  Systems factorial technology (SFT; Townsend & Nozawa, 1995) is regarded as a
useful tool to diagnose if features (or dimensions) of the investigated
stimulus are processed in a parallel or serial fashion. In order to use SFT,
one has to assume the speed to process each feature is influenced by that
feature only, termed as selective influence (Sternberg, 1969). This assumption
is usually untestable as the processing time for a stimulus feature is not
observable. Stochastic dominance is traditionally used as an indirect evidence
for selective influence (e.g., Townsend & Fifi\'c, 2004). However, one should
keep in mind that selective influence may be violated even when stochastic
dominance holds. The current study proposes a trackball movement paradigm for a
direct test of selective influence. The participants were shown a reference
stimulus and a test stimulus simultaneously on a computer screen. They were
asked to use the trackball to adjust the test stimulus until it appeared to
match the position or shape of the reference stimulus. We recorded the reaction
time, the parameters defined the reference stimulus (denoted as \alpha and
\beta ), and the parameters defined the test stimulus (denoted as A and B). We
tested selective influence of \alpha and \beta on the amount of time to adjust
A and B through testing selective influence of \alpha and \beta on the values
of A and B using the linear feasibility test (Dzhafarov & Kujala, 2010). We
found that when the test was passed and stochastic dominance held, the inferred
architecture was as expected, which was further confirmed by the trajectory of
A and B observed in each trial. However, with stochastic dominance only SFT can
suggest a prohibited architecture. Our results indicate the proposed method is
more reliable for testing selective influence on the processing speed than
examining stochastic dominance only.
