The Psychologist’s Microelectrode By, Armando Canela

 “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.

 In the ’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.

 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

Test StimulusAdaptation Stimulus Original Test Stimulus

 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:

 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.

 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?

 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

 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.

 Results: face w/increased spacing was perceived as “normal” in cases following adaptation in which the congruent-sex face was presented w/increased spacing.

 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

 Results: showed a direct relationship between face masculinity in adaptation and congruence of face- sex in the adaptation/post-adaptation preference test

 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.

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), Frisby, J.P., Stone, J.V. (2010). Seeing: The Computational Approach to Biological Vision. Massachusetts, MIT Press.