Re: Anosognosia

From: Stevan Harnad (harnad@cogsci.soton.ac.uk)
Date: Sun Nov 03 1996 - 14:07:04 GMT


> From: Sarah Potts <sep296@soton.ac.uk>
> Date: Fri, 1 Nov 1996 09:27:38 GMT
>
> I read part of "The Man Who Mistook His Wife For A Hat" by Oliver Sacks
> and on page 3 it says that anosagnosia is most common in right
> hemisphere syndromes. I am not quite sure why it affects the right
> hemisphere and not the left. I realise that left hemisphere syndromes
> (such as "The Man With A Shattered World") are not affected by this as
> much but I don't understand why and was wondering if someone could
> explain?

    First, a little aside about the word anosognosia and its etymology
    (origin). Although on that page Sacks says Babinski called it
    "anosAgnosia," and I also incorrectly gave you that spelling in
    class, the currently accepted spelling is anosOgnosia.

    The word comes from the Greek "gnosis," meaning knowledge and "a-"
    meaning without. An "agnostic" is someone who doesn't know, but
    since that has become associated mainly with agnosticism about
    religion, neurology has used "agnosic" (no "t") to refer to a
    patient who does not know or notice or recognise something. So
    patients can be agnosic for colours (colour agnosia), agnosic for
    objects (object agnosia), agnosic for faces ("prosopagnosia" -- from
    Greek "prosopon" and agnosia), etc.

    The reason it is anosOgnosia, rather that anosAgnosia is that the
    word comes from Greek "nosos," meaning disease. So, like
    prosopagnosia, it COULD have been called "nosagnosia," but the root
    "nos" is too ambiguous; "nosognosia" would mean that you DO
    recognise your disease, "nosOAgnosia" would be awkward as the
    word for NOT knowing your disease, so anosognosia was chosen for
    referring to that condition. (By the same token, prosopagnosia
    could have been called "AprosopOgnosia," but that's not the way
    neurologists chose to name the condition.)

    By the way, "cognition" (knowing), comes from Latin cognoscere (to
    know), which in turn comes from Greek gnosein (to know). If we
    had chosen a Greek rather than a Latin root for it,
    "cognitive psychology" would have been "gnosology"...

Now your question about why anosognosia occurs more with right-sided
brain damage than with left: No one knows for sure. One theory is that
a big component of anosognosia is a failure of attention: The symptoms
are there, but your brain distracts you from noticing them. This is
clearest in the weaker cases of anosognosia I described, for example,
hemispatial NEGLECT: This is not as bad as hemianopsia, when you are
blind in half of your visual field (but don't know it); hemispatial
neglect occurs when you can still see there, but you keep forgetting
and not noticing that half of space, so you have to actively keep
reminding yourself.

Now many of the anosognosias are agnosias for parts of visual space, or
parts of your body. The left hemisphere is specialised for language and
the right hemisphere is specialised for space perception. So maybe the
reason there is more left-field neglect (right brain injury) than
right-field neglect (left brain injury) is this specialisation
of the right for space, and perhaps the body image.

There are also those who think that it has to do with a left/right
asymmetry for emotions. When there is one-sided brain injury,
left-brain injury tends to be more often accompanied by
depressive-catastrophic reactions, whereas right-brain injury tends to be
more often accompanied by indifference or even euphoria. This is closer
to anosognosia, but let's not forget that left-brain injuries are also
often accompanied by aphasia, which is a pretty debilitating handicap,
hard to ignore, and perhaps something anyone would get depressed
about!

And yet there are left-brain injuries with aphasia AND anosognosia.
And there are right-brain injuries with depression. It all depends on
the details of the injury. I've been calling it "left-brain" and
"right-brain," but of course we are talking mostly about injuries to the
cerebral cortex. Injuries much further down, in the brain stem, tend
to produce coma or death. And the cortex consists of two hemispheres,
which have different regions: The back part, the occipital area, is
mainly visual. The sides, the temporal lobes, are to a great extent
auditory; there are also motor regions and regions specialising in touch
and smell.

And there are the language regions: Wernicke's area, usually in the
left temporal lobe, is mainly for language understanding; Broca's area,
usually in the left frontal lobe, is for language production.
Some of the strange conditions you are reading about are caused not only
by injuries to this areas, but by disconnections of one from the other
-- the split brain being the most dramatic disconnection of them all.

For more about the agnosia, apraxias, aphasias etc., see

http://www.uic.edu/~csjao/nc/sa/

In general, to search for information about the brain, go to:

http://www.acsiom.org/nsr/neuro.html

I've also put two bibliographies on the Web for you. One on anosognosia
and one on left/right differences in emotion.

Either go near the bottom of my home page, just after all the
skywriting and click on "Bibliographies", or go state to the
anosognosia references with:

ftp://cogsci.soton.ac.uk/pub/harnad/Bibliographies/96.anosognosia

or the emotion laterality references with:

ftp://cogsci.soton.ac.uk/pub/harnad/Bibliographies/emotion.laterality



This archive was generated by hypermail 2b30 : Tue Feb 13 2001 - 16:24:18 GMT