Reciprocating Bill
Posts: 4265
Joined: Oct. 2006
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| Quote (creeky belly @ Sep. 29 2007,03:14) | | Quote | | This brings up the issue of explaining how professional tennis can be played when a half a second delay in response is too long. |
I didn't hear about Libet's death, but here are some thoughts. There could be many other factors that tennis players pick up on: frequency of faults, favoring down the middle versus cross court, watching opponent's body orientation and habits, looking at the racket orientation. Having played tennis myself, much of it is knowing the best place position yourself. If you're talking about raw reaction time, simply trying to react to the ball is probably not a good strategy (around 186 ms for the ball to leave the racket and reach the net on a serve). |
The 500ms data is a challenge primarily to the notion that we consciously guide behaviors - that we become conscious of a visual scene, contemplate its contents, select actions based upon what is consciously apprehended, then act. There are many lines of evidence that suggest that motor planning, even that based upon visual information, includes major elements (both cortical and subcortical) that are entirely unconscious - and unconstrained by the 500ms delay. In the instance of very rapidly executed behaviors (such as in tennis), both the sensory information upon which rapid acts are based and the the motor plans that guide the behavior become available to conscious processing only after the fact - the first as a visual experience and the second as a sense of volition. Even visual-motor behaviors that are executed quite deliberately, such as inserting a card into a slot, are guided by extensive processing of visual information that is never conscious, and that does not contribute significantly to the experienced visual scene. See, for example, Milner, D. A., & Goodale, M. A. (1996). The Visual Brain in Action. Oxford: Oxford University Press.
This is not to say that conscious processing has no role to play. Much of what goes on in tennis (and other rapid motor activities) is driven not in response to a sensory scene that represents the present, but rather by means of a multilevel model - a constantly updated model that includes representations of the anticipated scene, the player's body and possible motor actions, and even the player's understanding of the strategic circumstances within which their opponent's play is embedded. Play is guided by the anticipatory model rather than real time sensory information, which at best enables last instant adjustments of the model for better fit.
A clear example of this is seen in hitting in major league baseball: There is usually insufficient time for a batter to determine which pitch is coming once the ball is thrown (fastball or something off speed?), so the hitter attempts to anticipate a particular pitch on the basis of strategy and "sits on it." This is a very high level activity that includes the batter's representation of a pitcher who understands he faces a batter who is attempting anticipate his next pitch, and so on. Upon anticipating a fastball in a given count, the batter initiates a subcortically executed motor plan for hitting a fastball as the pitch is delivered, using sensory information to perform last minute modifications of the plan to enable contact. If the guess is wrong there is often too little time to substitute another plan and the batter (batsman?) fans at the ball.
In short, while the 500ms data is a challenge to the ordinary notion that we are entirely conscious actors, it doesn't indicate that things are going too fast for forward causality in the nervous system to accommodate; much of what guides behavior is unconcious, while other elements are pre-processed.
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