Bristol, UK. When we are confronted by threatening siruations, the speed at which we react could have life or death implications. In our distant and more primitive past, the application of speed and wit could have meant escape from a wild animal. Today, it might mean swerving on a highway to avoid a head-on car crash.

 

Many scientists have thought for some years that mammals have two decision-making systems in their brains which operate at different speeds to cope with different situations. New research supports this theory. It shows that the evolutionary pressures arising from the older, faster, but less accurate, part of the brain may have shaped the more recent development of the slower-acting but more precise cortex, found in humans and higher animals. The research findings appear in the Proceedings of the Royal Society B.

 

Pete Trimmer, from the University of Bristol and lead author on the study, says: "If we compare the brain of a human with that of a reptile, we find they are very similar except that mammals have a large 'outer cortex' around the outside of the existing 'sub-cortical' brain, that is common to other vertebrates.

 

"The fact that lizards make decisions indicates that the sub-cortical brain in humans is also likely to be used in decision-making. However, fMRI scans now reveal that parts of the outer cortex (which developed more recently in our evolutionary past) are also used when making decisions."

 

There are a number of interesting questions that pose challenges for researchers.

To address these questions, Trimmer built theoretical models representing the two systems in which the sub-cortical system was assumed to act very quickly but inaccurately, whereas the cortex allowed information to be gathered before making an informed decision, and was therefore slower.

 

The results of their modelling showed that when the threat level is high, such as the risk of being attacked by a dangerous animal, it is very useful to have the fast-acting, if inaccurate, system. But when dealing with situations which don't occur very often, or complex scenarios with many conflicting cues such as social situations, the cortical system is of more use than the sub-cortical system. 

 

Trimmer commented: "As life became more complex, the benefit of gathering information before making a decision put an evolutionary pressure on the early brain. This may have led to the rapid development of the cortex in mammals. So if humans continue to live in a world of dangers such as wild animals or fast-moving cars, there will still be an evolutionary benefit to maintaining the sub-cortical system, and it is unlikely to atrophy in future humans."

 

The modelling was done by Trimmer and colleagues across the Departments of Computer Science, Maths, Biology and Veterinary Science at the University of Bristol.

 

Mammalian choices: combining fast-but-inaccurate and slow-but-accurate decision-making systems. Pete Trimmer, Alasdair Houston, James Marshall, Rafal Bogacz, Elizabeth Paul, Mike Mendl and John McNamara. Proceedings of the Royal Society B. doi: 10.1098 / rspb.2008.0417. [ Download PDF ]

Abstract

Empirical findings suggest that the mammalian brain has two decision-making systems that act at different speeds. We represent the faster system using standard signal detection theory. We represent the slower (but more accurate) cortical system as the integration of sensory evidence over time until a certain level of confidence is reached. We then consider how two such systems should be combined optimally for a range of information linkage mechanisms. We conclude with some performance predictions that will hold if our representation is realistic.

Supplemental Material

Additional Material A: Cortical drift and variance calculations. For the cortical system, we calculate the expected movement and variance in log-space caused by a single update. We then show the effect of updates becoming a continuous process. [ Download SuppA PDF ]

Additional Material B: Cortical threshold calculations. We derive an expression for the optimal false alarm rate of the cortical system, assuming that the system is operating independently. [ Download SuppB PDF ]

Additional Material C: Expected decision time and probability of action. We derive expressions for the expected cortical decision time and the overall probability of the animal taking action through both the thalamic and cortical systems. [ Download SuppC PDF ]

 

 

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