Figure 5.

Directional Reaching Instructed by an Arbitrary Vibratory Cue Is Rapidly Learned

(A) Schematics of the task structure showing the sequence of events in a trial. Proficient mice were required to hold the resting bar for 2 s in order to receive a vibrotactile cue through the paw: a 200 Hz or 60 Hz sinusoidal vibration. The vibratory cue delivered by the resting bar was followed by a randomly variable delay period during which mice had to continuously hold the resting bar. The end of the delay period was signaled by a “beep” sound (“go” cue), upon which mice could release the resting bar and perform a reach-to-grasp movement toward the left or the right target. Correct responses with grasping of at least 120 ms were rewarded with a water drop delivered through the same water spout and signaled with a “click” sound. Incorrect responses (reaching for the incorrect target, not shown) were followed by a time-out period signaled with white noise (6–10 s). A new trial was started when mice returned the right paw to the resting bar and held it for 2 s.

(B) Pictures of a mouse performing the instructed directional reaching task. Vibrotactile stimuli (cues) were delivered through the resting bar. Two fixed water spouts located to the left or to the right of the animal were the targets for the reaches. The tip of the water spouts was enlarged with a small ball of solder in order to facilitate grasping.

(C) Example raster plots depicting the behavior of a mouse during early and late sessions of the instrumental phase of training. Trials are aligned to the “go” cue (0 s) and ordered according to the outcome of the trial (correct, incorrect, aborted, and inactive) and trial type (left or right) to ease visualization. The vibrotactile stimuli are represented with dashed lines. When the mouse is holding the resting bar or touching the water spouts, it is indicated with continuous colored lines (gray, pink, or green). Reward, if gained, is represented as a blue circle. Note the increase in performed trials and proportion of correct trials between the early and late session. Also, the timings of the behavior, such as the reaction time and the reward time, become more stereotyped. After receiving a reward, mice retract the paw to the mouth and drink the water. Note that after gaining and drinking the reward, mice typically perform subsequent reach-to-drink movements to collect the remaining water. This is visualized as water spout touches in the raster plot after reward delivery.

(D–I) Learning curves showing performance across sessions (n = 14 mice). (D) Percentage correct is the percentage of correct responses (direction of reaching) of all valid trials. (E) Discrimination ability of the vibrotactile stimulus. (F) Rewards per minute gained across sessions. (G) The variability within each session of the timing when mice received the water reward decreases exponentially across sessions. Plotted dots are the mean of 14 mice; solid line is the exponential fit. (H and I) Reaction times to the “go” cue measured as the release time of the resting bar decreases exponentially across sessions in left and right trials. Plotted dots are the median of 14 mice for right (H) and left (I) trials; solid lines, exponential fit.