Fig. 2.

Validation of in vivo recording site. Following in vivo recordings, 500 μm thalamocortical slices were generated from fixed or fresh brains to validate recording sites (white arrows). Postnatal ages are indicated on top right of each panel. (A) The lesion site ~ 3.4 mm from the rostral end, corresponding to the region in fresh slices that respond to the stimulation of the auditory thalamus, MG, and thus ACx. This slice was derived from the animal recordings shown in Fig. 1. For anatomical landmarks, see B. (B) The lesion site indicates the lesion is ~ 1.4 mm from the rostral end, and therefore possibly somatosensory. MG: medial geniculate body, ACx: auditory cortex, H: hippocampus, Peri: perirhinal cortex. (C) The lesion site is 3.3 mm from the rostral end indicates a more temporal and deeper location and correspond to coordinates of thalamorecipient ACx as follows. (D). The lesion site 5.5 mm indicates a caudal location for the recording site, corresponding to the perirhinal cortex. This lesion had opened up appearing as a wedge during vibratome sectioning, and thus appears deeply cut. (E, F) Photomicrographs of thalamocortical slices from which coordinates of thalamorecipient ACx were measured. When the auditory thalamus (MG) was stimulated extracellularly (bipolar stimulating electrode immediately rostral to the MG at the emerging afferent fibers, blue arrows in the recordings indicate stimulus artifact, 5 mA/500 μS), brief field potentials are recorded, validating that the site of recordings was thalamorecipient ACx. Such responses are shown at the bottom panels of E and F where MG-evoked field responses are shown (blue traces). Note the several milliseconds time delay between the stimulus artifact and the onset of inward response. Recording sites were chosen 1 mm apart to illustrate the approximate caudorostral dimension of ACx. The gray traces represent failed synaptic transmission when subthreshold stimulation at lower intensities (100- 200 μA/100-200 μS duration, blue arrows indicate stimulus artifacts) did not elicit any detectable response. G and H are images of brain slices from 2 pups in which in which < 3 min in vivo recordings were first performed (Fig. 1B and C). Pups were then anesthetized and brain slices made immediately and taken to the recording chamber. When the auditory thalamus (MG) was stimulated field responses could be recorded in areas (asterisks) adjacent to the lesion (< 500 μm) created during the in vivo recordings (white arrows), confirming the in vivo recordings were from thalamorecipient auditory cortex (ACx). The bottom 2 traces show thalamically-evoked responses. Blue arrows indicate stimulus artifacts.