Multisensory integration of orally-sourced gustatory and olfactory inputs to the posterior piriform cortex in awake rats

Abstract

Flavour refers to the sensory experience of food, which is a combination of sensory inputs sourced from multiple modalities during consumption, including taste and odour. Previous work has demonstrated that orally-sourced taste and odour cues interact to determine perceptual judgements of flavour stimuli, although the underlying cellular- and circuit-level neural mechanisms remain unknown. We recently identified a region of the piriform olfactory cortex in rats that responds to both taste and odour stimuli. Here, we investigated how converging taste and odour inputs to this area interact to affect single neuron responsiveness ensemble coding of flavour identity. To accomplish this, we recorded spiking activity from ensembles of single neurons in the posterior piriform cortex (pPC) in awake, tasting rats while delivering taste solutions, odour solutions and taste + odour mixtures directly into the oral cavity. Our results show that taste and odour inputs evoke highly selective, temporally-overlapping responses in multisensory pPC neurons. Comparing responses to mixtures and their unisensory components revealed that taste and odour inputs interact in a non-linear manner to produce unique response patterns. Taste input enhances trial-by-trial decoding of odour identity from small ensembles of simultaneously recorded neurons. Together, these results demonstrate that taste and odour inputs to pPC interact in complex, non-linear ways to form amodal flavour representations that enhance identity coding. KEY POINTS: Experience of food involves taste and smell, although how information from these different senses is combined by the brain to create our sense of flavour remains unknown. We recorded from small groups of neurons in the olfactory cortex of awake rats while they consumed taste solutions, odour solutions and taste + odour mixtures. Taste and smell solutions evoke highly selective responses. When presented in a mixture, taste and smell inputs interacted to alter responses, resulting in activation of unique sets of neurons that could not be predicted by the component responses. Synergistic interactions increase discriminability of odour representations. The olfactory cortex uses taste and smell to create new information representing multisensory flavour identity.

Keywords: cross-modal; decoding; flavour; odour; olfactory cortex; retronasal; taste.

Figure 1. Histological reconstruction of recording sites

A, schematic from a rat brain atlas (Paxinos & Watson, 1986) indicating the general region of the posterior piriform cortex (1.4 mm posterior to bregma). B and C, coronal sections taken from two rat brains showing DAPI (blue) and DiI (pink) staining of nuclei and electrode tracts, respectively. Scale bar = 0.75 mm. Electrode tips are indicated by white arrowheads. The animal in (B) was implanted with a silicon probe; the animal in (C) was implanted with a microwire array. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 2. Unisensory responsiveness of single pPC neurons

A, responses of an example pPC neuron to intra‐oral delivery of a unisensory taste solution (sodium chloride), two unisensory odour solutions (citral, citronellal) and plain water. Top: 100 randomly selected waveforms (grey), as well as the average over all waveforms (black) in the single neuron cluster. Middle: a spike raster plot showing all action potentials for all trials aligned on stimulus delivery (t = 0). Note that, although trials are sorted by stimulus, stimuli were presented in random order during the experiment. Bottom: average firing rate over trials for each stimulus relative to baseline. Rate plots are calculated using a 250 ms sliding window to illustrate the temporal profile of responses, but lack the temporal resolution of raster plots as a result of the size of the smoothing window. Inset shows time‐averaged mean ± SEM normalized firing rate (stimulus period − baseline period). ^*Significant response relative to baseline; brackets indicate significant difference between odors. B, proportion out of all neuron‐odour (n = 820) and neuron‐taste (n = 528) pairs that elicited a significant response as a function of time. C and D, mean ± SEM effect size (response magnitude versus baseline) over all significant odour (n = 147) (C) and taste (n = 106) (D) responses, as well as water responses from the same neurons (n = 111 and 91, respectively). Analyses in (B) and (D) were performed using a 500 ms sliding window. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 3. Responsiveness on an individual mixture basis

A, component responsiveness for all neuron‐mixture pairs (n = 1168). B, mixture responsiveness for all neuron‐mixture pairs (n = 1168). [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 4. Examples of multisensory interactions in single pPC neurons

A–D, examples of pPC neurons that exhibited significant multisensory interactions. Top: 100 randomly selected waveforms (grey), as well as the average over all waveforms (black) in the single neuron clusters. Middle: spike raster plots showing all action potentials for all trials aligned on stimulus delivery (t = 0). Note that, although trials are sorted by stimulus, stimuli were presented in random order during the experiment. Bottom: average firing rate over trials for each stimulus relative to baseline. Insets show time‐averaged mean ± SEM normalized firing rate (stimulus period − baseline period). ^*Significant response relative to baseline; brackets indicate significant difference between mixture and best component. Taste + odour combinations: (A) sodium chloride + 2‐hexanone, (B) citric acid + citral, (C) sucrose + methyl valerate and (D) quinine + octanal, respectively. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 5. Mixture response magnitude as a function of best unisensory component response magnitude

For all mixtures that yielded a significant differences between mixture and best unisensory component response (n = 63). Dashes line indicates x = y.

Figure 6. Magnitude of odour‐specific responses and multisensory integration as a function of time

Traces show the effect size of odour versus water comparisons and mixture versus best unisensory component comparisons, showing the mean ± SEM over all odour responses (n = 820) and mixture responses (n = 1168), respectively. Effect sizes were computed using a 500 ms sliding window. Horizontal bars indicate significant differences from baseline for odour versus water (blue) and mixture versus best unisensory component (magenta) conditions. [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 7. Effect of taste on decoding of odour identity from pPC ensemble responses

A, average firing rate during the stimulus period in response to odors A and B in the absence and presence of taste for all neurons in an example ensemble consisting of 13 pPC neurons. Odour stimuli were octanal and methyl benzoate, taste stimulus was sucrose. B, change in decoding accuracy (stimulus period − baseline period) for all ensembles of a given size (n = 34, 45 and 14, respectively). ^*Significant increase in accuracy for stimulus period versus baseline (t test, P < 0.01). C, Decoding accuracy, showing the mean ± SEM over all cases where we recorded responses simultaneously from >12 pPC neurons to a pair of odors in the presence and absence of taste (n = 14) in a 500 ms sliding window over time. Horizontal bars on top indicate significant decoding above chance in the odour only condition (blue), odour + taste condition (magenta), and significant differences in decoding accuracy between the two conditions (cyan). [Colour figure can be viewed at wileyonlinelibrary.com]

Figure 8. Responsiveness of single pPC neurons to a battery of basic taste solutions

A, responses of an example pPC neuron to intra‐oral delivery taste solutions representing four basic taste qualities and plain water. Top: 100 randomly selected waveforms (grey), as well as the average over all waveforms (black) in the single neuron cluster. Middle: spike raster plot showing all action potentials for all trials aligned on stimulus delivery (t = 0). Note that, although trials are sorted by stimulus, stimuli were presented in random order during the experiment. Bottom: average firing rate over trials for each stimulus relative to baseline. Inset shows time‐averaged mean ± SEM normalized firing rate (stimulus period − baseline period). ^*Indicates significant response relative to baseline. B, proportion of single pPC neurons that exhibited taste‐selective and palatability‐selective responses as a function of time. C, breadth of taste tuning in single pPC neurons, quantified by the number of taste stimuli to which each single pPC neuron responded. Numbers inside the bars indicate the number of neurons. D, responses to all taste stimuli expressed as proportion of the stimulus eliciting maximum response. Reponses are ranked by magnitude, and grouped by neurons for which the maximum response was excited (n = 48) or inhibited (n = 34) relative to baseline. [Colour figure can be viewed at wileyonlinelibrary.com]