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www.pc.rhul.ac.uk/staff/y.saalmann/
Focusing on attention
29 Jun, 2007 03:15 pm
There are many things in the world around us, too many for the brain to process in detail at the same time. A lot of these things, however, are not always important to us. Ideally, we would spend more time and resources processing the important things and less time on the rest. Attention helps us to do just that, by selecting what is potentially important to us, so that it can be processed in detail by the brain.
aDepartment of Psychology, Royal Holloway, University of London
bInstitute for Information Transmission Problems, Russian Academy of Sciences
cDepartment of Optometry and Vision Sciences, University of Melbourne
Sometimes this can go wrong. For instance, a syndrome referred to as spatial neglect can occur after strokes. Patients with spatial neglect will not notice things in the space around one side of their bodies, usually their left side. Many patients are not even aware that they have this problem.
Let us now look more closely at how the brain allows us to pay attention to the things we see. Information from the world around us is sent by the eyes to the back of the brain, also known as the visual cortex. Different parts of the visual cortex process information from different locations around us. When we pay attention to a particular location, then there is a selective increase in the electrical activity of the brain cells that process information from this location.
The question is: what causes this increase in electrical activity in the visual cortex? A higher area of the brain called the posterior parietal cortex is involved in determining the importance of locations around us (in fact, the posterior parietal cortex is part of a network of brain areas involved in attention). To test how this area affects other parts of the brain, we simultaneously measured the electrical activity of brain cells in the posterior parietal cortex and visual cortex of macaque monkeys, while they played video games. Our research1,2 shows that electrical signals are sent from the posterior parietal cortex to the visual cortex, providing “feedback” as to which locations around us are important. It is these signals that increase the electrical activity in visual cortex, and it is this interaction between the brain areas that is critical for attention.
Our data suggest that the feedback can increase the electrical activity in visual cortex in three ways. First, more electrical signals can be sent from the posterior parietal cortex to the visual cortex. Incoming electrical signals increase the electrical activity in the visual cortex – the more incoming signals, the more activity in the visual cortex.
Second, many brain cells in the posterior parietal cortex send electrical signals to the visual cortex. If the signals from a number of different brain cells in the posterior parietal cortex are synchronized, then their impact on the visual cortex is increased. This is similar to a situation where your car breaks down and you have a number of friends to help push. The car moves more easily when you all push at the same time, rather than if you push one at a time. When incoming electrical signals act on the visual cortex at the same time, they more easily increase the electrical activity in the visual cortex.
Third and finally, the electrical activity of brain cells can go up and down in a repeating rhythm, that is, the activity can oscillate. If the electrical activity of brain cells in the visual cortex is up, then electrical signals arriving from the posterior parietal cortex have a greater effect (in comparison with the scenario where signals arrive when the activity in the visual cortex is down). This can be made more likely by synchronizing oscillations in the posterior parietal cortex and visual cortex.
The three mechanisms outlined above may well play a more general role in cognition than just attention. These mechanisms may be fundamental ways to increase the impact of electrical signals, that is, information, sent between any two brain areas. Different pieces of information will be important under different circumstances. So the ability to weight information according to circumstance allows us to flexibly interact with our environment.
1 Saalmann YB, Pigarev IN, Vidyasagar TR (2007) Neural mechanisms of visual attention: how top-down feedback highlights relevant locations. Science 316: 1612-1615.
2 Experiments were carried out at the University of Melbourne, Australia, and were supported by grants from the National Health and Medical Research Council of Australia.
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This is an interesting paper that appears to be overly simplified. The authors should mention that the work was carried out in monkeys using unicellular recordings. They should also indicate which of the three proposed mechanisms is the most likely to be involved in spatial attention.
[Response] In reply to sentence 1 by Dr. Lassonde - We have tried to follow the instructions given to us from editor Clementine Fullias: "Please be as simple as possible for a lay audience. As if you were talking to a high school student. The title should be catchy and not technical." Additional information about our research has now been added to the article, as discussed below. In reply to sentence 2 by Dr. Lassonde - We have added the following text to the 4th paragraph: "To test how this area affects other parts of the brain, we simultaneously measured the electrical activity of brain cells in the posterior parietal cortex and visual cortex of macaque monkeys, while they played video games." In reply to sentence 3 by Dr. Lassonde - Our results support all 3 mechanisms being involved in spatial attention, i.e. no one mechanism appears more likely. To make this clearer, we have added the following text to the start of the 5th paragraph: "Our data suggest that..."
Neural connectivity which binds synchronized activity within and between neurons into cell assemblies (NCA) forms the structure of stimulus representations. Consequently, selection mechanisms including stimulus salience are realised through synchronisation of activated NCA?s. Salience favouring selection of a stimulus are thought to involve higher-order brain regions. Taking into account top-down and bottom-up processing of sensory input, stimulus salience should hence be related by mechanisms as described by the present work.
Dear Editor, the authors wrote a well-written item on their latest research that was published in Science. However, I would like to see that the authors acknowledge that also regions other than the posterior parietal cortex can be involved in controlling spatial attention. For instance, the prefrontal cortex is likely to be important too. Thus, my advice is REVISE,
Best wishes,
Rufin Vogels
[Response] We have included the following brief statement in the article: "in fact, the posterior parietal cortex is part of a network of brain areas involved in attention". We do not specifically name other brain areas involved in attention, such as the pulvinar, cingulate and prefrontal cortex, because mentioning these in the article would contribute little to the main point that the posterior parietal cortex provides feedback to sensory areas about attention priorities.