FIG. 1.

(Color online) Spectra of synthetic vowel stimuli used to study formant-frequency discrimination. (a) Single-formant stimuli from experiment I used for estimation of F1 (top) at F2 (bottom) discrimination thresholds. Solid vertical lines are harmonic frequency components of the standard stimulus (black; thin lines) and a target stimulus with lower formant frequency (red; thick lines). Smooth curves are transfer functions of the Klatt synthesizer, which define the spectral envelope of each stimulus. Vertical dotted lines indicate the formant frequency of each stimulus. The formant frequency of the target stimulus was increased during tracking sessions (rightward pointing arrows) until the test stimulus could no longer be reliably discriminated from the standard. Note that a change in formant frequency alters the relative amplitude of individual harmonic components rather than their frequency. Harmonic frequencies remain unchanged because fundamental frequency (F0; voice pitch) was held constant (200 Hz) in all experiments. (b) Two-formant stimuli from experiment II used for estimation of F1 (top) and F2 (bottom) discrimination thresholds in the presence of a competing stationary formant. (c) Two-formant stimuli from experiment III used to study discrimination thresholds for simultaneously changing formants. The test stimulus shown is from the F1-weighted condition, for which the percent frequency change in F1 (15%) is 2 times the percent frequency change in F2 (7.5%).