The Multidimensional Spectrum of Imagination
Nigel J.T. Thomas
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Page 4
Source : http://www.imagery-imagination.com/spectrum.htm
But the prevalence of the passive conception of vision in modern thought is by no means entirely due to the influence of Aristotle and the impression metaphor. It is also bound up with one of the emblematic discoveries of the scientific revolution, that lies at the very roots of modern science and modern philosophy. The theory of the retinal image, and the optics of its formation, was first worked out in the early 17th century by Johannes Kepler (Lindberg, 1976), even better known, of course, for his major contribution toward our modern understanding of the solar system, and thus one of the leading hero-figures in our mythos of the Scientific Revolution. Before long, Kepler's retinal image theory was being confirmed experimentally, most notably, perhaps, by Descartes (not only "the father of modern philosophy ," but himself a key figure in the Scientific Revolution). In his Optics (discourse V: p. 166 in Cottingham et al., 1985) Descartes describes how to prepare and set up an eyeball taken from a recently deceased person or animal, in order to see for yourself the retinal image formed within it.
The fact that this to be is done with a dead eye, dissected out of the body of which it was once a part, serves to make it very clear that retinal image formation is a purely passive matter, driven not by the agency of a living animal or person, but entirely by the incoming light. Descartes built the rest of his theory of visual perception (and, to a considerable degree, his epistemology too) upon the foundation of this important scientific discovery. The optics of the retinal image became the basis for speculations about visual cognition that went far beyond the available empirical evidence. Nerve pathways from the retina, he suggested, terminate near the surface of the pineal gland, in the center of the brain, and are so arranged that they cause another image, isomorphic to the retinal image, to be projected there. The images from each retina are projected to the same place, to form a single, combined, internal image on the surface of this gland, which was, of course, notoriously, the place where he believed that the physical processes of the body managed, in some mysterious, unexplained fashion, to interact with the immaterial, conscious soul. Thus, it is not our retinal images, and still less the external world, that are the direct cause of our visual experiences, but the images formed in this mysterious, hidden, inner place. It is these images that we (our true selves, our souls) experience, rather than the world. Note also, however, that it is only at this last stage, as the soul is affected by the imaginal representation inside the brain, that anything possessing agency even enters the story. Everything leading up to it, in the eye and in the brain, are things our bodies passively suffer as a result of the impact of light.
The fact that seeing involves the formation of a retinal image does not, in fact, entail that seeing is fundamentally passive. Nobody has ever seriously believed, after all, that retinal image formation is the whole of the story. However, if it is the one big fact that you know about vision, the crucial, new, exciting discovery that has made your understanding of vision modern and scientific (as heliocentrism made astronomy and cosmology modern and scientific) then it is understandable that you might want to try to build the rest of your visual theory in its image. This was indeed the situation for Descartes, and for his successors, for quite a long time afterwards.
Very few philosophers or cognitive scientists today believe in the Cartesian immaterial soul, and almost every detail of Descartes' speculative neurophysiology has been superseded as brain science has advanced since his time. Nevertheless, although every brick in the original edifice has been replaced, the basic architecture of the Cartesian model of vision – the idea that the essence of seeing is the passive reception of an image by the eyes, followed by the projection (and, in modern versions, processing) of the visual information contained in that image back, deep into the brain, until it eventually becomes experience – continues to shape popular and introductory, and even not-so-introductory, accounts of visual science (e.g., Pringle, 2000; Boothe, 2002; Marr, 1982). Most of us are well socialized into believing that any other way of thinking about how we see is not only repugnant to common sense, but also thoroughly unscientific, and the findings of modern neuroscience are still, more often than not (and insofar as it is possible), interpreted in the light of this theoretical framework.
It is worth noting, however, that people who do not know about retinal images, do not necessarily find the idea that visual perception occurs passively particularly intuitively compelling. This is evidenced by the fact that theories depicting vision very much as dependent upon the active agency of the seeing organism, as a process of reaching into the visual world rather than passively waiting to be affected by it, flourished mightily in the ancient world (and in early medieval Islamic culture). For many centuries such theories seem to have dominated (though they did not entirely monopolize) the learned understanding of vision, and they were developed with considerable philosophical and mathematical detail and sophistication that, in fact, did much to pave the way for modern scientific optics (Lindberg, 1976; Smith, 1981). It is true that we now know that these theories were founded upon some fundamental misconceptions about the natures of both light and the eye. No "rays" emerge from the eyes, as the ancient theorists believed. Nevertheless, even in contemporary America, many children, and even many adults who have not yet had the fundamentals of retinal optics (and the accompanying ideology of Cartesian passivism) sufficiently dinned into them, apparently find it more intuitive to think of vision much as these ancients did, as fundamentally active rather than passive (Winer et al., 2002).
Perhaps these untutored intuitions have something to be said for them. Thanks to the sophisticated techniques and instruments now available to optical scientists, the optics of retinal image formation by no means remains (as it was in the 17th century) the only well-established, hard scientific fact that we have about the workings of our eyes. We know quite a lot of other things now, and although these newer facts cannot match the fame or luster that still attaches to retinal image optics, they are just as factual, and, I want to suggest, when given their due weight they point towards an understanding of the visual process as a whole that is really quite at odds with traditional Cartesian-style passivism.
In particular, recent visual science has now made it quite clear that the continual and purposeful movements of our eyes play a central and ineliminable role in vision. The fact that we turn our eyes every so often, in order to look in a different direction, is only the tip of the iceberg; "most human behaviors are eye movements" (Bridgeman, 1992 p. 76). If our eyes did not move, we would, quite literally, be unable to see. In order to extract the useful information from the light that surrounds us, our eyes (directed, for the most part, by sub-personal, unconscious brain processes) constantly flit about in large and small motions. There are several distinctive types of eye movement, but the best studied (and probably the most important) are the rapid, irregular "flicks" known as saccades. These normally occur several times per second, and are absolutely integral to human vision. Despite this, and despite that fact that a large saccade will very radically alter the optical image falling one's retina, we are normally unaware of their occurrence, and not just their frequency but their very existence came as some surprise to the late 19th century visual scientists who first formally described them (Wade & Tatler, 2005).
More recent technological advances have made the accurate measurement and recording of eye movements possible in the laboratory, and have revealed that, despite the fact that we are largely unaware of making them, they are nevertheless under purposeful cognitive control. The pattern of saccadic movement is complex and irregular, but it is far from random. It depends both on the detailed structure of the visual scene that a person is looking at, and on the larger purpose behind the looking: on what information we hope to discover, and on what we need to know in order successfully to pursue our ongoing behavioral goals (Stark & Ellis, 1981; Hayhoe & Ballard, 2005; Martinez-Conde & Macknik, 2007; Rothkopf et al., 2007).
The passive, Cartesian theory of vision is very often combined with the "snapshot conception of visual experience" that has recently been very effectively criticized by Noë (2004). The passive theory, and the understanding of the optics of the eye that is at its heart, opens up a conceptual space in which the seductive analogy between seeing and photography can take root. Thus we arrive at the idea (explicitly embraced by, for instance, Bergson (1907)) that it is appropriate to conceive of visual experience as consisting of a succession of essentially static images of what is before our eyes at a particular instant, like a sequence of still photographs, or the individual frames of a movie. After all, although optical images (such as retinal images, or those in a camera obscura) do not have to be static, they are, to all intents, formed instantaneously. If the eye is regarded (in the 17th century fashion) as preeminently a device for retinal image formation (setting aside the muscles that jerk it around in a way that no photographer would tolerate, and the complex structure of the retina, with, its unevenly distributed photoreceptors), and if retinal image formation is seen as the heart and soul of visual perception, it is tempting to think that once such an image has, in an instant, been formed, an act of seeing has been accomplished, and we are ready to move on to the next, and, a moment later, to the next, and so on.
Back in the 1960s, Neisser (1967) explicitly employed the metaphor of the "visual snapshot" in an attempt to reconcile the "information processing" theory of vision, a modernized version of Cartesian passivism, with what was then understood about saccadic eye movements: that the eyes move in quick saccades, interspersed with brief moments of relative immobility (fixations). He drew an explicit analogy between a fixation and the snapping of a photograph. Even then, however, Neisser clearly recognized both that this was at odds with the subjective experience of vision (which seems, surely, to be continuous), and that it would be very difficult to give a satisfactory account of how such snapshots might be integrated into a coherent and cognitively useful representation of the visible world. Later he firmly repudiated not only the snapshot metaphor, but the whole passivist, information-processing framework that had motivated it (Neisser, 1976).
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