Adaptation to a direction-dependent visuomotor gain in the young and elderly

Consistent with the widely accepted notion of separate specification of movement amplitude and direction, it has been argued that there is also a categorical difference between adaptation to novel visuomotor rotations and to novel visuomotor gains. In line with this view, ageing seems to affect rotation and gain adaptation differently in that age-related impairments are consistently found for the former, but not for the latter. In this study we ask whether the contrasting findings could also be ascribed to differences in the level of difficulty of gain and rotation adaptation tasks, respectively. In order to increase the difficulty of gain adaptation, younger and older participants had to adapt to a direction-dependent gain transformation. Results revealed direction-dependent adaptation in both groups. More importantly, we replicated the typical findings of age-related impairments of adaptation, but not of aftereffects, that were previously only reported for rotation adaptation. Younger participants also showed superior explicit knowledge regarding the novel visuomotor mapping as compared to the older participants. We show that this knowledge was used by younger participants to selectively augment adaptive shifts. Finally, our findings suggest that the difficulty of the novel visuomotor transformation and, related to this, the involvement of explicit knowledge in adaptation is critical for age-related changes to show up, but not the type of adaptation task, rotation and gain adaptation, respectively.     

Adult age differences in implicit and explicit memory: time course and encoding effects

Young and older adults were tested at three delays on word-stem completion or cued recall following semantic or structural word judgments. Identical three-letter stems were present at retrieval for both implicit (completion) and explicit (cued recall) tasks; only the intention to recall list words differed. The young adults outperformed the older adults on both implicit and explicit tasks at all test delays. Under some conditions, the older but not the young adults performed more poorly on cued recall than on stem completion, suggesting a possible failure to use implicitly available information to support explicit remembering. These results suggest that some forms of implicit memory decline with normal aging.     

Adaptation to visuomotor rotations in younger and older adults

Adaptation to a visuomotor rotation is known to be impaired at older adult age. The authors examined whether the impairment is present already at preretirement age and whether it depends on the difficulty of the adaptation task. Moreover, the authors tested predictions of the hypothesis that the age-related impairment pertains primarily to strategic corrections and the explicit knowledge on which they are based but not to the acquisition of an (implicit) internal model of the novel visuomotor transformation. The authors found an age-related impairment of adaptation and explicit knowledge already at preretirement age but no age-related change of aftereffects. With an incremental simplification of the adaptation task, age-related changes were able to be eliminated. Individual differences of the quality of explicit knowledge were associated with differences of adaptation, but not of aftereffects. When age groups were matched by explicit knowledge, age-related impairments of adaptation largely disappeared. However, a reliable difference remained in one of the experiments, suggesting that other processes of adjustment to a visuomotor rotation might be affected by aging as well.     

Functional independence of explicit and implicit motor adjustments

Adaptation to novel visuomotor transformations for example when navigating a cursor on a computer monitor by using a computer mouse, can be explicit or implicit. Explicit adjustments are made when people are informed about the occurrence and the type of a novel visuomotor transformation and intentionally modify their movements. Implicit adjustments, in contrast, are made without reportable knowledge of a novel visuomotor transformation and without a change intention. The relation of implicit adjustments to explicit adjustments needs further clarification. Here we show that these two types of adjustment occur at the same time and remain functionally independent. The size of total adjustment turned out to be the sum of explicit and implicit adjustments measured in isolation, even when both processes produce opposite outcomes. In perspective our results demonstrate that automatic, implicit processes of motor control are not superseded by intentional, explicit ones, but only superposed.     

Priming guesses on a forced-recall test

The forced-recall paradigm requires participants to fill all spaces on the memory test even if they cannot remember all the list words. In the present study, the authors used that paradigm to examine the influence of implicit memory on guessing--when participants fill remaining spaces after they cannot remember list items. They measured explicit memory as the percentage of targets that participants designated as remembered from the list and implicit memory as the percentage of targets they wrote but did not designate as remembered (beyond chance level). The authors examined implicit memory on guessing with forced recall (Experiment 1), forced cued recall with younger and older adults (Experiment 2), and forced free and cued recall under a depth-of-processing manipulation (Experiment 3). They conclude that implicit memory influences guesses of targets in the forced-recall paradigm.     

Implicit proactive interference, age, and automatic versus controlled retrieval strategies

We assessed the extent to which implicit proactive interference results from automatic versus controlled retrieval among younger and older adults. During a study phase, targets (e.g., "ALLERGY") either were or were not preceded by nontarget competitors (e.g., "ANALOGY"). After a filled interval, the participants were asked to complete word fragments, some of which cued studied words (e.g., "A_L_ _GY"). Retrieval strategies were identified by the difference in response speed between a phase containing fragments that cued only new words and a phase that included a mix of fragments cuing old and new words. Previous results were replicated: Proactive interference was found in implicit memory, and the negative effects were greater for older than for younger adults. Novel findings demonstrate two retrieval processes that contribute to interference: an automatic one that is age invariant and a controlled process that can reduce the magnitude of the automatic interference effects. The controlled process, however, is used effectively only by younger adults. This pattern of findings potentially explains age differences in susceptibility to proactive interference.     

Reflections of distraction in memory: transfer of previous distraction improves recall in younger and older adults

Three studies explored whether younger and older adults' free recall performance can benefit from prior exposure to distraction that becomes relevant in a memory task. Participants initially read stories that included distracting text. Later, they studied a list of words for free recall, with half of the list consisting of previously distracting words. When the memory task was indirect in its use of distraction (Study 1), only older adults showed transfer, with better recall of previously distracting compared with new words, which increased their recall to match that of younger adults. However, younger adults showed transfer when cued about the relevance of previous distraction both before studying the words (Study 2) and before recalling the words (Study 3) in the memory test. Results suggest that both younger and older adults encode distraction, but younger adults require explicit cueing to use their knowledge of distraction. In contrast, older adults transfer knowledge of distraction in both explicitly cued and indirect memory tasks. Results are discussed in terms of age differences in inhibition and source-constrained retrieval.     

Examining the Response Competition Hypothesis of Age Effects in Implicit Memory

ABSTRACT

Age reductions in priming have been explained by differences in processing demands across implicit memory tests. According to one hypothesis, older adults show reduced priming relative to younger adults on implicit tests that require production of a response because these tests typically allow for response competition. In contrast, older adults do not show reductions in priming on identification tests that contain little response competition. The following experiments tested the specific role of response competition in mediating age effects in implicit memory. In Experiment 1, younger and older adults studied a list of words and were then given an implicit test of word stem completion. They studied a second list of words and were given an implicit test of general knowledge. Each implicit test contained items with unique solutions (the low response competition condition) and items with multiple solutions (the high response competition condition). In Experiment 2, younger and older adults were given explicit versions of the word stem completion and the general knowledge tests. Results showed an effect of age on explicit memory (Experiment 2), but no effect of age or response competition on priming (Experiment 1). Results are inconsistent with the theory that response competition leads to age effects on production tests of implicit memory.     

Effects of depth-of-processing and ageing on word-stem and word-fragment implicit memory tasks: Test of the lexical-processing hypothesis

The joint effects of depth-of-processing and age on repetition priming in implicit memory tests of word-fragment completion (WFC) and word-stem completion (WSC) were investigated. The experiment consisted of three study tasks (perceptual, lexical, and semantic) and four memory conditions: implicit (WSC and WFC) and explicit (WS cued recall and WF cued recall). In the WSC condition, semantic and lexical study processing produced equal priming, both superior to the perceptual study processing, whereas the WFC test showed equal priming for these three study conditions. This finding provides clear evidence, consistent with the lexical-processing hypothesis, that depth-of-processing in WSC priming reflects a lexical rather than a semantic process. It also provides support for the view that WSC and WFC involve different processes. However, there was no evidence of an age effect on either of these two implicit tasks. The data also revealed an overall significant effect of age and depth-of-processing, and an interaction between these variables on explicit cued recall tasks, indicating that older adults benefited less than younger ones from a deep encoding condition.     

Do young adults show conceptual knowledge of previous distractors?

Using implicit tests, older adults have been found to retain conceptual knowledge of previously seen task-irrelevant information. While younger adults typically do not show the same effect, evidence from one study [Gopie, N., Craik, F. I. M., & Hasher, L. (2011). A double dissociation of implicit and explicit memory in younger and older adults. Psychological Science, 22, 634–640. doi:10.1177/0956797611403321] suggests otherwise. In that study, young adults showed greater explicit than implicit memory for previous distractors on a word fragment completion task. This was interpreted as evidence for maintaining access to previous conceptual knowledge of the distractors. Here, we report two failures to replicate that original finding, followed by a third study designed to test directly whether young adults use conceptual-level information that was previously irrelevant. Our findings agree with others that young adults show weak to no evidence of conceptual knowledge of previously irrelevant information.     

Implicit memory, age, and time of day: paradoxical priming effects

Memory retrieval can occur by at least two routes: a deliberate one, as when one attempts to retrieve an event or fact, and an unintentional one, as when one's behavior is triggered by the past without one's knowledge or awareness. We assessed the efficacy of these retrieval systems as a function of circadian arousal and time of day. Evening-type younger adults and morning-type older adults were tested at either peak (morning for old; evening for young) or off-peak times on implicit and explicit stem completion (Experiment 1) or on implicit category generation (Experiment 2). Results for explicit stem-cued recall replicated better performance for each age group at its peak time. In stark contrast, implicit performance was better at off-peak than at peak times of day, raising the possibility that the processes that serve explicit and implicit retrieval are on different circadian schedules, and highlighting the need to consider individual differences in circadian arousal when assessing either memory system.     

Contributions of processing ability and knowledge to verbal memory tasks across the adult life-span

This study investigated the relationships of processing capacity and knowledge to memory measures that varied in retrieval difficulty and reliance on verbal knowledge in an adult life-span sample (N = 341). It was hypothesized that processing ability (speed and working memory) would have the strongest relationship to tasks requiring active retrieval and that knowledge (vocabulary ability) would be related to verbal fluency and cued recall, as participants relied upon verbal knowledge to retrieve category items (fluency) or develop associations (cued recall). Measurement and structural equation models were developed for the entire sample and separately for younger (aged 20-54 years, n = 168) and older (aged 55-92 years, n = 173) subgroups. In accordance with the hypotheses, processing ability was found to be most highly related to free recall, with additional significant relationships to cued recall, verbal fluency, and recognition. Knowledge was found to be significantly related only to verbal fluency and to cued recall. Moreover, knowledge was more important for older than for younger adults in mediating variance in cued recall, suggesting that older adults may use age-related increases in knowledge to partially compensate for processing declines when environmental support is available in memory tasks.     

Aging affects word-stem completion and recognition but not category generation

In this study, we investigated the effect of aging on two implicit memory tasks, word-stem completion and category generation, and on explicit recognition. We compared the performance of young and older adults on these implicit memory tasks with those of explicit recognition. We expected better performance of young than older adults in the explicit memory task and similar priming in both implicit memory tasks. The results showed that young adults performed better than older adults in the recognition task. Moreover, both age groups showed priming in the implicit memory tasks, although priming was greater in young adults compared to older adults in the word-stem completion memory task, whereas both age groups showed similar levels of priming in the category generation task. The present results showed dissociations as a function of age not only between the explicit and the implicit tasks but also between the implicit tasks.     

A double dissociation of implicit and explicit memory in younger and older adults

This study examined whether age-related differences in cognition influence later memory for irrelevant, or distracting, information. In Experiments 1 and 2, older adults had greater implicit memory for irrelevant information than younger adults did. When explicit memory was assessed, however, the pattern of results reversed: Younger adults performed better than older adults on an explicit memory test for the previously irrelevant information, and older adults performed less well than they had on the implicit test. Experiment 3 investigated whether this differential pattern was attributable to an age-related decline in encoding resources, by reducing the encoding resources of younger adults with a secondary task; their performance perfectly simulated the pattern shown by the older adults in the first two experiments. Both older and younger adults may remember irrelevant information, but they remember it in different ways because of age-related changes in how information is processed at encoding and utilized at retrieval.     

Adult age differences in the implicit and explicit components of top-down attentional guidance during visual search

Two experiments investigated adult age differences in the explicit (knowledge-based) and implicit (repetition priming) components of top-down attentional guidance during discrimination of a target singleton. Experiment 1 demonstrated an additional contribution of explicit top-down attention, relative to the implicit effect of repetition priming, which was similar in magnitude for younger and older adults. Experiment 2 examined repetition priming of target activation and distractor inhibition independently. The additional contribution of explicit top-down attention, relative to the repetition priming of distractor inhibition, was greater for older adults than for younger adults. The results suggest that some forms of top-down attentional control are preserved as a function of adult age and may operate in a compensatory manner.     

Age differences in collaborative memory: the role of retrieval manipulations

In two experiments, we examined age differences in collaborative inhibition (reduced recall in pairs of people, relative to pooled individuals) across repeated retrieval attempts. Younger and older adults studied categorized word lists and were then given two consecutive recall tests and a recognition test. On the first recall test, the subjects were given free-report cued recall or forced-report cued recall instructions (Experiment 1) or free recall instructions (Experiment 2) and recalled the lists either alone or in collaboration with another subject of the same age group. Free-report cued recall and free recall instructions warned the subjects not to guess, whereas forcedreport cued recall instructions required them to guess. Collaborative inhibition was obtained for both younger and older adults on initial tests of free-report cued recall, forced-report cued recall, and free recall, showing that the effect generalizes across several tests for both younger and older adults. Collaborative inhibition did not persist on subsequent individual recall or recognition tests for list items. Older adults consistently falsely recalled and recognized items more than did younger adults, as had been found in previous studies. In addition, prior collaboration may exaggerate older adults’ tendency toward higher false alarms on a subsequent recognition test, but only after a free recall test. The results provide generality to the phenomenon of collaborative inhibition and can be explained by invoking concepts of strategy disruption and source monitoring.     

Effects of hearing words, imaging hearing words, and reading on auditory implicit and explicit memory tests

In four experiments, we examined the degree to which imaging written words as spoken by a familiar talker differs from direct perception (hearing words spoken by that talker) and reading words (without imagery) on implicit and explicit tests. Subjects first performed a surface encoding task on spoken, imagined as spoken, or visually presented words, and then were given either an implicit test (perceptual identification or stem completion) or an explicit test (recognition or cued recall) involving auditorily presented words. Auditory presentation at study produced larger priming effects than did imaging or reading. Imaging and reading yielded priming effects of similar magnitude, whereas imaging produced lower performance than reading on the explicit test of cued recall. Voice changes between study and test weakened priming on the implicit tests, but did not affect performance on the explicit tests. Imagined voice changes affected priming only in the implicit task of stem completion. These findings show that the sensitivity of a memory test to perceptual information, either directly perceived or imagined, is an important dimension for dissociating incidental (implicit) and intentional (explicit) retrieval processes.     

The effect of forced recall on illusory recollection in younger and older adults

The effect of an initial forced recall test on later recall and recognition tests was examined in younger and older adults. Subjects were presented with categorized word lists and given an initial test under standard cued recall instructions (with a warning against guessing) or forced recall instructions (that required guessing); subjects were later given a cued recall test for the original list items. In 2 experiments, initial forced recall resulted in higher levels of illusory memories on subsequent tests (relative to initial cued recall), especially for older adults. Older adults were more likely to say they remembered rather than knew that forced guesses had occurred in the original study episode. The effect persisted despite a strong warning against making errors in Experiment 2. When a source monitoring test was given, older adults had more difficulty than younger adults in identifying the source of items they had originally produced as guesses. If conditions encourage subjects to guess on a first memory test, they are likely to recollect these guesses as actual memories on later tests. This effect is exaggerated in older adults, probably because of their greater source monitoring difficulties. Both dual process and source monitoring theories provide insight into these findings.     

Direct comparison of auditory implicit memory tests

In this experiment, we examined the degree to which four implicit tests and two explicit tests, all involving auditory presentation, were sensitive to the perceptual characteristics of the stimuli presented during study. Presenting stimuli visually decreased priming in all the implicit memory tests, relative to auditory presentation. However, changing voice between study and test decreased priming only in the implicit memory tests requiring identification of words degraded by noise or by low-pass filtering, but not in those tests requiring generation from word portions (stems and fragments). Modality effects without voice effects were observed in cued recall, but the opposite pattern of results (voice effects without modality effects) was obtained in recognition. The primary new finding is the demonstration that auditory memory tests, both explicit and implicit, differ in their sensitivity to the perceptual information encoded during study.     

Impact of Parkinson’s disease and dopaminergic medication on adaptation to explicit and implicit visuomotor perturbations

The capacity to learn new visuomotor associations is fundamental to adaptive motor behavior. Evidence suggests visuomotor learning deficits in Parkinson’s disease (PD). However, the exact nature of these deficits and the ability of dopamine medication to improve them are under-explored. Previous studies suggested that learning driven by large and small movement errors engaged distinct neural mechanisms. Here, we investigated whether PD patients have a generalized impairment in visuomotor learning or selective deficits in learning from large explicit errors which engages cognitive strategies or small imperceptible movement errors involving primarily implicit learning processes. Visuomotor learning skills of non-medicated and medicated patients were assessed in two reaching tasks in which the size of visuospatial errors experienced during learning was manipulated using a novel three-dimensional virtual reality environment. In the explicit perturbation task, the visuomotor perturbation was applied suddenly resulting in large consciously detected initial spatial errors, whereas in the implicit perturbation task, the perturbation was gradually introduced in small undetectable steps such that subjects never experienced large movement errors. A major finding of this study was that PD patients in non-medicated and medicated conditions displayed slower learning rates and smaller adaptation magnitudes than healthy subjects in the explicit perturbation task, but performance similar to healthy controls in the implicit perturbation task. Also, non-medicated patients showed an average reduced deadaptation relative to healthy controls when exposed to the large errors produced by the sudden removal of the perturbation in both the explicit and implicit perturbation tasks. Although dopaminergic medication consistently improved motor signs, it produced a variable impact on learning the explicit perturbation and deadaptation and unexpectedly worsened performance in some patients. Considered together, these results indicate that PD selectively impairs the ability to learn from large consciously detected visuospatial errors. This finding suggests that basal ganglia-related circuits are important neural structures for adaptation to sudden perturbations requiring awareness and high-cost action selection. Dopaminergic treatment may selectively compromise the ability to learn from large explicit movement errors for reasons that remain to be elucidated.