Reward value effects on reward timing

Animals and people need to learn when important resources such as food, water, or a mate are going to be available. One of the ways they can do this is to recognise cues that precede the availability of a resource such as food. Over time, they can refine this ability and respond to these cues primarily around the time that food is going to be obtainable. To do this some kind of timing device is required. Humans can use a watch or a similar mechanical device; other species do not have this luxury. Work using primarily humans, rats and pigeons has determined that a form of internal clock, a sort of stopwatch in the brain, can be started, stopped and reset. Unlike the stopwatches humans use, these internal clocks are not always accurate; some drugs, modulations in body temperature and even some loud noises can affect the way in which people and animals time. Experiments in our laboratory have found that changing the value of the food, or 'reward', that rats receive seems to affect their perceptions of when the reward will occur. If we train rats on a small reward and then make it larger, the rats seem to expect it to occur sooner than it actually does. If, on the other hand we decrease the value of the reward, for instance making a larger reward smaller, or making the reward less palatable to the rat, they respond as if they believe the reward should be available later than it actually is. This may seem strange, but we humans do this in everyday life. If you know you are going on holiday, your last day at work or school seems to drag. It seems as though your holiday, the thing you have been looking forward to, should start sooner. But why should this be? This proposal aims to find the mechanism responsible for this altered perception of the time of reward delivery. Within the internal clock are two possible mechanisms that could be responsible; the pacemaker, the stopwatch itself; or the reference memory, where all the previously experienced durations are stored. We are also proposing to look at other possible influences on the perception of time, such as the attention given to the task; the motivation or arousal level of the rat; and also any behaviour patterns the rat may be using as an alternative way of tracking time. The results of these investigations will hopefully provide a better understanding of the way all species predict the time to reward and how changing the value of the reward may alter this perception. This is especially relevant to work involving impulsivity, such as investigations into gambling, addiction, attention deficit hyperactivity disorder (ADHD) and a host of other conditions. In these cases individuals have a distorted perception of the value of a reward when the delay to that reward is manipulated. Here we are looking at the reciprocal question of why the perception of the delay to a reward is distorted when the value of that reward is manipulated.

Research in the York timing lab has indicated that reward value and reward timing are not independent. Specifically, rats that were trained on a 60-s peak procedure displayed substantial peak shifts when the value of the reward increased, or the reward was devalued through either a taste aversion or satiety treatment. This proposal seeks to further examine this effect of reward value on timing. Initial experiments (section C1) will examine appetitive and aversive techniques whilst increasing and decreasing the value of the reward using the peak procedure. This will determine whether aversive or appetitive increases and decreases in value operate in the same manner. These experiments will also establish the most robust parameters for subsequent experiments. Successive experiments will utilise these parameters to assess value manipulations on the bisection procedure (section C2). Both between and within phase shifts in reward value can be examined here and should give an initial indication of the possible mechanism involved in the perceived shifts in the time to reinforcement; for example, clock speed fluctuations or reference memory storage. The following series of experiments (Section D) will further assess alterations in these timing mechanisms using modified versions of the peak procedure. Assessment of the shape and progression of changes in the peak functions will indicate which mechanism is likely to be involved in the altered perception of the time to reward. A final series of experiments (Section E) will investigate other the contribution of arousal and/or attention to the value shift effects on timing. Also examined are adjunctive behaviours; the pattern and duration of which may be disrupted by a change in the reward value. The results should give an insight into the mechanisms involved in interval time perception and will have implications for the study of delay discounting, impulsivity and related conditions such as addiction and ADHD.