Coordination of Orofacial Motor Actions into Exploratory Behavior by Rat

Abstract

The delineation of sensorimotor circuits that guide exploration begins with an understanding of the pattern of motor outputs [1]. These motor patterns provide a clue to the form of the underlying circuits [2-4] (but see [5]). We focus on the behaviors that rodents use to explore their peripersonal space through goal-directed positioning of their nose, head, and vibrissae. Rodents sniff in response to novel odors, reward expectation, and as part of social interactions [6-12]. Sniffing serves olfaction [13, 14], while whisking synchronized to sniffing serves vibrissa-based touch [6, 15, 16]. We quantify the ethology of exploratory nose and head movements in relation to breathing. We find that sniffing is accompanied by prominent lateral and vertical deflections of the nose, i.e., twitches, which are driven by activation of the deflector nasi muscles [17]. On the timescale of individual breaths, nose motion is rhythmic and has a maximum deflection following the onset of inspiration. On a longer timescale, excursions of the nose persist for several breaths and are accompanied by an asymmetry in vibrissa positioning toward the same side of the face. Such directed deflections can be triggered by a lateralized source of odor. Lastly, bobbing of the head as the animal cranes and explores is phase-locked to sniffing and to movement of the nose. These data, along with prior results on the resetting of the whisk cycle at the onset of inspiration [15, 16, 18], reveal that the onset of each breath initiates a "snapshot" of the orofacial sensory environment. VIDEO ABSTRACT.

Keywords: active sensing; bobbing; brain stem; neck; nose; olfaction; sniffing; twitching; vibrissae; whisking.

Figure 1. Motion of the Nose

(A) Schematic of the experimental setup for recording breathing and three-dimensional nose motion in a head restrained rat (top). A thermocouple is implanted in the nasal cavity to monitor breathing. A charge-coupled device (CCD) camera is mounted above rat, and a split frame is used to simultaneously image lateral and rostrocaudal nose motion from the top view, and lateral and vertical motion via a mirror in front of the rat that is set at 45 degrees (Figures S1A and S1B). Two example split frames and two additional front view frames are shown (bottom). (B) A time series of breathing (red) along with lateral (blue) and vertical (black) nose motion. The nose tends to move during sniffing epochs and moves little during basal respiration. Sniffing bout onsets are marked by a tick. (C) Inspiration triggered average of breathing (red) and nose motion (orange and green) for sniffing, i.e., 4 and 6 Hz breathing rate (left) and basal respiration, i.e., < 2 Hz breathing rate (right). Trials were selected for averaging as right-going (positive amplitude; orange) or left-going (negative amplitude; green) if the value at the time of an inspiration onset was at least 0.5 mm from the center. Right- and left-going trials were averaged separately. Calculation performed with 560 trials (sniffing) and 98 trials (basal) of data across three rats; all error bars are SEM. (D) Raster plot of nose lateral position peaks, thresholded with a 0.5 mm minimum displacement, in relation to the breath rise. Trials are sorted by breath duration and the direction of the lateral movement; right motion is orange and left motion is green. The histogram shows the time for the lateral motion to peak for intermediate breathing frequencies (2.0–4.0 Hz, highlighted in gray); only data prior to the second breath is included. The second peak in nose displacement (+) occurs 0.34 s after the onset of inspiration. Data are from a single animal. (E) Histograms of the lateral and vertical nose positions for six rats. Blue histograms include values for both basal respiration and sniffing, while red histograms include only sniffing epochs. (F) Average lateral position of the nose relative to the onset of a sniffing bout (B). Trials were sorted into right onset (orange) and left onset (green) by the average value within 100 ms of the sniffing bout onset. An exponential fit was calculated on the average of all traces, starting at time t = 0; black line. (G) Scatterplot of the speed of lateral displacements versus the length of the displacement. Each length is defined by a starting and ending with a change in the sign of the velocity. Log-log plot; the line is a fit to the logarithm of the data with slope 0.63 ± 0.03, where the uncertainty is the 0.95 confidence interval. See also Figure S1.

Figure 2. Relation of EMG from Nose, Mystacial Pad, and Vibrissa Muscles to Lateral Nose Motion

(A) Schematic of the muscles in the snout. Two arrangements of EMG signals were measured. EMG wires were placed either bilaterally in the d. nasi, muscles or unilaterally in the nasolabialis, d. nasi, and vibrissa intrinsic muscles. The position of the nose is recorded through a CCD camera, as in Figure 1A. (B) Time series of lateral (blue) and vertical (black) nose position along with bilateral d. nasi EMG recordings. The EMG envelopes, right (purple) and left (magenta), are overlaid on the raw signal (gray). (C) Diagram of nose movement with activation of the d. nasi muscles. (D) Top traces are the normalized autocorrelation and cross-correlations of the bilateral d. nasi EMG envelopes. Middle traces are cross-correlations of right d. nasi EMG envelope with deflections of the nose to the right (purple) and left d. nasi EMG envelope with deflections to the left (magenta). The insert is an expansion near zero lag. Bottom traces are cross-correlations of right and left d. nasi EMG envelopes with vertical displacement of the nose. Data pooled across 4,000 s of data across three rats. (E) Triggered average of lateral (blue) and vertical (black) nose position, vibrissa position, and the EMG envelope in facial muscles with respect to the onset of inspiration during 4 to 6 Hz sniffing. The EMG data from d. nasi were sorted by ipsilateral (orange) and contralateral (green) onset of movement prior to rectification. Data pooled from 530 epochs across six rats.

Figure 3. Changes in the Bias of Deflection of the Nose and Vibrissae Concurrent with Odor Presentation

(A) Schematic of the bilateral olfactometer setup. The rat is head-restrained, wired with electrodes for measuring the EMGs from the right and left d. nasi, and is fitted with a thermocouple to monitor breathing (Figure 2A). Bedding odor can be presented alternately on the right (orange) or left (green) side by activating one of two three-way solenoid valves that maintain constant air flow with or without an odorant. (B) Example time series of the change in lateral (blue) and vertical (black) position, together with the right and left EMG and breathing signal, is response to bedding odor. Each of the two sets of traces is from a single trial. (C) The average response of the envelope of the EMGs from the d. nasi, displayed relative to contra- versus ipsilateral presentation of odorant. We further show the average change in lateral and vertical nose position to laterally presented bedding odor, with trials pooled by right (orange) and left (green) odor presentation. The breathing is shown as the averaged, instantaneous rate. Data pooled over 120 presentations across three rats. (D) Example time series of lateral nose position with one naris blocked. For these measurements, the rat had two thermocouples implanted, one is each nasal cavity. The right (orange) or left (green) thermocouple signal was observed to go to zero when the corresponding naris was blocked while the other thermocouple faithfully reported breathing. (E) Probability density function of lateral nose position distribution with one nostril blocked during all recording time (blue) and during sniffing only (red). Data pooled over 1,040 s of recordings across 6 rats. (F) Schematic of the view of a CCD camera for tracking the vibrissae. In addition, breathing was measured with a thermocouple and the nose position was tracked in a single, front plane. (G) Time-series of breathing (red) and the position of the left (green) and right (black) C2 vibrissa. The midpoint of whisking was computed as the average between the upper and lower envelope of the cycle-by-cycle angle of the vibrissa. The difference between vibrissa midpoints (magenta) is scaled to overlap maximally with the lateral nose position (blue). (H) The olfactometer (A) was added to establish the response of the vibrissae to a bedding odorant. We show the time series of the whisking amplitude and midpoint along with the average nose position, rectified so that right and left deflections overlap. Trials were selected by criterion that lateral deflection of the nose in response to the odor exceeded 1 mm from rest. Data pooled from 177 trials across three rats. See also Figure S2.

Figure 4. Head Bobbing and Summary of Rhythmic Orofacial Motor Actions in Relation to Breathing

(A) Schematic of the setup for measurements of head angular motion relative to the breathing cycle. A three-axis gyrometer is mounted on the head to record pitch, roll, and yaw of head movement, and a thermocouple was implanted in the nasal cavity to measure breathing. All signals were routed through a commutator. (B) Time series of breathing (red), yaw velocity (ω_α; orange), pitch velocity (ω_γ; green), and roll velocity (ω_β; blue) of the head, along with EMG data from the right and left splenius capitis muscles during a sniffing bout. Angular speed (black) was computed as √ (ω_α)²+ (ω_β)²+(ω_β)². Both raw EMG data (gray) and the envelope (magenta and purple) are shown; the scale for the envelope is 125 μV. (C) Triggered average of breathing (red), individual angular velocities, angular speed (magenta), and the summed and difference EMG signals, with respect to the onset of inspiration. Data pooled over three animals with 20,500 sniff cycles. The error band is a 0.95 confidence interval. (D) Schematic of the setup for measurements of head position relative to the nose position. A three-axis gyrometer is mounted on the head to record pitch, roll, and yaw of head movement, as in (A). Nose velocity was measured by a pair of Hall-effect probes mounted around the nose and a small magnet implanted in the nasal cartilage. (E) Time series of head speed (black), left Hall probe output (green) and normalized nose speed computed from the Hall probe signals (blue). (F) Spectral coherence of head speed and nose speed. Data pooled from 28 epochs, lasting 830 s, over 3 animals. The error band is a 0.95 confidence interval. (G) Summary of orofacial movements and muscle activity with respect to phase in the sniff cycle. See also Figure S3.