Vowel discrimination in cats: thresholds for the detection of second formant changes in the vowel /epsilon/.

The ability of cats to discriminate changes in the second formant of the vowel /epsilon/ was examined across a range of stimulus levels. Cats were trained to press and hold down a lever to produce a pulsed train of the standard vowel /epsilon/, and to release the lever only when a variant of [epsilon] occurred. Six synthetic variants of /epsilon/ had the same first and third formants (F1 and F3), but with the second formant (F2) located between 1700 and 2000 Hz. All stimuli were tested at levels of 10, 30, 50, and 70 dB SPL. Average difference thresholds for changes in F2 (delta F2) of the vowel /epsilon/ ranged from 87 to 36 Hz across levels of 10 to 70 dB SPL, and were only slightly above those of humans. Further, the delta F2 values were lower than pure-tone delta F values in the same frequency range, whereas humans exhibit higher delta F2 values than pure-tone delta F values in the same frequency range. Changes in the second formant in a negative direction (downward F2 shifts) were also found to be more difficult to detect than upward F2 shifts. These results suggest that, compared to pure tones, cats are better able to discriminate small changes in more complex, vowel-like stimuli.