The hypothesis that there are two streams of visual information appears to have arisen from studies carried out in the late 1960s. One set of studies, carried out by Gerald E. Schneider, and published in 1967 and 1969, was based on research with hamsters in which removal of the visual cortex (areas 17 and 18) resulted in an inability to discriminate patterns, but with the ability to orientate toward objects remaining intact. In contrast, separating the superior colliculus from the visual cortex led to the reverse effect. These findings led to the conclusion that the hamster had two visual systems: one providing a response to the question of ‘What is it’?‚ and the other resolving the question of ‘Where is it?”. These became known as the cortical focal system (for object vision via foveal vision) and the sub-cortical ambient system (for guiding behaviour such as locomotion via peripheral vision) following the work of Colwyn Trevathen in 1968 with split-brain baboons. Further refinements to the hypothesis came in 1982 with the distinction between ‘what’ and ‘where’ streams put forward by Leslie G. Ungerleider and Mortimer Mishkin, with the origin of the former being assigned to the inferotemporal cortex and the latter to the posterior parietal cortex. Some ten years later, this distinction was transformed by David Milner and Mervyn Goodale into a ventral stream for the perception of ‘what’ and a dorsal stream dedicated to resolving the issue of ‘how’ based on their work with patient D.F. who suffered from visual form agnosia and had severe problems in recognizing visual shapes (thus indicating damage to ventral stream), but no problems when required to reach out and grasp them (thus implying an intact dorsal stream, and thereby a dissociation between the two streams). Particularly in cognitive neuroscience, this distinction has led to a large amount of both animal and human research to examine the functional significance of these two streams in intact visual systems in terms of whether they are segregated or in some way work in cooperation in the coupling between perception and action. So far, there is a lack of consensus about these issues, but a growing trend to see them as functionally connected. Developmentally, an issue has been the order in which the two streams become functional, but more importantly how they become integrated in the performance of goal-directed actions. Thus, how does a ‘vision-for perception’ system specialized for identification of objects (viz., the ventral stream) co-develop with a ‘vision-for-action’ system involved in controlling actions to objects arranged in an egocentric spatial framework (viz., the dorsal stream)? As yet, there are no clear-cut answers.
See Allocentric search, Cognitive neuroscience, Common coding, Dorsal visual pathway (or stream), Entorhinal cortex, Inferior parietal cortex (IPL), Inferior temporal cortex (ITC), Lumping (versus splitting), Object identity, Occipital cortex (or lobe), Parietal cortex, Perception-action coupling, Perception, Primary visual cortex (V1), Superior colliculus, Temporal lobe, Ventral visual pathway (or stream), Visual cortex