Can deja vu result from similarity to a prior experience? Support for the similarity hypothesis of deja vu

A virtual reality paradigm with dynamic scene stimuli for use in memory research

Episodic memory may essentially be memory for one's place within a temporally unfolding scene from a first-person perspective. Given this, pervasively used static stimuli may only capture one small part of episodic memory. A promising approach for advancing the study of episodic memory is immersing participants within varying scenes from a first-person perspective. We present a pool of distinct scene stimuli for use in virtual environments and a paradigm that is implementable across varying levels of immersion on multiple virtual reality (VR) platforms and adaptable to studying various aspects of scene and episodic memory. In our task, participants are placed within a series of virtual environments from a first-person perspective and guided through a virtual tour of scenes during a study phase and a test phase. In the test phase, some scenes share a spatial layout with studied scenes; others are completely novel. In three experiments with varying degrees of immersion, we measure scene recall, scene familiarity-detection during recall failure, the subjective experience of déjà vu, the ability to predict the next turn on a tour, the subjective sense of being able to predict the next turn on a tour, and the factors that influence memory search and the inclination to generate candidate recollective information. The level of first-person immersion mattered to multiple facets of episodic memory. The paradigm presents a useful means of advancing mechanistic understanding of how memory operates in realistic dynamic scene environments, including in combination with cognitive neuroscience methods such as functional magnetic resonance imaging and electrophysiology.

Associative asymmetry of the recognition without cued-recall effect in thematic relations

In the present study, two experiments were conducted to examine whether thematic relation can produce recognition without cued-recall effect and whether the direction of the association has an influence on this effect. The participants provided higher familiarity ratings for studied items than for unstudied items during target retrieval failure. Additionally, the thematic relation-elicited recognition without cued-recall effect was larger in the forward association than in the backward association. Collectively, these results indicated that thematic relations can elicit the recognition without cued-recall effect, and this effect is asymmetrical. The current findings support the features overlap hypothesis described in the global match model.

The Subjective Experience of Autobiographical Remembering: Conceptual and Methodological Advances and Challenges

The investigation of the phenomenology of autobiographical memories (i.e., how a memory is subjectively experienced and its meaning) has provided an important contribution to our understanding of autobiographical remembering. Over the last two decades, the study of phenomenology has received widespread scientific attention, and the field has undergone quite relevant conceptual and methodological changes. In the present work, we (1) review some basic and well-established research findings and methodological achievements; (2) discuss new theoretical and methodological challenges, with a special focus on the issue of the phenomenological experience of the retrieval process and its relationship with the phenomenology of the products of retrieval; and (3) propose an alternative way of conceptualizing and understanding it in the framework of experimental phenomenology.

Towards a conflict account of déjà vu: The role of memory errors and memory expectation conflict in the experience of déjà vu

Déjà vu can be defined as conflict between a subjective evaluation of familiarity and a concurrent evaluation of novelty. Accounts of the déjà vu experience have not explicitly referred to a "conflict account of déjà vu" despite the acceptance of conflict-based definitions of déjà vu and relatively recent neuroimaging work that has implicated brain areas associated with conflict as underpinning the experience. Conflict monitoring functioning follows a similar age-related trajectory to déjà vu with a peak in young adulthood and a subsequent age-related decline. In this narrative review of the literature to date, we consider how déjà vu is defined and how this has influenced the understanding of déjà vu. We also review how déjà vu can be understood within theories of recognition memory and cognitive control. Finally, we summarise the conflict account of déjà vu and propose that this account of the experience may provide a coherent explanation as to why déjà vu experiences tend to decrease with age in the non-clinical population.

Are involuntary autobiographical memory and déjà vu natural products of memory retrieval?

Involuntary autobiographical memories (IAMs) and déjà vu are phenomena that occur spontaneously in daily life. IAMs are recollections of the personal past, whereas déjà vu is defined as an experience in which the person feels familiarity at the same time as knowing that the familiarity is false. We present and discuss the idea that both IAMs and déjà vu can be explained as natural phenomena resulting from memory processing and, importantly, are both based on the same memory retrieval processes. Briefly, we hypothesise that both can be described as "involuntary" or spontaneous cognitions, where IAMs deliver content and déjà vu delivers only the feeling of retrieval. We map out the similarities and differences between the two, making a theoretical and neuroscientific account for their integration into models of memory retrieval and how the autobiographical memory literature can explain these quirks of daily life and unusual but meaningful phenomena. We explain the emergence of the déjà vu phenomenon by relating it to well-known mechanisms of autobiographical memory retrieval, concluding that IAMs and déjà vu lie on a continuum.

Specifying a relationship between semantic and episodic memory in the computation of a feature-based familiarity signal using MINERVA 2

Approaches to modeling episodic recognition memory often imply a separability from semantic memory insofar as an implicit tabula rasa (i.e., blank slate) assumption is apparent in many simulations. This is evident in the common practice of having new test probes correspond to zero memory traces in the store while old test probes correspond to traces representing instances of items' occurrence on a study list. However, in list-learning studies involving word lists, none of the test items would actually correspond to zero items in the person's memory, as all of the test words are generally known to participants, whether old or new. By focusing on a list-learning recognition phenomenon that likely results from feature-based familiarity detection and necessarily involves a role of preexisting knowledge in its mechanisms-the semantic-feature-based recognition without cued recall phenomenon-we show how incorporating preexisting knowledge into the MINERVA 2 model enables it to simulate previously shown empirical patterns with this phenomenon. The simulation patterns reported here raise new theoretical implications worth further exploration, such as the extent to which the variances change in the signal versus the noise distribution when preexisting knowledge is present versus absent in the simulations.

A conceptual space for episodic and semantic memory

I propose a model that places episodic, semantic, and other commonly studied forms of memory into the same conceptual space. The space is defined by three dimensions required for Tulving's episodic and semantic memory. An implicit-explicit dimension contrasts both episodic and semantic memory with common forms of implicit memory. A self-reference dimension contrasts episodes that occurred to one person with semantic knowledge. A scene dimension contrasts episodes that occurred in specific contexts with context-free semantic information. The three dimensions are evaluated against existing behavioral and neural evidence to evaluate both the model and the concepts underlying the study of human memory. Unlike a hierarchy, which has properties specific to each category, the dimensions have properties that extend throughout the conceptual space. Thus, the properties apply to all forms of existing and yet-to-be-discovered memory within the space. Empty locations in the proposed space are filled with existing phenomena that lack a clear place in current theories of memory, including reports of episodic-like memories for events reported to but not witnessed by a person, fictional narrative accounts, déjà vu, and implicit components contributing to personality, the self, and autobiographical memory.

Functional activity changes in memory and emotional systems of healthy subjects with déjà vu

Déjà vu (DV) is a fascinating and mysterious human experience that has attracted interest from psychologists and neuroscientists for over a century. In recent years, several studies have been conducted to unravel the psychological and neurological correlates of this phenomenon. However, the neural mechanisms underlying the DV experience in benign manifestations are still poorly understood. Thirty-three healthy volunteers completed an extensive neuropsychiatric and neuropsychological battery including personality evaluation. The presence of DV was assessed with the Inventory for Deja vu Experiences Assessment. Participants underwent episodic memory learning test, and 2 days later during event-related functional magnetic resonance imaging (fMRI), they are asked to rate old and new pictures as a novel, moderately/very familiar, or recollected. We identified 18 subjects with DV (DV+) and 15 without DV (DV-) matched for demographical, neuropsychological, and personality characteristics. At a behavioral level, no significant difference was detected in the episodic memory tasks between DV+ and DV-. Functional magnetic resonance imaging analysis revealed that DV+, independently from task conditions, were characterized by increased activity of the bilateral insula coupled with reduced activation in the right parahippocampal, both hippocampi, superior/middle temporal gyri, thalami, caudate nuclei, and superior frontal gyri with respect to DV-. Our study demonstrates that individuals who experienced DV are not characterized by different performance underlying familiarity/recollection memory processes. However, fMRI results provide evidence that the physiological DV experience is associated with the employment of different neural responses of brain regions involved in memory and emotional processes.

A postdictive bias associated with déjà vu

Recent research links reports of déjà vu - the feeling of having experienced something before despite knowing otherwise - with an illusory feeling of prediction. In the present study, a new finding is presented in which reports of déjà vu are associated not only with a predictive bias, but also with a postdictive bias, whereby people are more likely to feel that an event unfolded as expected after the event prompted déjà vu than after it did not. During a virtual tour, feelings of predicting the next turn were more likely during reported déjà vu, as in prior research. Then, after actually seeing the turn, participants exhibited a postdictive bias toward feeling that the scene unfolded as expected following déjà vu reports. This postdictive bias following déjà vu reports was associated with higher perceived scene familiarity intensity. A potential reason for this association may be that high familiarity intensity as an event outcome unfolds falsely signals confirmatory evidence of having sensed all along how it would unfold. Future research should further investigate this possibility.

Effects of scripted activities on false feelings of having previously visited a location

False memories of one's past are often created by unconscious influences of previous experiences. This study examined whether action sequences, scripts that are frequent in everyday life, might induce false beliefs of having visited a location. Participants were shown photos of places they had not previously visited. Next, they rated how strongly they felt that they had visited the locations in the target scenes. Results indicated that when typical actions were presented in canonical order before the target scene, the feeling of having previously visited the location increased, relative to a condition with a random ordering of typical actions or to one with no presented actions. This study suggests that activation of script knowledge and misattribution of the source of feelings of familiarity elicit false beliefs of having visited a specific location, even if one understands that one has never previously visited the location.

Déjà vu and the feeling of prediction: an association with familiarity strength

A recent laboratory study by Cleary and Claxton [2018. Déjà vu: An illusion of prediction. Psychological Science, 29(4), 635-644. http://journals.sagepub.com/doi/full/10.1177/0956797617743018] documented a relationship between déjà vu and feelings of premonition. During instances of retrieval failure, participants reported stronger feelings of prediction during déjà vu than non-déjà vu states, despite displaying no actual predictive ability in such instances. The present study further explored the link between déjà vu reports and feelings of prediction. Although feelings of prediction were more likely to occur during reports of déjà vu than non-déjà vu, they were not the sole defining feature of déjà vu, accounting for just over half of all reported déjà vu states. Instances of déjà vu that were accompanied by feelings of prediction were associated with greater feelings of familiarity than instances that were not. This was shown by a greater likelihood of reporting that the scene felt familiar and also by a higher rated intensity of the feeling of familiarity elicited by the scene when it did feel familiar. Though the present study was mainly descriptive in characterising the interrelations between déjà vu, feelings of prediction, and familiarity, the full pattern points toward the possibility that high familiarity intensity may contribute to the feeling of prediction during déjà vu.

fMRI evidence supporting the role of memory conflict in the déjà vu experience

Attempts to generate déjà vu experimentally have largely focused on engineering partial familiarity for stimuli, relying on an ensuing, but unprompted evaluation of conflict to generate the experience. Without verification that experimentally-generated familiarity is accompanied by the awareness of stimulus novelty, these experimental procedures potentially provide an incomplete déjà vu analogue. We used a modified version of the Deese-Roediger-McDermott (DRM) false memory procedure to generate both familiarity and novelty within a déjà vu analogue - we coupled experimentally-generated familiarity with cues indicating that the familiarity was erroneous, using this additional source of mnemonic information to generate cognitive conflict in our participants. We collected fMRI and behavioural data from 21 participants, 16 of whom reported déjà vu. Using univariate contrasts we identified brain regions associated with mnemonic conflict, including the anterior cingulate cortex, medial prefrontal cortex and parietal cortex. This is the first experiment to image an analogue of the déjà vu experience in healthy volunteers. The increased likelihood of déjà vu reports to DRM critical lures correctly identified as "new", and the activation of neural substrates supporting the experience of cognitive conflict during déjà vu, suggest that the resolution of memory conflict may play an integral role in déjà vu.

Overcoming familiarity illusions in a single case with persistent déjà vu

While occasional déjà vu is benign in the general population, rare neuropsychological cases with persistent déjà vu have been described in the literature. We report the case of MN, a 25-year-old woman, who suffered a cerebral haemorrhage in the right thalamo-callosal region and experienced recurrent déjà vu episodes. Through clinical interviews and memory tasks related to déjà vu, we assumed that source memory errors and an inappropriate feeling of familiarity (measured by the number of false recognitions) were critically involved in MN's déjà vu. Based on this, we developed the first neuropsychological intervention dedicated to déjà vu. The rationale was to train MN to detect elements that could produce an inappropriate feeling of familiarity and to promote metacognitive awareness about déjà vu. This intervention was effective at reducing the frequency of déjà vu episodes in MN's daily life, as well as the number of false recognitions in memory tasks. In addition to its clinical contribution, this single-case study contributes to the limited literature on patients whose déjà vu is not related to epileptic abnormalities and medial temporal brain damage, and provide supportive evidence of the role of an erroneous feeling of familiarity and of metacognitive processes in déjà vu.

Déjà Vu: An Illusion of Prediction

Déjà vu is beginning to be scientifically understood as a memory phenomenon. Despite recent scientific advances, a remaining puzzle is the purported association between déjà vu and feelings of premonition. Building on research showing that déjà vu can be driven by an unrecalled memory of a past experience that relates to the current situation, we sought evidence of memory-based predictive ability during déjà vu states. Déjà vu did not lead to above-chance ability to predict the next turn in a navigational path resembling a previously experienced but unrecalled path (although such resemblance increased reports of déjà vu). However, déjà vu states were accompanied by increased feelings of knowing the direction of the next turn. The results suggest that feelings of premonition during déjà vu occur and can be illusory. Metacognitive bias brought on by the state itself may explain the peculiar association between déjà vu and the feeling of premonition.

Abnormal rsFC and GMV changes in parahippocampal and DLPFC for high Déjà vu experienced subjects

How déjà vu works has long been a mystery, partially because of its characteristics of unpredictable occurrences and quick disappearances, which make it difficult to be explored. Previous studies have described the anatomical structures underlying déjà vu in healthy subjects; however, the functional mechanism of déjà vu remains unclear. Therefore, this study investigated the brain structural and functional components underlying déjà vu by combining voxel-based morphometry analysis (VBM) with resting-state functional connectivity (rsFC). The VBM analysis revealed that the anterior parahippocampal gyrus (PHG) had significantly less grey matter volume (GMV) in high déjà vu group than low group, confirming previous studies. Further functional connectivity analysis revealed that the frequency of déjà vu experiences was negatively correlated with the strength of the rsFC between anterior dorsal lateral prefrontal cortex (DLPFC) and anterior PHG but positively correlated with the strength of the rsFC between posterior DLPFC and posterior PHG. Moreover, the frequency of déjà vu experiences was negatively correlated with the strength of the rsFC between the anterior and posterior regions of the PHG. These findings indicated that familiarity without recollection (PHG) and superior context monitoring (DLPFC) are critical for real-life déjà vu experiences.

Investigating the Role of Assessment Method on Reports of Déjà Vu and Tip-of-the-Tongue States during Standard Recognition Tests

Déjà vu and tip-of-the-tongue (TOT) are retrieval-related subjective experiences whose study relies on participant self-report. In four experiments (ns = 224, 273, 123 and 154), we explored the effect of questioning method on reported occurrence of déjà vu and TOT in experimental settings. All participants carried out a continuous recognition task, which was not expected to induce déjà vu or TOT, but were asked about their experiences of these subjective states. When presented with contemporary definitions, between 32% and 58% of participants nonetheless reported experiencing déjà vu or TOT. Changing the definition of déjà vu or asking participants to bring to mind a real-life instance of déjà vu or TOT before completing the recognition task had no impact on reporting rates. However, there was an indication that changing the method of requesting subjective reports impacted reporting of both experiences. More specifically, moving from the commonly used retrospective questioning (e.g. “Have you experienced déjà vu?”) to free report instructions (e.g. “Indicate whenever you experience déjà vu.”) reduced the total number of reported déjà vu and TOT occurrences. We suggest that research on subjective experiences should move toward free report assessments. Such a shift would potentially reduce the presence of false alarms in experimental work, thereby reducing the overestimation of subjective experiences prevalent in this area of research.

What is French for déjà vu? Descriptions of déjà vu in native French and English speakers

Little is known about how people characterise and classify the experience of déjà vu. The term déjà vu might capture a range of different phenomena and people may use it differently. We examined the description of déjà vu in two languages: French and English, hypothesising that the use of déjà vu would vary between the two languages. In French, the phrase déjà vu can be used to indicate a veridical experience of recognition - as in "I have already seen this face before". However, the same is not true in English. In an online questionnaire, we found equal rates of déjà vu amongst French and English speakers, and key differences in how the experience was described. As expected, the French group described the experience as being more frequent, but there was the unexpected finding that they found it to be more troubling.

FAMILIARITY TRANSFER AS AN EXPLANATION OF THE DÉJÀ VU EFFECT

Déjà vu is often explained in terms of an unconscious transfer of familiarity between a familiar object or objects and accompanying new objects. However, empirical research tests more the priming effectiveness than such a transfer. This paper reviews the main explanations of déjà vu, proposes a cognitive model of the phenomenon, and tests its six major assumptions. The model states that a sense of familiarity can be felt toward an objectively new stimulus (point 1) and that it can be transferred from a known stimulus to a novel one (point 2) in a situation where the person is unaware of such a transfer (point 3). The criteria for déjà vu are that the known and the novel stimuli may have graphical or semantic similarity, but differences exclude priming explanations (point 4); the familiarity measure should be of an non-rational nature (sense of familiarity rather than recognition; point 5); and that the feeling of familiarity toward a novel stimuli produces a conflict, which could be measured by means of increased reaction (point 6). 119 participants were tested in three experiments. The participants were to assess the novel stimuli in terms of their sense of familiarity. The novel stimuli were primed or were not primed by the known stimulus (Exp. 1) or primed by the known vs a novel stimulus (Exp. 2 and 3). The priming was subliminal in all the experiments. Reaction times were measured in Exps. 2 and 3. The participants assessed the novel stimuli as more familiar when they were preceded by a known stimulus than when they were not (Exp. 1) or when they were preceded by a novel stimulus (Exps. 2 and 3). Reaction times were longer for assessments preceded by known stimulus than for assessments preceded by a novel stimulus, which contradicts the priming explanations. The results seem to support all six points of the proposed model of the mechanisms underlying the déjà vu experience.

The awareness of novelty for strangely familiar words: a laboratory analogue of the déjà vu experience

Déjà vu is a nebulous memory experience defined by a clash between evaluations of familiarity and novelty for the same stimulus. We sought to generate it in the laboratory by pairing a DRM recognition task, which generates erroneous familiarity for critical words, with a monitoring task by which participants realise that some of these erroneously familiar words are in fact novel. We tested 30 participants in an experiment in which we varied both participant awareness of stimulus novelty and erroneous familiarity strength. We found that déjà vu reports were most frequent for high novelty critical words (∼25%), with low novelty critical words yielding only baseline levels of déjà vu report frequency (∼10%). There was no significant variation in déjà vu report frequency according to familiarity strength. Discursive accounts of the experimentally-generated déjà vu experience suggest that aspects of the naturalistic déjà vu experience were captured by this analogue, but that the analogue was also limited in its focus and prone to influence by demand characteristics. We discuss theoretical and methodological considerations relevant to further development of this procedure and propose that verifiable novelty is an important component of both naturalistic and experimental analogues of déjà vu.

Déjà vu experiences in healthy subjects are unrelated to laboratory tests of recollection and familiarity for word stimuli

Recent neuropsychological and neuroscientific research suggests that people who experience more déjà vu display characteristic patterns in normal recognition memory. We conducted a large individual differences study (n = 206) to test these predictions using recollection and familiarity parameters recovered from a standard memory task. Participants reported déjà vu frequency and a number of its correlates, and completed a recognition memory task analogous to a Remember-Know procedure. The individual difference measures replicated an established correlation between déjà vu frequency and frequency of travel, and recognition performance showed well-established word frequency and accuracy effects. Contrary to predictions, no relationships were found between déjà vu frequency and recollection or familiarity memory parameters from the recognition test. We suggest that déjà vu in the healthy population reflects a mismatch between errant memory signaling and memory monitoring processes not easily characterized by standard recognition memory task performance.

Recall versus familiarity when recall fails for words and scenes: the differential roles of the hippocampus, perirhinal cortex, and category-specific cortical regions

This fMRI study examined recall and familiarity for words and scenes using the novel recognition without cued recall (RWCR) paradigm. Subjects performed a cued recall task in which half of the test cues resembled studied items (and thus were familiar) and half did not. Subjects also judged the familiarity of the cue itself. RWCR is the finding that, among cues for which recall fails, subjects generally rate cues that resemble studied items as more familiar than cues that do not. For words, left and right hippocampal activity increased when recall succeeded relative to when it failed. When recall failed, right hippocampal activity was decreased for familiar relative to unfamiliar cues. In contrast, right Prc activity increased for familiar cues for which recall failed relative to both familiar cues for which recall succeeded and to unfamiliar cues. For scenes, left hippocampal activity increased when recall succeeded relative to when it failed but did not differentiate familiar from unfamiliar cues when recall failed. In contrast, right Prc activity increased for familiar relative to unfamiliar cues when recall failed. Category-specific cortical regions showed effects unique to their respective stimulus types: The visual word form area (VWFA) showed effects for recall vs. familiarity specific to words, and the parahippocampal place area (PPA) showed effects for recall vs. familiarity specific to scenes. In both cases, these effects were such that there was increased activity occurring during recall relative to when recall failed, and decreased activity occurring for familiar relative to unfamiliar cues when recall failed.

Déjà experiences in temporal lobe epilepsy

Historically, déjà vu has been linked to seizure activity in temporal lobe epilepsy, and clinical reports suggest that many patients experience the phenomenon as a manifestation of simple partial seizures. We review studies on déjà vu in epilepsy with reference to recent advances in the understanding of déjà vu from a cognitive and neuropsychological standpoint. We propose a decoupled familiarity hypothesis, whereby déjà vu is produced by an erroneous feeling of familiarity which is not in keeping with current cognitive processing. Our hypothesis converges on a parahippocampal dysfunction as the locus of déjà vu experiences. However, several other temporal lobe structures feature in reports of déjà vu in epilepsy. We suggest that some of the inconsistency in the literature derives from a poor classification of the various types of déjà experiences. We propose déjà vu/déjà vécu as one way of understanding déjà experiences more fully. This distinction is based on current models of memory function, where déjà vu is caused by erroneous familiarity and déjà vécu by erroneous recollection. Priorities for future research and clinical issues are discussed.

Familiarity from the configuration of objects in 3-dimensional space and its relation to déjà vu: a virtual reality investigation

Déjà vu is the striking sense that the present situation feels familiar, alongside the realization that it has to be new. According to the Gestalt familiarity hypothesis, déjà vu results when the configuration of elements within a scene maps onto a configuration previously seen, but the previous scene fails to come to mind. We examined this using virtual reality (VR) technology. When a new immersive VR scene resembled a previously-viewed scene in its configuration but people failed to recall the previously-viewed scene, familiarity ratings and reports of déjà vu were indeed higher than for completely novel scenes. People also exhibited the contrasting sense of newness and of familiarity that is characteristic of déjà vu. Familiarity ratings and déjà vu reports among scenes recognized as new increased with increasing feature-match of a scene to one stored in memory, suggesting that feature-matching can produce familiarity and déjà vu when recall fails.

Tip-of-the-tongue (TOT) states: retrieval, behavior, and experience

The tip-of-the-tongue state (TOT) is the feeling that accompanies temporary inaccessibility of an item that a person is trying to retrieve. TOTs have been studied experimentally since the seminal work of Brown and McNeill (1966). TOTs are experiences that accompany some failed or slow retrievals, and they can result in changes in retrieval behavior itself, allowing us to study the interplay among experience, retrieval, and behavior. We often attribute the experience of the TOT to the unretrieved target, but TOTs are based on a variety of cues, heuristics, or sources of evidence, such as partial information, related information, and cue familiarity, that predict the likelihood of overcoming retrieval failure. We present a synthesis of the direct-access view, which accounts for retrieval failure, and the heuristic-metacognitive view, which accounts for the experience of the TOT. We offer several avenues for future research and applications of TOT theory and data.

Recognition without identification, erroneous familiarity, and déjà vu

Déjà vu is characterized by the recognition of a situation concurrent with the awareness that this recognition is inappropriate. Although forms of déjà vu resolve in favor of the inappropriate recognition and therefore have behavioral consequences, typical déjà vu experiences resolve in favor of the awareness that the sensation of recognition is inappropriate. The resultant lack of behavioral modification associated with typical déjà vu means that clinicians and experimenters rely heavily on self-report when observing the experience. In this review, we focus on recent déjà vu research. We consider issues facing neuropsychological, neuroscientific, and cognitive experimental frameworks attempting to explore and experimentally generate the experience. In doing this, we suggest the need for more experimentation and a more cautious interpretation of research findings, particularly as many techniques being used to explore déjà vu are in the early stages of development.