Are Theories of Imagery Theories of Imagination? An Active Perception Approach to Conscious Mental Content
Nigel J.T. Thomas
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12
Page 6
Source: http://cogprints.org/5018/1/im-im-cp.htm
A Perceptual Activity theory of imagery.
Dotted arrows represent processes operative during perception, but which may be suppressed or attenuated during imagery.
The right-hand box in figure 3 represents the array of perceptual instruments available to an organism. I take it that higher organisms are endowed (perhaps innately) with many such instruments, each specialized for making a particular type of perceptual test. In many cases different instruments may share many of the same anatomical resources: not only receptor cells and muscles, but even some of the same neural structures. Which particular instrument is in play at any one time will depend not so much on which anatomical structures are being used, as on which neural "algorithm" is in control of them. Switching between such algorithms may be possible "on a rapid (~100 ms) time scale" (Van Essen, Anderson, & Felleman, 1992). The seeming immediate presence of the visual world to consciousness does not arise because we have built a detailed internal representation of it, rather it is (like the ever shining fridge light) a product of the "immanence illusion" (Minsky, 1986): For the most part, the visual perceptual instruments ask and answer their questions so quickly and effortlessly that it seems as though all the answers are already, and contemporaneously, in our minds.
Haptic tests involve applying the perceptual instrument directly to an object. Other sense modes, however, apply their instruments to causal products of the real objects of interest: products that are reasonably reliably correlated with properties of the objects themselves. Thus we detect the presence of the gorgonzola by sniffing at the foetid air it produces. Likewise, our ears interact not with the speaking person but with the vibrating air, and our eyes interact directly not with the things we see but with the "optic array" (Gibson, 1966, 1979), the ambient structured light that is scattered from them. Figure 3 attempts to depict this situation by the shaded area around the object of perception, and by the fact that the double headed arrows that represent the perceptual tests reach into this area, but not necessarily right to the perceived object itself.
However, things are yet further complicated because it seems quite likely that some perceptual instruments may be applied to causal products within the perceiver's body (arrows within the right hand box in figure 3). Although some auditory tests (for example for locating a sound source) involve moving the head, and thus the ears, relative to the vibrating air, it may be that other tests are applied within the ear itself, to the vibrational state of the cochlear fluid (itself, of course, a causal product of the vibration of the sound source). The cochlea receives a considerable degree of efferent innervation, which seems to control movements of "hair cells" within its fluid-filled interior. These movements are thought to dynamically regulate the cochlea's sensitivity and frequency tuning, and possibly underlie active discrimination of specific speech components (Dallos, 1992, 1997).
Similar considerations may even apply to the retina of the eye. There is a good deal of evidence for efferent fibers in the optic nerve, fibers that carry signals from the brain to the retina (Repérant, Miceli, Vesselkin, & Molotchnikoff, 1989). According to Pribram (1991) these comprise about 8% of the fibers in mammalian optic nerve, and serve to modulate the afferent, sensory signals carried by the other fibers, bringing them under directionally specific attentional control. Although some of the evidence for this has been disputed (Mangun, Hansen, & Hillyard, 1986), it is certainly consistent with the notion that the optical image on the retina may have various different tests applied to it by selective tuning of receptor arrays under central control.
It may also remain useful to think of some perceptual instruments as working completely within the brain, making their tests upon "representations" which are, in effect, "bottom-up", passive causal products of the stimulus object. I think it is a serious mistake to think of such neural representations as "mental" merely because they occur in the brain, but they do carry information about their causative objects, much as a shadow, a photograph, or a TV signal does. Some visual tests (notably those involving eye movements) seem to be applied directly to the optic array (Gaarder, 1966; Gibson, 1966, 1979), and there is reason to think that these are, evolutionarily, more basic (Horridge, 1987). However, considerable information can be obtained from brief tachistoscopic displays, and this, together with the related phenomenon of "iconic memory" (Neisser, 1967), suggests that other tests are applied to bottom-up neural (as opposed to "mental") representations in retinotopically mapped cortex.
It would not contravene the core commitments of the PA research program if all perceptual tests were like this (although it seems empirically unlikely, from the PA perspective, to truly be the case). If it actually were so, then we would be returned to an understanding of visual perception consistent with Kosslyn's view as expressed in figure 1, with a visual buffer subject to examination by an active mind's eye. Note, however, that this would not amount to embracing a quasi-pictorialist imagery theory. Kosslyn's theory requires that an image be established in the visual buffer before the mind's eye can go to work and examine it. PA theory, even in this completely "internalized" form, makes no such demand: PA imagery no more requires an image in the buffer than it requires an object before the eyes; it is the testing activity that matters, rather than to what (if anything) the tests are applied.
Nevertheless, the likely fact that some perceptual tests are directed at neural, bottom-up representations may help to make sense of the otherwise contradictory findings, previously discussed (at the end of §2.1.1), concerning activity in retinotopically mapped brain regions during imagery. Such perceptual tests would involve sending efferent signals to retinotopic cortex (the pathways for this certainly exist [Felleman & Van Essen, 1991]), and we should thus expect some activity in such cortex if these tests are deployed during imagery. Any such activation, however, would be an effect, a by-product, of the process of imaging rather than its necessary causal antecedent. This would explain why, although visual imagery may often be accompanied by activation in the retinotopically mapped areas, it can sometimes occur without such activation (the relevant tests are not in play) (Mellet et al., 1996), or even when the relevant parts of these areas have been destroyed (Ramachandran & Hirstein, 1997; Weiskrantz et al., 1974).
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12
We Make it Easy to Succeed
Successwaves, Intl.
Brain Based Accelerated Success Audios
 |