Two different streams for
object vision and spatial vision were first inferred by contrasting
the effects of inferior temporal and posterior parietal lesions
in macaque monkeys (Ungerleider and Mishkin, 1982).
Since this early work, multiple visual cortical areas in
both monkeys and humans have been shown to be organised into
two functionally and anatomically distinct streams - a ventral
stream projecting to the inferior temporal cortex for object
perception, and a dorsal stream connected to the parietal
cortex for visuospatial control of movement. In an admittedly
simplified way of summarising the clinical and experimental
data one may attribute to the ventral stream the processing
of "what" and to the dorsal stream the processing
of "where" and "how".
Neuropsychologists accepted willingly the Ungerleider and
Mishkin proposal. Indeed, since the beginning of the 20th
century, the neuropsychological literature has distinguished
syndromes related to occipito-parietal lesions (Balint's syndrome,
Holmes syndrome) and those related to occipito-temporal lesions
(visual agnosia, prosopagnosia, alexia).
Some questions may arise from the relative independence
of these two postulated streams: is object vision possible
without any spatial vision? In other words, how to identify
complex shapes or objects without analysing the spatial relationships
between their different parts? Functional neuroimaging evidence
in any case indicates that visual object processing takes
place in both the dorsal and the ventral streams (e.g. Kraut,
1997).
Milner and Goodale (1993) have argued that the “What-Where”
model cannot account for some new behavioural data. Their
neuropsychological investigations have shown that a patient
with visual form agnosia remained able to grasp precisely
objects of different size and orientation, while perceptively
the patient could not appreciate size nor orientation. This
patient was able to process “what” to a limited
degree, but only when acting. In Goodale and Milner’s
proposal (1992), both streams are assumed to use information
about objects and their locations, but each stream, however,
uses this information in different ways.
According to these authors, the ventral stream carries out
transformations concerning characteristics of objects and
their internal spatial relations, therefore allowing the formation
of long-term perceptual representations that are necessary
to identify and recognise objects. In contrast the dorsal
stream carries out transformations which use moment-to-moment
information about objects and their locations in egocentric
frames of reference, thus mediating the visual control of
skilled actions (such as pointing, reaching, and grasping).
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