Topic 10 Parallel pathways Flashcards
What is the projection site of the major sub-cortical pathway?
Superior colliculus
What are the tuning properties of cells in that area?
1) Ill-defined ON & OFF regions: respond to almost anything
2) Multisensory: some cells also receive auditory input
What is the likely function of that area?
Detect the presence of objects in peripheral vision;
Guide orientating movements of the eye and head towards those objects.
What are the properties of cells in the magnocellular and parvocellular pathways?
1) M (large cells): high contrast sensitivity large RF size faster axon conduction speed (transmission speed) more transient response not sensitive to colour differences
2) P (small cells): low contrast sensitivity small RF size (better spatial resolution) slower axon conduction speed more sustained response sensitive to colour dfferences
Discuss the concept that these pathways are involved in the processing of different types of visual information, e.g. motion verses fine spatial information.
We shouldn’t view processing as being purely M or P mediated, though some tasks can be dominated by one of them.
Pathways are linked to specific tasks, e.g.
M pathways process motion
P pathways process fine spatial info
M & P systems extend the sensitivity of the visual system:
M in the temporal domain
P in the spatial domain
How could impaired functioning in the magnocellular pathway be a cause of dyslexia?
In term of dyslexia, M system is impaired possibly due to a dysfunction in saccadic suppression. People get blurred input when making eye movement in order to read the text. Another possibility is that ability of controlling spatial attention in which M cells involved in impaired.
What are the major functions of the ventral and dorsal streams? Provide evidence for your answer, citing the results of electrophysiological, lesion, functional mapping (PET and fMRI) and clinical studies.
1) Ventral stream (V4): “what” pathway for object vision
2) Dorsal stream (V5): “where” pathway for spatial vision
[1] Electrophysiological:
Numbers of colour sensitive cells found in V4; Colour and orientation tuning becomes tighter when performing a difficult discrimination task
Most cells in V5 are selective to motion
[2] Lesion evidence showed that
V4 lesions affected colour and pattern perception but not motion
V5 lesions affected motion but not pattern perception
[3] Functional mapping (PET & fMRI):
Spatial and face-matching tasks both lead to occipital-lobe activity
Spatial -> posterior parietal
Face-matching -> temporal-lobe activity
Similar results for motion and colour tasks
[4] Clinical studies:
Discrete lesions can results in selective functional loss.
What is the motion aftereffect, and what does it tell us about how motion is processed in the visual system?
MAE (also called waterfall illusion):
View motion in one direction for a time, then view a static image, it will appear to move in opposite direction.
Shows:
Have cells tuned to different directions of motion
Have an inhibitory connection between units tuned to opposite directions of motion
Spatial localisation and motion perception mediated by different pathways.
How can you use the motion aftereffect to determine whether humans have motion selective cells at the retinal or cortical level?
Retinal cells are monocular, while cortical cells are binocular.
Cause adaptation to motion with one eye, and test for a MAE with the other. If we get inter-ocular transfer of the MAE, we can say that motion units are in the cortex (i.e. at the binocular cell level).
What is the aperture problem, why does it occur and what processing limitation does it impose on the information that local-motion units can extract?
The aperture problem refers to the fact that the motion is ambiguous if it is viewed through a small aperture such that the ends of the stimulus are not visible.
Limitation is imposed by local motion units because of having spatially localised RFs.
For objects that extend beyond the RF, the cell can only extract the motion component that is orthogonal to the cell’s preferred orientation.
How are motion-sensitive cells in area V5/MT arranged and how has this area been linked to the processing of motion information?
Like V1, columns tuned to direction of motion.
Stimulating a V5 column tuned to a specific direction of motion will bias the monkey’s perceived motion towards that direction.
What is optic-flow information and how is it produced?
The movement of elements in a scene relative to the observer.
By self-motion through environment.
What is (the illusion) of induced self-motion and how can it be produced?
Seeing an optic-flow pattern typically results in the impression of self-motion.
Vestibular system only detects changes in motion. When driving at a constant speed and direction, visual response is the major source of motion info.
How can it be shown that optic-flow information can be used to maintain balance?
Removing visual feedback, or providing wrong visual cues.
e. g. People will lose balance if they stand on one foot with eyes closed.
e. g. People in the swinging room will fall backwards, if the wall swings towards them.
Humans could show differential sensitive to radially expanding and contracting optic-flow patterns. What are some theoretical reasons for predicting greater sensitivity to either expanding or contracting patterns?
We are more sensitive to expanding pattern if environmental bias drives sensitivity;
We are more sensitive to contradicting pattern if outcome consequence drives sensitivity.
