Let say I am holding, say, a pencil in my hand. How fast do a person has to be
- to grab the pencil without me seeing it?
- to grab the pencil before an average person could react?
I’d say 200ms.
Our visual processing system has a mechanism to suppress blurred vision that results from rapidly moving the eyes when our attention automatically shifts. These movements are called ‘saccades’, and they last 200ms. The interesting thing about saccades is that during a saccade, both your vision and your sense of time are suppressed. These little chunks of 200ms are ‘edited out’ before they ever reach your consciousness, and you are not aware of them. I would imagine that this is how a typical ‘up close’ magic trick works - the magician does something to grab the watcher’s attention, and during the saccade that results, actually palms the coin, or switches the card, or whatnot. The watcher was not aware of the switch even in their peripheral vision, because said vision was shut off for the duration of the saccade. And because their sense of time was suppressed, they are not aware that 200ms even happened.
There are lots of little ways to game the innate tradeoffs that our nervous system had to make to maintain functionality. See “Mind Hacks” by Tom Stafford and Matt Webb, from O’Reilly press.