Schematic of the training apparatus, shown from above. Ferrets made behavioral responses and received water rewards from the three stainless steel water spouts while standing on the aluminum footplate. In the first training stage, sounds were presented from the central loudspeaker (black) as well as the two peripheral loudspeakers (gray). In later training stages and during testing, sounds were presented from the central loudspeaker only.
Discrimination trial schematics. (A) In the first three stages of training, a reference stimulus was repeated until the ferret initiated a trial by activating the central spout. Then, a target stimulus of a different frequency was presented repeatedly until the animal responded. The ferrets’ task was to indicate whether the fundamental frequency of the target sound was higher or lower than that of the reference by activating the right or left peripheral spouts, respectively. In training stages 1 and 2, the stimuli were pure tones, and in stage 3 they were click trains. (B) In training stages 4 and 5 and in testing, a ready signal consisting of a repeating 5000 Hz tone pip was presented to indicate to the animal that a trial could be started by licking the central start spout. The reference and target sounds were each then presented only once, in quick succession. As above, the ferrets’ task was to indicate the direction of pitch change between the reference and target sounds by responding at the correct peripheral spout. The sounds to be discriminated were click trains in training stage 4 and an artificial vowel sound (formant-filtered click trains) in the final training and testing stages.
Power spectra of the artificial vowel stimulus used in this experiment , shown with a fundamental frequency of 350 (left panel), 500 (center panel), and 650 (right panel) Hz.
Performance of one ferret (subject F1) on the discrimination task when the reference vowel had a fundamental frequency of 700 Hz. The percentage of right spout choices is plotted as a function of the fundamental frequency of the target sound (black dots), and the reference F0 is indicated by an open circle plotted at 50% choice probability. The psychometric curve, a fitted cumulative Gaussian distribution function, is also shown (black line).
Psychometric curves describe the performance of four ferrets trained to discriminate the pitch of an artificial vowel. Each panel shows the psychometric functions for a single animal, and the style and grayscale of each psychometric curve correspond to the fundamental frequency of the reference vowel (see legend). The reference F0 for each curve is also indicated by a grayscale circle at 50% choice probability.
(A) Difference limens for pitch of four ferrets across a range of reference F0s. Data for each ferret are displayed with a unique symbol and connected with solid lines. (B) Weber fractions for the four ferrets (F1–F4) across a range of reference F0s, calculated using the difference limens in (A). The average Weber fractions across all ferrets are shown by the asterisks.
Bias of four ferrets on the pitch discrimination task. Biases were calculated as the distances, in hertz, between the 50% right choice probability obtained from the fitted psychometric function and the corresponding reference F0, so that positive values indicate a bias toward the left (“lower”) spout and negative values indicate a bias toward the right (higher) spout. These biases are normalized by the reference F0. Data for different animals are plotted with different symbols, and bias values that are significantly different from zero (99.87% confidence intervals; 0.05 alpha with Bonferroni correction) are shown in gray, with non-significant values in white. The average percentage of bias across all four animals is plotted with asterisks. A dotted line is shown at a bias of zero, for reference.
Performance of two ferrets when the reference roved between two F0 values across trials. Right spout choice probabilities ( deviation, across eight consecutive testing sessions) are plotted across the four reference and target combinations presented in the variable reference testing sessions. This type of testing was carried out with ferret 2 (black bars) and ferret 3 (white bars).
Fitted psychometric curves showing the performance of four ferrets on a pure tone frequency discrimination task. The reference frequency for each curve is coded in grayscale, and the value of this frequency is also indicated by the position of the filled grayscale circles along the -axis.
Performance of four ferrets on a tone discrimination task. (A) Weber fractions of four ferrets across a range of reference F0s, measured with either the artificial vowel (open symbols, solid lines) or with pure tones (filled symbols, dashed lines). Data for each ferret are displayed with a unique symbol, as shown in the legend. (B) Bias of four ferrets on the tone discrimination task. Biases were calculated and are presented as described in Fig. 7.
Weber fractions of four ferrets and five human listeners on the pitch discrimination tasks. Data for each ferret are displayed with black symbols connected by black lines, and human data are displayed with gray symbols connected by gray lines. Weber fractions for each subject were measured using artificial vowels (open symbols) and tones (filled symbols).
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