Magnetoencephalographic (MEG) investigation of brain function in humans with normal and disordered vision
Electrical current flow in individuals produces magnetic fields outside the individual that can be measured using SQUID (superconducting quantum interference device) magnetometers. Early research concentrated on the human heart, but technological advances have enabled magnetic fields generated by brain activity to be recorded. This is a remarkable achievement as the strength of the magnetic field emerging from the human brain is about seven orders of magnitude smaller than that in a typical urban environment. This technique is called MEG. The measured magnetic fields reflect neural activity. This is a clear advantage over PET and fMRI, which only provide an indirect measure of neural activity as both methods rely upon changes in local blood flow for functional imaging. The other main advantages of MEG are: (1) brain activity can be resolved on a time scale that is compatible with human thought processes; and (2) as magnetic fields are little distorted by brain tissue or bone, its main advantage over electrical measures (VEP's) is in determining the site of cortical activity. We have used MEG to examine the extent to which the primary visual cortex (area V1) is affected in amblyopia ('lazy eye'). A large volume of animal work suggests that V1 is affected in amblyopia, yet recent PET and fMRI studies on humans suggest that it is not. Our MEG evidence is clear: neural activity in area V1 is reduced in individuals with amblyopia. Reasons for the discrepancy between the PET/fMRI data and the MEG data will be discussed.