Brain Laterality, Split-Brain, Memory and H.M. Lecture Notes

Module 4: Brain Disorders

Introduction

• The focus is on how the brain normally works, learning from what happens when parts don't function correctly in specific disorders.
• This lecture covers brain laterality, the functions of the left and right hemispheres, split-brain patients, memory, and the role of the hippocampus through the case study of H.M.

Debunking Myths About Brain Hemispheres

• The idea of being a "left-brain" (analytical, logical) or "right-brain" (creative) person has no basis in neuroscience.
• Higher-order traits and abilities rely on both hemispheres working together.
• While there are differences in the functions of the left and right hemispheres, they typically work together. Surprisingly, even when they don't communicate (as in split-brain patients), people can function reasonably well.

Brain Laterality: Lateralization of Function

• Brain laterality refers to the functions that are more dominant in one hemisphere than the other.
• Few functions are exclusively in one hemisphere.

Language

• Expressive language (speech production): Strongly lateralized to Broca's area in the left hemisphere for most people.
• Nonverbal Communication: Tone of voice and prosody (rhythm and melody of speech) involve the right hemisphere more.

Face Perception

• Specific areas in the visual cortex are specialized for face perception, tending to be more dominant in the right hemisphere.

Perceptual Grouping

• The ability to see overall patterns versus individual parts tends to be more in the right hemisphere.

Contralateral Functions

• Functions are crossed in the brain, linked to the opposite side of the body or space (movement, sensation, and vision).
• Right hemisphere controls the left side of the body, and the left hemisphere controls the right side.
• Sensation from the right side of the body goes to the left hemisphere, and sensation from the left side goes to the right hemisphere.

Broca's Area and Aphasia

• Broca's area, located in the inferior frontal region of the left hemisphere, is essential for language production (speech, writing, sign language).
• Damage to this area results in Broca's aphasia, a deficit in language production.

Language Comprehension

• Both hemispheres can support language comprehension, but speech production relies on the left hemisphere for most people.

Hemisphere Dominance

• Right-handedness indicates that the left hemisphere is dominant for movement control.
• Most people have language areas in the left hemisphere and are right-handed, but this isn't always linked.
• Some people have Broca's area in their right hemisphere, resulting in reversed asymmetry.
• About 90% of people are right-handed.
• Of right-handed people, 95% have Broca's area in the left hemisphere.
• Left-handed people are more likely to have language areas in the right hemisphere.

Contralateral and Ipsilateral

• Contralateral: Connected to the opposite side.
• Ipsilateral: Connected to the same side.
• Primary motor and sensory areas of the cortex have contralateral connections to the opposite side of the body.

Vision

• The left side of vision maps to the right hemisphere, and the right side of vision maps to the left hemisphere.
• Vision from both eyes is divided so that each hemisphere receives input from both eyes (binocular vision).
• The left visual field maps to the right hemisphere, and the right visual field maps to the left hemisphere.
• Binocular input allows depth perception in each hemisphere.

Corpus Callosum

• The corpus callosum is the main nerve pathway for communication between the left and right hemispheres.
• It is a bundle of axons from neurons traveling between hemispheres.

Interhemispheric Communication

• The route of information in different tasks dictates the importance of interhemispheric communication.
• Stimuli presented very briefly to one side of the screen go only to one hemisphere if the person doesn't move their eyes.
• For speech, information needs to get to the left hemisphere's Broca's area.
• Example: A picture flashed on the left side of the screen (going to the right hemisphere) requires crossing the corpus callosum to reach the left hemisphere for verbal reporting.
• For movement, if the task is to point with the left hand to a picture on the left side of the screen, the information stays within the right hemisphere.
• If the task is to point with the right hand, the information needs to cross the corpus callosum to reach the left hemisphere.

Split-Brain Patients

• Split-brain patients have had their corpus callosum cut, preventing communication between the left and right hemispheres. This is typically done as a last resort treatment for severe epilepsy.
• Roger Sperry won the Nobel Prize for his research on brain laterality using split-brain patients.
• Michael Gazzaniga, Sperry's student, is considered the founder of modern cognitive neuroscience.

Classic Experiment

• Pictures of objects are flashed briefly to the left or right of a screen using a tokistoscope.
• Patients reach their hand through a gap under the screen to feel objects.

Scenario 1

• A picture of an object (e.g., a hammer) is flashed on the left side of the screen (goes to the right hemisphere).
• Verbal Report: The person says, "I saw nothing" because the left hemisphere (responsible for language) saw nothing on the right side of the screen. The left hemisphere is giving a literal answer.
• Action: When asked to use their left hand to pick up the object, they can do so because the right hemisphere (which saw the hammer) controls the left hand.

Scenario 2

• An object is presented on the right side of the screen and the goes to their left hemisphere, they are able to name the object.

Scenario 3

• A word (e.g., "face") is flashed on the left side of the screen.
• Verbal Report: The person says, "Nothing" because the left hemisphere saw nothing but then is able to use their right hand (controlled by their left hemisphere) to draw the object.

Implications of Split-Brain Studies

• The right hemisphere is capable of language comprehension.
• The person's left hemisphere (with language) starts to get visual information by watching what the left hand is doing and starts making guesses.

Perceptual Grouping Task

• The left hand (controlled by the right hemisphere) can perform the task effectively.
• The right hand (controlled by the left hemisphere) is incapable of doing it.
• When both hands are allowed to work together, they fight over each other.

Consciousness and Split-Brain Patients

• Raises questions about the nature of conscious awareness.
• The left hemisphere can verbalize what it has seen.
• The right hemisphere can only show and direct the left hand.
• It is not knowable of there is separate consciousnesses in the left and right hemispheres with the left being able to communicate and the right hemisphere unable to.

Memory and the Hippocampus

• The hippocampus is part of the limbic system and plays a critical role in memory.

Patient H.M. (Henry Molaison)

• Had both left and right hippocampi removed to control severe epileptic seizures.
• Resulted in severe memory loss. He was not able to form new memories from after the point of his surgery onward. This meant interacting with him was difficult ad he was unable to rememeber prior interactions with people.
• He could recall events before his surgery as well as mentally rehearse things
• He was not able to rememeber the act of practicing new skills, but could still learn them (procedural memory).

Short Term Memory

• Lasts for several seconds.
• Synonymous with working memory.
• The hippocampus is not needed for short term memory.

Long Term Memory

• A long term store for collecting at a later date information.
• Information is encoded in the brain and recalled in the future.

Declarative vs. Procedural Memory

• Declarative Memory: Things we can consciously recall and declare. This is divided into episodic and semantic memories. Patients with damage to the hippocampus like H.M. have issues with this type of memory.
• Procedural Memory: It is is also long term, but it's all the things not for conscious recall. They are typically skills we've learned that your body does you really don't have complete conscious awareness of.

Episodic Memory

• Memory of particular events and episodes in our lives.
• H.M.'s deficit was in this area.

Semantic Memory

• Memory of facts and basic knowledge.

Encoding and Retrieval

• Encoding: Laying down a new memory trace in the brain. It is the initial learning of information.
• Retrieval: Recalling the memory back to conscious awareness. It is the act of recalling a memory.
• The fact that H.M. could recall memories from before his surgery indicates that the hippocampus is not needed for memory retrieval or storage, but is necessary for encoding new memories.

Epilepsy and the Hippocampus

• Removing both left and right hippocampi leads to severe memory deficits.
• Removing one hippocampus has different effects depending on whether it's the left or right.