Sign-Tracking and the Integrated Reward System

How might the three functions of the integrated reward system encourage the performance of Sign-Tracking? Note that during Sign-Tracking procedures, the subject receives delivery of rewarding food, which presumably elicits the emotional response of pleasure. In addition, note that repeated pairings of the lever with the rewarding food are likely to lead to the association of the lever with the rewarding feelings of pleasure. The association of the lever with pleasure will increase the chances that the lever will become the target of the psychomotor activation syndrome. Thus, due to the integrated reward system, the subject will approach the lever, contact the lever, then lick, gnaw and chew the lever, performing the actions of the Sign-Tracking response. To illustrate further, we provide videos showing demonstrations of Sign-Tracking in the laboratory.

Sign-tracking video 1: Lever paired with food.
This video shows trials 5, 10, 15, 20 and 25 of the 25 total trials of a daily session of training with Sign-Tracking procedures. The subject is a Long-Evans hooded rat. The setting is a laboratory apparatus where the insertion of a metal lever is followed by the presentation of a pellet of food. The food pellet is delivered regardless of what the subject does. The lever is located to the left of the food tray and is inserted for 5 seconds. The removal of the lever is followed immediately by the delivery of the food pellet.

Note that on trials 1 and 2 on the video, the subject begins to show the first signs of associating the lever with the food. Note also that on later trials the behavior of the subject becomes more and more centered upon the lever. Finally, after many pairings of the lever with the food, the subject reacts to the lever by attempting to eat it, as though the lever was a pellet of food. This is the Sign-Tracking response. The performance of the Sign-Tracking response is perplexing because it serves no purpose. It is totally unnecessary and a complete waste of time and energy. But, due to pairings of the lever with the food, the presentation of the lever comes to reflexively trigger the actions of eating the lever. It seems unlikely that the intention of the subject is to eat the lever, because the lever is made of stainless steel and has no nutritional value. Yet, the action of the subject is to perform the Sign-Tracking response and to do so, on trial after trial. Although the behavior makes no sense, reports of Sign-Tracking in the rat, due to pairings of lever with food, have been extensively documented in scientific manuscripts published by researchers working in experimental laboratories around the world.