nystagmus

JoshCahoon joshcahoon at cs.com
Fri Aug 24 16:05:39 EST 2001


>
>Well, most of that was correct. However, Bisiach (the study I believe you
>refer
>to) did a left ear caloric test to alieviate the symptoms of the hemineglect.
>Two things are important to remember - one is that the anosognosia and
>hemineglect that result from right parietal damage show very high rates of
>spontaneous remission indicating that the deficit/abnormality is turned off
>or
>taken over by other brain regions. The second is that left ear caloric
>testing
>drives a rightward nystagmus simulating a leftward change in orientation. The
>leftward orientation is represented bilaterally in the vestibular nuclie
>which
>project up through the dorsal lateral tegmental nucleus and mammillary bodies
>to the anterodorsal thalamic nucleus and various parts of the rest of the
>brain
>including the intact left parietal cortex which may eventually be responsible
>for gating off the right parietal deficit. 
>Anyway, the vestibular stimulation is not likely to be of benefit 'enhancing'
>functioning for exams or driving.
>
>
>
>
>
>

I'm taking a class with V. Ramachandran. He described alleviating anosognosia
and hemineglect symptoms himself with nystagmus, although he also referred to
other studies.

I'm aware that these patients tend to recover from these symptoms relatively
quickly.

However, I don't know the vestibular pathways very well at all. You say 

> left ear caloric testing
>drives a rightward nystagmus simulating a leftward change in orientation. The
>leftward orientation is represented bilaterally in the vestibular nuclie

I'm a little confused about how this happens. The caloric testing in the left
ear generates signals in the left vestibular nucleus, which projects to the
right vestibular nucleus, resulting in the perception of leftward movement? 
You also suggest that it's left parietal activation that alleviates the
symptoms during a left ear caloric test, whereas I've been thinking it's right
parietal activation. I don't know the neurophysiology of this pathway well
enough to say which is more plausible. Do you know of any evidence that left
ear caloric testing preferentially activates left or right parietal cortex?

At any rate, if caloric testing does result in increased activity of either
left or right parietal cortex,  you  think it would be implausible that it
might enhance some normal parietal function? Why?

Rama posited a couple hypotheses to explain why left ear caloric testing might
alleviate neglect and denial. One is that it stimulates the right hemisphere,
one specialization of which he thinks might be "anomaly detection" whereas the
left might be more prone to rationalization. Another is the REM-like eye
movements of a person undergoing nystagmus might activate the neural pathways
that allow "repressed thoughts" into consciousness during dreaming. Both
hypotheses are highly speculative. At any rate, this is kind of a digression
from my main question--can caloric testing enhance any normal function? I've
outlined why I think this might be the case, and am still unclear about  why
the possiblity should be discounted. 




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