Michael is no doubt correct in saying that most of the delay is farther
upstream, but the details of his exposition are misleading.
Not sure what he means by saying visual input takes a path "through the
limbic system"--maybe "as the crow flies" ?? i.e., the main route may
pass by some limbic landmarks, but it is a non-limbic route on its way
to the thalamus.
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WAIT--NEVER MIND--JUST RE-READ HIS COMMENT! Sorry about that. Just
back from several days at INS mtg in Boston and dead tired, not reading
so carefully. Obviously, he is talking about route from thalamus to
amygdala vs. from thalamus to visual cortex. Well, my "correction"
which follows (i.e. I'm inserting this after writing that) is not a
correction but simply an elaboration of his point.
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BOTH visual and auditory "main streams" are relayed from the thalamus,
to primary visual and auditory cortex; as are all sensory modalities
except olfaction. For complexly-determined, context-relevant
responses, further intracortical relays in parallel streams through
associative cortex contribute further to the latency of the response.
There are detours and short-cuts, however. Joseph LeDoux, for example,
has described a sort of "short-circuit" from the thalamus to the
amygdala, and from there to motor/behavioral output AND (a separate
route) to autonomic/physiological output--providing a basis for very
rapid "reflexive" but learned responses.
(And of course both visual and auditory inputs also have side-road
routes bypassing the thalamus to superior and inferior colliculi, etc.)
F. Frank LeFever, Ph.D.
New York Neuropsychology Group
In <36C2E8D8.28B26C5D at mich.com> Michael Edelman <mje at mich.com> writes:
>>>>Didier A. Depireux wrote:
>>>R.Hill at iti.salford.ac.uk wrote:
>> : Why does the response time take so long compared with the time
>> : for a reflex.
>>>> Someone will give a more intelligent answer than me, but I think
that the biggest lag
>> is at the level of the retina, where receptors take a long time to
>> providing a potential strong enough to generate an action potential
(how much more
>> cursory can you get?). In the cochlea, all that happens is a
travelling wave that
>> propagates up the cochlea (2-3 ms) and a transduction in the inner
hair cells, which
>> takes a ms or so.
>>Of course the hair cells in the cochlea also have to build up enough
transmitter at a
>synapse to initiate an AP. There's no real difference in transduction
speed. And when
>you're dealing with a reaction moderated by the concious individual
the reaction time
>(perhaps 750ms or more) far outweighs any differences in receptor
>transduction should really be faster, since it takes virtually no time
for light to reach
>the back of the retina.
>>Hearing and vision are both rather complex systems. The optic nerves
for example, take
>two different paths, one through limbic system and one on to optic
cortex. Sound can be
>analyzed at different levels as well. There are different kinds of
>require different processing.
>>Responding to a flash of light is like responding to a loud sound-
this can be handled by
>your "reptilian" brain and doesn't need concious monitoring to
intercede. More complex
>task like pattern recognition is going to require activation of parts
of the cortex.
>You've got multiple hard-wired analysis stages in visual cortex before
you even get to
>the pattern extraction and comparison stage.
>Michael Edelman http://www.mich.com/~mje>>