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Fig. 2: Psychometric functions for pitch height. The psychometric function shows the dependence of the proportion of correct responses on the size of the change in pitch height. Psychometric functions for three subjects were derived from a two-interval, two-alternative, forced-choice experiment in which subjects were asked to detect ‘which note is higher?’ The ordinate shows the proportion of correct subject responses, where 50% is equivalent to performance at chance; the abscissa shows the attenuation of odd harmonics in the test stimulus relative to baseline intensity. The fundamental frequency, f0, was fixed at 80 Hz throughout the experiment (see Methods). The psychometric functions are based on change in attenuation of odd harmonics relative to standards in which the odd harmonics have fixed attenuation of 0 dB (left), 6 dB (center) and 12 dB (right). Each data point is based on at least 60 trials. Weibull functions were fitted using maximum likelihood estimation implemented in Matlab (© The Mathworks, Inc [fitting software at http://bootstrap-software.org/psignifit/]). The 75% threshold is defined as the attenuation value at which subjects achieve a score of 75% correct (halfway between chance and ceiling performance): the 75% thresholds and 95% confidence intervals for each threshold shown (+) were derived using bootstrapping with 999 simulations (17,18). In order to obtain 95% confidence intervals, the bootstrapping procedure employed a Monte Carlo method for estimating the variability of the fitted psychometric function. All subjects heard an increase in pitch height from the standard when the odd harmonics were attenuated by approximately 1 dB in the test stimulus, and the pitch height threshold is uniform along the dimension.
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