6 Hz, SE = 11.4) than 1000-Hz (M = 170. 2 Hz, SE = 13.4) test tone. At 1000 Hz, ERBs were similar in the tDCS and sham stimulation sessions (t6 = 1.15, P = 0.30, Cohen’s d = 0.05). However, tDCS significantly broadened frequency selectivity at 2000 Hz (t6 = 2.80, P = 0.031, Cohen’s d = 1.17). We examined in this experiment the effects of anodal tDCS applied over primary auditory cortex on TFS thresholds, a psychophysical measure relying on temporal coding. Fig. 6 shows TFS thresholds were markedly larger during tDCS mTOR inhibitor than sham stimulation sessions (t5 = 2.72, P = 0.04, Cohen’s d = 0.62). TFS thresholds were consistently greater in the

tDCS than the sham stimulation session with this effect shown in all but one subject. Our hypothesis that increasing excitability of auditory cortex with anodal tDCS would enhance rapid frequency discrimination learning was not supported. Both tDCS and sham stimulation groups showed similar decreases in thresholds with training. We found unexpectedly that tDCS

degraded frequency discrimination, CYC202 in vivo with subjects receiving tDCS stimulation having mean DLFs more than double those receiving sham stimulation. This effect persisted for at least 24 h after stimulation but had dissipated on retesting 2–3 months later. Two follow-up experiments that investigated the source of the tDCS-induced degradation of frequency discrimination almost showed that although tDCS did increase the ERB of the PTCs measured at 2000 Hz, it had no effect at 1000 Hz (the frequency tested in Experiment 1), and that tDCS increased TFS thresholds by ~30%. Together, these results suggest that tDCS degrades frequency discrimination by affecting temporal, rather than place, coding mechanisms. It is unclear why anodal tDCS over auditory cortex did not enhance frequency discrimination learning during stimulation given the many reports that such stimulation over motor

cortex enhances motor learning (Nitsche et al., 2003b; Antal et al., 2004a,b; Reis et al., 2009). It should be noted first the difference between the groups does not appear to be due to sampling error, biasing the allocation of differently hearing subjects. All subjects reported normal hearing and stimulus presentation levels were individually tailored to ensure consistency between subjects. There is additional evidence suggesting all subjects had normal frequency discrimination, as DLFs for all subjects during Block 1 were within normal levels (Moore, 2012) and subjects in both groups improved similarly with training. It is also unlikely the simultaneous degradation of frequency discrimination masked the enhancement of learning, as a previous study (Amitay et al., 2005) has demonstrated that subjects with initially poor frequency discrimination show the greatest improvements. The difference between groups is therefore likely to be a genuine experimental effect.