The Multidimensional Spectrum of Imagination
Nigel J.T. Thomas
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Page 5
Source : http://www.imagery-imagination.com/spectrum.htm
In fact, modern research shows that the eyes are not stationary, even during the periods of apparent fixation. Not only are there the large, relatively easily observable saccades, through which we turn our eyes to look at different parts of the scene before us, and over which we can exert at least a limited degree of voluntary control, there are also microsaccades which continue to occur, well below the threshold of consciousness, even during the periods of so-called "fixation" between larger saccades, when we seem, subjectively, to be holding our eyes still. Even in the intervals between microsaccades, the eyes continue to move, making slower movements of comparable amplitude known as drifts. Superimposed on all these other movements, a low amplitude, high frequency tremor carries on continually. As already noted, although most of them are not made by conscious volition, saccades are clearly under cognitive control, and serve important visual functions. The evidence is, as yet, less clear concerning drift and tremor, but there are good reasons to think that the same is true of them (Spauschus et al., 1999; Hennig et al., 2002; Martinez-Conde et al., 2004; Kagan et al., 2008). Indeed, counterintuitive as it may seem, "fixation eye movements" in general (microsaccades and/or drift, and possibly tremor too) seem to be necessary in order for us to discern fine levels of visual detail. Much as we need to move our fingers over a surface in order to feel its texture, we need to move the retinal image over the receptor cells in the retina in order to sense the fine details of its optical structure (Rucci & Desbordes, 2003; Martinez-Conde et al., 2004; Rucci et al., 2007; Martinez-Conde & Macknik, 2007; Kagan et al., 2008).
In addition to direct evidence of this sort there are more general considerations, arising from the anatomical structure of the human visual system, that point towards the crucial importance of eye movements to ordinary visual experience. Because of the structure of the human retina, at any one instant our eyes can only take in fine detail and rich color in a very small, central region of our visual field, corresponding to the fovea, the central region of the retina, where most of the color sensitive cone cells are located, and where these receptor cells are packed together most closely. The fovea comprises only about 1% of the total area of the light sensitive retina, and it takes in information from a visual angle of only about 2°, "about the size of a thumbnail at arm's length", as compared to about 200° for the eye (and retina) as a whole (Richardson & Spivey, 2004). Moving away from the fovea towards those parts of the retina that subserve the peripheral visual field, we find that the light sensitive cells are progressively spaced further and further apart, and that a decreasing proportion of them are color sensitive cone cells. Most of the cone cells are in or fairly close to the fovea, and the peripheral retina consists mostly of rod cells, which do not register color at all, but only discriminate light or dark, and even these are widely spaced compared to the densely packed fovea (Conway, 2009; Roorda & Williams, 1999; Curcio et al., 1991). The few scattered cones that do occur in the far periphery of the visual field do not seem to subserve color vision there (Wooten & Wald, 1973). Thus our capacity for color vision and for discriminating fine detail falls off sharply away from the central 2° of our visual field, and in the far periphery we can do little more than sense the mere presence or motion of some indefinite thing, and we must turn our eyes to bring it into foveal vision if we want to know what it is.Our normal impression that there is a richly detailed and colored visual world all around us is sustained only because of the way our eyes effortlessly and constantly turn in their sockets, rapidly moving this searchlight beam of detailed, foveal color vision from one spot to another. We do not normally notice that our peripheral vision is uncolored, but that is just one more token of the fact that eye movements are an integral and automatic (and so largely unnoticed) part of normal seeing.
I do not, however, mean to imply that the dependence of vision upon eye movement is entirely a consequence of the specific type of retinal anatomy possessed by humans. Most species of animal do not have a foveally structured retina, but most species that have a more than rudimentary visual capacity (including many invertebrates) do make saccade-like eye movements (Land, 1999). Even visual systems very different from (and much simpler than) ours have been shown to rely upon movements of the receptor cells relative to the source of their illumination. Insect eyes are very different from ours, and cannot be moved independently of the head. Nevertheless insect vision appears to depend very largely on the changes in visual stimulation that arise as the insect moves its head, or its whole body, relative to the things around it (Horridge, 1996). Even the visual capacity of simple plankton animals (whose "eyes" contain just a single light-sensitive cell, with no apparatus for forming an image whatsoever) has recently been shown to depend upon their self-movement. They can tell when they are moving in the right direction (toward the light) only if they rotate their bodies (thereby turning their eyes) as they swim (Jékely et al., 2008).
From within the still dominant conceptual framework of the passive, Cartesian theory of vision, eye movements, if they are considered at all, appear to be extraneous to the real processes of vision. According to Findlay & Gilchrist, "Many [college level] texts on vision do not even mention that the eyes can move" (2003 p. 1). Worse still, when they are discussed eye movements are often treated as if they are a "problem" that our visual system must somehow overcome (e.g., Ross et al., 2001): our brains must have developed elaborate ways to "compensate " for the movements of our eyes, so that the things we are experiencing do not seem to jump about wildly as the optical image on the retina (and the corresponding neural representation in the visual cortex: the modern counterpart of Descartes' image on the surface of the pineal gland) wobbles and jitters about with each little movement. Seeing, it seems, would be so much easier if only we were able to hold our eyes still, so as to produce a nice steady retinal image for the inner homunculus to analyze at its leisure!
As others have noted (O'Regan, 1992; Bridgeman et al., 1994; Noë, 2004), something is very wrong with that picture. Not only have we somehow failed to evolve the capacity to hold our eyes still, we have evolved an elaborate systems of muscles (and brain regions controlling those muscles) that actively keep them in constant and irregular motion. We do not need a stable retinal image in order to be able to see a stable world, nor do we need a stable representation in visual cortex, because we experience neither the image nor the representation, but the world itself (which, thankfully, is normally fairly stable). What we need in order to see it properly, it turns out, is an image that gets moved, in a suitable way, across the retina, thereby inducing an informative pattern of change in the firing rates of the receptor cells, and a corresponding pattern of change in the cortical representations. Eye movements are not a bug, they are a feature. Far from being unnecessary, extraneous, or problematic, they play an essential and integral role in normal vision. Gilchrist et al. (1998) describe the case of a young woman whose eye muscles are paralyzed by disease. In order to be able to see (relatively) normally, she has had to learn to make constant small jerky movements of her head, mimicking, as best she can, the movements of healthy eyes.
Reestablishing Continuity: 2. Recognition
Now that we are armed with an understanding of vision that is rooted in 21st rather than 17th century science, let us return to the consideration of the relationship between imagination and perception. Ichikawa (2009) apparently thinks that the criterion of will is quite sufficient to establish that there is a sharp distinction in kind between them. Sartre and Wittgenstein may have thought so too. McGinn, however, seems not to be so sanguine, and, as we have seen, offers us eight further criteria to do the same job. We are now ready to consider the rest of them.
In fact, however, McGinn justifies his claim about recognition as a corollary of his views about the will: if everything we imagine is something we have chosen to imagine, he suggests, we do not need to recognize it to know what it is. On the other hand, we do need to recognize the things we perceive, because they are not chosen but, as it were, imposed on us by the external world. As we saw earlier, however, our voluntary control over our imagery is far from absolute. If an image comes to mind unbidden, it may indeed take an effort of recognition to realize what it represents. Sometimes, an image of a face of someone from our past might drift into consciousness, and it might not be at all easy to put a name to it, or even remember the circumstances from which we recall it. Likewise, perhaps the most infuriating of the tunes that gets stuck in your head are the ones that you cannot readily identify. Even deliberately formed mental images are not always the images we want. Surely I am not alone in having had the experience of trying to recall the appearance of some particular place or person from my past, and thinking I had succeeded, only to realize later on that the image I had called to mind could not possibly be of what I had intended, and to recognize it as a memory of somewhere or someone else (or perhaps, even, from a dream).
Conversely, I am less than convinced that seeing something always calls for an act of recognition. Just as I can choose to imagine something, cannot I also sometimes choose to see something, by simply, once again, turning my eyes towards it? If I already knew what and where the thing was before I looked toward it, do I really actually need to recognize it when it comes into view? When I look at myself in the bathroom mirror in the morning, do I really need to recognize myself in order to know I am seeing me?
It is probably true that perceiving calls for recognizing much more often than imagining does, but perception may not always call for recognition, and imagining sometimes does have a place for it. The difference is one of degree, not kind.
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