Reverse Phi

 Do you see what I see?

[quicktime width=”350″ height=”350″]http://anstislab.ucsd.edu/files/2012/11/RevPhi4Spots.mov[/quicktime]

The four spots move back and forth in exact synchrony, in the direction shown by the arrow.  The two upper spots are correctly seen as moving in the direction of the arrow.  However, the two lower spots change their polarity between black and white as they shift.  These are perceived as moving backwards, toward the earlier stimulus and opposite to the true displacement.  This is reverse phi.  It is consistent with Ted Adelson’s motion energy model (JOSA 1985).

[quicktime width=”300″ height=”300″]http://anstislab.ucsd.edu/files/2012/11/RevPhiAnnulusDemo01.mov[/quicktime] [quicktime width=”300″ height=”300″]http://anstislab.ucsd.edu/files/2012/11/RevPhiAnnulusDemo11.mov[/quicktime]

Both movies are identical and both rotate clockwise.  But in the right movie the dots are black and white on alternate frames, and appear to rotate counterclockwise.  This is reverse phi (Anstis 1970: Anstis & Rogers 1975), in which the motion energy does go counterclockwise.
Gaze at the centre of each movie for 20s, then stop the movement.  Which way does the movement aftereffect go?  CCW in the left-hand movie of course.  But CW in the right-hand movie, appropriate to the perceived motion direction, not to the physical dot displacements.
This shows that reverse phi does adapt neural motion detectors; possibly in brain area MT (V5).

[quicktime width=”600″ height=”400″]http://anstislab.ucsd.edu/files/2012/11/RAM24s.mov[/quicktime]

In this reverse phi movie, made by PATRICK CAVANAGH, the spokes reverse their polarity on every movie frame.  Thus the inner ring actually steps counterclockwise (track a spoke with your eyes to check this) but it seems to rotate clockwise.  The opposite is true for the outer ring.  Adapt to the motion for 20s, then stop the motion.  In the motion aftereffect, the outer ring appears to move CW and the inner ring CCW — appropriate to the illusory reverse phi, not to the physical displacement.