On 27 Mar 2000 10:43:55 -0500, Bill Skaggs <skaggs at bns.pitt.edu>
wrote:
>Hmm. What about H.M.? His amnesia can't be attributed to ischemia,
>can it?
According to the recent MRI scan by Suzanne Corkin et al (1997, J.
Neurosci.), H.M. has 1/3 of the posterior hippocampus spared. The
major part of the excisions are constituted by the entorihnal and
perirhinal cortex cortex, the amygdala, and the anterior 2/3 of the
hippocampus. Normally, 1/3 sparing of the dorsal hippocampus would be
sufficient to support spatial learning in rats (the posterior
huppicampus in humans are homologous to the dorsal hippocampus in
rats), it is unlikely however if the remaining hippocampal tissue in
H.M. is functional as it may be disconnected from its cortical
afferents. Attributing the amnesia in H.M. to hippocampal damage is
difficult, as the lesion is unspecific and includes regions
(perirhinal cortex) that are required for the neurpsychological tests
of "declarative memory".
>Also, there is some difficulty in comparing the results of
>delayed-nonmatch-to-sample experiments using humans versus animals.
That is definately true.
>It isn't clear, as far as I
>know, that human amnesics given the same kind of training used in
>hippocampal-lesioned animals would do any worse than the animals.
There have been no reports of human amnesics with a selective
hippocampal lesion that do stem from cerebral ischemia. Animals do not
solve the DNMTS task if they have got a hippocampal lesion from
experimentally induced ischemia. The problem is to find a human case
that parallel the hippocampectomized animals that can solve the DNTS
task. As I may have metioned, the only visible neuropathology produced
by global ischemia is in the hippocampus, but the functional loss from
such ischemic brain damage is different from the functional loss
produced by ibotenate lesions of the hippocampus (in animals, that
is). This allows, in my opinion, no conclusion about human memory and
amnesia to be drawn. Hence the talk about hippocampal "memory
processing" in humans have no real foundation (actually, the
discussion about consolidation of hippocampal memories in the cortex
may suffer from the same fallacy). Elanor Maguire have reported (in
1996) that in humans, hippocampal activation is correlated with
"navigation" or "spatial memory" rather than non-navigatory "episodic
memory".
>Certainly there are many situations where amnesics show no conscious
>recollection of a previous experience but still show strong implicit
>effects on their behavior.
I personally hate the term "conscious recollection" as it has no
operational definition. In most instances "conscious recollection"
means "verbally reporting", as pointed out by Case Wanderwolf in a
review paper in Biobehavioral and Neuroscience Reviews (I don't have
the exact reference in my head, but I can look it up). That a human
can "recollect something consciously" means that it can give a verbal
report of the event. The question is whether something has to be
verbally reportable in order to be conscious. Try e.g. to describe a
particular odour. How does a perfume smell? This task is extreemely
difficult, nevertheless few would argue that olfaction is
"unconscious". And what about aniamls and aphasics, are they all
"zombies"?
So, this is why I confess to behaviourism. It avoids this
epistemological problem, which I would argue is an insolvable
pseudo-problem.
>I am personally convinced that both the memory and spatial models have
>substantial validity, and that the great task of hippocampal research
>is to come up with a "grand unified theory" that accounts for both
>(rather than expaining one aspect away, as the current theories do).
I am personally sceptical to the entire concept of "memory".
There is no organ that taskes care of sensing, nor are there any
organ that take care or perceiving. Common knowledge for any zoologist
is that evolution doesn't produce general purpose mechanisms, it
produces specialized solutions to particular problems. I think
nauroscientists should begin to listen to zoologists like Rüdiger
Wehner and cognitive psychologists like Randy Gallistel who have been
pointing this out at various occations. I am convinced that evolution
did not produce the hippocampus for the sake of recollecting
consciously. Rather, it did probably had something to do with solving
a particular ecological problem, that is, achieving a goal by
processing a particular type of information in a particular manner. So
hippocampal models should emphasise information processing and
descision making ther than conscious recollection.
Hence I prefer the "spatial" models because they are closer to
bahvioural ecology (rather than the "declarative memory" models as
they come pretty close to para-psychology and superstition.) But I do
admit that the current spatial models are insufficient to explain some
of the reported data.
Sturla Molden
