Re: Motor Imagery

From: Harnad, Stevan (
Date: Mon Oct 30 1995 - 20:03:21 GMT

> From: "Baden, Denise" <>
> Date: Mon, 30 Oct 1995 10:51:23 GMT
> I have heard that people who are blind from birth are able to
> appreciate changes in perceptual size as a function of distance. For
> example, a blind person when asked to point to the corners of a wall
> will be able to do this fairly accurately from most distances, i.e.
> their hands will be wide apart when near the wall and closer
> together as they move back. This would also follow from the idea
> that the brain has inbuilt spatial understanding.

Or from the fact that the same correlation between near and far width is
also available from touch and movement. Would you be less surprised if
the blind person could do it on a distance as small as a chessboard?
Than why assume an innate spatial sense at greater distances? (This is
not to deny that there might be an innate spatial sense: just that the
blind pointing evidence does not necessarily show it.)

> j> At least one clinical case of complete loss of visual imagery with
> j> preservation of normal visual perception has been reported (Charcot et
> j> Bernard, 1883)"
> That amazes me, I would not have thought it possible to have normal
> perception without the corresponding imagery. I thought it had been
> demonstrated that perception relies heavily on internal
> representations.

Wait till we discuss blindsight, in which people preserve the ability to
recognise objects presented visually even though they can't see at all
(not just that they don't have visual imagery).

> I don't necessarily think motor images are difficult to describe.

Depends what you take for granted in the way of shared experience, and
vocabulary to describe it. Red would be impossible to describe to someone who
saw only black and white, and movement descriptions would be impossible
to describe (or understand) for someone who could not move at all.

> j> Walking times were found to be closely similar to those measured in
> j> the actual walking condition for the same subjects and for
> j> corresponding distances.
> I'm not so sure that proves anything, maybe the subjects were
> consciously trying to match the imagined time taken to the likely
> actual time. They may have even believed that was what the
> experimenter wanted.

Maybe, but there is a lot of evidence, especially from mental rotation,
that the correlation between the metal operation and the behaviour is
too good to be just a deliberate conscious approximation. We can discuss
this in seminar tomorrow.

> Wow, that is an incredible result, I wonder if I could tone up my
> thighs by imagining a 5 mile jog every morning.

Yes, "mental" physical training, though it sounds like a contradiction
in terms (and in any case sounds unlikely) has been reported to work.
We can discuss this in seminar tomorrow.

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