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?
– 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
Searching for the NCC
• We’ve seen several examples of visual system
activity in which no awareness ensues, yet
information is represented and processed
– Blindsight
– Object Substitution Masking
– Neglect
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?