Towards the single cone transducer function

Abstract

Background. We measured the transducer function (d' and noise vs contrast) with both cone isolating and luminance increments of tiny dots in peripheral vision. We were interested in how the threshold, the multiplicative noise and the saturation point depend on the chromaticity and intensity of the background (specified in cone catch) and the chromaticity and intensity of the test dot. The present experiments were done using a CRT display to develop optimal methodologies for extension to single cone stimulation using the Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) of the Roorda Lab.

Methods. The rating scale method of constant stimuli was used with 6 contrasts going from zero to a stimulus of d' ≈ 4, and with 6 levels of response. The analysis involved 5 response criteria plus two approaches for fitting the psychometric function: a) 5 different d' values with the blank stimulus pegged at d'=0; b) a psychometric function given by d' = (c/th)^b where th is the threshold and b is the transducer exponent.

Results. For a variety of backgrounds the luminance and cone isolating stimuli had very similar thresholds. In addition the transducer exponent was between 1.3 and 1.7 for all conditions.

Discussion. A surprising finding was that the transducer exponent was lower than expected. The methodology being developed will be transferred to the AOSLO where higher test contrasts are possible for exploring the saturation regime of single cones with a substantially greater variety of background chromaticities.