Final Flashcards
Where does the visual system break things apart?
Point of concavity
Advantages of Elastic Graph Representation
- Invariant to position, size, orientation in plane (2D) and some degree of view-point
- Biologically plausible
- Nodes can be represented as hypercolumns in the brain and connections as synapses
- Accounts well for holistic and subordinate recognition
What makes Elastic Graph Representation different from template?
Representations are encoded in addition to local features
Feature-set representation characteristics
- Ex: Mel’s Seemore
- Parallel neurons fire for specific features and when the right combination of feature neurons fire, the object is identified
-Fast because of parallel processing
Potential disadvantage with feature-set representation and Seemore’s reply
- Visual system would erroneously identify an object if the features were scrambled
- Possibly visual system could require specific features that would prevent scrambling
Why is motion perception so important?
Retinal images are always moving even in a still scene
- Humans move
- Objects in retina move
- Micro saccades and deliberate saccades
Subject-relative motion
Word for when the only thing moving is the subject?
What is the slowest an object can move and still be perceived?
10-20 min (1/60 of a degree) per second w/o background (subject relative)
1-2 min per second w/ background (object relative)
Retina displacement
Changing position of an object position’s in your retina
optical pursuit
Term for when an object stays steady in our fovea and we track it with our eyes
Apparent motion, beta motion, stoboscopic motion
Perceiving an object when their is no motion (TV)
Interstimulus interval
Term for time between the end of one flash and the beginning of the next
Interstimulus interval thresholds
If ISI < 60 ms we perceive moving object as being in two places at once
If 60 > ISI < 200 ms we perceive fluid motion
if ISI > 200 ms we perceive succession
One part of the visual field or an object makes another part or the entire visual field of the viewer appear to move
Induced motion
Indirect Perception Theory
Motion perception depends on other processes in the brain
Direct Perception Theory
Motion perception is directly computed from inputs
How much time needs to pass between two stimuli for people to determine the presentation order?
> =45ms
How much time needs to pass between two stimuli for people to tell which direction the same stimulus has moved?
14ms
What was the conclusion of the Exner study and how did they reach that conclusion?
They found that people could tell motion direction at much smaller intervals than presentation order.
-Direct perception theory must be true
What was the conclusion of the Wertheimer study and how did they reach that conclusion?
We can perceive motion without perceiving an object (objectless motion)
-Ganglion cells (early in the visual system can perceive motion)
When IST is low and an object appears to be in two places at the same time
Phi motion
How does a Reichardt detector work?
- Neuron A and B have adjacent perceptive fields
- Neuron A synapses with D causing a delay
- Neuron X only fires when it receives simultaneous input from D and B
How are Reichardt detectors arranged to detect the complexity of motion in the visual field?
Reichardt detectors are arranged in opponency with right detector inhibitory and left excitatory or vise versa
What is the waterfall illusion and how is it caused?
If you stare at a waterfall and then avert your gaze to the surrounding rocks, you will see the rocks as moving upwards because downward reichardt system becomes fatigued
What is the stepping feet illusion and why does it occur?
- When a yellow block and a blue block move across a black and white stripped field at the same speed, they appear to move one at a time
- Motion is perceived by the magnocellular-ventral stream which is color blind
- When the yellow block passes over the white stripe it cannot be perceived because there is not enough contrast and same thing occurs with the blue block and the black stripe
- Consequently, the stripes seem to move out of phase with each, one at a time
- If you turn down the contrast on the background, the blocks appear to move synchronously and smoothly
term for change in position
First order motion
Term for change in change
Ex. in noisy square, black and white squares change color
Second order motion
-DIfferent mechanisms than first order motion
Two main problems when computing global motion from local motion detectors
Correspondence problem - identifying which points belong to the same object as they appear and disappear across the visual field
Aperture problem - When viewed through an aperature (receptive field), the movement of a local feature is ambiguous
Where in the brain is motion primarily processed after V1?
Medial temporal and medial superior temporal area or V5
Why does the world not shake when we move?
Motor signals stimulate corollary discharge signals that reach a comparador and then reach the visual cortex
The geometric term describing motion of an object in space during which there are no changes in distances between any two points on the object. (object does not get distorted as it moves)
Rigid Motion
Can the brain deal with non-rigid motion?
Yes, ill-posed motion. Elastic motion, joint motion
What is the difference between depth and distance?
Depth is the relative position of an object from the observer (nearer vs. farther).
Distance is absolute (how many arm lengths etc.)
What is the difference between metric and non-metric?
metric is qualitative
Motion parallax
When an observer moves, objects in the visual field move. The extent to which the objects move indicate how far they are from the observer.
Optic flow
Movement of the whole field (motion parallax)
What algorithm does the brain use to divine edges from motion?
Nonzero differences in optic flow values between an object and its neighbor indicate an edge.
Cilliary muscles
Muscles that manipulate the width of the lens
What is accommodation?
Ability of eyes to change lens shape to focus on objects at different distances
What do ciliary muscles do when the eye focuses on a distant object?
Cilliary muscles relax causing lens attachment fibers to become taut and flatten lens
What do ciliary muscles do when the eye focuses on a close object?
Cilliary muscles relax contract releasing tension on fibers and allowing lens to become more spherical
What is convergence?
Eyes converge towards each other when an object is close
How does the brain know how far apart the eyes are?
Proprioceptive information
What are oculomotor cues useful for?
Calculating how far objects are within in near space (1 - 3m) from us
Pictorial cues
Depth cues that can operate in flat pictures. All of them are monocular cues in that they can operate when you view with only one eye
Laws of optics
How a 3d world is projected on to a 2D retinal image
-Laws of physics that the perceptual system is implicitly aware of
Inverse optics
The brain reverse engineers the projection of the 3D world on to the retina
-Ill-posed problem
Law of linear perspective
- Brain judges on object further down a set of converging lines to be longer because it things the object must be farther away.
- Parallel lines in the image plane remain parallel in other planes they converge