Psychophysical properties of midbrain dopamine neurons and implications for the antidepressant effect of deep brain stimulation
Bibliographic record
Abstract
The discovery that animals engage in intracranial self-stimulation (ICSS) provided a direct way to study the neural networks that direct motivation. In ICSS experiments, animals are implanted with electrodes terminating in reward-implicated substrates. The development of optogenetics advanced the study of the brain reward system by confirming a causal role of midbrain dopamine firing in reward seeking. Since then, the correspondence of optical stimulation parameters to the neural signal of dopamine neurons causing operant behavior has been studied. In parallel, attention was paid to the application of deep brain stimulation on refractory mental illness, including depression. This thesis describes two psychophysical experiments that use optogenetic ICSS of midbrain dopamine neurons. The first experiment shows that, for a substantial range of powers (~12.6 mW - 31.6 mW), the trade-off between power and pulse duration undergoes temporal summation, aligning with Bloch’s Law. Pulse duration can be used to control the volume of activated opsin-expressing dopamine neurons. The second experiment provides a psychophysical measurement of firing fidelity of midbrain dopamine neurons. This study supports that pulse frequencies higher than 40 Hz are ineffective or counter-productive at improving the vigor of operant behavior. Together these experiments highlight the benefit of using measurable outcomes (e.g., operant response) as the basis for making inferences about the effectiveness of optical stimulation. These experiments contribute to the hypothesis that, similarly to electrical ICSS, the variable determining the intensity of reward seeking is the induced aggregate firing rate. Such insights can aid the understanding of how deep brain stimulation functions to alleviate depression. It is suggested here that the antidepressant effects of deep brain stimulation of the medial forebrain bundle (MFB) may involve activation of non-dopaminergic neural pathways. The reward platform hypothesis is presented, which suggests that MFB stimulation may cause antidepressant effects by facilitating reward seeking. This hypothesis is developed in relation to motivation parameters that promote involvement with response-contingent rewarding activities. Ways to test this hypothesis in both pre-clinical and clinical models are proposed. This thesis provides practical guidelines for optogenetic experiment designs, and it outlines original, theory-driven hypotheses about the structural and functional underpinnings of antidepressant deep brain stimulation.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".