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Dear Phil Tompkins,

You Asked:
"By the two categories of vision do you have in mind the common sense
experience of seeing vs. the process of the mind in creating the
unified visual/aural/tactile etc. representation?"

No I don't ...Perhaps an example will illustrate my point better.  The
first category, conscious seeing, is seeing in the ordinary sense like
seeing a face or a chair and recognizing them as such.   The second
category of seeing I call unconscious seeing.  For example you are in you=
r
car stopped at a red light at the top of hill.  The parked car to  the si=
de
of you begins to move forward.  Suddenly you become aware of this
motion (unconcious seeing)  and thinking it is you who are moving you
reflexively slam on the brakes.  Then you look directly  (conscious seein=
g)
at the moving "parked car" and resolve the conflict and you relax. =



The point is that the processing of motion is unconscious, ongoing,
peripheral and impacts on our motor activities.   As we walk through the
environment the motion we process isn't real but rather is apparent
resulting from our movement.  We don't "see"  this motion in the ordinary=

sense.  But due to conditioning all our lives we expect it to behave in a=

predictable manner and respond predictably to the motor function we are
performing.  If it is in conflict with how we would expect this motion to=

behave we must resolve this conflict (for example determine that in this
case the motion is real not apparent) or the motor function stops
(freezing). =

You also asked in regards to the enabling power of visual cues:  "Do you
have a theory as to why the additional cues work?
 I speculate that in  PD there is pathology in the perception or processi=

ng of motion data. (There is much in the literature to support this.)   I=
t
is not the doorway which results in freezing but rather the opaque walls
around it that block peripheral vision.  Augmenting the visibility of
apparent motion or accelerating it above the threshhold of perceptive
impairment usullly overcomes freezing gait and akinesia.  That is why man=
y
of us can run but cannot walk (running accelerates apparent motion).  Tha=
t
is also why walking on a moving sidewalk overcomes akinesia.  Also why
visual cues can enable gait - because the marks on the floor like white
dotted lines on the highway serve to augment the optical flow of apparent=

motion.

Hope this answers your question.  Remember the above is not to be taken a=
s
the gospel but rather a highly speculative theory.   On the other hand I
have given this A LOT of thought.

Tom