Manipulating mouse memory; London pollution; Nature of knowing; Snail fur

Manipulating mouse memory Optogenetics allows researchers to use light to turn the genes involved in memory, in the brain, off and on. It's a powerful tool for seeing exactly where specific types of memory are formed and processed. Researchers at MIT have been using the technique to manipulate fearful or pleasurable memories associated with a particular location, in mice. This is so they can see how memories are overwritten in the brain's processing regions.

London pollution Cities in Britain have moved on a great deal from air pollution events, like the London smog of 1952, where 4000 people died in a week. But a recent report has put London air pollution levels as bad as some of the worst in the world, on a level with Mexico City and Beijing. Pollution is a mixture of gases and tiny particulate matter (or PM) -particles too small to be filtered out by our noses, and which end up going straight into our lungs. Dr Rossa Brugha and reporter Marnie Chesterton take a bicycle ride through London's busy streets and parks with an air pollution monitor. Back in the studio, Rossa and Adam talk through the results...

Nature of knowing Philip Ball, the programme's on-call polymath and author of 'Invisible, the Dangerous Allure of the Unseen', comes into the studio to answer a listener's question about how science can possibly understand the unseeable, if it is supposed to be dealing with the observable universe.

Snail fur and how to grow a new head Why is it that some animals can regrow lost body parts and others, like us, can't? Even some closely related species, for instance salamanders, can regrow a lost tail, but fellow amphibians, the frogs, can't regrow lost legs? One of the best-studied 're-generators' is the sea creature - Hydractinia, or Snail fur, because it grows like fur on the back of the snail-shell homes of hermit crabs. By studying Hydractinia's regenerative powers at the cellular level, researchers think that most animals, including us, may have the potential to regrow lost limbs using stem cell systems lying dormant within us.

Producer: Fiona Roberts.