THE sensory overload of New Year's Eve in Durban - beach parties, tugboat horns, fireworks in the heavy coastal air, laughter, boisterous drinking and wired-up children somersaulting into the pool ...

But there's one kid in the party who sits under a table, fingers in her ears. She is curled up against the noise. A friendly adult reaches for her but the girl recoils at the light touch.

Elsewhere, a four-year-old boy is driving his mother crazy.

"He's so difficult and fussy. Anything too gloopy is off the menu. No mince, no oats, no custard, no sauces, no yoghurt. Nothing unusual and nothing with big chunks. Can he live on only bread and butter, plain pasta and chicken drumsticks?"

There are certain clothes he just won't wear. "They're scratchy," is all he will say.

On formal occasions he strips off at the first available opportunity, sometimes in the middle of someone else's garden. He hates wearing shoes, but becomes distressed if he has to walk barefoot over gravel or wet grass. He insists on wearing the same old soft T-shirt day after day.

These are two examples of sensory processing disorder (SPD).

The theory goes that the brain struggles to integrate the impact of normal multiple sensory inputs.

These sensations may be generated by external sources (sights, sounds, smells, touch and taste) or within the body itself (balance, awareness of the body's position in space).

This isn't a problem with the senses as such, but rather with the way in which sensations are processed by the brain.

There is huge individual variation and no two children or adults with SPD will manifest in exactly the same way. Broadly speaking, however, people either overreact or underreact (or a mixture of both) to sensory experiences.

The two examples I gave earlier are typical of overreactiveness or over-sensitivity, but underreactiveness is also common.

Think of the child who always bumps into things, rocks or hums incessantly, screeches in response to the most innocuous stimuli or throws tantrums loud enough to have neighbours dialling social services. This child is seeking out more sensation, not less.

SPD is a very common problem for children with developmental disorders, especially children on the autistic spectrum, who often have severe sensory processing difficulties. But it also occurs in ordinary, functional children. Some studies suggest that as many as one in six children suffers from significant sensory problems, while others put the incidence at one in 20.

Most of us will have some degree of sensory defensiveness and most of us learn to live with it. I hate the sensation of wearing stockings; I feel like my legs and toes are in prison. If I have the misfortune to be in a situation where I have to wear them (most of high school in fact), I experience a constant low-grade level of discomfort and can't wait to get home and change.

I have learnt to accommodate this. A career as an air hostess, with the necessary starched uniform and unladdered stockings, was never an option.

As most children mature they are able to process and modulate their sensory experiences more effectively, but it seems that there are children who do not adjust at all, or at least not normally.

They remain unable to process sensory information in an adequate way.

These children often experience difficulties planning and organising, staying calm in class or taking part in everyday fun activities. Some become highly avoidant, fearful and confused.

SPD was first highlighted in 1972 by the occupational therapist and psychologist Jean Ayres.

She believed that sensory problems were more than symptoms of other disorders. She argued that they were often the primary cause of many motor and behavioural problems such as anxiety and inattentiveness, and far more common than doctors recognised.

Treatment, for both children and adults, is usually conducted by occupational therapists trained in sensory integration therapy.

Therapy typically involves guided activities intended to make the client more comfortable as they engage the sensations that disturb them - or to make them more alert to those they usually do not notice.

However, there is controversy regarding diagnosis and treatment.

Critics say there isn't enough evidence-based research to support claims that this diagnosis exists as a separate entity, to explain what is happening in the brains of sufferers, or to support an effective treatment.

In an article about the issue in the New York Times (June 5 2007), journalist Benedict Carey comments that "sensory processing disorder is entering a kind of limbo state: present but not fully arrived; noticed but, like many of the children struggling with symptoms, not entirely accepted by peers."

Increasingly, however, research is emerging that supports what many parents and clinicians believe to be true: that sensory processing difficulties are real, often disabling and need to be treated and understood.