The charge is carried along the cells axon - a thin, snaking tentacle - to another neuron.

Most nerve cells fire because they are stimulated by their neighbours but certain sensory neurons - those in the retina or the skin for example - can be triggered by the effect of external stimuli such as light or heat. These cells pass on the message through pathways (often referred to as "wires" ) formed by the axons.

The pathways form an immensely complex network, which is constantly changing in response to experiences. The network develops to a genetically-determined blueprint which is similar in every normal brain. However, the precise pattern is different for everyone, just like the lines on one's palm. Indeed, it is the differences in brain anatomy that make us individuals.

Axons do not quite touch the cells they are signaling to, and the minute gap between them is bridged by chemicals called neurotransmitters. These are released from the tip of the axon, from which they drift over the gap to the receiving part of the second cell - the dendrite. If enough of the rght kind of neurotransmitter makes contact with the second cell, that will fire too - and so on.

There are hundreds of neurotransmitters - better-known ones include serotonin, dopamine and noradrenalin. Drugs for conditions such as depression generally work by altering the levels of one or more of these neurotransmitters, making signalling between cells more or less easy. Like tDCS, the idea of them is to change the brain's pattern of electrical activity , but unlike tDCS it is a very indirect way to do it. The drug has to get into the bloodstream, through the liver, into the brain and then find the right neurons to work on. Not surprisingly, such drugs have uncertain and sometimes unpleasant effects. tDCS , by contrast, works directly on the relevant neurons and does not therefore affect any other body systems. One crucial aspect of tDCS is that it is targeted correctly. Different parts of the brain, and different circuits within do quite different things. The anode and cathode must therefore be carefully positioned, depending on the aspect of brain function being addressed. For mood enhancement and memory, for instance, the electrodes are usually placed over an area of the forebrain. Pain requires that the electrodes are placed farther back on the cortex, and tinnitus has been relieved with another arrangement.

Although the current from tDCS reaches only surface neurons, the effect "knocks on" to deeper regions of the brain due to the long-distance signaling described above. Hence a local stimulation can encourage widespread activation involving many different groups of neurons. This sort of linked activity creates "whole" experiences , such as a pleasurable response to a smiling face. And the more intense the activity, the more likely it is to be conscious and clear rather than some vague, quickly forgotten impression.

When neurons fire together they start to bond, and the more that a particular group of neurons fire together - the "smiling face" group, say -the more likely they are to fire together in the future. Repeated synchronous firing causes the neurons involved to form physical connections - the "wires" that carry electrical information around the brain. As the T-shirt slogan puts it: "neurons that fire together wire together". Once a group of neurons have physically "wired" together they are even more likely to fire in unison. They have become a "habit" of mind. This is the basis of all learning, commonly referred to as neural plasticity. Hence tDCS triggers a natural process which leads to long-term change.17 To take the example above, by kick-starting the neuronal response to smiling faces it encourages the brain to rewire itself in such a way that it is quicker to register positive emotional signals. This in turn crates - literally - a new "you'.