A local anchor for the brain’s compass
Martin J. Chadwick & Hugo J. Spiers, University College London, based on Marchette et al (2014) :: Nature Neuroscience, Vol. 17, No. 11, November 2014 :: www.nature.com/neuro
Summary and review of the above article
INTRODUCTION: This study looks at neurons dealing with head direction and also location that are centred on the medial parietal cortex. However, it is admitted that both these neurons and the better known place cells provided only very fragmented information. The authors say that a more global code for direction and location must exist to allow navigation between the different locally-coded fragments of space.
Studies show that neurons in the limbic areas of the brain act in a way analogous to an internal compass. A head-direction neuron is active whenever a subject faces in a particular direction, with each head-direction neuron preferring a slightly different direction. For each direction, a unique population of neurons is active, and its compass-like direction can be utilised by other areas of the brain.
However, this system needs a way of fixing itself relative to the environment, analogous to the way in which a magnetic compass fixes itself relative to the magnetic north-pole. The study concentrates on the medial parietal cortex, sometimes known as the retrospenial complex (RSC). The brain appears to favour local fixes such as rooms or buildings, rather than more distant fixes such as the position of mountains.
The same RSC structure also responds to particular locations. The spatial code seen here is argued to be defined in terms of the immediate local environment rather than anything absolute. The system is able to generalise across environments that have a similar geometry. This study involved a series of similar museums in a park. Populations of head-direction cells have been observed in the RSC, and the authors consider the brain may possess what they call reference-vector cells that encode bearings to particular local landmarks.
A new class of specialised neuron?
This may or may not suggest the existence of a new class of specialised neurons. An alternative is that this function could be performed by the boundary vector cells located in the parahippocampal. It is also not known which brain regions signal the direction of future location goals. The entorhinal cortex encodes the distance along a particular vector, and this area might be more important than the RSC for navigation.
A problem with navigation
The better known place cells in the hippocampus encode current location in an environment, but this system has a similar problem to that for head direction, in that it is very dependent on local cues. The authors say that a more global code for direction and location must exist to allow navigation between different fragments of space that each have their local code.