What can MEG neuroimaging tell us about reading?

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Abstract

Learning to read is one of the most cognitively complex tasks we will ever learn to do. Thus understanding the reading process is not just intrinsically interesting, but can give us a number of valuable insights into the relationship between brain processes and cognitive behaviour. MEG neuroimaging allows us to investigate reading processes in terms of the spatial extent of cortical activations when reading, the timing between brain locations, and the frequency dynamics between different cortical areas. The big challenge now for neuroscience is to model all three components of neural behaviour in order to be able to really understand the complexity of human cognition.

Section snippets

What is MEG?

Magnetoencephalography (MEG) is a neuroimaging technique that measures the magnetic field patterns generated by the brain. MEG measures activity primarily from the post-synaptic potentials of pyramidal cortical cells. When large populations of neurons fire together, as is the case when the brain responds to some sensory input, then an electric current is measurable outside the head. This is the basis of the well-known EEG technique. The neuromagnetic correlate of the electric current measured

Spatial dynamics of reading

There are a number of excellent reviews that have mapped the functional components of the reading network (e.g., Fiez and Petersen, 1998, Jobard et al., 2003, Price, 2000, Pugh et al., 2001). The strength of MEG lies more in being able to combine spatial and temporal dynamics, so this section will serve more as a brief site-map to the more important components of the reading network.

Neuroimaging research has demonstrated a wide and complex network of cortical sites that are recruited in

Temporal dynamics of reading

Emerging MEG studies have allowed us to put tentative ‘time-tags’ on the areas of interest in the reading network, generally describing a posterior to anterior, and inferior to superior flow of activation. Earliest, pre-lexical activity occurs in the occipito-temporal areas (Pammer et al., 2004, Tarkiainen et al., 1999, Tarkiainen et al., 2002). Tarkiainen et al. (1999) showed distinct early components of the reading network within the first 200 ms, namely a midline occipital component (dark

Frequency dynamics – the new frontier

The reading network involves a number of spatially disparate sites and acquiring reading skills means that the brain must learn to communicate between these different sites, binding relevant information to access a coherent lexical entry and maintaining a smooth flow of dynamic processing. An emerging theme in reading research is the role of cortical connectivity in the neural networks that underlie skilled reading. MEG research in this area is still in its early stages and there are as yet a

What can MEG neuroimaging NOT tell us about reading?

One of the limitations of MEG is that accuracy in detecting sources decreases with increasing distance from the source (Hillebrand & Barnes, 2002). In other words, deeper sources are difficult to pick up and to localise accurately in MEG. This makes it more difficult to analyse contributions from deeper sources such as the thalamus for example. The thalamus has been consistently implicated in fMRI studies of reading and word recognition (e.g., Brunswick et al., 1999, Rumsey et al., 1997, Rumsey

Conclusion

In order to read fluently, the brain is required to recruit disparate cortical areas to function in a highly complex and dynamic way. Because MEG is sensitive to the spatial and temporal properties of cortical signals, it can provide a valuable contribution to the cortical processes involved in reading. MEG allows us to track the evolution of the time course of the information flow through the reading network, mapping the time course of the neuromagnetic signal to specific cortical locations.

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