Frequency difference limens were determined on two successive days with a median time of 22 h between sessions (range = 18–24 h). On Day 1, one group (n = 7) was given anodal tDCS over right auditory cortex and another group (n = 8) was given sham stimulation over the same region. The subjects were randomly assigned to either tDCS or sham stimulation groups and were blind to the existence of a sham
group until completion of testing. On Day 2, DLFs were determined in the same way as on Day 1 but without any tDCS for either group. DLFs were determined using an adaptive two-interval, two-alternative forced-choice (2I-2AFC) task with a two-down, one-up rule estimating the 70.7% point on the psychometric function (Levitt, 1971). One interval, selected at random, contained a 1000-Hz tone (the standard) and the other interval contained INK 128 chemical structure a tone with a frequency of 1000 + Δf Hz (the comparison). Tones were 100 ms long with 20-ms cosine rise/fall ramps, and were separated by a 500-ms interstimulus interval. The observation intervals were indicated by the numerals ‘1’
and ‘2’, which appeared successively on a computer screen coincident with the observation intervals. Subjects indicated the interval selleck chemicals containing the comparison tone by clicking the left or right button on a mouse to indicate the first or second interval respectively. Response feedback (illumination of a green or red light on the screen) was given immediately after the response. Following Hawkey et al. (2004), the initial frequency increment for the comparison stimulus (Δf) was 200 Hz. For the first six trials in each track, Δf was halved after two correct responses and doubled following an incorrect response; after the sixth trial, Δf was divided by √2 following two correct responses and multiplied by √2 after an incorrect response. Blocks of 180 trials were made up of three interleaved 60-trial tracks, with each track yielding an independent frequency discrimination threshold. Three 180-trial blocks were completed each day, with a self-paced break (typically < 1 min) between successive blocks. DLFs were calculated for each track as the geometric mean of Δf for the
last eight reversals and for each block as the geometric mean of DLFs obtained from each of the Thiamet G three tracks (Hawkey et al., 2004). Response times were measured as the time (in ms) between the onset of the second tone and the response. Median response times were calculated for each track and response times for each block were taken as the geometric mean of the three tracks. Both DLFs and response times were positively skewed and were subject to natural logarithmic transformation for analysis; back-transformed values are reported. All stimuli were presented 20 dB above each subject’s absolute threshold, which was determined immediately before testing each day with a 2I-2AFC procedure using a three-up, one-down rule to estimate a 79.