Data were recorded during behavior and pupillometry from 129 units in A1 and 66 units in IC. For one supplementary analysis (Figure 7—figure supplement 1), additional data were collected during behavior but without pupillometry (IC: 135 additional units, A1: 125 additional units). After animals demonstrated consistent above-chance performance (target versus reference d’ > 1, see below), a small craniotomy was opened to access either A1 or IC. Extracellular neuronal activity was recorded in non-anesthetized ferrets either using a tetrode (Thomas Recording Inc) or a linear 64-channel silicon probe (Shobe et al., 2015). The impedance of the tetrode was measured to be 1–2 MOhm, and the 64-channel probe was electroplated to reach a 0.7-MOhm impedance in each channel. The tetrode and probe were inserted using a motorized electrode positioning system (Alpha-Omega).

Amplified (tetrodes, AM Systems; arrays, Intan) and digitized (National Instruments) electrophysiological signals were stored using the open-source data acquisition software MANTA (Englitz et al., 2013) or Open Ephys (Black et al., 2017). Recording sites in A1 and IC were initially targeted stereotactically (Moore et al., 1983; Shamma et al., 1993). Sites in A1 were confirmed based on tonotopy—which changes from high to low BF as one moves from dorso-medial to ventro-lateral regions—and relatively reliable and simple frequency tuning (Shamma et al., 1993). Recording sites in the IC were classified based on tonotopy and response properties (Aitkin et al., 1975; Moore et al., 1983). Neurons in the central nucleus of the IC (here referred to as ICC) receive input from the auditory brainstem, and have a characteristic short response latency, dorsal-ventral tonotopy, and narrow bandwidth tuning. Conversely, regions around the central nucleus do not receive direct ascending input and present longer response latencies, considerably less sharp tuning, lack consistent tonotopic organization; these areas were grouped together as NCIC (Slee and David, 2015).

For tetrode recordings, upon unit isolation, a series of brief (100 ms duration, 200–400 ms inter-stimulus interval, 50 dB SPL) tones and/or narrowband noise bursts were used to determine the range of frequencies that evoked the strongest response, the BF of the unit(s). If a unit(s) in the site did not respond to the sound stimuli (an evoked increase or decrease in activity compared to spontaneous activity either during or after sound presentation), the electrode was moved to a new recording depth with small, 5 µm incremental steps. For the 64-channel recordings, we lowered the probe so that its entire extent (1 mm) spanned the target region (A1 or IC).

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