Pattern differentiation and tuning shift in human sensory cortex underlie long-term threat memory

Abstract

The amygdala-prefrontal-cortex circuit has long occupied the center of the threat system,¹ but new evidence has rapidly amassed to implicate threat processing outside this canonical circuit.^2-4 Through nonhuman research, the sensory cortex has emerged as a critical substrate for long-term threat memory,^5-9 underpinned by sensory cortical pattern separation/completion^10,11 and tuning shift.^12,13 In humans, research has begun to associate the human sensory cortex with long-term threat memory,^14,15 but the lack of mechanistic insights obscures a direct linkage. Toward that end, we assessed human olfactory threat conditioning and long-term (9 days) threat memory, combining affective appraisal, olfactory psychophysics, and functional magnetic resonance imaging (fMRI) over a linear odor-morphing continuum (five levels of binary mixtures of the conditioned stimuli/CS+ and CS- odors). Affective ratings and olfactory perceptual discrimination confirmed (explicit) affective and perceptual learning and memory via conditioning. fMRI representational similarity analysis (RSA) and voxel-based tuning analysis further revealed associative plasticity in the human olfactory (piriform) cortex, including immediate and lasting pattern differentiation between CS and neighboring non-CS and a late onset, lasting tuning shift toward the CS. The two plastic processes were especially salient and lasting in anxious individuals, among whom they were further correlated. These findings thus support an evolutionarily conserved sensory cortical system of long-term threat representation, which can underpin threat perception and memory. Importantly, hyperfunctioning of this sensory mnemonic system of threat in anxiety further implicates a hitherto underappreciated sensory mechanism of anxiety.

Keywords: acquired associative representation; sensory mechanisms of threat and anxiety; threat memory; threat perception; threat processing; threat representation; threat-related sensory cortical plasticity.

Figure 1.. Odor stimuli and experimental design

(A) Stimuli consisted of a continuum of five parametrically-morphed binary odor mixtures of neutral odors (acetophenone and eugenol labeled as Odor A and Odor B). The extreme mixtures (20% A/80% B and 80% A/20% B; to rule out confounds related to pure or mixture odorants, all stimuli consisted of binary mixtures) were differentially conditioned as CSt (threat) or CSs (safety) via paired presentation with aversive or neutral unconditioned stimuli (UCS: bimodal aversive or neutral pictures and sounds). SCR evoked by the aversive (vs. neutral) UCS confirmed their effectiveness (Aversive vs. Neutral: t = 2.88; P = .007). Assignment of CSt/CSs was counterbalanced across participants. The three intermediate mixtures (35% A/65% B, 50% A/50% B, and 65%A/35% B) were non-conditioned stimuli (nCS), representing the odor neighboring CSt (nCSt), the midpoint mixture (nCSm), and the odor neighboring CSs (nCSs). (B) Two-alternative-forced-choice (2-AFC) odor discrimination task (ODT) accompanied by fMRI and respiration acquisition. Each trial presented an odor mixture pseudo-randomly for 1.8 seconds, to which participants made judgments of “Odor A” or “Odor B” with button pressing. (C) Experiment schedule. Day 1 consisted of pre-conditioning 2-AFC ODT, conditioning, post-conditioning 2-AFC ODT, and odor risk rating. Day 9 consisted of post-conditioning 2-AFC ODT, odor risk rating, and an olfactory localizer scan. (D) Regions of interest (ROIs). Anatomical masks of the primary olfactory cortex (anterior piriform cortex/APC and posterior piriform cortex/PPC), the olfactory orbitofrontal cortex (OFC_olf), and the amygdala (AMG) are displayed on 3D T1 sections of one participant. These ROIs were further functionally constrained by the olfactory localizer. See also Figure S1.

Figure 2.. Behavioral effects of olfactory conditioning

(A) Hypothetical affective space over the odor continuum: the initial neutral baseline (gray line) would change to an ascending safety-to-threat line (black line) after acquiring affect (safety/threat) through differential conditioning. Inset shows changes (Δ) in odor affect over the continuum via conditioning, which conforms to a linear trend. (B) Hypothetical perceptual (quality) space over the odor continuum: the initial ascending trend (gray line; tracking the linear increase in the proportion of CSt) would be warped after conditioning due to expanded distances (i.e., enhanced perceptual discrimination) between the CS (CSt/CSs) and neighboring nCS (nCSt/nCSs) (black line). Inset illustrates the changes (Post – Pre) in perceived odor quality (solid line), which can be fitted by a cubic trend, anchored by respective increase and decrease in “CSt” rate for nCSt and nCSs (dotted line). (C) Empirical risk ratings (likelihood of aversive UCS) on both days conformed to the predicted profile of differential conditioning: below-chance risk for CSs and above-chance risk for CSt. Risks for the three intermediate mixtures remained chance-level (50%; indicated by the dotted line). (D) Empirical 2-AFC ODT performance (“CSt” responses rate) over the CSs-to-CSt continuum conformed to a linear trend before conditioning, which was warped after conditioning. Inset illustrates differential “CSt” rates (Post - Pre) over the odor continuum on Day 1 and Day 9, which largely conformed to the hypothesized cubic trend. Specifically, perceptual distances between the CS and neighboring nCS increased after conditioning, with the nCSt odor less endorsed as “CSt” and the nCSs odor more endorsed as “CSt” (i.e., less as “CSs”). Error bars represent s.e.e. (individually adjusted s.e.m.). *: P < .05.

Figure 3.. Olfactory cortical pattern differentiation between CS and neighboring nCS odors

(A) Group-average representational dissimilarity matrices (RDMs) for APC, PPC, OFColf and amygdala (AMG) at each phase. Each cell of the matrix indicates pattern dissimilarity (1-r), reflecting pattern differentiation, for a given odor pair. Cells right off the diagonal indicate pattern differentiation between neighboring odors: CSs and nCSs (d1), nCSs and nCSm (d2), nCSm and nCSt (d3), and nCSt and CSt (d4). Based on that, we derived a Pattern Differentiation Index (PDI) for the CS and the neighboring nCS [PDI = (d1 + d4) – (d2 + d3)]. (B) PDI for each ROI at pre-, Day 1, and Day 9 post-conditioning. Both APC and PPC (but neither amygdala nor OFC) demonstrated increased PDI from pre- to post-conditioning on Day 1, but not on Day 9. Center red line = group mean; red and blue boxes = 95% confidence interval and mean ± 1 SD, respectively. (C) Correlations between conditioning-induced PDI changes and anxiety. PDI changes on Day 9 (vs. Pre) in the APC and PPC correlated positively with anxiety, indicating persistent pattern differentiation in anxious individuals. *: P < 0.05; +: P < 0.1. See also Figure S2.

Figure 4.. Olfactory cortical tuning shift towards the CS

(A) Day 1 (dashed lines) and Day 9 (solid lines) post-conditioning tuning profiles of nCSs (green) and nCSt (pink) voxels (i.e., respectively tuned to nCSs and nCSt at the baseline). In PPC on Day 9, the nCS voxels exhibited a strong tuning preference for their respective CS: highest % of nCSs voxels tuned to CSs (shaded in green) and highest % of nCSt voxels tuned CSt (shaded in pink). (B) Tuning shift index (TSI; % of nCS voxels towards respective CS vs. the middle nCS) on Day 1 and Day 9 post-conditioning. On Day 9, PPC showed significant TSI for both nCSs and nCSt voxels towards their respective CS (CSs and CSt, respectively). The dotted line indicates zero tuning shift (TSI = 0). Center red line = group mean; red and blue boxes = 95% confidence interval and mean ± 1 SD, respectively. (C) Correlations between anxiety and tuning shift towards CS (collapsed across nCSs and nCSt). Day 9 TSI in the PPC correlated positively with anxiety, indicating amplified tuning shift in anxious individuals. The inset: in anxious (red dots) but not non-anxious (blue dots) participants, Day 9 PPC TSI correlated with Day 9 PPC PDI increase (more details in Supplemental Information, Figure S2). *P < 0.05; **P < 0.01. See also Figure S2.