Figure 5. Enhanced A1 responses at long delays following L6 CT activation can be attributed to short-term dynamics in thalamic sound processing.

(A) Coronal sections showing mCherry expression in auditory L6 CT neurons and medial geniculate body (MGB) axon terminals (top) as well as a more rostral section showing the L6 CT axon bundle in the internal capsule (ic) and collaterals in the thalamic reticular nucleus (TRN, bottom). (B) Schematic of procedure for simultaneous recordings of the A1 column and either MGB or TRN in awake, head-fixed mice. The ventral, medial and dorsal subdivisions of the MGB are illustrated (v, m and d, respectively). Parameters for L6 CT activation laser were a 50 ms duration laser pulse, 5 mW above threshold. (C) Frequency response areas (FRAs) from simultaneously recorded A1/MGB or A1/TRN sites. Recordings were topographically aligned such that frequency tuning was roughly matched between cortical and thalamic sites. (D) Iso-intensity frequency tuning functions from representative L4, MGB and TRN recording sites. The gray and blue functions correspond to the tone-alone and tone + laser conditions, respectively. Enhanced auditory responses are observed in both A1 and thalamus when tones and L6 CT activation are concurrent (orange). Divisive suppression is found in A1 shortly after L6 CT deactivation but not in either thalamic recording site (purple). Multiplicative enhancement is observed in A1 and MGB at long delays following L6 CT deactivation (green). TRN tuning is suppressed at this interval. (E) Mean (±1 SEM) tone-evoked firing rates normalized to the best frequency in the tone-alone condition (gray). Compared to tone-alone responses, firing rates were significantly elevated with concurrent L6 CT activation in A1, MGB and TRN units (paired t-test, p < 0.05); during the short delay period, A1 and TRN units showed significantly reduced firing rates (paired t-test, p < 0.05) while MGB units showed unchanged firing rates (p > 0.05); during the long delay period, A1 and MGB units showed significantly enhanced firing rates (p < 0.05) while TRN units showed significantly reduce firing rates (p < 0.05). (F) Mean (±1 SEM) shifting and scaling modulation computed for each multiunit site with the paired recording approach. Statistically significant shifting and scaling modulation for all permutations of laser duration, response period and brain structure was determined with one-sample t-tests against a population mean of 0 (shifting modulation) or 1.0 (scaling modulation). We observed significant additive gain during L6 CT activation in L2–5 and MGB and significant divisive gain in L2–5 (p < 0.05 for each). In the short-delay period following L6 CT deactivation, we observed significant subtractive and divisive gain only in L2–5 (p < 0.05 for both). In the long-delay period following L6 CT deactivation, we observed significant additive gain in MGB and L2–5, significant multiplicative gain in L2–5 and MGB, and significant subtractive and divisive gain in TRN (p < 0.05 for each).