Neural Correlates of Visual
Awareness
A Hard Problem
• Are all organisms conscious?
A Hard Problem
• Are all organisms conscious?
• If not, what’s the difference between those that are
and those that are not?
–
–
–
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Complexity?
Language?
Some peculiar type of memory?
All of these?
A Hard Problem
• Really what we’re asking is:
What is it about our brains that makes us conscious?
A Hard Problem
• Neuroscientists have deferred some of the difficulties
of that problem by focusing on a subtly different one:
What are the neural correlates of consciousness (NCC)
• What neural processes are distinctly associated with
consciousness?
– That is still a pretty hard problem!
Searching for the NCC
• When a visual stimulus appears:
– Visual neurons tuned to aspects of that stimulus fire action
potentials (single unit recording)
– Ensemble depolarizations of pyramidal cells in various parts
of visual cortex (and elsewhere) (ERP, MEG)
– Increased metabolic demand ensues in various parts of the
visual cortex (and elsewhere) (fMRI, PET)
– A conscious visual even occurs
Searching for the NCC
• We can measure all sorts of neural correlates of
these processes…so we can see the neural
correlates of consciousness right?
• So what’s the problem?
Searching for the NCC
• We can measure all sorts of neural correlates of
these processes…so we can see the neural
correlates of consciousness right?
• So what’s the problem?
• Not all of that neural activity “causes” consciousness
• We will explore some situations in which neural
activity is dissociated from awareness
Dorsal and Ventral Pathways
• But first some review and further consideration of
visual pathways
Dorsal and Ventral Pathways
• Different visual cortex
regions contain cells with
different tuning properties
represent different features
in the visual field
• V5/MT is selectively
responsive to motion
• V4 is selectively responsive
to color
Dorsal and Ventral Pathways
• V4 and V5 are doubly-dissociated in lesion literature:
Achromatopsia and Akinetopsia, respectively
Dorsal and Ventral Pathways
• V4 and V5 are key parts of
two larger functional
pathways:
– Dorsal or “Where” pathway
– Ventral or “What” pathway
– Ungerleider and Mishkin (1982)
• Magno and Parvo dichotomy
arose at the retina and gives
rise to two distinct cortical
pathways
Dorsal and Ventral Pathways
•
Pohl (1973) Early dissociations
of Temporal and Parietal
functions
•
Landmark task:
–
Monkeys trained to find reward in
well near a landmark
–
once they get the task the
contingency is switched – monkey
must find well opposite to the
landmark
–
#errors until relearning indicates
ability to use the spatial relationship
information to perform task
Dorsal and Ventral Pathways
• Pohl (1973) Early
dissociations of Temporal
and Parietal functions
• Landmark task:
– Dissociates Parietal and
Temporal lobes
– Parietal lesions impair
relearning of landmark task
Dorsal and Ventral Pathways
• Pohl (1973) Early
dissociations of Temporal
and Parietal functions
• Object task:
– Reward location is indicated by
one of two objects
– contingency is switched –
monkey must use other object
– # errors to relearn indicates
ability to use object distinction
to perform task
Dorsal and Ventral Pathways
• Pohl (1973) Early
dissociations of Temporal
and Parietal functions
• Object task:
– Adding this task doubly
dissociates Parietal and
Temporal lesions
– Temporal lesions impair object
task
Dorsal and Ventral Pathways
– do both of these
pathways equally
contribute their “contents”
to visual awareness?
V4
V5
Agnosia
• Lesions (especially in the left
hemisphere) of the inferior
temporal cortex lead to
disorders of memory for
people and things
• recognition and identification
are impaired
– prosopagnosia is a specific
kind of agnosia: inability to
recognize faces
• explicit (conscious) decisions
about object features are
disrupted
Agnosia
• Goodale and Milner –
Patient DF
• Patient could not indicate the
orientation of a slot using her
awareness
• Patient could move her hand
appropriately to interact with
the slot
– whether visually guided or
guided by an internal
representation in memory
Agnosia
• Single dissociation of action from conscious
perception
• Dorsal pathway remained intact while ventral
pathway was impaired
• Dorsal Pathway seems to guide motor actions, at
least for ones that need spatial information
• Activity within the Dorsal Pathway seems not to be
sufficient for consciousness
Blindsight
Lesions of Retinostriate Pathway
• Lesions (usually due to
stroke) cause a region of
blindness called a scotoma
• Identified using perimetry
• note macular sparing
X
Retinocollicular Pathway
independently mediates orienting
• Rafal et al. (1990)
• subjects move eyes to fixate
a peripheral target in two
different conditions:
– target alone
Retinocollicular Pathway
independently mediates orienting
• Rafal et al. (1990)
• subjects move eyes to fixate
a peripheral target in two
different conditions:
– target alone
– accompanied by distractor
Retinocollicular Pathway
independently mediates orienting
• Rafal et al. (1990) result
• Subjects were slower when presented with a
distracting stimulus in the scotoma (359 ms vs. 500
ms)
Retinocollicular Pathway
independently mediates orienting
• Blindsight patients have since been shown to posses
a surprising range of “residual” visual abilities
– better than chance at detection and discrimination of some
visual features such as direction of motion
• These go beyond simple orienting - how can this be?
Retinocollicular Pathway
independently mediates orienting
• Recall that the feed-forward sweep in not a single
wave of information and that it doesn’t only go
through V1
• In particular, MT seems to get very
early and direct input
Retinocollicular Pathway
independently mediates orienting
• Recall that the feed-forward sweep in not a single
wave of information and that it doesn’t only go
through V1
• In particular, MT seems to get very
early and direct input
• Information represented in dorsal
pathway guides behaviour but
doesn’t support awareness
Searching for the NCC
• What is needed is a situation in which a perceiver’s
state can alternate between aware and unaware in
ways that we can correlate with neural events
• One such situation is called Binocular Rivalry
Rivalrous Images
• A rivalrous image is
one that switches
between two
mutually exclusive
percepts
Binocular Rivalry
• What would happen if each eye receives
incompatible input?
Left Eye
Right Eye
Binocular Rivalry
• What would happen if each eye receives
incompatible input?
• The percept is not usually the amalgamation of the
two images. Instead the images are often rivalrous.
– Percept switches between the two possible images
Binocular Rivalry
• Rivalry does not entail suppression of one eye and dominance
of another – it is based on parts of objects:
Stimuli:
Left Eye
Percept:
Right Eye
Or
Binocular Rivalry
• Percept alternates randomly (not regularly) between
dominance and suppression - on the order of
seconds
– What factors affect dominance and suppression?
Time ->
Binocular Rivalry
• Percept alternates randomly (not regularly) between
dominance and suppression - on the order of
seconds
– What factors affect dominance and suppression?
– Several features tend to increase the time one image is
dominant (visible)
• Higher contrast
• Brighter
• Motion
Binocular Rivalry
• Percept alternates randomly (not regularly) between
dominance and suppression - on the order of
seconds
– What factors affect dominance and suppression?
– Several features tend to increase the time one image is
dominant (visible)
• Higher contrast
• Brighter
• Motion
• What are the neural correlates of Rivalry?
Neural Correlates of Rivalry
• What Brain areas “experience” rivalry?
• Clever fMRI experiment by Tong et al. (1998)
– Exploit preferential responses by different regions
– Present faces and buildings in alternation
Neural Correlates of Rivalry
• What Brain areas “experience” rivalry?
• Clever fMRI experiment by Tong et al. (1998)
– Exploit preferential responses by different regions
– Present faces to one eye and buildings to the other
Neural Correlates of Rivalry
• What Brain areas “experience” rivalry?
• Apparently activity in areas in ventral pathway
correlates with awareness
• But at what stage is rivalry first manifested?
• For the answer we need to look to single-cell
recording
Neural Correlates of Rivalry
• Neurophysiology of Rivalry
– Monkey is trained to indicate
which of two images it is
perceiving (by pressing a
lever)
– One stimulus contains
features to which a given
recorded neuron is “tuned”,
the other does not
– What happens to neurons
when their preferred stimulus
is present but suppressed?
Neural Correlates of Rivalry
• The theory is that Neurons in the LGN mediate
Rivalry
Neural Correlates of Rivalry
• The theory is that Neurons in the LGN mediate
Rivalry
• NO – cells in LGN respond similarly regardless of
whether their input is suppressed or dominant
Neural Correlates of Rivalry
• V1? V4? V5?
• YES – cells in primary and early extra-striate cortex
respond with more action potentials when their
preferred stimulus is dominant relative to when it is
suppressed
• However,
– Changes are small
– Cells never stop firing altogether
Neural Correlates of Rivalry
• Inferior Temporal Cortex (Ventral Pathway)?
• YES – cells in IT are strongly correlated with percept
Neural Mechanisms of Consciousness?
• So how far does that get us?
• Not all that far – we still don’t know what is the mechanism
that causes consciousness
• But we do know that it is probably distributed rather than
at one locus
• Thus the question is: what is special about the activity of
networks of neurons that gives rise to consciousness?