Neurophysiological Recording of Auditory Responses

Methods for neurophysiological recording are as described in Phan et al. (2006 (link)); Phan and Vicario (2010 (link)); Tsoi et al. (2014 (link)) and Bell et al. (2015 (link)). Briefly, the subject underwent surgical preparation for the neural recording prior to the presentation of auditory stimuli used for electrophysiological assessment of evoked responses to sound. Each bird was anesthetized (1.5%–2.0% isoflurane in oxygen), and surgically implanted with a head fixation pin and recording chamber over a craniotomy centered over the auditory forebrain using dental cement (Dentsply Caulk, Milford, DE, USA). A motorized microdrive (Eckhorn, Thomas Recording, Giessen, Germany) was used to advance independently 16 tungsten microelectrodes (quartz platinum/tungsten, impedance: 2–4 MΩ, Thomas Recording) bilaterally into the brain, targeting areas NCM (four electrodes per hemisphere) and the Field L complex (four electrodes per hemisphere; consisting of L1, L2a and L3; referred to as Non-NCM in the text). Both areas were defined prior to electrode placement from fiduciary landmarks centered on the bifurcation of the midsagittal sinus. Additionally, NCM and non-NCM areas were located electrophysiologically by their characteristic response patterns to white noise search stimuli shaped with the amplitude envelope of zebra finch song.