Neuronal discrimination of natural and synthetic vowels in field L of trained mynah birds

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Abstract
The discrimination of single neurons for vowels and vowel components was analyzed in the telencephalic field L which is a layered and tonotopically organized primary auditory projection area in the bird's neostriatum. Among 250 units, 132 (53%) were responsive to at least one out of nine vowels from one German speaker. The distribution of responsiveness to n (one to nine) vowels showed that a maximum of 33 out of the 132 neurons preferred n = one vowel. The mechanisms of vowel selectivity were analyzed with five synthetic vowels composed of two formants F1 and F2 which could be presented separately. Most of the selective units also responded to F1 or F2 of the preferred vowel alone. The suppression of the vowels could be explained by formants which fell into inhibitory ranges of that unit, independently demonstrated by the pure tone response. Other units had several excitatory bands which coincided with the formants of the preferred vowel. In some cases a certain amplitude ratio of F1 versus F2 gave the strongest response. Several qualitative models of excitatory-inhibitory interaction of inputs to field L neurons are presented which explain the described selectivities. It is interesting that the distribution of vowel-selective units relates to the most superficial and most basal layer of field L where units selective for species-specific calls have previously been located in a gallinaceous bird.