Introduction: The Great Divide
Have you ever wondered if your left hand truly knows what your right hand is doing? Usually, the two halves of our brain are in constant conversation. But what happens if we "cut the phone line" between them? In this chapter, we look at a famous study by Roger Sperry (1968), who investigated what happens to human consciousness when the two hemispheres of the brain are disconnected.
This study is a key part of Topic 4: The brain and neuropsychology. It helps us understand lateralisation—the idea that the left and right sides of the brain have different "specialist jobs."
Background: The Bridge of the Brain
In your brain, there is a thick bundle of nerve fibres called the corpus callosum. This acts like a bridge, allowing the left hemisphere and the right hemisphere to send messages to each other. For some people with very severe epilepsy, doctors perform surgery to cut this bridge to stop seizures from spreading. These people are known as "split-brain" patients.
Quick Review: Remember from our earlier lessons that the left hemisphere generally controls the right side of the body (and language), while the right hemisphere controls the left side of the body (and spatial tasks).
The Study: Sperry (1968)
Aims
Sperry wanted to investigate the effects of hemisphere deconnection. He specifically wanted to see how "split-brain" patients processed information compared to people with an intact corpus callosum. He aimed to show that each hemisphere has different functions and can actually operate independently.
Procedures
Sperry studied 11 "split-brain" patients who had already undergone surgery for epilepsy. He used a variety of tasks to test their visual and tactile (touch) abilities. Don't worry if these sound complex; think of them as specialized "hide and seek" games for the brain!
1. The Visual Task:
Participants sat in front of a screen. They were asked to fix their gaze on a central point. Sperry then flashed images or words onto either the left visual field (which goes to the right hemisphere) or the right visual field (which goes to the left hemisphere). The images were flashed for only \(0.1\) seconds—too fast for the eyes to move and share the info with both sides!
2. The Tactile (Touch) Task:
Participants were asked to reach under a screen (so they couldn't see their hands) and feel objects with either their left or right hand. They were then asked to name the object or find a matching one from a pile.
Findings (Results and Conclusions)
The results were fascinating because the patients behaved as if they had two separate minds!
Visual Results:
• When an image was shown to the right visual field (processed by the left hemisphere), the patient could easily describe what they saw. This is because the left hemisphere is the "language specialist."
• When an image was shown to the left visual field (processed by the right hemisphere), the patient insisted they saw nothing. However, if asked to use their left hand to point to or draw what they saw, they could do it perfectly! The right hemisphere saw it, but because it has no language, it couldn't "speak" the answer.
Tactile Results:
• If an object was placed in the right hand, the patient could name it.
• If an object was placed in the left hand, the patient could not name it, but they could pick it out of a bag of different objects using that same left hand.
Conclusions:
Sperry concluded that the left hemisphere is primary for speech and writing, while the right hemisphere is better at spatial tasks and recognizing objects through touch. In split-brain patients, each hemisphere appears to have its own "consciousness" and memories, and they cannot communicate with each other without the corpus callosum.
Strengths and Weaknesses of the Study
To do well in your exam, you must be able to evaluate the study using terms like reliability and validity.
Strengths
1. High Control (Reliability): Sperry used very standardized procedures. For example, flashing the images for exactly \(0.1\) seconds ensured that participants didn't have time to move their eyes. This makes the study easy to replicate by other scientists.
2. Scientific Evidence: The study provided clear, objective evidence for the lateralisation of function. It proved that the left and right hemispheres really do specialize in different tasks.
Weaknesses
1. Low Generalisability: Sperry only used 11 participants, all of whom had severe epilepsy. Their brains might have worked differently from "normal" brains even before the surgery. Therefore, we can't be sure if these results apply to everyone.
2. Low Ecological Validity: The tasks were very artificial. In real life, we use both eyes and move our heads, so information usually reaches both hemispheres anyway. The "split-brain" effect rarely causes problems in everyday life because patients adapt.
Memory Aid: The "Left is Language" Trick
To remember which side does what, just remember the L's:
Left Hemisphere = Language, Logic, and the Letters in a word.
The Right Hemisphere = Recognising faces and spatial Relationships!
Key Takeaways
• Sperry (1968) studied patients with a severed corpus callosum.
• The Left Hemisphere can speak and identify things in the right visual field.
• The Right Hemisphere can draw or point to things in the left visual field but cannot speak about them.
• This study is a classic example of how neuropsychology explains the link between brain structure and our behaviour.