From pouring a glass of milk to typing a news article, precise timing--down to the thousandth of a second--is key to the brain's control of movement.
By studying how monkeys track a visual target, Javier Medina and his colleagues have gained new insights into the strategies that the brain uses to measure time. Their findings indicate that the brain measures time by assessing the duration of a process--using the internal equivalent of a neural stopwatch--as well as by computing the distance that an object being tracked has moved. However, they found that the monkeys did not use the target position as a cue about when to expect its shift to vertical motion.
In their experiments, the researchers trained monkeys to track a target moving horizontally across a screen. After a fixed time interval, the target would abruptly move vertically. To probe how the monkeys were judging time, the researchers would infrequently insert an instance in which the target moved only horizontally. In such cases, because of their previous experience, the monkeys would briefly shift their gaze upward at the appropriate time to pursue the expected vertical target movement.
To learn how the monkeys' brains were processing the movement information, the researchers conducted a series of experiments. The scientists measured the animals' vertical-motion eye responses when they systematically varied (1) the time interval of the horizontal motion before vertical movement, (2) the position of the target when it went vertical, and (3) the speed of horizontal target movement.
In further tests, the researchers attempted to promote the animals' learning of horizontal distance by coordinating the horizontal distance and speed so that the distance traveled remained the same.
"We concluded that the pursuit system determines when to emit a learned eye movement by keeping track of elapsed time and distance traveled by the target and that the relative contributions that these two signals make to motor timing can be influenced by the conditions in the learning trials," wrote the researchers. They concluded, thus "that this temporal precision can be achieved explicitly, by measuring the duration of a time interval, and implicitly, by keeping track of the distance a target has traveled," the wrote.
They also wrote that their findings contradict other studies suggesting that visually guided movements cannot employ an explicit representation of time. And, they wrote, "our findings seem to contradict a recent hypothesis that the neural circuits representing time explicitly might be separate from those that make use of motion signals to keep track of the passage of time. Instead, our results demonstrate that the neural circuitry driving smooth pursuit in primates has access to both time and distance cues, and it appears that these two signals are used concurrently to guide learned timing."
Javier F. Medina, Megan R. Carey, and Stephen G. Lisberger: "The Representation of Time for Motor Learning"