Fixation disparity during reading : Fusion , not suppression

Towards the centre of the human retina there is a small area called the fovea that is responsible for providing very high acuity visual information to the visual system. While visual information is available from areas other than the fovea, it is less rich in detail since acuity falls off very rapidly from the centre of the fovea to the retinal periphery (see Balota & Rayner, 1991). Thus, in order that the human brain might receive high quality visual information, the eyeball must be oriented such that light from the specific point in space that a person wishes to view clearly falls precisely onto the foveal region. This requirement is possible since primates have eyes that are positioned frontally in the skull that can be rotated in three dimensions, the most important of which are the horizontal and vertical dimensions (I will not discuss torsion eye movements in this paper, as ordinarily during upright reading and scene viewing, they are not instrumental in bringing the eyes to fixate objects in space).