The animals were trained by instrumental conditioning to perform a positively reinforced, tone versus noise discrimination task (ferret L) (Slee and David, 2015) or tone-in-noise detection task (ferrets R, B, and T). During training, animals were provided access to water ad libitum on weekends, but were placed on water restriction during the weekdays (Monday through Friday), allowing them to maintain >90% of their baseline body weight long term. On weekdays, they were given the opportunity to receive liquid reward during behavioral training. Each behavioral trial consisted of a sequence of two to five broadband noise reference sounds (TORCs; 30 samples, five octaves, 0.75 s duration, 0.7 s inter-stimulus interval) (Klein et al., 2000), followed by a target tone, either alone (tone versus noise task) or overlapping with another TORC (tone-in-noise task). Animals reported the occurrence of the target by licking a water spout (Figure 1A). Licks were detected by a piezoelectric sensor glued to the spout. Licks occurring within the target window, 0.1–1.5 s after target onset, were rewarded with one to three drops of a 2:1 solution of water and a high-protein, high-calorie dietary supplement (Ensure). FAs, licks occurring before the target window, and misses, a failure to lick during the target window, resulted in a 5–8 s timeout. Regardless of trial outcome, the next trial began only after animals did not lick for a random period (2.5 ± 0.5 s), reducing the prevalence of accidental, short-latency FAs. The number of TORCs per trial was distributed randomly with a flat hazard function to prevent behavioral timing strategies (Heffner and Heffner, 1995). Behavioral performance was measured using d’, the z-scored difference between hit rate (HR) and FA rate (Green and Swets, 1966).

A behavioral block consisted of 60–100 trials with the same target tone frequency (100–20,000 Hz) and same distribution of target SNR. Animals completed one to three behavioral blocks per day, between which target frequency and/or SNR could change. During training, target frequency was chosen at random to span the audiogram of a ferret (Kelly et al., 1986). During electrophysiological recordings, target tone frequency was selected to match the BF of the recording site.

During the training on the tone-in-noise variant of the task, the TORC sample masking the target varied randomly in each trial to prevent animals from using TORCs’ spectro-temporal features to identify targets. At the beginning of training, the tone was presented at +40 dB SNR (ratio of peak-to-peak amplitude) relative to the TORCs. This difference was gradually reduced over the course of 2–3 weeks until the animal consistently performed above chance (three behavioral blocks with performance yielding d’>1, see below) at 0 dB SNR.

For the tone-in-noise task, the SNR of the tone with respect to the overall level of the TORCs (fixed at 55 or 60 dB SPL depending on the animal) varied between +5 and −20 dB SNR, in 5 dB steps. Each session included five target/noise SNRs. To manipulate task difficulty within each session, the probability of each of the five target/noise SNRs varied, yielding two difficulty conditions: a high SNR (‘easy’) condition in which 60% of the trials the target/noise SNR was either the highest or the second to the highest SNR within the target/noise SNR distribution; and a low SNR (‘hard’) condition in which the two lowest target/noise SNRs occurred in 60% of the trials. For example, for ferret B, the distribution after training was completed and kept between 0 and −20 dB SNR, so that in the easy condition 0 and −5 dB SNR targets would appear 60% of the time, −10 dB SNR 20% of the time, and the remaining −15 and −20 dB SNR would be presented another 20% of the trials. During electrophysiological tone-in-noise experiments, the tone was embedded in a single TORC sample, which also occurred in the reference period. We confirmed that animals were not biased to respond to this TORC exemplar in the reference phase.

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