 Competition-based mechanisms for achieving selectivity come in at least two varieties: In a simple race mechanism each of the competitors independently completes a process that is comparable, along some dimension of variation, with the processes that are completed by each of the other competitors. The competitor that achieves the highest value on the relevant dimension of variation is selected as the winner. In a struggle the competitors do not just get on with their own processing in the hope that they will do it better than each of the other competitors. Instead the active suppression of other competitors is a part of the process that each competitor carries out. Simple race models of attention have been proposed (Shibuya and Bundesen, 1988, Bundesen, 1987), but our best current theories supplement the simple race mechanism with some components of mutual struggle, or with additional processes of top-down control (see, for example, Bundesen and Habekost, 2008).