You are viewing a free preview of this lesson.
Subscribe to unlock all 10 lessons in this course and every other course on LearningBro.
The human brain has two cerebral hemispheres — left and right — connected by the corpus callosum. Although the hemispheres appear structurally similar, they differ in function. This functional asymmetry is known as lateralisation. The most dramatic evidence for lateralisation comes from research on split-brain patients — individuals whose corpus callosum has been surgically severed.
Key Definition: Lateralisation refers to the idea that the two hemispheres of the brain are functionally different, with certain cognitive processes and behaviours localised to one hemisphere more than the other.
Research over the past 150 years has revealed that the two hemispheres tend to specialise in different functions:
| Function | Left Hemisphere | Right Hemisphere |
|---|---|---|
| Language | Dominant for speech production (Broca's area) and comprehension (Wernicke's area) | Limited language; some understanding of simple words |
| Logic and analysis | Sequential processing, mathematical calculation | Holistic processing |
| Fine motor control | Controls right side of the body | Controls left side of the body |
| Spatial awareness | Limited | Dominant — navigation, face recognition, spatial reasoning |
| Emotional processing | Processes positive emotions (approach) | Processes negative emotions (withdrawal); prosody (emotional tone of speech) |
| Music | Lyrics, rhythm analysis | Melody, pitch, timbre |
Key Definition: The corpus callosum is a thick bundle of approximately 200 million nerve fibres connecting the left and right cerebral hemispheres, allowing communication and coordination between them.
A critical feature of brain organisation is that each hemisphere primarily receives sensory information from, and controls movement on, the opposite (contralateral) side of the body. The left hemisphere controls the right hand and receives visual information from the right visual field (and vice versa). This contralateral organisation is essential for understanding split-brain research.
Exam Tip: Before writing about Sperry's split-brain research, always explain contralateral processing. Without this concept, the findings make no sense. The left visual field projects to the right hemisphere, and the right visual field projects to the left hemisphere.
In the 1960s, Roger Sperry and his student Michael Gazzaniga studied patients who had undergone a commissurotomy — surgical severing of the corpus callosum — as a treatment for severe, intractable epilepsy. By cutting the corpus callosum, surgeons prevented epileptic seizures from spreading between hemispheres. However, this also meant the two hemispheres could no longer communicate directly.
Sperry used a tachistoscope — a device that projects visual stimuli to either the left or right visual field for a very brief duration (less than 0.1 seconds), too fast for the eyes to move and redirect the image to the other hemisphere.
The key experimental tasks included:
| Stimulus Presented To | Verbal Response | Manual Response (left hand) |
|---|---|---|
| Right visual field (left hemisphere) | Patient could name the object | N/A — right hand controlled by same hemisphere |
| Left visual field (right hemisphere) | Patient reported seeing "nothing" or could not name it | Patient could select the correct object by touch with the left hand |
Finding 1: Language lateralisation — When an image was presented to the right visual field (left hemisphere), patients could verbally describe what they saw. When the same image was presented to the left visual field (right hemisphere), patients could not name it verbally because the right hemisphere lacks the language centres needed for speech production. However, patients could point to or pick up the correct object with their left hand (controlled by the right hemisphere).
Finding 2: Two separate conscious entities — In one striking demonstration, the word "KEY" was flashed to the left visual field and "RING" to the right visual field. When asked what they saw, the patient said "RING" (left hemisphere, language). But when asked to select the object with their left hand (right hemisphere), they picked up a key. This suggested the two hemispheres were operating as two separate, independent conscious systems.
Finding 3: Right hemisphere capabilities — Although the right hemisphere could not produce speech, it demonstrated understanding of simple language, spatial processing, and emotional recognition. For example, when a provocative image was flashed to the right hemisphere, patients would laugh or show embarrassment but could not explain verbally why they were laughing.
Key Definition: A tachistoscope is a device used to present visual stimuli for very brief durations, ensuring that information is directed to only one hemisphere.
Subscribe to continue reading
Get full access to this lesson and all 10 lessons in this course.