Science: Mapping the sixth sense

Research on blind people's astonishing ability to orient themselves suggests that some parts of the brain, normally dedicated to sight, may be reallocated to make extra use of auditory cues

Watch a blind person thread their way along a crowded street, faultlessly skirting lampposts, litter bins and people, and you may be tempted to think that they are informed by some sixth sense. The scientific explanation has long been that their skill is due to enhanced hearing, but a recent study suggests a more subtle mechanism than simple amplification of sound.

Brigitte Roeder, a neuropsychologist at Philipps University in Marburg, Germany, joined colleagues at the Universities of California and Oregon to carry out one of the first experiments designed to find out what makes blind people so good at locating things by ear. They subjected two groups of people - one blind and the other sighted but blindfolded - to hearing tests while monitoring their brain functions via electrodes placed on the skull.

The tests showed that the blind subjects were better than the blindfolded volunteers at detecting subtle changes in tone from sound sources on either side. It is these tonal changes that indicate where sounds are coming from. However, the results revealed that the enhanced acuity shown was not just due to blind people consciously hearing the sounds more clearly. Instead they were "mapping" the locality of a sound before they heard it.

Reacting to a sound before it is heard sounds impossible, but previous studies have shown that people can respond to a sensory stimulus without being conscious of it. Joseph LeDoux, a brain researcher at New York University, trained rats to be fearful of a particular buzzing tone, then removed their auditory cortex - the part of the brain that turns signals from the ears into sounds consciously heard. The operation rendered the animals stone deaf in the normal sense, yet they still reacted with fear when the buzzer was sounded.

The rats could do this because nerve fibres from the ear do not go exclusively to the auditory cortex. Some branch off and connect with other parts of the brain, including the areas that generate emotions. These areas do not produce "hearing", but can identify the sounds and respond in their own way. In the rats' case, this meant producing fear.

The eyes, similarly, send signals to areas of the brain other than those concerned with consciously seeing. This can produce weird effects; there is a condition called "blindsight", for example, in which people who are blind can describe or react to a visual cue. Blindsight is seen in people whose sight impairment is due to damage to the primary visual cortex - a bit of the brain that is essential for conscious sight. If a pattern or a moving target is presented to them and they are prompted to "guess" its shape or direction, some are able to describe or follow it quite accurately, even though they claim not to see it.

Their ability to react without consciousness of the cue is thought to come about because a few neural strands from the eyes go directly to parts of the brain concerned with shape or movement, bypassing the area responsible for conscious vision. So they end up "knowing" the shape or direction of the thing in front of them, without knowing how.

Something similar seems to give blind people that curious "sixth sense" about what lies on either side of them. The electrode recordings in Roeder's study showed that blind subjects processed peripheral sounds in a different part of the brain from sighted subjects, and, furthermore, did this before the sounds had been consciously heard.

The brain area activated in the blind subjects during the test (though not in the sighted subjects) is in the parietal lobes, which lie just behind and to either side of the crown. These lobes are largely dedicated to "mapping" the outside world and locating what is in it. The process is independent of sight or hearing - a person with parietal lobe damage may be able to see something quite clearly, for example, but be unable to say whether it is up or down, in front of them or to the side.

A crucial part of the locating process involves shifting attention towards a new or unusual stimulus. In order fully to experience a sensory stimulus this attention shift has to happen before the stimulus is picked up and processed by the appropriate sensory cortex. Indeed, if our attention is not already in the right place to "catch" a sensation, we may miss it altogether. When our attention is accurately oriented, however, we can detect fine changes in it.

The "cocktail party" syndrome is a good demonstration of this. If you are in a room with lots of people talking at the same time, you will usually be able to make out only the speech of the person you are directly attending to. But if someone says your name - even on the other side of the room - you will be able to zoom in on them and hear every word.

The reason you can catch your name, and what follows, is that all incoming sensory information streams through the unconscious, emotional areas of the brain, which selects out new or important things - such as your name - and alerts the conscious processing areas to extract as much information as possible. Part of the selection process involves shifting attention to the source of the stimulus. All this happens 100-300 thousandths of a second in advance of the selected information being fully processed by the sensory cortex. So by the time you consciously hear your name you are already oriented to the conversation in which it cropped up - and every word may be clear.

Blind people seem to do this with peripheral sounds, and Professor Roeder thinks they are able to do so because their brains have hijacked parts of the parietal lobes that are usually sensitive only to vision .

When we are born, the parietal areas concerned with orienting and attention- shifting are wired up to respond to signals from many sense organs. Some neurones are connected to the ears, some to the eyes, others to the sensory pathways that carry information from the remote parts of the body. As we develop, though, the various senses appear to compete for a greater share of these neurones, and the most useful or powerful sense tends to win out. Hence, in normally sighted people, many of the parietal neurones that were originally wired up to react to auditory information may be annexed for vision. This gives sighted people an extremely acute "eye" for the location of things - most people can reach out and grasp an object with complete precision - but it greatly reduces their ability to tune in to the location of something using sound rather sight.

Roeder's study suggests that blind people not only retain the original parietal connections to the ear, but also reallocate the visual ones to give them even greater auditory acuity. So when they walk down a street they may react to the tiny tonal changes around them that signify where things are, even if they do not consciously hear them.

"Our study shows that the brain is very plastic," says Professor Roeder, " and that it has plenty of capacity for remoulding itself to compensate for sensory loss such as visual impairment. It also underlines the need to train blind people from an early age. The brain is much easier to mould when it is young."