If you have seen something briefly and you want to sketch it from memory, scan your remembered visual field about a second after seeing it, paying attention to the parts most important to your drawing. This will help fix them in memory. That's what I conclude from "Research Briefing: Attention restores forgotten items to visual short-term memory", a posting by Mark Stokes in his Brain Box blog.
Our everyday view of the world is necessarily biased: we focus our attention on information that is most relevant to our current goals, and ignore behaviourally irrelevant information. Without such bias, we would be lost in a world of information-overload, unable to accomplish even the simplest tasks.
One of the faculties this applies to is memory. The brain receives too much visual information to remember it all, so has to choose what to remember. Previous research has suggested, according to Mark's post, that paying attention to information in visual short-term memory helps one maintain it, in the same way that repeating a phone number to oneself helps one remember that. But the paper that's the subject of his post goes further. Paying attention to items sometimes restores them to memory even when they seem to have been forgotten. This may be because they were originally stored in a format in which they couldn't be retrieved. Paying attention to them converts them into a retrievable format.
The experiments that suggest this are described in "Attention Restores Discrete Items to Visual Short-Term Memory" by
Alexandra M. Murray, Anna C. Nobre, Ian A. Clark, André M. Cravo and Mark G. Stokes, in
Psychological Science published online 22 February 2013. Here's my cartoon summary of them:
(Apologies for the rough drawing, which I did with a mouse.)
In the first shot, the subject is shown a screenful of little coloured arrows in various orientations. In the last, the subject is given a "memory probe". This is a coloured arrow in the same position on the screen as it was before, but in a different orientation. The subject is asked to rotate it back to the original orientation, thereby testing their memory.
Where does the effect of attention come in? In some of the experiments, subjects were shown a "cue" as in the second shot: a small square that pops up somwehere on the screen. In these experiments, subjects showed more accuracy in the third shot than if the cue had not been given. As Mark says in his posting:
We combined behavioural and psychophysical approaches to show that attention, directed to memory items about one second after they had been presented, increases the discrete probability of recall, rather than a more perceptual improvement in the precision of recall judgements [...]
Full details of the experiments, and of the authors' conclusions, are given in the paper. What the research suggests to me is that if I'm trying to draw someone who I've only seen briefly, perhaps a person who has just walked past, then about a second after seeing them, I should deliberately scan my remembered visual field. And I should pay attention to those regions of it where I think the most salient details for my drawing are.