Happy and angry human pictures differentially affect dogs' postural stability

In humans, visual stimuli are known to elicit biomechanical balance responses, thereby influencing postural stability. These responses offer valuable insights into the relationship between visual perception and physical balance. This is the first study to examine the effect of human pictures on postural stability in dogs. Seventeen healthy pet dogs were evaluated standing on a pressure plate under three conditions: viewing happy human pictures, angry human pictures, and no-picture. Five conventional Center of Pressure (COP) parameters were analyzed; mediolateral displacement, craniocaudal displacement, support surface, average speed (AS) and statokinesiogram length. Although no significant differences in balance were found across the three conditions overall, a cluster analysis of relative COP changes (Delta COP_%, increase/decrease relative to the no-picture condition) revealed two distinct reaction patterns within both picture conditions. For happy pictures, 35% of dogs exhibited increases across all Delta COP_% parameters (cluster H2), while 65% showed decreases (cluster H1). Angry pictures led to increases across all Delta COP_% parameters in 47% of dogs (cluster A2), while 53% showed decreases (cluster A1). Comparisons within clusters H1/H2 and A1/A2 revealed significant differences in Delta COP_% parameters (ANOVA, all Delta COP_% parameters, p < 0.05, except AS within happy condition). No significant differences in Delta COP_% parameters were found between clusters H1/A1 (decrease in all Delta COP values) and between clusters H2/A2 (increase in all Delta COP values). This exploratory study is the first to suggest that emotional human pictures evoke varied postural responses in dogs, influenced by the type of stimulus (happy or angry) and individual differences in emotional reactivity and sensory processing. Both emotions (happy, angry) resulted in increases in all Delta COP_% parameters, interpreted as a destabilizing effect on balance, and decreases in Delta COP_% parameters, interpreted as stabilizing effect on balance.