  We introduce a sensitive test of memory effects in successive events. The
test consists of a combination K of binary correlations at successive times. K
decays monotonically from K = 1 for uncorrelated events as a Markov process;
whereas memory effects provide a temporal window with K > 1. For a monotonic
memory fading, K < 1 always. Here we report evidence of a K > 1 temporal window
in cognitive tasks consisting of the visual identification of the front face of
the Necker cube after a previous presentation of the same. The K > 1 behaviour
is maximal at an inter-measurement time {\tau} around 2 sec with inter-subject
differences. The K > 1 persists over a time window of 1 sec around {\tau};
outside this window the K < 1 behaviour is recovered. The universal occurrence
of a K > 1 window in pairs of successive perceptions suggests that, at variance
with single visual stimuli eliciting a suitable response, a pair of stimuli
shortly separated in time displays mutual correlations.
