Once they had established that the animals relied on their prior beliefs, the researchers set out to find how the brain encodes prior beliefs to guide behavior.
They recorded activity from about 1, neurons in a region of the frontal cortex, which they have previously shown is involved in timing. To make sense of these signals, the researchers analyzed the evolution of neural activity across the entire population over time, and found that prior beliefs bias behavioral responses by warping the neural representation of time toward the middle of the expected range. Researchers believe that prior experiences change the strength of connections between neurons.
The strength of these connections, also known as synapses, determines how neurons act upon one another and constrains the patterns of activity that a network of interconnected neurons can generate. The finding that prior experiences warp the patterns of neural activity provides a window onto how experience alters synaptic connections.
As an independent test of these ideas, the researchers developed a computer model consisting of a network of neurons that could perform the same ready-set-go task.
Using techniques borrowed from machine learning, they were able to modify the synaptic connections and create a model that behaved like the animals. These models are extremely valuable as they provide a substrate for the detailed analysis of the underlying mechanisms, a procedure that is known as "reverse-engineering. The model also had a warped representation of time according to prior experience.
The researchers used the computer model to further dissect the underlying mechanisms using perturbation experiments that are currently impossible to do in the brain. Using this approach, they were able to show that unwarping the neural representations removes the bias in the behavior. This important finding validated the critical role of warping in Bayesian integration of prior knowledge. The researchers now plan to study how the brain builds up and slowly fine-tunes the synaptic connections that encode prior beliefs as an animal is learning to perform the timing task.
Previous item Next item. Massachusetts Institute of Technology. Search MIT. Search websites, locations, and people. Enter keywords to search for news articles: Submit. Browse By. How expectation influences perception. Consistent with this, the manipulation of expectation at test showed more accurate recognition performance for expected than unexpected stimuli when familiarity was the basis of recognition Experiment 2a. Similarly, when recollection was the basis for recognition, unexpected stimuli at retrieval showed more accurate recognition performance than did expected stimuli Experiment 2b.
Overall, the findings show that expectations applied either at encoding or at retrieval have contrasting effects on familiarity and recollection responses, with expected stimuli selectively enhancing reliance on familiarity and unexpected stimuli selectively enhancing reliance on recollection. The mechanisms through which contextual expectation modulates memory are likely to be somewhat different for encoding and retrieval, although a degree of overlap might be predicted.
These two sources of expectation effect will, therefore, be discussed separately before concluding with a tentative interpretation that draws on both. Varying the expectedness of stimuli at study appears to have a strong impact on later memory performance and recognition decisions.
This probably relates to the qualitatively different type of processing that expected and unexpected stimuli receive when studied. In the current experiments, we used an expectation manipulation that is not contingent on the status of a stimulus as old or new as in Kafkas and Montaldi a , but is instead contingent on the contextual cues preceding each trial.
Enhanced learning for the unexpected stimuli was found in the present study as in our previous study only when recollection was the basis of recognition. This significantly extends previous findings that suggest that the more distinctive, or salient, stimuli within a list of items are remembered with greatest accuracy in recall tasks e. The mechanism by which distinctive information, such as the encounter of unexpected stimuli, results in greater recollection later at retrieval, may be explained using evidence from cognitive neuroscience regarding the way the hippocampus contributes to memory.
Specifically, unexpected stimuli, drive the creation of highly pattern-separated representations at encoding Kirwan and Stark, , Norman, At retrieval, pattern separation, a hippocampal computation LaRocque et al.
Our findings, therefore, suggest that the encounter of unexpected stimuli boosts pattern separation at encoding, thus creating more distinctive representations of these events, resulting in greater or more efficient pattern completion at subsequent retrieval. Such a memory updating mechanism agrees with theoretical models stressing the interplay between prediction, novelty detection and episodic memory when recollective memories are formed Kafkas and Montaldi, in press , Wahlheim and Zacks, in press.
Of particular novelty is the finding that alongside this unexpected-recollection effect, an expected-familiarity effect was found at encoding, whereby the subsequent recognition of expected stimuli was selectively enhanced for familiarity-based recognition but not for recollection-based recognition.
Therefore, stimuli that are consistent with a learned contextually predictive rule, and are consequently expected, show a familiarity advantage at later retrieval. This effect, while suggestive of the congruency effect, whereby stimuli that are re-experienced in contexts congruent with previous experience are better retrieved using recognition and recall tasks e.
The congruency effect has been found to affect predominantly recall and recollection, rather than familiarity as found here, and also to depend on links between stimuli which have strong pre-experimental semantic associations e. Therefore, although at first glance these effects might seem related, it is unlikely that the mechanism that underlies the congruency effect also explains our expected-familiarity effect.
Another, possibly more likely, explanation for the observed familiarity advantage for expected stimuli at encoding, may relate to the role of expectation in visual perception and its effect on enhancing the experiential regularity-driven similarity of the encoded stimuli.
Visual discrimination and processing are enhanced when cues that predict stimuli are presented than when no cues or invalid cues are presented e. In previous studies, visuospatial cues e. A similar attentional mechanism may also underlie the selective familiarity enhancement observed here at encoding resulting in more efficient processing of the expected information.
The expected stimuli may, therefore, be processed more readily due to attention-mediated enhanced processing efficiency. But why this mechanism appears to benefit familiarity more than recollection remains to be explained. The answer may relate to the role of similarity, or global matching, in familiarity detection Hintzman, In particular, it is argued that familiarity is modulated by the degree of similarity that exists between current sensory inputs and stored representations.
At encoding, the fact that expected stimuli current sensory inputs are processed more efficiently due to the aforementioned effect of expectation on visual perception may result in experiencing greater similarity between sensory inputs and stored representational characteristics that are consistent with expectation.
This form of experiential similarity thus selectively enhances later familiarity more than recollection, as the latter is driven by distinctiveness rather than similarity as discussed above.
Critically, this familiarity enhancement is likely to be more pronounced when encoding relies on the free viewing of information, as used in Experiment 1a.
In contrast, a semantic encoding task as used in 1b would ensure that the distinctive characteristics of the unexpected stimuli were more pronounced, thus leading to the creation of pattern-separated representations and recollection.
As the data clearly show, expectation not only modulated the encoding of information, but it also had an effect on recognition-based retrieval when manipulated post-encoding Experiments 2a and 2b.
In this way, manipulating the expectation status of stimuli at retrieval, just before participants made recognition decisions, again had opposing effects on familiarity and recollection. Consistent with the encoding effects, expected stimuli were more likely to be deemed familiar than were unexpected ones, whereas unexpected stimuli were more likely to be recollected than were expected stimuli.
Interestingly, these effects were only observed for truly old stimuli, as reflected by increased hit rates with no change in false alarm rates. Therefore, although manipulations of contextual expectation resulted in opposing effects on familiarity and recollection performance, these effects related to the modulation of recognition for already studied materials and did not lead to alterations in the criteria set for hit detection or for false alarms. Thus, one can conclude that the mechanism, or mechanisms, underlying these effects must be acting on the processes supporting the comparison between the sensory stimulus input and the stored stimulus representation for familiarity and the cued by the input stimulus recall of the stored representation for recollection , at the point of test, as they do not act on sensory stimulus inputs that have no previously formed internal representation i.
As described in the Introduction, previous studies have also shown that varying the context in which recognition occurs, either in terms of the stimulus characteristics within a test list e. Whittlesea, b , Whittlesea, has suggested that memory retrieval is the outcome of two interactive cognitive operations; a response to or performance on a current event, and an evaluation process which attributes the produced response to a prior occurrence in the past, taking into account prevalent contextual factors.
Similarly, the source monitoring approach suggested by Johnson and colleagues e. Jacoby and colleagues have also argued that attribution and inference are important components of recognition memory decisions to the extent that judgments on a past occurrence take into account characteristics of the current situation Jacoby and Whitehouse, , Jacoby et al.
Our retrieval findings are highly consistent with these arguments and further show that expectations generated within a retrieval context are subjected to evaluation and affect recognition judgments. Traditionally, theories of recognition memory have regarded familiarity as an acontextual form of memory, which responds to the absolute memory strength of a stimulus Tulving, Instead, the manipulation in Experiment 2a shows that familiarity judgments are sensitive to contextual factors, and more specifically to the expectations formed by the contextual characteristics of the retrieval setting.
However, the source of the effect of retrieval-related expectation on familiarity and recollection memory remains to be established. This effect cannot be attributed to differential encoding of expected and unexpected information, as the expectation manipulation occurs at retrieval and therefore both categories of stimulus received the same type of encoding-related processing. It is, therefore, the retrieval context that differentially triggered the kind of memory retrieval employed when encountering expected and unexpected stimuli.
For example, studied expected stimuli may have led to greater feelings of fluency than studied unexpected stimuli. This fluency may then result in the experience of greater similarity between the stored representation and the stimulus presented at retrieval, making it more likely that the stimulus be deemed familiar. In contrast, studied unexpected stimuli would not have been processed as fluently as the expected ones, albeit more fluently than new unexpected stimuli.
Therefore, the encounter of old unexpected stimuli may have prompted a memory search leading to the triggering of pattern-completion processes resulting in greater recollection.
One important aspect of the design of the current experiments was that participants were instructed to focus on the recognition tasks, and at debriefing, only 2 participants reported noticing the violation of the learned symbol-stimulus sequence in a subset of the trials.
This means that the effect of expectation on familiarity and recollection decisions does not necessitate conscious awareness of the operation of contextual cues but instead works implicitly. Indeed, awareness of the expectation manipulation may result in abolishing the effect on familiarity and recollection in the same way that the effect of fluency on recognition decisions is eliminated when participants can attribute it to another source, and not on past experience of the stimulus see e.
Taken together, these findings suggest that expectation, defined by the encoding or retrieval context, critically affects the processes and mechanisms drawn upon to support learning and memory.
The findings show that, at encoding, the level of expectation a stimulus carries ensures that the type of representation formed contains defining information that later, at test, emphasises either the similarity or the distinctiveness of a stimulus.
Furthermore, the level of expectation defined by the context in which retrieval takes place informs the kind of retrieval mechanisms that are spontaneously triggered; leading either to the application of global matching and an enhanced use of familiarity-based recognition, or to rigorous pattern completion and an enhanced use of recollection-based recognition.
A striking outcome of this research is that, not only is the accuracy of memory for the contextually distinctive, or unexpected events better, driven by contextual distinctiveness enhancement mechanisms at encoding and retrieval, but so too is the accuracy of memory for the expected events, driven instead by contextual similarity enhancement mechanisms at encoding and retrieval.
The finding that expected stimuli give rise to more accurate familiarity memory, whereas unexpected stimuli give rise to more accurate recollection, further highlights the discrete nature of these two kinds of memory. Experiences of familiarity and recollection respond to qualitatively different attributes of the test stimuli and their stored representations. Recollection is supported by the recovery of distinctive information, e.
In contrast familiarity is not simply the lack of such a recovery — and thus a weaker memory — but instead, reflects a mechanism that selectively boosts the similarity between a stored representation and the expected current sensory experience global matching resulting in enhanced feelings of memory. These findings have important implications for learning protocols and learning environments, as they suggest that exposure to information that is distinctive where distinctiveness can be defined and manipulated in a number of ways will boost the construction and recovery of rich associative memories.
On the other hand, exposure to information that follows previously learned regularities, that are readily predicted, or that can be easily accommodated into informational templates learned over time e. Therefore, the current findings argue that learning environments and learning protocols should be designed to take advantage of both the contextual distinctiveness and the contextual similarity enhancement mechanisms, proposed here, by triggering both mechanisms optimally.
More broadly, the findings also speak to previous theoretical views e. National Center for Biotechnology Information , U. Sponsored Document from. Author information Article notes Copyright and License information Disclaimer. Alex Kafkas: ku. This article has been cited by other articles in PMC. Abstract Our ability to make predictions and monitor regularities has a profound impact on the way we perceive the environment, but the effect this mechanism has on memory is not well understood.
Introduction A fundamental function of the human mind is the ability to infer predictions and form expectations Bar, , Hunt and Aslin, , Schacter et al. The effect of expectation on the encoding of information The processing of information that takes place at encoding is critical for memory formation, as it may determine the extent to which successful memories are formed and the type of memory experienced later, at retrieval Davachi and Dobbins, , Kafkas and Montaldi, , Paller and Wagner, , Schacter et al.
The effect of expectation on information retrieval Another outstanding question is how expectations operating at retrieval may affect memory and whether this effect may be similar or different from the effect of expectations at encoding. The present study The aim of the current set of experiments is, therefore, to directly investigate the effect of expectation on recognition memory performance, and more specifically on familiarity and recollection.
Methods 2. Open in a separate window. Procedure and design The design of the different experiments reported here was very similar, with the main difference being where the expectation manipulation would occur. Data analyses Memory performance for familiarity collapsed across the three levels of the rating scale and recollection responses was calculated by subtracting the false alarm rate FA from the corresponding hit rate for each response outcome.
Results Rule-learning task. Table 1 Mean proportions and response times RTs in ms for the different response outcomes at recognition for expected and unexpected at encoding stimuli in Experiment 1 encoding manipulation.
Experiment 2: effect of expectation at retrieval on familiarity and recollection performance In Experiment 2a, participants successfully completed the encoding matching-to-sample task with a mean accuracy score of 0.
Table 2 Mean proportions and response times RTs in ms for the different response outcomes at recognition for expected and unexpected stimuli in Experiment 2 retrieval manipulation. General discussion 4. Summary of the results While growing evidence supports the view that expectation modulates memory, the specific form that this modulation takes is not yet known.
Encoding expected and unexpected information Varying the expectedness of stimuli at study appears to have a strong impact on later memory performance and recognition decisions. The effect of expected and unexpected stimuli at retrieval As the data clearly show, expectation not only modulated the encoding of information, but it also had an effect on recognition-based retrieval when manipulated post-encoding Experiments 2a and 2b.
Conclusions and implications Taken together, these findings suggest that expectation, defined by the encoding or retrieval context, critically affects the processes and mechanisms drawn upon to support learning and memory. The authors declare no competing interests. Appendix A. Supplementary material The following are the Supplementary data to this article: Supplementary Data 1: Click here to view. Supplementary Data 2: Click here to view. References Bar M. The proactive brain: Memory for predictions.
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