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| George Wallis |
Research Briefing, by George Wallis
In almost any situation, there are hundreds
of things (or ‘stimuli’) that could attract our attention - just count the
number of objects you can see from where you are now. In order to get on with life and avoid total mental
chaos, we have
to be extremely selective about what we process – most stimuli are
essentially ignored. It is a
long-established finding that the number of things we can hold in mind (or hold
in ‘working memory’) is really very small – about 2-8 depending on the
experiment we run. Clearly we possess
powerful mechanisms that let us filter in only certain stimuli. The ways in which we can select what gets
into working memory is a much-studied topic in psychology, and often
psychologists run experiments in which they present ‘cues’ (e.g. arrows) that
tell people which items in the experiment to ‘select’. This is the experimental equivalent of
pointing out something with your finger.
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| Harry Styles of One Direction giving an attentional cue to the crowd |
However, most of the time, this isn’t how
we select what gets into working memory: in the real world people aren’t on
hand to continuously tell us what to pay attention to. One real-world factor psychologists think may
be important in determining whether an item gets into memory is its ‘reward
value’. For example, a twenty-pound note
is more likely to grab our attention than a piece of scrap paper, even if they
are about the same size and appearance.
Our paper recently published in Visual Cognition (Wallis, Stokes, Arnold, & Nobre, 2015) describe the results of two experiments we ran in which we looked
at how reward value affects the likelihood that a stimulus will get into
working memory.
Anderson and colleagues performed experiments
showing that items displayed in colours that the experimenters had previously
associated with a high monetary reward ‘grab’ attention as people look around a
visual scene for a particular target, slowing them down slightly (e.g. Anderson, Laurent, & Yantis, 2011). We adapted their experiment
to look specifically at memory, presenting four nonsense-shapes briefly, and
then testing how well people remembered which shapes had been presented a few
seconds later. Before running the memory
task we associated some of the shapes with high reward and some with low reward.
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| An experimental trial from our shapes experiment |
When we asked people to remember four shapes
out of which some items were worth more than others, people didn’t remember the
high value items any better than the low value items. This was a surprise! On the basis of the paper by Anderson, we
expected the high value items would be better remembered – after all, they
ought to grab attention. However, we did
find a curious effect. If all of the
shapes in a display were high-value (a ‘high value trial’), then any one of
them was remembered better than if all the shapes were low value (a ‘low value
trial’). More curious yet, if half the
shapes were high value, and half were low value, any shape was remembered about
equally well – but they were remembered a little less well than when all the
items were high value, and a little better than when all the items were low
value.
We reasoned that this could have been because
people simply made more effort when the shapes in the memory array were higher
in value, on average, and so they did a bit better. However, a more specific (and interesting)
explanation was also possible. We know that a chemical in the brain, dopamine, is involved
in processing reward – in studies on monkeys, where the dopamine neurons are
measured directly, experimenters see ‘pulses’ of dopamine release when rewarded
items are presented to monkeys. We also
know that the prefrontal
cortex (PFC) – the part of the brain thought to be most important in
controlling working memory, is ‘soaked’ in dopamine: dopamine is released
throughout the PFC. Some have suggested
that the dopamine pulses ‘open the gate’ to working memory, and the more
dopamine released at a given time, the wider the gate is opened (Braver & Cohen, 2000).
We couldn’t record dopamine firing in our
volunteers, so we couldn’t test this possibility directly. However, it made us wonder – what would
happen if instead of showing our memory items all at the same time, we
presented them very quickly one after the other, in a row? If subjects simply made more effort on those
trials where they had encountered a higher value item, then we would still
expect all of the items we showed to benefit from this. However, we know that dopamine pulses are
only a fraction of a second long (about a third of a second). If we presented each item for about this
length of time, one after the other, then a ‘reward pulse’ might be able to
‘pick out’ the high reward item, and not the other less valuable items. So, we
ran the experiment, with a few adaptations: rather than shapes, we used
coloured lines, presented one after the other, and asked people to remember the
orientation of the lines. Certain
colours were given high or low reward values.
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| An experimental trial from our second, sequential experiment |
We found that indeed, only the high-value
item in this experiment was more likely to be encoded, and not its
near-neighbours. This doesn’t prove that
dopamine pulses are responsible – we didn’t measure our volunteers’ dopamine
neurons – but it does suggest that the reward effect is quite tightly localized
in time: a ‘pulse’ tied to an item, not a more general ‘making an effort’
effect. This was an intriguing finding
and it opens up several questions.
Firstly, is this effect really down to dopamine, like we speculate? To find out, we’d need to see what dopamine
neurons are doing at the same time as running the task. Interestingly, there is some evidence that
the diameter of people’s pupils responds rapidly to dopamine release, so maybe
measuring pupil diameter would be a way of getting some more evidence without
having to get inside the brain.
Secondly – what’s the point of this rather
weird-seeming ‘pulse’ mechanism? And why
would it be useful – like in our first experiment – for unrewarded items to get
‘caught by the pulse’? Our speculative
answer to this question is that our experiment was unnatural – we asked our
volunteers to keep staring at the centre of the screen and flashed up the
shapes all together, just for a moment.
They had no chance to move their eyes (indeed we deliberately tried to
prevent that!). However, in more natural
settings, our eyes constantly flit around the scene. This is pretty hard to notice in yourself but
watch a friend’s eyes for a while (without freaking them out too much) – they
jump from place to place continually, ‘fixating’ first this object, then
that. In fact they move about 3 or 4
times per second, jumping from looking at one item to another. If our putative working-memory-updating
pulses were ‘tied’ to these fixations this might provide a mechanism by which
the more rewarded items in the scene were more likely to enter memory.
References:
Anderson, B. A., Laurent, P. A., & Yantis, S.
(2011). Value-driven attentional capture. Proceedings of the National
Academy of Sciences, 108(25), 10367–10371.
doi:10.1073/pnas.1104047108
Braver, T. S., & Cohen, J. D. (2000). On the control of control:
The role of dopamine in regulating prefrontal function and working memory. Control
of Cognitive Processes: Attention and Performance XVIII, 713–737.
Wallis, G., Stokes, M. G., Arnold, C., & Nobre, A. C. (2015).
Reward boosts working memory encoding over a brief temporal window. Visual
Cognition, 23(1-2), 291–312. doi:10.1080/13506285.2015.1013168