Biomedical Engineering Reference
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Like most [82, 83], though not all [79], sight-recovery patients, MM identified
colors easily (though he occasionally had difficulty remembering the correct
color name), and his equiluminance settings were normal. The only patient who
was thought to have had difficulty discriminating colors was Ackroyd et al.'s
patient HB [79]. However, this may have been due to uncertainty about what
property of the stimulus (brightness, hue, saturation) was referred to by the
color name, since HB spoke English as a second language. Although HB was
also blinded at the age of three, unlike MM, she had no experience of English
color names before the onset of blindness. Differences in color processing that
were observed between MM and visually normal observers were limited to color
illusions based on seeing images as being 3D.
Similarly motion processing also appears to be relatively normal in sight
recovery patients. “His [SB] only signs of appreciation were to moving objects,
particularly the pigeons in Trafalgar square”[82]. Similarly, Ackroyd et al.
(1974) [79] reported of their patient, “She could see the pigeons as they alighted
in Trafalgar Square but she said that they appeared to vanish as they came to rest.”
MM's motion processing also appears normal: he easily identified the direction
of motion of simple and complex plaids and was susceptible to the barber pole
illusion. He could segregate textured fields based on motion, and could distin-
guish rotational Glass motion patterns (in which two successive frames differ by
rotation) from random noise patterns. Surprisingly MM could use motion cues
to compute 3D shape, a computation thought to involve MT
[87-89]. This
sensitivity to form from motion was in striking contrast to his insensitivity to
most pictorial depth cues. For example, a stationary Necker cube immediately
“popped-out” as a cube when motion in depth was simulated. MM was also
sensitive to biological motion, recognizing a point-light “Johansson” biological
motion figure and even being able to determine whether the figure walked with
a male or female gait. Consistent with MM's ability to perform motion tasks,
when we measured fMRI responses in MM, we found that responses in MT
+
+
923 cm
2
,
were normal in area and amplitude (surface area: left MT
+
MM
=
706 cm
2
,914 cm
2
,778 cm
2
; right MT
76cm
2
, Controls
Controls
=
646 cm
2
,647 cm
2
,961 cm
2
). There was also evidence of retinotopic phase map
encoding in MT/MST [90].
MM could segment texture patterns based on luminance contrast but was
slightly worse than control observers at other form tasks requiring integration of
texture elements, such as identifying whether a field of line contours contained a
sequence of nearly collinear line segments [91] and discriminating Glass patterns
from random noise [92-94]. MM also had severe difficulty with “subjective
contours”; though he recognized outlined 2D shapes, he could not identify the
same shapes in “Kanisza figures”.
The literature on sight recovery seems to be in fairly good agreement that
patients have difficulties projecting their 2D retinal image into a 3D world.
Cheselden reported of his patient that he “thought no Objects so agreeable as
those which were smooth and regular, though he could form no Judgment of their
Shape”. It was reported of Sacks' patient Virgil, “Sometimes surfaces of objects
=
+
,MM
=
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