Limitations on position coding imposed by undersampling and univariance.

Position judgements, which are exquisitely precise in the fovea, are markedly degraded in the periphery. In a recent article [Hess & Field (1993) Vision Research, 33, 2663-2670] argue that the poor representation of positional information in peripheral vision is a consequence of uncalibrated spatial disorder of cortical connections rather than due to undersampling of the retinal image. Specifically, Hess and Field argued that if positional uncertainty is due to undersampling, then because of univariance, there should be an associated contrast uncertainty. In this report we show that the univariance model is limited in its generality since: (1) the Hess and Field data in which contrast and position discrimination are decoupled do not preclude undersampling with large univariant filters: (2) aliasing can decouple position from contrast; (3) undersampling or noise at a second stage of processing can lead to selective losses of position information without any degradation of contrast information.