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Visual Awareness: Back to the Future Terrence Sejnowski Howard Hughes Medical Institute

Visual Awareness: Back to the Future Terrence Sejnowski Howard Hughes Medical Institute The Salk Institute University of California, San Diego. Temporal binding problem : How does the brain keep track of events in the world?. brighter. Luminance. dimmer. Single Cell Recordings

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Visual Awareness: Back to the Future Terrence Sejnowski Howard Hughes Medical Institute

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  1. Visual Awareness: Back to the Future Terrence Sejnowski Howard Hughes Medical Institute The Salk Institute University of California, San Diego Temporal bindingproblem: How does the brain keep track of events in the world?

  2. brighter Luminance dimmer Single Cell Recordings from Monkey LGN J.H.R. Maunsell, et al Vis. Neurosci., 1999

  3. time The Flash-lag Illusion Motion extrapolation? (R. Nijhawan, Nature, 1994) ‘The visual system predicts where the moving object will be’

  4. The Flash-Lag Illusion

  5. Continuous Stopped Reversed 0o

  6. The Same Initial Trajectory Different Flash-lag Percepts 15 - - 1 0 - 5 Perceived Displacement (deg) 0 5 1 0 1 5 o -7 o 0 stopped reversed continuous o +7

  7. How Much of the Flash-lag Effect, If Any, Depends on What Happens Before the flash? Remove the Initial (Pre-flash) Trajectory.

  8. -7o 0o +7o -7o 0o +7o 15 - A - 1 0 - 5 Perceived Displacement (deg) 0 5 1 0 1 5 stopped reversed continuous - 15 No initial trajectory B - 1 0 - 5 Perceived Displacement (deg) 0 5 1 0 Figure 1 Eagleman & Sejnowski Science, 2000 1 5

  9. trev Spatio-Temporal Integration But averaging over what time? time To address this:

  10. Reversal After the Flash Can Modify the Percept

  11. Latency Difference Model Assume that a flash takes a longer time to be processed than a moving object (Whitney & Murakami, 1998; Purushothaman et al, 1998) According to this model the flash-lag effect is a temporal illusion Space flash perception of flash df dm Time flash appears to lag the moving object In the motion integration/postdiction model the flash-lag effect is spatial

  12. flash appears simultaneous with halt flash appears to precede halt Testing the Differential Latency Model perception of halt halt Space dm Time df flash appears to follow halt Time

  13. 100 Flash reported to precede halt (%) 50 High-luminance bar Low-luminance bar Flash precedes halt 0 -125 -97 -69 -42 -14 14 42 69 97 125 0 SOA between flash and halt (ms) The Flash is Not Perceived Later Than a Moving Object Subjects are able to correctly judge the order of flashed and moving objects (Eagleman & Sejnowski, Science, 2000c)

  14. d e r e t l i f s i n o i t i s o P Onset is coded Motion Integration / Postdiction Model Space Time

  15. The Flash-lag Effect • Neither a predictive model (motion extrapolation) nor an on-line model (latency difference) accounts for the flash-lag illusion. • The percept depends on events that occur in the next ~80 msec after the flash.

  16. Hypothesis The perceptual delay allows for the ‘best’ interpretation based on events near in time… …as required by the physics of the system How does the physiology relate to the percept?

  17. Single Cell Recordings from Monkey V1 T. J. Gawne, et al, J. Neurophys. 1996

  18. 80 msec Different Latencies for Different Luminances Data from J.H.R. Maunsell et al Visual Neuroscience, 1999 To integrate bright and dim visual stimuli, visual system must wait for the slowest signals to arrive. This allows independence from lighting conditions

  19. Optical Illusions That Should Exist But Don’t

  20. Perceptual Mistakes from Luminance Gradients?

  21. How Far in the Past Do We Live? 80 msec Two competing constraints: • Operate as close to the border of the present as possible • Account for latency differences in incoming information Why is temporal binding important?

  22. Predictability Hypothesis 1. The visual system has an internal model, which it compares against external measurements 2. When something unpredicted happens (flash), the internal model is temporarily devalued 3. The degree of devaluation depends on the salience of the stimulus.

  23. Test the Predictability Hypothesis Change the predictability of the flash

  24. Predictable

  25. Unpredictable

  26. unpredictable predictable Unpredictability Doubles the Flash-lag Effect 16 12 Lag angle (deg) 8 4 0 DE RS TC LS MS TY (Eagleman & Sejnowski, Science, 2000b)

  27. Conclusions • The same initial trajectory can lead to different percepts. • The percept depends on events that occur in the ~80 msec after the flash. • Luminance differences create latency differences • How are cortical signals related to visual awareness?

  28. David Eagleman

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