Stimulus configuration and the format of the normal sensitivity gradient.

The study investigated the effect of three distinct types of stimulus configuration on the format of the normal sensitivity gradient derived by computer assisted perimetry namely: projected stimuli; light emitting diodes (LED) with the same luminance as the perimeter bowl; and 'black hole' LED stimuli. The study comprised two separate parts: 22 age matched subjects were examined with the Dicon AP3000 and with the Topcon SBP-1000 along the 15 degrees-195 degrees meridian of the visual field of the right eye; a further 22 subjects matched for age and gender were examined along the same meridian in an identical manner with the Dicon AP2025 and with the Humphrey Field Analyser 620. The various stimulus parameters were chosen in order to provide uniformity as far as possible between the instruments. The Topcon evoked greater relative sensitivity than the Dicon at all eccentricities although the rate of change of sensitivity with increase in peripheral angle varied between the two instruments at different locations. Centrally the Dicon profile followed more closely that of Humphrey stimulus size II and beyond 5 degrees that of stimulus size I. The Topcon profile followed that of the Humphrey stimulus size II both centrally and peripherally in spite of being geometrically closer to the size III stimulus. It is proposed that the variations in the sensitivity gradient are not exclusively related to stimulus size and spatial summation; the accommodative stimulus of the 'black hole' LED stimuli, stimulus colour and thresholding strategy may all be contributing factors.