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The dorsal stream is a pathway for visual information which flows through the visual cortex, the part of the brain which provides visual processing. It is involved in spatial awareness: recognizing where objects are in space. The dorsal stream is one of two main pathways of the visual cortex, the other being the ventral stream.
Where it is located
The dorsal stream (also known as the parietal stream or the "where" stream) stretches from the primary visual cortex (V1) in the occipital lobe forward into the parietal lobe. It is interconnected with the parallel ventral stream (the "what" stream) which runs downward from V1 into the temporal lobe. The dorsal stream was first defined by Ungerleider and Mishkin (In: Ingle DJ, Goodale MA and Mansfield RJW (Editors), Analysis of Visual Behavior. MIT Press, Boston, 1982). An influential early review on the subject was published by Goodale and Milner in 1992 (Trends Neurosci. 15(1):20-5).
The neuroscience community is still not in agreement regarding the degree to which these streams are segregated (they are interconnected) or the functional significance which should be attached to them.
The dorsal stream is involved in spatial awareness and guidance of actions i.e. reaching. In this it has two distinct functional characteristics-it contains a detailed map of the visual field, and is also good at detecting and analysing movements.
The dorsal stream commences with purely visual functions in the occipital lobe before gradually transferring to spatial awareness at its termination in the parietal lobe.
The posterior parietal cortex is essential for, "the perception and interpretation of spatial relationships, accurate body image, and the learning of tasks involving coordination of the body in space" (from: Neuroscience: Exploring the brain, Bear, Connors, Paradiso).
It contains individually functioning lobules, one area of which contains neurons that produce enhanced activation when attention is moved onto the stimulus (LIP), and another section where visual and somatosensory information are integrated (VIP).
Effects of damage or lesions
Damage to the posterior parietal cortex causes a number of spatial disorders including:
- Simultanagnosia: where the patient can't focus on more than one object at a time.
- Optic ataxia: where the patient can't use visiospatial information to guide arm movements.
- Hemispatial neglect: where the patient is unaware of the contralesional half of space.
- Akinetopsia: inability to perceive motion
- Apraxia: inability to do discretionary or volitional movement in the absence of muscular disorders.
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