Chapter 6 Part 1 Flashcards
2 major object recognition pathways
o Dorsal stream “where”
o Ventral stream “what”
Dorsal Stream
o Dorsal stream “where”
V1 to superior longitudinal fasciculus to posteroparietal cortex
Ventral stream
o Ventral stream “what”
V1 to inferior longitudinal fasciculus to inferior temporal cortex
- Dissociation of parietal and temporal lobe lesions in monkeys
o Object discrimination“what” task o Landmark “where” task o Parietal lobe lesioned Monkeys could do “what” task Could not do “where” task o Temporal lobe lesioned Monkeys could not do “what” task Could do “where” task
Ventral stream cells demonstrate strong object selectivity
o Dorsal cells respond to stimuli in the fovea and the periphery
o Ventral cells respond to stimuli in the fovea
People’s faces
o Card trick represents this
o Example; pictures of hands
- PET Evidence
o Match based on the locations of the objects More dorsal stream areas activated (lateral) o Match based on what the objects were More ventral structures activated (bilateral)
Visual Agnosia (patient D.F.)
Failure of object recognition
Damage from carbon monoxide
Probably ventral stream damage
o Explicit matching task
Does not do well in matching the orientation of the slot
o Action task
Does well in inserting the card into the slot
Optic Ataxia
Can hold cards in the right orientation to the slot
Can’t correctly insert card into the slot
Inability to use visual information to guide movement
Patients can identify objects
Patients have difficulty picking up objects i.e. too high, too low, wrong hand position
Probably damage to the dorsal stream
Perception for identification vs perception for action
o Ventral is perception for identification
o Dorsal is perception for action
Object Constancy
o The same object looked at from different viewing angles can look vastly different on the retina
o However, we know the shape is constant despite what the retina perceives
Object Constancy: View dependent frame of reference
Recognition involves comparing the retinal image to multiple mental representations from different viewpoints
Object Constancy: View invariant frame of reference
An object’s properties are analyzed and compared to single representations
Evidence the brain uses both theories of object constancy
o Ventral stream neurons show object constancy
Angle of hand does not affect the firing of the cell
Hierarchical coding hypothesis
o Builds up a perception based on the features of an object and cell response
o Example: table (legs, surface, rectangle)
o Gnostic units (grandmother cells)
These cells allow us to know what an object is because they fire
o Gnosis means knowledge
Hierarchical coding hypothesis: Grandmother cells Controversy
The concept of grandmother cells is more problematic than it first seems
What objects you have never seen before? Do the neurons not fire until you see that object? What about before that object existed?
The loss of one neuron would mean you could not recognize that object anymore
No reason the perception of a table is in a certain neuron