Hypothesis of Two Visual Pathways
HYPOTHESIS OF TWO VISUAL PATHWAYS 10
Hypothesisof Two Visual Pathways
Hypothesisof Two Visual Pathways
Accordingto Milner and Goodale’s two visual pathways hypothesis, the neuralprocesses in charge of controlling the visuomotor movement areindependent of the ones that represent consciousness. The researchersasserted that the dorsal processing stream controls the sensorimotoraction while the ventral stream controls conscious awareness. Forexample, the ventral system helps a person to visualise a cup that isin front of him or her. The brain section helps an individual todetermine the shape, colour, handle, and other characteristics thatthe cup have. On the other hand, the dorsal stream enables a personto reach out and seize the cup, as well as control it as they wish.Milner and Goodale claim that two brain sections operateindependently, despite the fact that their functions areinterconnected (Coello & Magne, 2000).
Theideology that humans have a single visual system started beingchallenged by researchers back in the 1960s. Such a hypothesis backthen was revolutionary since scientists had believed that the humanvisual system was a single unit for many years. Nonetheless, furtherstudies in the field have proven that scientists only assumed thathumans had a single visual system, but that was not true (Coello &Iwanow, 2006). The first research to prove the hypothesis focused oninvestigating the functions of the superior colliculus in controllingvarious motor performances in rodents back in the 1970s (Goodale &Milner, 1982). According to Goodale and Milner, this was their firstjoint research. Most of these pioneer studies were conducted onprimates in the laboratory. A series of tests that extended from1980s to 1990s confirmed that the primates had a pair of visualstreams.
Oneof the supporting evidence Milner and Goodale gave to support thishypothesis is the fact that people with brain injuries can skillfullycontrol objects even though they are not able to experience thefeatures of the objects consciously, hence they cannot manipulate theobjects comprehensively (Carey, 2001). Additional data is coming inthe form of given visual illusions. It is possible to manipulate anillusion such that an individual appears to have a single visionhowever, in case he or she acts, the action will not be similar tothat of an individual who has not experienced an illusion. Thisdifference can be explained by the fact that a person experiences anobject as it is since only the consciousness was deceived, but thesensorimotor is not influenced.
Oncethe visual information gets out of the occipital lobe, it travelsthrough a couple of major pathways (Bautista & Korienek, 1999).The ventral pathway (also called the “what pathway”) passesthrough the temporal lobe and it assists in the recognition andidentification of the seen object. On the other hand, the dorsalstream (also called the “where pathway”) ends in the parietallobe, which helps in determining the spatial location of an object inrelation to the viewer’s position (Carey et al., 1996).
Themajor neuropsychological proof for Milner and Goodale (1992) proposeddetachment of vision for action and vision for perception refers to acollection of distinct behavioural impairment that originates fromfocal lesions and terminates in either the dorsal or ventral streams.The researchers based most of their proof on the evidence they madeon DF, a woman who was suffering from a damaged ventral stream(Clark, 2009). DF suffered from visual form agnosia that was causedby anoxic lesions, which destroyed the lateral occipital (LO) regiontwo-sided (Bruno, 2001). DF was able to use orientation and visualsize efficiently in when directing her hand to reach and graspobjects, although she is incapable to differentiate the propertiesclearly. Karnath et al. (2009) asserts that the spared herdoursalvisual pathway supported her retained visuomotor capability.
Tosupport this view, Milner and Goodale (1992) compared DF’s diseasewith that of patients suffering from optic ataxia. The latterportrays visually inaccurate movement after the SPL damage (Karnath &Perenin, 2005 Perenin & Vighetto, 1988). However, the naturallesions do not follow the standard boundaries set by the anatomists.Majority of the optic ataxic patients, including DF, have incompleteand imperfectly selective lesions of both visual pathways (James etal., 2003). However, gross analysis shows that vision-for-perceptionand vision-for-action are distinctly vulnerable to disruptionresulting from brain injury, thus signifying that they are isolatedin the regular brain (Brenner & Smeets, 1996). Obviously, thevision-for-action and vision-for-perception do combine at some pointconsidering that humans make decisions depending on the things thatthey are able to see consciously, but the streams the messages followbefore they reach the intersection points are different. Thisexplains the reason an individual can have a damagedvision-for-action, but a healthy vision-for-perception.
Accordingto the Milner and Goodale (2008), the difference between planning andprogramming an action is the most essential obstruction betweenventral and dorsal streams that influences behaviour. Thevision-for-perception feeds the awareness and controls actionplanning, though the actions are controlled through the conversion ofafferent visual data into motor commands that act as the realm ofvision-for-action (Bradshaw & Watt, 2002).
Althoughthis scheme seems well designed and set, it has some exceptions andqualifications. One of the exceptions that many critics have used tocounter the reliability of the approach is the fact that not everymetrical parameter of human actions can be designed on a bottom-upbasis (Buneo & Andersen, 2006). Each time a human being lifts anobject, memory cues and visual size control the force at thefingertips (Flanagan & Beltzner, 2000). Mon-Williams & Murray(2000) adds that the visual judgement of the material used forconstructing the object does also determine the force of anindividual will apply in lifting an object. This implies thatprogramming the force required to lift an object does also requirevisual recognition of the material used in constructing it orprevious knowledge of the same object (Beckers & Homberg, 1992).
Anothercriticism that is supporting the dorsal stream does not facilitatethe visual stream include diagnosis from patients suffering fromoptic ataxia. This condition results from dorsal passageway injury(Bruno et al., 2008). Clinical diagnosis for optic ataxianecessitates the exclusion of perceptual deficits when explainingmisreaching that is caused by under visual guidance, though this hasbeen accomplished roughly. More comprehensive analyses thatconcentrate in peripheral visual field in cases where misreaching ishigh have reported spoiled discrimination of object orientation orlocation (Andrews et al., 2002).
Anothercriticism that oppositionists have been using to contend feasibilityof the two-visual pathways is the fact that the physical things thatpeople touch are sometimes different from what one consciously sees(Andersen et al., 1985). In many cases, the actions human take areusually voluntary, as well as directly influenced by the will. On theother hand, the things humans experience consciously usuallydetermine the will. Aglioti et al. (1995) asserts that when Milnerand Goodale (1992) claimed that the object size visual illusion couldhelp the hand in estimating the size and force required to reach outto an object, many vision scientists across the globe begunconducting a chain of experiments that aimed at proving that thehypothesis was false (Mon-Williams & Bull, 2000).
However,Milner and Goodale do not completely object that the two-wayhypothesis is completely free from visual illusions. The researchersagree that some illusions appear very early, during the visualprocessing, such that they influence both action and perception(Bridgeman et al., 1979). The Milner and Goodale model alsoillustrates that illusions originating deep in the ventral passagewaydoes not influence visuomotor dispensation (Bruno & Franz, 2009).If, such a situation occurs, and it occurs many times, it portraysthat what humans often see might not be in charge of their action.
Insummary, biological psychologists claim that behaviors andexperiences of humans are determined by the nervous system activity. Recent biological studies have proven that many things that humansfeel, think, do, and say are to a given extent influenced byelectrochemical activities happening inside and between nervoussystem neurones. Although several biopsychologists concur that thebrain development is influenced to a certain extent by the genes anindividual inherits from his or her family line, they also note thatsome psychological and behavioural characteristics through evolution.David Milner and Melvyn Goodale developed a hypothesis that claimedhumans contain two independent visual systems. The pathways includethe ventral stream that helps in object recognition andidentification. The other pathway is the dorsal stream, whichassists in determining the spatial location.
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