All procedures were approved by Vanderbilt and Harvard Animal Care and Use Committees and followed the guidelines established by the National Institutes of Health for the care and use of laboratory animals. Female C57BL6 mice aged 4-7 weeks were brought to a surgical plane of anesthesia using a combination of pentobarbital sodium (50 mg/kg followed by 10-15 mg/kg supplements as needed) and chlorprothixene (0.2 mg). Multiunit responses were recorded from the middle cortical layers of AI (420-440 μm from pial surface) with epoxylite-coated tungsten microelectrodes (2.0 MΩ at 1 kHz, FHC) and from MGB with 16-channel silicon probes (177 μm2 contact area, 50 μm inter-contact separation, Neuronexus Technologies). Frequency response areas (FRAs) were measured with pseudo-randomly presented tone pips of variable frequency (5.5 to 45.3 kHz in 0.1 octave increments, 20 ms duration, 5 ms raised cosine onset/offset ramps, 600 ms intertrial interval) and level (0-60 dB SPL in 5 dB increments) delivered from a free field electrostatic speaker placed 12 cm from the contralateral ear.
Auditory core fields (AI and AAF) were identified by an unmistakable caudal-to-rostral mirror-reversal in tonotopy bounded by sites that with poor or abruptly-shifted frequency tuning. The tonotopic zone amounted to a narrow (< 0.5 mm) swath of cortical tissue with inter-animal variations that could not be predicted by vascular landmarks or position relative to bregma. For MGB recordings, the silicon probe was inserted through the auditory cortex at 15 degrees off the horizontal plane under stereotaxic guidance to match the plane of section used in tracer reconstruction and thalamocortical slice experiments. In order to avoid recording from the dorsal division of the MGB, the probe was initially inserted lateral to the auditory core fields, approximately 3.5 mm caudal to bregma. The ventral edge of the MGB was identified by documenting the most lateral cortical insertion point that yielded driven responses from the MGB, some 2.5 – 3.0 mm from the cortical surface. Reconstruction of lesions and electrode tracks confirmed that this corresponded to the ventral shell of the MGB. To target the MGBv and MGBm, the silicon probe was inserted 0.5 mm medial to this point, a trajectory that reliably corresponded to the center of the MGBv, as evidenced by histologic reconstruction of lesions and electrode tracks.
FRAs were reconstructed across the full rostral-to-caudal extent of the MGB by making successive penetrations rostral and caudal to the starting position (50-100 μm between penetrations), until the recording probe had advanced beyond the caudal and rostral poles of the MGB. FRAs were also compared along the full lateral-to-medial extent of the MGBv and MGBm by documenting variations in response properties across the linear array of contact sites spanning 0.75 mm. In some cases, electrolytic lesions were made at various rostral-caudal positions in the MGB identified with the silicon probe. FRAs were measured at different insertion depths with a tungsten microelectrode and small lesions were made by passing 0.8 μA of current for 12 seconds at one or two points of interest along the lateral-to-medial penetration (e.g. the lateral or medial extremes of tone-driven recording sites or reversals in frequency tuning).
Auditory core fields (AI and AAF) were identified by an unmistakable caudal-to-rostral mirror-reversal in tonotopy bounded by sites that with poor or abruptly-shifted frequency tuning. The tonotopic zone amounted to a narrow (< 0.5 mm) swath of cortical tissue with inter-animal variations that could not be predicted by vascular landmarks or position relative to bregma. For MGB recordings, the silicon probe was inserted through the auditory cortex at 15 degrees off the horizontal plane under stereotaxic guidance to match the plane of section used in tracer reconstruction and thalamocortical slice experiments. In order to avoid recording from the dorsal division of the MGB, the probe was initially inserted lateral to the auditory core fields, approximately 3.5 mm caudal to bregma. The ventral edge of the MGB was identified by documenting the most lateral cortical insertion point that yielded driven responses from the MGB, some 2.5 – 3.0 mm from the cortical surface. Reconstruction of lesions and electrode tracks confirmed that this corresponded to the ventral shell of the MGB. To target the MGBv and MGBm, the silicon probe was inserted 0.5 mm medial to this point, a trajectory that reliably corresponded to the center of the MGBv, as evidenced by histologic reconstruction of lesions and electrode tracks.
FRAs were reconstructed across the full rostral-to-caudal extent of the MGB by making successive penetrations rostral and caudal to the starting position (50-100 μm between penetrations), until the recording probe had advanced beyond the caudal and rostral poles of the MGB. FRAs were also compared along the full lateral-to-medial extent of the MGBv and MGBm by documenting variations in response properties across the linear array of contact sites spanning 0.75 mm. In some cases, electrolytic lesions were made at various rostral-caudal positions in the MGB identified with the silicon probe. FRAs were measured at different insertion depths with a tungsten microelectrode and small lesions were made by passing 0.8 μA of current for 12 seconds at one or two points of interest along the lateral-to-medial penetration (e.g. the lateral or medial extremes of tone-driven recording sites or reversals in frequency tuning).
