Different Multidimensional Representations across the Amygdalo-Prefrontal Network during an Approach-Avoidance Task

Abstract

The prelimbic (PL) area and basolateral amygdala (lateral [LA] and basolateral [BL] nuclei) have closely related functions and similar extrinsic connectivity. Reasoning that the computational advantage of such redundancy should be reflected in differences in how these structures represent information, we compared the coding properties of PL and amygdala neurons during a task that requires rats to produce different conditioned defensive or appetitive behaviors. Rather than unambiguous regional differences in the identities of the variables encoded, we found gradients in how the same variables are represented. Whereas PL and BL neurons represented many different parameters through minor variations in firing rates, LA cells coded fewer task features with stronger changes in activity. At the population level, whereas valence could be easily distinguished from amygdala activity, PL neurons could distinguish both valence and trial identity as well as or better than amygdala neurons. Thus, PL has greater representational capacity.

Keywords: amygdala; conditioning; fear; population coding; prelimbic cortex; punishment; reward; striatum; valence.

Figure 1.. Risk Reward Interaction (RRI) Task, Histological Verification of Recording Sites, and Neuronal Classification

(A) RRI task apparatus: a rectangular arena with high walls, no ceiling, and a floor made of metal bars. Light-emitting diodes (LEDs) at different positions signal an upcoming reward (blue; behind left and right walls, CS-R1 and CS-R2, respectively) or an impending foot shock (red; under each of three different floor sectors, CS-S1, CS-S2, and CS-S3). (B) Acquisition of conditioned active avoidance (B1) and reward seeking (B2). Gray lines, individual rats (n = 4). Colored lines, average ± SEM. (C) Histological verification of recording site. Coronal section stained with cresyl violet. A small electrolytic lesion (arrow) marks the last recording site. (D) Classification of recorded PL cells as presumed projection cells (PNs; blue) or fast-spiking interneurons (ITNs; red) on the basis of their spike duration (y axis, peak-to-trough interval, cutoff of 0.5 ms) and firing rate (x axis, cutoff of 10 Hz). Cells that did not meet both criteria (UNCL; gray) were not considered further. Cg, cingulate cortex; DP, dorsal peduncular cortex; fmi, forceps minor of the corpus callosum; IL, infralimbic cortex; PL, prelimbic cortex. See also Figure S1.

Figure 2.. CS-Evoked Activity in LA, BL, and PL Neurons

(A–C) Heatmaps of activity evoked by CS-Rs (top) or CS-Ss (bottom) in all available principal LA (A) (n = 284), BL (B) (n = 214), and PL (C) (n = 452) neurons. Firing rates were Z-scored on the basis of activity during the pre-CS period. Data are plotted with 200 ms bins. Warmer colors indicate higher firing rates (see color bar). In (A1)–(C1), neurons were rank ordered on the basis of the amplitude of their responses to CS-R1, and the same order was kept for CS-R2, respectively. In (A2)–(C2), cells were rank ordered on the basis of the amplitude of their responses to CS-S1, and the same order was kept for CS-S3. See also Figures S2 and S3.

Figure 3.. CS-, CR-Related Activity, and Selectivity of PL Neurons

(A–C) Three different PL neurons. Top: rasters, where ticks represent individual spike times and each row represents a trial. Bottom: Z-scored peri-event histograms (PEHs) of firing rates. In (A1)–(C1), rasters and PEHs are referenced to CS onset, whereas in (A2)–(C2), they are referenced to CR onset. (A) Cell with a CS response but no CR correlate. (B) Cell with no CS response that fires in relation to the CR. (C) Cell with both, a CS response and a CR correlate. (D) Comparison between Z-scored averaged firing rate ± SEM of PL cells during AA (black) versus no avoidance (green), referenced to CS onset. Only cells increasing their firing rates during AA are included in this panel. (E) Comparison between Z-scored average firing rate ± SEM of PL cells during correct (black) and error (red) CS-S trials. The number of cells included in this analysis is smaller than in (D) because error trials did not occur in all recording sessions. This figure is based on ten error trials from seven recording sessions. (F) Comparison between Z-scored average firing rate ± SEM of PL cells during correct (black) and error (red) CS-R trials (60 error trials from 12 sessions). (G1) Comparison between the incidence of cells with excitatory (red) or inhibitory (blue) responses to only one CS-R (top) or CS-S (bottom) among LA (left), BL (middle), and PL (right) neurons. Percentages represent selective cells out of total number of responsive cells. The total numbers of CS-R excited cells were 24 in LA, 22 in BL, and 40 in PL. The total numbers of CS-S excited cells were 31 in LA, 26 in BL, and 47 in PL. The total numbers of CS-R inhibited cells were 25 in LA, 52 in BL, and 49 in PL. The total numbers of CS-S inhibited cells were 20 in LA, 46 in BL, and 38 in PL. (G2) Example of PL cell with inhibitory response to CS-R2 but not to CS-R1. Comparison between the incidence of cells with excitatory (red) or inhibitory (blue) CR-related activity in relation to only one CS-R (top) or one CS-S (bottom). The total numbers of RA-excited cells were 28 in LA, 27 in BL, and 60 in PL. The total numbers of AA-excited cells were 47 in LA, 37 in BL, and 68 in PL. The total numbers of RA-inhibited cells was 35 in LA, 44 in BL, and 148 in PL. The total numbers of AA-inhibited cells were 47 in LA, 37 in BL, and 68 in PL. (H1) Comparison between the incidence of cells with excitatory (red) or inhibitory (blue) responses to only one RA (top) or AA (bottom) among LA (left), BL (middle), and PL (right) neurons. Percentages represent selective cells out of total number of responsive cells. The total numbers of RA excited cells were 28 in LA, 27 in BL, and 60 in PL. The total numbers of AA excited cells were 47 in LA, 37 in BL, and 68 in PL. The total numbers of RA inhibited cells were 35 in LA, 58 in BL, and 94 in PL. The total numbers of AA inhibited cells were 45 in LA, 44 in BL, and 148 in PL. (H2) Example of PL cell activated in relation to AA elicited by CS-S1 but not CS-S3. (G2 and H2) Top: rasters, where ticks represent individual spike times and each row represents a trial. Bottom: Z-scored PEHs of firing rates.

Figure 4.. Coding of Task Variables by Example PL Cell as Estimated by the GLM

The first six columns show GLM-estimated spiking (blue lines) for each task variable (gray lines). The last column on the right consists of the estimated spiking (blue lines) superimposed on the observed spiking of the cell (red lines) for each CS. CS-Rs and associated behaviors (blue letters) are shown in the first two rows, and CS-Ss and associated behaviors (red letters) shown in the bottom two rows. AA, active avoidance; Frz, freezing; RA, reward approach; RAnt, reward anticipation. See also Figure S4.

Figure 5.. Comparison between Coding of RRI Task Variables by Striatal, LA, BL, and PL Neurons as Determined Using a Generalized Linear Model (GLM)

(A1–A4) Proportion of presumed PNs (y axis) in the striatum (A1), LA (A2), BL (A3), and PL (A4) that exhibited excitatory (red) or inhibitory (blue) coding of different task variables used in the GLM (x axis). (B) Absolute average ± SEM modulation of the firing rates of striatal (B1), LA (B2), BL (B3), and PL (B4) PNs in relation to each variable used in the GLM. AA, active avoidance; Frz, freezing; RA, reward approach; RAnt, reward anticipation. See also Figure S5.

Figure 6.. Contrasting Representation of Task Variables in the Striatum, Amygdala, and PL

Presumed PNs in the striatum (1), LA (2), BL, (3), and PL (4). (A) Spearman correlation matrices between activity elicited by different task variables. As indicated by the color scale on the right, warmer and cooler colors indicate positive and negative correlations, respectively. (B) Frequency distributions of Gini index (top) and dimensionality index (bottom). AA, active avoidance; Frz, freezing; RA, reward approach; RAnt, reward anticipation. See also Figure S6.

Figure 7.. Principal Component Trajectories of Trial Types

(A) Time-normalized trajectories in three-dimensional PCA space starting 5 s before to 10 s after CS-onset for STR (A1), LA (A2), BL (A3), and PL (A4) neurons. Colors go from pale to dark across time. The black circle denotes the onset of the CS. The three-dimensional space has been rotated for each structure to show the best dimension of separation. (B) Mean Euclidean distances calculated between each trial type for striatal (B1), LA (B2), BL (B3), and PL (B4) neurons. See also Figure S7.

Figure 8.. Trial Type and Valence Decoders

(A1) Decoder accuracy of the decoders for trial type (CS-R1 versus CS-R2 and CS-S1 versus CS-S3) during the CS period before behaviors are initiated. Dashed line represents shuffled trial type, indicating chance. Error bars represent the SEM on the basis of 50 decoder repetitions. (B1) Decoder accuracy of trial type during active CRs (CS-R1 RA versus CS-R2 RA and CS-S1 AA versus CS-S3 AA). (C1) Decoder accuracy of valence (CS-Rs versus CS-Ss) during the CS period before behaviors are initiated. (D1) Decoder accuracy of valence active CRs (RA versus AA). (A2–D2) Decoder accuracy as population size increases. Dark-colored lines represent observed decoder performance. Light-colored lines represent decoder performance with shuffled trial labels. Related to Figure S8. (E) Gradients of representation indicated by color scale for valence, active-passive code, distributed code, dimensionality, mixed selectivity coding, and representational capacity in the striatum, LA, BL, and PL. The measures indicated on the right were used to calculate the color gradients for each representation.