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Accounts of mechanistic explanation have emphasized the importance of looking down—decomposing a mechanism into its parts and operations. Using research on visual processing as an exemplar, I illustrate how productive such research has been. But once multiple components of a mechanism have been identified, researchers also need to figure out how it is organized—they must look around and determine how to recompose the mechanism. Although researchers often begin by trying to recompose the mechanism in terms of sequential operations, they frequently find that the components of a mechanism interact in complex ways involving positive and negative feedback and that the organization often exhibits highly interactive local networks linked by a few long-range connections (small-worlds organization) and power law distributions of connections. The mechanisms are themselves active systems that are perturbed by inputs but not set in motion by them. Researchers also need to look up—situate a mechanism in its context, which may be a larger mechanism that modulates its behavior. When looking down is combined with looking around and up, mechanistic research results in an integrated, multi-level perspective.
Notes
Notes
[1] Machamer, Darden, and Craver (2000) distinguish mechanism sketches (accounts with known gaps between operations) and schemas (accounts exhibiting productive continuity but with many details remaining to be specified). The current accounts of visual processing as shown in Figure 3b below suggest productive continuity, but it is recognized that many other brain areas figure in the pathways of visual processing even though the operations they perform are not known.
[2] Theorists such as Marr (1982) construed the task of vision as constructing a representation of the three-dimensional world, the evidence is compelling that organisms don’t reconstruct the visual scene, but actively sample it to extract that information that is pertinent to action (Churchland, Ramachandran, & Sejnowski, 1994).
[3] Previous researchers had succeeded in identifying cells that responded most strongly to contrast between in brightness between the center of their receptive fields and the surround. These included cells in the optic nerve of frogs (Hartline, 1938) and retinal ganglion cells in frogs (Barlow, 1953) and cats (Kuffler, 1953).
[4] Lashley as well as several other brain investigators in the first half of the 20th century embraced a holistic view where what mattered for most of the brain was not specific neural tissue but how much of it there was. Thus, while he gave the name prestriate region to the area in front of striate cortex, he construed it as a general association area not limited to specific types of information processing.
[5] Working with hamsters, Gerald Schneider (1967) had proposed a similar pair of pathways, with object discrimination requiring a pathway from geniculate areas to cortex and location processing relying on tectofugal regions. Colin Trevarthen (1968) proposed a similar division of pathways for primates. The difference in Ungerleider and Mishkin's proposal is that both pathways are cortical.