Module 4 Flashcards
What does the dorsal stream do, and what region of the brain processes information?
AKA the cortical system pathway, the ‘where’ system.
It is responsible for perceptions of spatial location and relations of objects. Processing happens in the posterior parietal cortex.
What does the ventral stream do, and what region of the brain processes information?
AKA the visual cortex pathway, the ‘what’ system.
It is responsible for perceptions of qualities and features of objects. Processing happens in the inferior temporal cortex.
How was DB able to implicitly move his eyes/reach somewhat correctly in his bad field of vision (blindsight)?
Because information can still get to the brain via sub-cortical pathways. Either it goes to the primary visual cortex, or to retinal maps in the midbrain.
What is the difference between the ‘what vs. where’ system and the ‘what vs. how’ system?
The purpose of the ventral system stays the same between the two models. The ‘how’ system changes the focus on the dorsal stream, where the focus is now on goal-directed actions, and the purpose of visual input.
How does patient DF perform in perceptual and motor tasks?
Patient DF: has visual form agnosia, so she has problems with the ventral stream (perception).
Posting task: she has trouble matching her card to the slot position before the movement, but is able to correctly insert the card once she starts moving.
Grip aperture task: she shows the correct grip adjustments to smaller and larger objects when making a reaching movement. But, she cannot calibrate her grip when asked to estimate her grip width.
Shape recognition: able to judge the size of a shape only when grasping it, but unable to judge the size by just looking at it.
How does patient RV perform in perceptual and motor tasks?
Patient RV: has optic ataxia, and lesions to the posterior parietal cortex. So, he has problems with the dorsal stream (motor).
Posting task: very good at estimating/matching his card position to the slot position. However, he has problems with accuracy while performing the movement.
Grip aperture task: unable to calibrate grip while performing a reaching movement, but very good at estimating grip aperture prior to movement.
Shape recognition: unable to judge the size of a shape only when grasping it, but is able to judge the size by just looking at it.
How did Haffenden and Goodale manipulate the Ebbinghaus illusion? What were their findings?
They tested the regular illusion (i.e. same sized middle circles surrounded by either small or large circles) to make them look different, but they were actually the same. Then, they manipulated the size of the middle circles to make them look the same, but they were actually different (i.e. smaller middle circle surrounded by small circles, larger middle circle surrounded by large circles).
Findings: in all conditions, the ventral/perceptual stream is tricked, and the dorsal/motor stream is not tricked.
Altered illusion: People’s estimations of grip aperture were the same between conditions, but grip aperture changed when reaching out to grasp the circles.
Normal illusion: People’s estimations of grip aperture were different between conditions (larger for the small surrounding circles, smaller for the large surrounding circles), but grip aperture was similar when reaching out to grasp the circles.
How did Westwood and Goodale use the flanker illusion to separate dorsal and ventral stream reliance?
Flanker illusion: placing a smaller or larger object next to an object of focus (that stays the same size). People are asked to reach out and grab to focus object. If there is no illusion effect, the grip aperture difference between the 2 conditions will be 0. Otherwise, an illusion exists.
Manipulations: They did reaching tasks with visual information always available, and reaching tasks when vision is occluded (image is never brought back after the initial showing). They also did trials where there was a delay between the initial viewing of the objects and the movement cue (for both vision and occlusion).
Results: for both vision trial blocks, there was a negligible illusion effect. For both occlusion trial blocks, there was a significant illusion effect.
Conclusion: dorsal stream needs visual information in real time. If made to rely on memory, we rely on the ventral stream for movement tasks.
In what conditions can performance be improved with delays and occlusion (Milner study)?
When the person has dorsal stream damage. They have better performance in perceptual tasks, where they rely on the ventral stream.
Perceptual size matching of an object is good. When reaching out to grasp an object with a delay, grip aperture varies a lot, because they are relying on their damaged stream. In the pantomime trials with a delay, performance is good.
What is the double-step paradigm?
When movements are smoothly adjusted to a ‘double step’ of a target, outside of conscious awareness.
Pellisson et al. study - when target jumping is timed with saccades, you can hide the second jump of the target. Verbal reports if the object moved or not are up to chance.
What is saccadic suppression?
A reduction in visual sensitivity during saccadic eye movements. This reduces our awareness of ‘blur’ and visual flow so the world seems stable.
What experiments did Crossman et al. perform? What were the results?
First experiment: 2 blocks of trials. Instructions were given to either ‘GO’ to the target if it jumped, or ‘STOP’ the movement if the target jumped. People were successful in doing both.
Second experiment: 2 differently colored objects - always aiming for just one of them. Instructions were given to either ‘GO’ to the target if the targets switched colors, or ‘STOP’ the movement if the targets switched colors. People were successful in doing both. However, success in the ‘GO’ condition was lower compared to the jumping experiment, as color changes are slower for humans to process compared to location changes.
How do the color switching and target jumping results differ?
The hand paths were different between experiments: location changes trigger the automatic process (dorsal stream), while color changes don’t (ventral stream). This is seen as participants automatically deviate towards the target in the location jump trials, while the corrections were late in the color change trials.
Why is the automatic process only triggered during movements?
Day and Lyon study: automatic deviations are present in the reach tasks. Reach (-) trials showed an initial direction towards the new target location, but then corrected to move to the opposite one.
These deviations and corrections were not observed in the simple tracking task (just selecting a response).
How can the automatic process be disrupted?
If the posterior parietal cortex is probed with TMS during movement initiation, or if the posterior parietal cortex is damaged. In both cases, people will reach straight ahead first, and make a whole separate movement to reach the target that has changed its position.
