Memory and its role in appetite regulation

Abstract

The importance of memory processes for the formation and expression of conditioned food preferences and satieties has long been appreciated. Recently, based on the eating of multiple meals in amnesic patients, it has been suggested that information about a recent eating episode may be factored into decisions about how much to consume at the next meal. In support of this, it has been shown that enhancing memory for a recent meal, by cueing neurologically intact participants to recall items eaten at lunch, suppresses intake at a taste test later in the afternoon. This effect is specific to recalling food eaten that day, since asking participants to think about lunch consumed the previous day had no effect on intake. These studies suggest that memory for recent eating has a role to play in controlling everyday eating. However, the involvement of memory and cognition does not exclude learnt control by physiological after effects of the recent meal; indeed, this seems likely from the known functions of the hippocampal system that is damaged in amnesic patients.

Introduction

Memory refers to a range of processes that enable information from the past to be used to guide present and future behaviour. Many contemporary theories of memory propose that, rather than being localised in a specialised memory system, the encoding storage and retrieval of the information that is required for memory are due to plastic neural activity in a distributed network comprising many different brain areas [1], [2].

Learning is the acquisition of memories, and associative conditioning processes are thought to be fundamental to the ability of humans and other animals to respond in an adaptive manner to the consequences of ingesting foods and fluids. Extensive evidence has been found that dietary behaviour in rats, monkeys and human beings is learnt [3], [4], [5], and that associative conditioning (which relies on memory processes) is the most likely mechanism underlying the learning of food likes and dislikes [6]. Although there may be some differences between conditioned food preferences and other classically conditioned responses (e.g., conditioned preferences are usually resistant to extinction), much of the evidence is consistent with the suggestion that their acquisition involves some initial processing in memory of the relationship between a flavour (conditioned stimulus) and its postingestive consequences (unconditioned stimulus). This facilitates storage of the association, such that when the flavour is next encountered, it acts as a cue for retrieval, and behaviour can be modified based on the predicted consequences of ingestion [7], [8].

There is also a large body of literature relating to the learning and memory processes involved in motivated responding for food and other rewarding stimuli [9], [10]. Recently, much progress has been made in identifying the associative mechanisms that underlie responding for rewards such as food and drugs of abuse, and the neural substrates that might underpin such processes in laboratory animals. For example, there is evidence that memorial representations of the affective and sensory properties of rewarding stimuli, and their incentive value, play an important role in determining motivated behaviour directed towards obtaining such rewards, and that these processes are likely to be mediated by activity in brain areas such as the amygdala, nucleus accumbens, and prefrontal and gustatory cortex [11], [12].

While the importance of memory in learnt appetites and food motivation has been appreciated for some time, the wider role that memory plays in the control of eating behaviour has yet to be investigated fully, either by ingestive behaviour researchers, or by cognitive psychologists interested in memory [13]. This is surprising for several reasons. First, eating and drinking form a large part of our daily life, meaning that there are plenty of opportunities for information concerning past eating episodes to be gathered and processed. Second, decisions concerning what, when, and how much to eat, often occur in the absence of direct sensory contact with foodstuffs, suggesting that representations of prior eating are required to determine which items are appropriate for consumption. Relatedly, human eating is a highly complex behaviour that is the outcome of the integration in memory of many different inputs, including sensory, somatic, affective, socio-cultural and contextual information [5]. It is also noteworthy, that there has been little cross-talk between researchers interested understanding the physiological controls of eating, and those working on learning and memory theory. Perhaps as a consequence of this, many models of ingestive behaviour consider physiological and cognitive controls of feeding as separate rather than fully integrated components of the same system.

The aim of this paper is to explore the suggestion that memory of a recent eating episode is important in the control of food intake in humans [14]. Whereas previous work has examined the influence of memory for the predicted consequences of eating acquired over repeated experiences, the novel perspective adopted here will be to examine whether memory for the specific attributes of recently eaten foods has a role to play in determining subsequent consumption.