Prefrontal dopamine and the dynamic control of human long-term memory.

Dopaminergic projections to the prefrontal cortex support higher-order cognitive functions, and are critically involved in many psychiatric disorders that involve memory deficits, including schizophrenia. The role of prefrontal dopamine in long-term memory, however, is still unclear. We used an imaging genetics approach to examine the hypothesis that dopamine availability in the prefrontal cortex selectively affects the ability to suppress interfering memories. Human participants were scanned via functional magnetic resonance imaging while practicing retrieval of previously studied target information in the face of interference from previously studied non-target information. This retrieval practice (RP) rendered the non-target information less retrievable on a later final test-a phenomenon known as retrieval-induced forgetting (RIF). In total, 54 participants were genotyped for the catechol-O-methyltransferase (COMT) Val(108/158)Met polymorphism. The COMT Val(108/158)Met genotype showed a selective and linear gene-dose effect on RIF, with the Met allele, which leads to higher prefrontal dopamine availability, being associated with greater RIF. Mirroring the behavioral pattern, the functional magnetic resonance imaging data revealed that Met allele carriers, compared with Val allele carriers, showed a greater response reduction in inhibitory control areas of the right inferior frontal cortex during RP, suggesting that they more efficiently reduced interference. These data support the hypothesis that the cortical dopaminergic system is centrally involved in the dynamic control of human long-term memory, supporting efficient remembering via the adaptive suppression of interfering memories.

Response deadline and subjective awareness in recognition memory.

Level of processing and generation effects were replicated in separate experiments in which recognition memory was tested using either short (500 ms) or long (1500 ms) response deadlines. These effects were similar at each deadline. Moreover, at each deadline these effects were associated with subsequent reports of remembering, not of knowing. And reports of both knowing and remembering increased following the longer deadline. These results imply that knowing does not index an automatic familiarity process, as conceived in some dual-process models of recognition, and that both remembering and knowing increase with the slower, more controlled processing permitted by the longer response time.

Conjoint dissociations reveal involuntary "perceptual" priming from generating at study.

Incidental perceptual memory tests reveal priming when words are generated orally from a semantic cue at study, and this priming could reflect contamination by voluntary retrieval. We tested this hypothesis using a generate condition and two read conditions that differed in depth of processing (read-phonemic vs read-semantic). An intentional word-stem completion test showed an advantage for the read-semantic over the generate condition and an advantage for the generate over the read-phonemic condition, and completion times were longer than in a control test, prior to which there was no study phase. An incidental word-stem completion test showed equivalent priming for the read-semantic and read-phonemic study conditions, despite considerable power, and completion times were no longer than control, indicating that retrieval was involuntary, and insensitive to prior conceptual processing. The generate condition produced less priming than the read conditions, but significant priming nonetheless. The results show that priming from generating can be involuntary and suggest that lexical processes are responsible. They are also the first results conjointly showing a crossed double dissociation, a single dissociation, and a parallel effect across memory tests with identical physical retrieval cues.

Depth-of-processing effects on priming in stem completion: tests of the voluntary-contamination, conceptual-processing, and lexical-processing hypotheses.

Depth-of-processing effects on incidental perceptual memory tests could reflect (a) contamination by voluntary retrieval, (b) sensitivity of involuntary retrieval to prior conceptual processing, or (c) a deficit in lexical processing during graphemic study tasks that affects involuntary retrieval. The authors devised an extension of incidental test methodology--making conjunctive predictions about response times as well as response proportions--to discriminate among these alternatives. They used graphemic, phonemic, and semantic study tasks, and a word-stem completion test with incidental, intentional, and inclusion instructions. Semantic study processing was superior to phonemic study processing in the intentional and inclusion tests, but semantic and phonemic study processing produced equal priming in the incidental test, showing that priming was uncontaminated by voluntary retrieval--a conclusion reinforced by the response-time data--and that priming was insensitive to prior conceptual processing. The incidental test nevertheless showed a priming deficit following graphemic study processing, supporting the lexical-processing hypothesis. Adding a lexical decision to the 3 study tasks eliminated the priming deficit following graphemic study processing, but did not influence priming following phonemic and semantic processing. The results provide the first clear evidence that depth-of-processing effects on perceptual priming can reflect lexical processes, rather than voluntary contamination or conceptual processes.

Experiences of remembering, knowing, and guessing.

This article presents and discusses transcripts of some 270 explanations subjects provided subsequently for recognition memory decisions that had been associated with remember, know, or guess responses at the time the recognition decisions were made. Only transcripts for remember responses included reports of recollective experiences, which seemed mostly to reflect either effortful elaborative encoding or involuntary reminding at study, especially in relation to the self. Transcripts for know responses included claims of just knowing, and of feelings of familiarity. These transcripts indicated that subjects were often quite confident of the accuracy of their decisions, compared with those for guess responses. Transcripts for decisions associated with guess responses also expressed feelings of familiarity but additionally revealed various strategies and inferences that did not directly reflect memory for studied items. The article concludes with a historical and theoretical overview of some interpretations of the states of awareness measured by these responses.

Cross-modality priming in stem completion reflects conscious memory, but not voluntary memory.

A comparison of incidental and intentional stem-completion tests confirmed that cross-modality priming occurs when performance conforms completely to the retrieval intentionality criterion, indicating involuntary-not voluntary-retrieval in the incidental test. However, an on-line measure of awareness in the incidental test, and a process-dissociation analysis of the intentional test, indicated only within-modality, but not cross-modality, transfer of involuntary retrieval that is unaccompanied by memorial awareness. These results imply that conscious memory should not be equated with voluntary retrieval, and unconscious memory should not be equated with involuntary retrieval, because involuntary retrieval can be accompanied by memorial awareness.

Retrieval volition and memorial awareness in stem completion: an empirical analysis.

Facilitation in an incidental test of stem completion shows little influence of depth of processing at study, whereas facilitation in an opposition test (in which subjects give the first word coming to mind, but omit studied words) occurs following graphemic processing, but not following semantic processing. We argue that completions come to mind involuntarily in both tests. Involuntary conscious memory causes studied words to be omitted in an opposition test, but not in an incidental test, so that the difference in priming between tests is a measure of involuntary conscious memory. We obtained data consistent with this hypothesis by making overt the mental activities that occur covertly in an opposition test: (1) an on-line recognition measure in an incidental test showed a strong advantage of semantic over graphemic processing, even though depth of processing exerted little influence on priming; (2) conditionalizing on recognition failure resulted in accurate estimates of opposition performance; and (3) stems were completed much more rapidly in incidental and opposition tests than in an intentional test, in which voluntary retrieval was engaged. The data provide further evidence that retrieval volition (voluntary vs. involuntary) is dissociable from memorial state of awareness (conscious vs. unconscious). We contrast our approach with the process-dissociation approach, which confounds conscious awareness of the past with voluntary retrieval, overlooking involuntary conscious memory.

Intention and awareness in perceptual identification priming.

We show that inferences can be made about differences in retrieval intentionality between direct and indirect tests, even when those tests involve different physical cues. When the presence of old items was not mentioned (indirect test condition), we observed a crossed double dissociation between perceptual identification priming and recognition memory as a function of a manipulation of data-driven versus conceptually driven processing at encoding. When subjects were instructed to use their memory to help them identify test items (intentional retrieval condition), priming could be expressed as a monotonically increasing function of recognition memory performance. This reversed association (Dunn & Kirsner, 1988) between priming and recognition memory cannot be accommodated by a model that views intentional retrieval as common to the tests and attributes the crossed double dissociation to an intertest difference in physical cues and associated processes. A posttest questionnaire measure indicated that awareness of the presence of previously encountered items was ubiquitous among indirect test subjects. Crossed double dissociations between direct and indirect measures can therefore be ascribed to differences in retrieval intentionality but not necessarily to differences in subjective awareness of the past.

Involuntary conscious memory and the method of opposition.

Priming in an indirect test of stem completion should reflect involuntary memory, but can be accompanied by conscious awareness of the past (involuntary conscious memory) or unaccompanied by such awareness (involuntary unconscious memory). We adapted the method of opposition developed by Jacoby, Woloshyn, and Kelley (1989) to obtain a measure of stem-completion priming that should reflect only involuntary unconscious memory. Subjects completed stems with the first word coming to mind, but wrote down a different word if the word that came to mind first had been previously encountered. Facilitatory priming was expected only when involuntary unconscious influences outweighed inhibitory effects of involuntary conscious memory, or of intentional retrieval. We observed a facilitation effect for items processed graphemically at encoding, in conjunction with an inhibition effect for items processed semantically at encoding. In contrast, a standard indirect test showed similar levels of priming following graphemic and semantic encoding, whereas a direct test showed a strong advantage of semantic over graphemic encoding. We argue that the two encoding activities produced approximately equivalent involuntary influences of memory, but that items encoded semantically were associated with involuntary conscious memory to a greater extent than were items encoded graphemically. Comparing indirect and opposition test performance can provide a quantitative index of relative levels of involuntary conscious and involuntary unconscious memory.

Maintenance rehearsal affects knowing, not remembering; elaborative rehearsal affects remembering, not knowing.

In a directed-forgetting paradigm, each word in a study list was followed by a cue designating that word as eitherlearn orforget. This cue appeared after either a short or a long delay. It was assumed that a long delay would increase maintenance rehearsal of all the words, and that only the words followed by a learn cue would be rehearsed elaboratively. Moreover, because the interval between the words was constant, a short cue delay should allow more time for elaborative rehearsal. In a subsequent test, subjects maderemember orknow responses to indicate whether recognition of each word was accompanied by conscious recollection or by feelings of familiarity in the absence of conscious recollection. The hypothesis was that remembering depends on elaborative rehearsal, and knowing depends on maintenance rehearsal. In accord with this hypothesis, the learn-versus-forget designation influenced remember but not know responses, and there were more remember responses after the short cue delay; cue delay influenced know responses, regardless of word designation.