This is an excerpt from Motor Learning and Performance-5th Edition by Richard A. SchmidtT & im Lee.
Limitations in Movement Programming
As you’ll recall from chapter 2, the movement programming stage is the third in the sequence of information-processing stages. Here, after the performer has perceived what the environment will allow, and after having chosen a response that meets those demands, the performer must still organize the motor system in order to actually execute the action. In this stage, the performer must make critical adjustments that occur at various levels (e.g., in the limbs, muscles, and spinal cord). These adjustments take time, of course. A good example is the action of a fencer, who must preprogram a movement despite having to execute the movement in the face of a potentially changing environment. In the following example the programmed action is somewhat complicated, involving a move toward the center shoulder and then followed by a sudden change in the action.
A fencer moves the foil toward the opponent’s shoulder but then quickly alters the direction and contacts the waist instead. Often, responding to the first move, the fake, seems to have interfered with the opponent’s speed of responding to the second move, and the point is lost. The delay in responding suggests strong interference between activities in the later stages of information processing. Specifically, the delay occurs because of interference to the movement programming stage, which has the task of organizing the motor system to make the desired movement.
Some of this view comes from considerable research evidence using the so-called “double-stimulation paradigm,” where the subject is required to respond, with separate responses, to each of two stimuli presented very closely together in time (see Focus on Research 3.2). This paradigm is in many ways analogous to the problem facing the fencer’s opponent, who must respond to one move and then another in rapid succession. The delays in responding occur because of the interference that arises in programming the first and second movements as rapidly as possible.
Psychological Refractory Period
The delay in responding to the second of two closely spaced stimuli is termed the psychological refractory period (PRP). An important question concerns how soon a person can switch from (a) making a goal-directed response to one stimulus to (b) making a different goal-directed response to a different stimulus. The motor system processes the first stimulus and generates the first response. Then, if the experimenter presents the second stimulus during the time the system is processing the first stimulus and its response, the onset of the second response can be delayed considerably (the PRP effect).
One explanation for the PRP is that there is a kind of “bottleneck” in the movement programming stage, and that this stage can organize and initiate only one action at a time, as diagrammed in figure 3.6. Any other action must wait until the stage has finished initiating the first. This delay is largest when the time between stimuli (SOA) is short, because at this time the movement programming stage has just begun to generate the first response; this response must be emitted before the stage can begin to generate the second response. As the SOA increases, more of the first response will have been prepared by the time the second stimulus is presented, so there is less delay before the movement programming stage is cleared.
One more finding is of interest here. When the SOA is very short, say less than 40 ms, the motor system responds to the second stimulus in a very different way. The system responds to the first and second stimulus as if they were one, which produces both responses simultaneously. In this phenomenon, termed grouping, the early processing stages presumably detect both stimuli as a single event and organize a single, more complicated action in which both limbs respond simultaneously.
The phenomenon of psychological refractoriness just discussed accounts for many of the underlying processes in faking. In basketball, for example, the player taking the shot preprograms a single, relatively complex action that involves a move to begin a shot, a delay to withhold it, and then the actual shot—all done in rapid succession. The shooter’s movement is organized as a single unit and is prepared as any other movement would be in the movement programming stage. However, the defensive player sees only the first part of this action; this can be thought of as the first stimulus (S1) in the double-stimulation paradigm, and it triggers the response to block the shot, which does not occur until later. The processing of the first stimulus leads to large delays in responding to the new information that the shot has been withheld, that it is a fake (as indicated by the fact that the second stimulus, S2, the actual shot, is now being made). The result is that the first response (movement to block) cannot be withheld, and it occurs essentially as originally planned. This creates a very large delay in initiating a second, corrective response to block the actual shot, which is made at about the same time that the defensive player is dropping back to the floor after “taking the fake.” This all makes the shot very easy for the offensive player and makes the defensive player look a little foolish at the same time.
Some principles of faking emerge from research on psychological refractoriness. First, the fake—the first response—should be realistic, distinct, and clear, so the defensive player treats it as an actual shot. Second, for the effective fake, the single programmed action that contains both the fake and the actual shot is planned so as to separate the fake (stimulus 1) and the actual shot (stimulus 2) sufficiently to generate a relatively large delay for the response to stimulus 2. From the data in figure 3.5, for the fake to be maximally effective, this separation is somewhere around 60 to 100 ms. If this separation is too short, the defensive player can ignore the fake and respond instead to the actual shot (grouping). If the separation is too long, the defender will respond to the second stimulus with an essentially normal RT, and the shooter will have lost the advantage of the fake.
The Double-Stimulation Paradigm
Research on the psychological refractory period (PRP) uses the “double-stimulation paradigm,” in which the subject is asked, for example, to respond to a tone (Stimulus1) by lifting the right hand from a key as quickly as possible. A very short time following the tone a light (Stimulus2) might appear; the subject is to respond by lifting the left hand from a key as quickly as possible. The separation between the onsets of the two stimuli, called the stimulus-onset asynchrony (SOA), might range from zero to a few hundred milliseconds. Researchers are usually interested in reaction time (RT) to the second stimulus (RT2) as a function of the SOA. (See the paradigm timeline shown in figure 3.5a.)
The general findings from one study using this paradigm are graphed in figure 3.5b, where RT2 is plotted as a function of the SOA. The horizontal line (labeled RT2) is the value of RT2 when the first stimulus is not presented at all; it represents the “usual” (without interference) RT to this stimulus using this response. Depending on the length of the SOA, there is a marked delay in the RT2 while the first stimulus is being processed. When the SOA is about 50 ms, the delay is very large, and it can more than double the value of RT2, as compared to its control value. As the SOA lengthens, the delay in RT2 decreases, but there is still some delay in producing RT2 even with SOAs of 200 ms or more. The single-channel hypothesis (Welford, 1952), which was originally proposed to account for effects like these, argues that the processing of the first stimulus and response completely blocks the processing of the second stimulus and response until such time that the processing of the first stimulus and response has been completed. More recent thinking about data such as these holds that the major delay in RT2 arises from interference between the movement programming stages of these actions.
1. Why is the most important comparison in this paradigm the RT to the second of two closely spaced stimuli rather than to the response to the first stimulus?
2. How would the magnitude of the PRP effect be expected to change depending on the number of choices involved in responding to the first stimulus?
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