NEUR1020 Module 4 Content

What are lateralised brain functions?

Brain functions that rely more on one side of the brain than the other

• Left Hemisphere: language, speech

• Right Hemisphere": tone of voice/prosody, face perception, perceptual grouping (perceiving groups of objects as a whole)

Very few functions are specifically lateralised to 1 hemisphere and most complex human functions coordinate both

What are crossed/contralateral brain functions? What is ipsilateral?

Functions where each hemisphere controls the opposite side of the body

• primary motor/sensory/visual areas are crossed over to represent movement/sensation/vision on the opposite side of the body

• each half of the eye retina is split so left vision from both eyes goes to the right hemisphere and vice versa. This is important for 3D depth perception.

Ipsilateral=same side

What does the corpus callosum do? Importance example?

Connects the Left and Right hemispheres to allow communication

• axons of neurons (nerve fibres) cross to the opposite (contralateral) hemisphere to connect with opposite hemisphere neurons: allowing transfer of information between them

• inter-hemispheric information is crucial

eg. Experiment done where we briefly flash stimuli to one side of visual space so fast it is before the participant moves their eyes (only one side of visual space catches the image)

• flash to right=stimuli goes to LH.

  • LH is where language functions are lateralised, so if you ask for a verbal report of the stimuli there is no communication between the 2 hemispheres (information stays in LH)

• flash to left=stimuli goes to RH

  • To make a verbal report now, the information needs to cross through the corpus callosum to the other hemisphere where language functions are lateralised. No corpus callosum= can’t make the verbal report.

Split brains: what is it? why is it done? consequences?

Split brain patients have their corpus callosum cut, so their left and right hemispheres can no longer communicate.

• this procedure is a last resort surgical treatment for severe epilepsy as it stops seizures activity from spreading to the other hemisphere

• studying patients helps inform about lateralisation

consequences example: the same flashing experiment. under the flashing screen is a gap where people could feel objects but couldn’t see them.

• hammer image flashed left (goes to RH)

  • person says ‘i saw nothing’ as language is left lateralised and left doesn’t see it

  • person reaches with left hand and grabs the hammer object they ‘didn’t see’: Left sensation is right lateralized and right saw the hammer.

    • they could also draw the hammer with their left hand if asked to: then they could look at their drawing and go ‘oh it must have been a hammer!’ this allows external interhemispheric communication

Overall: left can tell (verbally), right can show (via left hand)

Hippocampus importance?

Part of the limbic system that lies in the medial temporal lobe that is crucial for encoding new declarative long term memories.

• damage/removal=can’t make new memories

• also contains ‘place cells’ that give us the ability of spatial navigation (mentally mapping familiar environments)

eg. HM had hippocampus removed in both hemispheres and:

• couldn’t form memories/recall anything after the time of the surgery (can’t encode new declarative long term memories)

• could remember things from before surgery (can retrieve long term memories)

• could “mentally rehearse” to remember things for a few seconds (short term memory intact)

• could learn new skills but couldn’t remember learning them (can encode new procedural long term memories)

Memory: types?

Memory=different components mediated by different brain parts. 2 main divisions:

1. Short term memory lasts several seconds (eg. remembering phone numbers long enough to type them into contacts)

2. Long term memory has 2 divisions:

   1. Declarative= memories we can bring back to conscious recollection and declare/describe. Includes:

      • episodic memories= memories of past events/episodes you’ve seen or done

      • semantic memories=facts and basic knowledge you can recall/declare (eg. Paris is the capital city of France)

   2. Procedural= memories that aren’t for conscious recall

      • eg. learnt skills, often motor, like how to ride a bike.

      • body knows how to do it but we don’t have conscious recollection. eg. can’t recall everything our muscles have to do to ride a bike.

   • long term memory process= encoding (laying down memories for long term storage) + retrieval (retrieving memories for conscious recall)

Epilepsy: discharge types? common onset site? hippocampus/memory consequences?

Involves abnormal discharges in the brain’s electrical activity

• discharges can impact the whole brain or just specialised areas

  • whole brain= generalised seizures=causes convulsions

    • absense seizures= people just switch off

  • small circumscribed seizures= specific symptoms depending on the region (eg. hearing noises, visual hallucinations)

• common onset site=medial temporal lobe around hippocampus area as epilepsy ‘piggy backs’ on the mechanisms that allow memory to occur (and memory=temporal localised)

  • memory connections are strengthened by repeat recall/neuron firing

  • if temporal regions starts firing abnormally, these connections will be strengthened and the abnormal discharges will occur more and more frequently cyclically. This is essentially epilepsy

  hippocampus/memory consequences

  • repeated networks firings (aka repeated seizures) create physiological changes that leads to a loss of neurons in that region, causing the hippocampus to progressively shrivel (atrophy)

  • this creates memory deficits that worsen as the hippocampus is further atrophied

How are patients assessed for surgery for medial temporal lobe epilepsy?

Surgical option if patient’s can’t find appropriate medication to alleviate symptoms= remove the ‘focus’ area of the brain producing the seizures

1. deficits analysed by testing verbal memory (left temporal love) and visual memory (right temporal lobe)

2. if only one hemisphere has deficits, that lobe can be removed and the other will likely pick up it’s function (cognitive reserve).

   • If deficits are found in both, removing could make a HM situation. Not a surgery candidate.

3. If only one hemisphere is found to have deficits, the water test is used to determine if there is risk of cognitive dysfunction after surgery (other hemisphere not picking up it’s function)

   • one hemisphere is anaesthetized, cognitive function is retested and if there is a complete loss of memory function the patient isn’t a candidate for surgery (other lobe won’t pick up it’s function so removal would cause total amnesia)

Mediation of behaviour

• the frontal and parietal lobes together mediate attention and cognitive control of behaviour

• behaviour is a combination of bottom-up and top-down processes:

  1. bottom up= driven by external stimuli or unconscious states (drives/urges)

  2. top down= cognitive control or volitional choice, modulated by prior knowledge and experience

Parietal lobe roles. What is spatial neglect?

Attention: directing attention via eye movements to explore the visual world

• damage causes spatial neglect= patients are completely unaware of one side of visual space due to deficit in attention to that side

  • follows lesion in 1 hemisphere, often by stroke

  • deficit is on the side contralateral to the lesion

  • vision is completely normal: it is purely attentionally driven

Spatial Awareness:

• linking vision to action

• represents spatial location of objects

Attention characteristics

1. Selectivity

   • “select” and prioritise stimuli based on location/features (what is relevant)

   • 2 main ways we selectively attend/prioritise information for processing:

     1. Shift attention to location our eyes are on like spotlights

     2. selectively attend to specific relevant features like colour and shape (eg. looking for lost keys, keeping attention on objects of the same size and colour as them)

2. Capacity limited

   • resources for attention are limited: we can’t attend to everything so we use attention to filter and prioritise sensory information

3. Voluntary and involuntary

   • Voluntary/Conscious controlled attention= Top-down function

     • prioritising attention to a task or goal

     • voluntarily shifting visual attention (spotlight) (1)

     • choosing features for focus of attention (2)

   • Involuntary/automatic attention= bottom-up

     • attention “captured” involuntarily by highly salient stimuli that stand out (Eg. sudden movements, bright contrasting colours/shapes)

     • advertisers use this tactic to draw eyes

4. Mediated by networks of prefrontal and parietal cortex

   • top down, goal driven attention involves a more dorsal network

   • bottom up, stimulus-driven attention involves more ventral network