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Lecture 6

PSYB51H3 Lecture Notes - Lecture 6: Projective Geometry, Correspondence Problem, Binocular Summation

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Matthias Niemeier

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PSYB51 - Perception Lecture Slides
Textbook Notes
Lecture 6
Eye Movements
-6 muscles are attached to each eye and are arranged in three pairs, which are controlled
by an extensive network of structures in the brain:
oInferior/superior/lateral/medial rectus
oInferior/superior oblique
-Controlled by 3 cranial nerves
-Superior colliculus: structure in the midbrain that plays an important role in initiating and
guiding eye movements
-When stimulated with electrical signals, eye movements can be observed
oSmooth pursuit: eyes move smoothly to follow moving object
oSaccade: rapid movement of eyes that change fixation from one object or location
to another; we make saccades in order to bring text into our fovea; no
information is processed during
oVergence eye movements: type of eye movement in which two eyes move in
opposite directions, done deliberately
oReflexive eye movement: automatic and involuntary eye movement
Vestibular eye movements: when the eyes move to compensate for head
and body movement while maintaining fixation on a particular target
Optokinetic nystagmus: eyes involuntarily track a continuously
moving object
oFixational eye movements, microsaccades
-Function of smooth pursuit eye movements: to keep the object of interest stable and on
the fovea
-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?
oBecause in one case there is an eye movement
-Similar effects can be observed with saccadic eye movements
-Function of saccades: to move (rotate) the fovea to the object of interest, to move as
quickly as possible to reduce travel time during which vision is blurred
-Yarbus (1967) scanpaths reveal intentions and interests
-3-4 saccades/second
-False motion and retinal smear during saccades

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-Why dont we notice that?
-Spatial constancy: tricky problem of discriminating motion across the retina that is due to
eye movements vs. object movements
oWhy does this happen?
oSaccadic 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
oHow does the brain know when to suppress?
-Compensation theory: perceptual system receives information about the eye movement
and discounts changes in retinal image that result from it
oMotor system sends motor command to eye muscles
oA copy of that command (“efference copy”) goes to an area of visual system that
has been dubbed “comparator”
oComparator 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
-Limits of the compensation theory:
oPerisaccadic misperceptions: briefly flashed stimuli appear as shifted in the
direction of the saccade, well before the saccade
Introduction to Space Perception
-The ability to perceive and interact with the structure of space is one of the
fundamental goals of the visual system
-Realism: a philosophical position arguing that there is a real world to sense
-Positivism: philosophical position arguing that the world might be nothing more
than an elaborate hallucination
-Euclidian geometry: geometry of the real world; parallel lines remain parallel as they
are extended in space
oObjects maintain the same size and shape as they move around in space
oInternal angles of a triangle always add to 180 degrees
oWhich sense is governed by Euclidian geometry?
oThe geometry of retinal images of thereal” world are non-Euclidean
oGeometry becomes non-Euclidean when the 3D world is projected onto the
curved 2D surface of the retina
oWe reconstruct the world from 2 non-Euclidean inputs: 2 distinct retinal
images (left/right retinal image) which always differ (they differ because the
retinas are in slightly different places)
-Problem for vision: recover 3D information from 2D projections
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