Acute stress enhances sensitivity to a highly attractive food reward without affecting judgement bias in laying hens

Affective states can be evaluated by assessing shifts in the animal's expectation of a positive and negative outcome in response to ambiguous cues, also known as judgement bias (JB). The aim of this study was to use a JB methodology, using a go/go type of task where animals are required to make an active choice, to assess the effects of acute stress on affective states in hens. Thirty ISA-Brown hens were trained in a two-choice (left-right) test in an arena to associate a high-value (H) reward (four mealworms) with a 100% black and a low-value (L) reward (one mealworm) with 5% black (visually white) cues. Twenty hens that learnt the tasks were randomly allocated to either a control (C) or stress (S; 5 min social isolation in a novel environment) group. During testing, hens were presented with H and L (rewarded) and three novel ambiguous (un-rewarded) cues: 75%, 50% and 25% black. Order of cue presentation was balanced between treatments to either having ambiguous cues always preceded by L cues (L-Ambiguous) or by H cues (H-Ambiguous). Latency to approach a reward and active choice made (i.e. reaching side associated with either H or L reward) were recorded.