Fig. 2

Neuronal activation within the anterolateral pallium in response to the CS in learner fish and to the US in naïve fish. (A) Intense immunostaining of pERK in the pallium (magenta highlighted region, Inset) of an L fish exposed to 5 CSs following TFC. The strong signal in an anterolateral region (yellow outline) of this optical section reveals regional neuronal activation. Relatively less immunostaining is present in the medial pallium (cyan outline). (B) An NL fish shows a lack of pERK staining in the anterolateral region (yellow outline) after exposure to 5 CSs in this equivalent optical section. (C) A naïve fish reveals strong pERK staining in the same anterolateral region (yellow outline) after exposure to 10 USs. Equivalent optical section to those in A and B. (D) A naïve fish exposed to 10 CSs does not show concentrated pERK labeling in the anterolateral region (yellow outline). Optical section equivalent to those in A–C. (E) A naïve fish not exposed to a CS or US (NS) does not show concentrated pERK labeling in the anterolateral region (yellow outline). Optical section equivalent to those in A–D. (F) L and US-exposed naïve subjects show a significantly higher lateral:medial pERK intensity ratio compared to NL and naïve untreated subjects (*P < 0.02, ***P < 0.005, n = 5 fish per group, Kruskal–Wallis multiple comparison test). White dashed lines mark the border of the pallium (midline = M) in A–E. (Scale bar for A–E, 20 µm.) Data available at https://doi.org/10.25551/1/1-1JP0 (50).