Please see the entry on the idea of this development blog.
As part of our efforts to implement STDP with heterogeneous delays, which I'll write more about some other time, we realised we needed to access recent values of a variable, but never covering more than a fixed duration T. That is, we needed access to V(t-s) for 0<s<T, where t is the current time. We could get access to this by recording all the values of V, but that's a big memory hog for a long running simulation. Instead, we came up with a new object, the RecentStateMonitor, which works in a very similar way to StateMonitor.
For the user, you can use RecentStateMonitor in exactly the same way as a StateMonitor (but with some extra options for allowing more efficient access to the data). We imagine it might be useful for things like network operations that monitor the dynamics of a variable as a simulation runs, and inject different currents depending on those dynamics. It could also be used together with a custom SpikeMonitor for a very simple spike triggered average monitor.
The implementation uses a cylindrical array, a 2D generalisation of a circular array that is circular in only one dimension. It is a 2D array V(i,j) where i represents a time index, and j a neuron index. The time index is circular and relative to a pointer into the array which updates at each time step, when the pointer reaches the end of the fixed size array, it resets back to the beginning in a circular fashion. This cylindrical array structure is good for memory access because no memory needs to be allocated or deallocated as it runs.
At the moment, RecentStateMonitor only records the values for a single variable, but it will very likely be extended, or an additional class added which handles multiple variables (since this seems to be a very likely use case). Any ideas on other features you would like this to have? or more efficient implementations?