The colour blue; birdsong, moving traffic and the rustling of leaves: all that and much more is alarmingly new for two of the Austrian children who have spent their lives locked in an underground bunker.

The full scope of what has to be recovered and repaired in the minds of the two imprisoned boys is only just becoming apparent to Austria's top psychiatrists. “It will take between four and eight years of intensive therapy,” said Professor Max Friedrich, who advised on the treatment of Natascha Kampusch, who was imprisoned by her abductor for eight years.

Josef Fritzl, 73, the father of the cellar children, told them that they would be gassed if anything happened to him, investigators say.

Bernhard Kepplinger, the supervising doctor for the Fritzl family - the three relatively well-adjusted children who were brought up above ground and the three from the cellar - is more cautious: “Each child will need individual therapy and we should be careful not to overdo it.”

Commentary: a society that nurtures secrets

What society would accept the probability of three children being left on the doorstep of Josef F and grant him adoption?

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• Pictures: house of horror

BACKGROUND

• Why are child abuse scandals happening in Austria?

• Analysis: damage caused may never be healed

• Brutal double-life of Josef Fritzl emerges

• Police search other Fritzl properties in Austria

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• Police flounder as Fritzl clues suggest accomplice

• Could the Fritzl case happen here?

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• Video: how Fritzl led a double life

Dr Kepplinger has reserved a space in his Amstetten clinic for the five children - the sixth, a cellar child aged 19, is still in a coma - and has erected a special dark chamber where they can retreat if the sunlight and noise become overwhelming. One problem has been separating the two boys from their mother, Elisabeth. “They were probably overprotected by their mother in the cellar,” Dr Friedrich said.

Elisabeth, 42, had to shield them from the sexual abuse of her - and their - father, Josef. When they heard the sounds of their father having sex with their mother they would retreat to one of the bedrooms and watch television to blank out the noise. Now, influenced by so much television, the children's view of the world is two, rather than three-dimensional.

The older boy reportedly has problems with balance and co-ordination.

The youngest boy is given the best chance of recovery because of his relatively short period in the dungeon. He is curious rather than threatened by the outside world. When he was first taken out of the cellar he asked a policeman: “Is that God up there?” pointing at the sky.

The hospital lifts have been frightening for the boys. When the doors close, they panic: fears of the cellar dungeon are still very fresh.

The younger boy is said to be confused by the ringtones of mobile phones. As for the older one, he has grown to adolescence in the improvised prison created by his father. The television has given him some idea about cars, computers and politicians, but it is all very abstract.

Some things are familiar - the smell of food from the cooking ring - and the psychiatric team is trying to build on the recognised sensations while extending his range.

Never in modern Europe has there been a case of such sustained sensory deprivation. Even Natascha Kampusch, who escaped from her kidnapper 18 months ago, spent part of her captivity upstairs, was allowed occasionally into the garden and was even taken on a trip to the mountains by her abductor. Moreover, she was already 10 years old when she was first kidnapped, already quite articulate, able to read and write and aware that she was a captive. None of the cellar children were told they were prisoners; Elisabeth presented their cellar existence as normal life.

Some of the initial therapy will come spontaneously in the form of contact with their upstairs family. This carries a risk. All the children are the products of an incestuous relationship and have been lied to over the years. Their trust in their father has collapsed. But the articulate upstairs children are reportedly communicating well with the other two.

The upstairs children - 16 and 15-year-old girls and a 12-year-old boy - are good at school. All three play the trumpet. The boy takes karate lessons and all are socially outgoing. In the upstairs world, birthdays and Christmas were celebrated; downstairs they were not, although Josef sometimes took presents. Both parts of the family were entirely focused on Josef, even though he was absent for most of the time. The upstairs children were allowed to join in his 70th birthday party in the garden three years ago. Downstairs, they heard nothing.

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