Chapter Four · failure evidence
What Optogenetics & Neural Stimulation got wrong, from 23 dissertations
The records detail technical and operational challenges encountered during optogenetic and neural stimulation experiments across various model systems. Many attempts failed due to insufficient stimulation efficacy, non-specific physiological perturbations, off-target behavioral artifacts, or limitations in kinetic and hardware parameter regimes. These records come from PhD theses at 12 institutions, 2021 to 2026. Each links to its thesis. They were extracted by language models reading the full text, so treat each as a lead to read, not a verdict.
Stimulation kinetics and frequency limits restrict effective modulation
High optical stimulation frequencies exceeded channelrhodopsin kinetics, leading to decaying and uncorrelated neural responses. Specific frequency boundaries also caused issues where out-of-range parameters provoked seizure-like activity or produced highly variable dopamine traces.
Tried and failed
high frequency optogenetic stimulation applied to neuronal activity modulation. Outcome: no signal. Reason: optical stimulation frequency exceeded channelrhodopsin kinetics leading to decaying and uncorrelated responses
Tried and failed
optogenetic stimulation parameter tuning applied to cortical seizure suppression. Outcome: unstable. Reason: frequencies and pulse widths outside a narrow window provoked seizure-like activity or had no effect
DEAFENING SILENCE: NOVEL ANTIEPILEPTIC TREATMENT FOR MEDICALLY REFRACTORY PARTIAL-ONSET EPILEPSY · Cornell
Considered and rejected
Considered and rejected: Decided against 4 Hz and 50 Hz optogenetic stimulation frequencies because 4 Hz did not reliably evoke dopamine and 50 Hz produced highly variable traces.
Mesolimbic dopamine and circuit level mechanisms in action initiation and restraint · Oxford
Electrical and physical delivery constraints limit targeting compared to alternative modalities
Direct-contact micro-LEDs failed to modulate deep targets because optical power density and light penetration were insufficient. Electrical stimulation methods faced issues where high current thresholds triggered non-specific excitation or temporal interference failed to modulate neurons independently of sum amplitude.
Tried and failed
direct-contact micro-LED optogenetic stimulation applied to deep brain target neurons. Outcome: no signal. Reason: optical power density or light penetration was insufficient to modulate targeted deep neuronal activity
Sleep studies in mice - open and closed loop devices for untethered recording and stimulation · Imperial
Tried and failed
temporal interference electrical stimulation applied to cortical neuronal modulation. Reason: Response strength did not depend on difference frequency or envelope amplitude independently of sum amplitude
Biophysical investigation of temporal interference neuromodulation · Imperial
Considered and rejected
Considered and rejected: Rejected direct electric neural stimulation for localized VTA modulation due to high current thresholds (≥50 µA) causing non-specific excitation.
Optical stimulation yields inferior output compared to standard baselines
Optogenetic stimulation evoked substantially lower dopamine release than electrical stimulation in transduced micro-TENNs. Similarly, optogenetic inhibition of subcortical inhibitory neurons was weaker and less reliable than pharmacological blockade or excitatory photoactivation.
Tried and failed
optogenetic inhibition of targeted inhibitory neurons applied to modulating subcortical arousal circuitry. Outcome: worse than baseline. Reason: produced weaker and less reliable behavioral and neural modulation than pharmacological blockade or excitatory photoactivation
INTEGRATED CORTICO-MOTOR FEATURES ASSESS AROUSAL IN COMA EMERGENCE IN RODENTS · Cornell
Lost to a baseline
Electrical stimulation vastly outperformed optical stimulation in ChR2-transduced micro-TENNs, evoking ~90 nM dopamine compared to ~4 nM by optogenetic stimulation
Tissue Engineered Nigrostriatal Pathway For Treatment Of Parkinson’s Disease · Penn
Non-specific motor arrest or broad activation interferes with localized circuit interrogation
Optogenetic inhibition of brainstem motor nuclei produced general bilateral motor arrest rather than specific directional choice bias. In other circuits, localized focal stimulation produced spatially diffuse downstream responses rather than discrete somatotopic activation.
Tried and failed
optogenetic inhibition of brainstem motor nuclei applied to probing choice bias and switching behavior. Outcome: no signal. Reason: caused non-specific bilateral motor arrest (miss trials) rather than directional choice bias or switching errors
Neural circuit mechanism of choice exploration in the basal ganglia · Harvard
Tried and failed
localized optogenetic stimulation for somatotopic mapping applied to basal ganglia projection pathways. Outcome: no signal. Reason: focal stimulation produced spatially diffuse downstream neuronal responses rather than discrete somatotopic activation
Substantia nigral activity in self-timed movements · Harvard
Stimulation cues introduce sensory artifacts and unintended behavioral disruptions
Optogenetic activation assays had to be rejected because the stimulating light physically woke up flies across all genotypes. In other sensory protocols, bifocal in-phase electrical stimulation decreased perceptual discrimination performance relative to anti-phase modulation.
Tried and failed
in-phase bifocal oscillatory electrical stimulation applied to sensory perceptual task performance. Outcome: worse than baseline. Reason: in-phase stimulation decreased discrimination performance relative to anti-phase modulation
Orchestration of oscillatory activity to improve visual motion discrimination · EPFL
Considered and rejected
Considered and rejected: Decided against optogenetic activation assays because stimulating light inherently woke up flies across all genotypes.
Cross-species investigation of hypersomnia genetics: a role for synaptic adhesion molecules · ScholarlyCommons at Penn
Methodological dropouts and viral delivery failures undermine experimental cohorts
Multiple subjects had to be excluded from optogenetic studies due to lack of viral transduction, missed cannula placements, or optic fiber infections. In other cohorts, animals expressing engineered opsins failed to exhibit baseline target behaviors during testing and were dropped from primary analyses.
Lost to a baseline
In Cre-ON/Flp-ON optogenetic experiments targeting VMHvl-projecting MeApd GABA neurons, 6 out of 8 ChETA-expressing mice showed no spontaneous baseline aggression during resident-intruder testing and had to be excluded from primary analysis.
The role of medial amygdala inhibitory neurons in regulating social behavior · EPFL
Lost to a baseline
Experiment 2 (NAcSh D2R optogenetics): 6 females and 1 male in the eNpHR3.0 group were excluded due to lack of viral transduction (n=3), missed cannula placement (n=3), or optic fiber site infection (n=1).
Roles of sex and gonadal hormones in the modulation of risk-based decision making · UT Austin
Opsin properties induce post-stimulation firing artifacts or poor molecular expression
Halorhodopsin was rejected for optogenetic silencing due to problematic changes in synaptically evoked spiking following light exposure. Other optogenetic systems like EL222 were rejected due to substantially lower expression levels compared to alternative constructs.
Considered and rejected
Considered and rejected: Rejected continuing with the EL222 (LOV-based) optogenetic system due to significantly lower luciferase expression compared to CRY2.
Entwicklung einer lichtinduzierten Proteinsynthese in optogenetisch aktivierbaren Säugerzellen für therapeutische Applikationen · Leibniz Universität Hannover Repository
Considered and rejected
Considered and rejected: Rejected using halorhodopsin (NpHR) for optogenetic silencing due to reported problematic changes in synaptically evoked spiking activity following light activation.
Examination of a locus coeruleus to dentate gyrus noradrenergic circuit in aversive contextual processing · ResearchWorks
Left open by the authors
Problems the authors named and did not get to.
Left open
Optogenetically stimulate axonal arbors of piriform feedback fibers in olfactory bulb slices to rescue trailing beta oscillations. Blocker: Requires wet lab, transgenic mice (Ntsr1-cre GN209), brain slice preparation, optogenetic stimulation rig, and MEA recording equipment
DYNAMICS OF SPIKE-TIMING AND GAMMA OSCILLATIONS IN THE OLFACTORY BULB · Cornell
Left open
Perform optogenetic influence mapping across varying visual stimulus contrasts and natural movies to test shifts from neural amplification to competition. Blocker: Requires two-photon optogenetic photostimulation and calcium imaging in transgenic mice (wet lab / physical optics rig).
Optical investigation of microcircuit computations in mouse primary visual cortex · Harvard
Left open
Test silent callosal synapses by measuring failure rates of AMPAR and NMDAR EPSCs under minimal optogenetic stimulation. Blocker: Requires a wet lab, animal models, electrophysiology rigs, and optogenetic apparatus
Experience-Dependent and Input-Specific Regulation of Neocortical Circuit Development by Genes Linked to Neurodevelopmental Disorders · DSpace at UTSWMED
Left open
Test intraneural optic nerve stimulation in non-human primates and assess phosphene localization through behavioral tasks. Blocker: Requires non-human primate animal facility, surgical implantation, behavioral testing setup, and specialized neural hardware
Investigation of electrical stimulation of the optic nerve for artificial vision · EPFL
Left open
Replace unpredicted background rewards with optogenetic stimulation/inhibition of dopamine fibers innervating the olfactory tubercle during contingency learning experiments. Blocker: Requires a wet lab, live mice, optogenetic apparatus, and specialized neural recording equipment
Neural circuit mechanisms underlying contingency learning · Harvard
Left open
Determine the functional logic and organizing principles of non-canonical striatonigral projections using electrophysiology and optogenetic stimulation. Blocker: Requires mouse in vivo electrophysiology wet lab, optogenetic stimulation hardware, and transgenic animal lines.
Substantia nigral activity in self-timed movements · Harvard
Left open
Train animals in a Pavlovian task using projection-specific dopamine optogenetic stimulation instead of water rewards to measure cortical belief dynamics. Blocker: Requires a wet lab, animal subjects, and optogenetic stimulation equipment.
Biological constraints and mechanisms for reinforcement learning · Harvard
Left open
Test whether non-human primate discrimination of retinotopic-scale optogenetic stimulation in V1 improves with behavioral training extended beyond two weeks. Blocker: Requires non-human primates, optogenetics/imaging rig, surgical preparation, and an animal behavior lab
Left open
Photostimulate fewer neural targets across more stimulation trials to quantify influence mapping on a single-pair basis. Blocker: Requires a wet lab, transgenic mice, two-photon imaging, and optogenetic photostimulation hardware.
Optical investigation of microcircuit computations in mouse primary visual cortex · Harvard
Left open
Perform optogenetic and chemogenetic manipulation of vCA1/vCA3 and LSCrhr2 neurons during arousal and defensive behavioral assays. Blocker: Requires a wet lab, animal subjects, stereotaxic surgeries, viral opsin vectors, and behavioral/photometry apparatus.
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