Quiz 3

What is perception?

a cognitive process that uses our previous knowledge to construct, identify, and interpret the stimuli that our senses register

Why we need perception?

�for object recognition
�for action

the inverse problem

detecting a signal away from its source, then trying to determine or reconstruct the source
(light detected by receptor cells at the retina forms a 2D image, while the real world is in 3D! - present in all perceptual modalities)

distal stimulus

object in the world that emits or reflects energy

proximal stimulus

�pattern of energy that reaches sensory receptors

perceptual object

what we perceive as a result of our sensory activity and post-sensory processing

from retina to brain

retina optic nerve lateral geniculatenucleus (LGN) primary visual cortex (V1) higher visual cortices association cortices

the likelihood principle

given all the possible stimuli that could produce the proximal stimulus, assume the most likely one to be the distal stimulus

frame of reference

apply the likelihood principle
in context!

in perceiving brightness and color what are 5 assumptions we make?

- surfaces are uniformly coloured
- gradual changes in brightness are caused by shadows
�light comes from above
- any light source producing a shadow is stationary
�local contrast

monocular depth clues: pictoral cues: 6

1. occlusion
2. linear perspective
3.atmospheric perspective
4. texture gradient
5. height-in-plane relative size
6. familiar size


binocular depth dues: the extent to which thetwo eyes are turned inward to fixate an object


Binocular Depth Dues: the visual perception of depth due to:
�retinal disparity-the difference betweenwhat the two eyes see(where on the two retinasan image projects).

retinal disparity

the difference betweenwhat the two eyes see(where on the two retinasan image projects). (the closer the object,the larger the retinal disparity)

What is perception?

a cognitive process that uses our previous knowledgeto construct, identify, and interpret the stimuli that our senses register

top-down processing

perceptual and associative knowledge acquired from past experienceguides our current perceptions.

bottom-up processing

assumptions and cues we use to go from sensory input to percept are learned!

ventral pathway goes from..

from visual cortex (V1)
to inferior temporal cortex (IT)

dorsal pathway goes from..

from visual cortex (V1) to posteriorparietal cortex (PPC)

ventral stream is considered the _______pathway

what pathway : object recognition
(conscious) perception of the visual properties and identity of objects; view-invariant

dorsal stream is considered the _______ pathway

where pathway (also how): location and movement for action
(unconscious/automatic) visual guidance of movement; egocentric

lesion to the _____ causes visual form agnosia

inferior temporal lesions : ventral stream

lesion to the _____ causes optic ataxia

posterior parietal lesions ; dorsal stream

visual form agnosia

unable to recognize object size/shape/orientation. BUT able to use visual features to guide precise hand and finger movements (finger-thumb grasp)
the dorsal stream must have access to information about the visual features of the object (e.g. shape and or


single neuron recordings in monkeys:
V1 V2 & V4 inferior temporal (IT) cortex
(feedback connections too!)
�each area represents the entire visual field
�neurons in these areas progressively increase
the sizeof their receptive fields
the com

v1 is the...

primary visual cortex

selectivity of v1 neurons: findings were...

cells in the primary visual cortex are selective for line orientation, direction,and speed of movement

what are the two models of visual recognition (ie larger theories) their definitionand their subcatagory models

1. viewer centered theories:
the perceptual representation of the object depends on the viewpointof the perceiver
�template-matching model
�feature-matching model
2. object centered theories
the perceptual representation of the object is independent of th

what are the two viewer centered theories?

template-matching model
feature-matching model


must find an exact object-level match
problem: very rarely will you see the exact same object from the exact same perspective twice!
changes in viewpoint produce drastic changes in the proximal stimulus


match some collection of individual features
problem: deals with size variance but how about orientation variance? being egocentricis a problem!
�how do you know which are the critical features?
changes in viewpoint produce drastic changes in the proximal

biedermans geon model

recognition by components

what are geons

geometric ions
�36 simple shapes
�distinguishable from almost every viewing perspective
�recognizable even when parts are obscured
�fundamental for object recognition
�building blocks of every object

geon model: objects are defined by... (2)

objects are defined by:
�their constituent set of geons
�the spatial relationship between those geons

geon model: object recognition (2)

1.extract appropriate geonfrom image
2.match to similar representations stored in long-term memory

geon model experiment

vertices -where two or more line segments come together
-are critical for the detection of geons
�if the vertices are deleted, object perception is impaired; if they are not, it's normal (even if slower)

does viewpoint matter?

usually can recognize objects at even the oddest of viewing angles, visual identification is fastest from certain viewpoints.
very small differences in object properties can make a world of difference!

most objects have a canonical view - what is this?

the most common viewing angle; the view at which recognition is fastest and most accurate

results of canonical object test:

familiarized subjects with novel objects at one viewpoint, and tested recognition at different viewing angles
recognition was slower the more the object is rotated from its learned viewpoint

viewer based model

whole-object views are stored in memory
not ALL possible viewpoints, only the most common (canonical) ones
in matching-to-template, the current object representation can be mentally rotated/zoomed

face recognition: findings

found that participants were better at identify whole faces than parts
also found that this part/whole difference did NOT hold for other kinds of stimuli: scrambled faces, inverted faces, houses
concluded that faces are represented holistically-parts are

what is the FAA

the fusiform"face" area - in right inferior temporal cortex
when damaged leads to prosopagnosia-a profound selective deficit in face recognition

two views: no there is no FAA because

faces are only different from other objects because we identify them not at the superordinateor subordinate level but at the specificlevel.
faces are represented holistically because this is how expertsrepresent their objects of expertise.