Dynamics of auditory cortical activity during behavioural engagement and auditory perception

Behavioural engagement improves auditory perception

We used a behavioural training paradigm to control the mode and level of behavioural engagement in a total of 25 rats. Eight of these animals were first trained to nosepoke for food reward following a target tone and to withhold from nosepoking after non-target tones (Fig. 1a,b). We tested animal performance on consecutive blocks of two variants (‘self-initiate d′ and ‘uncued') of this frequency recognition go/no-go task. In the ‘self-initiate d′ variant, rats voluntarily engaged in the task by self-initiating the trials, with tones occurring at 0.5, 1 or 1.5 s after self-initiation (Fig. 1b and Supplementary Movie 1). In the ‘uncued' variant, trials were externally triggered by tone presentation at pseudo-random time intervals between 6 and 10 s (Fig. 1b and Supplementary Movie 2). Except for the mode of initiation, the rest of the behavioural task was identical between the self-initiated and uncued conditions. All stimuli were 0.5–32 kHz pure tones at one octave intervals, presented at 70 dB sound pressure level (SPL) and 100 ms in duration. The target frequency was either 4 or 16 kHz and other frequencies were non-targets.

As expected, animals reliably responded to the target tone and rarely responded to non-target tones during self-initiated trials compared with uncued trials (Fig. 1c left; responses to 4 kHz target increased from 61.6±10.4% during uncued trials to 97.3±1.5% during self-initiated trials in this animal, n=3 behavioural sessions, P=0.02, Student's unpaired two-tailed t-test; false alarm rate did not change significantly: 8.1±1.8% during uncued trials and 12.3±3.0% during self-initiated trials, P=0.3; d′ increased from 1.7 during uncued trials to 3.1 during self-initiated trials). This led to higher hit rates and improved the d′ discriminability index when the non-target frequencies were 1+ octaves from the target tone on a less-challenging ‘wideband' version of the stimulus set (Fig. 1c, right; hit rates during self-initiated trials: 85.3±3.7% and during uncued trials: 66.0±5.4%, N=8 rats, P=0.003, Student's paired two-tailed t-test; d′ during self-initiated trials: 2.4±0.4 and during uncued trials: 1.4±0.3, N=8 rats, P=0.02, Student's paired two-tailed t-test). Similar enhancements were observed when the target and non-target tones were spectrally closer together on a more-challenging ‘narrowband' version (Fig. 1d; left, example animal, responses to 4 kHz target did not change significantly from 52.1±14.5% during uncued trials to 70.6±15.1% during self-initiated trials, n=3 sessions, P=0.4, Student's unpaired two-tailed t-test, but false alarm rate decreased from 33.3±0.9% during uncued trials to 17.2±2.5% during self-initiated trials, P=0.002; d′ increased from 0.3 during uncued to 0.8 during self-initiated trials; right, summary, hit rates during self-initiated trials: 71.4±4.3% and uncued trials: 57.0±5.2%, N=8 rats, P=0.03, Student's paired two-tailed t-test; d′ during self-initiated trials: 1.0±0.1 and during uncued trials: 0.6±0.1, P=0.01).

To determine whether self-initiation could also improve detection abilities, we varied the amplitude of target and non-target tones between 20 and 80 dB SPL, against the background noise level in the behaviour box (30–40 dB SPL). During self-initiated trials, animals detected the target tones at lower amplitudes than during uncued trials (Fig. 1e; top left panel, example animal: hit rates at 50–60 dB SPL increased from 20.0±20.0% during uncued trials to 85.7±14.3% during self-initiated trials, n=29 trials, P=0.003, two-tailed Fisher's exact test). False alarms remained low at all tone amplitudes, indicating that rats correctly recognized the target from non-target tones even at low sound levels during both self-initiated and uncued trials (Fig. 1e; top right panel, false alarm rates at 50–60 dB SPL for uncued trials: 21.4±7.1% and self-initiated trials: 34.2±0.8%, n=133 trials, P=0.05, two-tailed Fisher's exact test; bottom left panel, d′ values for example animal for 50–60 dB SPL increased from −1.2±1.8 during uncued to 2.6±1.9 during self-initiated trials; bottom right panel, summary d′ values calculated for 50–60 dB SPL increased from 0.2±0.6 during uncued trials to 1.8±0.5 during self-initiated trials, N=4 rats, P=0.02, Student's paired two-tailed t-test).

Although the overall structure of the task and the significance of tones were the same between self-initiated and uncued trials, there could be differences in performance between these two versions related to the position of the rat in the behaviour box at tone onset, or the movement of the rat during the trial and the duration of inter-tone intervals. Therefore, we next controlled for the possible contribution of these parameters to the improved performance during self-initiated trials.

First, we quantified differences in the duration of inter-tone intervals and tone presentation rate between self-initiated and uncued trials; we found no significant differences between the two task variants (Supplementary Fig. 1). This means that during behavioural testing, the total number of trials was similar for self-initiated and uncued sessions.

Next, to better control the sound level irrespective of the position of the animal relative to the speaker, we bilaterally implanted small speakers in the ear canals of four rats (Supplementary Movie 3). Rats with these implanted headphones also had improved performance during self-initiated trials (Fig. 1f; left panel, example performance, hit rate to 4 kHz target increased from 34.2±10.9% during uncued trials to 71.4±7.5% during self-initiated trials, n=4 sessions, P=0.03, Student's unpaired two-tailed t-test; false alarm rates did not change significantly for this animal from 10.4±7.0 during uncued trials to 11.2±1.9 during self-initiated trials, P=0.9; d′ values increased from 0.9 during uncued to 1.8 during self-initiated trials; top right panel, summary data, hit rates increased from 38.2±10.8% during uncued trials to 81.1±3.3% during self-initiated trials, N=4 rats, P=0.02, Student's paired two-tailed t-test; bottom right panel, d′ increased from 0.8±0.3 during uncued to 1.7±0.2 during self-initiated trials, P=0.03). Incidentally, it appeared that performance using headphones was lower compared to stimuli presented through the free-field speaker. It is unclear why this might be, however we speculate that animals may have difficulty adapting to the change in ear pressure with chronic headphones in place. Nonetheless, there was a significant enhancement in behaviour on the self-initiated trials versus uncued trials, indicating that the neural mechanisms engaged by self-initiation remain intact in headphone-implanted animals.

To control for the movement of animals in the behavioural box, we tracked the x–y position in real time during both self-initiated and uncued tasks. As expected, there was more movement before some self-initiated trials (generally following hits or false alarms, when the animal had been moving from the nosepoke port or the food tray). As such, this usually occurred in a period of 2 s before trial initiation, which we refer to as ‘Interval A' for the self-initiated trials (Supplementary Fig. 2a–c, Interval A x-motion during self-initiated trials: 10.8±0.7 cm, Interval A y-motion during self-initiated trials: 4.4±0.3 cm, n=91 trials). However, during the following interval between trial self-initiation and tone onset 0.5–1.5 s later (‘Interval B'), animals maintained a relatively fixed position with minimal movement (Supplementary Fig. 2a,b, Interval B x-motion during self-initiated trials: 4.4±0.5 cm, Interval B y-motion during self-initiated trials: 4.0±0.4 cm, n=91 trials, one-way analysis of variance (ANOVA) with Tukey's multiple comparison test). Throughout the uncued trials, we defined Interval A as the interval between 3 and 1 s before tone onset and Interval B as the one second preceding tone onset. The animals had little movement in either Interval in both coordinates (Supplementary Fig. 2a–c Interval A x-motion in uncued trials: 2.7±0.5 cm; Interval B x-motion: 2.6±0.5 cm; Interval A y-motion in uncued trials: 4.4±0.4 cm; Interval B y-motion in uncued trials: 3.4±0.3 cm, n=78 trials, one-way ANOVA with Tukey's multiple comparison test). Differences in movement during self-initiated trials compared to uncued trials did not explain the improved behavioural performance, as the x- and y-motion during correct trials was similar to the motion during error self-initiated trials (Supplementary Fig. 2c,d, x-motion during correct trials: 3.4±0.8 cm and during error trials: 4.0±0.6 cm, N=4 rats, P=0.6, Wilcoxon matched-pairs two-tailed signed-rank test; y-motion during correct trials: 3.7±0.6 cm and during error trials: 3.3±0.4 cm, N=4 rats, P=0.6). Thus, differences between self-initiated and uncued task performance are unlikely to result just from variability or changes of animal position in the training box.

Self-initiation modulates cortical tone-evoked responses

How might behavioural engagement modulate neural activity for task performance? Recent reports show that auditory cortex is important for various forms of acoustic behaviour in rodents⁵,³⁹,⁴⁰,⁴¹. Moreover, we previously showed that either cholinergic or the noradrenergic modulation produced plasticity within the rat auditory cortex that could improve behavioural performance on this task for hours to weeks⁴,²⁹. As the pure tone stimuli used in our task are highly processed by subcortical stations before reaching the cortex, the auditory cortex might encode the context dependence or behavioural significance of these sounds. Supporting this hypothesis, we found that neural activity in the auditory cortex was required for this behaviour. Bilateral muscimol infusion into the auditory cortex substantially impaired performance, whereas the same animals were unimpaired following saline infusion (Supplementary Fig. 3).

To determine how self-initiation modulates neural activity for improving sensory perception, we performed single-unit recordings from the auditory cortex of behaving rats chronically implanted with tetrode microdrives⁵,³²,⁴². We recorded spiking activity from 227 neurons in 6 rats, including 117 units that were monitored during consecutive blocks of self-initiated and uncued trials. The advantage of this comparison is that the external context, the motor output, and the significance and value of tones are the same in both cases, allowing us to isolate neural processes that may be recruited during behavioural engagement in the self-initiated trials.

We first examined whether self-initiation affected tone-evoked responses (Fig. 2). For comparison across units, we normalized responses by calculating z-scores for all trials aligned to either target or non-target tone onset, and considered the ‘evoked response' as the peak z-scored firing rate up to 100 ms after tone onset (Fig. 2a–c). We found that, for the same units, evoked responses were different in the self-initiated versus uncued versions of the task. In some cells, the evoked response to tones was lower during self-initiated trials than during uncued trials (Fig. 2a, left; z-scored response to target was 0.8±0.3 during self-initiated trials and 1.8±0.4 during uncued trials, P=0.001, Student's unpaired two-tailed t-test; Fig. 2a, right, z-scored responses of the same unit to non-targets decreased from 3.4±0.5 during uncued trials to 2.1±0.3 during self-initiated trials, P=0.0003). In some other cells, however, evoked responses were higher during self-initiation (Fig. 2b, z-scored target responses increased from −0.2±0.0 during uncued trials to 0.9±0.6 during self-initiated trials, P=0.02; Fig. 2c, z-scored responses of a different unit to non-targets increased from 0.9±0.2 during uncued trials to 1.6±0.2 during self-initiated trials, P=0.0001).

To determine how self-initiation modulates evoked responses at the population level, we calculated a modulation index between ‘Self' and ‘Uncued' evoked responses: (R_self−R_uncued)/(R_self+R_uncued), where R is the firing rate. For the majority of cells, (74/117 units or 63.2%), self-initiation decreased the evoked response to targets (leading to negative modulation indices), a proportion similar to the suppression detected during the transition from passive to active listening in a different auditory behaviour³². For the remaining 43/117 cells, the evoked response was larger during self-initiated trials, represented by positive modulation indices (Fig. 2d, left). Confirming that the majority of neurons had suppressed responses to target tones during self-initiated trials, the median modulation index was negative and significantly different from zero (Fig. 2g, median modulation index: −0.14, interquartile range: −0.42 to 0.18, n=117 cells, P=0.003, Student's one-sample two-tailed t-test). Self-initiation similarly affected responses to non-target tones: evoked responses were suppressed in 73/117 neurons and enhanced in 44/117 neurons (Fig. 2d, right). For the majority of cells, most non-target evoked responses were also suppressed during self-initiation, leading to negative modulation index values (Fig. 2g, median modulation index: −0.07 interquartile range: −0.23 to 0.14, n=117 cells, P=0.003). We observed similar modulation patterns when we plotted the difference in z-score values between responses during ‘Self' and ‘Uncued' trials, indicating that the modulation of evoked responses by trial self-initiation does not result from global changes in spontaneous neuronal activity between the two versions of the task (Fig. 2e). Movement during behavioural trials did not significantly contribute to the modulation of evoked responses at the population level (Supplementary Fig. 4). For the majority of neurons, the correlation coefficient between target or non-target evoked response and the x- or y-motion was small and not significant (only 8/32 neurons had significant correlation between the evoked responses and x, y-movement preceding self-initiation).

We wondered whether the improved performance during self-initiated trials related to the modulation of evoked cortical responses. To examine this, we compared the evoked responses during self-initiated trials with the evoked responses during correct uncued trials. Interestingly, evoked responses during correct uncued trials resembled evoked responses during self-initiated trials (Fig. 2f). At the neuronal population level, the median modulation index between self-initiated trials and correct uncued trials was not significantly different from zero for target evoked responses (Fig. 2g, median modulation index: −0.03, interquartile range: −0.4 to 0.1, n=117 cells, P=0.07, Student's one sample, two-tailed t-test) or non-target evoked responses (Fig. 2g, median modulation index: −0.01, interquartile range: −0.3 to 0.2, P=0.2). Thus, the magnitudes of evoked responses are adjusted in a manner that predicts successful auditory detection and recognition. Therefore, it appears that self-initiation improved performance by recruiting brain states that are optimal for behavioural engagement or for stimulus expectation.

Self-initiation controls cortical auditory receptive fields

We next asked whether, in the same neurons, targets and non-target responses were modulated in the same direction—that is, responses to targets and non-targets were both enhanced or both suppressed in individual units (Supplementary Fig. 5). Co-suppression of both target and non-target responses was observed in 50/177 recordings (42.7%; Supplementary Fig. 5a, lower left quadrant). In 21 other recorded neurons, self-initiation increased responses to both targets and non-targets (Supplementary Fig. 5a, upper right quadrant). For the other 47 cells, self-initiation differentially affected target and non-target tones, such that one set of responses was enhanced while responses to the other stimulus category was reduced (Supplementary Fig. 5a, upper left and lower right quadrants).

Surprisingly, these changes in target versus non-target tones during self-initiated trials could transform frequency tuning profiles of these neurons, including the best frequency. We measured frequency tuning of each cell during self-initiated (solid lines) and uncued (dashed lines) trials, fitting Gaussians to parametrize the peak and width of auditory cortical frequency tuning profiles (Supplementary Figs 5b and 6). When both target and non-target responses were similarly affected, tuning curve amplitudes were either increased or decreased during self-initiation. In contrast, when target and non-target responses were differentially modulated, this could sharpen or broaden tuning curves (that is, increasing or decreasing the width of the Gaussian fits). These changes in cortical frequency tuning can be observed in the example cells shown in Supplementary Figs 5b and 6: self-initiation sharpens the tuning profile either by increasing the response at a specific sound frequency (left) or by suppressing responses for most but not all sound frequencies (middle and right). To quantify this change in the sharpness of neuronal receptive fields, for each cell we aligned the tuning profiles to the best frequency and normalized them to the best frequency response (Supplementary Fig. 7a). The area under the curve calculated for the best frequency aligned plots is inversely correlated with how sharp the tuning profile of each cell is. For the example cells in Fig. 3b (from left to right), the area under the curve is 1.9, 1.2 and 1.4 during self-initiated trials and 2.3, 2.1 and 1.8 during uncued trials.

On average across the population of recorded neurons, self-initiation decreased the width of tuning curves (Supplementary Fig. 5c, left: mean self-initiated σ 5.6±1.4 octaves, uncued σ 12.5±3.0 octaves, n=41 cells, P<0.02, Student's paired two-tailed t-test), increased the dynamic range measured as the distance between the tuning curve maxima and minima (Supplementary Fig. 5c, middle: 0.7±0.0 during ‘Self', 0.6±0.0 during uncued trials, n=41, P<0.005), increased the sharpness of best frequency-aligned tuning profiles (Supplementary Fig. S7b, area under the curve for self-initiated trials was 0.7±0.07 and for uncued trials was 0.9±0.05, n=41, P<0.04), and improved the neuronal d′ values between targets and non-targets (Supplementary Fig. 5c, right: mean d′ for self-initiated trials was 4.1±1.1 and for uncued trials was 2.8±1.9, n=41, P<0.04).

To determine whether behavioural performance depends on the shape of cortical tuning profiles, we separately looked at how tuning during correct uncued trials compares with tuning during self-initiated trials. For some cells, tuning during correct uncued trials had an intermediate shape between self-initiated and uncued profiles (Supplementary Fig. 7b, left). At the population level, we find that tuning curves are equally sharp during correct uncued and self-initiated trials (Supplementary Fig. 7, the area under the curve for the best frequency-aligned tuning curves of correct uncued trials was 0.7±0.07, not significantly different from self-initiated trials).

To ask whether changes in evoked response magnitude and in receptive field structure are specific to the mode of trial initiation, as well as to exclude the possibility that the observed changes in receptive field structure resulted from degradation of the recording over time, we examined neural activity during a sequence of three consecutive sessions: uncued—self-initiated—uncued. For the cell in Supplementary Fig. 5d, the magnitude of evoked responses was comparable between the two uncued sessions but different during the self-initiated session (Supplementary Fig. 5d, z-score of evoked responses was similar between the two sessions of uncued trials: 0.5±0.2 for ‘Uncued 1'and 0.9±0.2 for ‘Uncued 2', and different for ‘Self' trials: −0.1±0.1, n=46 trials, P=0.005, one-way ANOVA and Dunnett's multiple comparison test). Similarly, the tuning profiles for this cell were comparable between the two uncued sessions but were sharper during the self-initiated session (Supplementary Fig. 5e, ‘Uncued 1' σ: 6.1 octaves, ‘Self' σ: 3.4 octaves, ‘Uncued 2' σ: 7.8 octaves). At the population level, evoked responses were highly similar between the two ‘Uncued' sessions, but decreased during the ‘Self' session (Supplementary Fig. 5f, ‘Uncued 1' z-score: 0.4±0.1, ‘Self' z-score: 0.1±0.0, ‘Uncued 2' z-score: 0.4±0.1, n=25 recordings, one-way ANOVA and Dunnett's multiple comparison test). Taken together, these data show that self-initiation induces a rapid and flexible restructuring of receptive fields related to improved behavioural performance.

Self-initiation regulates cortical ongoing activity

It has been previously shown that cortical responses to visual and somatosensory stimuli correlate with patterns of spontaneous activity such as the ‘up' and ‘down' cortical states¹³,⁴³,⁴⁴,⁴⁵. In addition to modulations of spiking during tone presentation, we also observed that self-initiation decreased the ongoing activity prior to tone onset (Fig. 3a, ongoing activity for an example neuron; top, tone-aligned raster plot for this cell; bottom left, tone-aligned z-score peristimulus time histogram (PSTH) and inset with waveform). At the population level, ongoing activity was gradually suppressed after self-initiation (Fig. 3b, z-score values progressively decreased from the time of self-initiation and were lowest before tone onset). The decrease in ongoing activity during the 0.5 s interval before tone (between −0.5 and 0 s before tone), was significant compared with a 0.5 s baseline measured before trial initiation (between −2.5 and −2.0 s), for both the example neuron (Fig. 3c, left: trial-to-trial z-scores for the ongoing activity decreased from 0.01±0.02 before self-initiation to −0.15±0.06 before tone, n=63, P=0.0007, Wilcoxon matched-pairs signed-rank test) and at the population level (Fig. 3c, right: mean z-score for ongoing activity:−0.061±0.008, mean z-score for baseline activity: 0.003±0.002, n=117 cells, P<0.0001). We calculated the modulation of spiking due to self-initiation versus uncued trials for the 0.5 s of ongoing activity preceding the tone onset. Self-initiation suppressed spontaneous activity in 71/117 (60.6%) neurons (Fig. 3d top and Fig. 3e, median modulation index: −0.07, interquartile range: −0.25 to 0.09, n=117 cells, P<0.0001, one-sample two-tailed t-test). Similarly, the difference in trial-by-trial ongoing activity z-scores between self-initiated and uncued trials was negative in the majority of neurons, confirming that these changes in spontaneous activity result from trial specific modulations rather than from global changes in neuronal firing (Fig. 3d, bottom). We calculated modulation indices between self-initiated and correct uncued trials for each cell and found that they were similar (Fig. 3e, left: median modulation index between ‘Self' and correct uncued trials was −0.005, with an interquartile range between −0.157 and 0.081, n=117 cells, P=0.07, Student's one-sample two-tailed t-test). Moreover, ongoing activity was significantly suppressed during correct but not error uncued trials (Fig. 3e, right: for correct uncued trials the median z-score was −0.019 with interquartile range between −0.041 and 0.015, n=117, P=0.005, Student's one-sample two-tailed t-test; for error uncued trials the median z-score was −0.009, with an interquartile range between −0.038 and 0.029, P=0.5). The magnitude of ongoing activity suppression varied from cell to cell and was significantly correlated with the modulation of target and non-target evoked responses in the same units (Fig. 3f). The suppression of ongoing activity did not depend on the movement of the animal during the self-initiated trials (Supplementary Fig. 8). This suggests that self-initiation regulates the rate of ongoing activity, to more precisely control sensory-evoked responses and boost signal-to-noise ratios.

Optogenetic disruption of auditory perception

To investigate the role of ongoing activity patterns in sensory perception, we used optogenetics to manipulate neuronal activity in the auditory cortex. In six adult rats, we injected adeno-associated virus (pAAV5-CaMKII-ChETA-EYFP) bilaterally in the auditory cortex for expression of the channelrhodopsin-2 ‘ChETA' variant under the CaMKII promoter⁴⁶. Animals also had either optical fibres or optical microdrives implanted in the auditory cortex. Location of viral injections and fibre implantation was established during surgery by multiunit electrode mapping of the auditory cortex and confirmed with immunohistochemistry at the end of each experiment (Fig. 4a).

During behavioural testing in the injected rats, we delivered pulses of blue light at 20 Hz via the optical implants. We recorded from the auditory cortex during this procedure, to determine how optical stimulation affected cortical activity (Fig. 4b, example neuron recorded during stimulation with 5 ms blue light pulses). Optical stimulation began at self-initiation and occurred either throughout the trial (starting at self-initiation and ending at tone offset) or only during the ongoing activity period (ending just before tone onset), or during the tone, or only post-tone (for one second after tone-offset). Optogenetic stimulation of the auditory cortex was capable of disrupting normal patterns of activity, as can be observed during ongoing activity (Supplementary Fig. 9, left; z-score for control trials was significant lower than 0: −0.01±0.00, n=30 cells, P=0.04, Student's one-sample two-tailed t-test; z-score for optogenetically disrupted trials was not significantly smaller than 0: −0.006±0.00, n=30 cells, P=0.4) and during evoked activity (Supplementary Fig. 9, right; the mean modulation index between optically disrupted and control trials was 0.08±0.02, n=30 cells, P<0.008, one sample t-test). Stimulation during ongoing and evoked periods decreased hit rates (Fig. 4c, example rat hit rate was 67.3% during light OFF trials 40% and during light ON trials, n=26 trials, P=0.04, Fisher's exact test; Fig. 4d, summary data: hit rate was 68.1±15.0 during control, light OFF trials and 50.0±9.6 during light ON trials, N=6 rats, P=0.03, Student's ratio paired two-tailed t-test). In contrast, stimulation only during the ongoing activity period (light OFF hit rate was 83.0±9.8 and light ON hit rate was 72.6±16.3, P=0.3), the tone period (light OFF hit rate was 94.2±3.0 and light ON hit rate was 83.0±7.0, P=0.1) or post-tone period (light OFF hit rate was 92.8±3.8 and light ON hit rate was 97.6±2.3, P=0.4) did not significantly affect responses to targets (Fig. 4d). Thus, disrupting both ongoing and evoked activity during self-initiated trials impaired behavioural performance.

Behavioural training

All animal procedures were performed in accordance with National Institutes of Health standards and were approved by the New York University School of Medicine Institutional Animal Care and Use Committee.

The behavioural task used here was similar to that we used previously⁴,²⁹,⁵⁴ with the addition that animals could self-initiate trials during some testing sessions⁵⁴. Animals were trained on a go/no-go task to nosepoke in response to a target tone frequency for a food reward in 9" × 10" × 12" operant conditioning chambers (Med Associates, Inc.). Each chamber contained a speaker (on the right wall) calibrated across frequencies at 70 dB SPL, a food dispenser on the left wall and three nose poke ports (two on either side of the food dispenser and one on the wall opposite). Each chamber was placed in a larger wood enclosure and insulated with foam. The measured background noise in each chamber was <30–40 dB SPL.

Twenty-five adult, female Sprague–Dawley rats were used in these behavioural studies. Animals were food restricted to maintain the weights at 80–85% of their initial pre-training weights. First, animals were shaped with two days of training to nosepoke for one food pellet. Next, rats were trained to nosepoke within 2.5 s after a target tone was played (for 23 rats the target tone was 4 kHz and for two rats the target tone was 16 kHz). When the rats had hit rates of >50%, six non-target tones were introduced (0.5–32 kHz at one octave intervals excepting the target frequency) and animals were trained to hit rates >65%. Target and non-target pure tones were 100 ms in duration presented in a pseudo-random order at 70 dB SPL. For correct trials, each trial ended at either the time of food pellet delivery (hit trials for targets) or 2.5 s after the tone (correct reject trials for non-targets). On error trials, failure to respond (miss trials for targets) as well as incorrect responses (false alarm trials for non-targets) were punished with a time-out of 7 s before the next trial began. Random nose pokes were punished with time-out as well.

In some blocks of behavioural testing, rats self-initiated the trials by nosepoking in a different port than the ‘response' port. After 0.5–1.5 s, either a target or non-target tone was played. In other blocks of trials (‘Uncued'), the trials started 6–10 s after the end of the previous trial or after the time-out interval. Overall, the trial-to-trial interval for uncued trials was 6–10 s when the previous response was correct or 13–17 s when the previous response was incorrect. The trial-to-trial interval increased above these values when the animal made random nose pokes.

For the detection task, the amplitude of target and non-target frequencies was varied randomly between 20 and 80 dB SPL (at 10 dB SPL intervals). For testing performance on the narrowband version of the task, the non-target frequencies were changed to spectrally closer (1/6 of an octave), whereas the target frequency remained the same. In the experiments where the sounds were presented via headphones, we built special adapters from copper mesh and grip cement that were then implanted over both ear canals of the rats. After recovery, commercial headphones with a 20 kHz cutoff frequency were connected to the sound generators of the MedAssociates chambers and then fitted in the adapters.

In five trained adult rats, we bilaterally implanted cannulas above the auditory cortex at the following stereotactic coordinates from Bregma: −5.5 anterior-posterior (AP), 6.5 medial-lateral (ML), −1.5 dorsal-ventral (DV). Following recovery from surgery, the rats were trained to recover behavioural performance and were tested as follows: before any injection, 20 min following injection of 2 μl muscimol solution (1 mg ml⁻¹) in each cannula or 20 min following 2 μl saline injections in each cannula.

Behavioural performance was estimated with hit rate measurements (per cent of times the rats respond to the target frequency) and the discriminability index d′ (the difference in the z-scores for the distribution of responses to targets and for the distribution of responses to non-targets).

Movement data collection and analysis

To monitor the movement of the rats in the behavioural chamber, we used video tracking to measure their position in the operant conditioning box during behavioural testing. An infrared camera (OptiTrack) was mounted on the ceiling of the wood enclosure. The camera was connected to the video tracking system (NeuroMotive), which recorded the position in x and y coordinates of a luminescent tag placed on the head of the animals, with a resolution of 640 × 480 pixels and a frame rate of 25 Hz. To calculate the movement during different behavioural trials, we calculated the difference in the x and y coordinates during Interval A (2 s before trial initiation, and from 3.5 to 1.5 s before tone during uncued trials) and during Interval B (the interval between self-initiation and tone, and 1.5 s before tone during uncued trials).

Microdrive assembly and implantation

Eight tetrode Versadrives (Neuralynx) were assembled using 12.5 μm nichrome wire. Tetrodes were spun using the OpenEphys tetrode spinner (80 turns forward and 40 turns backwards) and were consolidated using a heat gun. Tetrodes were cleaned and electroplated in gold solution until their impedance was ∼0.2–0.5 MΩ (NanoZ electroplater, Neuralynx). For simultaneous recordings and optogenetic stimulations, optical microdrives were prepared, where a 200 μm optic fibre was inserted in the middle of the microdrive.

For implantation, six of the rats tested behaviorally were anaesthetized with ketamine/dexmedetomidine (75–100/0.5–1 mg kg⁻¹, intramuscularly) A craniotomy was performed over the right auditory cortex (AP: −5.5 to −6.5 mm, ML: 6 to 6.5 mm). Primary auditory cortex (AI) was coarsely mapped by recording multi-unit responses with a tungsten electrode. After confirmation of location within AI, the microdrive was lowered down 0.9–1.1 mm and secured with bone screws and dental cement. Reference and ground electrodes were soldered to separate bone screws above the cerebellum. Rats were allowed to recover for ∼7 days after surgery.

Neuronal data collection and analysis

Extracellular spiking data was collected with a 32 channel digital headstage and a neural signal processor (Blackrock Microsystems). Neuronal responses were recorded at 30 kHz, bandpass filtered at 0.25–5 kHz and artefacts rejected based on refractory period violations (inter-spike interval (ISI) <1 ms) and amplitude violations (>1–2 mV). The waveforms were sorted offline using a time-amplitude algorithm (nPlay Sorter). Spike trains were aligned using Neuroexplorer software either to the onset of all tones, to the onset of target tones alone or to the onset of non-target tones. For error trial analysis, we separated the ‘Uncued' trials in correct and error trials. The spike counts between −3 and 3 s were exported in 5 ms bins. Tetrodes were lowered by ∼75 μm at the end of each recording session.

Neuronal responses were analysed using Matlab (Mathworks). Changes in firing rate were quantified by calculating the trial-by-trial z-score for each 5 ms time bin of the tone-aligned spike trains, using the following formula: z=(x−μ)/σ, where μ represents the mean firing rate and σ represents the s.d. of the trial (3 s before tone onset). The modulation index for the evoked activity of each cell was calculated as the difference over sum of evoked firing rates R during self-initiated versus uncued trials: (R_self−R_uncued)/(R_self+R_uncued). We used a 15 ms window centred around the peak of the evoked response for calculating this modulation index. Similarly, the modulation index for the ongoing activity was calculated as the difference over sum of the firing rates during the 500 ms preceding the tone in self-initiated versus uncued trials.

Neuronal receptive fields were constructed by separating the evoked responses to each individual frequency tone (in the subset of recordings when we registered the timestamps of individual tone frequencies). We included in this analysis only those neurons that had a normal distribution of responses to different tone frequencies, as measured with the Shapiro–Wilk normality test (α=0.05). Based on these criteria, we excluded 24 cells from receptive field analysis. Other 52 neurons could not be included in this analysis, because the timestamps of different tone frequencies were not recorded separately. Gaussian curves were then fitted on the tuning profiles and the s.d. (in octaves) of each distribution was calculated. The dynamic ranges of the tuning curves were calculated as the differences between the maximum and the minimum normalized spiking responses. The neuronal d′ was calculated only for those cells that had a significant modulation of z-score values between the self-initiated and uncued trials (Student's t-test, P<0.05), as the difference between the z-score for non-targets and the z-score for target tones.

Optogenetic stimulation

We injected bilaterally in the auditory cortex an AAV5 virus encoding ChETA (an optimized version of channelrhodopsin-2) under the CaMKII promoter (to limit expression to excitatory neurons). ChETA was fused with enhanced yellow fluorescent protein (EYFP), to allow for immunohistochemical confirmation. The virus was purchased from Addgene via the Penn Viral Core. The auditory cortex was targeted based on stereotactic coordinates (see above) and the location was confirmed electrophysiologically by multiunit recordings in anesthetized rats. For each hemisphere, we injected 1 μl of virus at two different sites in the auditory cortex. Following the injection, on each side of the brain a 200 μm fibre optic (Thor Labs) was implanted down to 1 mm below the brain surface and secured in place using Metabond and grip cement. We allowed two weeks for viral expression. During behavioural testing we connected a blue laser (400 nm) to the bilaterally implanted ferules. The light intensity at the tip of the fibre was ∼2 mW. The laser was triggered by a transistor-transistor logic (TTL) pulse sent by the Med Associate boxes when the animal self-initiated. The light ON trials were randomly interleaved with light OFF trials in a 1/5 ratio. When the laser was triggered, it delivered a 20 Hz pulse train for 0.5 s, until tone onset (in this experiment we only used 0.5 s delays between self-initiation and tone).

At the end of behavioural studies, rats were perfused with 4% paraformaldehyde, brains collected, cryoprotected in 30% sucrose and then sliced in 15 μm thin sections using a cryostat. We then performed immunohistochemistry using a rabbit anti-GFP antibody (1:500, Abcam) and Cy2-conjugated anti-rabbit secondary antibody (Jackson Immunoresearch). The sections were visualized using a Zeiss LSM 700 confocal microscope.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.