Monday, August 07, 2006

Specialization and Breakdown of Spatial Representations

"Specialization and Breakdown of Spatial Representations: Mind, brain, and development" by Barbara Landau, John Hopkins University

Williams Syndrome: unusual cognitive profile, studied by Ursula Bellugi, article published in 1994.

Phenotypic characteristics:
--Unusual facial morphology
--Small stature
--Defects of organs, esp. viscera & heart
--Commonly mild to moderate retardation (ave. IQ ~65)

Brain characteristics:
--Reduced overall volume
--Substantial abnormalities of sulcal folding (van Essen, 2006)
--Structural abnormalities at occipital-parietal juncture and in hippocampus

Genetics of Williams syndrome:
(from Meyer-Lindenberg et al, 20?? review)
Region of deletion: key genes include LimK1 (most strongly implicated in spatial deficits), Elastin, GTF2IRD1 (implicated in retardation), CYLN2.

Cognitive profile: severe impairment in "visuoconstructive" spatial tasks.
--comparison of children copying images. Original pictures are just circles broken up into 2-4 colors. Control children successfully duplicate general gist (although not particularly well), drawings by Williams syndrome children present at most vague similarity to an aspect of the image (realizing that the image requires different colors, for instance, and therefore just drawing a series of lines of different colors).
--assembly of puzzles: simple block puzzle, ~8 blocks (all yellow except for 2 black), subject can't assemble to match original.
----Where are subjects looking when they do the block puzzle? Simple two-block puzzle: Assembling puzzle inverted over and over again: successfully checking vs. real puzzle to realize blocks are wrong, but continually still assemble pieces that way. Even patients with normal IQs have this problem: the puzzle problem is the definitive trait of Williams syndrome.

Puzzle: How do genes influence cognitive structure?

Interactionist view: missing genes -> abnormal brain -> abnormal cognitive structures (almost by definition) -> Behavior.

Dr. Landau's question: do you have normal cognitive structures? If so, how else could behavior be occurring?

General Research Plan:
1. Look for breakdown in spatial functions.

2. Determine whether breakdown reflects abnormal cognitive structures.

3. Something something (uh... Profit?)


Object recognition: show someone varying perspectives on an object, they identify it
--Ventral stream
--multiple viewpoint function.
--four versions of object presented: "canonical" view (orientation) vs. "unusual" view, clear vs. blurred image.
----images all seem very identifiable
--minimal difference with clear canonical view, alternate orientation creates a spread with Williams Syndrome on bottom--roughly matched with mental age matches. But when blurred, Williams Syndrome beat mental age matches and match chronological age matches.
----Compare this to puzzle assemly and drawing copying, where they perform at ~a 4-year-old level.
(Population of Williams Syndrome subjects: 8-16 years old; control: children matched to mental age, sometimes normal adults.)

Question: can they perceive biological motion?
--requires ability to integrate global motion.

Showed us image of assemblage of dots "walking" in dot field. 1:1 Signal to Noise Ratio: still noise, very easy to identify. Random noise, harder but still easily doable. 1:3 Signal to Noise Ratio/Yoked Noise: 3 noise dots for every signal dot, movement of noise dots yoked to movement of signal dots: Really, really hard. Only a couple of us got this last one when presented for ~5 sec.
--WS subjects performed as well as adults on 1:1 tests, beating mental age matches. WS somewhat worse than adults on 1:3, but still slightly outperforming mental age matches.

Other domains of sparing examined: face perception, spatial language, motion coherence.

Next question: So what's wrong?
Return to the block construction task. Components:
"--segment overall design into blocks
--match individual blocks
--place blocks in adjacent copy space"

People with Williams syndrome have problems with all three of these components separately.

Matching individual blocks: problem with selecting mirror images. "Handedness" presents problems. Placing blocks in copy space: given single block, asked to place in space correlating to spot in competing puzzle. Issues with swapping left-right axis.

Realization: areas causing problems may be in dorsal stream and parietal lobe. Examined as series of parietal roles.

Vision for action. First task: take a dollar bill and stick it in a piggy bank. Second task is perceptual match: researcher turns a dial that moves a mannequin hand up to the piggy bank slot, subject has to tell them when to stop. Second task: Williams syndrome subjects do worse (not too horribly bad, but ~equivalent to 3-4 year olds). First task: quite poorly, also on part with 3-4 year olds. Also, once delay is introduced, ability breaks down entirely for both tasks.

Coordination of reference frames: last example, very new so only a little bit fo sample data.
Work by Laura Carlson: looking at activation of reference frames during attention tasks; normal adults activate multiple reference frames at once then either promote one or inhibit the others.
Task: place a dot in the same place relative to a square. Can't use body reference system. When box in same place in both target and test (can use screen reference system), do fairly well. When square in different place--can only use object reference frame--ability starts to deteriorate.
--introduced possibility of rotation by placing profile in square. With 0 rotation, subjects do quite well--some distance scatter, but on right axis. 135 degree rotation: have to dissociate object and screen reference systems. Havok induced, even when dot is near enough to be directly on top of profile's head. Scatters seem to be centered either around object or screen reference system. No consistency at all. When dot is further away, results scattered widely around screen reference, little around object reference point.

Summary:

Domains of breakdown (worse than MA; same as 3-4 yo)
--Vision for action (reaching, eye movements)
--use and coordination of reference frames
--more

Q: Is there any relation between this syndrome and hemispheric neglect?
--A: They do fine on neglect tests. Nonetheless, there is clearly some relationship going on but it isn't clear what yet.

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