Object Recognition

Question 1

Object Recognition

Answer 1

What’s involved? (visual object recognition)
– Intact vision (acuity)
– Ability to perceive shapes (perception)
– Discrimination
– Identification
– Recognition (memory)
– Naming (language)
– Object meaning/function
(often motor)

• Where can it fail?

Question 2

Object
Recognition
Deficits

Answer 2

• “Agnosia” = “failure of knowing”

• Visual agnosia = restricted to visual domain
– Can’t recognize objects visually
– Can still recognize objects using other senses (like touch)
– Visual acuity, memory, language, intellect intact

• (More on different types of agnosias later)

Question 3

Brain areas important for Object Recognition:

Answer 3

Occipital Lobe
Fusiform Gyrus (a face region)
Parahippocampal Area
Superior Temporal Sulcus (a face region)
Posterior Parietal 
Lateral Occipital & Posterior Inferior Temporal
Anterior Inferior Temporal

Question 4

The two visual processing streams

Answer 4

Dorsal Pathway (Where)
Ventral Pathway (What)

V1 and Posterier Parietal Cortex

Question 5

Evidence
for
2
streams

Answer 5

• Monkey lesions:
Trained monkeys on 2 tasks:
Landmark task and Object task
Lesioned parietal or temporal cortex; retested
Ungerleider, Mishkin et al., 1982

• Human lesion patients

• Neuroimaging
PET study Compare
a & b:
-­Object Task (same)
-­Position Task (different)

Question 6

The Dorsal and Ventral Streams

Answer 6

• “What” & “Where”
– How separate?
Where is information combined?

• Converge in frontal cortex

• Connections between streams
• Reality:
both “what” & “where” in both streams; the functions of the streams are not totally separate

• Other interpretations:
– “what” & “How”
– Identification vs action

Question 7

Patient

D.F.

Answer 7

Lesion to ventral “what” stream

“ashtray”
“long, black, & thin”

Visual identification impaired, but visually­‐guided
action intact.

Not a visual acuity deficit or a naming deficit

LOC Lesions

Question 8

Lateral Occipital Cortex

LOC

Answer 8

There is greater fMRI activity in LOC for images of intact objects than for scrambled pictures of the same objects.

Pattern of fMRI acLvity in LOC can differentiate
between objects

Question 9

Computational

challenges

Answer 9

• Variability in sensory information
(pink elephant, plaid apple)
– Object constancy (cars in different positions in space, differences in color due to shadows, knowing that something is there even if you can’t see it)
– Ambiguous information/bi-­stable percepts
(like the picture in which people perceive either a old woman or a young woman)

• View-­‐dependent or view­‐invariant recognition?
• Other types of invariance (location, size, viewpoint, lighting, etc)

• Shape encoding \ binding of parts
– how?
(three lines composing a triangle or an arrow)

• How are objects represented at neural level?
–Hierarchy of Coding Hypothesis
– “Grandmother cells” vs ensemble coding

Question 10

Ventral Pathway

Answer 10

The ventral stream begins with V1, goes through visual area V2, then through visual area V4, and to the inferior temporal cortex. The ventral stream, sometimes called the “What Pathway”, is associated with form recognition and object representation. It is also associated with storage of long-term memory.

Question 11

Dorsal Pathway

Answer 11

The dorsal stream begins with V1, goes through Visual area V2, then to the dorsomedial area and Visual area MT (also known as V5) and to the posterior parietal cortex. The dorsal stream, sometimes called the “Where Pathway” or “How Pathway”, is associated with motion, representation of object locations, and control of the eyes and arms, especially when visual information is used to guide saccades or reaching.

Question 12

Hierarchy of Coding Hypothesis

Answer 12

Features&raquo_space;> Conjunction of Features&raquo_space;> Component Shapes&raquo_space;> Object

Question 13

“Grandmother cells” vs ensemble coding

Answer 13

Ultra-­‐selective hypothesis:
Single neuron for “your grandmother”
-­ What if that cell dies?
‐ How do we perceive novel objects?
-­ How does it adapt over time (e.g., as grandma gets old)?
-­ Do we have enough neurons to represent every single object we might encounter?

AlternaLve:
Ensemble coding
-Collective activation of many neurons

Question 14

– 2
visual
processing
streams
(what
&
where/how)

– Grandmother
cells
vs
ensemble
coding

Answer 14

?

Question 15

What
is
the
evidence
for
“what”
informaFon
being
processed
in
the
ventral
stream?

Answer 15

?

Question 16

What
is
and
isn’t
impaired
in
visual
agnosia?

Answer 16

?

Question 17

Visual

agnosia

Answer 17

• Visual agnosia = restricted to visual domain
– Can’t recognize objects visually
– Can still recognize objects using other senses
– Memory, language, intellect intact

Question 18

Types
of
agnosia

Answer 18

• Visual agnosia = restricted to visual domain
– Can’t
recognize
objects
visually
– Can
sFll
recognize
objects
using
other
senses
– Memory,
language,
intellect
intact
• AppercepFve
agnosia
– Failure
of
perceptual
processing
– Can’t
perceive
forms,
trouble
with
object
constancy
• AssociaFve
agnosia
– Can
perceive
shapes/forms,
but
inability
to
associate
them
into
recognizable
objects
• IntegraFve
agnosia
• Category-­‐specific
agnosia

Question 19

Agnosia

Answer 19

“Agnosia” = “failure

of knowing”

Question 20

Apperceptive

agnosia

Answer 20

– Failure of perceptual processing

– Can’t perceive forms, trouble with object constancy

Question 21

Associative

agnosia

Answer 21

– Can perceive shapes/forms, but inability to associate them into recognizable objects

Question 22

Integrative

agnosia

Answer 22

Deficit in integrating parts of an object into a coherent whole

When asked to draw 2 circles and a square as they are in a picture, the patient draws parts of the circle and squares at a time, instead of just replicating the image shape by shape.

Question 23

Category-­Specific

agnosia

Answer 23

Selective deficit for living vs non-­‐living objects

Question 24

Face

Recognition

Answer 24

• Faces are important
– We see A LOT of them
– Ecologically important for survival & procreation
– Social implications
– Emotional cues

• …But are they special?
– Different neural mechanisms from other objects?
– Distinct parts of the brain?
– Different types of processing?

Question 25

Neural
mechanisms
for
face
percepFon
fMRI:
Fusiform
Face

Answer 25

fMRI: Fusiform Face Area (FFA)
Faces vs. Objects
Intact Faces vs. Scrambled Faces

ERP: N170 response specific to faces

Single cell recording:
Face‐selective responses in IT & STS

Question 26

Fusiform
Face
Area
(FFA)

Answer 26

The FFA is located in the lateral fusiform gyrus. This area is involved in holistic processing of faces and it is sensitive to the presence of facial parts as well as the configuration of these parts. The FFA is also necessary for successful face detection and identification. This is supported by fMRI activation and studies on prospagnosia, which involves lesions in the FFA.

Question 27

Special Properties of Face Perception

Answer 27

• Face inversion effect
– It’s much harder to recognize faces upside-­down
– Not the case with other types of objects

• Thatcher Illusion

• Whole vs parts:
“Holistic Processing”
(This is Larry; Is this Larry’s nose?)
We perceive whole faces better than parts of faces; But the difference is much much smaller when it comes to objects and parts of those objects (like house and then door)

Question 28

Thatcher

Illusion

Answer 28

The Thatcher Effect or Thatcher Illusion is a phenomenon where it becomes more difficult to detect local feature changes in an upside-down face, despite identical changes being obvious in an upright face. It is named after British former Prime Minister Margaret Thatcher on whose photograph the effect has been most famously demonstrated. The effect was originally created by Peter Thompson in 1980.

The effect is illustrated by two originally identical photos, which are inverted. The second picture is obviously altered so that the eyes and mouth are vertically flipped, though the changes are not immediately obvious until the image is viewed in normal orientation.

This is thought to be due to specific psychological cognitive modules involved in face perception which are tuned especially to upright faces. Faces seem unique despite the fact that they are very similar. It has been hypothesised that we develop specific processes to differentiate between faces that rely as much on the configuration (the structural relationship between individual features on the face) as the details of individual face features, such as the eyes, nose and mouth. When a face is upside down, the configural processing cannot take place, and so minor differences are more difficult to detect.

This effect is not present in people who have some forms of prosopagnosia, a disorder where face processing is impaired, usually acquired after brain injury or illness. This suggests that their specific brain injury may damage the process that analyses facial structures.

There is plenty of evidence that rhesus monkeys and chimpanzees exhibit the Thatcher effect.

The basic principles of the Thatcher Effect in face perception have also been applied to biological motion. The local inversion of individual dots is hard, and in some cases, nearly impossible to recognize when the entire figure is inverted.

Question 29

Holistic Processing

Answer 29

What role do external facial features (hair, moustaches, beards, etc.) play in face recognition? Many of us have experienced the difficulty of recognizing a friend or a colleague who had changed her hairstyle or had shaved his beard. Such a change seems like not just a change in the facial hair, but rather the whole face looks different. This phenomenon of face processing is called holistic processing, meaning that we perceive the face as a whole and not as a set of separate, independently processed features. Holistic processing is generally accepted to be unique to faces and provides strong support for the notion that faces are processed differently relative to all other object categories.

Question 30

Prosopagnosia

Answer 30

Prosopagnosia = inability to recognize faces

A type of selective face processing deficit

– “Face blindness”
– Like visual agnosia but only for faces
– Acquired prosopagnosia:
induced by brain damage
– Developmental prosopagnosia: congenital
(no obvious history of brain trauma/disease)

Question 31

Prosopagnosia

symptoms

Answer 31

• Cannot identify friends/colleagues
(social embarrassment, career difficulties)

• Cannot identify family members
(“which child is mine?”)

• Cannot recognize self in mirror

• Use other cues
(voice, hairstyle, clothing, unique facial features)

• Avoid social interactions
• Spectrum of severity

A double-­‐dissociation?
• Prosopagnosics can identify objects but not faces
• Any evidence of the reverse?
– Patient C.K. (Integrative Agnosic)

Question 32

Are faces special,

or just important?

Answer 32

• Expertise counter-­‐hypothesis (mixed evidence)

• Possible “modules” in the brain for specialized
processing:
– Faces
– Places
– Bodies
– Motion
– Tools
– Words
– Others?

Question 33

Other Aspects
of
Face Perception

Answer 33

• Recognition of facial identity
• Recognition of facial expression
• Social cues (joint attention, eye contact)

Question 34

A patient who mistakes a clock for a combination lock because they have similar features would be an example of what?

Answer 34

associative

Question 35

A patient who confuses a circle with a triangle would be an example of what?

Answer 35

apperceptive

Question 36

What are some examples of neural specialization

for face processing?

Answer 36

Whil
e
th
e
evidenc
e
reviewe
d
abov
e
migh
t
see
m
to
suppor
t
th
e
existenc
e
of
a
neura
l
syste
m
dedicate
d
to
processin
g
huma
n
faces
,
severa
l
recen
t
report
s
questio
n
thi
s
conclusion
.
Fo
r
example
,
th
e
sam
e
fusifor
m
area
s
tha
t
ar
e
preferentiall
y
activate
d
by
face
s
ca
n
als
o
be
preferentiall
y
activate
d
by
othe
r
categorie
s
wit
h
whic
h
th
e
perceive
r
ha
s
extensiv
e
experience
.
Th
e
‘
‘
fac
e
re
-
sponsive
’
’
are
a
is
activate
d
by
car
s
in
ca
r
expert
s
an
d
by
bird
s
in
bir
d
expert
s
(Gauthier
,
Skudlarski
,
Gore
,
&
Anderson
,
2000)
.
Thi
s
suggest
s
tha
t
visua
l
experienc
e
in
discriminatin
g
individua
l
member
s
of
a
particula
r
categor
y
play
s
an
importan
t
rol
e
in
formin
g
category
-
sensitiv
e
cortica
l
responses
.
On
e
hypothesi
s
is
tha
t
th
e
developmen
t
of
expertis
e
in
fac
e
processin
g
is
no
differ
-
en
t
fro
m
th
e
developmen
t
of
expertis
e
in
recognizin
g
individua
l
member
s
of
othe
r
categorie
s
of
comple
x
visua
l
stimuli
.
It
ma
y
onl
y
be
tha
t
face
s
ar
e
on
e
of
th
e
only
,
or
th
e
only
,
category
,
fo
r
whic
h
thi
s
abilit
y
devel
-
op
s
consistentl
y
acros
s
th
e
huma
n
population
.
How
-
ever
,
studie
s
of
adult
s
canno
t
full
y
tes
t
thi
s
hypothesi
s
becaus
e
the
y
examin
e
th
e
syste
m
onl
y
in
it
s
en
d
‘
‘
ex
-
pert
’
’
state
.
Merel
y
showin
g
tha
t
visua
l
expertis
e
ca
n
be
acquire
d
fo
r
nonfac
e
categorie
s
late
r
in
lif
e
doe
s
no
t
necessaril
y
prov
e
tha
t
visua
l
expertis
e
fo
r
fac
e
process
-
in
g
develope
d
in
th
e
sam
e
way
.
An
alternativ
e
possibilit
y
remain
s
tha
t
ther
e
ar
e
innat
e
mechanism
s
devote
d
specificall
y
to
fac
e
processin
g
bu
t
tha
t
thes
e
area
s
ca
n
late
r
be
recruite
d
fo
r
processin
g
in
othe
r
situation
s
tha
t
pos
e
simila
r
processin
g
demand
s
(e.g.
,
requirin
g
mem
-
or
y
fo
r
multipl
e
highl
y
simila
r
exemplar
s
withi
n
a
com
-
ple
x
visua
l
category)
.
Becaus
e
infant
s
begi
n
to
lear
n
abou
t
face
s
fro
m
ver
y
earl
y
in
life
,
th
e
rol
e
of
expertis
e
in
th
e
developmen
t
of
fac
e
recognitio
n
ca
n
onl
y
be
investigate
d
full
y
by
studyin
g
th
e
fac
e
processin
g
syste
m
as
it
develops
.
Studie
s
of
fac
e
recognitio
n
in
youn
g
infant
s
sho
w
tha
t
newbor
n
infant
s
respon
d
preferentiall
y
to
certai
n
simpl
e
face-lik
e
pattern
s
(Valenza
,
Simion
,
Cassia
,
&
Umilta
,
1996
;
Johnson
,
Dziurawiec
,
Ellis
,
&
Morton
,
1991
;
Go
-
ren
,
Sarty
,
&
Wu
,
1975)
,
raisin
g
th
e
possibilit
y
tha
t
th
e
face-specifi
c
regio
n
of
th
e
corte
x
observe
d
in
adult
s
is
activ
e
fro
m
birth
.
However
,
an
alternativ
e
vie
w
is
tha
t
th
e
tendenc
y
of
th
e
newbor
n
to
orien
t
to
face
s
is
largel
y
mediate
d
by
subcortica
l
structures
,
an
d
tha
t
ther
e
is
subsequentl
y
an
experience-dependen
t
specializatio
n
of
circuit
s
in
th
e
ventra
l
occipito-tempora
l
pathwa
y
fo
r
processin
g
face
s
(Johnso
n
&
Morton
,
1991
;
de
Schone
n
&
Mathivet
,
1989)
.
Thi
s
proces
s
of
specializatio
n
migh
t
be
see
n
as
increase
s
in
th
e
selectivit
y
of
respons
e
pattern
s
of
cortica
l
tissu
e
in
respons
e
to
a
particula
r
input
.
Fo
r
example
,
a
give
n
regio
n
of
cortica
l
tissu
e
ma
y
originall
y
respon
d
to
a
wid
e
rang
e
of
objects
,
bu
t
wit
h
experience
,
on
e
ma
y
narro
w
th
e
rang
e
to
jus
t
on
e
class
,
suc
h
as
faces
.
If
thi
s
is
true
,
on
e
woul
d
expec
t
devel
-
opmenta
l
change
s
in
th
e
specificit
y
of
cortica
l
process
-
in
g
of
face
s
ove
r
th
e
firs
t
year
s
of
life
.
Th
e
purpos
e
of
th
e
presen
t
stud
y
wa
s
to
examin
e
th
e
developmen
t
of
cortica
l
specializatio
n
fo
r
fac
e
process
-
ing
.
Sinc
e
functiona
l
magneti
c
resonanc
e
imagin
g
(fMRI
)
an
d
positro
n
emissio
n
tomograph
y
(PET
)
imagin
g
meth
-
od
s
canno
t
presentl
y
be
use
d
to
stud
y
health
y
huma
n
infants
,
we
use
d
ERP
s
to
addres
s
thi
s
questio
n
by
examinin
g
whethe
r
huma
n
infants
,
lik
e
adults
,
sho
w
cortica
l
potential
s
selectiv
e
fo
r
uprigh
t
huma
n
faces
.
A
prio
r
stud
y
ha
s
show
n
tha
t
an
N17
0
componen
t
tha
t
is
of
large
r
amplitud
e
an
d
shorte
r
latenc
y
fo
r
face
s
tha
n
object
s
is
observabl
e
in
childre
n
as
youn
g
as
4
year
s
of
ag
e
(Taylor
,
McCarthy
,
Saliba
,
&
Degiovanni
,
1999)
.
Th
e
sam
e
stud
y
showe
d
tha
t
th
e
N17
0
undergoe
s
develop
-
menta
l
chang
e
involvin
g
a
decreas
e
in
pea
k
latenc
y
an
d
an
increas
e
in
pea
k
amplitud
e
wit
h
age
.
In
vie
w
of
thi
s
developmenta
l
trend
,
we
expec
t
that
,
if
th
e
N17
0
is
presen
t
in
infants
,
it
wil
l
be
of
longe
r
latenc
y
an
d
smalle
r
amplitud
e
tha
n
in
adult
s
or
children
.
We
examine
d
th
e
specificit
y
of
th
e
respons
e
in
tw
o
ways
.
First
,
we
compare
d
response
s
to
huma
n
face
s
wit
h
response
s
to
face
s
of
anothe
r
species
,
monkeys
,
whos
e
face
s
hav
e
a
simila
r
configuratio
n
of
feature
s
as
huma
n
faces
.
We
predicte
d
tha
t
adults
’
N170
s
woul
d
be
of
large
r
amplitud
e
an
d
longe
r
latenc
y
fo
r
monke
y
face
s
tha
n
fo
r
uprigh
t
huma
n
faces
.
Ou
r
predictio
n
is
base
d
on
prio
r
finding
s
showin
g
tha
t
manipulation
s
of
th
e
fac
e
tha
t
disrup
t
configura
l
encoding
,
suc
h
as
inversio
n
(Rossio
n
et
al.
,
2000b
;
Eime
r
&
McCarthy
,
1999)
,
increas
e
th
e
amplitud
e
an
d
latenc
y
of
th
e
N170
.
We
expecte
d
mon
-
ke
y
face
s
to
be
simila
r
to
inverte
d
huma
n
face
s
in
thi
s
regard
,
sinc
e
it
is
know
n
tha
t
adult
s
ar
e
wors
e
in
recognizin
g
facia
l
identit
y
in
bot
h
monke
y
face
s
an
d
inverte
d
huma
n
face
s
tha
n
in
uprigh
t
huma
n
face
s
(e.g.
,
Pascali
s
&
Bachevalier
,
1998
;
Yin
,
1969)
.
fMR
I
studie
s
wit
h
adult
s
indicat
e
tha
t
inverte
d
huma
n
facs
activate
both
face-sensitiv e
region
s
of
th
e
cortex
,
suc
h
as
th
e
so-calle
d
fusifor
m
fac
e
are
a
(Kanwisher
,
Tong
,
&
Nakayama
,
1998)
,
an
d
additiona
l
adjacen
t
area
s
of
th
e
corte
x
involve
d
in
objec
t
recognitio
n
(Haxb
y
et
al.
,
1999)
.
Thus
,
in
adults
,
stimul
i
tha
t
ar
e
recognizabl
e
as
face
s
bu
t
ar
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Question 37

What special property of faces does the “Thatcher illusion” illustrate?

Answer 37

?

Question 38

What’s the difference between apperceptive

agnosia & associative agnosia?

Answer 38

Apperceptive agnosias (also known as visual space agnosias) refer to a condition in which a person fails to recognize objects due to a functional impairment of the occipito-temporal vision areas of the brain. Other elementary visual functions such as acuity, colour vision, and brightness discrimination are still intact. Apperceptive agnosics are unable to distinguish visual shapes and so have trouble recognizing, copying, or discriminating between different visual stimuli. When patients are able to identify objects, they do so based on inferences using colour, size, texture and/or reflective cues to piece it together. For example an apperceptive patient may not be able to distinguish a poker chip from a scrabble tile despite their clear difference in shape and surface features.

Associative agnosias are also known as visual object agnosias. Although they can present with a variety of symptoms, the main impairment is failure to recognize visually presented objects despite having intact perception of that object. A patient with an associative agnosia may be able to replicate a drawing of the object but still fail to recognize it. Errors in misidentifying an object as one that looks similar are common. Three specific criteria are associated with a diagnosis of associative agnosia (Farah,1990):

1) Difficulty recognizing a variety of visually presented objects (e.g., naming or grouping objects together according to their semantic categories).
2) Normal recognition of objects from a verbal description of it or when using a sense other than vision such as touch, smell, or taste.
3) Elementary visual perception intact sufficient to copy an object, as exemplified in original and copied picture below.

Overall, this loss can be thought of as “recognition without meaning”.

The neuropathology of associative agnosia does not show much consistency, perhaps different subtypes involve different perceptual impairments. Some cases seem to involve bilateral occipito-temporal damage. Other cases appear to be associated with unilateral damage to the occipital lobe and adjacent posterior temporal or parietal lobe, sometimes on the right or the left.

They may, for example, know that a fork is something you eat with but may mistake it for a spoon.

an associative agnosic who is shown a padlock will be able to recognize it and use it, but won’t be able to say what it is.

Because primary visual processing was intact, Lissauer considered the possible diagnostic distinction between deficits in perception (apperceptive agnosia) and in recognition (associative agnosia).

Question 39

primary visual cortex, V1

Answer 39

The primary visual cortex, V1, is the koniocortex (sensory type) located in and around the calcarine fissure in the occipital lobe. Each hemisphere’s V1 receives information directly from its ipsilateral lateral geniculate nucleus.

V1 transmits information to 2 primary pathways, dorsal stream and the ventral stream:

The primary visual cortex is the best-studied visual area in the brain. It is located in the posterior pole of the occipital cortex (the occipital cortex is responsible for processing visual stimuli). It is the simplest, earliest cortical visual area. It is highly specialized for processing information about static and moving objects and is excellent in pattern recognition.

The functionally defined primary visual cortex is approximately equivalent to the anatomically defined striate cortex. The name “striate cortex” is derived from the line of Gennari, a distinctive stripe visible to the naked eye that represents myelinated axons from the lateral geniculate body terminating in layer 4 of the gray matter.

The primary visual cortex is divided into six functionally distinct layers, labeled 1 through 6. Layer 4, which receives most visual input from the lateral geniculate nucleus (LGN), is further divided into 4 layers, labelled 4A, 4B, 4Cα, and 4Cβ. Sublamina 4Cα receives most magnocellular input from the LGN, while layer 4Cβ receives input from parvocellular pathways.

V1 has a very well-defined map of the spatial information in vision. For example, in humans, the upper bank of the calcarine sulcus responds strongly to the lower half of visual field (below the center), and the lower bank of the calcarine to the upper half of visual field. In concept, this retinotopic mapping is a transformation of the visual image from retina to V1. The correspondence between a given location in V1 and in the subjective visual field is very precise: even the blind spots are mapped into V1. In human and animals with a fovea in the retina, a large portion of V1 is mapped to the small, central portion of visual field, a phenomenon known as cortical magnification. Perhaps for the purpose of accurate spatial encoding, neurons in V1 have the smallest receptive field size of any visual cortex microscopic regions.

The tuning properties of V1 neurons (what the neurons respond to) differ greatly over time. Early in time (40 ms and further) individual V1 neurons have strong tuning to a small set of stimuli. The neuronal responses can discriminate small changes in visual orientations, spatial frequencies, and colors. Furthermore, individual V1 neurons in human and animals with binocular vision have ocular dominance, namely tuning to one of the two eyes. In V1, and primary sensory cortex in general, neurons with similar tuning properties tend to cluster together as cortical columns. David Hubel and Torsten Wiesel proposed the classic ice-cube organization model of cortical columns for two tuning properties: ocular dominance and orientation. However, this model cannot accommodate the color, spatial frequency, and many other features to which neurons are tuned. The exact organization of all these cortical columns within V1 remains a topic of current research.

Current consensus seems to be that early responses of V1 neurons consists of tiled sets of selective spatiotemporal filters. In theory, these filters together can carry out neuronal processing of spatial frequency, orientation, motion, direction, speed (thus temporal frequency), and many other spatiotemporal features.

Later in time (after 100 ms), neurons in V1 are also sensitive to the more global organisation of the scene. These response properties probably stem from recurrent processing (the influence of higher-tier cortical areas on lower-tier cortical areas) and lateral connections from pyramidal neurons. While feedforward connections are mainly driving, feedback connections are mostly modulatory in their effects. Evidence shows that feedback originating in higher-level areas such as V4, IT, or MT, with bigger and more complex receptive fields, can modify and shape V1 responses, accounting for contextual or extra-classical receptive field effects.

The visual information relayed to V1 is not coded in terms of spatial (or optical) imagery but rather as the local contrast. As an example, for an image comprising half side black and half side white, the divide line between black and white has strongest local contrast and is encoded, while few neurons code the brightness information (black or white per se). As information is further relayed to subsequent visual areas, it is coded as increasingly non-local frequency/phase signals. Note that, at these early stages of cortical visual processing, spatial location of visual information is well preserved amid the local contrast encoding.

Question 40

• Monkey lesions: Trained monkeys on 2 tasks: Landmark task and Object task Lesioned parietal or temporal cortex; retested Ungerleider, Mishkin et al., 1982

Answer 40

In 1982, Ungerleider and Mishkin argued that the 2 streams of visual processing play different but complementary roles in the perception of incoming visual information. According to their account, the ventral stream plays a critical role in the identification and recognition of objects, while the dorsal stream mediates the localization of those same objects. Some have referred to this distinction in visual processing as one between object vision and spatial vision-“what” versus “where.” Support for this idea came from work with monkeys. Lesions of inferior temporal cortex produced deficits in the animal’s ability to discriminate between objects on the basis of their visual features but did not affect their performance on a spatially demanding “landmark” task. Conversely, lesions of the posterior parietal cortex produced deficits in performance on the landmark task but did not affect object discrimination learning. Recent findings from a range of studies in both humans and monkeys are more consistent with a distinction not between subdomains of perception, but between perception on the one hand and the guidance of action on the other.

One source of evidence for the perception-action distinction comes from the study of the visual properties of neurons in the ventral and dorsal streams. Neurons in ventral stream areas such as IT are tuned to the features of objects, and many of them show remarkable categorical specificity; some of these category-specific cells maintain their selectivity irrespective of viewpoint, retinal image size, and even color. They are little affected by the monkey’s motor behavior, but many are modulated by how often the visual stimulus has been presented and others by whether or not it has been associated with reward. Such observations are consistent with the suggestion that the ventral stream is more concerned with the enduring characteristics and significance of objects than with moment-to-moment changes in the visual array.

Neurons in the dorsal stream show quite different properties from those in the ventral stream. In fact, the visual properties of neurons in this stream were discovered only when methodological advances permitted the experimenter to record from awake monkeys performing visuomotor tasks. Different subsets of neurons in PP cortex turned out to be activated by visual stimuli as a function of the different kinds of responses the monkey makes to those stimuli. For example, some cells respond when the stimulus is the target of an arm reach; others when it is the object of a grasp response; others when it is the target of a saccadic eye movement; others when the stimulus is moving and is followed by a slow pursuit eye movement; and still others when the stimulus is stationary and the object of an ocular fixation. In addition, of course, there are many cells in the dorsal stream, as there are in the ventral stream, that can be activated passively by visual stimuli-indeed logic requires that the visuomotor neurons must receive their visual inputs from visual cells that are not themselves visuomotor. These purely visual neurons are now known to include some that are selective for the orientation of a stimulus object. One important characteristic of many PP neurons is that they respond better to a visual stimulus when the monkey is attending to it, in readiness to make a saccadic or manual response. This phenomenon is known as neuronal enhancement.

htrsht

Question 41

N170

Answer 41

The N170 is a component of the event-related potential that reflects the neural processing of faces.