Lecture 6- PSYB51
What is the problem of univariance in colour vision? Please explain. What does it mean for
vision based on a single type of colour- sensitive cone? How can the problem of
univariance be solved?
The problem of univariance can be solved by having tricolor rods
Firing rate varies—universally based on: wave lengths, and intensity of the light.
>cones will response differently depending on wavelength.
Large = red
More cones= more richer colour vision
– 6 muscles controlled by cranial nerves
– Oculomotor control
– Different eye movements
– How do we achieve spatial constancy?
Why talk about eye movements in a course on perception?
- It could affect how we see the world
- It is driven by attention
- Motion -> quickly look at something ugly, dangerous, interesting, etc.
- Move your eyes to points of interest so that you have a higher resolution
- Eye movements are important to enhance our vision
Eye movements: six muscles are attached to each eye and are arranged in three
– Inferior/superior/lateral/medial rectus
– Inferior/superior oblique
Rectus muscle runs straight
Inferior rectus= bottom
Superior rectus= top
Boney hook= trochlea
> As soon as you rotate your head more than 5degrees, your eyes counter rotate (you don’t
really notice that- vision stays stable).
> Passed 5 degrees, eyes make a quick motion, so a little disruption in vision (gaze is not stable)
> Third dimension is rotation along sight
> For each dimension of rotation, you would need 2 muscles
> Muscles don’t do anything but pull
Controlled by 3 cranial nerves
– Frontal eye fields & parietal cortex => LOOK AT DIAGRAM
– Superior colliculus: Structure in midbrain that plays important role in initiating
and guiding eye movements
– When stimulated with electrical signals, eye movements can be observed
– Frontal eye field is the most important eye field in the cerebral cortex
– Parietal cortex: somewhere around that area, there’s a region called lateral
– Smooth pursuit: Eyes move smoothly to follow
– Saccade: Rapid movement of eyes that change fixation from one object or
location to another
-- Vergence eye movements (convergence and diversion movements): Type of eye
movement in which two eyes move in opposite directions, done deliberately
– Fixational eye movements, microsaccades (tiny versions of the saccades;
it refreshes the eye) -> the eye never stays stable, it slides a bit
Function of smooth pursuit eye movements: keep object of interest stable and on the
Demonstration: Look at the background with the pencil moving across in the
front you see that the eraser is clearer than the background
Why do we perceive the pencil to be in motion in the first case, but perceive the dot
to be stationary in the second case?
– Because in one case there is an eye movement
Similar effects can be observed with saccadic eye movements
Function of saccades: move (rotate) fovea to object of interest, move as quickly as
possible to reduce travel time during which vision is blurred.
Yarbus (1967): scanpaths reveal intentions and interests.
False motion & retinal smear during saccade
Why don’t we notice that? Spatial constancy: Tricky problem of discriminating motion across the retina
that is due to eye movements vs. object movements
– Demonstrate this
– Why does this happen?
– Saccadic suppression (of vision, incl. motion): Reduction of visual sensitivity that
occurs when one makes a saccadic eye movement; eliminates smear from retinal image
motion during an eye movement
the vision starts shaking if you close one eye
When you push the eye, it moves
– Remaining questions:
> How does the brain know when to suppress?
> Insufficient: displacements across saccades should result in apparent motion
illusions (but doesn’t)
> Botox was injected to the eye muscles to make it unusable. It appeared to him as
if things were jumping around
Compensation theory: Perceptual system receives information about the eye movement
and discounts changes in retinal image that result from it (proposition)
– Motor system sends motor command to eye muscles
– A copy of that command (―efference copy‖, or colorily discharge) goes to an area of
visual system that has been dubbed ―comparator‖ -> efference is anything that comes out
of the brain
-- Comparator compensates for image changes caused by the eye movement, inhibiting
any attempts by other parts of the visual system to interpret changes as object motion
[LOOK AT DIAGRAM]
Space Perception and Binocular Vision
Euclidian geometry: Parallel lines remain parallel as they are extended in space
– Objects maintain the same size and shape as they move around in
space they don’t shrink
– Objects get bigger as they approach our eyes
– Which sense is governed by Euclidian geometry?
Problem for vision: recover 3D info from 2D projections
– Most depth cues can be derived from geometrical consequences of the
The two retinal images of a three-dimensional world are not the same! -> Parallax
Binocular disparity: The differences between the two retinal images of the same
scene. It is the basis of stereopsis; a vivid perception of the three- dimensionality of
the world that is not available with monocular vision. Monocular depth cues vs. Binocular depth cues: One eye vs. two eyes
Binocular depth cues (from overlapping visual fields) provide:
– Stereopsis: is the impression of depth that is perceived when a
scene is viewed with both eyes by someone with normal binocular vision
– Ability of two eyes to see more of an object than one eye
Monocular Cues to Three-Dimensional Space
– Relative size
– Position cues
– Familiar size
– Aerial perspective
– Linear perspective
– Motion cues
Occlusion: A cue to relative depth order when, for example, one object obstructs
the view of part of another object
– Nonmetrical depth cue: provides information about depth order but not
magnitude. -> ex. we know there’s a square behind a circle; we just don’t know how far it
-- (Metrical depth cues: Provide quantitative information about distance)
Size and position cues:
– Relative Size: A comparison of size between items without knowing the
absolute size of either one (things UP on a picture means it’s closer to the
horizon, and DOWN means closer to us)
Size and position cues cont’d:
– Texture Gradient: A depth cue based on the geometric fact that items of the same size
form smaller images when they are farther away
– Relative Height: Objects at different distances from the viewer on the ground plane will
form images at different heights in the retinal image
The more remote parts in planes situated below the eye, appear higher (the
projection EF of BC appears higher than the projection DE of AB).
• Natural scene statistics.
> There are two pictures of a smokey landscape. Th