ABSTRACT
Optical neural stimulation has become a key neurophotonic technology, providing multiple benefits over electrical stimulation due to its non-contact nature, high spatiotemporal resolution, and potential for cell-type selective targeting. Early work on multi-site optical stimulation focused on sequentially accessing stimulation loci (Shoham et al., 2005; Nikolenko et al., 2007), an approach with relatively limited temporal resolution due to scan and dwell durations. To avoid scanning, the utilization of spatial light modulators (SLMs) for parallel cell activation was explored. First, amplitude SLMs like digital mirror devices were used (Knapczyk et al., 2005; Wang et al., 2007; Farah et al., 2007). Digital mirror-type amplitude modulating projectors shape the projected light pattern by directly switching each pixel on or off. These devices can generally produce high temporal and lateral resolution patterns but suffer from very high losses when the projected pattern is sparse (light hitting the “off” pixels is lost). In contrast, phase SLMs modulate the phase (wavefront) of the incoming beam such that the diffracted light power is divided solely between the “on” regions with minimal light loss in the modulation process. This enables efficient parallel scan-less optical “holographic” stimulation where arbitrary light patterns are created in the Fourier plane of a phase SLM simply by displaying the desired Computer-Generated Hologram (CGH) on the SLM. This benefit prompted several research teams independently to pursue the development of such holographic excitation optical neural interfaces (Lutz et al., 2008; Nikolenko et al., 2008; Golan et al., 2009).
