A multi-pattern medium is a mechanism that can take inputs from multiple sources and generate a pattern of output which is unique to each combination. The paradigm sources for purposes of this theory would be unitrackers. In essence, this mechanism as described instantiates a COPY mechanism for any single input. The mechanism could also, possibly, perform various operations on more than one input, such as an AND operation, generating an output unique to any combination of inputs.
[begin wild speculation]
A multi-pattern medium would support the requirements of a Global Neuronal Workspace. Suppose there is a group of neurons that can take inputs from most, if not all, unitrackers, and generate a unique firing pattern relative to a given unitracker. Suppose, under the right circumstances, this group of neurons can take inputs from more than one unitracker at a time and generate a firing pattern that constitutes a unique pattern relative to that combination. Suppose the outputs from this group go out to most of the unitrackers in the cortex, and including especially all of the unitrackers which potentially provide inputs.
This functionality of a multi-pattern medium has been demonstrated in biologically plausible neuronal networks by Chris Eliasmith (see semantic pointers.)
So where would this semantic pointer network be in the brain? My working hypothesis is the thalamus. (Note: I don’t think Chris Eliasmith puts it there.). The thalamus seems to have the connectivity described and is central to the entire cortex.
[/end wild speculation]