A practical challenge for the successful application of the synthetic assembly method is to selectively activate neurons. This would require an a priori knowledge of the spatiotemporal pattern of the assembly members and selective light delivery only to these member neurons in the appropriate temporal order. Currently used ‘optrodes’ are not up to this task because light is delivered to orders of magnitude more neurons than the few observed (Cardin et al., 2009; Han et al., 2009; Sohal et al., 2009; c.f., Miesenböck, 2009). More localized delivery of light limited to the volume of recorded neurons is a necessary requirement to this end (Royer et al., 2010). An alternative solution is to express light sensitivity in those neurons only, which are active in a given specific task and test whether their subsequent activation elicits the same behavior. While such activity markers may identify the members of neural words (Claridge-Chang et al., 2009), appropriate temporal sequencing may still be necessary since different temporal ordering of the same assemblies may be interpreted differently by the reader-actuator. Finally, even successful elicitation of a
