Damage Responses and Brain Hemispheres
Neuroplasticity and Brain Damage Responses
Definition of Neuroplasticity
Neuroplasticity is the brain's ability to modify itself and change in response to experience, especially during childhood, by reorganizing after damage or by building new pathways based on experience.
Enduring Effects of Brain Damage
Two hard facts explain why brain damage effects often endure:
Lack of Regeneration: Severed brain and spinal cord neurons, unlike cut skin, usually do not regenerate. A severed spinal cord usually results in permanent paralysis.
Preassigned Brain Functions: Certain brain functions appear preassigned to specific regions. For example, a newborn who experienced damage to temporal lobe facial recognition areas was never able to recognize faces.
Reorganization After Damage
Undamaged regions can develop extra connections to reorganize the brain in response to damage, particularly in young children.
Plasticity provides significant advantages for individuals who are blind or deaf by freeing up unused brain areas for other sensory processing, such as sound and smell.
Mechanisms of Brain Reassignment and Neurogenesis
Sensory Reassignment Examples
Braille Reading: When a blind person uses one finger to read Braille, the brain area dedicated to that finger expands as touch invades the visual cortex typically used for sight.
Enhanced Auditory and Visual Skills: Blind individuals often display a superior ability to locate sounds. Deaf individuals who learn sign language before another language may exhibit enhanced peripheral and motion detection vision.
Temporal Lobe Adaptation in Deafness: In deaf individuals whose native language is signed, the temporal lobe area typically dedicated to hearing waits and wanes for auditory stimulation before reassigning itself to process visual sign signals.
Language Compensation: If a slow-growing left hemisphere tumor disrupts language (which resides mostly in the left hemisphere), the right hemisphere may compensate.
Amputation and Cortical Remapping: When a finger is amputated, the somatosensory cortex receiving its input begins receiving input from adjacent fingers, making those fingers more sensitive.
In one documented case, a patient whose lower leg was amputated described a phantom experience during sexual intercourse: "I actually experienced my orgasm and my phantom foot, and there and there it's much bigger than it used to be because it's no longer just confined into my genitals."
Neurogenesis
Definition: Neurogenesis is the formation of new neurons.
Process: The brain attempts self-repair through neurogenesis by producing new neurons that may mature more and grow larger than the neurons present at birth.
Stem Cells: Stem cells are special cells capable of developing into various cell types, including brain cells.
Evidence: Baby neurons have been discovered deep in the brains of adult mice, birds, monkeys, and humans, where they can form connections with neighboring neurons.
Hemispheric Lateralization and Split-Brain Research
Lateralization of Function
Left and right hemispheres look alike but serve distinct functions, a phenomenon known as lateralization.
Effects of hemispheric damage:
Left Hemisphere: Damage from accidents, strokes, or tumors impairs reading, writing, speaking, arithmetic reasoning, and understanding.
Right Hemisphere: Similar damage produces less visibly dramatic effects.
Surgical Splitting of the Brain
In the early 1960s, neurosurgeons speculated that major epileptic seizures resulted from abnormal brain activity bouncing back and forth between the cerebral hemispheres in an amplified biological feedback loop.
Corpus Callosum: The wide band of axon fibers connecting the two hemispheres that carries messages between them.
Animal Precedents: Psychologists Roger Sperry, Ronald Myers, and Michael Gazzengia had previously divided the brains of cats and monkeys by severing the corpus callosum without serious ill effects.
Surgical Outcome: Severing the corpus callosum caused seizures to all but disappear. Patients remained surprisingly healthy, with intellect and personality hardly affected. Upon waking from surgery, one patient joked that he had a "splitting headache."
Definition of Split Brain: A condition resulting from the surgical separation of the brain's two hemispheres by severing the connecting fibers, mainly those in the corpus callosum.
Visual Processing Mechanics
Each eye receives visual information from the entire visual field.
Information from the left half of the visual field goes to the right hemisphere.
Information from the right half of the visual field goes to the left hemisphere (which usually controls speech).
In intact brains, information received by one hemisphere is instantly transmitted across the corpus callosum to the other hemisphere. A severed corpus callosum prevents this information sharing.
Cognitive and Behavioral Effects in Split-Brain Patients
Testing Hemispheric Isolation
Visual Field Experiment: Michael Gazzengia flashed the word "HE.ART" on a screen while a split-brain patient stared at a center dot.
"HE" appeared in the left visual field (transmitted to the right hemisphere).
"ART" appeared in the right visual field (transmitted to the left hemisphere).
Verbal Response: Patients reported seeing "ART" because the left hemisphere controls speech.
Motor Response: When asked to point with their left hand (controlled by the nonverbal right hemisphere) to what they saw, patients pointed to "HE".
Tactile Identification Experiment: When a picture of a spoon was flashed to the right hemisphere, patients could not state verbally what they viewed.
When asked to identify the object by feeling hidden items with their hand, they readily selected the spoon.
When experimenters confirmed the selection as correct, the talking left hemisphere replied in bewilderment: "What? Correct? How could I possibly pick out the correct object when I don't know what I saw?"
Unruly Independence and Dual Minds
The nonverbal right hand and left hand can act independently and at cross-purposes (e.g., the left hand unbuttoning a shirt while the right hand buttons it, or the left hand putting grocery items back on a shelf right after the right hand places them in the cart).
Split-brain patients can comprehend and follow instructions to simultaneously draw or copy different figures with their left and right hands.
Roger Sperry concluded that split-brain surgery leaves people with two separate minds, resembling a river that has branched into separate streams, each unaware of its influence on the other (allowing possibilities such as playing a solitary game of rock, paper, scissors left hand versus right hand).
The Left Hemisphere as Interpreter
Rationalization of Unconscious Actions
When the two hemispheres receive conflicting cues, the left hemisphere performs mental gymnastics to rationalize behaviors it did not initiate or understand.
Walking Command Example: If an instruction such as "walk" is sent exclusively to the right hemisphere, the patient will stand up and begin walking.
When asked why they are walking, the left hemisphere (unaware of the original command) does not state "I don't know."
Instead, the left hemisphere instantly improvises a narrative: "I'm going to the house to get a coke."
Michael Gazzaniga concluded that the conscious left hemisphere acts as an interpreter constructing instant explanations, demonstrating that the brain often operates on autopilot, acting first and explaining itself later.
Hemispheric Specialization in the Intact Brain
Functional Differences in undivided Brains
Over of individuals have undivided brains, yet functional specialization remains distinct:
Perceptual Tasks: Tasks such as recognizing facial emotions cause increased activity, brain waves, blood flow, and glucose consumption in the right hemisphere.
Verbal and Mathematical Tasks: Speaking or calculating increases activity in the left hemisphere.
Sedative Anesthesia Pre-Surgery Demonstration
To locate language centers before brain surgery, a surgeon injects a sedative into the neck artery feeding blood to a specific hemisphere.
Left Hemisphere Anesthesia: Injected into the left neck artery, the patient's right arm instantly falls limp, and the patient becomes speechless within seconds until the drug wears off.
Right Hemisphere Anesthesia: Injected into the right neck artery, the patient's left arm falls limp, but the patient retains the ability to speak.
Universal Language Processing
Language processing operates similarly whether spoken or signed. Deaf individuals process sign language in the left hemisphere just as hearing individuals process spoken language.
A left hemisphere stroke disrupts a deaf person's signing capacity in the same way it impairs a hearing person's speech.
Right Hemisphere Contributions
While the left hemisphere performs quick, literal interpretations of language, the right hemisphere excels at specific subtle tasks:
Making Inferences: Helps solve insight problems quickly (e.g., recognizing that the single word connecting "boot", "summer", and "ground" is "camp"). One patient who suffered a right hemisphere stroke noted: "I understand words, but I'm missing the subtitles."
Speech Moderation: Helps modulate speech to clarify meaning (e.g., distinguishing between vocalizing "let's eat grandpa" versus "let's eat, grandpa").
Orchestrating Self-Awareness: Damage to the right hemisphere can impair self-awareness, leading partially paralyzed patients to stubbornly deny their impairment and claim they can move paralyzed limbs.