9 - The Human Visual System

Abstract

Brain mapping techniques have made significant inroads into understanding the mechanisms of human vision. Both positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) have defined loci and networks in the brain, but structural imaging can still play an important part. Originally, gadolinium contrast injection was used to obtain a detectable fMRI signal, but now it is almost universal to rely on the so-called blood oxygen level dependent technique. When regional cerebral blood flow (rCBF) increases, the local concentration of deoxygenated hemoglobin decreases, and this small change is detected by the magnetic resonance scanner. Radiochemists are able to label various tracers with PET isotopes and thus allow a vast array of functional imaging approaches. These include measures of local metabolism, blood flow and blood volume, the determination of presynaptic transmitter concentration, postsynaptic receptor density, and the site of action of drugs. Almost all the work on the visual system has been done by measuring metabolism or rCBF. The techniques of event-related potential, magnetoencephalography, and transcranial magnetic stimulation allow information on the fine timing of cerebral events to be obtained. Each method has strengths that suit some experimental questions more than others, but they all should be thought of as complementing each other.