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 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 EyeRight 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 EyeRight 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?