system using optogenetics can be challenging. indeed, the light used for optogenetic control may lead to the activation of photoreceptors, as a result of the proximity between primary visual circuits and these photoreceptors. in this case, spatial selectivity is difficult to achieve (particularly in the case of the fly optic lobe). thus, the study of the visual system requires spectral separation, using channels that are activated by different wavelengths of light than rhodopsins within the photoreceptors (peak activation at 480 nm for rhodopsin 1 in drosophila). red-shifted cschrimson or bistable channelrhodopsin are used for optogenetic activation of neurons (i.e. depolarization), as both allow spectral separation. in order to achieve neuronal silencing (i.e. hyperpolarization), an anion channelrhodopsin discovered in the cryptophyte algae species guillardia theta (named gtacr1). can be used. gtacr1 is more light sensitive than other inhibitory channels such as the halorhodopsin class of chlorid pumps and imparts a strong conductance. as its activation peak (515 nm) is close to that of rhodopsin 1, it is necessary to carefully calibrate the optogenetic illumination as well as the visual stimulus. the factors to take into account are the wavelength of the optogenetic illumination (possibly higher than the activation peak of gtacr1), the size of the stimulus (in order to avoid the activation of the channels by the stimulus light) and the intensity of the optogenetic illumination. it has been shown that gtacr1 can be a useful inhibitory tool in optogenetic study of drosophila's visual system by silencing t4/t5 neurons expression. these studies can also be led on intact behaving animals, for instance to probe optomotor response. sensorimotor system optogenetically inhibiting or activating neurons tests their necessity and sufficiency, respectively, in generating a behavior. using this approach, researchers can dissect the neural circuitry controlling motor output. by perturbing neurons at various places in the sensorimotor system, researchers have learned about the role of descending neurons in eliciting stereotyped behaviors, how localized tactile sensory input and activity of interneurons alters locomotion, and the role of purkinje cells in generating and modulating movement. this is a powerful technique for understanding the neural underpinnings of animal locomotion and movement more broadly. precise temporal control of interventions the currently available optogenetic actuators allow for the accurate temporal control of the required intervention (i.e. inhibition or excitation of the target neurons) with precision routinely going down to the millisecond level. the temporal precision varies, however, across optogenetic actuators, and depends on the frequency and intensity of the stimulation. experiments can now be devised where the light used for the intervention is triggered by a particular element of behavior (to inhibit the behavior), a particular unconditioned stimulus (to associate something to that stimulus) or a particular oscillatory event in the brain (to inhibit the event). this kind of approach has already been used in several brain regions: hippocampus sharp waves and ripple complexes (swrs) are distinct high frequency oscillatory events in the hippocampus thought to play a role in memory formation and consolidation. these events can be readily detected by following the oscillatory cycles of the on-line recorded local field potential. in this way the onset of the event can be used as a trigger signal for a light flash that is guided back into the hippocampus to inhibit neurons specifically during the swrs and also to optogenetically inhibit the oscillation itself. these kinds of "closed-loop" experiments are useful to study swr complexes and their role in memory. cellular biology/cell signaling pathways analogously to how natural light-gated ion channels such as channelrhodopsin-2 allows optical control of ion flux, which is especially useful in neuroscience, natural light-controlled signal transduction proteins also allow optical control of biochemical pathways, including both second-messenger generation and protein-protein interactions, which is especially useful in studying cell and developmental biology. in 2002, the first example of using photoproteins from another organism for controlling a biochemical pathway was demonstrated using the light-induced interaction between plant phytochrome and phytochrome-interacting factor (pif) to control gene transcription in yeast. by fusing phytochrome to a dna-binding domain and pif to a transcriptional activation domain, transcriptional activation of genes recognized by the dna-binding domain could be induced by light. this study anticipated aspects of the later development of optogenetics in the brain, for example, by suggesting that "directed light delivery by fiber optics has the potential to target selected cells or tissues, even within larger, more-opaque organisms." the literature has been inconsistent as to whether control of cellular biochemistry with photoproteins should be subsumed within the definition of optogenetics, as optogenetics in common usage refers specifically to the control of neuronal firing with opsins, and as control of neuronal firing with opsins postdates and utilizes distinct mechanisms from control of cellular biochemistry with photoproteins. photosensitive proteins utilized in various cell signaling pathways in addition to phytochromes, which are found in plants and cyanobacteria, lov domains(light-oxygen-voltage-sensing domain) from plants and yeast and cryptochrome domains from plants are other natural photosensory domains that have been used for optical control of biochemical pathways in cells. in addition, a synthetic photosensory domain has been engineered from the fluorescent protein dronpa for optical control of biochemical pathways. in photosensory domains, light absorption is either coupled to a change in protein-protein interactions (in the case of phytochromes, some lov domains, cryptochromes, and dronpa mutants) or a conformational change that exposes a linked protein segment or alters the activity of a linked protein domain (in the case of phytochromes and some lov domains). light-regulated protein-protein interactions can then be used to recruit proteins to dna, for example to induce gene transcription or dna modifications, or to the plasma membrane, for example to activate resident signaling proteins. cry2 also clusters when active, so has been fused with signaling domains and subsequently photoactivated to allow for clustering-based activation. the lov2 domain of avena sativa(common oat) has been used to expose short peptides or an active protein domain in a light-dependent manner. introduction of this lov domain into another protein can regulate function through light induced peptide disorder. the aslov2 protein, which optogenetically exposes a peptide, has also been used as a scaffold for several synthetic light induced dimerization and light induced dissociation systems (ilid and lovtrap, respectively). the systems can be used to control proteins through a protein splitting strategy. photodissociable dronpa domains have also been used to cage a protein active site in the dark, uncage it after cyan light illumination, and recage it after violet light illumination. temporal control of signal transduction with light the ability to optically control signals for various time durations is being explored to elucidate how cell signaling pathways convert signal duration and response to different outputs. natural signaling cascades are capable of responding with different outputs to differences in stimulus timing duration and dynamics. for example, treating pc12 cells with epidermal growth factor (egf, inducing a transient profile of erk activity) leads to cellular proliferation whereas introduction of nerve growth factor (ngf, inducing a sustained profile of erk activity) leads to differentiation into neuron-like cells. this behavior was initially characterized using egf and ngf application, but the finding has been partially replicated with optical inputs. in addition, a rapid negative feedback loop in the raf-mek-erk pathway was discovered using pulsatile activation of a photoswitchable raf engineered with photodissociable dronpa domains. optogenetic noise-photostimulation professor elias manjarrez´s research group introduced the optogenetic noise-photostimulation. this is a technique that uses random noisy light to activate neurons expressing chr2. an optimal level of optogenetic-noise photostimulation on the brain can increase the somatosensory evoked field potentials, the firing frequency response of pyramidal neurons to somatosensory stimulation, and the sodium current amplitude.