The representation of concurrent vowels in the cat anesthetized ventral cochlear nucleus: evidence for a periodicity-tagged spectral representation.

Chopper units of the ventral cochlear nucleus (VCN) provide a rare representation of stimulus spectrum and a temporal representation of fundamental frequency (F0). This dual representation may be useful in segregating competing speech sounds, where differences in F0 are a cue. Responses to the vowel portion of concurrently presented pairs of syllables /bV integral/ with different F0's (88, 98, and 112 Hz) were studied in the VCN of anesthetized cats; 11 English vowels were used for V. Vowels were chosen so that one had a formant frequency just above the unit's best frequency (BF) and the other had a formant just below BF. By changing the stimulus sampling rate, formant peaks were shifted relative to the unit's BF, producing a range of stimuli, varying in the relative power of the two vowels within the unit's tuning curve. Results show that units' discharge rates reflect the energy within their tuning curves and the relative synchronization of units' responses to the two F0's favors the dominant vowel. A method of segregating two vowels is provided in which relative synchronization to the F0's is used to apportion discharge rate between the vowels. Best results were obtained in chopper units, although primarylike units showed similar behavior.