Maps in the Brain

Understanding Brain Mapping

The brain, despite being locked in silence and darkness, constructs detailed maps of the body and its surroundings. This lecture explores how these maps form, change, and contribute to human perception and action.

Wilder Penfield’s Discovery

In 1951, Canadian neurosurgeon Wilder Penfield conducted experiments on patients undergoing neurosurgery by stimulating different regions of the brain with electrodes. His findings revealed a striking discovery: the brain contains a mapped representation of the body.

The somatosensory cortex contains a sensory map, detecting touch and other sensations from the body.
The motor cortex contains a motor map, responsible for controlling movements.
The size of body parts on these maps is not proportional to their actual size but instead reflects their importance in sensation and movement (e.g., lips and fingertips have larger representations than the knees).

Penfield’s discovery led to the term homunculus (Latin for “little person”), which visually represents the body’s distorted proportions as mapped in the brain.

The Dynamic Nature of Brain Maps

Initially, scientists believed that brain maps were genetically pre-programmed. However, later research demonstrated that these maps are highly adaptable. A pivotal study with the Silver Spring monkeys showed that when a nerve to a limb was severed, the brain map readjusted, erasing the absent limb’s representation.

Human Examples

Phantom Limb Phenomenon: Amputees often feel sensations, including pain, in their missing limb. This happens because while the primary somatosensory cortex quickly adapts, downstream areas of the brain continue to interpret missing limb signals.
Reorganization in Response to Injury: When a limb is lost, the neural space it occupied in the brain is repurposed by neighboring regions. For example, after Admiral Lord Nelson lost his arm, his brain’s representation of the arm was taken over by nearby sensory inputs.

How Maps Change

The brain continuously modifies its maps based on sensory input and experience. If sensory signals decrease or disappear, the brain reallocates resources:

Pressure or Anesthesia: If a limb is numbed or tightly constrained, its representation in the brain shrinks.
Tying Fingers Together: The brain starts treating the fingers as one unit because they no longer provide independent sensory feedback.
Blindness or Deafness: When one sense is lost, its cortical real estate is repurposed for other senses. In blind individuals, the visual cortex is utilized for touch, sound, and even mathematical reasoning.

Neural Plasticity and Synaptic Strength

Neural networks change through synaptic strengthening and weakening, which follow a principle known as Hebbian Learning:

“Neurons that fire together, wire together”: If two neurons are consistently active at the same time, their connection strengthens.
Long-Term Potentiation (LTP): Repeated stimulation of a neural connection enhances its signal transmission.
Long-Term Depression (LTD): If two neurons rarely fire together, their connection weakens.

This mechanism enables the brain’s flexibility in remapping sensory inputs and motor outputs.

Implications for Sensory and Motor Adaptation

Blind individuals develop enhanced tactile and auditory skills due to increased neural representation in their remaining senses. Braille readers, for instance, use their visual cortex to process touch.
Echolocation in the Blind: Some blind individuals, like Ben Underwood, use mouth clicks and sound echoes to navigate their surroundings. Studies show that their visual cortex processes sound, highlighting the brain’s adaptability.
Colorblind Individuals: While they lack full color perception, they often excel in detecting subtle shades of gray, giving them an advantage in distinguishing camouflage patterns.

The Role of Sleep and Dreaming in Brain Mapping

Given the brain’s rapid plasticity, sensory deprivation (e.g., blindness) can lead to cortical reorganization. This raises the question: why does the visual system not get taken over by other senses during sleep?

The Defensive Activation Theory suggests that dreams exist to prevent sensory takeover.
During REM sleep, activity is injected into the visual cortex, keeping it engaged and preventing repurposing by other senses.
This theory explains phenomena such as tinnitus (ringing in the ears after hearing loss) and phantom limb pain, which may stem from the brain generating artificial signals to compensate for lost input.

Predictions and Cross-Species Comparisons

Animals with higher plasticity have more REM sleep. Studies of primates show that species with longer development periods require more REM sleep.
Elephants, which sleep only 1-2 hours per night and have good nocturnal vision, exhibit minimal REM sleep.
Blind individuals still dream, but their dreams involve touch and sound instead of vision.

Summary of Key Points

The brain maintains dynamic maps of the body and senses.
These maps adapt based on input; lost sensory or motor function leads to cortical reorganization.
Neurons that fire together strengthen their connections, shaping perception and skill acquisition.
More brain real estate dedicated to a task results in enhanced ability.
Dreams may function to preserve neural real estate for the visual system.
Sensory deprivation leads to cross-modal plasticity, where unused brain areas are repurposed.

This dynamic nature of brain mapping underscores the remarkable adaptability of the human brain, enabling individuals to adjust to injuries, sensory loss, and environmental changes.