Another Remembered Present

摘要

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 (<5) systems. Why is this the case?