  Recent crowdsourcing experiments have shown that true contextuality of the
kind found in quantum mechanics can also be present in human behavior. In these
experiments simple human choices were aggregated over large numbers of
respondents, with each respondent dealing with a single context (set of
questions asked). In this paper we present experimental evidence of
contextuality in individual human behavior, in a psychophysical experiment with
repeated presentations of visual stimuli in randomly varying contexts
(arrangements of stimuli). The analysis is based on the
Contextuality-by-Default (CbD) theory whose relevant aspects are reviewed in
the paper. CbD allows one to detect contextuality in the presence of direct
influences, i.e., when responses to the same stimuli have different
distributions in different contexts. The experiment presented is also the first
one in which contextuality is demonstrated for responses that are not
dichotomous, with five options to choose among. CbD requires that random
variables representing such responses be dichotomized before they are subjected
to contextuality analysis. A theorem says that a system consisting of all
possible dichotomizations of responses has to be contextual if these responses
violate a certain condition, called nominal dominance. In our experiment
nominal dominance was violated in all data sets, with very high statistical
reliability established by bootstrapping.
