neural correlates of visual awareness

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


• 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?


• 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 recall some details about visual 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


• V4 and V5 are doubly-dissociated in lesion literature: Achromatopsia and Akinetopsia, respectively


• 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


– 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


• Rafal et al. (1990)

• subjects move eyes to fixate a peripheral target in two different conditions:– target alone– accompanied by distractor


• Rafal et al. (1990) result

• Subjects were slower when presented with a distracting stimulus in the scotoma (359 ms vs. 500 ms)


• 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?


• Recall that the feed-forward sweep is 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


• 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


• 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


• 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


• 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


• 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?


• The theory is that Neurons in the LGN mediate 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


• 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


• Inferior Temporal Cortex (Ventral Pathway)?

• YES – cells in IT are strongly correlated with percept


• Inferior Temporal Cortex (Ventral Pathway)?

• YES – cells in IT are strongly correlated with percept• Why does area IT sound familiar to you?

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? – that’s still a very hard problem

neural correlates of visual awareness

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