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 neural processes are distinctly associated with consciousness?– That is still a pretty hard problem!

What are the neural correlates of consciousness (NCC)

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:

Left Eye Right Eye

Stimuli:

Percept: 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?