  With resurgent interest in individual differences in perception, cognition
and behavioural control, as early indicators of disease, endophenotypes, or a
means to relate brain structure to function, behavioural tasks are increasingly
being transferred from within-subject settings to between-group or
correlational designs. The assumption is that where we know the mechanisms
underlying within-subject effects, these effects can be used to measure
individual differences in those same mechanisms. However, between-subject
variability can arise from an entirely different source from that driving
within-subject effects, and here we report a clear-cut demonstration of this.
We examined the debated relationship between the visibility of a masked-prime
stimulus and the direction of priming it causes (positive or reversed). Such
reversal of priming has been hypothesized to reflect an automatic inhibitory
mechanism that controls partially activated responses and allows behavioural
flexibility. Within subjects, we found an unambiguous systematic transition
from reversed priming to positive priming as prime visibility increased,
replicated seven times, and using different stimulus manipulations. However,
across individuals there was never a relationship between prime discrimination
ability and priming. Specifically, these data resolve the controversial debate
on visibility and reversed priming, indicating that they arise from independent
processes relying on partially shared stimulus signals. More generally, they
stand as an exemplar case in which variance between individuals arises from a
different source from that produced by stimulus manipulations.
