The ups and downs of conscious visual perception

We present a new view of the organization of implicit and explicit mechanisms subserving visual perception. We propose that initial feed-forward processing along the hierarchy of visual cortical areas is implicit, not available to consciousness and not under conscious control. Explicit perception begins at the large generalizing receptive fields of high cortical levels, and later cascades in the reverse direction along the same hierarchy, seeking detailed information available at lower levels. This Reverse Hierarchy Theory (RHT) explains quite naturally a variety of perceptual phenomena and is in accord with recent physiological findings. RHT attributes rapid feature search "pop-out" to higher cortical areas, where large receptive fields underlie spread attention and detection of "what without where" with poor acuity, but also detection of object categories following complex computations. RHT also predicts the characteristics of RSVP and Change Blindness, as well as of a number of other seemingly bizarre perceptual phenomena. The return to lower areas, with focussed attention for fine discriminations and localizations, may underlie late binding effects, incorporating fine details and overcoming illusory conjunctions. These two modes of explicit (conscious) perception, rapid "vision at a glance" and slower "vision with scrutiny" may reflect the extrema of a continuum of mechanisms corresponding to integration of different levels along the reverse visual hierarchy.