The speed of sight : neural correlates of perceptual reports in RSVP

Abstract

Macaque monkeys were presented with continuous rapid serial visual presentation (RSVP) sequences of unrelated naturalistic images at rates of 14ms/image to 222ms/image, while neurones that responded selectively to complex patterns (e.g. faces) were recorded in the anterior superior temporal sulcus (STSa). Stimulus selectivity was preserved for 65% of these neurones even at surprisingly fast presentation rates (14ms/image=72images/s). Such rapid processing constrains theories of visual processing. Five human subjects were asked to detect or remember specific images in the RSVP sequences under equivalent conditions. Their performance in both tasks was above chance at all rates (14-11lms/image). The neurometric performance of single neurones was quantitatively comparable to the psychophysical performance of human observers and responded in a similar way to changes in presentation rate. Large sections (51 or 93ms) of the stimulus presentation duration in the RSVP sequences were then replaced with gaps of blank screen. This manipulation affected neither the neurometric performance of single neurones nor the recognition performance of human observers. This indicates that in STSa neural persistence after a stimulus has been turned off is quantitatively identical to the response that occurs if the stimulus stays on for up to 93ms longer: a neural correlate of human visual persistence. This maintained performance in judging the identity of a stimulus is surprising considering how different sequences with and without gaps appeared to the observers: introducing gaps as short as 23ms consistently created a perception of flicker. Together these findings indicate that the perception of stimulus identity is dissociated from other aspects of perception such as flicker, and that the responses of STSa cells are a neural correlate of visual identity perception but not of other aspects of visual perception such as flicker.