Aims of the study

Olds and Milner set out to investigate specific areas in the rat's brain that might control feelings of pleasure and pain. They wanted to see if rats would repeatedly stimulate these areas themselves if given the chance, even over extended periods.

Method and participants

The study used a laboratory experiment with rats to test brain stimulation effects.

Participants

The participants were 15 male rats.

Procedure

1. Electrodes were surgically implanted into specific areas of each rat's brain. These implants were designed not to harm the rats' health or restrict their normal movements.
2. Each rat was placed in a Skinner box with a lever that the animal could press.
3. When the rat pressed the lever, it triggered a mild electrical current to the implanted electrode, stimulating the brain.
4. The experiment included two main phases:
   • Acquisition testing: This lasted 6–12 hours per rat. During this time, lever presses delivered electrical stimulation, acting as a potential reward.
   • Extinction testing: This followed for 1–2 hours. Here, lever presses no longer produced stimulation, to see if the behaviour persisted without the reward.
5. Researchers observed and recorded the rats' behaviours, including how much time they spent pressing the lever. They calculated a percentage score for each rat, showing the proportion of acquisition time devoted to lever-pressing.
6. Notably, during testing, rats often ignored food, water, and even other rats in the box, focusing solely on obtaining the stimulation.

Results of the study

• Rats spent over 75% of their acquisition time pressing the lever when the septal area of the brain was stimulated by the electrode.
• During the extinction phase, rats spent only about 22% of their time at the lever.
• This demonstrated that stimulation of the septal area acted as a primary reward.

Conclusions and implications

From the results, Olds and Milner concluded that specific pleasure centres exist in the brain, primarily in the septal area and other lower regions. When stimulated, these centres drive animals to repeat behaviours that produce the sensation, acting as a form of self-reward.

Key conclusions:

• This provided the first direct evidence of dedicated reward areas in the brain.
• The findings indicate that these pleasure centres lead to self-rewarding behaviour in rats.