Table 2.

Photostimulation substrates.

	Caged neurotransmitters	Light-sensitive ion channels
Cell-type specific/genetically targetable	No	Yes
Bidirectional	Yes – demonstrated with 2P only (Kantevari et al., 2010)	Yes – with expression of multiple channels and multicolor photostimulation (Han and Boyden, 2007)
Activates axons of passage	No	Ectopic: yes – Modified endogenous: no
In vivo applicability	No	Yes
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