Date of Award
Summer 8-4-2023
Degree Name
Doctor of Philosophy (PhD)
Degree Type
Dissertation
Abstract
Olfaction is a primitive sensory modality used by animals to perform basic functions such as searching for food or mates. Olfactory cues are highly temporally dynamic and therefore time is an important encoding dimension in this sensory modality. How olfactory information is encoded by the temporal patterns of neural responses remains a fundamental question in the field of sensory neuroscience. Furthermore, it is not understood whether different organisms employ similar neural encoding principles. In this work, I investigated how neural activity is patterned over time in the primary olfactory system of the American bird locust (Schisctocerca americana). Previous studies have shown that odorants evoke neural responses that outlast the duration of the stimulus. Notably, the neurons that were activated during odor presentations (ON responses) became silent after odor termination. Intriguingly, many of the neurons that were silent during odor presentation became activated after odor termination (OFF responses). As a result, the neural activity during these two epochs were decorrelated. While this was previously observed for a few odorants, I investigated whether this generic coding scheme is applicable to a wide panel of diverse odorants. My results reveal that this is indeed a ubiquitous coding feature in the locust olfactory system. Furthermore, collaborating with researchers working on rodent olfaction, I found that this temporal coding feature was also present in the odor-evoked calcium signals of the mouse main olfactory system. As a direct consequence, two emergent computations arise in the locust and mouse olfactory systems: novelty contrast enhancement and short-term olfactory memory. I also examined how odor-evoked neural activity changes on longer timescales due to the persistence of stimulus-specific information in the neural network. I investigated whether previous encounters of the same odor confounded stimulus intensity information in the locust olfactory system. Repeated stimulation of the same odor induced reductions in the ensemble odor-evoked neural response, a phenomenon known as sensory adaptation. Consistent with previous studies, I found that stimulus intensity decrements also induced reductions in the ensemble neural response. However, I show that decrements in odor intensity activated unique ensembles of neurons and this combinatorial activation pattern can be maintained robustly to encode information about odor intensity. I also found that locusts’ innate behavioral responses to odorants varied with repetition and stimulus intensity. Counter-intuitively, the locusts’ perception of stimulus intensity differences 2 became significant only after adaptation had set in. My neural and behavioral data convincingly show how information about odor intensity can be preserved in an adaptation-invariant manner. In sum, in this work I reveal key ideas regarding how the temporal dimension could be used to encode olfactory information.
Language
English (en)
Chair
Baranidharan Raman