References: <1559000001 at igc.org>
Sender: M Bartos at uk.ac.ed
Organization: Edinburgh University (Scotland, U.K.)
In article <1559000001 at igc.org> apakaln at igc.org (Alan Pakaln) writes:
>>Question: I extend my hand and touch my fingers together.
>I feel my fingers touching and I also experience it happening "out there"
>a distance from my body.
>Is "out there" a sensation? And is that why I "feel" in my fingers
>instead of "in" my head?
The perception of location of limbs/body position in space is called
proprioception. I think it's quite valid to say that "out there" is
something which we sense. The way we derive the information relating to
where out there our limbs can come from two possible routes
1) Information from sensors (e.g. muscle spindle signals returning to the
spinal cord - these signal information relating to stretch and tension)
2) Information from within (i.e. if you know how you tell something to
move and know where it starts then you should be able to derive where it
is - assuming it is unhindered)
These two routes are known as EXAFFERENCE and REAFFERENCE. (Within the
second category there is something called corollary discharge, but no
need to confuse the issue!)
Anyway, as ever neither one nor other is completely correct (e.g. you
need to know something about how you are moving your muscles to modulate
your muscle spindle sensitivity and you need to know something how you
are modulating your muscle spindle sensitivity to work out when they
sense that you've encountered an obstacle and you need to know about
obstacles to judge where your limb because your movement might not have
reached where it was going => you need to have 1 and 2 ...)
Interestingly proprioception does have relatively localisable neural
pathways in the spinal cord... proof of this comes from syphilitic
patients who suffer 'tabes dorsalis', in this condition the dorsal
(those closest to the back) columns of the spinal cord suffer some
degeneration and the patients lose the sensation of locality of their
limbs. This is most noticeable when they try to walk. In order to walk
they adapt by using vision to give them the needed information about
limb/foot position. A diagnostic test (suggested by one anatomy
demonstrator) was to stand a suspected sufferer in a room, feet together
and then snap off the lights... a positive result was if 'you hear a
thud' as the patient topples :-) :-o positive from the diagnostic point
of view - not the patient's! If you think about it, when we stand
vertically feet together we have stability like a pencil standing on one
end, given that we sway a little and have more joints around which we
can slip from a perfectly vertical posture we are very lucky to stay
upright. Our good fortune comes from a clever 'fly-by-wire' or more
appropriately 'stand-by-nerve' set of postural reflexes which
continuously reset the tone of important muscle groups. (Other clever
systems dear to my heart include those which manage the position of the
head and eyes - ever wondered why we manage to keep our eyes locked onto
a target even when we move quickly (especially if turn)? or why our head stays
pretty stable even though it's so heavy and our necks are so maneouverable?)
Ouch, this post is getting over long for such a brief question...
Just to leave you thinking about other senses which aren't normally
thought of ...
Have you ever wondered how it is that we can tell durations of events?
i.e. if you ask someone to tell you when ten seconds has passed then
they will succeed to within a certain reasonable accuracy; also you can tell
when a length of time varies (try dropping a ball onto a hard surface and
listening - you can perceive that the length of time intervals changes
and you can tell that they are getting shorter); ever switched off an
alarm just before it went off e.g. a 1 minute beeper on a microwave
Still, enough! Too much wonderful neuroscience to type about, too little time...
"Hear what they say about tabes dorsalis?"
"No... what do they say about syphilis... (do I really want to hear this....)"
"Dangerous behaviour in the dark leads to the dark being dangerous for
"That really wasn't worth waiting for."
Asking questions, thinking and clearly explaining concepts is worth more to
science than citing names and dates of papers. So why isn't this always
reflected in present academia? After all facts are free and will increasingly
be at people's fingertips but thinking and explaining costs effort you can't
yet successfully get a computer to spend...