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Flashcards in Week 2 Deck (23):
1

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

2

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.

3

Models of Object Recognition

Explain how information from early stages of
visual processing gets to a 3d layout of the world from observers viewpoint

4

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))

5

Models of Object Recognition - Initial representation

Marr - primal sketch)

deals with Intensity (brightness) changes across field of
vision

Features such as edges

6

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

7

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 Name

8

models 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

9

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

10

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

11

Perceptual Classification Stage

Match a viewpoint-centred object to an object-
centred representation


12

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

13

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

14

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

15

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

16

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

17

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

18

Colour Agnosia:

colour perception deficit of CNS -

central and general loss of
knowledge about colour.

19

Colour Anomia:

colour perception deficit of CNS -
inability to produce colour
names.

20

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.

21

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

22

Visual Agnosia

object perception failure with
intact shape colour and motion processes

23

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.