Positive animal emotion (affect) is a key component of good animal welfare [1] and plays an important role in stress-coping and resilience [2]. Methods for reliably inducing and measuring positive affect are critical, but both have been limited in availability. In rats, one promising way of inducing positive affective states is by human-simulated rough and tumble play or ‘tickling’ [3,4]. However, in humans tickling induces both pleasure and displeasure, and neither an established non-verbal indicator of positive affect, the Duchenne smile, nor laughter detects this variation [5,6]. Rats also show individual differences in response to tickling [7], and this variation needs to be readily quantified if we are to ensure that tickling is only implemented where it generates positive affect. Here, we use a validated and objective measure of affective valence, the affective bias test [8], to show that 50 kHz ultrasonic vocalizations provide a quantifiable and graded measure of positive affect that accurately reflects the positive state induced by this human–rat interaction.

积极的动物情绪（情感）是良好动物福利的关键组成部分 [1]，在应对压力和恢复力方面发挥重要作用 [2]。可靠地诱导和衡量积极影响的方法至关重要，但两者的可用性都受到限制。在大鼠中，诱导积极情感状态的一种有前途的方法是通过人类模拟的粗暴和翻滚游戏或“挠痒痒”[3,4]。然而，在人类身上，挠痒痒会引起愉悦和不快，而且既定的积极影响的非语言指标、杜兴微笑和大笑都无法检测到这种变化 [5,6]。老鼠对挠痒痒的反应也表现出个体差异 [7]，如果我们要确保挠痒痒只在产生积极影响的地方实施，就需要很容易地量化这种变化。这里，