Friday, August 15, 2008

Anticipation of movement suppresses sensory awareness.

Here is an interesting tidbit...Voss et al. show that the mere expectation of moving a part of our body suppresses that body part's openness to sensory input. Here is the abstract:
When a part of the body moves, the sensation evoked by a probe stimulus to that body part is attenuated. Two mechanisms have been proposed to explain this robust and general effect. First, feedforward motor signals may modulate activity evoked by incoming sensory signals. Second, reafferent sensation from body movements may mask the stimulus. Here we delivered probe stimuli to the right index finger just before a cue which instructed subjects to make left or right index finger movements. When left and right cues were equiprobable, we found attenuation for stimuli to the right index finger just before this finger was cued (and subsequently moved). However, there was no attenuation in the right finger just before the left finger was cued. This result suggests that the movement made in response to the cue caused ‘postdictive’ attenuation of a sensation occurring prior to the cue. In a second experiment, the right cue was more frequent than the left. We now found attenuation in the right index finger even when the left finger was cued and moved. This attenuation linked to a movement that was likely but did not in fact occur, suggests a new expectation-based mechanism, distinct from both feedforward motor signals and postdiction. Our results suggest a new mechanism in motor-sensory interactions in which the motor system tunes the sensory inputs based on expectations about future possible actions that may not, in fact, be implemented.


  1. This finding brings to mind similar phenomena with regard to perception in the external world. One that I continually am reminded of is the perceptual opponent-process paradigmatic case of the stationary escalator stair case appearing to move backward given our implicit expectation that it be moving the other direction. I've always found this one really neat because even though I've read that this has been a demonstrated psyhological effect, I still often step onto stationary escalators with a bit of hesitation - indicating that my unconscious perceptual system does, if just for a split second, suggests that this staircase is mobile and is moving toward me rather than away.

  2. This phenomenon can (I think) be explained by forward models in the cerebellum. See:

    "Central cancellation of self-produced tickle sensation"

    Basically, your prediction of the sensory consequences supersedes actual sensory input because the loop is much tighter. When the sensory input does make it in hundreds of milliseconds later, it is used to train up that sensory forward model, which is in turn used to bootstrap an inverse model of the motor command.

    Does this paper really show something new?