Space Takes Time: Concentration Dependent Output Codes from Primary Olfactory Networks Rapidly Provide Additional Information at Defined Discrimination Thresholds

Characterizing neural coding performance for populations of sensory neurons: comparing a weighted spike distance metrics to other analytical methods

The identity of sensory stimuli is encoded in the spatio-temporal patterns of responses of the encoding neural population. For stimuli to be discriminated reliably, differences in population responses must be accurately decoded by downstream networks. Several methods to compare patterns of responses have been used by neurophysiologists to characterize the accuracy of the sensory responses studied. Among the most widely used analyses, we note methods based on Euclidean distances or on spike metric distances. Methods based on artificial neural networks and machine learning that recognize and/or classify specific input patterns have also gained popularity. Here, we first compare these three strategies using datasets from three different model systems: the moth olfactory system, the electrosensory system of gymnotids, and leaky-integrate-and-fire (LIF) model responses. We show that the input-weighting procedure inherent to artificial neural networks allows the efficient extraction of information relevant to stimulus discrimination. To combine the convenience of methods such as spike metric distances but leverage the advantages of weighting the inputs, we propose a measure based on geometric distances where each dimension is weighted proportionally to how informative it is. We show that the result of this Weighted Euclidian Distance (WED) analysis performs as well or better than the artificial neural network we tested and outperforms the more traditional spike distance metrics. We applied information theoretic analysis to LIF responses and compared their encoding accuracy with the discrimination accuracy quantified through this WED analysis. We show a high degree of correlation between discrimination accuracy and information content, and that our weighting procedure allowed the efficient use of information present to perform the discrimination task. We argue that our proposed measure provides the flexibility and ease of use sought by neurophysiologists while providing a more powerful way to extract relevant information than more traditional methods.

Flight motor networks modulate primary olfactory processing in the moth Manduca sexta

Across vertebrates and invertebrates, corollary discharge circuits (CDCs) project to and inform sensory networks about an animal’s movements, which directly impact sensory processing. Failure of CDCs likely underlie sensory hallucinations in schizophrenia, Parkinson’s disease, and dyspnea, highlighting the fundamental importance of CDCs for successfully interpreting sensory cues to adaptively interact with the external world. Ultimately, understanding the role of CDCs in integrating sensory motor function will be vital to understand these diseases, but mechanistically little is known about how CDCs function. CDCs have been identified in most sensory domains except olfaction. Our findings indicate that a histaminergic CDC enhances the ability of the olfactory system to more precisely encode stimulus temporal structure, resulting in enhanced olfactory acuity.

Nervous systems must distinguish sensory signals derived from an animal’s own movements (reafference) from environmentally derived sources (exafference). To accomplish this, motor networks producing reafference transmit motor information, via a corollary discharge circuit (CDC), to affected sensory networks, modulating sensory function during behavior. While CDCs have been described in most sensory modalities, none have been observed projecting to an olfactory pathway. In moths, two mesothoracic to deutocerebral histaminergic neurons (MDHns) project from flight sensorimotor centers in the mesothoracic neuromere to the antennal lobe (AL), where they provide the sole source of histamine (HA), but whether they represent a CDC is unknown. We demonstrate that MDHn spiking activity is positively correlated with wing-motor output and increased before bouts of motor activity, suggesting that MDHns communicate global locomotor state, rather than providing a precisely timed motor copy. Within the AL, HA application sharpened entrainment of projection neuron responses to odor stimuli embedded within simulated wing-beat–induced flows, whereas MDHn axotomy or AL HA receptor (HA-r) blockade reduced entrainment. This finding is consistent with higher-order CDCs, as the MDHns enhanced rather than filtered entrainment of AL projection neurons. Finally, HA-r blockade increased odor detection and discrimination thresholds in behavior assays. These results establish MDHns as a CDC that modulates AL temporal resolution, enhancing odor-guided behavior. MDHns thus appear to represent a higher-order CDC to an insect olfactory pathway; this CDC’s unique nature highlights the importance of motor-to-sensory signaling as a context-specific mechanism that fine-tunes sensory function.