The popular phrase that the brain “lights up” when a person thinks or feels is a metaphor for a complex biological event. While the brain does not literally glow, thought involves an intense, measurable surge of activity in specific regions. This activity is a rapid, localized shift in energy demand that modern technology can detect and map. Scientists use the resulting, often brightly colored images, to understand which parts of the interconnected cells are working hardest during a mental task.
The Electrical and Chemical Signals
This measurable activity begins with a rapid sequence of electrical and chemical signals between specialized cells. Neurons communicate by sending electrical impulses down their length, triggering the release of chemical messengers at junctions known as synapses. This process—the firing of an electrical signal and the transmission of a chemical signal—constitutes brain activity.
When a neuron fires, it requires immediate energy to restore its chemical and electrical balance. This constant demand makes the brain a significant consumer, using approximately 20% of the body’s total resting energy and oxygen supply. The sudden increase in localized neural firing creates an equally sudden increase in metabolic demand for oxygen and glucose.
The brain responds to this heightened energy need by triggering a rapid, localized increase in blood flow, a phenomenon known as the hemodynamic response. Local blood vessels dilate to deliver an oversupply of oxygenated blood to the active region, exceeding the actual oxygen consumption rate. This localized change in the ratio of oxygenated to deoxygenated blood is the indirect signal that scientists measure to create the images of a “lit up” brain.
Tools for Observing Neural Activity
The primary method for visualizing this hemodynamic response is functional Magnetic Resonance Imaging (fMRI). This technology does not measure the electrical firing of neurons directly, but tracks the resulting change in blood oxygenation. The fMRI machine detects the Blood-Oxygen-Level Dependent (BOLD) signal, based on the distinct magnetic properties of oxygenated versus deoxygenated hemoglobin.
When a brain region becomes active, the rush of fresh, oxygen-rich blood contains more oxygenated hemoglobin, which is diamagnetic. The computer translates this localized shift in magnetic properties into the bright colors seen on a scan, indicating the areas with the highest blood flow. The BOLD signal, therefore, is an indirect marker reflecting the vascular response to neural activity, providing high spatial resolution to pinpoint activity within millimeters.
Positron Emission Tomography (PET) tracks metabolic activity by focusing on the brain’s energy consumption. A PET scan involves injecting a radioactive tracer, often attached to a glucose molecule, into the bloodstream. Because active cells burn more glucose, the tracer accumulates in highly engaged brain regions. The PET scanner detects the radiation emitted, creating an image that maps the areas of highest glucose metabolism. While PET provides a direct measure of energy use, fMRI is preferred for its non-invasive nature and ability to map activity changes rapidly.
Localizing Cognitive Functions
By using these imaging tools, scientists have been able to consistently map specific mental tasks to distinct areas of the cerebral cortex, supporting the concept of functional specialization. For instance, tasks involving language processing reliably activate two major regions in the left hemisphere: Broca’s area in the frontal lobe, associated with speech production, and Wernicke’s area in the temporal lobe, linked to language comprehension.
Complex thought processes like planning, decision-making, and impulse control consistently show increased activity in the prefrontal cortex, situated at the front of the brain. When recalling a past event, the hippocampus, deep within the temporal lobe, often shows heightened activity related to memory retrieval. Visual processing, from recognizing shapes to perceiving color, is concentrated in the visual cortex, located at the back of the brain in the occipital lobe.
However, the “lighting up” rarely occurs in a single isolated spot, as higher cognitive functions are managed by distributed networks of regions working together. For example, while the amygdala is strongly associated with emotional responses like fear, a full emotional experience involves coordinated activity with the prefrontal cortex for regulation and the hippocampus for memory context. Multiple areas collaborate in a synchronized pattern to execute even simple mental tasks.