Mark asked:

We've all heard that direct electric stimulation of the brain can elicit memories and sensations in the
subject undergoing such a procedure — someone being thus probed might remember a past incident,
or smell the scent of a peach.

Assuming these reports are true, (I know nothing of the science of it) would it follow that this kind of
direct stimulation of the brain could cause new, unprecedented sensations in the subject? That is, is
the scent of a peach somehow there, already, in the brain, awaiting to be switched on? If so, could
new knowledge (e.g. the taste of mint to one who had never tasted it) arise in this situation?

This seems improbable, but the alternative seems to be that the act of smelling a peach causes that
smell to be recorded somehow by the brain. The problem here is what is happening the second time
you smell a peach...are you smelling the peach or is your brain providing the sensation for you? It
seems you could never know, which makes the first idea not so far-fetched. Any ideas?

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The easy answer to "pre-sensation" is: No, there is no scent (or any other sensory stimulus) hidden in
your nervous system. To put it extremely simply: Smell is communicated to the nervous system by
molecules, so it's an altogether physical impingement. You may know that Hobbes promoted the view
that all our senses are variants of the sense of touch; and science has confirmed this. What this
means in relation to your question is that our sensory modalities are each "preconfigured" (by long
massaging from evolutionary factors) to have sensitivity to "their" species of input, and being physical,
stimulation modifies the affected nerve strands (this is called "hardening" in neurophysiology). This
physical alteration acts like a marker to pathways in your memory facilitating recall. What your
question deals with are memories that the subjects have forgotten — but the hardened nerve strands
are still in place and therefore accessible under certain conditions (including direct stimulation of the
dedicated nerve strands).

There's much more to it, of course, but I can't produce a thesis here. The only other thing that might
help your understanding is that memories are not stored, computer fashion, as complete (tactile,
audio or video) images, but as instructions for the mind to reconstruct the whole memory from its
internal resource of stored stimuli.

Jurgen Lawrenz

Sydney

Yes, those reports are pretty much true, depending on where in the brain the stimulation is. It is the
case that the act of smelling the peach causes the smell to be "recorded" (ugh, a bad term, really...
it's much more complex than that) by the brain. So when you smell a peach when your brain is being
stimulated, you are recreating the neural effects of smelling a real peach... and you can't tell the
difference, since those neural effects arewhat smelling a peach is. Now, the last two sentences are
rough. There are very many qualifications I could put on them, about how complete the experience is,
where in the brain the stimulation must happen, other things accompanying smelling the peach
(seeing it, etc.), philosophy about mind/ brain differences... and so forth. But very roughly that's what's
going on. You might check out a recent book:The World in Your Headby Lehar.

Now, as far as stimulating the brain and producing a newsensation. An interesting idea... but not
unprecedented. Think of the similar effects of drugs, for example. Don't we have there stimulation of
the brain, not identical to stimulation by electrodes, perhaps, but similar, and external in the sense of
not willed, i.e., not self-induced? And doesn't it result, sometimes, in strange ideas? Think about LSD
trips, about how we think better (or so we hope) on caffeine... etc. That's really pretty much the same,
isn't it. So really, you don't need to think about electrodes here, and people have been doing this for
quite a long time.

Steven Ravett Brown