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Study of Human Classification using Psychophysics and Machine Learning




We attempt to reach a better understanding of classi cation in humans using both psychophysical and machine learning techniques. In our psychophysical paradigm the stimuli presented to the human subjects are modi ed using machine learning algorithms according to their responses. Frontal views of human faces taken from a processed version of the MPI face database are employed for a gender classi cation task. The processing assures that all heads have same mean intensity, same pixel-surface area and are centered. This processing stage is followed by a smoothing of the database in order to eliminate, as much as possible, scanning artifacts. Principal Component Analysis is used to obtain a low-dimensional representation of the faces in the database. A subject is asked to classify the faces and experimental parameters such as class (i.e. female/male), con dence ratings and reaction times are recorded. A mean classi cation error of 14.5% is measured and, on average, 0.5 males are classi ed as females and 21.3females as males. The mean reaction time for the correctly classi ed faces is 1229 +- 252 [ms] whereas the incorrectly classi ed faces have a mean reaction time of 1769 +- 304 [ms] showing that the reaction times increase with the subject's classi- cation error. Reaction times are also shown to decrease with increasing con dence, both for the correct and incorrect classi cations. Classi cation errors, reaction times and con dence ratings are then correlated to concepts of machine learning such as separating hyperplane obtained when considering Support Vector Machines, Relevance Vector Machines, boosted Prototype and K-means Learners. Elements near the separating hyperplane are found to be classi ed with more errors than those away from it. In addition, the subject's con dence increases when moving away from the hyperplane. A preliminary analysis on the available small number of subjects indicates that K-means classi cation seems to re ect the subject's classi cation behavior best. The above learnersare then used to generate \special" elements, or representations, of the low-dimensional database according to the labels given by the subject. A memory experiment follows where the representations are shown together with faces seen or unseen during the classi cation experiment. This experiment aims to assess the representations by investigating whether some representations, or special elements, are classi ed as \seen before" despite that they never appeared in the classi cation experiment, possibly hinting at their use during human classi cation.

Author(s): Graf, ABA. and Wichmann, FA. and Bülthoff, HH. and Schölkopf, B.
Volume: 6
Pages: 149
Year: 2003
Month: Febuary
Day: 0
Editors: H.H. B{\"u}lthoff, K.R. Gegenfurtner, H.A. Mallot, R. Ulrich, F.A. Wichmann

Department(s): Empirical Inference
Bibtex Type: Poster (poster)

Digital: 0
Event Name: 6. T{\"u}binger Wahrnehmungskonferenz (TWK 2003)
Event Place: T{\"u}bingen, Germany
Organization: Max-Planck-Gesellschaft
School: Biologische Kybernetik

Links: PDF


  title = {Study of Human Classification using Psychophysics and Machine Learning},
  author = {Graf, ABA. and Wichmann, FA. and B{\"u}lthoff, HH. and Sch{\"o}lkopf, B.},
  volume = {6},
  pages = {149},
  editors = {H.H. B{\"u}lthoff, K.R. Gegenfurtner, H.A. Mallot, R. Ulrich, F.A. Wichmann},
  organization = {Max-Planck-Gesellschaft},
  school = {Biologische Kybernetik},
  month = feb,
  year = {2003},
  month_numeric = {2}