Neural Correlates of an Experiential Event
The actions we perform and what exactly we become consciously aware of are limited and shaped by the minds obligation to the body. The figure illustrates an experiential event* for an awake human being. Each experiential event unfolds from an antecedent event in lived time and merges with a following one. The two modes of perception, the direct, non-sensory "perception in the mode of causal efficacy" (PMCE) and more consciously intrusive perception in the mode of presentational immediacy" (PMCI), are, of course, both present, as indicated by the downward pointing arrows. For a conscious human being, each experiential event begins with a Yes/No decision to embark towards the satisfaction of a goal. I call this an "assent." This decision, if it is to be coherent to the world, requires PMCE, PMPI, and an intact brain. The decision, once made, sets off a complex sequence of neural processing events involving recruitment of neurons at all levels into subensembles, each of which represents one or another possible action that could potentially satisfy the goal. Initially, much of the neural processing is accomplished without conscious awareness ("preconscious processing"). The body is adjusted via "motor sets," in the context of the world and the intention, for the forthcoming action. Postural and directional sets (anticipatory muscular adjustments) are established as various neural circuits "instruct" the cerebral cortex. That is, the neurons which are to act as executives for the action are selected and preprogrammed and the conscious action, accompanied by a more-or-less distinctly conscious imperative, "Now!" blossoms out of preconscious neural processing. This process is accompanied by a relative shift in the activation patterns of upper motor neuron (UMN) pathways. In particular, the activity of corticobulbar neurons from premotor cortex and supplementary motor cortex, which control reticulospinal and other brainstem motor pathways (ventromedial activation pathways) lessens or plateaus. The activity of these UMNs, whose activation was one of the first, preconscious neural hints of the action to come, is now joined by increased activity in primary motor cortex, the cells of origin of the lateral corticospinal and corticorubral tracts (dorsolateral activation pathways). The horizontal arrows (and the dots between labels) linking left and right sides of the figure are to be read "merges into, influences, shapes, or limits." Please be so kind as to imagine a series of downward pointing arrows between levels and stretching from left to right for each set of influences, rather than the one arrow that is shown.
Thus, preconscious neural processing in various basal ganglia circuits can be seen as a necessary foundation for purposeful movement. And, this preconscious neural processing shapes not only the movement to come, but also the neural systems whose activity provides for perception and cognition. It cannot be any other way, so long, that is, as one desires to have a body. There is a constant interplay of the mind obligating body/brain and body/brain obligating mind.
I have left out of this consideration a description of the important role of corticopontocerebellothalamocortical loops, which are critical for preprogramming and formation of anticipatory muscular adjustments. In addition, the role of the amygdala and orbitofrontal prefrontal cortex in signaling positive or negative assents - which initiate each experiential event - has not received the attention it deserves. Thus, a more adequate description of the neural correlates an experiential event would speak of the intersection of limbic, basal gangia, and cerebellar loops.
*This experiential event is based on data from monkeys described by DeLong in Principles of Neural Science, 4th Ed., 2000, pgs 860-61. In experiments on motor system function, a monkey is set up for single neuron electrical recording and must respond with a forelimb movement in one or another direction when cued by a particular light ("Go light"). A warning light that specifies the direction of movement that will be required is shone a few seconds before the Go light. Results show that some 30% - 50% of movement-related neurons in the forelimb region of supplementary motor, motor cortex, putamen, and pallidum fire in relation to the direction of movement regardless of the exact pattern of muscle activity. Now, some of cells in these same brain areas only fire (or change their activity) when the warning light comes on and these changes in activity persist until the movement-triggering Go light comes on. In the basal ganglia, such "motor set" neurons receive their input from the supplementary motor cortex, whereas neurons that discharge in relation to the actual movement receive their input from the motor cortex.
We can, for this monkey, imagine a series of experiential events and Yes/No decisions that led up to the one diagramed in the figure above. These include acquiescing to leaving his home cage and getting in the monkey chair, as well as agreeing to participate at least some of the time by paying attention and responding with a forelimb movement. Decisions like these, at a preconscious or conscious level, had to have been made before the trial shown, which I have begun at the warning light. This is true, I believe, even if the monkey's overarching goal is to obtain a desired food pellet, which is the usual case.
There is another good example of the importance of preconscious processing in Principles of Neural Science, pgs. 817-18, including Fig. 41-1. Here's the task, which I recommend that you try: standing upright, raise one leg laterally (sideways) off the floor. No problem, huh? Next try it with your shoulder (opposite the leg to be raised) held firmly against a wall or door frame. Ah, ha! There is no way to avoid losing your balance because you have obviated the effects of anticipatory motor set. Normally, we unconsciously give a little push with the leg to be raised so as to shift our center of gravity off-center sufficiently to counterbalance the effect of the sideways leg movement. I am told by a research worker in the field, Dr. Raymond Chong, that Parkinsonism patients lose or have greatly diminished anticipatory postural adjustments of this type. What they lose is the ability to make rapid changes between different motor sets. And, I would argue, the loss stems from the absence of enabled neural subensembles that 'represent' the necessary anticipatory postural adjustment. Akinesia and bradykinesia, therefore can be describe as a state of little or no movement while the posterior frontal cortex waits for its appropriate instruction via corticostriatopallidothalmocortical loops, an instruction that must be "in place" before an "actualizing movement" can occur.