‘Surely you’re not saying—’ began Karene.

‘What if all of this is about Harry’s memory? If we set the bomb aside, Harry disappeared for seventy-two hours then reappeared with no recollection of the past four years.’

‘You’re thinking something happened in the last four years that someone wants him to forget?’ said Jameson.

‘Maybe he did something. Or witnessed something.’

‘Or they got him to do something in the time he was missing,’ said Jameson. ‘And they needed him to forget that.’

‘This is ridiculous,’ said Karene. ‘You can’t decide which memories someone is going to lose from a head injury. It doesn’t work like that. You know that as well as I do!’

‘You say that.’ Bloom opened her notes on memory loss from the research she’d done that morning. ‘But there are case studies of retrograde amnesia limited to one or two years where there is specific damage to the CA1 field of the hippocampus.’

‘But head injuries affect different people in different ways. You can’t predict it,’ said Karene.

‘We know which parts of the brain are involved in episodic memory and which control procedural memory. We know that certain regions lead to anterograde amnesia and others retrograde.’

‘Wait,’ said Jameson. ‘You’ve lost me.’

‘Sorry,’ said Bloom. ‘We have two main types of memory, procedural – which is how we learn skills and the performance of tasks – and declarative – which covers our recollection of events and facts about our lives, like the names and faces of family and friends.’

‘And they are managed by different areas of the brain?’ he said.

‘Yes. Harry has lost his episodic memory. He can’t remember what he’s experienced or learned over the past four years but he will still recall how to drive, for example. So only his declarative memory is affected. And we know that the hippocampus in the brain is heavily involved in that specific function. For instance, there was one unfortunate chap who underwent brain surgery to reduce his epilepsy and, because little was known about what different brain regions did at that time, his hippocampus was removed which resulted in severe retrograde amnesia.’

‘H.M. The most studied patient in neurology,’ said Karene.

‘And retrograde?’ asked Jameson.

‘Memories of the past,’ said Karene. ‘As opposed to anterograde amnesia, which is our ability to form new memories.’

‘So he has a very specific type of memory loss, controlled by a very specific part of the brain?’ said Jameson.

‘Exactly,’ said Bloom.

He looked at her. ‘And you’re saying that if you wanted someone to lose the last few years of their memory, you could cause localized damage to the hippocampus.’

A deep frown creased Karene’s brow. ‘But that would require surgery not a head trauma.’