gating of movement-related neural activity ?

[MGLinWS]

Andrew R. Mitz (arm at helix) wrote:

: The main problem here is that it is unclear if a rat has a premotor
: cortex (PM).  Activity in monkey PM is often related to an upcoming 
: movement, independent of how the movement is cued or triggered (but only
if : the cue or trigger is well-learned).  There are a number of papers on
this
: phenomenon.  Search for papers by S. P. Wise for reviews on PM.  
: Basically, if a monkey knows what movement to make (i.e. receives
: a cue signal) before it is permitted to make the movement (i.e. receives
a
: separate trigger signal), PM cells associated with that movement
: are active just after the cue and shut down just after the
: trigger.  (Actually, many PM neurons also fire during the movement).
: Thus, within (certain parts of) PM the unit activity is in preparation
: for an upcoming movement.  During a choice reaction-time task this PM 
: activity would look "movement-related", since the cue signal and
: the trigger signal are one and the same.  Note that preparatory
: activity has also been observed in the supplementary motor area
: SMA, an area that rats apparently do have.

I guess I was not clear on the task and activity I see in the striatum and
frontal cortex of freely moving rats.  The animals perform a simple
reaction-time task.  Each trial begins with a sustained nose-poke response
into a hole on one wall of the experimental chamber.  There is only one
movement that the animals can make (withdraw from a nose-poke operandum)
in response to a tone.  Single neurons exhibit "preparatory" activity like
that described by Schultz and collegues in the monkey: either a sustained
pattern of firing over the preparatory interval or a build-up of activity
that terminates with the tone or the animal's response.  Other neurons
fire in relation to the animal's movements, which include a rapid,
whole-body movement (withdrawal) and a rapid turning towards the water
fountain, located on the opposite wall of the chamber.

My interest in this task is to explain the delay-dependent speeding of
reaction-times that occurs after long preparatory intervals.  This
phenomenon is lost following lesions of the striatum, either with ibotenic
acid or with 6-hydroxydopamine.  Lesions of the medial agranular cortex,
where I am also recording neural activity simultaneously, likewise impairs
reation-time performance.

The variation in neural activity that I see relates, I believe, to this
"motor readiness".  Peak rates of firing and temporal patterns of spikes,
i.e., bursts, of both movement-related and preparatory neurons vary with
the length of the preparatory interval and with the animal's
reaction-time.  (I know this because I record the same neurons over many
hundreds of trials, thanks to many neuron recording methods.)

In light of this information, I ask again, is this phenomenon something
that others have seen previously ?  Do the firing rates of
movement-related neurons in the monkey premotor, supplementary motor, or
primary motor cortex vary with the length of the interval prior to the
trigger stimulus ?  What about the basal ganglia ?

Furthermore, I do not understand why the use of the rat is a "problem". 
Why must the animal have a premotor cortex akin to the macaque monkey to
exhibit neural activity in relation to motor preparation ?  Does anyone
know if the nervous systems of invertebrates or lower vertebrates show
alterations in neural activity in preparation for movement ?


Mark Laubach
Department of Physiology & Pharmacology
Bowman Gray School of Medicine
Wake Forest University