Two Streams hypothesis
Encyclopedia
The two-streams hypothesis is a widely accepted, but still controversial, account of visual processing. As visual information exits the occipital lobe
, it follows two main channels, or "streams". The ventral stream (also known as the "what pathway") travels to the temporal lobe
and is involved with object identification. The dorsal stream (or, "where pathway") terminates in the parietal lobe
and process spatial locations.
The hypothesis was originally proposed by Leslie Ungerleider
and Mortimer Mishkin in 1982 and reviewed by George Ettlinger in 1990. An influential early review on the subject was published by Melvyn Goodale and David Milner in 1992.
However, recent experimental work has undermined the empirical basis of the dissociation, and a recent review has concluded that it is "difficult if not impossible to test" and that, contrary to the two-streams hypothesis, a more "general and parsimonious one posits the existence of one single processing stream."
The dorsal stream is proposed to be involved in the guidance of actions and recognizing where objects are in space. Also known as the parietal stream, the "where" stream, or the "how" stream, this pathway 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 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".
It contains individually functioning lobules, one area of which contains neurons that produce enhanced activation when attention is moved onto the stimulus or the animal saccade
s towards to a visual stimulus (LIP), and another section where visual and somatosensory information are integrated (VIP).
), the limbic system
(which controls emotions), and the dorsal stream (which deals with object locations and motion).
The ventral stream gets its main input from the parvocellular
(as opposed to magnocellular
) layer of the lateral geniculate nucleus
of the thalamus
. These neurons project to V1 sublayers 4Cβ, 4A, 3B and 2/3a successively. From there, the ventral pathway goes through V2 and V4 to areas of the inferior temporal lobe
: PIT (posterior inferotemporal), CIT (central inferotemporal), and AIT (anterior inferotemporal). Each visual area contains a full representation of visual space. That is, it contains neurons whose receptive fields together represent the entire visual field. Visual information enters the ventral stream through the primary visual cortex and travels through the rest of the areas in sequence.
Moving along the stream from V1 to AIT, receptive fields increase their size, latency, and the complexity of their tuning.
All the areas in the ventral stream are influenced by extraretinal factors in addition to the nature of the stimulus in their receptive field. These factors include attention
, working memory
, and stimulus salience
. Thus the ventral stream does not merely provide a description of the elements in the visual world—it also plays a crucial role in judging the significance of these elements.
may distort judgements of a perceptual nature, but when the subject responds with an action, such as grasping, no distortion occurs. However, recent work suggests that both the action and perception systems are equally fooled by such illusions.
Occipital lobe
The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...
, it follows two main channels, or "streams". The ventral stream (also known as the "what pathway") travels to the temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
and is involved with object identification. The dorsal stream (or, "where pathway") terminates in the parietal lobe
Parietal lobe
The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...
and process spatial locations.
The hypothesis was originally proposed by Leslie Ungerleider
Leslie Ungerleider
Leslie Ungerleider is an experimental psychologist and neuroscientist, currently Chief of the Laboratory of Brain and Cognition at the National Institute of Mental Health...
and Mortimer Mishkin in 1982 and reviewed by George Ettlinger in 1990. An influential early review on the subject was published by Melvyn Goodale and David Milner in 1992.
However, recent experimental work has undermined the empirical basis of the dissociation, and a recent review has concluded that it is "difficult if not impossible to test" and that, contrary to the two-streams hypothesis, a more "general and parsimonious one posits the existence of one single processing stream."
Dorsal stream
Occipital lobe
The occipital lobe is the visual processing center of the mammalian brain containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1...
forward into the parietal lobe
Parietal lobe
The parietal lobe is a part of the Brain positioned above the occipital lobe and behind the frontal lobe.The parietal lobe integrates sensory information from different modalities, particularly determining spatial sense and navigation. For example, it comprises somatosensory cortex and the...
. It is interconnected with the parallel ventral stream (the "what" stream) which runs downward from V1 into the temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
.
General features
The dorsal stream is involved in spatial awareness and guidance of actions (e.g., 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 analyzing 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".
It contains individually functioning lobules, one area of which contains neurons that produce enhanced activation when attention is moved onto the stimulus or the animal saccade
Saccade
A saccade is a fast movement of an eye, head or other part of an animal's body or device. It can also be a fast shift in frequency of an emitted signal or other quick change. Saccades are quick, simultaneous movements of both eyes in the same direction...
s towards to a visual 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:- SimultanagnosiaSimultanagnosiaSimultanagnosia is a rare neurological disorder characterized by the inability of an individual to perceive more than a single object at a time. It is one of three major components of Bálint’s syndrome, an uncommon and incompletely understood variety of severe neuropsychological impairments...
: where the patient can only describe single objects without the ability to perceive it as a component of a set of details or objects in a context (as in a scenario, e.g. the forest for the trees). - Optic ataxia: where the patient can't use visuospatial information to guide arm movements.
- Hemispatial neglectHemispatial neglectHemispatial neglect, also called hemiagnosia, hemineglect, unilateral neglect, spatial neglect, unilateral visual inattention, hemi-inattention or neglect syndrome is a neuropsychological condition in which, after damage to one hemisphere of the brain, a deficit in attention to and awareness of...
: where the patient is unaware of the contralesional half of space (that is, they are unaware of things in their left field of view and focus only on objects in the right field of view; or appear unaware of things in one field of view when they perceive them in the other). For example, a person with this disorder may draw a clock, and then label it from 12, 1, 2, ..., 6, but then stop and consider their drawing complete. - AkinetopsiaAkinetopsiaAkinetopsia, also known as cerebral akinetopsia or motion blindness, is an extremely rare neuropsychological disorder in which a patient cannot perceive motion in their visual field, despite being able to see stationary objects without issue. For patients with akinetopsia, the world becomes devoid...
: inability to perceive motion. - ApraxiaApraxiaApraxia is a disorder caused by damage to specific areas of the cerebrum. Apraxia is characterized by loss of the ability to execute or carry out learned purposeful movements, despite having the desire and the physical ability to perform the movements...
: inability to produce discretionary or volitional movement in the absence of muscular disorders.
Ventral stream
The ventral stream is associated with object recognition and form representation. It has strong connections to the medial temporal lobe (which stores long-term memoriesLong-term memory
Long-term memory is memory in which associations among items are stored, as part of the theory of a dual-store memory model. According to the theory, long term memory differs structurally and functionally from working memory or short-term memory, which ostensibly stores items for only around 20–30...
), the limbic system
Limbic system
The limbic system is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly support a variety of functions including emotion, behavior, long term memory, and olfaction. The term "limbic" comes from the Latin...
(which controls emotions), and the dorsal stream (which deals with object locations and motion).
The ventral stream gets its main input from the parvocellular
Parvocellular
Parvocellular can refer to:* Parvocellular cell, located in the lateral geniculate nucleus* Parvocellular pathway of the visual system* Parvocellular neurosecretory cell* Parvocellular red nucleus* Parvocellular reticular nucleus...
(as opposed to magnocellular
Magnocellular
Magnocellular can refer to:* Magnocellular cell, part of visual system* Magnocellular red nucleus* Magnocellular neurosecretory cell* Magnocellular pathway* Magnocellular theory of dyslexia...
) layer of the lateral geniculate nucleus
Lateral geniculate nucleus
The lateral geniculate nucleus is the primary relay center for visual information received from the retina of the eye. The LGN is found inside the thalamus of the brain....
of the thalamus
Thalamus
The thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
. These neurons project to V1 sublayers 4Cβ, 4A, 3B and 2/3a successively. From there, the ventral pathway goes through V2 and V4 to areas of the inferior temporal lobe
Temporal lobe
The temporal lobe is a region of the cerebral cortex that is located beneath the Sylvian fissure on both cerebral hemispheres of the mammalian brain....
: PIT (posterior inferotemporal), CIT (central inferotemporal), and AIT (anterior inferotemporal). Each visual area contains a full representation of visual space. That is, it contains neurons whose receptive fields together represent the entire visual field. Visual information enters the ventral stream through the primary visual cortex and travels through the rest of the areas in sequence.
Moving along the stream from V1 to AIT, receptive fields increase their size, latency, and the complexity of their tuning.
All the areas in the ventral stream are influenced by extraretinal factors in addition to the nature of the stimulus in their receptive field. These factors include attention
Attention
Attention is the cognitive process of paying attention to one aspect of the environment while ignoring others. Attention is one of the most intensely studied topics within psychology and cognitive neuroscience....
, working memory
Working memory
Working memory has been defined as the system which actively holds information in the mind to do verbal and nonverbal tasks such as reasoning and comprehension, and to make it available for further information processing...
, and stimulus salience
Salience
Salience or saliency may refer to:* Salience , the state or quality of an item that stands out relative to neighboring items* Salience , relative importance or prominence of a piece of a sign...
. Thus the ventral stream does not merely provide a description of the elements in the visual world—it also plays a crucial role in judging the significance of these elements.
Problems with the hypothesis
The dichotomy of the dorsal/ventral pathways is still contentious among scientists. The neuroscience community is still not in agreement regarding the degree to which these streams are segregated (they are, in fact, heavily interconnected) or the functional significance which should be attached to them. The hypothesis is probably an over-simplification of the true state of affairs in the visual cortex and a recent review concluded that the hypothesis is "difficult if not impossible to test."Evidence from illusions
Much support comes from the findings that visual illusions such as the Ebbinghaus illusionEbbinghaus illusion
The Ebbinghaus illusion or Titchener circles is an optical illusion of relative size perception. In the best-known version of the illusion, two circles of identical size are placed near to each other and one is surrounded by large circles while the other is surrounded by small circles; the first...
may distort judgements of a perceptual nature, but when the subject responds with an action, such as grasping, no distortion occurs. However, recent work suggests that both the action and perception systems are equally fooled by such illusions.