2.3. Intersubjective avoidance apparatus

The testing apparatus, adapted from a previous report (Preobrazhenskaya & Simonov, 1970), was made of clear acrylic plastic. One inner chamber was 12.0″ L × 9.5″ W to fit a metal grid floor purchased from Med Associates (ENV‐008). Two outer chambers (12.0″ L × 7.0″ W and 17.0″  × 7″) each shared a separate, but adjacent transparent wall with the inner chamber (Figure 1a). The transparent walls between chambers had holes to allow visual, auditory, olfactory, and nose‐to‐nose tactile contact between the inner chamber and both outer chambers. The outer chambers were connected by an open door (3.0″ W × 4.0″ H). One outer chamber (outer chamber 1 in Figure 1a) had three darkened walls and a black floor made out of LEGOs^® for texture, while the other outer chamber (outer chamber 2 in Figure 1a) had two white walls, one transparent wall, and a white plastic floor. One 60W light bulb was placed outside the transparent wall of outer chamber 2 to illuminate it with approximately 1050 lux (Stanford animal facility 1), 950 lux (Stanford animal facility 2) or 350 lux (Duke animal facility); illumination was adjusted so that pilot cohorts preferred the dark chamber by 60–120 s during baseline. In all cases the dark outer chamber was kept at approximately 12 lux. The Receiver's chamber was held at approximately 250 lux.

Receiver distress is aversive and motivates avoidance. (a) Intersubjective Avoidance (IA) test design: Baseline and Testing 1. (b) Testing Observers (N = 33) reduce their light avoidance/dark preference during Testing 1. *Testing v. Control Observers, p < .01 in repeated measures ANOVA (with cohort as a covariate). (c) Histogram of all Observers’ behavior during Testing 1 (color‐coded by cohort). Intersubjective avoidance was demonstrated whenever an Observer's dark preference was reduced during testing (Δ Dark Preference is negative). (d) IA of “Natural Avoider” shown in Movie S1. (e) Observers’ IA is spatially specific; Observers’ dark preference increased when going into the light chamber was paired with shocks to Receivers (N = 6), but decreased when going into the dark chamber was paired with shocks to Receivers (N = 6). * Experimental group × testing phase interaction in repeated measures ANOVA, p < .05. All error bars indicate s.e.m.