Event A is believed to cause event B if it regularly precedes B in time and if in its absence B fails to occur. Thus, while the activity of a reader-actuator (event B) may regularly follow a unique trajectory of assemblies (event A), providing circumstantial evidence for a cause-effect relationship, definite evidence requires either artificial recreation or elimination of the cause (event A). Since methods for selective and fast activation and inactivation of multiple single neurons and synapses by light activation are on the horizon (Boyden et al., 2005; Deisseroth et al., 2006; Luo et al., 2008; Miesenböck, 2009; Zhang et al., 2007), discussion of their potential use in identifying assemblies, neural words and sentences are warranted.
