Two Visual Systems
Goodale and Milner amassed an array of anatomical, neuropsychological, electrophysiological, and behavioural evidence for their model, according to which the ventral ‘perceptual’ stream provides the rich and detailed representation of the visual world required for cognitive operations whereas the dorsal ‘action’ stream transforms incoming visual information into the required coordinates for skilled motor behaviour. The model also posits that visual perception encodes the object properties (e.g. size) relative to the properties of other visible objects; in other words it utilises relative metrics and scene-based frames of reference. Whereas the visual control of action uses absolute metrics determined via egocentric frames of reference, computing the actual properties of objects relative to the observer. Thus, grasping movements directed towards objects embedded in size-contrast illusions have been shown to escape the effects of these pictorial illusions, as different frames of references and metrics are involved in the production of the illusion versus production of the grasping act.
Norman proposed a similar dual-process model of vision, and described eight main differences between the two systems consistent with other two-system models.
| Factor | Ventral system | Dorsal system |
|---|---|---|
| Function | Recognition/identification | Visually guided behaviour |
| Sensitivity | High spatial frequencies - details | High temporal frequencies - motion |
| Memory | Long term stored representations | Only very short-term storage |
| Speed | Relatively slow | Relatively fast |
| Consciousness | Typically high | Typically low |
| Frame of reference | Allocentric or object-centered | Egocentric or viewer-centered |
| Visual input | Mainly foveal or parafoveal | Across retina |
| Monocular vision | Generally reasonably small effects | Often large effects e.g motion parallax |
Read more about this topic: Two-streams Hypothesis
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