Selb, D. M.; Barreiro, A. K.; Gautam, S. H.; Shew, W.; Ly, C.

A fundamental question in neuroscience is how sensory signals are decoded from noisy cortical activity. We address this question in the olfactory system, decoding the route by which odorants arrive into the nasal cavity: through the nostrils during inhalation or sniffing (orthonasal), or through the back of the throat during exhalation (retronasal). We recently showed with modeling and novel experiments on anesthetized rats that orthonasal versus retronasal modality information is encoded in the olfactory bulb (OB, a pre-cortical region). However, key questions remain: is modality information transmitted from OB to anterior piriform cortex (aPC)? How can this information be extracted from a much noisier cortical population with overall less firing? With simultaneous spike recordings of populations of neurons in OB and aPC, we show that an unsupervised and biologically plausible algorithm we call Shared Covariance Decoding (SCD) can indeed linearly encode modality in low dimensional subspaces. Specifically, our SCD algorithm improves encoding of ortho/retro in aPC compared to Fisher\'s linear discriminant analysis (LDA). Consistent with our theoretical analysis, when noise correlations between OB and aPC are low and OB well-encodes modality, modality in aPC tends to be encoded optimally with SCD. We observe that with several algorithms (LDA, SCD, optimal) that the decoding accuracy distributions are invariant when GABA_A (ant-)agonists (bicuculline and muscimol) are applied to OB. Overall, we show modality information can be encoded efficiently in piriform cortex.