Photoreceptors controlling the sight (i.e. cones and rods) are located in the retina, together with the ipRGC (intrinsically photosensitive retinal ganglion cells). Unlike cones and rods, the ipRGC do not influence the sight but regulate the circadian rhythm. Since the spectral sensitivities of these photoreceptors partially overlap, one of the main challenges is to properly stimulate only the photoreceptors of interest, i.e. ipRGC.
By exploiting the silent substitution technique, the cones stimulation can be kept constant, while varying the ipRGCs one. Furthermore, by suitably controlling the environmental illumination conditions, the rods stimulation can be kept constant, too. Indeed, ipRGCs receive synaptic inputs from cones and rods and in the absence of those inputs, these cells themselves are photosensitive, due to the expression of the melanopsin photopigment in the cell membrane.
The study of the effect of mammalian ipRGCs photoreceptors on the pupillary light reflex is nowadays a hot research topic. Moreover, up to our knowledge there is a lack on the developement of suitable instrumentation to properly address the latter.
In this work a new instrument as been designed to evaluate the effect of the metameric stimulation by measuring pupil variations. In nature, there exist colors having different spectral components but equally perceived by the human eyes, namely metamers. In particular, they are characterized by the same RGB coordinates. By exploiting the aforementioned prototype and stimulating the subject's eye with different metamers alternated with different frequencies, the pupil dimension variation has been evaluated. Several problems have been faced, like the study of the suitable components, such as the surface the light source impinges in and its reflectance, the light source itself (i.e. LEDs and their distribution), together with the appropriate drivers, the microcontroller and the camera. The LabVIEW environment has been employed, to develop an easy to use interface.The experimental sessions revealed that, considering a long-term observation, pupil constriction occurs. This result matches the expectations, as the ipRGCs are much less light-sensitive than the other photoreceptors. A secondary effect derives from a tollerable approximation in the synthesis of metamers, due to an instrinsically difficult computation of colors. This yields to minimal yet inevitable stimulation of cones, that express in sudden variation of the pupil diameter.