Flexible Sensory Representations in Auditory Cortex Driven by Behavioral Relevance

Abstract

Animals require the ability to ignore sensory stimuli that have no consequence yet respond to the same stimuli when they become useful. However, the brain circuits that govern this flexibility in sensory processing are not well understood. Here we show in mouse primary auditory cortex (A1) that daily passive sound exposure causes a long-lasting reduction in representations of the experienced sound by layer 2/3 pyramidal cells. This habituation arises locally in A1 and involves an enhancement in inhibition and selective upregulation in the activity of somatostatin-expressing inhibitory neurons (SOM cells). Furthermore, when mice engage in sound-guided behavior, pyramidal cell excitatory responses to habituated sounds are enhanced, whereas SOM cell responses are diminished. Together, our results demonstrate the bidirectional modulation of A1 sensory representations and suggest that SOM cells gate cortical information flow based on the behavioral relevance of the stimulus.

Figure 1. Imaging sensory representations in A1 layer 2/3 pyramidal cells

(A) Top: a viral vector expressing GCaMP6s is injected in auditory cortex of mice heterozygous for GAD2-ires-Cre and Rosa-LSL-tdTomato. Bottom: initial mapping of auditory cortical areas by intrinsic signal imaging is followed by cellular-level two-photon calcium imaging of A1 in awake mice. (B₁) Intrinsic signal imaging from one mouse showing responses to pure tones (3, 10, and 30 kHz) superimposed on the auditory cortex surface vasculature. Three auditory cortical areas (A1, A2, and AAF) are identified based on tonotopic patterns. Red squares indicate the locations of subsequent two-photon imaging in (B₄). (B₂) In vivo two-photon image of GCaMP6s (green) and tdTomato (red) expressing cells in L2/3 of A1 of an awake mouse. (B₃) Responses of a representative L2/3 pyramidal cell to tone pips of 17 different frequencies (columns) at three intensities (rows). Average trace across five trials is shown for each tone and red asterisk indicates the best frequency. (B₄) Activity map showing the best frequency of individual pyramidal cells within fields indicated in (B₁). (C) Top: heat map of inferred spikes for a representative pyramidal cell in response to a seven-second 18.9 kHz tone over 20 trials. Bottom: average trace of inferred spikes across trials. (D) Prolonged tones evoke responses with diverse temporal patterns in awake mice but elicit only transient excitation under anesthesia. Average responses of four pyramidal cells to an 18.9 kHz tone in the awake (black) and anesthetized (green) state. (E) Pyramidal cell responses are more temporally dynamic in the awake state. Left: comparison of the inferred spike rates during the first one second and the last one second of the tone in individual cells in the awake state. Each point represents a single cell (n = 3 mice, 239 cells). Right: the same cells in the anesthetized state. (F) Top: fraction of cells with significant excitatory (Exc) and inhibitory (Inh) responses across mice. NR, non-responding cells. Bottom: fraction of excited and inhibited cells shown separately for individual mice. **p < 0.01. See also Figure S1.

Figure 2. Daily passive sound experience induces a long-lasting habituation of sensory representations

(A) Top: schematic of the protocol for sound experience and chronic imaging. Mice were passively exposed to 5–9 sec tones for 200 trials/day over five consecutive days. Bottom: maps of L2/3 pyramidal cell activity from one animal in response to 18.9 kHz tones show that excitation becomes sparser and inhibition becomes denser after five days of sound experience. Left panel shows map of all imaged pyramidal cells. (B) Average sound-evoked responses from three cells on Day 1 (black) and Day 5 (blue). (C) Top: fraction of cells with significant excitatory and inhibitory responses across mice. Bottom: fraction of cells shown separately for individual mice (n = 8 mice, 519 cells). (D) Average (solid) and SEM (shading) of the change index across days. Each data point represents a block of 50 trials. Excitatory response amplitudes gradually decrease and inhibitory responses increase over days. (E) Daily change index of excitatory responses in individual cells reveals a reduction in the strength of excitation from Day 1 to Day 5 (n = 104 excited cells). Error bar represents SEM. ***p < 0.001. See also Figure S1–S5.

Figure 3. Sound experience causes only a weak and balanced reduction of excitation and inhibition in L4 thalamorecipient cells

(A) Left: canonical cortical circuit diagram. Top right: GCaMP6s targeting approach. Bottom right: in vivo two-photon image of GCaMP6s-expressing L4 neurons. (B) Top: protocol for sound experience and chronic imaging. Bottom: maps of L4 cell responses in one mouse to 13 kHz tones reveal only a small reduction in the fraction of excited and inhibited cells after daily experience. Left panel, all imaged cells. (C) Average sound-evoked responses from three L4 cells on Day 1 (black) and Day 5 (blue). (D) Top: fraction of L4 cells with significant excitatory and inhibitory responses across mice. Bottom: fractions of cells shown separately for individual mice (n = 4 mice, 398 cells). (E) Daily experience causes only a small and balanced decrease in the strength of excitatory and inhibitory responses. Average (solid) and SEM (shading) of change index for blocks of 50-trials plotted across days. (F) Daily change index of excitatory responses in individual cells does not show a significant effect of experience from Day 1 to Day 5 (n = 99 excited cells). Error bar represents SEM. See also Figure S2 and S5.

Figure 4. Passive experience selectively enhances excitatory responses of SOM cells

(A) In vivo two-photon image of GCaMP6s-expressing L2/3 PV cells. (B) PV cell responses to prolonged tones and the effect of experience mirror pyramidal cells. Average sound-evoked responses from four PV cells on Day 1 (black) and Day 5 (blue). (C) Like pyramidal cells, PV cell excitation becomes sparser and inhibition becomes denser after five days of sound experience. Top: fraction of PV cells with significant excitatory and inhibitory responses across mice. Bottom: fraction of cells shown separately for individual mice (n = 5 mice, 174 cells). (D) Daily change index of excitatory responses in individual PV cells shows a strong reduction in excitation from Day 1 to Day 5 (n = 29 excited cells). Error bars represent SEM. (E) In vivo two-photon image of GCaMP6s-expressing L2/3 SOM cells. (F) Sensory experience increases tone-evoked SOM cell activity. Average sound-evoked responses from four SOM cells on Day 1 (black) and Day 5 (blue). (G) Top: fraction of SOM cells with significant excitatory and inhibitory responses across mice. Bottom: fraction of cells shown separately for individual mice (n = 5 mice, 112 cells). (H) Daily change index of excitatory responses in individual SOM cells reveals a significant increase in the strength of excitation from Day 1 to Day 5 (n = 91 excited cells). *p < 0.05, **p < 0.01, ***p < 0.001. See also Figures S2, S4, and S5.

Figure 5. Auditory cortex contributes to a sound offset detection task

(A) Schematic of the task structure. After the inter-trial interval (ITI), a target sound (5–9 second pure tone) is delivered. Licking during a 1-sec answer period immediately following the target offset triggers a water reward. Licks during the last one second of the target sound are false alarms (FA) and terminate the trial. On randomly interleaved trials, the auditory cortex of VGAT-ChR2 mice is illuminated with LED pulses to silence the cortex. (B) Left: learning curve from a representative mouse. Right: lick latency averaged across trials within each day for the same mouse shown on the left panel. Error bars represent SEM. (C) Left: schematic of experimental setup. Right: representative psychometric curves for behavioral performance during LED_off trials (black) and LED_on trials (blue) for one mouse. (D) Psychometric curves averaged across all mice (n = 5 mice) with contralateral silencing shows a significant increase in detection threshold in LED_on trials. (E) Left: schematic of control experiments in the same mice using ipsilateral cortical silencing. Right: psychometric curves for ipsilateral silencing in the same mouse shown in (C). (F) Psychometric curves averaged across all mice with ipsilateral silencing show no change in detection threshold between LED_on trials and LED_off trials. See also Figure S6.

Figure 6. Engagement in sound-guided behavior enhances L2/3 sound representations

(A) Top: schematic showing the protocol for comparing sound representations between behaving and passive blocks within the same day. Bottom: maps of L2/3 pyramidal cell activity during 18.9 kHz tones show that excitation is denser during behavior. (B) Average sound-evoked responses from three cells during behaving (red) and passive (black) conditions. (C) Engagement in the behavioral task causes a significant increase in the fraction of cells with excitatory responses (n = 5 mice, 414 cells). (D) Change index indicates an increase in strength of excitatory response during task engagement compared to the passive condition (n = 31 excited cells). Error bar represents SEM. *p < 0.05. See also Figure S7.

Figure 7. Engagement in sound-guided behavior suppresses evoked SOM cell activity

(A and B) Imaging L4 excitatory cells reveals no effect of task engagement on the fraction of excited or inhibited cells (n = 5 mice, 482 cells). (C) Change index shows no change in excitatory response strength between passive and behaving conditions (n = 105 excited cells). Error bars represent SEM. (D and E) Imaging L2/3 SOM cells indicates a significant decrease in the fraction of cells with tone-evoked excitation during behavior (n = 4 mice, 109 cells). (F) Change index for individual SOM cells shows a marked decrease in the strength of excitatory responses in the behaving vs. passive condition (n = 69 excited cells). (G and H) Sound-guided behavior has no obvious effect on fraction of excited or inhibited PV cells (n = 4 mice, 92 cells; excitation: p = 0.97; inhibition: p = 0.26). (I) Change index shows no change in excitatory response strength between passive and behaving conditions (n = 4 excited cells; p = 0.90). (J) Schematic illustrating the proposed bidirectional modulation of A1 sound representations by SOM cells. Habituation increases SOM cell activity, which leads to decreased sound representations in L2/3 pyramidal cells. In contrast, engagement in sound-guided behavior suppresses SOM cell activity, leading to the enhancement of sound representations in L2/3 pyramidal cells. **p < 0.01. ***p < 0.001.