Prof. Greg Francis 5/27/08

Transcription

1 Visual Perception : Motion IIE 269: Cognitive Psychology Dr. Francis Lecture 11 Motion Motion is of tremendous importance for survival (Demo) Try to find the hidden bird in the figure below ( How movies work. Motion We can derive a lot of information about the world from motion In simple animals (like flies and frogs), we know how motion is detected Detects movement of a specified direction and speed When something moves in the proper direction two signals meet at same time, strong response 1

2 Here s a fancier version of the same thing When something moves in the wrong direction no response When something goes at the wrong speed, the timing is off e.g., too fast of movement, no response Change the order of the to get the other direction Change the spacing of the detectors to get different speeds Bigger separation detects faster motion Humans have something like Reichardt motion detectors at lots of different positions in the visual field sensitive to lots of different motion directions sensitive to lots of different motion speeds Many aspects of how we perceive motion follow from the properties of Reichardt motion detectors 2

3 Apparent Motion When objects move, there is a continuous path of motion Reichardt motion detectors do not require continuous motion and, continuous paths are not necessary for motion to be seen Time1 Time2 Apparent Motion For just two stimuli, it does not depend on color shape attention cognitive priming The percept of motion does depends on stimulus duration interstimulus interval ( msec) distance Korte (1915) noted that to get good motion, you needed to this is what would be expected from a Reichardt detector ISI just right Korte (1915) noted that to get good motion, you needed to this is what would be expected from a Reichardt detector ISI too brief (simultaneity) Korte (1915) noted that to get good motion, you needed to this is what would be expected from a Reichardt detector ISI too long (separate dots) Compare to CogLab data (137 subjects) 3

4 Movies Movies Moving pictures are convincing because of apparent motion Time 1 Time 2 Movie film does not run continuously through the projector (you would just see smear!) Instead, a shutter makes the light flash on and off for each frame (thus the noise!) Apparent motion is the basis for motion on TV, computers, and moving lights If a bar stimulus goes beyond a cell s receptive field, it is impossible to completely determine the direction and speed of the bar this is a property of optics that also applies to neurons with a limited receptive field Despite this limitation people tend to report motion in such situations in a consistent way implies that higher order systems are interpreting the motion signals in a particular way Barber Pole illusion ( Competition Similar things are going on here Breathing square illusion competition between opposite directions of motion Left-right Up-down habituating gate offset of one direction leads to rebound in other 4

5 After effect Motion can be adopted by non-moving stimuli Motion illusions Some images produce movement on their own (generally require eye movements) Motion illusions Many other aspects of perception depend on motion Conclusions Motion Perception Reichardt motion detector Apparent motion» many characteristics consistent with motion detector Lots of other issues that we haven t discussed Next time Attention Magic Information processing How could you not see it? 5