Do Intense Perimetric Stimuli Saturate the Healthy Visual System?

PURPOSE

A recent proposal for why glaucomatous perimetric sensitivities of approximately 15 to 19 dB or less are unreliable involves the idea that the neural response of normal retinal ganglion cells saturates for intense perimetric stimuli. A predicted consequence of this saturation is that the neural response for two different high intensity stimuli will be the same, leading to an inability to discriminate between them. We test that prediction.

METHODS

We used a two-interval forced-choice method of constant stimuli (7 steps, 40 presentations/step) to measure the ability of four healthy observers to discriminate between different intensity Size III perimetric stimuli at 0°, 9°, and 21° eccentricity. The lower intensity stimulus for each discrimination was either 27, 23, 19, 15, or 11 dB (Humphrey Field Analyzer equivalents).

RESULTS

Foveally, discrimination performance exceeded 90% for all observers provided the more intense stimulus was made sufficiently intense, even if the lower intensity stimulus in the pair was itself already intense (≤19 dB). The shapes of the curves were similar across all lower intensity stimulus values investigated. At 21°, discrimination performance exceeded 90% in three of the four observers despite the lower intensity stimulus being 19 dB.

CONCLUSIONS

Observers can reliably discriminate between two different, but both very intense, perimetric stimuli, indicating that responses of the human visual system are not saturated by such stimuli. Therefore, the cause of high perimetric test-retest variability is not readily predicted from our current knowledge of how normal ganglion cells respond to high intensity stimuli.