Department of Psychology, Bucknell University, Lewisburg, PA 17837, USA.
Physiol Behav. 2011 Mar 28;102(5):466-74. doi: 10.1016/j.physbeh.2010.12.016. Epub 2010 Dec 23.
Rats learn to prefer flavors that are followed by postingestive effects of nutrients. This experiment investigated whether the timing of a flavor (specifically, in the first or second half of the meal) influences learning about that flavor. Stronger learning about earlier or later flavors would indicate when the rewarding postingestive effects of nutrients are sensed. Rats with intragastric (IG) catheters drank saccharin-sweetened, calorically-dilute solutions with distinct flavors added, accompanied by IG infusion of glucose (+sessions) or water (-sessions). In both types of sessions, an "Early" flavor was provided for the first 8 min and a "Late" flavor for the last 8 min. Thus, rats were trained with Early(+) and Late(+) in high-calorie meals, and Early(-) and Late(-) in low-calorie meals. Strength of the learned preference for Early(+) and Late(+) was then assessed in a series of two-bottle choice tests between Early(+) vs. Early(-), Late(+) vs. Late(-), Early(+) vs. Late(+), and Early(-) vs. Late(-). Rats preferred both Early(+) and Late(+) over the respective (-) flavors. But Early(+) was only preferred when rats were tested hungry. Late(+) was preferred when rats were tested hungry or recently satiated. This indicates qualitatively different associations learned about flavors at different points in the meal. While not supporting the idea that postingestive effects become most strongly associated with later-occurring ("dessert") flavors, it does suggest a reason dessert flavors may remain attractive in the absence of hunger.
老鼠学会喜欢那些伴随着营养物质餐后效应的味道。本实验研究了味道的时间（具体来说，是在餐的前半部分还是后半部分）是否会影响对味道的学习。对早期或晚期味道的学习能力越强，就表明营养物质的奖赏性餐后效应被感知的时间越早。有胃内（IG）导管的老鼠饮用添加了不同味道的含糖（+ 餐）或水（- 餐）的蔗糖甜味、低热量稀释溶液。在这两种类型的餐中，“早期”味道在前 8 分钟提供，“晚期”味道在后 8 分钟提供。因此，老鼠在高卡路里餐中接受早期（+）和晚期（+）的训练，在低卡路里餐中接受早期（-）和晚期（-）的训练。然后，在一系列两瓶选择测试中，通过早期（+）与早期（-）、晚期（+）与晚期（-）、早期（+）与晚期（+）以及早期（-）与晚期（-）之间的比较，评估对早期（+）和晚期（+）的习得偏好的强度。老鼠更喜欢早期（+）和晚期（+）而不是各自的（-）味道。但只有在老鼠饥饿时才会更喜欢早期（+）。当老鼠饥饿或最近吃饱时，晚期（+）会被优先选择。这表明在进餐的不同时间点对味道的学习产生了不同的关联。虽然这并不支持餐后效应与后来出现的（“甜点”）味道关联最强的观点，但它确实表明了为什么在没有饥饿感的情况下，甜点味道仍然具有吸引力的原因。
