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 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
• 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

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 Right Eye
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 Correlates of Rivalry
• Inferior Temporal Cortex (Ventral Pathway)?
• YES – cells in IT are strongly correlated with percept
• Why does area IT sound familiar to you?

• 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…

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

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

What are Mirror Neurons?
• 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

What are Mirror Neurons?
• Mirror neurons are in regions immediately adjacent to these frontal and parietal areas
• Motor properties of mirror neurons are same as “nonmirror” 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

• 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!

Mirror Neurons in Humans?
• 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.

Iacoboni et al (2006)

Mirror Neurons in Humans?
• 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

Mirror Neurons in Humans?
• 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

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

• 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