Introduction

The electroretinogram (ERG) and the visual evoked potential (VEP) are massed electrical potentials, the result of the summed electrical activity of the cells of the retina (ERG) and the cells in the occipital cortex (VEP). ERGs and VEPs have been recorded in both the clinic and the laboratory to study the normal and abnormal activity of the retina and visual pathways. Chapters 12 and 15 describe the basis of the ERG and VEP, and the standard clinic tests are described in chapter 20. However, it is often desirable to study the local electrical activity of the retina or optic nerve. Conventional ERG and VEP tests involve stimuli that stimulate relatively large areas of the retina and therefore do not easily lend themselves to the study of localized activity. With the multifocal ERG and VEP techniques,^40,43 local ERG or VEP responses can be recorded simultaneously from many regions of the visual field. This chapter provides an introduction to these techniques.