nystagmus

MEShinder meshinder at aol.com
Sun Aug 26 13:27:03 EST 2001


In article <20010824170539.18593.00000839 at mb-fk.news.cs.com>, joshcahoon at cs.com
(JoshCahoon) writes:

>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? 

The vestibular nuclear complex is extremely bilateral in anatomy due to
extensive commissures, and bilateral in function by creating velocity storage,
neural integration, and inhibitory sensory tuning through bilateral networks.
The representation or interpretation of movement is a balance of the bilateral
inputs - hence caloric testing drives the sensation of movement while only
changing responding in one ear. Perception of leftward movement is represented
simultaeously throughout the nuclie bilaterally, so I refer to it as bilateral
representation.

>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?

I suggested the left parietal as plausible, because the right is damaged
(usually by stroke) which is causing the deficit. However, there is to my
knowledge no physiological study of this phenomena, so I chose the nearest
neighbor with similar functional characteristics. The projections to the
aterior thalamic feed the various cingulate pathways (frontocingulate,
temporocingulate, and parietocingulate) which become difficult to navigate, but
can signal much of the brain so not too many possibilities can be ruled out at
this time.
>
>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?
>
I'm not suggesting increased activity at all. I'm stating that it has been
proposed that orientation signals from the vestibular system play a significant
role in the hippocampal place/orientation and parietal interpretation or
integration of orientation. Such processing is assumed to be important for
navigation, coordination, spatial memory, etc. Such functions can integrate
vision, somethetic, auditory, and vestibular information. (A way to think of
this is navigating through one's house where it is easier to do during the day
or with lights, but it can be done by memory in the dark given you know which
way your facing.) These are specific signals being used as cues, components, or
atributes. There is nothing as yet that indicates to me that this causes
generalized local activation that could enhance normal function. 

>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. 
>
I'm more of a systems physiologist/functional anatomist at heart, so someone is
going to have to convince me that damage from a right parietal stroke can be
alieviated by right parietal stimulation. And I'm also sceptical of the
mechanistic pathway for nystagmus to  activate dream states on their own when
nystagmus is such a normal and prevalent function - you can drive it by looking
out a car window at the poles by the side of the road. 
You could do an experiment to the effect - does vestibular activation enhance
the probability of complex sensory integration. However, before you start
trying to develop such a difficult experiment, it might be worth reviewing that
stimulation of any sensory modality in relationship to general functioning
tends to have an effect - usually by novelty or arousal. 
Like the visual system and such things as edge detection, orientation and
orientation changes are specific sensory stimuli. 




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