Optical and neural contribution on the visual function in mesopic and photopic adaptations

Abstract

We study the relative contribution of optical and neural factors on contrast sensitivity for mesopic to photopic lighting. MTFs were determined from double-pass images on a group of seven young normal vision subjects and the calculation of the neural transferfunction (NTF) was performed by mean of amodel that includes the photon noise, the neural noise and the lateral inhibition. In order to analyze the changes in visual sensitivity resulting from lighting, the areas under the curves (AUC) of both the MTFs,NTFs and CSFs were computed for seven lighting levels (0.005, 0.025, 0.5, 5, 35, 320, and 950cd/m2). Besides, Strehl ratio, MTF cut-off frequency and CSF cut-off frequency were determined. The NTFAUC as a function of luminance shown the steepest slope in the mesopic range and then flattening towards the maximum measured luminance for the photopic range, while the MTFAUC shows small variations in the mesopic and undergoes a sustained increase in the photopic. The variation of MTF and CSF cut-off frequencies shown that, for the mesopic range, the MTF cut-off frequencies are above to the CSF cut-off frequencies suggesting the optical response is limited by the neural processing but, in the photopic range, both curves continue to grow reaching a coincident response suggesting the neural response account for the high spatial resolution provides by the optics of the eye.