Do Intense Perimetric Stimuli Saturate the Healthy Visual System?

Area of Science:

• Ophthalmology
• Neuroscience
• Visual Psychophysics

Background:

• Glaucomatous perimetry results below 15-19 dB sensitivity are often unreliable.
• A proposed explanation is neural saturation of retinal ganglion cells by intense stimuli.
• This saturation predicts an inability to discriminate between high-intensity stimuli.

Purpose of the Study:

• To test the prediction that neural responses saturate for intense perimetric stimuli.
• To determine if healthy observers can discriminate between high-intensity visual stimuli.

Main Methods:

• Used a two-interval forced-choice method with constant stimuli.
• Tested four healthy observers on discriminating different intensity Size III perimetric stimuli.
• Stimuli were presented at 0°, 9°, and 21° eccentricity with varying lower intensities (11-27 dB).

Main Results:

• Foveal discrimination exceeded 90% for all observers with sufficiently intense stimuli, even when the lower stimulus was 19 dB.
• Discrimination performance curves were consistent across different lower stimulus intensities.
• At 21° eccentricity, three out of four observers achieved over 90% discrimination with a 19 dB lower stimulus.

Conclusions:

• Healthy observers can reliably discriminate between two different, intense perimetric stimuli.
• This indicates the human visual system's responses are not saturated by such stimuli.
• The cause of high perimetric test-retest variability is not explained by current understanding of ganglion cell responses to intense stimuli.