Can Visual Attention Truly Be Divided?

Psychologists have devised all sorts of ingenious experiments for delving into the mind. And nowhere is that ingenuity better tested than in the psychology of attention.

Take the attentional spotlight metaphor. This suggests we have a beam of attention that is independent of gaze direction sweeping across our visual field, enhancing the cognitive processing of things that are most relevant to us. But, ah, no, say some, why is it just one beam? Surely we can attend to two or more things at once: say, a child wandering out into the road and the car bearing down on her.

Two problems come to mind. Firstly people can't be trusted to introspect accurately (cf. the hidden workings of the mind) and secondly people can move their attention between two locations quite quickly. It might feel like you're attending to the speeding car and the hapless child at the same time, but it's just consciousnesses up to its old tricks.

Measuring 'visual evoked potentials'

One solution to these two problems was used in a study by Muller at al. (2003) published in Nature which relies on a quirk of the brain's electrical response to certain visual stimuli. This quirk is that some flashing lights -- or in the jargon, steady-state visual evoked potentials -- produce reliable patterns of electrical activity in the brain.

When people are attending to the flashing light, these electrical frequencies increase in amplitude. So by measuring the electrical activity of the brain it is possible to tell if people are attending to a flashing light. This provides the crucial objective measure of whether or not someone is attending to an object.

Muller at al. (2003) showed participants four symbols in a row and measured the electrical activity in their brains. Each symbol flashed with a different frequency so that the researchers could tell from the evoked electrical potentials in the brain to which symbol the participants were attending. Participants were then told to either concentrate on two of the symbols that were next to each other, or two symbols that were divided by another symbol they were told to ignore.

Divided attention

The telling conditions were when participants tried attending to two symbols that were separated by another symbol. If their attention was really divided this symbol in the middle should not activate its characteristic electrophysiological response. And this was exactly what they found. The results suggested that people were in fact successfully dividing their attentional between two different locations for several seconds, while ignoring what was in-between.

So it seems we really can pay attention to both the child and the car at the same time. This ability to divide our attention may be particularly useful because it probably takes us up to half a second to switch our attention from one location to another. And sometimes half a second is the difference between life and death.

→ Part 4 of a series on the psychology of attention →

[Image credit: gnackgnackgnack & gnackgnackgnack]

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