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Prof. Greg Francis 1/2/12 PSY 200: Intro. to Cognitive Psychology 1 Purdue University Visual dynamics PSY 200 Greg Francis Lecture 10 Why (CRT) computer monitors work. Purdue University Flicker  A flashing light looks constant if it is presented rapidly enough  The frequency of flashing at which subjects do not detect flicker is called the Critical Flicker Frequency (CFF)  about 50 Hertz (50 on-off cycles in a second)  20 millisecond durations Purdue University CFF  Establishes minimum characteristics of electronic devices  Lights flicker at 120 Hz  we spend a lot of time in darkness  Computer (Cathode Ray Tube, CRT) monitors and TV’s flicker at around 60 Hz  better monitors go faster  Liquid Crystal Display (LCD) monitors work differently » Although some still flicker Purdue University Phosphor  The phosphor on a computer screen typically glows less than 10 milliseconds  ten thousandths of a second  The gun reactivates the phosphor every 17 milliseconds  Thus, at any given time 1/3 of the screen is dark  the percept persists in your head! Purdue University Persistence  What is the source of the persisting percept?  Receptors in the eye?  Receptive fields?  Network interactions?  Cognitive (memory)?  Studies support network interactions Purdue University Experiment  Bowen, Pola & Matin (1973)  subjects adjust duration of a blank stimulus so onset of probe matched perceived offset of the target +Prof. Greg Francis 1/2/12 PSY 200: Intro. to Cognitive Psychology 2 Purdue University Strange property  As the target’s duration or luminance increases  its persistence decreases Purdue University Networks  Feedback is important  Produces a persisting response  Demonstration Input from eyes Purdue University Explanation  Francis, Grossberg & Mingolla (1994)  Something has to reset the network  else it would keep “persisting” forever  Two mechanisms  (1) new inputs inhibit old responses  (2) afterimages act as new inputs  Note: afterimages get stronger as duration and luminance increase! Purdue University Explanation  Offset of input from the eyes produces an after response  e.g., due to competition from orthogonally tuned cells  Offset response inhibits persisting response Input from eyes Purdue University Explanation  As the target’s duration or luminance increases  the afterimage produced at target offset increases in strength  so there is stronger inhibition to break the feedback  so the persistence of the original percept decreases Purdue University Wait a minute  If visual percepts persist for over 100 milliseconds, why doesn’t the world seem blurry?  There should be smears of objects as they move or as we move  There must be something else preventing such blurring  maskingProf. Greg Francis 1/2/12 PSY 200: Intro. to Cognitive Psychology 3 Purdue University No mask  Write down all the letters you see Purdue University No mask  Write down all the letters you see R K S M W D Purdue University No mask  Write down all the letters you see Purdue University Masking demonstration  Write down all the letters you see Purdue University Masking demonstration  Write down all the letters you see Q F D J R P Purdue University X X X X X X X X X X X X Masking demonstration  Write down all the letters you seeProf. Greg Francis 1/2/12 PSY 200: Intro. to Cognitive Psychology 4 Purdue University Significance  The mask appeared after the target turned off  The target was presented all by itself for a brief period of time  However, our visual system is unable to develop a complete percept of a scene in a such a period of time  Thus, the XXX mask interferes with processing of the letters by shortening their persisting responses  And prevents perceived blurring of changing scenes  Masking is often used to investigate other aspects of cognition (we’ll see examples later) Purdue University Motion  In simple animals (like flies and frogs), we know how motion is detected  Demo on web page  Reichardt detector http://neurovision.berkeley.edu/Demonstrations/matthew/reichardt.html Purdue University Detecting 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  Think of them as receptive fields that vary in both space and time  Many aspects of how we perceive motion follow from the properties of Reichardt motion detectors Purdue University 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 Purdue University Apparent Motion  For just two stimuli, it does not depend on  color  shape  attention  cognitive priming Purdue University Apparent Motion  The percept of motion does depends on  stimulus duration  interstimulus interval (50-200 msec)  distanceProf. Greg Francis 1/2/12 PSY 200: Intro. to Cognitive Psychology 5 Purdue University Timing  For a Reichardt detector to indicate motion, the signal from the second area must follow the signal from the first by just the right length of time  Vary the Interstimulus Interval (ISI) between the stimuli  The time between offset of the first stimulus and onset of the second stimulus ISI too short"ISI about right"ISI too long"Purdue University Korte’s laws  Apparent motion was highly studied at the turn of the 20th century  Korte (1915) noted that to get good motion, you needed to increase the ISI between the stimuli as the distance between them increased  CogLab data  (169 participants- from class, 12,824 from global) Purdue University Motion representation  One conclusion of studies of apparent motion is that motion is a fundamental percept  It has an explicit representation in the visual system  You could imagine otherwise, we can be aware of something moving without actually seeing the


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