  The disruption of coupling between brain areas has been suggested as the
mechanism underlying loss of consciousness in anesthesia. This hypothesis has
been tested previously by measuring the information transfer between brain
areas, and by taking reduced information transfer as a proxy for decoupling.
Yet, information transfer is a function of the amount of information available
in the information source-such that transfer decreases even for unchanged
coupling when less source information is available. Therefore, we asked whether
impaired local information processing leads to a loss of information transfer.
An important prediction of this alternative hypothesis is that changes in
locally available information (signal entropy) should be at least as pronounced
as changes in information transfer. We tested this prediction by recording
local field potentials in two ferrets after administration of isoflurane in
concentrations of 0.0 %, 0.5 %, and 1.0 %.
  We found strong decreases in the source entropy under isoflurane in area V1
and the prefrontal cortex (PFC)-as predicted by our alternative hypothesis. The
decrease in source entropy was stronger in PFC. Information transfer between V1
and PFC was reduced bidirectionally, but with a stronger decrease from PFC to
V1. This links the stronger decrease in information transfer to the stronger
decrease in source entropy, suggesting reduced source entropy reduces
information transfer. This conclusion fits the observation that the synaptic
targets of isoflurane are located in local cortical circuits rather than on the
synapses formed by interareal axonal projections. Thus, changes in information
transfer under isoflurane seem to be a consequence of changes in local
processing more than of decoupling between brain areas. We suggest that source
entropy changes must be considered whenever interpreting changes in information
transfer as decoupling.
