Post-test review session Tuesday Nov 22 4-5 in TH241.

Post-test review session

Tuesday Nov 22

4-5 in TH241

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

• 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

From Perception to Action

And what’s in between?

A thought problem

• Mary the vision scientist has had a few drinks after a long day at a neuroscience conference. She boldly announces to the group at the table that Vision Science is the most important branch of neuroscience because “50% of the brain is visual cortex”.

• Charlie the motor control guy has also had a few drinks. “nonsense”, he exclaims, “50% of the brain is motor cortex so we must be equally important!”

• The rest of the group, comprised of auditory, memory and attention researchers start rolling up their sleeves and fixin for a fight, because that doesn’t leave any brain left for them to study.

A thought problem

• “Hold it!”, you shout, because you spot the problem immediately…

• The brain exhibits functional specialization, but the functions are complex and dynamic

– E.g. catching a ball, driving a car, reading a book, running

– Each example requires tight integration of perception and action

• “motor systems” are perception systems as well

• Some cells in basal ganglia have visual receptive fields that are locked to arm location rather than

Mirror Neurons

The perception – action system

What are Mirror Neurons?

• Mirror neurons are cells that fire when a monkey (or person?) performs an action or when it views another animal performing that same action


• Located in conjunction with a frontoparietal action planning network:– posterior inferior frontal

• pre-motor areas that represent impending actions• Many cells are “motor dominant”: they fire when monkey grasps

or reaches, but are independent of visual input – inferior parietal

• sensory area that integrates somatosensory, visual and auditory signals

• Many cells are “visuo-dominant” or “visuomotor-dominant”: they fire when monkey sees a graspable object or a stimulus that could be interacted with

– These regions are densely interconnected


• Mirror neurons are in regions immediately adjacent to these frontal and parietal areas

• Motor properties of mirror neurons are same as “non-mirror” neurons but…

• Sensory properties are different– These cells do not fire when monkey sees a graspable

object– They do fire when monkey sees another monkey (or a

person!) perform actions relative to objects

What can Mirror Neurons Represent?

• Some cells are precisely tuned to the specific actions (strictly congruent – about 1/3 of cells)– e.g. using two fingers to pick up an object

• Others are broadly tuned to any action that accomplishes the same goal (broadly congruent – about 2/3 of cells)– E.g. using any combination of hand and fingers to pick up an

object


• Mirror neurons represent abstract actions and goals– Cells will not fire when grasping is pantomimed– Cells will fire when grasping is real and visible– Cells will also fire when grasping happens behind an

occluding screen, as long as the monkey has seen that there is an object to be grasped behind the screen!


• Mirror neurons represent abstract actions and goals– Some mirror neurons will even fire in response to the sound

of an action being performed• e.g. the sound of breaking a peanut shell

What are mirror neurons for?

• Two theories:– Action imitation

– Understanding actions of others

Mirror Neurons in Humans?

• The existence of mirror neurons in humans is inferred– We don’t record directly from human cortex!


• EEG and MEG evidence suggests a Mu desynchronization (9 – 12 Hz over premotor areas) when someone observes grasping

• TMS evidence shows that observed grasping decreases the threshold for triggering motor potentials for grasping muscles in the hand.

• The putative Mirror Neuron System in humans

Iacoboni et al (2006)


• fMRI studies have found evidence for two brain regions associated with imitative actions

Task 1: watch action

Task 2: perform action

Task 3: imitate action


• Based on work with monkeys, it was presumed that mirror neuron areas would exhibit a specific pattern of BOLD response:

• Task 1 < Task 2 < Task 3

• Two regions exhibited such a response

Mirror Neurons Dysfunction in Autism?

• Some evidence supports a theory that a dysfunction of the MNS underlies social isolation disorders such as autism spectrum disorder (ASD)

–Some structural abnormalities revealed with anatomical MRI


• fMRI data shows that BOLD signal in MNS during viewing and imitating emotional expressions is negatively correlated with severity of Autism Spectrum Disorder


• Some evidence supports a theory that a dysfunction of the MNS underlies social isolation disorders such as autism spectrum disorder (ASD)

– MEG data shows abnormal propagation of signals in the MNS when imitating lip movements in individuals with Asperger’s Syndrome

Post-test review session Tuesday Nov 22 4-5 in TH241.

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Transcript of Post-test review session Tuesday Nov 22 4-5 in TH241.