Tutorial 3 Flashcards
what effect does increasing the reward associated with a target have on eye movements towards that target?
It increases the velocity of the eye movement
what effect does blinking have on eye movements?
It pushes the eye, altering its trajectory
what problems do increasing rewards and blinking cause for the oculomotor system>
The motor commands that initiate eye movements are variable
due to variations in context and situation.
The problem is how the oculomotor system is able to fixate on a target accurately
despite this variation in initial state.
what is corollary discharge and how does it deal with this problem?
Corollary discharge is an internal copy of an action command
which predicts the sensory consequences of an action
before sensory feedback is available.
Corollary discharge is able to compensate for variability in the initial motor commands
and produce accurate eye movements.
what problem does delayed sensorimotor feedback cause for the motor system?
Slow sensory feedback can destabilize movements.
results in continual over-adjustments,
whereby the absence of feedback leads to additional movements
and then when the feedback arrives it shows the movement has gone too far
so a big counter-adjustment is made, and so on.
how might a forward model solve the problem of delayed sensorimotor feedback?
A forward model would generate a prediction of the position and velocity of a limb
based on prior motor commands
this would enable feedback control to happen in real-time
and avoid the instability associated with feedback delays.
what additional advantage do forward models have?
can account for perturbations affecting not only the directly perturbed muscle,
but also knock-on effects on other, connected muscles
forward models can take into account the more complex physical dynamics of the limb.
what evidence is there to suggest the cerebellum might generate such forward models
TMS to the cerebellum disrupts the ability to make accurate movements to a target that has disappeared.
Subjects made arm movements to the side and when a tone was played they had to make a forward movement to a remembered target location.
If the movement was based just on the sensory feedback from the target then they should be inaccurate, as by the time they move their arm the feedback will be out of date.
However, subjects are able to make accurate movements, which are disrupted by TMS to the cerebellum.
Suggests the cerebellum computes the likely position of the arm in the future (i.e. when the movement is initiated) and compensates by adjusting the trajectory.
why would the oculomotor system have a better idea of the position of the eye after a movement than before?
After making a movement, the brain is able to combine two sources of information
the prediction of the sensory consequences of an eye movement (provided by the forward model)
and the sensory feedback after completion of the movement,
providing a more precise estimate than if it had to rely on a single source of information.
what is the role of the parietal cortex in this process?
Neurons in the parietal cortex have been shown to remap their receptive fields before an eye movement occurs,
based on a prediction of the sensory consequences of the movement
essentially, the parietal neurons provide a model of what the world will look like after the eye movement.
what would happen if we couldn’t predict the sensory consequences of making eye movements?
When we move our eyes across a scene the brain is normally able to predict that the stationary image will move across the retina and adjust our perception of the world accordingly.
Without this ability the world would appear to move every time we move our eyes.
This is equivalent to what happens when you move a camera while taking a picture – it blurs the image because the camera has no way to compensate for the motion.
what is the optimal way to combine 2 sources of info?
Combine the two sources of information in a way that minimizes the variance of the resulting estimate.
The idea is that the brain should incorporate a measure of uncertainty into its predictions.
how should each source of info be ‘weighted’ when being combined?
The information should be weighted in inverse proportion to the variance.
So, the weighting will be higher for a source of information with lower variance, and the weighting will be lower for a source of information with higher variance.
how does the brain form a ‘belief’ about the current state of the world?
By combining predictions about the sensory consequences of a movement with the actual measurements made at the time of movement.
how might physical development of the body affect sensorimotor predictions?
It would change the relationship between motor commands and the actual motion of the limb, thus affecting the ability to predict the consequences of any particular motor command.
