Careers for Psychology and Neuroscience Majors Oct. 19th5-7pm in SU 300 Ballroom B.

Careers for Psychology and Neuroscience Majors

• Oct. 19th 5-7pm in SU 300 Ballroom B

WebCT and Your Exam

• Your grade as it currently appears on WebCT might be off by up to 2 points.

• WebCT programmers are working to fix the bug.

• Don’t panic.

“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

Shape Identification Requires Interaction between Top-Down and

Bottom-Up Processes

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Single Target - Identify shape

Single Target - Identify shape

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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

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Many targets - Identify shape inside dots

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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

– Two Physiological Depth Cues1. accommodation

2. convergence

Physiological Depth Cues

– Accommodation

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

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

– Motion1. Parallax

Motion Depth Cues

– Parallax

Motion Depth Cues

– Parallax– points at different locations in the visual

field move at different speeds depending on their distance from fixation

Motion Depth Cues

– Parallax

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Seeing in Seeing in StereoStereoSeeing in Seeing in StereoStereo

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

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

• Corresponding retinal points

Binocular Disparity

• Corresponding retinal points

Binocular Disparity

• Corresponding retinal points

Binocular Disparity

• Corresponding retinal points

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?

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

Binocular Disparity

• Panum’s Area extends just in front of and just behind the horopter

Stereopsis

• Our brains interpret crossed and uncrossed disparity as depth

• That process is called stereoscopic depth perception or simply stereopsis

Stereopsis

• Stereopsis requires that the brain can encode the two retinal images independently

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

Next Time:

• More about stereo vision

• MagicEye stereograms

• Pinker chapter