One can, for example, learn associations between directions of motion and many arbitrary visual stimuli (in addition to the arrows used by Schlack and Albright [2007]), such as colors, shapes, faces, or alphanumeric characters, as well find more as with non-visual stimuli, such as tones (A. Schlack et al., 2008, Soc. Neurosci., abstract) or tactile movements. The obvious implications are that the source of top-down signaling has access to a wide range of types of sensory information, and that this range may be manifested in the recall-related responses in visual cortex. Third, the feedback signals would appear to be temporally

flexible, inasmuch as cued associative recall is context-dependent. The visual images recalled by the sight of a shovel, for example, may depend upon whether the shovel is viewed in the garden or the cemetery. Although it remains to be seen whether recall-related neuronal responses in areas MT and IT are context dependent (but see Naya et al., 1996), the context dependence of imagery itself implies that the relevant top-down signals are dynamically engaged rather than hardwired. The task of identifying feedback mechanisms and circuits SCH772984 price that satisfy these multiple constraints is daunting, to say the least, but their recognition casts new light on cortical visual processing.

Additional insights into top-down signaling and its contribution to perceptual experience may come from consideration of what purpose it serves. Much has been written about the functions of visual imagery (e.g., Farah, 1985, Hebb, 1968, James, 1890, Kosslyn, 1994, Neisser, 1976, Paivio, 1965 and Shepard and Cooper, 1982). To understand these functions, it is useful to consider two types of imagery: explicit and implicit. Scientific and colloquial discussions second of visual imagery have most commonly focused on a class of operations that enable an individual to evaluate the properties of objects or scenes that

are not currently visible. This type of imagery is typically both explicit and volitional—corresponding to the “active” retrieval process described above (see Miyashita, 2004)—and is conjured on demand to serve specific cognitive or behavioral goals. Explicit imagery may be retrospective or prospective. The retrospective variety involves scrutiny via imagery of material previously seen and remembered, such as the examination in one’s mind’s eye of the kitchen counter in order to determine whether the car keys are there. Prospective imagery—what Schacter et al. (2007) call “imagining the future”—includes the evaluation of visual object or scene transformations, or wholesale fabrication of objects and scenes based on information from other sources, such as language. For example, one might imagine the placement of the new couch in the sitting room, without the trouble of actually moving the couch.