Lateralization and Consciousness

• Lateralization

  • Information is exchanged between the two hemispheres through commissures: white matter tracts → are white because of the amount of myelinated axons
  • Most major commissure: corpus callosum
  • Others: ex: anterior commissure, hippocampal etc…
• • Left and right hemispheres
  • Lateralization: division of labor between the two hemispheres, some functions have overlap in the hemispheres but some are isolated to a single hemisphere
  • Ex: left side is generally specialised for language production → where broca’s is
  • It's the dominant side in 95% of right handers
  • Most left handers have left hemi-dom but some have right/mixed dom
  • Comprehension is NOT lateralized
  • Why are they usually on the opposite side? Contralateral control (no shown benefits)
  • EXCEPT SMELL AND TASTE
  • Skin and muscles on the right side are connected to the left hemisphere
• • Each hemisphere processes visual information from the opposite visual field
  • However, since HUMAN eyes face forward, it it not ONLY connected contralaterally
  • Wdym? 
  • Visual field: what can be seen
  • Light from one side of the visual field shines on both retinas and vice versa
• • Our retinas in both eyes are separated into the right visual field and left visual field, these fields then connect to their contralateral parts respectively
  • Ex: the left eye will have a portion of retina for left and right visual field - the right visual field feeds to the left hemisphere and vice versa 
  • Half of the axons from either eye cross at the optic chiasm
• Animals that have eyes on the side of their head, their eyes are completely connected with one hemisphere
• • Cutting the corpus callosum
  • Damage to the major commissure = prevents a major amount of hemisphere information exchange
  • Damage can be unintentional or intentionally
• • Intentional corpus callosum case study: epilepsy
  • Epilepsy: repeated episodes of seizures (from neural activity) (unregulated depolarization)
  • Can be from genetic mutations
  • Some medications are not helpful
  • Doctors attempt to find the focus of it but it's hard to remove all of them without harming patient’s QOL
• Intentional corpus callosum case study: epilepsy

  • Last resort? Cutting the corpus callosum
  • Why?
  • It restricts the seizure to one side as there is no communication between hemispheres: less frequent seizures
• Intentional corpus callosum case study: epilepsy

  • Immediately after surgery
  • Each hemisphere quickly responds to information that directly reaches it → like 2 people in one brain
  • Thus, hemispheres can be in conflict
  • Smaller commissures don't work as fast as the corpus callosum
• • Intentional corpus callosum case study: epilepsy
  • Over time
  • Brain recovers and adapts to smaller commissures and conflicts less often
  • But integration between hemispheres remain difficult
  • Partial recovery
• • “Split brain people”
  • These patients remain normal in intellect, motivation, walking and speech
  • Kinda like HM, where many things were intact but memory
  • Split brained people were mostly fine
  • Gain ability to use their hands independently in ways people can't
• • Difference?
  • When stimuli is limited to one side of the body = altered ability in naming the object/saying what object
  • Ex: right hemisphere sees hat, left hemisphere sees band
  • With your right hand, write what you saw → right hand is controlled by left → left hemisphere saw band: writes band - left for language
  • With your left hand point to what you saw → left hand is controlled by right → right hemisphere saw hat: point to hat - right for motor
  • But they have no idea why they pointed: left is for language
  • It's like two people inhabiting a single body
• • Right hemisphere
  • Left is thought to be “more dominant” and gave you sense of self - the master
  • Better at perceiving emotions of others, spatial relationships and overall context
  • Damage to right hemisphere: you struggle to do the things it’s good at (above)
  • Ex: emotions of others, humour, pragmatics
  • Being more left brained or right brained isn't real lmao
• • Studies
  • Ex: struggle to detect who is happier in an image with a smile on one side and is neutral 
  • Ex: an H made out of Bs, slight elevation in activity in right when seeing overall context, slight elevation in activity when looking for what is the smaller letter
• • Development of lateralization
  • Corpus callosum gradually matures, axons gradually become myelinated/some projections are pruned
  • Children <6 have an immature corpus callosum - resemble split brained people
  • Struggle to compare objects from the left and right hands
  • Ex: putting an apple in their left hand - may struggle to answer in comparison to right hand
• Development of lateralization

  • Corpus callosum may not develop completely, different than injury/cutting
  • In development = more neuroplasticity: the brain can easily adapt to having no corpus callosum
  • Compensatory hypertrophy (overgrowth) of other commissures to compensate for lack of corpus callosum
  • This allows for better performance on tasks compared to split brained people
• Consciousness

  • Consciousness can be seen neuroscientifically and philosophically however, philosophy wise - it's difficult and requires assumptions
• Assumptions

  It exists

  It seems obvious but ex: in split brain patients - they struggle to do ex: explain why they touched something with their right hand: demonstrates that we dont need conscious access to information to report why you made a conscious action
• Assumptions

  It’s physical

  Consciousness does not arise from spirituality or religion - doesnt hold much ground in science but it is still a popular belief
• Assumptions

  Not everything is conscious

  Some argue that everything is conscious - not falsifiable → not scientifically testable
• • Studying consciousness
  • “What's it like to be a bat?”
  • Argues that consciousness is if you can have an experience that is fundamentally like something else
  • But no one knows what someone else’s conscious state is → ex: what if my red is your green
  • In this definition, because “experience” is subjective and science is objective, we cannot study it
• • The problem with studying consciousness
  • We don’t know someone else’s consciousness and we can't understand why we’re conscious at all
  • “Why do we have any experience at all?
  • Easy problem: how does the brain produce behavior
  • Hard problem: why do we have experience at all?
  • This can't be addressed!
• • Argued that you can again information about subjective information from objective information
  • She studied the physics and behaviors of bats → previously: bats use echolocation to navigate the world: this representation is not available to us - their “experience” → but we can narrow down the behavior and why they do it: ex: stereotyping echolocation for pray capture
  • There are neurons for direction to pray, speed of bat, etc… this doesnt form a 3D representation of the world though
• • Narrowing consciousness
  • Consciousness distinction
  • Phenomenological: experience, what it feels to be conscious → hard to research
  • Access consciousness: consciousness that is available for use in reasoning and rationally guiding speech and action
  • This lets us make closer neuroscientific theories
• • Theories must be:
  • Consistent with current observations
  • Precise
  • Has explanatory power - should provide a mechanism for a concept
  • Falsifiable
  • Parsimonious (least complicated as possible)
  • Promotes further scientific process
• • Measuring consciousness with attention
  • At different levels of visibility: subject performance and subjective report are correlated - they perform slightly better than chance → suggests that the barely visible target, despite not being consciously perceived, still has a subliminal effect (unconscious processing → action)
• • Eeg activity: temporal specificity and kinda okay spatial specificity, when and generally where, can't get deep brain specificity
  • Identified 5 major events:
  • P = positive
  • N= negative
  • Numbers = if the signal happens in the first whatever hundred millisecond
  • P1a, P1b, N1, N2, P3
  • The early signals (P1a/b) - in occipital realm, in early visual processing
  • Intermediate signals (N1) - signal moves down to other cortical sites, ex: frontal lobe
  • Late (N2, P3) - correlate to visibility of stimulus - activity increases as visibility increases
• • Measuring consciousness with attention
  • Using 50 SOA (50/50 seen vs not seen stimuli)
  • Subjects that saw them: greater amplitude in P3 than those who didnt
  • Shows us neural correlate of consciousness
  • Something that is different in the brain between situations of conscious perception
• • Fmri signature for specificity
  • Seen vs unseen
  • Masked words: activation in occipital lobe (P1, P2) visual processing areas
  • Visible words: shows distributed activation in temporal, parietal and frontal cortex
  • This was repeated in several other studies ex: auditory/motor masking and show the same results
• • Observing effects in single units of the brain
  • Epilepsy patients are recorded in HPC (structures with memory) when shown familiar vs unfamiliar images
  • Results:
  • Almost perfect selectivity in this one neuron → it fires only when the person knows the image, tells us info is not routed to the HPC unless the person knows the image
  • Neurons on recognized trials: show high rate of firing regardless of presentation variation
  • Unrecognised trials: low rate of activation
• All together

  • Visual stimuli is being processed in the brain regardless of conscious perception
  • We all have multiple streams of visual processing, attention of conscious perception and unconscious perception
  • Only conscious perception triggers global activation: routing of information from one subliminal processing stream to a distributed network in the brain
• All together

  • Conscious access is mediated by distributed access and simultaneous processing of a stimuli or a thought by a neural network - at any time, we have many processing streams that are competing for our attention → having higher strength and allocating energy to it, that network can enter the global neuronal workspace (network of brain areas) to process that information → global neuronal workspace = this is the neural correlate of consciousness
  • It isn't the only theory of consciousness but it's one of the popular ones
• • Implications
  • Those in minimally conscious states may be able to form clear distinct neural activities in accordance to the specific scenarios
  • This allows us to measure responses in minimally conscious states ex: imagining 2 scenarios as a proxy for “yes” and “no”
  • Not very reasonable to do and we hope to look for an easier way to do it
  • Infographic - using tone changes and pattern changes to measure the different levels of activity in specific brain regions
  • Vegetative responsive patients show similar activity to conscious controls → may show some form of consciousness