  The oculomotor delayed-response (ODR) task is a common experimental paradigm
of working memory (WM) study, in which a monkey must fixate its gaze on the
center of a screen and, following a brief cue that flashes on the screen, keep
fixating for several more seconds before shifting its gaze to the location
where the cue flashed. Consequently, in the delay period between the cue and
the response the monkey must maintain a memory of cue location.
Electrophysiological recordings from the prefrontal area of the cortex (PFC)
revealed neurons that display selective persistent activity: their firing rate
change induced by the cue persists through delay period, but only in response
to a confined range of cue locations. This suggests that the representation of
the cue is maintained in the network by a change in network activity profile.
In this work, we study a network of rate-model neurons that is capable of
preserving information about a past input, owing to structured connectivity and
nonlinearities in the neuronal transfer function (TFs). Particularly, we focus
on the acceleration of the TF close to firing threshold and the concavity
around TF saturation. Any memory mechanism which exploits TF saturation means
that some neurons must fire close to their saturation rates; with our model,
however, we show that a certain relation between the excitatory and inhibitory
neurons' TFs can cause an effective saturation in the network without forcing
the neurons into the saturating parts of their TFs. In addition, this mechanism
enables the erasure of memory at the end of the delay by a global excitatory
signal. Finally, we demonstrate the mechanism in a model network of spiking
neurons which describes with more detail the oscillatory dynamics in the state
transition due to the interaction of membrane and synaptic time constants which
is neglected in the rate model.
