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The spontaneous spike activity of the primary afferents was statistically analysed in order to gain insight into its origin and to provide a basis for examining its modulation by sound. The responses to pure tone stimulation were investigated by examining the change in spike rate, degree of synchrony and the phase locking angles, to pressure and velocity stimuli at different stimulus intensities, over a range of frequencies. The 3-dimensional directional characteristics of the units were studied by presenting vibrational stimuli on different axes around the animal. It was found that most spontaneous spike activity showed serial dependence and thus the ISI histogram is an incomplete description of the activity. There is evidence of resonance of the hair cell/primary afferent complex, both from certain patterns of spontaneous spike activity and from the relationship between the driven spike activity and the phase of the stimulus. The phase angle of response to pure tone stimulation increased or decreased as the stimulus intensity increased depending on the whether the stimulus frequency was above or below the best frequency of the unit. It is shown for the first time that cod primary afferent units respond to sound in 3 dimensions.

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Ultrasound (US) has become a standard procedure used during pregnancy to document the health and development of a fetus. When ultrasound was first developed, some researchers urged caution, suggesting that the possibility of hazard should be kept under constant review. Given the routine application of fetal ultrasound imaging, any possibility of deleterious developmental effects resulting from its use is an important public health issue. Rats have a well characterized central nervous system whose neurochemical pathways and neuronal electrophysiology qualitatively correspond to those of humans. Because of this, we opted to use Wistar rats as an animal model to document effects from ultrasound exposure. We exposed one group of rats on prenatal days 15 and 20 for fifteen minutes. A control group was exposed subjected to similar conditions, however no ultrasound exposure was given. A third group was exposed for ten minutes each on post natal days (PND) 2 and 3 while a fourth control group was exposed to the same conditions as group three with no ultrasound exposure. The rats were then watched for developmental delays. When the rats reached the appropriate age, they were given a locomotor task to test for appropriate motor responses. Acoustic startle and prepulse inhibition tests were administered to test for sensorimotor gating, hearing, and motor response. Finally, a brainstem auditory evoke potential test was given to track auditory threshold and appropriate neural firing at various auditory nuclei. Postnatally US exposed rats showed a decreased acoustic startle response and prenatally exposed rats exhibited a speeding up in components of the brainstem auditory evoked potential test.

Thesis (Ph.D.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / This study stems from reports in the experimental literature that selective destruction of portions of the ventral acoustic system of the medulla produced differential effects on sound-based behavior. It has been shown that destruction of the nucleus of the trapezoid body or its afferent fibers resulted in a decrement in the strength of responding which terminated an intense noise signal. whereas auditory threshold to noise was not significantly affected. Bxtensive destruction of all the nuclear and fiber components of the ventral acoustic system raised the threshold substantially and produced a reduction in responding on the noise-termination task. The two problems approached in this thesis were to discover other aspects of audition which would be differentially affected by destruction of selected portions of the ventral acoustic system. and to define precisely the neural damage required to produce substantial shifts in auditory threshold. The subjects were albino rats. The experimental chamber was a wire mesh cage which contained a lever and food cup. A loudspeaker and light source were mounted on the roof of the cage. This apparatus and a pellet dispenser were enclosed in a sound-resistant, light-proof bax. [TRUNCATED] / 2999-01-01

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Once the sound reaches the ear, hearing can no longer be exclusively described as a me- chanical phenomenon of wave propagation. As we follow the auditory pathways deeper into the brain, neuronal action potentials shape our perception of sound. But how exactly do the spectrotemporal characteristics of the sound wave a ect perception? We investigated human auditory perception and decisional behaviour with reverse correlation. This technique yields richer datasets than classical methods based on performance metrics alone, providing classi - cation images (CIs) that display observers' task-dependent strategies while potentially serving as templates for computational modeling. We found that observers use the same strategies to detect peaks and dips in sound pressure on a narrow time scale but rst and second-order CIs reveal di erent temporal dynamics within each strategy. When observers detected a speci c frequency on a similar timescale, we were able to expose signatures of neuronal-like spectrotemporal tuning. Detailed modeling of the results showed that observers were not able to rely on the explicit output of these channels. In auditory motion experiments, CIs presented distinct spectrotemporal dynamics between sounds moving from one side of the observer to the other and sound moving towards or away from the observer. In stark contrast, an artificial detector program returned identical CIs. When stimuli were embedded in fragments of human speech and natural sounds, observers used a knowledge- based strategy; as long as fragments were perceived as meaningful, CIs displayed robust tuning e ects which diminished when speech was presented in a temporally reversed order. Overall, we can conclude that reverse correlation is a powerful tool for probing the human auditory system. It reflects task-dependent strategies imposed by the underlying neuronal circuitry rather than statistics or task speci cation.

Modelling of the mammalian auditory system is valuable in understanding perception processes and has benefits in the design of signal processing systems and human prosthetic implants. However, as models increase in complexity, traditional methods of modelling using general purpose computers become very slow. One method of overcoming this is to use electronic implementations of these models. This thesis looks into the feasibility of auditory system implementations in digital technology, through the implementation of the Four-Stage Pitch System for pitch detection in hearing proposed by Hewitt and Meddis.