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Summary Action- 2021,1 CORTICAL CONTROL OF VOLUNTARY MOVEMENT
This first problem is concerned with the global functional organization of voluntary action system.
Understanding this system starts with the dorsal visual pathway, often called the ‘how’ pathway- processing
and selecting possible targets for action. What counts as a target for action depends on the environment but
also the task-demands. The task-related issues are called the cognitive side of action.
1.1 ANATOMY
What brain structures and systems are involved in planning an action in response to visually
presented objects?
1.1.1 Representation of relevant stimuli
in order to execute a movement we need to first select our motor goals after which we make a
motor plan. Identification of a motor goal involves the perception of an object of interest and the
determination of what is to be done to that objects- object location, application of task constraints,
identifying the motor goal and the choice of whether to
initiate an action.
Attention
Attention is required to select an object of interest to the
exclusion of all other parts of the environment. We pay
attention to an array of coffee cups in the coffeeshop
instead of all the cookies on display. Attentional effects
are thought to be caused by the lateral intraparietal (LIP)
area which contains a salience map for spatial attention.
Visual processing streams
Output from the V1 and V2 to the medial temporal area
and V4 initiate 2 anatomically and functionally distinct channels of visual information processing; the
dorsal and ventral streams. Generally, the dorsal stream is assumed to mediate navigation and the
visual control of skilled actions directed at objects in the visual world. The ventral stream transforms
visual inputs into representations that embody enduring characteristics of objects and their spatial
relationships.
Neural processing in the dorsal stream is best characterized by:
➢ Motion analysis related to locomotion and
pursuit/ tracking in the superior temporal
sulcus (STS)
➢ Computations informing target selection for
arm and eye movements, object manipulation
and visuospatial attention in the intraparietal
sulcus, which divides the inferior parietal
lobule (IPL) and the superior parietal lobule
(SPL)
Evidence has indicated that this dorsal parieto-frontal
circuit can be divided in 2 substreams:
, 1. Dorso-dorsal stream- from V3a to V6, to V6a
and the medial intraparietal area in the SPL,
to the dorsal pre-motor areas
2. Ventral-dorsal stream- from the medial
superior temporal area to the IPL to the
ventral premotor cortex
a. Seems to constitute an interface
between the ventral and dorsal
stream
Object processing in the dorso-dorsal stream
This stream is the most direct visual pathway for
action: direct goal-directed visuo-motor transformations involving short-lived processes. This stream
can be called the ‘reach-and-grasp’ stream because lesions here cause misreaching to visual targets.
The parietal foci are organized along a posterior-anterior gradient of visual-to-somatic information
integration.
Object processing in the ventro-dorsal stream
This stream seems to underlie processing of sensorimotor information based on longer-term object
use representations→ called the ‘use’ or ‘tool’ stream. Lesions of the ventral-dorsal stream impair
more cognitive aspects of action representation requiring knowledge of skilled object use. The left
IPS plays a crucial role in making correct inferences about the function of an object from its structure.
1.1.2 Translating visual targets into motor goals and the representation of motor goals
Critical to the formation of a motor goal is the application of appropriate task rules which are
thought to be encoded by the prefrontal cortex. The PFC represents the association between a
specific cue and the goal it indicates- it encodes the relationship between objects and the goals they
specify according to arbitrary task rules. The PFC also determines whether to proceed with or inhibit
a response, as well as the overall task structure such as to encode identity.
The selection of motor goals is fundamentally a decision-making process.
Decision-making
For simple tasks the object of interest= the motor goal. If there is only 1 coffee cup, our goal is to
obtain that cup.
More complex tasks may require a decision about which goal to select. If there are 2 coffee cups, we
need to decide between the one on the left and the one on the right, thus we have 2 motor goals
between which we need to decide.
A drift diffusion model: decision-making processes that slowly accumulate evidence about the nature
of the stimulus. evidence in favour of a particular goal gradually accrues as a random walk until it
surpasses a threshold, at which a goal is selected and a movement generated. If we consider the 2
coffee cups; we see that the right coffee cup is a small size (we ordered a large) and we see that the
left coffee cup has the words ‘cappuccino’ on them (we ordered a cappuccino). Evidence in favour of
the left coffee cup has accumulated and now passed the threshold, we decide that our motor goal is
to obtain the left cup of coffee.
These models have prompted a search for neural evidence of a signal that gradually accumulates
prior to movement onset→ for saccadic eye movements this is the frontal eye fields (FEF)-neural
, activity reflects a decision about a motor goal to guide
upcoming actions. Neural activity initially represents several
alternative target choices and task-relevant stimulus
features but evolves over time to reflect only the final motor
goal.
The FEF is the site in which information about the
attentionally selected object of interest is
transformed- via the application of appropriate task
rules- into a goal about the desired location to which
a movement should be directed.
1.1.3 Selecting actions
The formulation of a motor goal is critical to gate the onset of motor planning, during motor planning
the specific movement to achieve that goal must be defined. These ‘how’ processes are
characterized as processes that reduce ambiguity about how the motor goal will be achieved by
specifying all remaining details of the movement. Some of these features can be constrained by task
goals.
Motor planning translates the abstract concepts of a motor goal into a concrete course of action. I
have decided that obtaining the left coffee cup is my motor goal, now I need to specify HOW I am
going to obtain it. I need to take into account that there is a chair in between me and the counter
where the cups are.
Lesion studies in monkeys lead to the conclusion that the supplementary motor area (SMA) and the
pre-supplementary motor area pre(SMA) are involved, particularly, in self-generated movement. In
a study whit 2 conditions; memory-condition where monkeys had to memorize the order of keys
they had to press, and an externally specified condition. More cells in the SMA were active in the
memory-condition than in the visually-specified condition. However, other studies have found the
SMA to fire in both conditions, yet, with time-resolved fMRI it could be shown that for self-generated
movements the SMA and pre-SMA start their firing earlier. Thus, it can be concluded that the SMA
and pre-SMA reflect preparation for action.
There is a distinction between the function of the pre-SMA, SMA and supplementary eye fields (SEF)
which became apparent in later studies (Sumner).
➢ The SMA and SEF are responsible for the automatic inhibition of primed actions
o This is effector-specific such that the SMA is involved with manual tasks and the SEF
with saccadic tasks
➢ The pre-SMA is responsible for selecting the action and switching from automatic to more
controlled action
o Not effector-specific
I have decided that I want to obtain the left coffee
cup. I can obtain this in multiple ways; reach over the
chair and grab it, move the chair and grab the coffee,
walk around the chair and take the coffee, etc… the
SMA and SEF inhibit all these actions until the pre-
SMA decides that the best action to obtain my coffee
is to move around the chair and grab my coffee, now
I still need to decide how I am going to do this.
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