Direct Memory Tasks Research Articles

Oscillatory brain activity in the human EEG during indirect and direct memory tasks

Up to today, the neuronal bases of activating and establishing a memory trace are not well understood. Several findings point towards the idea that the activation of an engram is mirrored in induced oscillatory bursts in the gamma frequency range (> 20 Hz; induced gamma band responses; iGBRs). In the present study, we further investigated this hypothesis. Volunteers performed two tasks on repeated pictures of familiar and unfamiliar objects. They either made a familiarity decision with repetition being task irrelevant (indirect memory task), or a recognition judgment with repetition being task relevant (direct memory task). Furthermore, we complemented iGBR analyses by investigating other brain responses known to be modulated by mnemonic manipulations, namely, evoked gamma oscillations, lower frequency oscillations, and event-related potentials (ERPs). The results obtained for the indirect task replicated previous findings of repetition suppression of iGBRs for repeated familiar stimuli and an increase of iGBRs for repeated unfamiliar objects. These effects might be linked to the 'sharpening' of a cell assembly representing a familiar object and to the formation of a new object representation for unfamiliar stimuli, respectively. In contrast, the direct task revealed no repetition-related modulations of iGBRs. Thus, modulations of iGBRs are not a mere automatic consequence of repeated stimulus processing but might rather mirror changes within cortical object representation according to use-dependent plasticity principles. Furthermore, evoked gamma responses, lower frequency bands, and late components of the ERPs correlated with more specific subprocesses during mnemonic functioning.

ERP correlates of retrieval orientation: Direct versus indirect memory tasks

The neural correlates of the processes involved in cueing memory retrieval were investigated using ERPs to unstudied (new) items in a yes/no recognition test (direct memory task) and a semantic judgement task (indirect memory task). Subjects encoded either pictures or auditory words and were tested on visual words. We replicated previous findings that ERPs to correct rejections of new items in a yes/no recognition test differ according to the study material, with ERPs to words encoded as pictures being more negative (relative to a average mastoid reference) than words encoded auditorily [Hornberger, M., Morcom, A.M., et al., 2004. Neural correlates of retrieval orientation: effects of study-test similarity. J. Cogn. Neurosci. 16 (7), 1196–1210]. This difference was sustained from approximately 450–1200 ms. An effect of study material on the ERP to new items was found in the indirect memory task but was both earlier onsetting and shorter lived, ca. 250–600 ms. These findings add weight to the concept of ‘retrieval orientation’ [Rugg, M.D., Wilding, E.L., 2000. Retrieval processing and episodic memory. Trends Cogn. Sci. 4 (3) 108–115] – differential processing of retrieval cues according to the form of the sought-for information – by showing that a putative ERP correlate of retrieval orientation is restricted to direct memory tasks.

Remembering What One Intended to Forget: The Lack of Directed Forgetting Effects in Implicit Memory

The results of previous studies on directed forgetting effects in implicit memory have led to the conclusion that the underlying mechanism for such effects is an inhibition of retrieval or an encoding difficulty. Differences in procedure and stimuli may contribute to these different conclusions. In 2 studies we standardized the procedure while varying the stimuli. As in previous research, directed forgetting results were obtained for direct memory tasks. However, the magnitude of the directed forgetting effect varied with the stimuli. In Experiments 1 and 2, medium- and low-frequency to-be-remembered (R) and to-be-forgotten (F) words appeared to be equally accessible using the indirect task. In Experiment 3, we devised a procedure in which a single stem could lead to an R-word or F-word, but space was provided for only one word. In that way, we could examine the relative accessibility of R- and F-words. Results are discussed in light of Mandler's (1980) conceptualization of the contributions of identification and familiarity to memory retrieval processes.

Directed forgetting affects both direct and indirect tests of memory.

In two experiments, subjects were instructed either to remember or to forget each word. Following study, two tests were given, one a direct test of memory requiring conscious recollection of the study list and the other an indirect test that could be performed without awareness of the study list. In Experiment l, subjects recognized more remember than forget words (direct test) and completed more remember than forget fragments (indirect test) on both immediate and 1-week delayed tests. In Experiment 2, subjects showed superior recall (direct test) and greater repetition priming in lexical decision (indirect test) for remember than for forget words. The consistent directed forgetting effect on both types of tests is in accord with the idea that forget items are inhibited at the time of retrieval and that retrieval manipulations, unlike elaboration manipulations at encoding, affect direct and indirect tests in similar ways. In their review chapter, Johnson and Hasher (1987) identified the relations among different memory tasks as a central problem for memory researchers to address. One distinction they particularly emphasized was that between direct and indirect measures of memory. In the words of Johnson and Hasher (1987), Direct memory tasks (free recall, cued recall, recognition) require conscious expressions of remembering; indirect memory tasks (e.g., perceptual identification, homophone spelling, word completion, skill learning) do not (p. 64 l). Johnson and Hasher went on to survey the considerable research effort that had been directed at this distinction.

Measures of Memory

Measures of Memory

Recognition and recall in amnesics.

Although there is considerable agreement that performance in direct memory tasks (e.g., recall, recognition) is more disrupted by amnesia than performance in indirect memory tasks (e.g., mirror reading, word completion), one may be able to further circumscribe the deficit within the domain of direct memory tasks. The present article explores whether recall is disproportionately disrupted by amnesia compared to recognition. If amnesia affects memory uniformly across different direct memory measures, recall of normal controls should not differ from the recall of amnesics when recognition scores of these two groups are equated. On the other hand, if recall is disproportionately disrupted, normal recall should be superior to amnesic recall even when recognition is equated. The present study equated amnesic recognition with that of controls by providing amnesics with 8 s of study time and normal subjects with 0.5 s. Amnesics with Korsakoff's syndrome, amnesics with other etiologies, and appropriate controls were examined. Normal recall was superior to amnesic recall even when no differences were found in recognition. The results further specify the selective nature of amnesia.