Steve Palmer wrote:
>> seaknight navigation inc. wrote in message
> <89hph3$m8a$1 at medousa.forthnet.gr>...
> >What is the approximate minimum time required to perceive e.g. touch, from
> >the onset of the stimulus, assuming that the cerebral cortex is necessary
> >for conscious perception?
>> That depends on 2 factors:
> 1. The distance of the stimulated area from the brain (i.e. transit
> time is quicker from your face than from your toe).
> 2. The type of nerve fiber involved. Myelinated fibers transmit much
> faster than unmyelinated fibers (ex. sharp, cutting pain, which is
> transmitted by myelinated A-delta fibers, gets to your brain more quickly
> than dull, burning pain, which is carried by unmyelinated C fibers).
For example, from the foot of a tall person to the first synaptic
relay in their brainstem might be 1.5m.
Fast fibres such as touch conduct at around 60 m/s, pain fibres
can be less than 1 m/s.
The conduction delay is then 1.5/60 * 1000 = 25 ms for touch and
1.5 s for pain. There are then addditional delays in
transmission (allow 1 to 2 ms per synapse) and conduction up to
cortex. This is part of the explanation as to why you know you
have stubbed your toe (touch) before it starts to throb (pain).
It is hard to measure just the time to perception - unless the
subject indicates the experience you are only guessing from brain
waves (evoked potentials) that they have perceived it. This then
requires that you include some motor unit loop in your
measurement. If you do this, people can respond to a stimulus in
just over 100ms, so the time to perception must be substantially
less than this. If you were thinking 20-60ms for the earliest
perception you would be in the right ball park.
Cheers
Richard
--
Dr Richard Vickery
School of Physiology & Pharmacology, UNSW, Australia, 2052
ph. 61 2 93851676, fax 61 2 93851059
http://www.med.unsw.edu.au/Physiology/School/staff/vickery/Welcome.html
