America has deliberately driven hundreds, perhaps thousands, of prisoners insane. Now it is being held to account in a Miami court.

Something remarkable is going on in a Miami courtroom. The cruel methods US interrogators have used since September 11 to "break" prisoners are finally being put on trial. This was not supposed to happen. The Bush administration's plan was to put José Padilla on trial for allegedly being part of a network linked to international terrorists. But Padilla's lawyers are arguing that he is not fit to stand trial because he has been driven insane by the government.

Arrested in May 2002 at Chicago's O'Hare airport, Padilla, a Brooklyn-born former gang member, was classified as an "enemy combatant" and taken to a navy prison in Charleston, South Carolina. He was kept in a cell 9ft by 7ft, with no natural light, no clock and no calendar. Whenever Padilla left the cell, he was shackled and suited in heavy goggles and headphones. Padilla was kept under these conditions for 1,307 days. He was forbidden contact with anyone but his interrogators, who punctured the extreme sensory deprivation with sensory overload, blasting him with harsh lights and pounding sounds. Padilla also says he was injected with a "truth serum", a substance his lawyers believe was LSD or PCP.

According to his lawyers and two mental health specialists who examined him, Padilla has been so shattered that he lacks the ability to assist in his own defence. He is convinced that his lawyers are "part of a continuing interrogation program" and sees his captors as protectors. In order to prove that "the extended torture visited upon Mr Padilla has left him damaged", his lawyers want to tell the court what happened during those years in the navy brig. The prosecution strenuously objects, maintaining that "Padilla is competent" and that his treatment is irrelevant.

The US district judge Marcia Cooke disagrees. "It's not like Mr Padilla was living in a box. He was at a place. Things happened to him at that place." The judge has ordered several prison employees to testify on Padilla's mental state at the hearings, which began yesterday. They will be asked how a man who is alleged to have engaged in elaborate anti-government plots now acts, in the words of brig staff, "like a piece of furniture".

It's difficult to overstate the significance of these hearings. The techniques used to break Padilla have been standard operating procedure at Guantánamo Bay since the first prisoners arrived five years ago. They wore blackout goggles and sound-blocking headphones and were placed in extended isolation, interrupted by strobe lights and heavy metal music. These same practices have been documented in dozens of cases of "extraordinary rendition" carried out by the CIA, as well as in prisons in Iraq and Afghanistan.

Many have suffered the same symptoms as Padilla. According to James Yee, a former army Muslim chaplain at Guantánamo, there is an entire section of the prison called Delta Block for detainees who have been reduced to a delusional state. "They would respond to me in a childlike voice, talking complete nonsense. Many of them would loudly sing childish songs, repeating the song over and over." All the inmates of Delta Block were on 24-hour suicide watch.

Human Rights Watch has exposed a US-run detention facility near Kabul known as the "prison of darkness" - tiny pitch-black cells, strange blaring sounds. "Plenty lost their minds," one former inmate recalled. "I could hear people knocking their heads against the walls and the doors."

These standard mind-breaking techniques have never faced scrutiny in an American court because the prisoners in the jails are foreigners and have been stripped of the right of habeas corpus - a denial that, scandalously, was just upheld by a federal appeals court in Washington DC. There is only one reason Padilla's case is different - he is a US citizen. The administration did not originally intend to bring Padilla to trial, but when his status as an enemy combatant faced a supreme court challenge, the administration abruptly changed course, charging Padilla and transferring him to civilian custody. That makes Padilla's case unique - he is the only victim of the post-9/11 legal netherworld to face an ordinary US trial.

Now that Padilla's mental state is the central issue in the case, the government prosecutors are presented with a problem. The CIA and the military have known since the early 1960s that extreme sensory deprivation and sensory overload cause personality disintegration - that's the whole point. "The deprivation of stimuli induces regression by depriving the subject's mind of contact with an outer world and thus forcing it in upon itself. At the same time, the calculated provision of stimuli during interrogation tends to make the regressed subject view the interrogator as a father-figure." That comes from Kubark Counterintelligence Interrogation, a declassified 1963 CIA manual for interrogating "resistant sources".

The manual was based on the findings of the agency's notorious MK-ULTRA programme, which in the 1950s funnelled about $25m to scientists to carry out research into "unusual techniques of interrogation". One of the psychiatrists who received CIA funding was the infamous Ewen Cameron, of Montreal's McGill University. Cameron subjected hundreds of psychiatric patients to large doses of electroshock and total sensory isolation, and drugged them with LSD and PCP. In 1960 Cameron gave a lecture at the Brooks air force base in Texas, in which he stated that sensory deprivation "produces the primary symptoms of schizophrenia".

There is no need to go so far back to prove that the US military knew full well that it was driving Padilla mad. The army's field manual, reissued just last year, states: "Sensory deprivation may result in extreme anxiety, hallucinations, bizarre thoughts, depression, and antisocial behaviour" - as well as "significant psychological distress".

If these techniques drove Padilla insane, that means the US government has been deliberately driving hundreds, possibly thousands, of prisoners insane around the world. What is on trial in Florida is not one man's mental state. It is the whole system of US psychological torture.

Naomi Klein's book on disaster capitalism will be published this spring; a version of this article appears in the Nation www.nologo.org