For the first time, we attempted to quantify the independent role of cognition (episodic memory) as a determinant of satiety in humans. This was achieved by covertly manipulating the amount of soup entering or leaving a soup bowl during a meal. Immediately after consuming the soup, hunger ratings were suppressed. Participants who consumed 500 ml reported a greater reduction in hunger than those who consumed 300 ml. We attribute this to the immediate proximal effect of the food promoting neural and endocrine signalling 
. By contrast, at meal termination, we found little evidence that hunger was mediated by a memory for the amount of food that had been presented at the beginning of the meal (300 ml or 500 ml). This result contrasts a previous finding interpreted as marked insensitivity to the physical volume of food served from a self-filling soup bowl 
Further into the inter-meal interval, a different pattern of results was observed. Two and three hours after the meal, hunger was no longer predicted by the actual amount consumed. This was the case despite the fact that participants ate either 300 ml or 500 ml of soup. Instead, where differences in hunger were observed, these related to the perceived amount at the beginning of the meal. Specifically, participants who were shown 500 ml of soup experienced greater satiety than those who were shown 300 ml. This result accords with Higgs’ original proposition that satiety is influenced by memory for a recently consumed meal 
. In particular, it fits well with a previous observation that effects of memory recall are evident three hours after eating but not after only one hour 
. More generally, our findings are noteworthy because they reveal that the role for memory processes is substantial and that it can be exposed even without the need to explicitly cue a memory of recent eating.
The prospect that satiety is influenced by memory for recent eating is consistent with studies exploring the role of expectations around mealtime. Several studies show that beliefs about the content or energy density of a meal can have a marked effect on subsequent hunger and fullness 
. One suggestion is that information about a meal may trigger ‘meal schema’ that influences intake at a subsequent meal 
. For example, Capaldi and colleagues have reported differences in subsequent intake after eating a food described as either a ‘meal’ or as a ‘snack’ 
. Our findings also add to emerging evidence that the retrieval of food-related imagery can impact appetite and energy intake directly 
One possibility is that memory for recent eating serves a purpose. Specifically, it may help to interpret post-ingestive signals by attributing them to a recently consumed meal 
. We suggest that this process enables a ‘tuning’ or contextualisation of the interoceptive signals that are generated by ‘satiety hormones’ such as cholecystokinin (CCK) 
. One of the important advantages of our approach is that it enables us to isolate and estimate the impact of post-ingestive feedback after controlling for effects of memory for recent eating. Two and three hours into the inter-meal interval, participants who consumed very different amounts of soup (300 ml or 500 ml) reported broadly similar hunger, suggesting blunted sensitivity based on post-ingestive feedback alone. Consistent with this interpretation, amnesiacs appear to experience a disconnect between feelings of hunger and appetitive behaviour 
. This failure to recognise and integrate visceral sensations also accords with animal hippocampal-lesion studies showing an impaired ability to use interoceptive states to predict a shock or a reward in ambiguous environments 
. In particular, one suggestion is that the hippocampus is responsible for inhibiting food intake and that this process can be conceptualised as an example of negative occasion setting 
These ideas are important and well grounded. Nevertheless, they remain largely untested in humans. In particular, an opportunity exists to explore obese/lean differences in memory function and appetite control. One hypothesis is that diets that are high in saturated fat impair hippocampal function and that this leads to a deficit in memory performance 
. This results in weakened inhibitory control, leading to greater consumption of high-fat foods, further impairment, and further weight gain 
. In future, this proposition might be explored by comparing the effects of our manipulation in consumers of high- and low-fat diets and/or in patients with Alzheimer’s disease.
A potential concern is that the manipulation of perceived intake amounts to a form of deception that tells us little about normal appetite regulation. In response, we note that our volunteers were unaware that the volume of soup had been manipulated. This makes it very difficult to attribute our findings to a simple demand characteristic. An important issue relates to whether the memory for recent eating can be modified. Many everyday behaviours are supported by implicit memory 
- in other words, memories that facilitate performance without the need for conscious or intentional recollection of those experiences (e.g.,
walking to work). In this context, learning is often regarded as ‘incidental’ because it occurs without conscious effort. Memory for recent eating might also be influenced by explicit processes that are under conscious control. If this is the case then an opportunity exists to enhance satiety by avoiding distraction and encouraging encoding during a meal. In a recent study, Higgs and Donohoe showed that focusing on the sensory characteristics of a food (while eating) leads to a reduction in intake at a subsequent meal 
. They do not distinguish between implicit and explicit processes. Nevertheless, this is an exciting finding and one that holds promise as the basis for a novel therapeutic intervention.
Our manipulation check indicates that participants are able to discriminate between a 300 ml and a 500 ml portion of soup and that this ability is also expressed in memory for these portions, both immediately after exposure and after a two-hour interval. This is critical, because it shows that memory is differentially influenced by our manipulation, even though participants are never instructed to encode the amount that they have consumed. In of itself, this does not demonstrate a causal relationship between hunger and memory for recent eating. However, this would seem a parsimonious explanation for our findings. Nevertheless, two alternatives merit consideration. First, the effect of perceived volume reflects subtle differences in the capacity of the (perceived) large and small portion to elicit a conditioned cephalic phase response at the time of ingestion 
. In relation to this idea, we note that the effect of perceived volume was evident only after a delay of two hours and not immediately after eating, as might be expected were this the case. Nevertheless, it is clear that ‘high level’ beliefs and cognition can influence stomach emptying and endocrine responses to foods and beverages 
and we recommend that these measures should be included in future research. A second possibility is that perceived volume influenced mood. Specifically, participants who saw a 300-ml portion may have been disappointed by its small size and responded to this negative response by rating their hunger higher than those who saw a 500-ml portion. In relation to this proposition, again, we suggest that a ‘protest vote’ would be more likely immediately after receiving the small portion. Instead, both immediately and at one-hour post-consumption, participants experienced a similar reduction in hunger irrespective of whether they saw 300 ml or 500 ml. Notwithstanding this point, to eliminate this hypothesis with certainty we would recommend that measures of mood and ‘portion satisfaction’ should be included in future protocols.
In addition to the immediate effects of memory on post-meal hunger and fullness, we also assessed effects on the expected satiation of a fixed portion of soup (400 ml) at a subsequent test session. Regardless of amount eaten, those participants who initially saw a smaller portion of soup (day 1) then went on to expect the 400 ml portion to be relatively less satiating (day 2). A likely explanation is that participants were biased by their recent post-ingestive experience. Those who initially saw a large portion then went on to experience a greater reduction in hunger. This memory for hunger then biased estimates of expected satiation 24-hours later. Although this interpretation remains to be tested formally, it is consistent with models that characterise the retrieval of abstract knowledge (expected satiation included) in terms of multiple activation of episodic memory traces 
. Indeed, recent models emphasis the importance of recency in this form of learning 
Previously, we have shown that expected satiation is dynamic and it ‘drifts’ over time 
. Shifts are more likely to involve an increase than a decrease in expectations. However, once learned, these expectations may be preserved over long periods 
, perhaps even permanently. In future, it would be interesting to measure the extent to which our manipulation leads to sustained changes in expected satiation and the extent to which this generalises to other types of soup. We also note that memories are more likely to be retrieved if retrieval takes place in the same environment in which a memory was encoded 
. In our experiment, participants were tested in the same environment on both days. It remains to be determined whether a shift in expected satiation is dependent on this kind of context-dependent memory.
Finally, memory for recent eating is helpful because it enables us to draw on beliefs about a food, and in particular, beliefs relating to post-ingestive consequences. These expectations are likely to be governed by flavour-nutrient associations that are refined over time as we interact with individual foods 
. It follows that any disruption to flavour-nutrient learning will promote imprecise caloric regulation (impoverished adjustment of energy intake from one meal to the next). In a series of intriguing studies, Davidson and Swithers have experimentally manipulated the extent to which sweet tastes, viscosity, and fat predict positive nutritive post-ingestive consequences 
. Consistently, animals that are exposed to an ‘inconsistent’ diet are found to increase food intake and bodyweight. This raises important questions about our own diet and the use of fat substitutes and artificial sweeteners in many manufactured foods. The prospect that these foods disrupt our memory for recent eating warrants attention and this represents a natural extension of the work that we present here.