Midget retinal ganglion cell surrounds in macaque: cone-selective or not?

Despite decades-long study of macaque midget retinal ganglion cells (mRGC), significant knowledge gaps exist regarding their receptive field (RF) properties. One example is the controversy regarding cone pooling in mRGC surrounds. Anatomy and in-vitro physiology, the latter in peripheral retina, indicate that L- and M-cones contribute non-selectively to mRGC RF surrounds, whereas in-vivo physiology in more central retina indicates that the RF surrounds are highly cone-type selective. To better understand the mRGCs, we developed a model of their linear spatiochromatic RFs. We model the cone inputs to the mRGCs based on anatomical and physiological data, taking into account the impact of physiological optics. Knowledge of these factors allows us to model the mRGC RFs across a large part of the visual field. We use the model to compute responses of synthetic mRGCs to cone-isolating grating and m-sequence stimuli, matched to those that have been employed by in-vivo physiological studies. Simulation enables us to compute the expected in-vivo responses for mRGCs with different surround L- to M-cone ratios. We perform the simulations over a range of eccentricities, taking into account the eccentricity dependence of the physiological optics, the cone fundamentals used to derive cone-isolating stimuli, and the mRGC RF structure. Our results reveal that near the fovea, where centers receive one or two cone inputs, physiological optics significantly enlarges the stimulus-referred RF center, thereby attenuating the antagonistic responses from surround-cones of the same type as the center cone. For this reason, the surround measured in vivo can appear heavily biased toward selective pooling of cones of the non-center cone type. In particular, this happens for models in which the simulated RF surrounds draw indiscriminately on L- and M-cones. This phenomenon, which we observed with both m-sequence and drifting grating simulations, provides a plausible explanation for the discrepancy in conclusions across studies.