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visual system's representation of the world
-visual system generates an internal model (representation) of the world
-Two questions:
1. How accurately is the world represented?
2. How detailed is this representation?
Perceptual Illusions
illusion: something that it is not what it appears to be
-cafe wall illusion
-Hermann Grid Illusion
retinal ganglion cells
-have a center-surround receptive field
Is visual perception veridical?
-veridical perception: direct perception of stimuli as they exist
-visual world is not always accurately mapped (internally represented)
other evidence that visual perception is not veridical:
visual "filling in" of missing information
Blind spot
-where the optic nerve leaves the retina
-6 degree visual angle
-What do you see at the blind spot?
-not a blank space
-if background is boring gray, you see gray
-if background is blue/yellow stripes, you see blue/yellow stripes at blind spot
What if it's a complex scene: population of people/faces
-pebbles on the beach
-it is filled in with the appropriate stimulus (people/pebbles)
-note: blind spot is 15 degrees away from fovea
the brain paints in properties at this location (fovea)
active cognitive processes:
-completion
-interpretation
-filling in
How does filling in work?
-isomorphic
-symbolic
isomorphic filling
low level visual processes fill in the details
symbolic filling
filling in occurs higher up in the visual system and is more conceptual
Oliver Sacks
-tumor destroyed much of the retina of his right eye
-had a scotoma
-if he looked at the sky he would see a grey dot
scotoma
large blind spot
Oliver Sacks: filling in
-plain colors fill in quickly
-patterns on carpet in his office: 10 to 15 seconds
-size of pattern mattered
-2 feet from brick wall: scotoma filled with the brick's red color, but no details
-20 feet from brick wall: perfectly formed bricks
Oliver Sacks: foot
-his foot would look like a stump then one minute later becomes a phantom foot
V. S. Ramachandran (UCSD)
-blind spot experiments
-two vertical lines with a gap
-gap falls on blind spot
-subjects report continuous line
V. S. Ramachandran (UCSD): Bicycle wheel
-spokes are completed and no gap is perceived
V. S. Ramachandran (UCSD): Yellow doughnuts
-if blind spot falls on a hole, doughnut is perceived as a solid disk
-"pop out" effect
Josh (patient)
-right primary visual cortex (V1) injured by steel rod in industrial accident
-Large scotoma in left visual field
-position scotoma on a solid vertical line
-First, I see a gap
-After a few seconds, gap closes and I see a continuous line
X experiment
-filling in did not work for column of large Xs
-worked for column of small x
-Different levels of processing:
-large "X"s activated object recognition area (ventral/temporal)
-column of small "x" activated lower level texture coding
fMRI studies on filling in
studies show that early visual areas (V1, V2) are not involved in "filling in"
-areas V3 and V4 are involved
change blindness
-a demonstration of the sparseness of visual representation
eye saccades
cause massive blue
-trans-saccadic memory is very poor
Change blindness: early vision theories:
-successive pictures (from multiple saccades) must be integrated into one big representation
-requires massive computations
-Claim: we have a detailed view of the world
-change blindness challenged this assumption
Change blindness: we do not have a detailed view of the world
-massive integration of successive views is not necessary
-we overestimate our ability to detect change
Inattentional blindness
-if you are not directly paying attention to something, will you see it?
-easy to miss something obvious (does not enter your consciousness)
-the most important factor affecting inattentional blindness is a person's own attentional goals
-hard to see something that is truly unanticipated
Inattentional blindness: Mack & Rock (1988)
-circles and cross
-which is longer, horizontal or vertical?
implications for theories of vision
-vision is not a process of building up detailed inner representation that allow comparison from one moment to next
-we do not hold on to nearly as much information as we seem to think
Simons and Levine (1997)
-initial rich visual experience
-extract meaning ("gist")
-if eyes move (saccade) we get a new experience
-if "gist" is the same, you assume details are the same
-will not notice change
a second theory, Resink (2000)
-you never form a complete representation of the world
-not even during fixations
-object representations are build one at a time
-focused attention takes a few basic objects from low-level processing to a "coherence field"
-objects that make it there will persist for a few seconds
-when attention is released, the object loses coherence and "dissolves"
a third theory, Kevin O'Regan (1992)
-no need to store large amounts of information
-the outside world acts as an external memory storage
-no internal models
-visual world is not something we build up, it's something we do
-sensorimotor theory of vision
-what you see are those aspects of the scene that you are currently "visually manipulating"
-counter intuitive because seeing does not feel like manipulating
-what about "pop out" effects
