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. 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. We speculate that memory effects provide a temporal window with K > 1 and this experiment could be a possible first step towards a better comprehension of this phenomenon. The K > 1 behaviour is maximal at an inter-measurement time tau around 2 s with inter-subject differences. The K > 1 persists over a time window of 1 s 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.
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