Date of Award

8-1-1970

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Articles have recently appeared by Erickson (1968) and Pfaff (1969) which propose that the study of the activity of a large population of neurons may contribute information about sensory coding which is supplementary to data obtained from single neuron studies. These authors propose that there are not enough neurons in the primary sensory pathways so that every stimulus can be represented by a separate neuron. They maintain that over-all measures of neuronal activity will be more fruitful in the study of sensory coding. The present study used such a measure (multiple unit techniques) to monitor the over-all discharge characteristics of neuronal pools at the cochlear nucleus and inferior colliculus of cat brains to aural stimulation.

Measures of spike discharge frequencies were obtained from four adult cats. Stimuli consisted of recorded male and female voices presenting the words "one" through "ten". Data was collected from four sites: left and right cochlear nuclei and left and right inferior colliculi. The dependent variable was the number of neuronal spike discharges counted during each 500 millisecond recording period. The basic hypothesis of the experiment was that the large multiple unit spike bursts to aural stimulation consistently observed at.the sites investigated represented a stimulus coding mechanism. If such a mechanism was operating, it was predicted that analysis of discharge patterns would reveal that the subcortical auditory nuclei discharge differentially to the different stimuli. It was also thought that different voices presenting the same stimuli would evoke very similar patterns of activity both within and between anatomical sites tested. The following effects were investigated: laterality of stimulation; stimuli; voices presenting the stimuli; and anatomical sites both within and between cats.

Results clearly indicated that at the level of the cochlear nuclei ipsilateral stimulation evoked a sustained multiple unit spike burst while contralateral stimulation had no observable effect on the on-going neuronal activity. The importance of laterality of stimulation was also clearly confirmed at the inferior colliculi level. Only rarely did ipsilateral stimulation at the inferior collicular level cause any detectable change in the on-going level of neuronal discharge. Contralateral stimulation, however, was consistent in evoking a sustained spike burst.

The experiment demonstrated that the multiple unit spike bursts ! observed at the cochlear nucleus and inferior colliculus have a functional significance. Different stimulus words generated discharge patterns which differed from each other. Different voices presenting identical stimuli also generated responses having a moderate to very high similarity to each other. The spike discharge patterns were highly similar at both cochlear nucleus and inferior colliculus in all experimental animals. There was also a very close association between anatomical sites within each cat.

The results contributed evidence that the original hypothesis of the existence of a neuronal coding mechanism was confirmed. This phenomenon appeared to be a prominent feature of the neuronal response to low frequency complex stimuli through the level of the inferior colliculus. Low frequency.sounds which are coded at the cochlear nerve by frequency of nerve fiber discharge appear to be encoded in the same way by groups of neurons at the dorsal cochlear nucleus and the inferior colliculus. The fact that the encoding mechanism occurs only below the level of the thalamus suggested that pitch discrimination may be essentially completed before the neuronal discharges arrive at the medial geniculate body of the thalamus.

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