Following the results of Experiment 1, we conducted two experiments to determine the effects of tDCS on the processes underlying frequency discrimination, place and temporal coding. We first examined
the effects of tDCS on frequency selectivity, a psychophysical measure of place coding, at both 1000 and 2000 Hz. According to place coding theory (Zwicker, 1974), the bandwidth of frequency selectivity determines frequency discrimination, with smaller bandwidths producing smaller DLFs. Psychophysical tuning curves (PTCs) are commonly used to measure frequency selectivity, with wider PTCs indicating broader frequency selectivity (Moore et al., 1984). PTCs were determined at two frequencies to examine frequency-specific effects of tDCS Epacadostat on auditory perception. If tDCS degrades frequency Proteasome inhibitor discrimination by affecting place coding it will be evident in broader PTCs. A within-subjects design was employed with the effects of tDCS on PTCs being assessed in separate sessions where either anodal or sham tDCS stimulation were applied over auditory cortex. A fast method was used to determine PTCs for 1000- and 2000-Hz test tones using the SWPTC program, which quickly and reliably measures frequency selectivity (Sęk et al., 2005; Sęk & Moore, 2011). A fast method
was used rather than a lengthy constant-stimulus method as ethical guidelines recommend tDCS only be delivered for 20 min in a session (Bikson et al., 2009). The fast method allowed each frequency to be assessed in 10 min and was appropriate for the study. Tones were presented 10 dB above each Thymidine kinase subject’s 70.7% absolute threshold, estimated immediately prior to each testing session. The sampled point of the psychometric function for absolute thresholds was changed from Experiment 1 for consistency with previous measures of frequency selectivity (Sęk et al., 2005; Sęk & Moore, 2011). The PTC task required subjects to detect a test tone (with a frequency referred to as fc) in the presence of a narrow-band noise whose center frequency was gradually swept across a range of frequencies.
As frequency selectivity represents the auditory system’s ability to resolve frequencies, noise will interfere with detection only when it cannot be resolved from the test tone. The bandwidth of narrow-band noise was 200 Hz for the 1000-Hz test tone and 320 Hz for 2000-Hz test tone. Simultaneously with presentation of the test tone, which was pulsed on and off for 200 ms (with a 50% duty cycle), the center frequency of the narrow-band noise was swept at a constant rate from 0.5fc to 1.5fc over 5 min. At the start of the procedure the narrow-band noise was presented at 0.5fc at 25 dB above absolute threshold so it was clearly audible. Subjects were required to hold a key down while the tone was audible and release it when it was not. The amplitude of narrow-band noise increased by 2 dB/s if audible and decreased by 2 dB/s if inaudible.