gating of movement-related neural activity ?

[MGLinWS]

ewass at codon.nih.gov (Eric Wassermann) wrote:

:I think it is far more likely that the rise in excitability in M1
:preceding movement is conditioned by activity in the basal ganglia
:via the ventral thalamus, since it is abnormal in Parkinson's 
:disease where the motor cortex is not directly affected 
:(Hallett et al. Electroenceph Clin Neurophysiol 1991;?76:?76; 
:Pascual-Leone et al. 1994 Neurology;44:884)

The intention behind my studies was to address the neural basis
of motor control using a simple reaction-time task that arguably
tests a critical function of the basal ganglia: the mediation
of responding to a trigger stimulus and the delay-dependent speeding
of reaction-times (Brown and Robbins, Brain, 114, 513-525, 1991).  
Most patients with Parkinson's disease are clearly impaired in 
performing simple reaction-time tasks (Evarts et al., Brain, 
104:167-186, 1981). The fact that patients with PD show an 
abnormality in motor cortical excitability in relation to movement 
suggests a possible mechanism for the slowed movements associated 
with this disease.  Perhaps multi-site multi-electrode studies of 
the basal ganglia and motor-related areas of the frontal cortex 
in behaving animals, together with imaging studies in human subjects, 
may provide the essential information for understanding what 
goes wrong in PD.

I disagree with Dr. Wasserman's conclusion that the basal ganglia are
the "conditioning" source for motor cortical excitability.  Studies in 
primates have shown that there are substantial projections to the 
primary motor cortex from the ventral tegmental area and the substantia 
nigra pars compacta (Gaspar et al., JCN 325:1-21, 1992).  In addition, 
there is degeneration in ALL areas of the midbrain dopamine complex 
in PD (German et al. Ann. Neurol. 26:507-514, 1989; and more recent 
papers).  These anatomical data suggest that the motor cortex may 
indeed be affected in PD.  

In any case, the studies mentioned by Dr. Wasserman point to a potential 
means by which neural activity could be gated during the reaction-time.
My question is:  HAS ANYONE STUDIES HOW THE EXCITABILITY OF PRIMARY MOTOR
CORTEX VARIES WITH THE SUBJECT'S REACTION-TIME OR WITH THE LENGTH OF TIME
BETWEEN THE START OF A TRIAL AND THE OCCURRENCE OF THE TRIGGER STIMULUS ?


Mark Laubach