Neuropsychology
Split-Brain Laboratory
Set up a lateralised trial, predict whether the answer can get out, and then find the conditions the classic result quietly depends on.
Simulated - a deliberately simplified model, original items, no published stimuli and no real participants
Learning objective
By the end you should be able to work out what a split-brain trial shows about routing, rather than about what each hemisphere is for.
About 20 minutes, in two experiments. Nothing you do here is saved or sent anywhere.
Read this before you start
Nothing on this page is flashed to one visual field. A web page cannot control where your eyes are, and lateralised presentation needs fixation control and timing that a browser cannot provide. You build a trial and the tool reports what a simplified model predicts. The model is an argument about routing, not a measurement.
The findings behind it come from a very small number of people who had surgery for severe epilepsy, whose brains differed from typical brains in other ways too. Results varied between them and between tasks.
- Experiment 1. Choose where the item appears and how you ask for a response. Predict the outcome, then run the trial. Then open Cut the corpus callosum and run the same trial again.
- Keep going until you have found a trial where cutting the callosum changes the outcome and one where it changes nothing.
- Experiment 2. Take the one trial that classically fails and switch off, one at a time, the conditions it depends on.
First, a judgement
In a fictional study, a word is presented in the left visual field of a person whose corpus callosum has been completely divided. The person says they saw nothing at all. Moments later, their left hand picks the matching object out of a bag they cannot see into.
The laboratory
- 1 Building a trial
- 2 What the result depends on
Need a hand?
Nothing here changes the controls for you, and none of it says what you are going to find.
Key terms
- Corpus callosum
- The band of fibres carrying information between the hemispheres. Cutting it is the manipulation here.
- Visual field
- Which side of the fixation point something appears on. It decides which hemisphere receives it first, whichever eye is open.
- Response channel
- How the answer is given — speech, left hand or right hand. Each one is reachable from different parts of the system.
- Lateralisation
- A function being handled more by one hemisphere than by the other. It is a tendency across people, not a switch inside one.
Experiment 1 - building a trial
Goal
Where the information can and cannot go
The trial that is supposed to fail
A word in the left visual field, a spoken report, a complete section of the corpus callosum.
Why, condition by condition
Trials you have run
Every combination at once
| Item appears in | Response channel | Callosum intact | Callosum sectioned |
|---|
With the callosum intact every cell succeeds, because both hemispheres receive the item within a few tens of milliseconds. After a complete section, a response succeeds only when the hemisphere that received the item also controls the channel you asked with. That is the whole model.
Challenge - two claims to weigh
What this demonstrates
What the patient can report depends on which hemisphere received the stimulus and which one controls the channel you ask through. The same item is available or unavailable depending on the route, not on which hemisphere is cleverer. Cut the callosum and the two halves stop sharing; leave it intact and the difference disappears. Lateralisation is a tendency in a connected brain, which is why “left-brained people” does not follow.
For teaching elsewhere: take this activity as one self-contained block of HTML, on the clipboard or as a file. Either way it is styled so that it will not disturb the page you put it into.