Sensory memory is that memory you have of the word you just heard spoken or when you glance and close your eyes and you can just about remember some of what was in front of you. And arguably you might not even think of that as memory. So it's sort of like a memory, but not memory, but it really does actually make a difference to your life.
The most obvious way in which sensory memory happens that you might notice is in visual imaging. So if you stare at an image, say a Red Cross on a green background and you stare at it and you stare at it, you stare at it for a long time. And then you look to a white background, you'll often see an afterimage. On the white background you see the opposite colors often. It's particularly true if it's going to be a bright image.
If it's dull, you don't tend to notice it; it's got to be quite a bright image. And the reason for that is most of our senses have a very low-level dulling. So it's actually we've got multiple kinds of sensory memory here, so there's a dulling. If you have the same thing happening all the time, most of our senses don't notice it after a while. They notice change. That's true of touch. If you're touching something for a long while, you can stop feeling it. And it's true of sounds. If you've got the same sound going all the time,
it just fades into the background. And it's true of our visual senses. Sometimes it's called *habituation*. Your senses get used to things, and it's the *change* that you notice most. You can almost say we only ever see change; that's not entirely true – we can focus. But to a large extent it's change which drives the sensory processes. This is used, whether it's a computer movie but if you look at old sort of Victorian toys,
and also think of cine film, it's still the way a lot of high-quality film is taken even in a digital age. What you have is a series of still images that go after each other, one after another. So in fact, if you see, just like a dot move across the screen, you see... You think you're seeing a movement, but actually what you've got is a series of still points of that dot as it moves.
If you're looking at a person – if you're looking at me moving now – every movement I make, you're seeing probably 25 frames a second, perhaps 30 frames a second. So basically lots of stills and your brain fills in the gaps and makes these persistent. So that's happening at the sensory level and it's sort of a memory-ish sort of effect. But then as things get deeper into your brain, most of your senses have a buffer, a temporal buffer.
So particularly this is true of your sight to support the *iconic memory*, so that business of not just the after effects, which is really about the sensors in your eye, but the fact that I said, you can close your eyes and I've got a pretty good idea of what's in front. It doesn't last very long, and it varies from person to person. That's quite crucial. We don't all experience that in the same way. You have this thing called an *echoic memory*, which is this aural thing.
And you might have noticed this where somebody says something to you and you think you didn't quite grasp it and you start to say "Pardon?" or "What did you say?" And then you realize, and it's because you've replayed it in your head and then the second time around, you actually heard it. Just like you might play a replay if you're watching a video on the computer or listening to it on the computer. Now again, that varies. A personal thing, I actually have great difficulty in reproducing a sound I hear.
I obviously have a slight failing, and somewhere between my echoic memory connects into my speech production – I can hear the sounds really well; I can produce the sounds pretty well, but I struggle to connect them up. So different people, you know, I wouldn't call that a disability. Perhaps it is, but we're different. We're very different in the way these things get together. But most of us have a level of these things. And then you also can get the same thing with haptic memory...
There's a sensitizing we talked [about], where if you've touched the same thing for a long time, you don't notice. It's the change you tend to feel as your finger moves over a rough patch. But also you have a sense of what you just felt. So if you feel something, it's your brain sort of remembers it for a moment after your body stops actually physically sensing it. However, these are really, really, really short term. They're just constantly getting overwritten. So it's a bit like having a loop on an old tape recorder
but one that's so short it has to be [re]written. So it is very simple. That echoic memory, that ability to say, "What did you say?" that... Almost to re-hear it disappears very, very rapidly. Now you might remember what they said and be able to replay that in your head. But the actual sound memory disappears exceedingly rapidly.