P3D Left Brain/Right Brain

[Ray Zone <r3dzone@xxxxxxxxxxxxx>]

On Sat, 27 Mar 1999, Bruce Springsteen wrote:

>The corpus callosum, a structure which connects the left and right
>hemispheres of the brain, is sometimes surgically severed as a way to
>alleviate extreme chronic seizures, essentially disconnecting the right
>brain and its functions from the left brain.  Persons who have had this
>procedure done have been the subject of very interesting studies in
>perception which I have occasionally seen described on educational
>television shows (my chief source of scientific edification).  But I don't
>recall if any effects of this procedure on stereo perception were studied
>and reported.  Does anyone have information on this question?  (Last time I
>asked this kind of question, I got some great references to research on
>eidetic memory and stereo!)

March 28, 1999  Eric J Fleischer, MD wrote:

With no references at hand, but speaking as an ophthalmologist,
severance of the corpus callosum should have no effect on stereopsis.

Visual information from the right half of the retina in the right eye
(perceiving the left half of the visual field) stays on the right side
of the head as it follows the visual pathways to the visual centers in
the occipital cortex at the back of the brain. Visual information from
the right half of the retina in the left eye ( also perceiving the
left half of the visual field) crosses over to the right side of the
head at the optic chiasm, a few inches behind the eyes. The
information pathways also ends up in the right occipital cortex. The
pathways for the left halves of the retina are arranged similarly.

Therefore, each side of the brain gets information from both of the
eyes, and would be able to perceive stereopsis even with no
communication from the other half of the brain. While I have no
experience with people that have had surgical disruption of the corpus
callosum, I have treated many patients who have had strokes, and lost
all vision to one side (e.g., the right half of the vision in each
eye, called a homonymous hemianopia), and still retain stereopsis.

Ray Zone responds:

Thanks to both Bruce and Eric for illuminating postings on the tantalizing
and essential issue of stereo perception.

In a book titled "Left Side/Right Side, A Review of Laterality Research" by
Alan Beaton (Yale University Press, 1985) under the chapter "Hemispheric
Assymetry in Normal Subjects" there is a subheading for "Stereopsis" which
reads as follows:

"Stereoscopic depth perception was studied by Durnford and Kimura (1971) by
asking subjects to judge whether two rods, one presented at the point of
fixation and one presented in the left or right visual field, were
horizontally aligned.  With monocular viewing no field difference was found
but with binocular viewing a left field superiority was obtained.
Binocular viewing of random dot stereograms also yielded higher scores for
the left field in identification of the fused forms.  Durnford and Kimura
therefore concluded that the right hemisphere is specialised for the
perception of depth cued by retinal disparity.  Dimond, Bures, Farrington
and Brouwers (1975), using a different technique, also claimed a right
hemisphere dominance for stereoscopic depth perception.  This supports
certain findings from brain damaged patients (Carmon and Bechtoldt, 1969;
Benton and Hecaen, 1970; Hamsher, 1978) but the issue of whether there is a
hemispheric dominance for stereopsis is unresolved (Julesz, Breitmeyer and
Kropft, 1976; Danta, Hilton and O'Boyle, 1978; Pitblado, 1979b)."

More recent studies may have made more specific determinations.



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