reaction time

reaction time Your hand accidentally touches the hot plate of an oven and is withdrawn immediately. A young child runs out in front of your car and you hammer on the brakes. A lottery ball falls into its position upside down and you have to shout out the correct number as fast as you can to a colleague who is checking off the numbers for your syndicate. All three examples of reaction time are the time it takes to make a movement in response to a sensory stimulus. However, even if we try to respond as fast as possible in each situation, the reaction time is quite different.

In this context, time is measured in milliseconds (ms) — thousandths of a second. It may take only 100 ms to withdraw our hand from the stove, 200 ms to stamp on the brakes, and 500 ms to read out the number on the ball. The difference occurs because of the different amount of time it takes for the central nervous system (CNS) to process the sensory signals and to choose the appropriate course of action.

The quickest reaction times have the simplest neuronal circuitry. Tap the knee and the leg moves. This is the tendon jerk beloved of clinical neurologists. The tap excites receptors in the quadriceps muscle at the front of the thigh and these send signals back to the lumbar part of the spinal cord. There, a direct connection is made to the motor neurons that innervate the quadriceps muscle and cause it to contract, making the leg kick forwards. It takes a total of about 30 ms for this to happen. The receptors take 1–2 ms to respond, and another 1–2 ms is needed for the connections to operate in the spinal cord. The remaining 27 ms or so is taken up with the time it takes nerve impulses to travel from muscle to spine and back again. There is of course a price to pay for such a fast circuit. The circuit is so simple that the same thing happens every time the tendon is tapped; it is impossible to control what happens no matter how hard we try. Because of this we refer to this type of reaction as a reflex, and the time it takes as the reflex response time. In electronic jargon we can imagine that it is a hard-wired input–output circuit.

There are rather few examples where the circuit is so simple. The corneal reflex, which causes an eye blink when a speck of dust hits the cornea, is one of the few other familiar examples. Most other very rapid reactions turn out to be more complex. Withdrawal of a hand from a hot cooker is certainly automatic, but can, with great effort of will, be controlled. The neural circuit is more complex than that for the tendon jerk, and this gives it more adaptability at the expense of a longer response time. However, like the tendon jerk, this is a circuit that is innate, and ready for action from the moment we are born.

More complex reactions, like hitting the brakes to stop a car in an emergency, are neither innate nor hard-wired. After all, a person who had never been in a car before would have no idea how to stop the vehicle. They are learned responses that can be selected with remarkable speed in the correct conditions. In the simplest situation we may be asked to press a button as soon as possible after a light is illuminated. There is no ready-made circuit to do the job. Instead, the motor system prepares in advance the instructions for the response (move the arm), and all our attention is concentrated on the light. As soon as a change in illumination is detected, the instructions for movement are released and the button is pressed. In this situation the CNS narrows down the total possible number of movement options and sensory events to just one of each, and links them together with high security. Of the millions of possible connections between sensation and movement, one is highlighted by the preparation to respond in a particular manner. In the case of driving a car, there may be several circuits that have a particularly high probability of being called into action. One of them may link the operation to press the brakes hard with the unexpected arrival of an object in the path of the vehicle. Such very fast responses are sometimes referred to as voluntary, to indicate the necessary involvement of volition in preparing to respond in a particular way to what may well be an arbitrary event. The term ‘voluntary’, however, does not mean that we need consciously identify the sensory signal before issuing the instructions to move. Drivers will often volunteer that they pressed the brakes before knowing what it was that was in front of the car. They may well say that it was a ‘reflex’ response, presumably indicating that conscious appreciation of the action occurred only after the event.

There are some responses that require much more careful evaluation of the sensory input before an appropriate movement can be selected. These have longer reaction times, since the circuits cannot be prepared in advance with any certainty. Calling out upside-down numbers on lottery balls is probably in this category. First of all the visual field must be rotated mentally by 180 degrees, and even then, fifty possible responses are available, perhaps narrowed down to 10 if the colour of the ball is known. All of this takes the CNS a good deal of processing, and by the time the response (vocalization of the number) is selected, the sensory impression has probably reached consciousness.

In summary, reaction times span a spectrum of response types. At one end are very fast, predefined neural circuits such as the tendon jerk and the withdrawal reflex that always operate, but which can be modulated, depending on how complex their connections are, by volitional control. At the other end, sensations must first be evaluated and then assigned the correct motor response, which prolongs the response time by a factor of ten or more. In the middle, situations occur in which the CNS can accurately predict what to do when a certain simple sensation is received. In these circumstances, sensory and motor circuits are selected in advance and joined with high probability so that processing time is reduced to an absolute minimum.

J. C. Rothwell

See also reflexes.