  In this article we present a biologically inspired model of activation of
memory items in a sequence. Our model produces two types of sequences,
corresponding to two different types of cerebral functions: activation of
regular or irregular sequences. The switch between the two types of activation
occurs through the modulation of biological parameters, without altering the
connectivity matrix. Some of the parameters included in our model are neuronal
gain, strength of inhibition, synaptic depression and noise. We investigate how
these parameters enable the existence of sequences and influence the type of
sequences observed. In particular we show that synaptic depression and noise
drive the transitions from one memory item to the next and neuronal gain
controls the switching between regular and irregular (random) activations.
