Specialists in deaf education heard from a leading international expert of “terrifying” results from a study of 800 American university students which led him to question a policy in which he had long believed.

“I used to be a strong supporter of mainstream education for deaf children,” Marc Marschark of the National Technical Institute for the Deaf in New York said. “But now I feel mainstream may not be optimal for many deaf students.”

The research study showed that more than half of deaf students who use spoken language to communicate often do not understand what is said to them, while only three out of five using sign language fully comprehend what they are told. It was based on face-to-face interviews between individuals who use the same form of communication.

Speaking at a Scottish Sensory Centre conference on language and deaf education, Professor Marschark, an honorary professor in the department of psychology at Aberdeen University, warned that there was worse still - deaf students did not realise they didn’t understand.

The research found that only 42 per cent of students using oral language understood when a peer using the same form of communication asked them a single sentence question. When a student who used signing was asked in sign by a deaf peer who also used sign, only 62 per cent understood.

“What was more worrying was that they did not realise they had not understood completely what was said to them,” Professor Marschark said.

“That is terrifying. It is not just a cognitive issue, but also one of basic language.”

While recognising that the move to mainstream deaf children could not be reversed, Professor Marschark called for further research into understanding and improving deaf education. In Scotland, there are around 1,300 deaf children and young people and more than 80 per cent are educated in mainstream schools.

Professor Marschark said: “We are now at the threshold of significant progress in improving deaf education. We need to learn from the past, but not dwell on our earlier assumptions.”

Currently, deaf children in Scotland dramatically underperform compared to their hearing peers. Statistics collected by the Achievements of Deaf Pupils in Scotland showed that, in 2003-04, only 35.5 per cent attained a level D in maths at P7 compared to 69.7 per cent of all pupils. In reading, it was 41.9 per cent compared to 74.4 per cent, and in writing 30.2 per cent compared to 60.6 per cent.

“I wouldn’t single out Scotland, in so far as our Scottish statistics only confirm what we know to be the overall picture for deaf children generally,” Marian Grimes, acting co-ordinator of the postgraduate programme for deaf education at Edinburgh University’s Moray House School of Education, said.

“The big issue is whether we can rise to challenges such as the need to take on board all we know, and all we are coming to know, about the different ways that deaf children learn.”