35-36 What is Perception? Flashcards
What percentage of the brain is involved in vision?
30% of 100,000,000,000 brain cells
What is the function of the V4 area of the visual cortex?
It’s the colour module
How about the medial temporal area (MT) of the cortex? What’s it for, doofus?
Detecting motion
Where is the fusiform face area (FFA)?
It’s located in the fusiform gyrus, which runs from the temporal to occipital lobe
What happens as we ascend the hierarchy of the visual cortex?
Receptive fields get larger - less spatial resolution
Receptive fields get more complex - complex selectivities
Receptive field responses correlate with perception (V4 - colour, FFA - face etc.)
What is a Reichardt detector?
It’s a delay system neuron used for detecting direction and speed of movement
How many saccades per second does the eye make?
Around 3 per second.
What is the Waterfall Effect?
The effect whereby if you look at something moving in constant unidirectional motion for a long time (such as waterfall or movie credits), then look away, you see static objects move in opposite direction
How can the Waterfall Effect be explained?
Motion detecting cells adapt to downward movement, causing less downward activity in sum of motion detectors. When stimulus ceases, cells return to below their resting level firing rate , causing the opposite effect (population of motion detectors is biased. Upward sum). It’s a motion aftereffect.
How does colour constancy work?
While reflected wavelengths changes, relative brightness remains constant - brain processes colour in context.
How can we focus on blue if there are no blue cones at the fovea?
Blue is filled in - brain extrapolates. Plus, their small number (5%) is enhanced by “blue amplifier” somewhere in brain.
Why can’t we see 180 degrees in high resolution?
Brain not big enough to process all that data! We compensate by eye saccades.
What features of the face are (not) detected by the FFA?
Size, colour, contrast don’t matter.
What’s important is oval exterior shape, presence of eyes and mouth.
FFA cells are also sensitive to viewpoint - some like front on, others 3/4, others profile. They show view dependence.
What does FFA encode apart from face identity?
The FFA also encodes
expression
gaze direction
What disorders are associated with face recognition deficits?
Autism; schizophrenia
What does a local legion of FFA cause?
Prosopagnosia - inability to recognise faces
What do inverted face tests such as the Thatcher illusion demonstrate?
Our facial recognition detectors show:
Strong orientation dependence (inverted face is harder to recognise)
Strong tendency to holistic processing (overrides sensitivity to parts - Thatcher illusion; composite illusion)
What is perception?
Perception is the process of attaining awareness of our environment from sensory information
What evidence is there for the existence of separate perceptual modules for vision?
Neurophysiology, fMRI and lesion studies
What are the three kinds of colour blindness?
Protanopia – L-cone (red) missing
Deuteranopia - M-cone (green) missing
Tritanope - S-cone missing.
What is perceptual bistability (rivalry)?
The bistable state produced by visual ambiguity
What is binocular rivalry?
When two different images are simultaneously presented to the two eyes, you will only be conscious of one of the two images at a time.
Why study binocular rivalry?
If you find the brain activity fluctuating with the switches in percept you’ve found the “neural correlate of consciousness”
How does the Ames room work?
Viewer looks through a pinhole, and so the only depth cues are monocular - all of these suggest a square room. Interpretation: If the two people are at the same distance, but very different in retinal size, one must be a giant, and the other tiny!
What are the two neural pathways for vision?
The dorsal and ventral
What is the dorsal action (where) pathway?
The dorsal stream is proposed to be involved in the guidance of actions and recognizing where objects are in space. It’s fast & veridical motor “action” path. It goes directly from the retina, via the superior colliculus and the pulvinar, into the posterior parietal lobe, where it also gets input from V1.
What is the ventral (what) pathway?
The ventral pathway is the slow perceptual path, prone to illusions. It is associated with object recognition and form representation. It stretches from the retina to the lateral geniculate nucleus to V1 to the inferotemporal cortex.
If someone with lesion to lateral occipital cortex tries to match a letter to a letter slot, what happens?
Interference, as this task activates the ventral “what” pathway
What is someone with lateral occipital lesion tries to post a letter?
Fine, as this action activates the dorsal “where” pathway - lateral occipital cortex not involved.
