Freethought & Rationalism Archive

ps418 · 06-27-2003, 10:50 AM

Hemineglect (aka, unilateral neglect, hemispatial neglect) is a strange neurological condition that often develops afters stroke, in which people are not consciously aware of, and are strangely indifferent to, what is taking place in one spatial hemisphere. Usually it is a right brain stroke that results in left-hemispace neglect. This is not simply a perceptual problem. Patients for instance may apply makeup to or shave only one side of their face, comb hair on one side of their head, eat food on one side of their plate. V.S. Ramachandran has some interesting descriptions of neglect patients in his book 'Phantoms in the Brain.' For more info:

Unilateral Hemineglect and Anosognosia of Hemiparesis and Hemiplegia

Hemispatial Neglect Syndrome

Review of Unilateral Neglect: Clinical and Experimental Studies, by Robin Walker

A new paper by Jiang et al (2003) offers new insights into the physiological basis of hemineglect. As I understand the model they proposed in their paper, the basal ganglia are supposed to modulate the activity of the superior colliculi on each side of the brain by means of two inputs, on inhibiting and the other disinhibiting. The SC are bilateral structures in the midbrain that have been shown to be involved with the processing of spatial information, i.e. the orientation of objects in space. The idea is that in hemineglect, the disinhibiting input to the SC is obliterated, so that the SC only recieves the inhibiting input, and thus the SC on that side of the brain can not respond to visual input.

Quote:

One of the intriguing implications of this organizational scheme is to provide a potential physiological basis for the Sprague effect, a paradoxical case of the amelioration of a lesion-induced deficit by a second lesion28. Contralateral visual neglect produced by large unilateral lesions in the visual cortex can be 'reversed' by lesions that affect crossed nigrocollicular neurons15, 28, 29. According to the present data, eliminating the crossed nigrocollicular pathway removes the widespread inhibition conveyed by these neurons onto the cortically deafferented SC, which presumably reinstates responsiveness in that SC. This, then, would allow previously ineffectual visual inputs to initiate SC activity and SC-mediated orientation behaviours again. Thus, the amelioration of hemineglect can be interpreted as the disruption of an essential component of the circuitry that is normally used by the basal ganglia to coordinate visuomotor activity in the two halves of the neuraxis. This is consistent with the developing understanding that the basal ganglia are critical structures in the manifestation of human hemineglect syndromes30.

Jian et al, 2003. Opposing basal ganglia processes shape midbrain visuomotor activity bilaterally. Nature 423, 982 - 986.

I admit that this article is a wee bit beyond me, neurologically. Nonetheless, what I could understand seems quite interesting. For one thing, I wasn't aware that hemineglect-like conditions could be experimentally produced in other animals and reversed with a second brain lesion. It will be interesting to see if hemineglect in humans can be reversed by interfering with basal ganglia inputs to the SC. In the press release, coauthor Stein comments:

Quote:

"The potential site of treatment has now been identified," said Barry E. Stein, Ph.D. professor and chairman of the Department of Neurobiology and Anatomy and a co-investigator. "Once you know where the site is, you can speculate about cures and strategies for therapy."

One likely treatment is to disrupt some actions of the basal ganglia so the remaining intact visual areas can function again. The malady itself is among the most bizarre of medical conditions. "These people have very strange perceptions," said McHaffie. "They can go so far as to say, 'that's not my arm.'"

Wake Forest Scientists Figure Out Bizarre Visual Condition

Patrick

doghouse · 06-29-2003, 07:19 AM

Thanks for the links. As your reviews indicated, neglect involves sensory functions other than just vision as well as motor deficits. I wonder if similar mechanisms (unopposed inhibition) that Jian�s group found in the visual system are involved in these other deficits as well.

Gurdur · 06-29-2003, 09:08 PM

Yaaarrr, thanks Patrick, for the info.

echidna · 06-29-2003, 10:40 PM

Sacks also wrote some on this IIRC, although I don't remember it by that name. On occasion his patients would become very distressed at this intruding arm which would keep invading their space & try to push it away with the other.

Wounded King · 06-30-2003, 01:05 AM

I remeber sacks recounting of his patient who atee half the food on his plate and would then rotate the plate, or did someone else do that(?), so he could eat the rest.

ps418 · 06-30-2003, 07:16 AM

Quote:

Originally posted by echidna
Sacks also wrote some on this IIRC, although I don't remember it by that name. On occasion his patients would become very distressed at this intruding arm which would keep invading their space & try to push it away with the other.

This is referred to as 'Intermanual Conflict,' and is part of the Alien Hand Syndrome. I first heard about it in association with callosotomy patients (i.e. split-brain patient), though it can occur with other types of brain injury as well. There's two interesting reviews and case studies available for free:

Chan and Ross, 1997. Alien hand syndrome: Influence of neglect on the clinical presentation of frontal and callosal variants. Cortex 33, 287-299,

Fisher, 2000. Alien Hand Phenomena: A Review with the Addition of Six Personal Cases. Canadian Journal Neurological Sciences 27, 192-203.

Patrick

doghouse · 07-01-2003, 08:58 AM

Quote:

Originally posted by Wounded King
I remeber sacks recounting of his patient who atee half the food on his plate and would then rotate the plate, or did someone else do that(?), so he could eat the rest.

Dogs with hemineglect do this too. They will eat all the food on one side of their bowl, then stop. You turn the bowl around and they eat the rest. I suspect drinking would be the same, but I can't figure out how to keep the water on only one side of the bowl

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06-29-2003, 07:19 AM	#2
doghouse Veteran Member Join Date: Feb 2003 Location: Standin in the rain, talkin to myself Posts: 4,025	Thanks for the links. As your reviews indicated, neglect involves sensory functions other than just vision as well as motor deficits. I wonder if similar mechanisms (unopposed inhibition) that Jian�s group found in the visual system are involved in these other deficits as well.

06-29-2003, 09:08 PM	#3
Gurdur Contributor Join Date: Jun 2000 Location: Buggered if I know Posts: 12,410	Yaaarrr, thanks Patrick, for the info.

06-29-2003, 10:40 PM	#4
echidna Veteran Member Join Date: May 2001 Location: Melbourne, Australia Posts: 2,832	Sacks also wrote some on this IIRC, although I don't remember it by that name. On occasion his patients would become very distressed at this intruding arm which would keep invading their space & try to push it away with the other.

06-30-2003, 01:05 AM	#5
Wounded King Veteran Member Join Date: Mar 2003 Location: Edinburgh Posts: 1,211	I remeber sacks recounting of his patient who atee half the food on his plate and would then rotate the plate, or did someone else do that(?), so he could eat the rest.

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