Perimetry and visual psychophysics

Patterns of visual loss are the key to anatomic localization. Strictly monocular defects map to the prechiasmal sensory visual system. Bitemporal hemianopia is the signature of damage to the optic chiasm. Incongruous homonymous hemianopia points to an optic tract lesion. The closer a lesion gets to the occipital cortex, the more congruous or similar are the patterns of homonymous hemianopia. Understanding these patterns will add an important dimension to the clinician's localization arsenal. Perimetry is the quantitation of the visual field. This is done with a perimeter and usually involves estimating visual thresholds with a range of light stimuli. It is used clinically to map patterns of visual loss due to damage to the sensory visual system. This chapter discusses the types of perimetric testing and the underlying psychophysics of perimetry. This is followed by the interpretation of perimetric results, the relevant visual system anatomy, and patterns of loss helpful for neurologic localization. Lastly, the difficult issue of deciding whether the visual field has changed is reviewed.