Seeing both sides: dogs' eyes reveal valence-sensitive shifts between emotional states

Left-right asymmetries in behaviour, reflecting underlying asymmetries in brain activity, can serve as indirect markers of internal states. However, the underlying mechanisms driving asymmetry patterns remain unclear. This study tested two competing hypotheses concerning behavioural lateralization, using pet dogs as a model: the Valence hypothesis, which posits that lateralization is shaped by emotional valence (positive vs. negative), and the Approach-Withdrawal hypothesis, which suggests that left-right asymmetries reflect motivational tendencies (approach vs. withdrawal). We examined dogs' visual behaviour asymmetries in two distinct emotional states: positive anticipation and frustration, while awaiting a reward. Anticipation has a positive and frustration has a negative emotional valence, but both contexts motivate reward-directed approach tendencies. This allowed us to distinguish between the competing hypotheses. Dogs showed a right visual field/left-brain bias when inspecting the reward during states of positive anticipation, whereas states of frustration were associated with a left visual field/right-brain bias. Our results provide the first evidence favouring the Valence over the Approach-Withdrawal hypothesis in a non-human animal. Additionally, this study is the first to demonstrate the dynamic and fluid nature of lateralization in a non-human species, showing that dogs' visual field preference shifts within seconds as emotional states change.