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Taste Quality Representation in the Human Brain

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Taste Quality Representation in the Human Brain bioRxiv preprint doi: https://doi.org/10.1101/726711; this version posted August 6, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. bioRxiv (2019) Taste quality representation in the human brain Jason A. Avery?, Alexander G. Liu, John E. Ingeholm, Cameron D. Riddell, Stephen J. Gotts, and Alex Martin Laboratory of Brain and Cognition, National Institute of Mental Health, Bethesda, MD, United States 20892 Submitted Online August 5, 2019 SUMMARY In the mammalian brain, the insula is the primary cortical substrate involved in the percep- tion of taste. Recent imaging studies in rodents have identified a gustotopic organization in the insula, whereby distinct insula regions are selectively responsive to one of the five basic tastes. However, numerous studies in monkeys have reported that gustatory cortical neurons are broadly-tuned to multiple tastes, and tastes are not represented in discrete spatial locations. Neu- roimaging studies in humans have thus far been unable to discern between these two models, though this may be due to the relatively low spatial resolution employed in taste studies to date. In the present study, we examined the spatial representation of taste within the human brain us- ing ultra-high resolution functional magnetic resonance imaging (MRI) at high magnetic field strength (7-Tesla). During scanning, participants tasted sweet, salty, sour and tasteless liquids, delivered via a custom-built MRI-compatible tastant-delivery system. Our univariate analyses revealed that all tastes (vs. tasteless) activated primary taste cortex within the bilateral dorsal mid-insula, but no brain region exhibited a consistent preference for any individual taste. How- ever, our multivariate searchlight analyses were able to reliably decode the identity of distinct tastes within those mid-insula regions, as well as brain regions involved in affect and reward, such as the striatum, orbitofrontal cortex, and amygdala. These results suggest that taste quality is not represented topographically, but by a combinatorial spatial code, both within primary taste cortex as well as regions involved in processing the hedonic and aversive properties of taste. Key words: Taste; fMRI; 7T; MVPA 1 INTRODUCTION are intermingled, with little evidence for topographic organization (Simon et al., 2006) prior to reaching their cortical target in the The sensation of taste begins on the tongue with sensory receptor mid-dorsal region of the insular cortex (Small, 2010). cells tuned to one of the five basic tastes: sweet, salty, sour, bitter, and umami (Roper and Chaudhari, 2017). These taste receptor cells There are two basic models that can account for the repre- signal to afferent sensory neurons, and although some are dedicated sentation of taste quality within the insular cortex: A topographic to one specific taste, most receive input from multiple taste cells model, where a specific taste, such as sweet, is represented at (Roper and Chaudhari, 2017). Thus taste coding in the periphery a discreet spatial location, and a combinatorial coding model in is a mixture of labelled-line transmission and a complex combi- which taste is represented by a combination of broadly tuned cor- natorial code across afferent neurons that are both specifically and tical neurons that are differentially sensitive to different tastes. The broadly-tuned (Simon et al., 2006; Roper and Chaudhari, 2017). In topographic model would be analogous to somatotopy in the so- the taste afferent pathway, taste fibers travel along cranial nerves matosensory system, or retinotopy in the visual system. It should VII, IX, and X to reach their first synapse at the nucleus of the soli- be noted, however, that unlike retinotopy and somatotopy, where tary tract in the medulla (Beckstead et al., 1980). In primates, these sensory signals are represented according to their well-defined lo- second-order taste afferents are relayed to the thalamic gustatory cation in space, the sensation of taste has no such spatial layout, nucleus (knows as either the VMb or VPMpc nucleus) (Pritchard as previous theories of a taste map on the tongue have been dis- et al., 1986). At both of these sub-cortical relays different tastes proven (Chandrashekar et al., 2006). Currently, although there is some support for the topographical model in rodents (Chen et al., 2011), human neuroimaging studies have failed to provide evi- ? Corresponding author at: Laboratory of Brain and Cognition, National dence for this possibility. These previous studies, however, have Institute of Mental Health, 10 Center Drive, MSC 1366, Building 10, Rm. relied on small sample sizes and relatively low resolution imag- 4C101, Bethesda, MD 20892, United States. ing (Schoenfeld et al., 2004; Prinster et al., 2017). Higher resolu- E-mail address: [email protected]. tion neuroimaging methods may be needed in order to identify to- bioRxiv preprint doi: https://doi.org/10.1101/726711; this version posted August 6, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. 2 J.A. Avery et. al pographically distinct insula regions selective for particular tastes. during which they performed our Taste Perception task. Five par- Relatedly, another possibility suggested by a previous human neu- ticipants returned for a second session where they once again com- roimaging study (Schoenfeld et al., 2004) is that, while distinct pleted the Taste Perception task. The time between these sessions tastes may be represented topographically, this representation may varied from 6 to 98 days (average 34 days). not be uniform from one subject to another. If so, then a taste topog- raphy may not be observable at the group level but may be revealed in individuals scanned on different days. 2.2.1 Taste Stimuli In contrast to topographical models, many neurophysiology Participants received four tastant solutions during scanning and the studies in rodents and primates have provided evidence for some pre-scan taste assessment: Sweet (0.6M sucrose), Sour (0.01M cit- form of a combinatorial coding model. These studies report that the ric acid), Salty (0.20M NaCl), and Neutral (2.5mM NaHCO3 + neurons in gustatory cortex are responsive to a complex variety of 25mM KCl). To reduce within-subject variability, all participants orosensory, viscerosensory, and motor signals, with relatively few received tastants at the same molar concentration. The specific con- neurons responsive solely to taste (Scott and Plata-Salamn, 1999; centrations used for this study were derived from median concen- Scott and Giza, 2000). Indeed, in primates, most taste-responsive trations used in previous neuroimaging studies of taste perception, neurons appear to be broadly tuned to multiple tastes, and there compiled in previous neuroimaging meta-analyses (Veldhuizen et appears to be no observable topographic organization for specific al., 2011; Yeung et al., 2017). All tastants were prepared using ster- tastes (Scott et al., 1991). If, as these primate studies suggest, (Scott ile lab techniques and USP-grade ingredients by the NIH Clinical and Plata-Salamn, 1999; Kaskan et al., 2018), insula gustatory re- Center Pharmacy. gions are broadly responsive to multiple tastes, taste-specific neu- ral responses might be discernable within those regions using mul- tivariate pattern analysis (MVPA) techniques (Kriegeskorte et al., 2.2.2 Gustometer Description 2006). In fact, some evidence for this possibility in the human brain has recently been reported (Chikazoe et al., 2019). A custom-built pneumatically-driven MRI-compatible system de- To distinguish between these competing models, we examined livered tastants during fMRI-scanning (Simmons et al., 2013b; Av- taste-evoked hemodynamic responses within the human brain using ery et al., 2015; 2017; 2018), see Figure 1. Tastant solutions were ultra-high resolution functional magnetic resonance imaging (MRI) kept at room temperature in pressurized syringes and fluid delivery at high magnetic field strength (7-Tesla). During scanning, partic- was controlled by pneumatically-driven pinch-valves that released ipants tasted sweet, salty, sour and tasteless (control) liquids, de- the solutions into polyurethane tubing that ran to a plastic gusta- livered via a custom-built MRI-compatible tastant-delivery system. tory manifold attached to the head coil. The tip of the polyethy- To identify brain regions exhibiting shared and distinct responses lene mouthpiece was small enough to be comfortably positioned for each taste, we employed both standard univariate analyses as between the subjects teeth. This insured that the tastants were al- well as multivariate analysis techniques to examine distributed ac- ways delivered similarly into the mouth. The pinch valves that re- tivation patterns across multiple brain voxels. leased the fluids into the manifold were open and closed by pneu- matic valves located in the scan room, which were connected to a stimulus delivery computer, which controlled the precise timing and quantity of tastants dispensed to the subject during the scan. 2 METHODS Visual stimuli for behavioral and fMRI tasks were projected onto a screen located inside the scanner bore and viewed through a mirror 2.1 Participants system mounted on the head-coil. Both visual stimulus presenta- tion and tastant delivery were controlled and synchronized via
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