Neural coding during active somatosensation revealed using illusory touch

Journal name:
Nature Neuroscience
Year published:
Published online


Active sensation requires the convergence of external stimuli with representations of body movements. We used mouse behavior, electrophysiology and optogenetics to dissect the temporal interactions among whisker movement, neural activity and sensation of touch. We photostimulated layer 4 activity in single barrels in a closed loop with whisking. Mimicking touch-related neural activity caused illusory perception of an object at a particular location, but scrambling the timing of the spikes over one whisking cycle (tens of milliseconds) did not abolish the illusion, indicating that knowledge of instantaneous whisker position is unnecessary for discriminating object locations. The illusions were induced only during bouts of directed whisking, when mice expected touch, and in the relevant barrel. Reducing activity biased behavior, consistent with a spike count code for object detection at a particular location. Our results show that mice integrate coding of touch with movement over timescales of a whisking bout to produce perception of active touch.

At a glance


  1. Overview of the experimental system and whisking strategy during object location discrimination.
    Figure 1: Overview of the experimental system and whisking strategy during object location discrimination.

    (a) Recordings were made from L4 neurons while mice localized objects with the C2 whisker. Whisker movements were measured with high-speed video. The mouse indicated its decision by licking for a water reward. Whiskers were detected as they crossed a virtual pole (infrared laser). A real-time system controlled photostimulation of ChR2-positive L4 neurons on the basis of whisker position. (b) Schematic of the object location discrimination task. θ is the azimuthal angle of the whisker at the base. Gray shading indicates the θROI. (c) Whisking during object location discrimination (data from one representative session are shown). Left, whisker θ at touch onset (408 touches; YES trials, blue; NO trials, red). Right, distribution of whisker positions during task-related whisking (θamp > 2.5°). Occupancy (s) is the time spent at a particular θ (bin size, 1°). Deg, degree. (d) Whisker movements (gray, θ) in two example behavioral trials (top, NO trial; bottom, YES trial). The black trace segments correspond to contact periods. Pole entry (gray) and availability (black) are indicated in the top line. Protraction corresponds to increasing θ. Ticks, spikes; asterisk, lick. (e) The number of contacts per trial for all sessions (36,910 trials; YES trials, blue; NO trials, red). Dots, means.

  2. L4 neurons spike with precise latencies during object location discrimination.
    Figure 2: L4 neurons spike with precise latencies during object location discrimination.

    (a) Spike rasters and a peritouch spike histogram for one L4 neuron aligned to the first touch (same session as in Fig. 1d). (b) Peritouch spike histogram averaged across all rapidly touch-excited L4 neurons <250 μm from the C2 center (13 neurons) for the first touch per trial.

  3. Decoding object location and behavioral choice on the basis of L4 spikes.
    Figure 3: Decoding object location and behavioral choice on the basis of L4 spikes.

    (a) Neural coding of object location. Top, whisker position (θ, gray) and the two pole locations (blue, YES; red, NO). Bottom, schematic spike probability for the two object locations. (b) Spike-triggered θ (for every spike in the exploration window adjusted by the spike latency, 9 ms) (YES trials, blue, n = 73; NO trials, red, n = 42; same data as in Fig. 2a). (c) Spike count during the exploration window (same data as in Fig. 2a). Dashed line, mean reaction time. (d) L4 neurons discriminate object location equally on the basis of spike count and spike-triggered θ (circles, individual neurons; cross, population mean and standard error; P = 0.57 by paired two-tailed t test). Discrimination performance is the area under the receiver operating characteristic curve for a linear classifier. Dotted lines, chance discrimination performance and equal discrimination performance. (e) L4 neurons discriminate behavioral choice better on the basis of spike count than spike-triggered θ (P = 0.0085).

  4. Optogenetic stimulation mimics touch-evoked spiking in L4 neurons.
    Figure 4: Optogenetic stimulation mimics touch-evoked spiking in L4 neurons.

    (a) Targeting ChR2 to L4 neurons. Left, ChR2 expression (magenta) in one barrel (white dotted outline). Right, genetic scheme. (b) Single example of a neuron responding to different light intensities. Cyan, photostimulus. (c) Population peristimulus time histograms recorded in different cortical layers (n = 85 neurons total) after a ChR2 stimulus. Responses are averaged across light intensities (Online Methods). (d) Overlay of the population peristimulus time histograms (gray, touch; same data as in Fig. 2b; magenta, photostimulation, delayed by 5 ms from the stimulus). (e) Comparison of L4 activity evoked by touch and photostimulation.

  5. Closed-loop photostimulation causes illusory perception of object location.
    Figure 5: Closed-loop photostimulation causes illusory perception of object location.

    (a) Four trial types during a photostimulation behavior session depending on pole location and photostimulation (cyan lightning bolts). The virtual pole (magenta) was in the θROI. Mice reported object location by licking or not licking. (b) Photostimulation (blue circles) coupled to whisker movement (gray, θ) during object location discrimination. Asterisk, answer lick. (c) Responses in the four trial types across one behavioral session. Green, yes responses; gold, no responses. (d) Photostimulation in NO trials (red) in the C2 barrel increases the fraction of yes responses. Blue, YES trials; stim, photostimulation. Error bars, s.e.m. Each line represents an individual mouse. (e) Fooling index (as defined in d). Black circles, individual mice; gray circle, first session averaged across all mice. Error bars, s.e.m. Gray bar, mean maximum possible fooling index. (f) Same experiment as in ae but without ChR2 expression. Error bars, s.e.m. (g) Same experiment as in ae but with ChR2 expression and photostimulation in the E3 barrel. Error bars, s.e.m. (h) Symmetric response task; both object locations were indicated by licking at one of two lickports (left or right lick). Black circles, individual mice. Gray bar, mean maximum possible fooling index. The performance of each mouse was different from zero as determined by one-tailed permutation test. Error bars, s.e.m.

  6. Precise millisecond-timescale spike latencies are not required for detecting an object at a particular location.
    Figure 6: Precise millisecond-timescale spike latencies are not required for detecting an object at a particular location.

    (a) Top, delayed photostimulation of L4 neurons was triggered by whisker crossings with varying delays (Δt). Bottom, whisker movements with whisker crossings (red circles) and corresponding photostimuli (cyan circles) for Δt = 50 ms. (b) Fooling index as a function of the delay between whisker crossing and photostimulation. (c) Fooling index as a function of azimuthal angle at the time of stimulation.

  7. Optogenetic silencing of the C2 column biases behavioral choice toward no responses, which is consistent with spike count coding.
    Figure 7: Optogenetic silencing of the C2 column biases behavioral choice toward no responses, which is consistent with spike count coding.

    (a) Left, silencing the C2 cortical column using ChR2-based stimulation of GABAergic neurons. Right, recordings from putative excitatory neurons under control (black) and photostimulation (gold) conditions (the peristimulus time histogram is aligned to the first touch; bin size, 2 ms; n = 6 neurons from four mice; Online Methods). The photostimulus (1.4 mW) began approximately 200 ms before first touch. (b) Reducing spike count in the C2 column reduces performance in YES trials and improves performance in NO trials. Lines, three individual mice. (c) Same as in b but for a symmetric response version of the object location discrimination task. The lines show data from three individual mice in two photostimulation conditions with an average power of 2 mW (continuous illumination, dashed lines; illumination with 1-ms pulses at 80 Hz at the same average power, solid lines). (d) We speculate that mice monitor spike count within the ensemble of L4 neurons in the C2 column normally activated by contact in YES trials (blue 'neurons', upper left corner of the table). The table shows a schematic of the L4 ensemble under the conditions tested in this study (Figs. 5,6,7). For example, in NO trials, a distinct but overlapping ensemble is activated (red neurons; the YES trial ensemble is indicated with blue outlines). In photostimulated NO (virtual pole, cyan) trials, activity is evoked in a subset of the YES ensemble, fooling mice into making yes responses. Bottom, hypothetical distribution of the decision variable (spike count in YES ensemble) used by mice to decide between a yes and a no response. Red, NO trials; blue, YES trials; cyan, NO trials with virtual pole and photostimulation (Figs. 5 and 6); gold dashed lines, silencing. If spike count in the YES ensemble of neurons exceeds a threshold value (the 'decision boundary'), the mouse makes a yes response; otherwise the mouse makes a no response.

  8. Illusory object location can be evoked only during periods of tactile exploration marked by whisking bouts.
    Figure 8: Illusory object location can be evoked only during periods of tactile exploration marked by whisking bouts.

    (a) Example of whisking bouts in relation to trial start and pole motion. Shown are whisker movement (θ, gray) and whisking amplitude (θamp, black). (b) Eight example trials showing the time course of whisking (θamp, black) and the corresponding photostimulation pattern (cyan circles). Left, trials in which photostimulation occurred during periods without whisking. Right, trials in which photostimulation occurred during whisking. (c) Fooling index for the whisking and not whisking trials. Also plotted are interleaved standard virtual pole trials (Δt = 5 ms, as in Fig. 5e). Error bars, s.e.m.


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Author information

  1. These authors contributed equally to this work.

    • Daniel H O'Connor &
    • S Andrew Hires


  1. Janelia Farm Research Campus, Howard Hughes Medical Institute, Ashburn, Virginia, USA.

    • Daniel H O'Connor,
    • S Andrew Hires,
    • Zengcai V Guo,
    • Nuo Li,
    • Jianing Yu,
    • Qian-Quan Sun,
    • Daniel Huber &
    • Karel Svoboda
  2. Present addresses: The Solomon H. Snyder Department of Neuroscience and Brain Science Institute, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA (D.H.O.), Department of Zoology and Physiology, University of Wyoming, Laramie, Wyoming, USA (Q.-Q.S.) and Department of Basic Neurosciences, University of Geneva, Geneva, Switzerland (D.H.).

    • Daniel H O'Connor,
    • Qian-Quan Sun &
    • Daniel Huber


D.H.O., S.A.H., Z.V.G., N.L., J.Y. and Q.-Q.S. performed experiments. D.H., Z.V.G. and N.L. developed the symmetric response task paradigm. D.H.O., S.A.H. and K.S. planned the project. D.H.O., S.A.H., Z.V.G., N.L. and K.S. analyzed the data. D.H.O., S.A.H. and K.S. wrote the paper with comments from the other authors.

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