Spatial segregation of different modes of movement control in the whisker representation of rat primary motor cortex

Abstract

What is mapped on the surface of the primary motor cortex (M1)? The classic somatotopic map holds true on the level of limb representations. However, on the small scale (at within-limb representations), neither somatotopy nor movement dynamics/kinematics seem to be organizational principles. We investigated the hypothesis that integrated into the body representation of M1 there may be separate representation of different modes of motor control, using different subcortical computations but sharing the same motor periphery. Using awake rats and long intracortical stimulation trains in M1 whisker representation (wM1) revealed that natural-like, rhythmic whisking (normally used for tactile exploration) can be evoked from a posteromedial subregion of wM1. Nonrhythmic whisker retraction, on the other hand, was evoked in an adjacent but more anterolaterally located region within wM1. Evoked whisker retraction was always accompanied by complex movements of the face, suggesting that the respective subregion is able to interact with other representations in specific behavioral contexts. Such associations were absent for evoked rhythmic whisking. The respective subregion rather seemed to activate a downstream central pattern generator, the oscillation frequency of which was dependent on the average evoked cortical activity. Nevertheless, joint stimulation of the two neighboring subregions demonstrated their potency to interact in a functionally useful way. Therefore, we suggest that the cause of cortical separation is the specific drive of subcortical structures needed to generate different types of movements rather than different behavioral contexts in which the movements are performed.

Figure 4.

Electrically evoked movements last as long as the stimulus. Stacked single trajectories evoked from a site within RW (A) and in RF (B) are shown. The gray fields indicate the stimulus duration (350, 700, and 1400 ms; 60 Hz, 50 μA). Note the highly variable whisking movements after stimulus offset in RF (D) (supplemental movie, available at www.jneurosci.org as supplemental material). C, Quantitative analysis of duration of rhythmic whisking (as shown in A) for three rats. The ordinate plots the integral of a whisking trace from 0 to 1400 ms after stimulus onset after normalizing the trace to its maximum, setting the mean position to zero, and rectifying it. This measure corresponds to the duration of rhythmic whisking and rises monotonically with stimulus duration. D, Characteristics of rhythmic whisking are different during stimulation of RW and after stimulation in RF. Left, Overlay plot of single trial power spectra from whisker trajectories during RW stimulation (A, gray area). Right, Same type of plot, but data were taken from the period after stimulation in RF (to the right of the gray area in B).

Figure 2.

Whisker retraction is elicited from RF in conscious animals. A, The top two images show the rat's head at stimulus onset (0 ms) and 160 ms thereafter (60 Hz train at 50 μA). The whiskers have moved backward after 160 ms (asterisks indicate the whisker carrying the gelfoam marker). To make face movements that are well visible in the movie (see supplemental material, available at www.jneurosci.org) evident in the freeze image, we generated an inverted absolute difference of the images at 160 ms from the one obtained at 0 ms (bottom row of images). Black features in the different images are the ones that moved between time 0 and 160 ms. Immobile regions appear white (see, for instance, parts of the restrainer marked by the diamond). It is evident that, together with the whisker (asterisks), snout and whisker pad (s), eye (e), ear (au), and paw (p) moved as well. Three different images from three subsequent stimulation trials are shown to demonstrate the repeatability of the evoked movements (the freeze images shown correspond to the first stimulus). B, Typical trajectory of whisker retraction evoked from RF in an awake animal. The whisker was retracted at a latency of ∼50 ms and quickly left the measuring range of the CCD array. For the entire stimulus duration (horizontal line on top), it stayed pulled up alongside the face. After the end of the stimulus train (indicated by the horizontal line on top), the whisker moved swiftly back to the null position.

Figure 1.

Whisker protraction and retraction are elicited from different subregions within M1 under ketamine anesthesia. A, Whisker trajectories evoked from an anterolateral area (RF area). Already, short pulses evoked a visible whisker retraction (light gray trace; 90 μA). Long stimulation at 100 Hz evoked a maximal retraction, followed by slow relaxation forward toward null position (medium gray trace). Long stimulation at 60 Hz evoked a slower but constant retraction movement (black trace). B, Whisker trajectories evoked from a posteromedial area (RW area). Short pulses evoked only a minimal whisker protraction (arrow) sometimes not visible by the eye. Long stimulation at 100 Hz (medium gray trace) evoked a clear protraction that quickly reaches a maximum and then relaxes back toward the null position. Long stimulation at 60 Hz evoked a slower monotonic protraction (black trace). Note that the whisker stayed protracted even after the stimulus command was turned off. C, Photomicrograph of the surface of M1 in one animal. Movement of body parts and directions of whisker movements are marked by colored dots at the stimulation sites (R, rostral; M, medial; bregma is indicated by the white dot in the bottom right corner). D, Line drawing showing the limits of the compound map for retraction and protraction as found in four animals mapped at a spatial precision similar to C (220 sites in total). Scaling and location of bregma is aligned in C and D.

Figure 3.

Rhythmic whisking is elicited from RW in conscious animals. A, Typical trajectory obtained from long stimulation (60 Hz, 50 μA) in RW. The average set point in this case was protracted (indicated by the dotted line). B, Pictures taken from a video sequence during one cycle of electrically evoked whisking. The movie taken from the front shows the typical asymmetry in duration of protraction (seen in 4 frames) and retraction (last frame). C, Traces of one whisking cycle as recorded using the CCD array. The black trace is a self-initiated spontaneous whisk performed by the rat before electrical stimulation started. The gray trace is an electrically evoked one (same rat). Note the similarity in shape and duration. D, Comparison of whisker movements evoked from a site in RW at stimulus frequencies of 60, 100, and 200 Hz. Stimulation at 60 Hz evoked rhythmic whisking at constant amplitudes. Higher stimulation frequencies evoked a few whisking cycles at comparable frequency and amplitudes that then (during ongoing stimulation) were followed by cycles tapering in amplitude (most evidently at the highest frequency of 200 Hz).

Figure 5.

Synchronous activation of RW and RF modulates set point of rhythmic whisking. Synchronous activation of two sites within RW modulates whisking frequency. A, Single-electrode stimulation (stim.) from a site within RW (60 Hz, 50 μA). The bottom plot shows 10 single trials evoked the same way and overlaid. B, Double stimulation of the site shown in A and a site in RF that gave rise to whisker retraction in a window starting 670 ms after onset of RW stimulation and lasting for 500 ms (60 Hz, 12 μA). Note the combined effect of rhythmic whisking and retraction after RF stimulation was turned on. The bottom plot shows 13 trials evoked by double stimulation and overlaid. Note that the whisker left the CCD array in several trials, leading to clipping of the trajectory. C, Power spectra from single-electrode (black trace) and double-electrode (gray trace) stimulation from sites within RW. Single-electrode stimulation evoked rhythmic whisking at 8 Hz. The rhythm evoked by double-electrode stimulation shows a significant (see Results) elevation of the whisking frequency to 10 Hz.

Figure 6.

Inherent whisking frequency is evoked regardless of variation in the temporal structure of the stimulus command. The horizontal line on top of the trajectories indicates stimulus duration. The insets depict power spectra of the trajectories. A, Continuous stimulation (60 Hz, 50 μA) leads to rhythmic whisking at ∼7 Hz. B, Burst stimulation (5 Hz) also evokes rhythmic whisking at ∼7 Hz. C, Burst stimulation at 2.5 Hz reflects the stimulus frequency in the whisker movement. However, the rhythmic whisking at ∼7 Hz is evoked as well (note the double-peaked trajectory evoked by each burst and the double-peaked power spectrum in the bottom inset). Calculating the power spectrum from a narrow time window after one burst (frame in the overlay plot at the bottom, depicting 10 trajectories obtained under these conditions) extracts the whisking frequency of 7 Hz evoked independently of the temporal structure of the stimulus (top inset). The axes labels of all insets are the same as in A.