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Neurons in substantia nigra pc in basal ganglia release dopamine. These neurons signal expected reward. Neurons at all levels of saccadic eye movement circuitry are sensitive to reward. This provides the neural substrate for learning gaze patterns in natural behavior, and for modeling these processes using Reinforcement Learning.
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What is meant by “top-down” and “bottom-up” processing? Give examples of both.
Bottom up processes are evoked by the visual stimulus.
Top down processes are operations that reflect the subject’s current cognitive goals.
In the case of eye movements, fixations that are for the purpose of getting specificinformation to accomplish a task are said to reflect top down control.
Fixations that are evoked automatically by the occurrence of a stimulus are said tobe under bottom up control.
Examples?
What is “Neuroeconomics”? Explain how the saccadic eye movement circuitry is influenced by reward.
Humans/primates exhibit behaviors that lead to expected reward. Reward is provided by the release of dopamine.
Neurons in substantia nigra pc in basal ganglia release dopamine.These neurons signal expected reward.
Neurons at all levels of saccadic eye movement circuitry are sensitive to reward.
This provides the neural substrate for learning gaze patterns in natural behavior, and for modeling these processes using Reinforcement Learning.
Dopaminergic neurons in basal ganglia signal expected reward. (Schultz, 2000)
Response to unexpected reward
Increased firing for earlier or later reward
Expected reward is absent.SNpc
Conditioned stimulus predicts reward
target selection
signals to muscles
inhibits SC
saccade decision
saccade command
planning movements
Neural Circuitry for Saccades
Substantia nigra pc
Substantia nigra pc modulates caudate
Neurons at all levels of saccadic eye movement circuitry are sensitive to reward.
LIP: lateral intra-parietal cortex. Neurons involved in initiating a saccade to a particular location have a bigger response if reward is bigger or more likely
SEF: supplementary eye fieldsFEF: frontal eye fieldsCaudate nucleus in basal ganglia
Monkey makes a saccade to a stimulus - some directions are rewarded.
Cells in caudate signal both saccade direction and expected reward.Hikosaka et al, 2000
This provides the neural substrate for learning gaze patterns in natural behavior, and for modeling these processes using Reinforcement Learning. (eg Sprague, Ballard, Robinson, 2007)
Give some examples that eye movements are learned.
Jovancevic & Hayhoe 2009 Real Walking
• Occasionally some pedestrians veered on a collision course with the subject (for approx. 1 sec)
• 3 types of pedestrians:
Trial 1: Rogue pedestrian - always collides Safe pedestrian - never collides Unpredictable pedestrian - collides 50% of time
Trail 2: Rogue Safe Safe Rogue Unpredictable - remains same
Experimental Design (ctd)
Learning to Adjust Gaze
• Changes in fixation behavior fairly fast, happen over 4-5 encounters (Fixations on Rogue get longer, on Safe shorter)
Time fixatingIntersection.
“Follow the car.”
or“Follow the car and obey
traffic rules.”
Car RoadsideRoad Intersection
Shinoda et al. (2001)
Detection of signs at intersection results from frequent looks.
Top Down strategies: Learn where to look
Give some examples that reveal attentional limitations in visual processing
1. Difficult to detect color change in one of 8 colored squares.
2. Invisible gorilla
3. Color-changing card trick
What are these examples called?
What conclusions has been drawn from these experiments.
• Experimental Question: How sensitive are subjects to unexpected salient events (looming)?
• General Design: Subjects walked along a
footpath in a virtual environment while avoiding pedestrians.
Do subjects detect
unexpected potential collisions?
Briefly summarize the experiment by Jovancevic, Hayhoe, & Sullivan. What did they find?
Pedestrians’ paths
Colliding pedestrian path
What Happens to Gaze in Response to an Unexpected Salient Event?
• The Unexpected Event: Pedestrians on a non-colliding path changed onto a collision course for 1 second (10% frequency). Change occurs during a saccade.
Does a potential collision (looming) attract gaze?
Probability of Fixation During Collision Period
Pedestrians’ paths
Colliding pedestrian path
More fixations on colliders in normal walking.
No effect in Leader condition
Controls Colliders
Normal Walking
Follow Leader
Small increase in probability of fixating the collider.
Failure of collider to attract attention with an added task (following) suggests that detections result from top-down monitoring.
Why are colliders fixated?
Detecting a Collider Changes Fixation Strategy
Longer fixation on pedestrians following a detection of a collider
“Miss” “Hit”
Time fixating normal pedestrians following detection of a collider
Normal Walking
Follow Leader
To make a top-down system work, Subjects need to learn statistics of environmental events and distribute gaze/attention based on these expectations.
Subjects rely on active search to detect potentially hazardous events like collisions, rather than reacting to bottom-up, looming signals.