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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