ACTIVE VISION
Experimental psychologists Iain Gilchrist and John Findlay (2003: 1-6) champion a paradigm for vision, based on coupling between the structural “inhomogeneity of the retina” and the ubiquity of eyeball movements. Previously these striking phenomena had been regarded as “incidental” rather than fundamental features of the visual system. Their “viewpoint is not widely current.” They note that “[m]any texts on vision do not even mention that the eye can move.” They declare that:
Active vision takes as its starting point the inhomogeneity of the retina, seeing the fovea not simply as a region of high acuity, but as the location at which visual activity is centred.
It has been calculated that a hypothetical human retina supporting the concentration of cone cells actually found the fovea would require a brain visual cortex “weighing perhaps ten tons.” Taking an evolutionary approach, Gilchrist and Findlay (2003: 5) hold that:
A mobile eye constructed on the principle of the vertebrate eye is not a co-incidence or a luxury but is very probably the only way in which a visual system can combine high resolution with the ability to monitor the whole visual field.
John M. Findlay and Iain D. Gilchrist (2003) Active Vision: The Psychology of Looking and Seeing. Oxford University Press. Oxford, UK.
A fool sees not the same tree that a wise man sees.
William Blake
The Marriage of Heaven and Hell. From Plate VII: Proverbs of Hell (1789-1790)
DYNAMIC EYEBALLS
Our synchronized eyeballs shift in their sockets relentlessly. Stimuli of interest are captured by peripheral vision and processed at lightning speed to determine the precise trajectory of saccadic shifts. Saccadic movements place selected points of interest precisely in line with the foveae, the tiny portions of each retina which provide visual acuity.
The spherical shape of the eyeballs provides maximum maneuverability. The protruding corneas provide the correct focal lengths without compromising the spherical structure of the embedded portions of the eyes. (The corneas are responsible for about 70% of focusing.) Again, mostly in service of optimizing fixation at the foveae, the antagonistic muscles of the irises and the ciliary muscles of the lenses also make constant fine adjustments. The irises optimize the amount of incoming light by adjusting the pupil apertures. The ciliary muscles stretch the lenses (which in their relaxed states resemble transparent jelly-filled bags) to refract incoming light patterns to the foveae. Focusing by adjusting the shape of the lenses is known as accommodation.
Department of Experimental Psychology, University of Bristol.
Centre for Vision and Visual Cognition, Dept of Psychology, University of Durham.
