  A large number of eukaryotic cells are able to directly detect external
chemical gradients with great accuracy and the ultimate limit to their
sensitivity has been a topic of debate for many years. Previous work has been
done to understand many aspects of this process but little attention has been
paid to the possibility of emergent sensing states. Here we examine how
cooperation between sensors existing in a two dimensional network, as they do
on the cell's surface, can both enhance and fundamentally alter the response of
the cell to a spatially varying signal. We show that weakly interacting sensors
linearly amplify the sensors response to an external gradient while a network
of strongly interacting sensors form a collective non-linear response with two
separate domains of active and inactive sensors forming what have called a
"1/2-state" . In our analysis we examine the cell's ability to sense the
direction of a signal and pay special attention to the substantially different
behavior realized in the strongly interacting regime.
