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Seeing Aftereffects :

The Psychologist’s Microelectrode By, Armando Canela. Seeing Aftereffects :. Definition. “Illusory phenomena experienced after prolonged or intense constant stimulation” (e.g. accidental exposure to a bright light)

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Seeing Aftereffects :

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  1. The Psychologist’s Microelectrode By, Armando Canela Seeing Aftereffects:

  2. Definition • “Illusory phenomena experienced after prolonged or intense constant stimulation” (e.g. accidental exposure to a bright light) • After-image – following this exposure the light source will appear super-imposed on whatever seen we happen to gaze upon next.

  3. As psychologists’ microelectrodes • In the 1960-70’s aftereffects were first implemented as tools for the indirect probing of the sensory mechanisms discovered by neurophysiologists. • Term refers to the relationship between psychological and neurophysiological findings using aftereffects and actual microelectrodes respectively.

  4. Procedure: Test-Adaptation-Test Cycle • Step 1: participant looks at a test stimulus and observes some particular property (e.g. color) • Step 2: next, the participant is shown an adapting stimulus for an extended period of time (i.e. adaptation period). • Step 3: participant reverts to the test stimulus and reports its appearance once again. • The effect of the adaptation is measured by comparing the pre and post-adaptation appearance of the test stimulus

  5. Example: The McCullough Effect Test Stimulus Adaptation Stimulus Original Test Stimulus

  6. Grating Stimuli • The stimuli most frequently used in visual psychophysics. • A repeating collection of bars varying on a varying on several properties such as, orientation, color, contrast etc. • Contrast:

  7. Adaptation • Recalibration Hypothesis – neurons re-tune themselves to meet new challenges in the visual world. • Fatigue Hypothesis– cells become tired after being over-stimulated for a particular period of time and do not respond as usual after exposure to a weaker stimulus.

  8. Little, DeBruine & Jones: Sex-Contingent Face Aftereffects • Exposure to faces biases perceptions of subsequently viewed faces. • Faces similar to those encountered previously are perceived as more normal/attractive than prior to exposure. • Question: Only one group of neurons to represent both male and female faces?

  9. Sex-Contingent Face Aftereffects (cont.) • Experiment 1: eye spacing was decreased/increased in one sex vs. the other • Experiment 2: (ignore) • Experiment 3: sexual dimorphism (masculinity/femininity) of face shape manipulated

  10. Experiment 1: Eye Spacing Manipulation • Stimuli : a pair of composite faces were chosen: 1 original, 1 in which eye-spacing was altered one standard deviation from the mean position. • Adaptation: 10 male & 10 female faces manipulated by increasing/decreasing eye-spacing by 200% from composite. Each presented twice for duration of 3 sec. • Test: 5 NEW male & 5 female faces (increased/decreased by 100%); observers asked to chose the more normal looking of each pair.

  11. Experiment 1 (cont.) • Results: face w/increased spacing was perceived as “normal” in cases following adaptation in which the congruent-sex face was presented w/increased spacing.

  12. Experiment 3: Sexual Dimorphism • Stimuli: 20 male/female faces masculinized & feminized by transforming them by +/-50% the differences in shape between two male/female composites • Procedure: Masculinity preference testing (may affect detection in shifting during adaptation) • Adaptation: Participants viewed each of 20 faces (in all 6 possible adaptations) 3 sec/each • Test: Masculinity preference test

  13. Experiment 3 (cont.) • Results: showed a direct relationship between face masculinity in adaptation and congruence of face-sex in the adaptation/post-adaptation preference test

  14. Outcome • Experiments demonstrated sex-contingent aftereffects for specified range of face transformations/perceptions. • These findings are indicative of separate neural populations for coding male/female faces respectively. • If the same neural population coded for both face types, then exposure to either face type should affect perception equally not just in the cases of face sex-congruency.

  15. Thank you!

  16. References DeBruine, L.M., Jones, B.C., Little, A.C. (2005). Sex –Contingent Face After-effects Suggest Distinct Neural Populations Code Male and Female Faces. The Royal Society, 272(1578), 2283-87. Frisby, J.P., Stone, J.V. (2010). Seeing: The Computational Approach to Biological Vision. Massachusetts, MIT Press.

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