Priming effects in a letter-by-letter reader depend upon access to the word form system

Implicit Memory, Constructive Memory, and Imagining the Future: A Career Perspective

In this article I discuss some of the major questions, findings, and ideas that have driven my research program, which has examined various aspects of human memory using a combination of cognitive, neuropsychological, and neuroimaging approaches. I do so from a career perspective that describes important scientific influences that have shaped my approach to the study of memory and discusses considerations that led to choosing specific research paths. After acknowledging key early influences, I briefly summarize a few of the main takeaways from research on implicit memory during the 1980s and 1990s and then move on to consider more recent ideas and findings concerning constructive memory, future imagining, and mental simulation that have motivated my approach for the past 2 decades. A main unifying theme of this research is that memory can affect psychological functions in ways that go beyond the simple everyday understanding of memory as a means of revisiting past experiences.

Preserved priming of novel objects in patients with memory disorders

Amnesic patients perform poorly on explicit memory tests that require conscious recollection of recent experiences, but frequently show preserved facilitations of performance or priming effects on implicit memory tasks that do not require conscious recollection. We examined implicit memory for novel visual objects on an object decision test in which subjects decide whether structurally possible and impossible objects could exist in three-dimensional form. Patients with organic memory disorders showed robust priming effects on this task---object decision accuracy was higher for previously studied objects than for nonstudied objects---and the magnitude of priming did not differ from matched control subjects or college students. However, patients showed impaired explicit memory for novel visual objects on a recognition test. We argue that priming is mediated by the structural description system, a subsystem of the perceptual representation system, that operates at a presemantic level and is preserved in amnesic patients.

Memory and Working-with-Memory: A Component Process Model Based on Modules and Central Systems

Abstract A neuropsychological model of memory is proposed that incorporates Fodor's (1983) idea of modules and central systems. The model has four essential components: (1) a non-frontal neocortical component that consists of perceptual (and perhaps interpretative semantic) modules that mediate performance on item-specific, implicit tests of memory, (2) a modular medial temporal/hippocampal component that mediates encoding, storage, and retrieval on explicit, episodic tests of memory that are associative/cue dependent, (3) a central system, frontal-lobe component that mediates performance on explicit tests that are strategic and on procedural tests that are rule-bound, and (4) a basal ganglia component that mediates performance on sensorimotor, procedural tests of memory. The usefulness of the modular/central system construct is explored and evidence from studies of normal, amnesic, agnosic, and demented people is provided to support the model.

Hemisphere differences in conscious and unconscious word reading

Hemisphere differences in word reading were examined using explicit and implicit processing measures. In an inclusion task, which indexes both conscious (explicit) and unconscious (implicit) word reading processes, participants were briefly presented with a word in either the right or the left visual field and were asked to use this word to complete a three-letter word stem. In an exclusion task, which estimates unconscious word reading, participants completed the word stem with any word other than the prime word. Experiment 1 showed that words presented to either visual field were processed in very similar ways in both tasks, with the exception that words in the right visual field (left hemisphere) were more readily accessible for conscious report. Experiment 2 indicated that unconsciously processed words are shared between the hemispheres, as similar results were obtained when either the same or the opposite visual field received the word stem. Experiment 3 demonstrated that this sharing between hemispheres is cortically mediated by testing a split-brain patient. These results suggest that the left hemisphere advantage for word reading holds only for explicit measures; unconscious word reading is much more balanced between the hemispheres.

Visual implicit memory in the left hemisphere: evidence from patients with callosotomies and right occipital lobe lesions

Identification of visually presented objects and words is facilitated by implicit memory for past visual experiences with those items. Several behavioral and neuroimaging studies suggest that this form of memory is dependent on perceptual processes localized in the right occipital lobe. We tested this claim by examining implicit memory in patients with extensive right occipital lobe lesions, using lexical-decision mirror-reading, picture-fragment, and word-fragment-completion tests, and found that these patients exhibited normal levels of priming. We also examined implicit memory in patients with complete callosotomies, using standard and divided-visual-field word-fragment-completion procedures, and found that the isolated left hemisphere exhibited normal priming effects. The results indicate that the right occipital lobe does not play a necessary role in visual implicit memory, and that the isolated left hemisphere can support normal levels of visual priming in a variety of tasks.

Developmental dissociation between visual and auditory repetition priming: the role of input lexicons

Contrasting theories posit the source of verbal repetition priming in the activation of preexisting memory representations in the input lexicons or, alternatively, in the formation of new episodic memory traces. The two hypotheses predict different outcomes from the comparison of developmental rates of visual and auditory verbal repetition priming. The activation theory predicts a developmental dissociation between the early maturation of auditory priming and the later maturation of visuo-verbal priming, contingent upon the discrepant acquisition rates of the auditory and visual input lexicons. The episodic theory, instead, does not make such an assumption. We administered visual and auditory implicit Stem Completion to 40 reading beginners (first-graders), 40 third-graders and 20 fifth-graders. Consistent with previous reports, auditory priming was stable across different age groups. Visual priming and a measure of lexicality in reading, instead, showed a parallel developmental increase passing from reading beginners to third-graders and to fifth-graders. In the overall group, visual priming and the measure of lexicality in reading were significantly associated. These data describe a new developmental dissociation in the memory abilities of normal children and provide further support for the hypothesis that repetition priming for words reflects facilitated access to previously established memory representations.

Further evidence for sublexical components in implicit memory for novel words

This study provides further evidence for the notion, proposed by Dorfman (1994), that activation of sublexical components plays an important role in implicit memory for novel words. Priming in an identification task was observed consistently when nonwords were formed out of morphemes; weaker, more variable effects were observed when nonwords were composed of syllables or pseudosyllables. Priming was insensitive to changes in the surface features (type case) of the stimuli and was disrupted by rearrangement of the stimulus components. Results are interpreted in terms of the activation and integration of preexisting sublexical representations and as evidence against the role of newly established episodic or perceptual representations. It is concluded that findings of priming for novel information should not be taken as evidence against an activation view.

Brain potentials associated with perceptual priming vs explicit remembering during the repetition of visual word-form

Priming of visual word-form was studied using a reading manipulation in which some words appeared in a backward format (e.g., d-r-o-w) instead of the usual forward format. In Experiment 1, subjects discriminated occasional targets (common first names) from other words with a speeded response. Reaction time was faster for words that had also appeared earlier in the forward format compared to the backward format. Event-related potentials (ERPs) recorded in response to word presentations showed a corresponding difference, a positive offset present during the time interval beginning about 300 ms after word onset from electrodes over occipital and parietal cortex. In Experiment 2, the task was changed to a recognition test, and a later and more widespread ERP response was observed, thus confirming the association between the ERP difference in Experiment 1 and priming rather than explicit remembering. ERP measures were presumably sensitive to neural events underlying the specific influence of recent reading experiences on the processing of visual word-form, thus providing real-time evidence on the neural mechanisms of priming.

Do chronic alcoholics have intact implicit memory? An ERP study

In order to investigate whether visual object priming differs from visual word priming and whether the visual repetition priming in chronic alcoholic patients is impaired, we performed an ERP study on 27 male control and 67 male alcoholic subjects. Sixty-one electrodes were employed to record ERPs that were elicited by random presentations of object pictures, words, and scrambles for both pictures and words. We also used an implicit task that required subjects to identify whether each stimulus was recognizable. The current experiment revealed that (1) the reaction times to both recognizable picture and word stimuli were significantly shortened by the prior exposures of the same stimuli, (2) control subjects reflected visual object and word priming in different ERP components with different topographic patterns, (3) alcoholic subjects manifested visual word priming in the same ERP component as controls, and (4) the differences in ERP components, both in amplitude and topographic distribution, between the two groups occurred mainly in the different stimuli. These data suggest that the visual object and word priming have distinctive neural processes. The visual object priming in alcoholic subjects may be impaired while the visual word priming seemed to be intact.

Visual object priming differs from visual word priming: an ERP study

Implicit memory is inferred from repetition priming effects in tasks such as word identification, word fragment completion, and perceptual recognition with masking or brief exposures. In this experiment, we explored whether the visual word and object repetition priming effects can be reflected by features of ERP and whether visual word repetition priming differs from visual object repetition priming. We have observed that (1) pre-exposure to recognizable stimuli (both word and object picture) shortened the response time in identifying their repetitions; (2) repetition of unrecognized scrambles of words or object pictures did not show any effects on ERP patterns; (3) ERPs distinguished recognizable from unrecognizable stimuli; and, (4) repetitions of both words and pictures strongly influenced the patterns of ERPs, though the ERPs to word stimuli differed from the ERPs to picture stimuli.

Form-specific priming and functional brain asymmetries in perceptual identification

Word form representations in intact cerebral hemispheres were studied by a lateralized perceptual identification priming task. During the study phase, word forms were primed by displaying words visually in uppercase or lowercase letters. During the test phase, perceptual identification of non-studied baseline words and studied words (presented in same or different lettercase as studied) was tested by displaying targets in the left or right visual field. Experiment 1 showed that the hemispheric pattern of priming effects was dependent on the lettercase at test. For uppercase test items, only the left visual field/right hemisphere was sentitive to study-test changes in lettercase, replicating an earlier result obtained in word-stem completion (Marsolek, Kosslyn and Squire, 1992). However, lowercase test items did not reveal any asymmetries in the form-dependent priming component indicating that in some conditions form-specific representations are computed in the left hemisphere also. No asymmetries were found in the abstract, form-independent component of priming. Experiment 2 revealed that use of explicit memory in the perceptual identification task eliminated the form-specific priming effects and suggested that the results of Experiment 1 were uncontaminated by explicit memory.

Normal perceptual priming of orthographically illegal nonwords in amnesia

This study examined priming in perceptual identification of orthographically illegal nonwords in control subjects and patients with global amnesia. Subjects studied a list of orthographically illegal nonwords and then performed a perceptual identification task in which half of the stimuli were from the prior study list and half were new (unstudied) stimuli. Priming was reflected in enhanced identification accuracy of studied compared to unstudied nonwords. Amnesic patients showed significant and normal priming despite impaired recognition memory performance. Because the experimental stimuli were dissimilar to real words in terms of orthography and phonology, it is unlikely that this priming effect was mediated by activation of pre-existing representations of orthographically or phonologically similar words, morphemes, or syllables. These results demonstrate that intact perceptual priming in amnesia is not limited to stimuli that are premorbidly represented in long-term knowledge, nor to novel stimuli that conform to the rules that characterize familiar items. Further, because the experimental stimuli comprised novel letter assemblies, the results suggest that amnesic patients can show normal priming for new perceptual associations. These findings demonstrate that processes spared in amnesia can support the creation and subsequent retrieval of novel stimulus representations.

Explicit memory and repetition priming in dementia: evidence for a common basic mechanism underlying conscious and unconscious retrieval deficits

The present study was designed to assess performance of Alzheimer's (AD) and Multi-infarct (MID) demented patients on the explicit and implicit versions of two memory tasks, namely Word-Stem Completion and Word-Pair Learning. Consistently with previous studies, the AD patients were deficient on the explicit and implicit versions of both tasks. In MID patients, a dissociation emerged between normal implicit and deficient explicit Word-Stem Completion. Two multiple regression analyses were performed to evaluate patients' ability on measures of lexical-semantic competence, explicit memory, and global intellective efficiency in predicting level of repetition priming. The results demonstrate a close association between explicit and implicit memory performance in AD patients but no relation between repetition priming level and measures of lexical-semantic competence or general intelligence. Overall, the results of the present study do not support previous conceptualizations suggesting that a breakdown in the structure of semantic memory is at the root of deficient priming in demented patients. Alternative interpretations of the deficient repetition priming effect in dementia, based on a common mechanism underlying conscious and unconscious retrieval deficits, are discussed.

Visual repetition priming for words relies on access to the visual input lexicon: evidence from a dyslexic patient

In this study we tested the hypothesis that visual repetition priming for words depends upon the accessibility of lexical units in the visual input lexicon. For this purpose, we investigated a dyslexic patient, A.M., whose neuropsychological performances suggested an impaired access to the lexical route of reading. According to the predictions, Experiments 1 and 2 demonstrated deficient priming in tests involving the visual presentation of words (Word Identification and Stem Completion). In Experiment 3, we demonstrated that A.M.'s deficient priming was specific for visually presented words, in that the auditory presentation elicited a normal priming effect (auditory Stem Completion). These data are discussed in the light of a theoretical framework suggesting a fractionation of the modalities by which repetition priming can be elicited, each mediated by a particular memory subsystem.

Toward the neural basis of verbal priming: a cognitive-neuropsychological synthesis

Repetition priming is a mnemonic phenomenon that has attracted considerable attention from neuropsychologists and cognitive scientists. In an attempt at elucidating the putative mechanisms of priming, the present review draws on evidence from both domains. The review is restricted to verbal priming of visually presented stimuli--an area that accounts for the majority of empirical studies of priming. A number of theoretical accounts are presented. The interim conclusion is that neither multiple systems nor unitary system-multiple process theories can adequately explain the data on priming, although both contain many valid components. An integrative model is proposed to improve the explanation of the empirical evidence. The central assertion of the proposed model is that repetition priming depends on perceptual processes that can be mapped on specific neural systems. It is postulated that individual differences in perceptual processing ability predict variability in memory performance. It is proposed that data-driven priming of verbal stimuli critically depends on the activity of primary and secondary visual cortices in the right hemisphere, whereas conceptually-driven priming is hypothesized to rely on the activities of higher order tertiary association cortices in language areas and more anterior neocortical areas.

Spelling dyslexia: a deficit of the visual word-form

A patient with spelling dyslexia read both words and text accurately but slowly and laboriously letter by letter. Her performance on a test of lexical decision was slow. She had great difficulty in detecting a 'rogue' letter attached to the beginning or end of a word--for example, ksong--or in parsing two unspaced words, such as applepeach. By contrast she was immune to the effects of interpolating extraneous coloured letters in a word, a manipulation that affects normal readers. Therefore it is argued that this patient had damage to an early stage in the reading process, to the visual word form itself.

Age differences in implicit memory: more apparent than real

Elderly subjects and a group of young subjects identified fragmented picture sequences under conditions of focused attention. Two other groups of young subjects carried out this task under divided-attention conditions. Implicit memory, as measured by item-specific savings, was found in all groups, but this effect was smaller in the elderly group. The young subjects, but not elderly subjects, performed better on new items. The divided-attention conditions equated recall and recognition by the young and the elderly, but only the young subjects showed greater savings for recalled items. The elderly subjects' reduced implicit memory therefore stemmed from their inability to facilitate implicit memory with explicit memory. A second experiment, involving only young subjects tested after delay, produced findings similar to those for the young divided-attention subjects. Implicit memory, as measured by savings in picture completion, does not show an age-related change when the role of explicit memory is considered. Age does, however, reduce skill learning.

Does letter-by-letter reading involve the spelling system?

Warrington and Shallice (Brain 103, 99-112, 1980) proposed that letter-by-letter readers can no longer access the visual word-form system, and read by using the spelling system 'in reverse'. Contrary to their suggestion, this paper presents the case of a letter-by-letter reader who appears to use quite different strategies in word reading and spelling. Reading and spelling are both impaired, but they appear to be impaired in different ways. Most strikingly, the patient's spelling errors consist largely of phonological regularizations--indeed there is little evidence that he has any lexical spelling knowledge available--whereas there are virtually no regularizations amongst his reading errors. These are largely visual paralexias and letter misidentifications. It is suggested that this pattern is more easily explained in terms of compensatory mechanisms that access the reading lexicon than by use of the spelling system 'in reverse'.