Methods
10 healthy subjects were trained to discriminate deviant tones presented with a slightly higher pitch than a standard tone, using a Go/No Go paradigm. To ensure that discrimination was performed at equivalent levels of performance for each subject, individual psychometric curves were assessed beforehand using a two-step psychoacoustic procedure. The auditory stimuli were harmonic complexes with a fundamental frequency (f 0) and 20 harmonics (3dB/octave roll-off in the spectrum level), 300 ms duration (rise/fall time = 80 ms), presented through earphones at a level of 75 dB SPL and with a 1 sec inter-onset interval. Each subject underwent an anatomical MRI scan for coregistration purposes, and 12 independent PET scans, 2 during each of 6 conditions: Rest (silent rest, eyes closed); Passive Listening (attending to the standard tones); Pitch Discrimination at 4 different performance levels (mentally detecting deviant tones slightly higher in pitch among these standard tones). To control for motor-related confounds subjects were required to respond covertly. However, they were required to indicate subjective pitch discriminability at the end of each scan involving a Pitch Discrimination condition. Moreover, before and after a scan involving a PD condition, subjects performed a test-retest Pitch Discrimination run of the same performance level as that in the corresponding scan, during which mouse button responses were acquired.