Flashcards
Topic 10: parallel pathways
Cards in this deck (22)
- 11) What is the projection site of the major sub-cortical pathway?Superior colliculus
- 22) What are the tuning properties of cells in that area?Ill defined On & Off regions: respond to ~ anything
- Suggests cells concerned with where the object is, not what it is
• •Multisensory: some cells also receive auditory input
- for the cell to fire, the auditory & visual stimulation must originate from the same region of space - 33) What is the likely function of that area?Seems to detect the presence of objects located away from the point of fixation (in peripheral vision) and guide orientating movements of the eye & head towards those objects
- 4Magnocellular pathways15% of total cells)
• •High contrast sensitivity (but saturates)
• •Large RF size
• •Fast axon conduction speed
• •More transient response
• •Not sensitive to colour differences - 5Parvocellular pathways80% of total cells)
• •Low contrast sensitivity (less saturation)
• •Small RF size (better spatial acuity: see fine detail)
• •Slower axon conduction speed
• •More sustained response
• •Sensitive to colour differences - 6Discuss the concept that these pathways are involved in the processing of different types of visual information, e.g. motion verses fine spatial information.No simple segregation in their function
• Overlap in sensitivity of M & P cells
• M & P systems extend the sensitivity of the visual system
– M: in the temporal domain
• i.e. sensitive to rapid variation in the stimulus over time
– P: in the spatial domain
• i.e. sensitive to rapid variation in the stimulus over space
• For many tasks, the system does a coarse to fine analysis (low to high spatial frequencies)
– e.g. determine spatial form at a coarse scale (large receptive fields), & then use that as a guide to fill in the finer detail (small RFs)
– Means that both M & P cells would be involved
– A reason why (large RF) M cells have a faster transmission speed than (small RF) P cells - 7How could impaired functioning in the magnocellular pathway be a cause of dyslexia??
- 8What are the major functions of the ventral and dorsal streams?• Ventral stream
– Projects to the Inferotemporal cortex
– Includes cortical area V4
– Input is parvocellular dominated
• Dorsal stream
– Projects to the posterior-parietal cortex
– Includes cortical area V5 (MT)
– Input is magnocellular dominated - 9What is the function of V4?• Originally thought to be a colour processing area due to the number of colour sensitive cells found in it
• Cells arranged into clusters of colour selective and orientation selective cells
• Colour & orientation tuning of cells become tighter when the animal is performing a difficult discrimination task
• Colour and form subsystems within V4?
Inferotemporal cortex
• Also contains multiple subdivisions
• Cells sensitive to shape, colour or texture anywhere within large receptive fields
• Some cells appear to be selective for particular faces
• Input is parvocellular dominated - 10What is the function of V5• Most cells selective to motion
• Appears to play a particular role in motion processing
Posterior-parietal cortex
• Implicated in locating objects in space & directing attention to a particular part of the visual field
• Input is magnocellular dominated - 11Present evidence from lesion studies (monkeys)• Dorsal (V5) lesion
– Motion not pattern perception affected
• Ventral (V4) lesions
– Colour & pattern perception affected, not motion - 12Present evidence from human studies (PET and fMRI)– Spatial vs face-matching tasks
– Both lead to occipital-lobe activity
– Spatial -> posterior parietal
– Face-matching -> temporal-lobe activity
– Similar results for motion & colour tasks - 13What is the motion aftereffect?• View motion in one direction for a period of time
• Then view a static image, it will appear to move in the opposite direction, even though it doesn’t appear to markedly change its location
• Also called the waterfall illusion - 14what does the motion after effect tell us about how motion is processed in the visual system?• Shows :
– Have cells tuned to different directions of motion
– Have an inhibitory connection between units tuned to opposite directions of motion
• Inhibition not limited to opposite directions of motion
– Spatial localisation & motion perception mediated by different P/Ws
• The initial cells in the visual system could be (& are) common to both P/Ws - 159) How can you use the motion aftereffect to determine whether humans have motion selective cells at the retinal or cortical level?Inter ocular transfer of the MAE
• Cells in each eye are monocular
• Cells in the cortex are binocular
• Adapt to motion with one eye (e.g. left)
• Test for a MAE with the other (right)
• Get inter ocular transfer of the MAE, shows that motion units are in the cortex (i.e. at the binocular cell level) - 16What is the aperture problem, why does it occur and what processing limitation does it impose on the information that local-motion units can extract?• Limitation imposed by (local) motion units having spatially localised RFs (apertures)
• For objects that extend beyond the RF, the cell can only extract the motion component that is orthogonal to the cell’s preferred orientation
• To extract the actual motion of the object, need to pool the output of many local-motion cells - 17How are motion-sensitive cells in area V5/MT arranged and how has this area been linked to the processing of motion information?• Need to pool local-motion signals to extract the true motion of spatially extended (i.e. 2+ Deg.) objects
• Pooling occurs at the ‘Global-Motion’ stage
• In area V5/MT (middle temporal) - 1812) What is optic-flow information and how is it produced?• Produced by self motion through an environment
• Three types:
– Translational: produced by frontoparallel motion
– Radial: produced by motion in depth
– Rotational: produced by rotation
• Provides a rich source of information about
– Our motion through the environment
– The environment itself - 1913) 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
• In most cases it is a veridical percept
• Can be illusionary, i.e. induced self-motion
• What circumstances can lead to this illusion ?
– In a stationary car & the car beside you moves
– Works best when the other car moves slowly
• Otherwise is a cue conflict: visual & vestibular responses - 2014) How can it be shown that optic-flow information can be used to maintain balance?• Ways to show its use
– Remove visual feedback
• Stand on one foot with eyes open/closed
– Provide wrong visual cues - 21Humans 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?– If environmental bias drives sensitivity
• More sensitive to expanding patterns
– If outcome consequences drives sensitivity (i.e. in balance control, have greater control to stop falling forward than backwards
• More sensitive to contracting patterns - 2216) How can the relative sensitivity to radially expanding and contracting optic-flow patterns be determined psychophysically, and to which pattern are humans more sensitive to?Sensitivity asymmetries
• Global-Motion thresholds are lower for contracting patterns than for expanding ones
• Edwards & Badcock, 1993
– We are more sensitive to contracting patterns (BW motion) then expanding ones (FW motion)
– Possible issue: stimuli were relatively small (24 Deg annulus). Need to test using larger stimuli
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- Flashcards on Topic 10: parallel pathways , created by Catrina Dorrell on 07/11/2018.
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