Intact implicit memory in patients with frontal lobe lesions.

Patients with frontal lobe lesions and control subjects were administered tests of word-stem completion priming. In this implicit memory test, subjects are first presented words (e.g. MOTEL, PARADE) in an incidental learning paradigm. Following word presentation, subjects are shown word stems (e.g. MOT, PAR) and asked to produce the first word that comes to mind. Patients with frontal lobe lesions exhibited normal levels of word-stem completion. These findings indicate that implicit memory can operate normally despite damage to the prefrontal cortex. The present results substantiate previous neuropsychological and positron emission tomography findings which indicate that word priming depends critically on posterior cortical areas.     

The neural substrates of visual implicit memory: do the two hemispheres play different roles?

Identification of visually presented words is facilitated by implicit memory, or visual priming, for past visual experiences with those words. There is disagreement over the neuro-anatomical substrates of this form of implicit memory. Several studies have suggested that this form of priming relies on a visual word-form system localized in the right occipital lobe, whereas other studies have indicated that both hemispheres are equally involved. The discrepancies may be related to the types of priming tasks that have been used because the former studies have relied primarily on word-stem completion tasks and the latter on tasks like word-fragment completion. The present experiments compared word-fragment and word-stem measurements of visual implicit memory in patients with right occipital lobe lesions and patients with complete callosotomies. The patients showed normal visual implicit memory on fragment completion tests, but essentially no visual priming on standard stem completion tests. However, when we used a set of word stems that had only one correct solution for each test item, as was true of the items in the fragment completion tests, the patients showed normal priming effects. The results indicate that visual implicit memory for words is not solely dependent upon the right hemisphere, rather it reflects changes in processing efficiency in bilateral visual regions involved in the initial processing of the items. However, under conditions of high lexical competition (i.e., multiple completion word stems), the lexical processes, which are dominant in the left hemisphere, overshadow the visual priming supported by the left hemisphere.     

Cross-modality word matching in letter-by-letter readers

Letter-by-letter readers can match spoken words to printed words they cannot read. Good word-matching ability may be attributable to a priming effect. However, since letter-by-letter readers have intact spelling ability, an alternative hypothesis is that word matching depends on a strategy of scanning printed words for recognizable letters known to be in the target items. In the present study, experimental manipulations that taxed the ability of the two subjects to process key letters (those that distinguished target words from foils) or scan for them caused a decline in word-matching performance. Thus, letter-by-letter readers may rely on sequential letter processing to accomplish cross-modality word matching.     

Preserved priming in auditory perceptual identification in Alzheimer's disease

To examine the status of auditory perceptual priming in Alzheimer's disease (AD), this study examined the performance of AD patients in auditory perceptual identification of words. In Experiment 1, the processing operations required to perform the tasks at study and test were matched, whereas in Experiment 2, processing operations at study and test were mismatched. AD patients showed normal priming in both experiments, despite impaired recognition memory. These findings extend to the auditory domain the finding of intact perceptual implicit memory in AD. Preserved auditory priming in AD may reflect the operation of a pre-semantic, phonological representation system, localized to posterior neocortical areas that are functionally spared in AD.     

Neuroanatomical organization of perceptual memory: an fMRI study of picture priming

Neuroanatomical organization of perceptual representation in human memory system is unclear primarily because it has been studied using paradigms that have both, perceptual and conceptual components (e.g., word stem completion and word fragment completion). In the present experiment, functional magnetic resonance imaging (fMRI) technique was used to examine the pattern of cortical activation in a picture identification test in which subjects were asked to identify subliminally presented primed and novel pictures. This test is a modification of the word identification test that is considered a "pure" form of perceptual priming. Results indicate that perceptual priming is associated with reduced activation in the extrastriate cortex and that the memory for subliminally presented stimuli is processed by the same brain areas that process adequate stimuli. The activation pattern observed in picture identification test is different from that reported in the experiments of conceptual priming, suggesting that perceptual and conceptual representation of memory are supported by separate brain mechanisms.     

Event-related brain potentials elicited by morphological, homographic, orthographic, and semantic priming

The morphological structure of words, in terms of their stem morphemes and affixes, could influence word access and representation in lexical memory. Three experiments were carried out to explore the attributes of event-related potentials evoked by different types of priming. Morphological priming, with pairs of words related by their stem (hijo/hija [ son/ daughter]), produced a sustained attenuation (and even a tendency to positivity) of the N400 shown by unrelated words across the three experiments. Homographic priming (Experiment 1), using pairs of words with a superficially similar stem, but without morphological or semantic relation (foco/foca [floodlight/seal]), produced an initial attenuation similar to the morphological pairs, but which rapidly tended to form a delayed N400, due to the impossibility of integration. However, orthographic priming (rasa/rana [flat/frog]) in Experiment 2 does not produce attenuation of the N400 but an effect similar to that of unrelated pairs. Experiment 3 shows that synonyms advance more slowly than morphological pairs to meaning coherence, but finally produce a more positive peak around 600 msec.     

A further analysis of perceptual identification priming in alcoholic Korsakoff patients

Alcoholic Korsakoff patients have been shown to have normal repetition priming for words, but impaired priming for pseudowords, on perceptual identification tasks. Because real words, but not pseudowords, have semantic representations, successful priming of real words has been attributed to semantic activation. Alternatively, however, priming might reflect the perceptual, i.e. orthographic and phonological familiarity associated with real words in comparison with pseudowords. In order to examine which of these factors accounts for normal priming by amnesics, Experiment 1 was designed to contrast perceptual identification priming for words, pseudowords and pseudohomonyms (e.g. "phaire"). On this task, Korsakoff patients showed normal priming for real words and for pseudohomonyms, but significantly impaired priming for pseudowords. It was felt that since pseudohomonyms are orthographically unfamiliar but could access semantics through their phonological route, activation of a semantic representation rather than perceptual familiarity might be the critical factor underlying successful priming. Experiment 2 was then designed to further explore the robustness of this semantically mediated priming by presenting different word types within the same study list. Under these conditions, Korsakoffs' pseudohomonym priming was consistently below normal. This suggests that priming in Korsakoff patients depends upon the conceptual salience of the stimuli and not solely on semantic activation.     

Paired-associate learning and memory interference in schizophrenia

Patients with frontal lobe damage have been shown to exhibit disproportionate impairments of second list learning as a result of interference effects. Based upon the assumption that schizophrenia is associated with frontal dysfunction, we attempted to explore how various manipulations of paired-associate learning tasks would interfere with schizophrenic patients' memory performance. Patients with schizophrenia were administered four tests of paired-associate learning, in which cue and response words were manipulated to increase interference across two study lists. In two tests of paired-associate learning (AB-AC test), cue words used in one list were repeated in a second list but were associated with different response words (e.g. lion-hunter, lion-circus). One version of this test employed moderately related word pairs and the other version employed unrelated word pairs. In the other two tests (AB-ABr test), all words used in one list were repeated in a second list but were rearranged to form new pairs. Again, one version of this test used moderately related word pairs and the other version used unrelated word pairs. We hypothesized that patients with schizophrenia would exhibit disproportionate impairment of second-list learning as a result of interference effects and that they would do especially poorly in the AB-ABr task, where the word pairs were unrelated. However, these predictions were not supported. Furthermore, it was difficult to tease apart a specific problem in list discrimination from the generally poor memory of the schizophrenic patients. We suggest that the susceptibility to these interference effects in patients with schizophrenia is not a specific problem in cognition, but rather one that is confounded by general memory problems.     

Lateralised covert attention in word identification

The right visual field superiority in word recognition has been attributed to an attentional advantage by the left brain hemisphere. We investigated whether such advantage involves lateralised covert attention, in the absence of overt fixations on prime words. In a lexical decision task target words were preceded by an identical or an unrelated prime word. Eye movements were monitored. In Experiment 1 lateralised (to the left or right of fixation) prime words were parafoveally visible but foveally masked, thus allowing for covert attention but preventing overt attention. In Experiment 2 prime words were presented at fixation, thus allowing for both overt and covert attention. Results revealed positive priming in the absence of fixations on the primes when these were presented in the right visual field. The effects of covertly attended primes were nevertheless significantly reduced in comparison with those of overtly attended primes. It is concluded that word identification can be accomplished to a significant extent by lateralised covert attention alone, with right visual field advantage.     

Lateralised covert attention in word identification

The right visual field superiority in word recognition has been attributed to an attentional advantage by the left brain hemisphere. We investigated whether such advantage involves lateralised covert attention, in the absence of overt fixations on prime words. In a lexical decision task target words were preceded by an identical or an unrelated prime word. Eye movements were monitored. In Experiment 1 lateralised (to the left or right of fixation) prime words were parafoveally visible but foveally masked, thus allowing for covert attention but preventing overt attention. In Experiment 2 prime words were presented at fixation, thus allowing for both overt and covert attention. Results revealed positive priming in the absence of fixations on the primes when these were presented in the right visual field. The effects of covertly attended primes were nevertheless significantly reduced in comparison with those of overtly attended primes. It is concluded that word identification can be accomplished to a significant extent by lateralised covert attention alone, with right visual field advantage.     

Lexical and semantic priming deficits in patients with Alzheimer's disease

Two experiments utilizing priming procedures examined the status of semantic memory in demented and amnesic patients. In the first investigation, lexical priming was assessed in patients with dementia of the Alzheimer type (DAT), Huntington's Disease (HD), alcoholic Korsakoff's syndrome (KS), and in intact control subjects. Subjects were first exposed to a list of words in a rating task and then required to complete three-letter stems with the "first word that comes to mind". Half of the stems could be completed with the previously presented words and the other half were used to assess baseline guessing rates. Recall and recognition of incidentally exposed words was also assessed. Although all three patient groups were impaired on tests of recall and recognition, only the DAT patients exhibited a priming deficit on the stem-completion task. In the second experiment, DAT, HD, and intact control subjects were administered a semantic priming test which required the subject to "free associate" to the first words of previously presented semantically associated word pairs. The results for this association task showed that DAT patients were significantly less likely to produce the second word of the semantically related pair than were the other subject groups. The results of these two experiments suggest that the memory capacities of DAT patients are characterized by a breakdown in the structure of semantic memory and that this impairment is evident on some "automatic" as well as "effortful" processing tasks.     

A critical evaluation of the lateralizing significance of material-specific memory deficits in patients with mesial temporal lobe epilepsy with hippocampal sclerosis

To critically assess the value of material-specific memory deficits in lateralizing temporal lobe dysfunction preoperatively, we compared the neuropsychological data of 50 consecutive patients with unilateral mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS; right: 31, left: 19) with those of 50 age- and education-matched healthy control subjects. On case-control comparison, both the subcohorts with left and right MTLE-HS performed poorly on intelligence tests, in addition to individual memory tests. However, comparison of the verbal and visual memory functions between subcohorts with right and left MTLE-HS revealed that learning trials and delayed word list recall were the only tests that hypothesized left temporal lobe dysfunction. We conclude that material-specific memory deficits are largely test driven, but there is a lateralizing role for task-specific deficits in left MTLE-HS. Although neuropsychological data help to define baseline neuropsychological impairment, caution should be exercised in interpreting the lateralizing value of material-specific memory deficits prior to surgery.     

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.     

Cross-modal semantic priming in schizophrenia.

Work on implicit memory in normal subjects has demonstrated the influence of stimulus modality on the retrieval of semantic information. The present study examined the effects of auditory and visual semantic priming on the recognition of visual words using a lexical decision task. Performance was studied in a group of 20 patients with DSM-IV schizophrenia and 26 normal volunteers of similar age and sex. There were two versions of the task: ipsimodal, in which the word or nonword visual target followed 400 ms after the onset of a visual word prime which may or may not be semantically related to the target; and cross-modal, in which the visual target followed 400 ms after the onset of an auditory word prime. Both groups showed significant priming in both modality conditions, although the schizophrenia patients exhibited significantly greater priming in the cross-modal condition. Priming effects in the ipsimodal condition did not differ substantially between patients and controls. The priming effects in the two conditions correlated with each other in the schizophrenia patients only. The results suggest that priming may occur through amodal semantic representations. In schizophrenia, there appears to be increased cross-modal connectivity (reduced modality modularity and informational encapsulation) between lexical representations that could result in impaired language, particularly speech, processing.     

Exploring different routes of recovery from memory in Alzheimer's disease: evidence for preserved long-term priming

Studies investigating implicit memory in Alzheimer's disease suggest that priming abilities disappear over time. This study investigates long-term priming in Alzheimer's disease. A total of 20 patients with Alzheimer's disease and 20 controls underwent the Free and Cued Selective Reminding test. After a 30-minute delay, participants were submitted to a word identification task comprising the studied words and new words. The patients exhibited a long-term priming effect similar to that presented by the comparison group. Therefore, whereas explicit retrieval is definitely impaired in Alzheimer's disease, implicit processes allowing the retrieval of that same information operate relatively normally, including after a long delay.     

Cerebral hemispheric differences in memory of emotional and non-emotional words in normal individuals

The present study was designed to examine the cerebral hemispheric differences in memory of positive, negative and non-emotional words using a new method of successive presentation to each visual half-field in which perception of each item was nearly perfect thereby allowing laterality differences for effects of emotion on memory to emerge unconfounded by perception (Experiment 1). This procedure was compared with traditional perceptual identification (Experiment 2). In Experiment 1, 12 words were presented successively in each half field in each trial followed by free recall at the end of the trial. The results showed that recall of positive and negative emotional words was better than that of non-emotional words in both visual fields. Recall of positive and negative emotional words was not different in left and right visual fields (RVFs) although the recall of non-emotional words was better in the RVF than in the left visual field (LVF). The differences in recall between emotional and non-emotional words was greater in the LVF than in the RVF. Experiment 2 used the more traditional method of perceptual identification following each visual half-field presentation of a single item. Perceptual identification was better in the RVF than the LVF in each word condition. There were no visual field differences in perceptual identification between emotional and non-emotional words, as there was for memory in Experiment 1. The results supported the hypothesis that explicit memory for emotional words was dependent more on the right hemisphere, whereas perception of both emotional and non-emotional words was more dependent on the left hemisphere.     

Successive memory test performance and priming in Alzheimer's disease: evidence from the word-fragment completion task

This study assessed the performance of patients with Alzheimer's disease and healthy controls in a successive memory test paradigm. Subjects studied lists of words. Following study, tests of recognition (an explicit memory task) and primed word fragment completion (an implicit memory task) were administered. Since the same words were used in the two tasks, we were able to calculate the degree of dependence between recognition performance and primed word fragment completion. AD patients evidenced impaired recognition memory. In contrast, priming was intact. The pattern of correlation between the two tasks was similar in healthy controls and in AD. Independence between recognition and fragment completion was obtained when recognition preceded the fragment completion task, but not when fragment completion preceded recognition.     

Lexical and conceptual components of stem completion priming in patients with Alzheimer's disease.

This study evaluated the hypothesis of dissociation between normal lexical but deficient conceptual repetition priming in patients with Alzheimer's disease (AD). For this purpose, we administered to patients with AD and age-matched normal controls the Stem Completion task. In Experiment 1, the level of word processing during study was manipulated by requiring subjects to count vowels (graphemic condition) or generate meanings (semantic condition) of target words. In Experiment 2, the presentation modality was varied during the study to obtain an intramodal and crossmodal repetition priming. Probably due to a floor effect of performance in the graphemic condition, in Experiment 1, AD patients exhibited lower priming than normal controls for the semantically processed words but comparable priming for the graphemically processed ones. In contrast, in Experiment 2, AD patients were poorly primed both in the intra- and crossmodal conditions. Results question the hypothesis of a lexical/conceptual dissociation in the repetition priming exhibited by AD patients and call for other explicative hypotheses of the dissociation between normal and deficient forms of repetition priming in degenerative dementia.     

Reading speed in pure alexia

This study investigated possible causes of differences in reading speed between two alexic patients who read words letter by letter. As both patients appeared to rely on serial left-to-right processing of letters within words, the difference in reading speed did not seem to be related to any differences in the extent to which the patients could recognize letters in words in parallel or ‘ends-in’. Differences in reading speed also seemed to be unrelated to the patients ability to identify individual letters since their letter recognition accuracy was very similar. Furthermore, although patient PD was significantly slower at reading words aloud than patient DC, PD was in fact significantly quicker than DC on a test that has previously been used to assess letter recognition skills in letter-by-letter readers. It is therefore concluded that PD reads words more slowly because of an additional impairment at the level of the word form system. The results therefore reinforce the distinction between Type 1 and Type 2 letter-by-letter readers that was first drawn by Patterson and Kay [Q. J. exp. Psychol.34A, 411–441, 1982].     

Repetition effects in a lexical decision task: the role of episodic memory in the performance of alcoholic Korsakoff patients

To examine the contribution of episodic memory to the successful verbal priming performance of Korsakoff patients, we adapted a lexical decision task which holds constant the processing demands during study and testing. Words and nonwords were repeated at a lag of 0, 1, 3, 8 and 15 items and the decrease in response latency was taken as a measure of priming. In Experiment 1, Korsakoff patients showed repetition priming of a magnitude similar to that obtained by alcoholic controls, even at a lag of 15 intervening items. Experiment 2 explored the effect of word frequency on repetition priming. Korsakoff patients again showed normal priming up to lags of 15, and, as expected, these repetition effects were larger for low frequency than for high frequency words. This outcome was felt to be more consistent with an episodically based familiarity account than with a semantic activation account. Finally, Korsakoff patients were found to be impaired in their ability to use episodic memory in an explicit memory task that used the same material under comparable presentation conditions (Experiment 3). Contemporary processing models of episodic memory are discussed as possible explanations for the successful implicit memory performance by amnesic patients on this task.