Psychophysical Study of Central Auditory Processing with Peripheral and Neural Deficits
The present dissertation includes three studies collectively investigating perceptual consequences of peripheral and neural deficits in central auditory processes occurring in the ascending pathway from cochlea to cortex. Chapter 1 describes a study of how electrical stimulation delivered from a cochlear implant affects processing of co-varying frequency (spectral-variance) and temporal cues. We hypothesized post-lingually deaf cochlear implant listeners retain central processing abilities, which are hampered by degraded peripheral, cochlear inputs. In eight cochlear implant listeners, we measured auditory nerve compound action potentials to estimate peripheral filters, quantifying implant-induced spread of current and resultant spread of neural excitation. Then, we measured psychophysical detection thresholds in the presence of multi-electrode maskers placed either inside or outside the peripheral filter to determine peripheral and central contributions on processing. Results from actual and simulated implant listening support the hypothesis broad peripheral filters greatly limit central processing of spectral-variance, but not of temporal cues. Chapter 2 reports four experiments investigating effects of nicotine (6 mg) gum on auditory tasks with varying attentional demand in healthy, normal hearing subjects. Lack of drug effects on central gain (tone-in-noise detection), temporal acuity (auditory gap detection), frequency resolution (spectral ripple discrimination), or auditory discrimination (attended listening) seem largely due to ceiling performance. Variability in the most demanding gap condition just reaching a significant decrease with nicotine could reflect improved temporal summation. Presumably, low task demand and maximum individual baseline attentional processing limit nicotine effects. Correlation analyses propose separability of attention and auditory processes and dissociable gap detection mechanisms. Chapter 3 describes a study in auditory neuropathy— disorders typically disrupting synaptic encoding and/or neural transmission of auditory signals in the cochlea and auditory nerve– differentiating temporal gap processes. The data seemingly support our hypothesis desynchronized neural discharges and/or reduced neural input limit peripheral temporal acuity but not central temporal acuity. Comparing subjects with disrupted auditory nerve activity with control subjects, a significant gap delay on the order of tens of milliseconds and insignificant delay on the order of hundreds of milliseconds may differentiate peripherally- and centrally-based temporal processing, respectively. We conclude by summarizing results and exploring future research for medical interventions.