A theory of implicit and explicit knowledge

The implicit-explicit distinction is applied to knowledge representations. Knowledge is taken to be an attitude towards a proposition which is true. The proposition itself predicates a property to some entity. A number of ways in which knowledge can be implicit or explicit emerge. If a higher aspect is known explicitly then each lower one must also be known explicitly. This partial hierarchy reduces the number of ways in which knowledge can be explicit. In the most important type of implicit knowledge, representations merely reflect the property of objects or events without predicating them of any particular entity. The clearest cases of explicit knowledge of a fact are representations of one's own attitude of knowing that fact. These distinctions are discussed in their relationship to similar distinctions such as procedural-declarative, conscious-unconscious, verbalizable-nonverbalizable, direct-indirect tests, and automatic-voluntary control. This is followed by an outline of how these distinctions can be used to integrate and relate the often divergent uses of the implicit-explicit distinction in different research areas. We illustrate this for visual perception, memory, cognitive development, and artificial grammar learning.     

Shifting from implicit to explicit knowledge: different roles of early- and late-night sleep

Sleep has been shown to promote the generation of explicit knowledge as indicated by the gain of insight into previously unrecognized task regularities. Here, we explored whether this generation of explicit knowledge depends on pre-sleep implicit knowledge, and specified the differential roles of slow-wave sleep (SWS) vs. rapid eye movement (REM) sleep in this process. Implicit and explicit knowledge (insight) related to a hidden regularity were assessed in an associative motor-learning task (number reduction task, NRT), which was performed in two sessions (initial practice and retest) separated by 3 h of either early-night sleep, rich in SWS, or of late-night sleep, rich in REM sleep. About half of the participants developed signs of implicit rule knowledge (i.e., speeded reaction times for responses determined by the hidden regularity) at initial practice preceding early or late sleep. Of these, half developed explicit knowledge across early-night sleep, significantly more than across late-night sleep. In contrast, late-night subjects preferentially remained on the level of implicit rule knowledge after sleep. Participants who did not develop implicit knowledge before sleep had comparable rates of transition to implicit or explicit knowledge across early and late sleep. If subjects gained explicit knowledge across sleep, this was associated with lower amounts of REM sleep, specifically in the late-night group. SWS predominant during the early night may restructure implicit memory representations in a way that allows creating an explicit representation afterward, whereas REM sleep in the late night appears to stabilize them in their implicit form.     

Probing memory with conceptual cues at multiple retention intervals: a comparison of forgetting rates on implicit and explicit tests

The time courses for implicit and explicit conceptual tests of memory were compared in two experiments. In Experiment 1, participants encoded target words by judging the apparent pleasantness of their meaning. Immediately thereafter or 48 h later, retrieval cues were presented to different groups of participants for either an implicit or an explicit free-association task. Whereas explicit test performance showed a decline over the 48-h delay, implicit test performance was statistically unaltered. In Experiment 2, memory was tested at five retention intervals, lasting up to 3 weeks. The forgetting functions of both implicit and explicit tests conformed to a logarithmic function. Despite the large conceptual priming effect, which resulted from relational encoding instructions, implicit memory performance still declined at a much slower rate than did performance on the cued-recall test. We argue that because nominal conceptual cues were held constant across the implicit and the explicit conditions, the observed dissociation in performance supports a memory systems approach.     

The emergence of explicit knowledge from implicit learning

Substantial evidence has highlighted the ability of observers to incidentally extract statistical contingencies present in visual environments. This study examined whether the knowledge extracted regarding statistical contingencies is unconscious initially, even when it becomes fully accessible to conscious awareness after extensive training. Using a “typical” contextual cuing procedure adapted to real-world scenes, we first observed that, after extensive training in searching for a target within repeated scenes, knowledge about regularities was associated with conscious awareness (Experiment 1). However, both subjective and objective measures of consciousness revealed that in the early phase of training, learning of regular structures first takes place at an unconscious level (Experiments 2 and 3). These results are discussed in the light of the causal relationships between learning and consciousness.     

Visual statistical learning produces implicit and explicit knowledge about temporal order information and scene chunks: Evidence from direct and indirect measures

We examined whether visual statistical learning (VSL) produced implicit and/or explicit knowledge about temporal order information and scene chunks, using a rapid serial visual presentation target detection task and a two-alternative forced-choice (2AFC) familiarity test as indirect and direct measures of VSL, respectively. In the familiarization phase, participants viewed a visual stream of natural scenes consisting of four triplets (i.e., three images that always appeared in the same order). In the subsequent target detection task, participants were required to detect target items embedded in a stream of 12 images or 12 words representing each natural scene. In the final 2AFC familiarity test, participants observed two test sequences (statistically related triplets vs. unrelated foils) and decided whether the first or second sequence was more familiar based on the familiarization phase. In both test phases, we included the same (forward) and reverse (backward) order of scenes as presented during the familiarization phase to examine whether the expression of VSL was based on temporal order of scenes or scene chunks. The results of the target detection task showed a learning effect for both temporal order in the forward condition and chunks in the backward condition, irrespective of whether stimuli were images or words; in contrast, we did not observe a learning effect in the backward condition for scene images in the familiarity test. Our findings are compatible with a learning mechanism that has both implicit and explicit components based on temporal order information and scene chunks.     

The impact of different directed forgetting instructions on implicit and explicit memory: New evidence from a modified process dissociation procedure

In contrast to previous research on directed forgetting, the present studies adopted a recent modification of the process dissociation procedure (Jacoby, 1991; Richardson-Klavehn & Gardiner, 1995) to accommodate the cross-contamination of memory test performance by implicit and explicit memorial factors. In Experiment 1, 120 subjects were compared in global directed forgetting, item-by-item directed forgetting, and control conditions on estimates of voluntary conscious memory, involuntary conscious memory, and involuntary unconscious memory performance. In Experiment 2, 80 subjects were compared in specific directed forgetting and control conditions on estimates of voluntary conscious memory, involuntary conscious memory, and involuntary unconscious memory performance. Subjects showed significant decrements in voluntary and involuntary conscious memory performance following instructions for directed forgetting in all conditions. None of the directed forgetting conditions showed a decrement in involuntary unconscious memory performance. Results suggest that, regardless of instruction type, directed forgetting prevents the conscious expression of memorial information (both voluntary and involuntary) while leaving unconscious memory intact.     

Role of prior knowledge in implicit and explicit learning of artificial grammars

Artificial grammar learning (AGL) performance reflects both implicit and explicit processes and has typically been modeled without incorporating any influence from general world knowledge. Our research provides a systematic investigation of the implicit vs. explicit nature of general knowledge and its interaction with knowledge types investigated by past AGL research (i.e., rule- and similarity-based knowledge). In an AGL experiment, a general knowledge manipulation involved expectations being either congruent or incongruent with training stimulus structure. Inconsistent observations paradoxically led to an advantage in structural knowledge and in the use of general world knowledge in both explicit (conscious) and implicit (unconscious) cases (as assessed by subjective measures). The above findings were obtained under conditions of reduced processing time and impaired executive resources. Key findings from our work are that implicit AGL can clearly be affected by general knowledge, and implicit learning can be enhanced by the violation of expectations.     

Effects of aging on implicit sequence learning: Accounting for sequence structure and explicit knowledge

The present research was intended to examine the sequence learning ability of elderly people — with a focus on comparing sequences with different structural characteristics and on properly assessing explicit knowledge. Experiment 1 showed that learning-related improvements in serial reaction time task performance were greater for young than elderly subjects, and elderly subjects were especially poor at learning a sequence with complex structural characteristics. Measures of recognition memory showed that neither young nor elderly subjects showed above-chance explicit knowledge of the sequences. Experiment 2 was designed to test the validity and sensitivity of the explicit recognition measures by comparing young subjects in groups given all random trials, given sequence trials with implicit instructions, or given sequence trials with explicit instructions. Experiment 2 confirmed the sensitivity of the recognition measures to explicit knowledge, so it is concluded that group effects in Exp. 1 reflect age-related differences in implicit learning.     

Distinguishing implicit and explicit learning

I agree with Dienes and Berry’s (1997) and Neal and Hesketh’s (1997) call for investigations of the qualitative differences between implicit and explicit learning and note that such investigations must be guided by a workable definition of what is implicit and by theories that predict what the qualitative differences might be. Following Schacter, Bowers, and Booker’s (1989) retrieval intentionality criterion, I propose using anencoding intentionality criterion to distinguish implicit from explicit learning; we can reasonably infer that implicit learning has occurred when a variable known to influence explicit learning has no effect in a comparable implicit learning condition. I then suggest that implicit learning depends on noncognitive, nonhierarchical associations, whereas explicit learning depends on cognitive, hierarchical associations, and briefly describe an experiment that confirms a qualitative difference between implicit and explicit learning predicted by this hypothesis.     

Selective attention affects implicit and explicit memory for familiar pictures at different delay conditions

Three experiments investigated the effects of two variables -selective attention during encoding and delay between study and test- on implicit (picture fragment completion and object naming) and explicit (free recall and recognition) memory tests. Experiments 1 and 2 consistently indicated that (a) at all delays (immediate to 1 month), picture-fragment identification threshold was lower for the attended than the unattended pictures; (b) the attended pictures were recalled and recognized better than the unattended; and (c) attention and delay interacted in both memory tests. For implicit memory, performance decreased as delay increased for both attended and unattended pictures, but priming was more pronounced and lasted longer for the attended pictures; it was still present after a 1-month delay. For explicit memory, performance decreased as delay increased for attended pictures, but for unattended pictures performance was consistent throughout delay. By using a perceptual object naming task, Experiment 3 showed reliable implicit and explicit memory for attended but not for unattended pictures. This study indicates that picture repetition priming requires attention at the time of study and that neither delay nor attention dissociate performance in explicit and implicit memory tests; both types of memory require attention, but explicit memory does so to a larger degree.     

Obtaining separate measures for implicit and explicit memory

In this paper a new model, the Implicit/Explicit Separation (IES) model, is developed and applied. The model is designed to obtain separate probability measures for explicit memory storage, implicit memory storage, fractional storage, and non-storage. The model is needed because memory research has shown the importance of the distinction between a conscious memory of a target event and a memory residue that is unable to support conscious recall or confident recognition, but it is still able to support guessing at rates above chance. Maximum likelihood and population-parameter mapping estimates for the parameters of the IES model are provided. The accuracy of parameter estimates is studied as a function of sample size. Three experiments are reported to demonstrate how the IES model is used to achieve a more fine grained assessment of the quality of information storage. These experiments also provide strong validation support for the IES model itself. Across the three experiments, each of the four components for representing target information (explicit, implicit, fractional, and non-storage) demonstrates a different pattern. The IES model is discussed in terms of alternative models such as the dual-process model and the process-dissociation model.     

Impetus beliefs as default heuristics: Dissociation between explicit and implicit knowledge about motion

We examined the extent to which findings from the literature on naive physics and representational momentum studies are consistent with impetus beliefs postulating imparted internal energy as a source of motion. In a literature review, we showed that, for situations in which impetus theory and physical principles make different predictions, representational momentum effects are consistent with impetus beliefs. In three new experiments, we examined people’s implicit and explicit knowledge of the effect of mass on the rate of ascending motion. The results suggest that implicit knowledge is consistent with impetus theory and is unaffected by explicit knowledge. Expert physicists, whose explicit knowledge is in accord with Newtonian principles, exhibited the same implicit impetus beliefs as novices when asked to respond in a representational momentum paradigm. We propose that, in situations in which an immediate response is required and one does not have specific contextual knowledge about an object’s motion, both physics experts and novices apply impetus principles as a default heuristic.     

Evaluating the relationship between explicit and implicit knowledge in a sequential reaction time task

Can knowledge underlying a simple perceptual-motor skill be unconscious? Three experiments (a) trained participants on a 4-choice reaction time (RT) task in which the stimulus on each trial was determined by a repeating 12-element sequence and (b) studied the extent to which participants' knowledge of this sequence was implicit, that is, unavailable for conscious access. Participants proved via an indirect test to have acquired knowledge of the sequence, because their RTs increased when the sequence was changed. To evaluate whether this knowledge was consciously accessible, participants were asked to perform an "objective" free-generation or recognition test of sequence knowledge. Results show that sequence knowledge is fully accessible on these objective tests. Moreover, it is demonstrated in this procedure that old-new recognition cannot be explained by unconscious attribution of perceptual-motor fluency. The question is raised whether distinct implicit (procedural) and explicit (declarative) forms of knowledge are acquired when participants learn a perceptual-motor skill.     

From implicit skills to explicit knowledge: a bottom-up model of skill learning

This paper presents a skill learning model CLARION. Different from existing models of mostly high-level skill learning that use a top-down approach (that is, turning declarative knowledge into procedural knowledge through practice), we adopt a bottom-up approach toward low-level skill learning, where procedural knowledge develops first and declarative knowledge develops later. Our model is formed by integrating connectionist, reinforcement, and symbolic learning methods to perform on-line reactive learning. It adopts a two-level dual-representation framework (Sun, 1995), with a combination of localist and distributed representation. We compare the model with human data in a minefield navigation task, demonstrating some match between the model and human data in several respects.     

Processing implicit and explicit representations.

This article summarizes the results of a 15-year research program dedicated to understanding how implicitly activated memories affect remembering and proposes a model for describing such influences. Implicit memories are manipulated by varying the number of associates preexperimentally linked to test cues or to studied words. Assumptions of the model specify when implicit memories of various types are likely to contribute to performance in various tasks. The main assumptions are that encoding involves both explicit and implicit processing components and that these components provide mutually exclusive sources of information during testing. Experiments designed to evaluate the exclusivity assumption are reported, and implications of the findings for several theoretical frameworks are discussed.     

Implicit and explicit knowledge decay at different rates: a dissociation between priming and recognition in artificial grammar learning

An experiment tested the common assumption that implicit and explicit knowledge are forgotten at different rates. In a training phase participants responded to sequences of letters generated by a finite-state grammar by pressing corresponding letters on a keyboard. A control group responded to randomized sequences. Participants were tested immediately following training and after intervals of seven and fourteen days. During each test participants responded to the letters of old and new sequences, and performed a concurrent recognition test. Priming was indexed by the time taken to respond to the sequences. In the immediate test both priming and recognition were substantially greater than the control group. In the delayed tests the level of priming remained unchanged but recognition had declined. The data indicate that priming and recognition decay at different rates, and are discussed with reference to both single and dual process models of memory.