The Dorsal Stream as the "How" Pathway

The human visual system is a complex variety of organs, sensors, and receptors that work together to detect and process the world around us. One part of this visual system is the dorsal stream.

The dorsal stream is one of two different cortical streams involved in visual processing. According to Bear, Connor, and Paradiso (2007) the dorsal stream serves to analyze visual motion, as well as the visual control of action (p. 333). Kalat (2009) describes this as the "how" or "where" pathway due to its job aiding the motor system finding and using a wide variety of devices. Research done on macque monkeys by Newsome and his Stanford University collegues (as cited in Bear, Connors, and Paradiso, 2007) noted that electrical stimulation in the MT area (an area that helps to make up the dorsal stream) altered the monkey's perceived direction in which small dots of lights moved (p. 335). This research suggests that stimulation in areas associated with a specific directional movement will impact the brain, making it believe it has sensed movement in that particular direction (i.e. stimulation in a "left" preferential area causes a perceived movement in that direction).

Kalat (2009) notes that individuals with brain damage to the dorsal stream are unable to "accurately reach out to grasp an object, even after describing its size, shape, and color," and that "although they can remember what the furniture looks like, they cannot remember how it is arranged." This lack of ability to arrange objects according to memory of their position seems also to carry over to other familiar objects such as body parts, patterns, and the like that are not visible. One particular study verifying this concept was done by Goodale and Milner (1992) in a study on patient "DF." This patient suffered from damage to the brain which severed the connection to the dorsal stream, causing the patient to experience an inability to indicate the dimensions of various objects as indicated by researchers (as qtd in Arbib, 2006). To further complicate matters, patient DF was able to reach out and grasp an object, although DF was unable to consciously adjust her fingers to the size of the object beforehand. Thus, the dorsal stream was dubbed the "how" pathway, as it served to tell the body how to move and how to interact with the world.

A lesion to the dorsal stream, affect the areas the posterior parietal cortex and the superior parietal cortex, thus causing a what is known as "optic ataxia." Optic ataxia is a deficit of the visuomotor system which causes the sufferer to loose the ability to perform "goal-directed actions to visual targets" with any sort of precision and accuracy (Snider, et al., N.d). However, it is interesting to note that optic ataxia leaves the location (the where) of the object in-tact, even though the individual is unable to perform accurate actions with the object.

References:

Arbib, M. A. (2006). Action to Language via the Mirror Neuron System. New York: Cambridge University Press.

Bear, M. F., Connors, B. W., & Paradiso, M. A. (2007). Neuroscience: Exploring the Brain, 3^rd ed. Baltimore: Lippincott Williams & Williams.

Goldstein, E. (2007).Sensation and Perception. Belmont: Wadsworth.

Kalat, J. W. (2009). Biological Psychology. Belmont: Wadsworth.

Snider, B., Arthur, S., Thompson, D., & LeSage, M. (N.d). The Dorsal Stream. Retrieved November 4, 2009, from http://ahsmail.uwaterloo.ca/kin356/dorsal/dorsal.htm

Werner, J., Chalupa, L. M. (2004). The Visual Neurosciences, Vol. 1. Bakerville:Massachusetts Institute of Technology.