Mancosu - Philosophy of Mathematical Practice (Oxford, 2008).pdf

54 marcus giaquintoAttention shiftingVisual bufferCategoryspecificationactivationAttentionwindowImage generatingfunctionFig. 2.7.visual representation of the whole of such a line appears to be plain wrong. Butit may not be wrong if visual representations include not only visual imagesbut also visual category specifications. Recall the distinction: a visual image is atransient item of visual experience of specific phenomenological type, whereasa visual category specification is a stored representation, consisting of an ensembleof feature descriptions.¹⁰ What is impossible is an infinitely extended visualimage. But it is possible that a category specification specifies a line with noright end, one that continues rightward endlessly. To explain this I need tosay more about the relation of category specification to image, which I will dowith reference to Kosslyn’s view of the functional architecture of the humanvisual system, a part of which is represented in Fig. 2.7. This figure is adaptedfrom Kosslyn (1994).¹¹Category specifications are representations in the category specificationactivation subsystem and images are identified with patterns of activity inthe visual buffer. When attention shifts to an image part, the correspondingactivity (now in the attention window) is augmented. An internal ‘instruction’to visualize something is input to the category specification activation system;the eventual output is a pattern of activity in the visual buffer that constitutesan image. More than one image can be generated from a given categoryspecification; in fact, a continuously¹² changing image can be generated overan interval of time from one category specification. When the category¹⁰ Here ‘image’ is used broadly to include percepts. Where ‘image’ is used strictly for products ofvisual imagination, the representation results from activation of a category specification by ‘top-down’processing, as opposed to ‘bottom-up’ processing originating with retinal stimulation.¹¹ I have omitted all arrows representing bottom-up processing; only a few parts of the system arerepresented. See the figure on p. 383 of Kosslyn (1994). The subsystem I call ‘category specificationactivation’ Kosslyn calls ‘category pattern activation’; what I call ‘image generating function’ he calls‘spatiotopic mapping’.¹² Here I use ‘continuously’ in the ordinary non-mathematical sense.