined, gradual acoustic reflex reduction and knockout of outer hair cells, such that interaction between efferent reflexes could be investigated. This manipulation suggested that at normal speech levels, one efferent reflex may somewhat compensate for the loss of another. Removing both efferent reflexes led to an SRT elevation (~ 4dB) comparable to that seen in moderately impaired patients. Overall, MAPsim opens the path to the differential diagnosis of sensorineural pathologies because it enables the testing of psychophysical tools in a search for the psychophysical signatures of specific pathologies.