Phantom Limbs

Hi, I’m back after a break for Christmas and exams with a topic I find fascinating – phantom limbs. My interest on this topic stems from reading ‘Phantoms in the Brain’ by V.S. Ramachandran – I’f you’re at all interested in psychology and neuroscience then I really recommend it 🙂

So what is a phantom limb?

Phantom limbs can occur after a patient has a limb amputated. Their limb is gone, but they still feel as though they have one. This is strange enough, but these phantom limbs can itch, hurt, and even sense touch. This might seem bizarre, but there is a neurological explanation for how this occurs.

Remember the homunculus from my previous post about the motor cortex? If you need a reminder, here it is.

This is a map of how the human body is represented in our motor cortex. Now, say for example that a patient has a phantom arm and hand. As you can see in the picture above, the areas for ‘hand’ and ‘face’ are located next to each other. There are two explanations for what happens next:

1. It is thought that when the input to the hand area is gone (when the hand was amputated) the area processing information about the face encroaches into this area, which is now free.

2. The face area encroached into the hand area anyway, but then after the hand was amputated there were no signals to override those coming from the face.

The outcome of both of these explanations in the same: when a person’s face is touched in a certain place, they feel this touch as though it is coming from their phantom hand (remember the ‘hand’ area in the brain is still present).

But why do some people feels as though their phantom limb is paralysed or in pain? In most cases of people having a paralysed phantom, their real limb was paralysed for a period of time before the amputation. It is thought that the brain learns that although motor signals are sent to the limb, no movement occurs. This learned paralysis still occurs after amputation, causing the patients to believe their phantom limb is frozen. The same kind of thing occurs for patients who have pain in their phantom limbs: this learnt pain is transferred from before the limb was amputated.

Ramachandran has developed a pioneering treatment to help people suffering from learned paralysis and learned pain. It uses a mirror box (like the one shown below) in which the patient puts their phantom limb in the side of the box behind the mirror, and places their other normal hand so that its reflection fits in with the location of the phantom. When the patient moves their normal hand, it feels as though the phantom is moving too. This removes the learnt paralysis, and can often remove the phantom limb completely, as the brain is confused by the conflicting sensory input.

I hope you found this post interesting, check back for more soon!

The Motor Cortex

The motor cortex is the part of the brain which controls most of our movement (a few reflexes are regulated by the spinal cord). There is a motor area on both the left and right hemispheres, with the left hemisphere motor cortex controlling the right side of our body, and vice versa.

Here is a diagram of the brain showing where the motor cortex is located:

The motor cortex can be divided into several different sub-sections:

the primary motor cortex is the main area controlling the execution of movement.

the premotor cortex is thought to be responsible for the preparation of movement – cells in this area fire just before movement takes place.

the supplementary motor area (SMA) is thought to be involving in planning sequences of movement and coordinating movement on both sides of the body.

However, it is not just the motor cortex which is responsible for controlling our movement. The cerebellum, basal ganglia, posterior parietal cortex and the primary somatosensory cortex have also been shown to be involved.

It has also been shown that different areas of the motor cortex are involved in controlling movement in different parts of the body. Much of this work was carried out by Penfield during the 1930’s. In his most famous experiment, he applied electrical stimulation to different parts of the motor cortex in the brains of patients who were undergoing surgery for epilepsy. As the patients were conscious, they could tell him which part of their body they felt movement in when each part of the motor cortex was stimulated. From this research, he mapped the body onto the motor cortex – he called this a homunculus.

As you can see from the homunculus map, different parts of the body are represented over larger areas of the motor cortex, regardless of the size of the body part. The fact that the hand covers such a large area reflects how the hand is one of the most mobile areas of the body.

If a part of the body has to be amputated then this causes disruption to the motor cortex. Due to neural plasticity, patients can feel as though they have ‘phantom limbs’ – the feeling that the limb that has been amputated is still there.

Check back soon for my next post about phantom limbs 🙂