At the heart of the debate over using monkeys in scientific research is the primate paradox: their closeness to humans makes them the ideal experimental model for the study of the brain, yet because they look so much like us, they also appear to feel pain in the same way.
Primates are given special status under the law. It is more difficult for scientists to get a licence to work on monkeys – which can only be used for the study of serious medical conditions – than on rats and mice.
The brain remains the least understood part of the body, which means that scientists have to turn to our closest animal relatives if they are to have any chance of finding effective treatments for diseases such as Parkinson's and Alzheimer's, as well as certain forms of blindness.
Part of the problem is that "lower" animals, such as rats, do not have a well-developed cortex, the outer structure of the brain linked with the "higher" mental processes of humans.
The Research Defence Society, a body representing the charities sponsoring medical research involving animals, said the similarity between the cortex and frontal lobes of primates and those of human beings makes monkeys the best subjects to model brain disorders in humans. "The study of neurological and neuropsychiatric disorders, which generally implicate higher cognitive functions and brain structures such as the frontal lobes, depend much more on studies of primates than of other animals," the society says.
"For example, disorders such as depression, schizophrenia, attention-deficit hyperactivity disorder, autism, drug addiction and obsessive compulsive disorder all implicate malfunctioning of the frontal lobes and their interactions with other structures.
"It has proved largely impossible to understand the wiring of the human brain from post-mortem studies, and studies of rodents are only of limited applicability because their brain structure is not sufficiently similar to humans."
In Britain, experiments involving the higher primates – great apes such as chimps and gorillas – have been banned, even though in theory their brains would be better experimental models than those of monkeys.
The scientific consensus in the UK – but not in the US – appears to be that chimps are just too close to man to justify using them as experimental "guinea pigs". Laboratory chimps, for instance, can show total terror at the sight a man in a white coat holding a hypodermic needle – any parent of a child waiting to vaccinated would empathise with that.
Monkeys, however, have not yet achieved such exalted status. "We understand that many people find the use of monkeys in medical research distressing," admitted Cambridge University.
"[But] this work must continue it we are to make essential life-saving advances in medicine," the university said.