One purpose of perimetry is to estimate the spatial density of functional retinal neurons in eyes suspected of retinal diseases which kill photoreceptors (e.g. retinitis pigmentosa, macular degeneration) or ganglion cells (e.g. glaucoma, optic neuritis).

Conventional perimetry does not allow for such estimations because there are a variety of mechanisms besides the postulated loss of neurons which could be responsible for the loss of contrast sensitivity measured by this clinical technique. For example, sick neurons may become less responsive and less sensitive before they die. Alternatively, the diseased retina may be noisier, thus increasing the amount of stimulus contrast needed to reach threshold.

Even if neural dropout were the cause of reduced sensitivity on the perimetric test, there is no currently accepted theory which would allow the clinician to draw inferences about functional cell density from increment threshold measurements. However, such a theory does exist for another kind of visual task which is very familiar to clinicians, namely, resolution acuity. The purpose of this lecture is to review this theory and to show how it may be applied to perimetry in order to perform 'functional neuroanatomy' (i.e. determine the number of functioning neurons per degree on the retina) on patients.

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WWWaveTM 1996
World Wide Web automated virtual environment TM 1996
Kevin Haggerty, Indiana University.