Two-streams hypothesis
The two-streams hypothesis is a model of the neural processing of vision as well as hearing. The hypothesis, given its initial characterisation in a paper by David Milner and Melvyn A. Goodale in 1992, argues that humans possess two distinct visual systems. Recently there seems to be evidence of two distinct auditory systems as well. As visual information exits the occipital lobe, and as sound leaves the phonological network, it follows two main pathways, or "streams". The ventral stream leads to the temporal lobe, which is involved with object and visual identification and recognition. The dorsal stream leads to the parietal lobe, which is involved with processing the object's spatial location relative to the viewer and with speech repetition.
History
Several researchers had proposed similar ideas previously. The authors themselves credit the inspiration of work on blindsight by Weiskrantz, and previous neuroscientific vision research. Schneider first proposed the existence of two visual systems for localisation and identification in 1969. Ingle described two independent visual systems in frogs in 1973. Ettlinger reviewed the existing neuropsychological evidence of a distinction in 1990. Moreover, Trevarthen had offered an account of two separate mechanisms of vision in monkeys back in 1968.In 1982, Ungerleider and Mishkin distinguished the dorsal and ventral streams, as processing spatial and visual features respectively, from their lesion studies of monkeys – proposing the original where vs what distinction. Though this framework was superseded by that of Milner & Goodale, it remains influential.
One hugely influential source of information that has informed the model has been experimental work exploring the extant abilities of visual agnosic patient D.F. The first, and most influential report, came from Goodale and colleagues in 1991 and work is still being published on her two decades later. This has been the focus of some criticism of the model due to the perceived over-reliance on findings from a single case.
Two visual systems
Goodale and Milner amassed an array of anatomical, neuropsychological, electrophysiological, and behavioural evidence for their model. According to their data, the ventral 'perceptual' stream computes a detailed map of the world from visual input, which can then be used for cognitive operations, and the dorsal 'action' stream transforms incoming visual information to the requisite egocentric coordinate system for skilled motor planning.The model also posits that visual perception encodes spatial properties of objects, such as size and location, relative to other objects in the visual field; in other words, it utilizes relative metrics and scene-based frames of reference. Visual action planning and coordination, on the other hand, 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-ambiguous scenes have been shown to escape the effects of these illusions, as different frames of references and metrics are involved in the perception of the illusion versus the execution 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 |
Dorsal stream
The dorsal stream is proposed to be involved in the guidance of actions and recognizing where objects are in space. The dorsal stream projects from the primary visual cortex to the posterior parietal cortex. It was initially termed the "where" pathway since it was thought that the dorsal stream processes information regarding the spatial properties of an object. However, later research conducted on a famous neuropsychological patient, Patient D.F., revealed that the dorsal stream is responsible for processing the visual information needed to construct the representations of objects one wishes to manipulate. Those findings led the nickname of the dorsal stream to be updated to the "how" pathway. The dorsal stream is interconnected with the parallel ventral stream which runs downward from V1 into the temporal lobe.General features
The dorsal stream is involved in spatial awareness and guidance of actions. 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. The lateral intraparietal sulcus contains neurons that produce enhanced activation when attention is moved onto the stimulus or the animal saccades towards a visual stimulus, and the ventral intraparietal sulcus where visual and somatosensory information are integrated.
Effects of damage or lesions
Damage to the posterior parietal cortex causes a number of spatial disorders including:- Simultanagnosia: 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.
- Optic ataxia: where the patient cannot use visuospatial information to guide arm movements.
- Hemispatial neglect: where the patient is unaware of the contralesional half of space. For example, a person with this disorder may draw a clock, and then label all twelve of the numbers on one side of the face and consider the drawing complete.
- Akinetopsia: inability to perceive motion.
- Apraxia: inability to produce discretionary or volitional movement in the absence of muscular disorders.
Ventral stream
The ventral stream gets its main input from the parvocellular 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, CIT, and AIT. 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. For example, recent studies have shown that the V4 area is responsible for color perception in humans, and the V8 area is responsible for shape perception, while the VO2 area, which is located between these regions and the parahippocampal cortex, integrates information about the color and shape of stimuli into a holistic image.
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.
Damage to the ventral stream can cause inability to recognize faces or interpret facial expression.
Two auditory systems
Auditory ventral stream
The two streams hypothesis of audition was largely proposed mainly by Hickok and Poeppel in 2004, from which they have continued to publish empirical evidence and updated reviews for this model. This comes off the heels of the visual two streams hypothesis, building on the idea that cortical structure and processing in both different areas of the brain as well as systemically merging perceptual circuitry with motor coordination and output to “match sound onto meaning and matching sound onto articulatory-based representations.” Hickok and Poeppel also theorize that there is a possibility both dorsal and ventral streams have a level of “bidirectionality” – meaning that both pathways can function in a variety of ways with and apart from each other to facilitate speech perception and production.Along with the visual ventral pathway being important for visual processing, there is also a ventral auditory pathway emerging from the primary auditory cortex. In this pathway, phonemes are processed posteriorly to syllables and environmental sounds. The information then joins the visual ventral stream at the middle temporal gyrus and temporal pole. Here the auditory objects are converted into audio-visual concepts.The ventral system is colloquially referred to as the “what” pathway which is due to the function of this pathway largely existing to merge sensory stimuli into digestible information for linguistic information. The ventral system is able to do this due to its focus on lexical interface – this interface consists of clusters of words in and out of context and their meanings as well as syntactic changes. Based on this, through several projections in the brain specifically to the Superior Temporal Gyrus, the ventral pathway helps to “map sound onto meaning.”