A New Mode of Perception?
In Rensink's change blindness flicker paradigm, subjects are presented with two different visual scenes which alternate back and forth, separated only by a brief (80ms) blank screen during the switch. Forty subjects underwent nearly 48 trials each, 42 of which actually contained a change from one scene to the other; the remaining six trials consisted of only two identical pictures switching back and forth which served as "catch trials."
Subjects were to give one response when they first "sensed" a change, and a second response when they were sufficiently sure of a change that they could verbally identify the changing object and its location. The results were separated into two trial types: alpha trials were those in which the first response occurred within 1 sec of the second response, and hence there was effectively no "sensing;" beta trials were those in which "sensing" responses occurred more than 1 sec before the "seeing" response. Subjects were then divided into three groups: the only-see group were those who had a very low proportion of alpha trials, while the remaining subjects were divided between the can-sense group (if they performed above 50% on the catch trials) and the guess group.
The results showed that the can-sense group was able to "sense" a change more than 2 seconds before they were able to identify that change, and given the hit rate for "sensing" it's clear that these responses were not merely the result of a guessing process. Further analyses suggest that "sensing" responses are not simply the a result of a lower change detection threshold; rather the pattern of results more strongly implicate a distinct mechanism of visual perception. A second experiment was conducted to rule out the possible effects of transients in the display. No one knows the specific mechanism by which this non-specific "sensing" might occur, although one possibility proposed in Rensink's paper is the disturbance of some non-attentional, global representation of scene layout.
Nearly 30% of participants were able to sense changes without having actually seen them - which Rensink calls "mindsight." In contrast to the phenomenon of blindsight (in which people with damage to V1 will declare themselves totally blind, but can nonetheless perform well above change in identifying visual information), mindsight involves a conscious awareness of information, but no visual experience. Rensink concludes by conjecturing that mindsight may underlie the commonly held belief in a "sixth sense," and while there's no need to posit an "extrasensory" modality, it's likely that similar phenomena would occur for the other senses as well.
EDIT: be sure to see the next article in this series, "Mindsight Reconsidered," for a different perspective on this data, offered by Dan Simons, et al..