Week 2 Flashcards
What is Object Recognition?
Recognise objects regardless of:
Orientation
Distance
Lighting conditions
Important: objects encountered everyday
presented under a variety of circumstances
Impaired viewing conditions: object recognition
more difficult but can do it
Two-dimensional images: photos, paintings ie.
depth is absent
Brain’s object recognition system automatically
“copes” with a number of object transformations
Two things about our object recognition
system
Object descriptions of structures of all known
objects must be stored in the brain
Object recognition involves a comparison of the
seen object with the stored structural description
of the object.
Models of Object Recognition
Explain how information from early stages of
visual processing gets to a 3d layout of the world from observers viewpoint
What information is stored within the object recognition system?
visual knowledge of an object’s shape
describe from alternative viewpoints
knowledge of semantic functions and object’s
associates
functions and use of an object from a familiar
category of objects and know other objects that
are associated with the object just seen
knowledge of verbal names (labels)
Our cognitive system is flexible
e.g. describe an object at a number of levels but
some levels of object description are easier than
others
Rosch and colleagues: demonstrated that
identification on the basis of category
membership easier than identification at either
sub-ordinate or super-ordinate levels. (eg cat versus sub-ordinate(siamese) or superordinate (mammal))
Models of Object Recognition - Initial representation
Marr - primal sketch)
deals with Intensity (brightness) changes across field of
vision
Features such as edges
Models of obj reco - Viewer-centred representation
Marr - 2.5d sketch
Spatial locations visible surfaces
Uses information such as depth and location
Object only represented from the viewpoint of
viewer
Ellis and Young model
Object
|
V
Initial raw visual
representation
|
V
Viewer centred description
|
/ \
/ \
OCD>Object recognition units
|
V
semantic system
|
V
Speech output Lexicon
|
V
Spoken Namemodels of object reco - Object-Centred Represntation
Marr - 3d model
Real shape of objects and surfaces
independent of view-point of viewer
could use this “standard” info to look up object in object store
Object Recognition Units
stored structural object representations
comparison of viewer and object centred
representation
interface between visual and semantic object
representations
one recognition unit for each known visual
object
describe what an object looks like
Semantic Representation
describes an object’s properties and attributes
Object Recognition Unit can access semantic
representations when match between visual
representation and stored object description at
levels of object recognition unit
Access to Semantic representation occurs at
number of levels
Number of different types of input gain semantic
access
object, picture, written name, spoken name
Semantic Representation does not contain
name code for objects
Object Name is represented in separate store –
Lexicon
Must access object name via access to
semantics
Object naming task is typically used with
neuropsychological patients
there is Evidence for distinct stages in processing of an
object at the levels of:
Perceptual classification
Semantic classification
Name retrieval
Perceptual Classification Stage
Match a viewpoint-centred object to an object-
centred representation
Repetition Priming:
items, which have been
previously encountered at an earlier phase of an
experiment, are more easily recognised than
those that have not been previously
encountered
Effect of RP – modality specific
Warren and Morton (1982)
Briefly presented pictures of objects – task
naming aloud
Found that subjects were able to identify a
picture of an object at a shorter exposure
duration if they had either …
Seen the same object in a picture previously or
A different picture of the same object previously
in the experiment
Compared to the control condition – no prior
exposure to object picture
Previous exposure to object label – no effect on
recognition performance
As RP appears to be modality specific –
processing facilitation must be due to stage that
does not involve picture label (assume object
label and picture access same semantic
representation)
RP must effect a stage in the model that is
influenced by representations of the appearance
of an object
Semantic Classification
Previous exposure to object label – no effect on
recognition performance
As RP appears to be modality specific –
processing facilitation must be due to stage that
does not involve picture label (assume object
label and picture access same semantic
representation)
RP must effect a stage in the model that is
influenced by representations of the appearance
of an object
Semantic Classification
stage at which object’s functions and associates
are retrieved from memory
Access to semantics occurs – object seen,
heard, felt or if written name or spoken name
encountered
Why would this be important?
Semantic Priming – effect of related item
context on performance
Name Retrieval
access the names of objects via semantics
In normals
classification of pictures faster than decisions
requiring access to object labels (Potter & Faulconer,
1975)
word recognition – words named faster than they are
categorised
interference effects – irrelevant names interfere with
object naming but not with picture classification
(Glasser & Glasser, 1995)
Objects and pictures have direct access to their
meanings but an indirect access to their labels which
occurs via semantics
Note: Stages of object recognition operate in
sequence there is also cascading of the levels of
processing
Later stage in the model will start before an
earlier stage has been completed
Neuropsychological Impairments of Visual and
Spatial Processing
Problems in object recognition can arise due to
deficits within or between any of these stages in
the framework
Patterns of impairment in object recognition in
brain damaged patients reveal the relationship
between these different stages and the internal
organisation of each stage
Neuropsychological impairments of low-
level vision
initial raw visual representation
Impaired shape/form perception
—-intact colour, motion, location of object in space
but misperceives the form of an object.
—-Presented with a circle and a square patient is
unable to judge if the two stimuli have the same
shape or not
—-Reports of Patients with impaired form, shape or
depth perception are ambiguous
Deficits of Colour Perception ---Colour Blindness: Person who has a genetic defect resulting in an abnormality in the photoreceptor system ---Dichromats: people with only two photopigments ----red-green colour blind – missing photosensitive pigment for medium or long wavelengths ----blue-yellow – short wavelengths ----Anomalous trichromat: hav e all 3 photopigments but one of them has abnormal sensitivity
Colour Perception deficits from the CNS
Cerebral achromatopsia: patient cannot
perceive colours of object or scenes. Vision is
based on shades of grey
Due to damage in the colour-analysing regions
of the cortex, which is due to brain damage in
person that prior to the brain damage were able
to perceive colours normally (lesions in V4 but
may also extend to lesions in neighbouring
extra-striate cortex) or V8
Test for achromatopsia
–present with three colour patches: two are
identical and the third varies in hue, brightness
or saturation, task is to identify the patch that is
different
–Can do brightness but has difficulty with hue
Colour Agnosia: central and general loss of
knowledge about colour.
Colour Anomia: inability to produce colour
names.
So if you presented each of these patients with a colour patch, none of them would be able to name the colour of the patch BUT for three different reasons. defective colour perception defective colour knowledge defective colour name production
Colour Agnosia:
colour perception deficit of CNS -
central and general loss of
knowledge about colour.
Colour Anomia:
colour perception deficit of CNS -
inability to produce colour
names.
Impairment Movement Perception
Akinetopsia:
selective loss of motion
perception
One case reported to date, patient MP (Max
Plank Institute Munich, 1983).
Can perceive colour, shape and location of an
object but not object movement
Instead of continuous motion this type of
patient sees an object as jumping
discontinuously from one position to the next.
View the world as series of snapshots
eg pouring coffee into a cup, seen as static
brown column
MP has intact colour and form perception and
can identify briefly present letters
CT scans - bilateral lesions of the
temporoparietal cortices, lesions included
posterior and lateral portions of the middle
temporal gyrus, areas that were lesioned
roughly correspond to those responsible for
motor perception
Distinct forms of impairment at each of these low
levels of visual processing is evidence to
support neural recordings on primates that there
are distinct regions in the visual cortex
responsible for analysing these four dimensions
of visual input.
Akinetopsia
selective loss of motion
perception (intact colour and form perception and
can identify briefly present letters)
CT scans - bilateral lesions of the
temporoparietal cortices, lesions included
posterior and lateral portions of the middle
temporal gyrus, areas that were lesioned
roughly correspond to those responsible for
motor perception
Visual Agnosia
object perception failure with
intact shape colour and motion processes
Visual Agnosia (more info)
Visual Agnosia: object perception failure with
intact shape colour and motion processes
means failure of knowledge and recognition
adequate low-level vision, intellectual function,
language
impaired object recognition presented in visual
modality
can identify objects via other modalities such as
auditory, tactile
can get other forms of sensory modality agnosia
eg. tactile.
Agnosia can occur not only for one particular
sensory modality but may also be found for a
particular class of objects within a modality
eg. inability to recognise faces but ok at other visual objects (prosopagnosia)
problems with object recognition despite
registration of visual information at a cortical level
PT had a stroke
normal language function, no co-ordination
problems
normal visual acuity, intact colour, shape and
motion perception
Severe object recognition problems
Colour name to definition
Could identify objects using other sensory
modalities
Therefore deficit modality specific
Number of different stages in object recognition
from low-level visually processing level to the
semantic level
Follows that there are a number of different
types of visual agnosia.
