Several postings yesterday referred to observations that certain activities,
those learned over a long period of time seem relatively easy to perform
relative to more recently learned activities.
Over the past several years I have spent a  lot of time investigating this and
related topics and have created a conceptual model of gait.  The report which
describes this model is too long for a posting here (24 pages) but I would like
to post an excerpt from this report for general feedback,
 
overlearned motor package: a functional and probably neuroanatomical assemblage
of neuronal circuits which results in a learned motor event such as a step or a
tennis serve.  It is a result of so-called overlearning and once cued can be
performed in its entirety, automatically, without conscious attention and is not
dependent on visual sensory feedback.  It is externally cued.
 
self-cueing overlearned motor package: when the end of an overlearned motor
package contains a cue (internal cue) that prompts the brain to replay the same
motor package it is said to be self-cueing or looping.  An example of the latter
would be unconscious, automatic walking.  By this I mean the kind of walking
that allows one to carry on a conversation or read a book at the same time.  It
doesn't require conscious attention. It is internally cued by a motor package
whose characteristics are sufficiently matched to that of the overlearned motor
package (a kind of template).  The essential parameters which determine whether
the self-cueing motor package is accessed seems to be stride length, which must
exceed a certain minimum (or threshold) and an environment that does not impose
a necessity for change in velocity beyond a certain range which would require a
change in stride length. The minimal threshold stride length will remain
constant over a range of different gait velocities.  However, at a certain
threshold velocity the minimal threshold stride length increases in order to
access a faster gait motor package..  This next incremental change would occur
when walking becomes running.  As long as we perceive  the environment as being
compatible with this stride it will continue to play itself (self-cue).  It is
accessed by a template and sustained by unconscious peripherally perceived
virtual motion.
 
 
A Proposed Model For Normal Gait
 
General Concepts
 
Gait can be thought of as learned motor activity of the lower extremities which
when performed results in movement of the subject in the environment. By varying
the various parameters of gait, (stride length, velocity, heel and toe strike
timing etc.) one can perform an infinite variety of gaits. Like any other
learned information (motor or sensory or cognitive) a particular gait can be
newly learned or overlearned.  Newly learned  gait exists in  short term memory.
Characteristics of a given environment determine the particular characteristics
of a newly learned gait.  Once the environment changes, the newly learned gait
is no longer needed and like short term memory is quickly forgotten and replaced
by a different newly learned gait tailored to. new environmental conditions..
While the environment does vary, there are a few environment conditions that
recur repeatedly and are present most of the time.  This results in repeated
learning of a select few newly learned gaits until the learned information is
transformed in the brain into what is termed overlearned information.
Overlearned information differs from newly learned information in several
important ways.  The most  important distinction is that once initiated, an
overlearned motor package can be performed unconsciously or automatically.  This
allows our conscious attention to be directed to another task and is the basis
for efficient simultaneous task performance. This  is mediated by the basal
ganglia.   In addition freshly learned gait is defined or cued by the external
environment and is ephemeral.  The overlearned motor package is predefined and
permanently stored in the brain. An internal cue prompts the performance of the
entire package as a unit.
 
To summarize, gait can be performed in two different modes; conscious mode and
unconscious (automatic)  mode.  Conscious, newly learned gait is externally cued
and its parameters change constantly to adapt to the changing conditions of the
environment.  Automatic gait is prerecorded and predefined.  An externally cued
gait serves as a template and when certain critical characteristics match an
automatic (overlearned) motor package that package is accessed.
 
Whenever the environment permits we function in the automatic mode (it can be
thought of as the default mode).  This allows us to perform more complex tasks
by assembling overlearned motor packages.  When the environment is incompatible
with the automatic mode we function in the conscious mode..and then revert to
the automatic mode when the environment allows it.
 
 
The postings on this subject  are consistent with the above conceptual model.
Essentially the point is that in PD the melody (motor package) is not damaged
but rather the PLAY button is damaged.  This means that if an alternative means
of pressing the PLAY button can be found normal function can be restored.
 
critical comments welcomed.
Tom