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“Top-Down” and “Bottom-Up” Processes in Perception
Light & Dark Line Orientation
Surfaces and Background Identity/Meaning
A “Bottom-Up” System
Why should this model be called into question?
“Top-Down” and “Bottom-Up” Processes in Perception
Light & DarkLine Orientation COMPARE
Possible InterpretationsSurfaces and
Background
Identity/Meaning
-- Sensory System --“Top-Down” or “Look-up” System
A Top-Down System
Single Shape/Brief Dots
• Shape is identified by top-down system before information in the bottom-up system gets changed
Single Shape/Brief Dots
• Shape is identified by top-down system before information in the bottom-up system gets changed
• Let’s push the system: overload top-down system AND change the signal in the bottom-up system
Single Shape/Brief Dots
• Shape is identified by top-down system before information in the bottom-up system gets changed
• Let’s push the system: overload top-down system AND change the signal in the bottom-up system
• What would you predict of a strictly bottom-up system?
Many targets - Identify shape inside dots
QuickTime™ and a decompressorare needed to see this picture.
Many targets - Identify shape inside dots
QuickTime™ and a decompressorare needed to see this picture.
Object Substitution
• Strictly bottom-up system should still identify the shape! but top-down model accounts for the phenomenon:
• Bottom-up info gets changed before top-down process completes
• all that’s left in the bottom-up signal is the four dots
More Depth Cues
• Pictorial Depth Cues
• Physiological Depth Cues
• Motion Parallax
• Stereoscopic Depth Cues
Physiological Depth Cues
– Accommodation– relaxed lens = far away– accommodating lens = near
– What must the visual system be able to compute unconsciously?
Physiological Depth Cues
– Convergence– small angle of convergence = far away– large angle of convergence = near
– What two sensory systems is the brain integrating?
– What happens to images closer or farther away from fixation point?
Physiological Depth Cues
– Convergence and accommodation are reflexively linked
Under what circumstances might this be a problem?
Motion Depth Cues
– Parallax– points at different locations in the visual
field move at different speeds depending on their distance from fixation
Seeing in Stereo
It’s very hard to read words if there are multiple images on your retinaIt’s very hard to read words if there are multiple images on your retina
Seeing in Stereo
It’s very hard to read words if there are multiple images on your retinaIt’s very hard to read words if there are multiple images on your retina
But how many images are there on yourretinae?
Binocular Disparity
• Your eyes have a different image on each retina– hold pen at arms length and fixate the spot
– how many pens do you see?
– which pen matches which eye?
Binocular Disparity
• Your eyes have a different image on each retina– now fixate the pen
– how many spots do you see?
– which spot matches which eye?
Binocular Disparity
• Binocular disparity is the difference between the two images
• Disparity depends on where the object is relative to the fixation point:– objects closer than fixation project images that
“cross”– objects farther than fixation project images
that do not “cross”
Binocular Disparity
• Points in space that have corresponding retinal points define a plane called the horopter
The Horopter
Binocular Disparity
• Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points)
Binocular Disparity
• Points not on the horopter will be disparate on the retina (they project images onto non-corresponding points)
• The nature of the disparity depends on where they are relative to the horopter
Binocular Disparity
• points nearer than horopter have crossed disparity
• points farther than horopter have uncrossed disparity
Binocular Disparity
• Why don’t we see double vision?
• Images with a small enough disparity are fused into a single image
Binocular Disparity
• Why don’t we see double vision?
• Images with a small enough disparity are fused into a single image
• The region of space that contains images with close enough disparity to be fused is called Panum’s Area
Stereopsis
• Our brains interpret crossed and uncrossed disparity as depth
• That process is called stereoscopic depth perception or simply stereopsis
Stereopsis
• Primary visual cortex (V1) has bands of neurons that keep input from the two eyes seperate
Stereopsis
• If the brain only gets normal signals from one eye early in life, that eye’s neurons crowd out the other eye’s neurons
Amblyopia
• Amblyopia is a visual deficit in which one eye has poor vision because the brain never developed the ability to use signals from that retina
Amblyopia
• Amblyopia is a visual deficit in which one eye has poor vision because the brain never developed the ability to use signals from that retina
• Usually caused by – strabismus - when eyes don’t lock onto the same
point – anisometropia - when one eye has very bad optics
and the other is normal