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Intuition tells us that perceptual experience—the seamless flow of conscious images of vision, sound, touch, and so forth—reflects the external world. Accordingly, information flow along the brain's sensory pathways has been thought to follow a caudo-rostral direction, away from the ports of entry, toward integrative cortices in the anterior parts of the frontal and temporal lobes. However, this view of a unidirectional, “bottom-up” processing cascade is challenged by findings which suggest that there is also information transfer in the opposite, “top-down” direction, from association areas toward early sensory cortices. A particularly intriguing observation is that while the initial bottom-up activation sweep along the sensory pathways can accomplish stimulus processing of considerable complexity and yield certain automated behaviors, conscious awareness of a sensory object appears to depend on top-down signals (1–3), as observed in the visual (4), auditory (5), and somatosensory (6) systems. Why is this the case?