Problem 8 Flashcards
Action selection
Selects from specified action plans
–> what action to perform
Action specification
Accumulation of sensory info to create several potentially useful action plans
–> How to perform an action
BUT: only action plans that are currently available are specified (=related to the situation)
When and why is action specification needed ?
Any given situation may have different outcomes,
–> thus in order to be prepared:
- sensory info will continuously be used to specify several currently available potential actions
- simultaneously other kinds of info are collected to select from among these specified action plans.
–> the selected one will be released into overt execution
In which way are action selection vs specification related ?
These 2 processes occur simultaneously and continue even during overt performance of movements
The visual system can be organized into parallel processing pathways.
Name them.
- Occipito-temporal pathway
- -> ventral stream - Occipito-parietal pathway
- -> dorsal stream
Function of the occipito-temporal pathway ?
Ventral pathway
Sensitivity to info about the identity of objects
–> what
Function of the occipital-parietal pathway ?
Dorsal pathway
Sensitivity to spatial info
–> mediates various visually guided actions,
THUS: parkt of action specification
Traditionally the dorsal pathway should build a representation of “where” things are in the environment.
Is this the case here too ?
No, it does not have a unified representation of the space around us since it diverges into a number of sub streams
–> each is specialized towards the needs of different kinds of actions
e.g. LIP + MIP
Lateral intraparietal area
LIP
Involved in specifying potential saccade targets
through
- eye gaze control
- representing space in a body-centred reference frame
–> interconnected with parts of the oculomotor system
- FEFs
- Superior colliculus
Medial intraparietal area
MIP
Involved in specifying possible directions for reaching through
- arm reaching actions
- representing target locations with respect to the current hand location
–> interconnected with fontal regions involved in reaching
- dorsal premotor cortex (PMd)
- primary motor cortex
Now being established that the dorsal stream is involved in action specification; does it represent a single unique movements that has already been selected ?
Seen that action specification + selection occur simultaneously ?
No,
it offers a variety of options to choose from
- multiple saccade targets
- multiple reaching movements
BUT: does not represent all possible movements, since attentional modulation
Does Action specification represent every single possible movement ?
No,
since attentional modulation will enhance the interest of information from particular regions of interest
–> meanwhile info from other regions is suppressed
THUS: selective attention is an early mechanism for action selection
Fronto-parietal system
Consists of the reciprocal connections between the parietal cortical areas with the frontal regions that are involved in movement control
–> thus,
- a set of loops spanning over the central sulcus
- each loop processing info related to a different aspect of movement
Give a more detailed explanation of the affordance competition hypothesis.
How do the different loops, associated with different action-related outcomes, work in tandem?
What is meant with the biasing input and how does it influence the activity in these loops?
The competition between potential actions plays out in the fronto-parietal system
–> cells with different movement preferences will mutually inhibit each other, based on the biasing input that the individual receives
THUS: specific info will favor a given action which will cause increased activity related to that action, while info against an action will cause it to decrease
Give a more detailed explanation of the affordance competition hypothesis.
How do the different loops, associated with different action-related outcomes, work in tandem?
What is meant with the biasing input and how does it influence the activity in these loops?
The competition between potential actions plays out in the fronto-parietal system
–> cells with different movement preferences will mutually inhibit each other, based on the biasing input that the individual receives
THUS: specific info will favor a given action which will cause increased activity related to that action, while info against an action will cause it to decrease
Sensory info in the dorsal stream is used to specify the spatial parameters of several currently available potential actions in PARALLEL
How is this demonstrated in functional imaging ?
Whenever multiple peaks appear simultaneously within a single frontal or parietal cortical region, they compete against each other through mutual inhibiton
–> cells with similar parameter preferences excite each other, while cells with different ones inhibit each other
When is a decision between the potential specified actions made ?
When neural activity reaches some threshold
Why does the alternative framework according to Krauzlis view attention as an effect rather than a causal agent ?
Because,
attention rises as a functional consequence of circuits that are centered on the BG
–> which in turn is involved in value-based motor + non-motor decision making
State estimation
Is crucial to decision making
–> since it entails the sensed features of the external world + internal status of subject
THUS: at each moment the subject must consider several possible estimates of the sate
Which brain regions are responsible for state estimation ?
- Input nuclei of the BG
- -> especially striatum - Striatum receives converging inputs from
a) cortex
b) amygdala
c) thalamus
+ d) dopamine signals, chi guide reinforcement learning
The affordance competition hypothesis suggested that there is competition to determine how sensory data is represented in the neocortex.
The alternative framework (Krauzalis) suggests otherwise.
Elaborate.
It suggests there is competition between possible interpretation of the current state by the basal ganglia
–> each possible state differs in the weights it assigns to the various inputs
THUS: context-dependent decision making
competition determines which estimate of the ‘state’ provides the best match to
- the current sensory data
- prior knowledge
- internal status of the subject
–> The dominant estimate of the state then determines which decision policy is followed
According to the alternative framework, each state can be viewed as a candidate template (CV), where the best-matching template will dominate the competition.
Now if the circumstances change, how does this metaphor still apply?
As circumstances will change a candidate that would match good previously, may match poorly in another round
–> This results in a linked chain of states, where the transition from one to the next is based on some event, or change in an internal variable, that gives the new state more support than the preceding one and carries along with it a new decision policy
Which role does attention play in the alternative framework ?
The filter-like properties associated with attention result from the particular weights applied to the sensory and non-sensory inputs that define the current state
THUS: attention occurs when a state dominates and then only to that state
How do shifts of attention occur, according to the alternative framework ?
Shifts of attention correspond to the transition from one dominant state to another
- Stimulus-driven/Bottom up
- -> triggered by unexpected sensory data - Endogenous/Top-down
- -> triggered by a change in internal state or knowledge
Sensory cortex/SC
Parietal lobe
Controls orienting movements of the eyes and head + spatial attention
–> retinotopically organized map
It has been previously assumed that attention operates by regulating HOW the sensory signals are represented in the neocortex.
This view has been proved wrong.
Explain.
The enhanced responses go sensory neurons to attended stimuli were preserved during the inactivation of the SC
–>well-known signatures of attention in the cortex remained intact despite behavioral deficits in the covert attention task
THUS: only at behavioral level, there is no attention, but on critical level there certainly is
Why does the inactivity of the SC only lead to attention deficits on a behavioral ?
Because, it will suppress spatially specific signals that provide crucial support for these states that involve parts of the visual field
–> w/o input from the SC (spatial info), it is hard for these crucial spatial states to win against the other competing states
Through which routes does the SC affect attention ?
- SC
- medials dorsal Thalamus
- FEF
- also sneds signals to striatum directly
- -> or indireclty via PFC
or
- intermediate SC
- thalamus
- parafiscular nucleus (Pf)
- caudate nucleus
Parafiscular nucleus
Is part of the thalamus, and its neurons are modulated by spatial cues
–> inactivation increases reaction times during attention tasks
Necortex is not necessary for selective attention.
Why
Because,
here, the process of extracting salient features has been outsourced
BUT: doesn’t fundamentally changes the circuit
Give clinical evidence for the fact that disruptions of the BG cause problems with attention
- Spatial neglect, which is the failure to process objects adequately across space
–> occurs when striatum , right parietal cortex is damaged
- Parkinsons, also asossociated with attentional + visual perception deficits
How does state estimation occur?
Is it learned or is it innate ?
According to Krauzalis it is learned
–> the particular propensities of attention in an individual would develop over time through learning mechanisms that determine which states and their associated value functions are most likely to result in rewarding outcomes
