Retinal and extra-retinal contribution to position coding

Though considerable effort has been expended on demonstrating the importance of extraretinal cues in distance perception (e.g. state of vergence), recent studies have shown that enriching the visual image brings about a decrease of perceptual underestimation of distance as observed otherwise, providing that contextual information is situated in the proximal space with regard to target position. The fact that a similar effect was observed when viewing monocularly was suggesting a prevalence of retinal input in distance coding. The present study, investigating reaching movements performed monocularly or binocularly in three successive visual scenes (dark-structured-dark), gave evidence for this assumption. Whatever the vision condition, a dark environment gave rise to an underestimation of target distance, which disappeared instantaneously when a structured background was unexpectedly provided. The sudden return to the dark condition resulted in a progressive drift towards underestimation. These findings strongly suggest that structured retinal information influences widely the perception of target distance. They show in addition that retinal signals may contribute to the calibration of non-retinal sources of information. The putative implication of the posterior parietal cortex in this dual influence is discussed.