Does priming with awareness reflect explicit contamination? An approach with a response-time measure in word-stem completion

Inefficient stimulus processing at encoding affects formation of high-order general representation: A study on cross-modal word-stem completion task

Priming is an implicit memory effect in which previous exposure to one stimulus influences the response to another stimulus. The main characteristic of priming is that it occurs without awareness. Priming takes place also when the physical attributes of previously studied and test stimuli do not match; in fact, it greatly refers to a general stimulus representation activated at encoding independently of the sensory modality engaged. Our aim was to evaluate whether, in a cross-modal word-stem completion task, negative priming scores could depend on inefficient word processing at study and therefore on an altered stimulus representation. Words were presented in the auditory modality, and word-stems to be completed in the visual modality. At study, we recorded auditory ERPs, and compared the P300 (attention/memory) and N400 (meaning processing) of individuals with positive and negative priming. Besides classical averaging-based ERPs analysis, we used an ICA-based method (ErpICASSO) to separate the potentials related to different processes contributing to ERPs. Classical analysis yielded significant difference between the two waves across the whole scalp. ErpICASSO allowed separating the novelty-related P3a and the top-down control-related P3b sub-components of P300. Specifically, in the component C3, the positive deflection identifiable as P3b, was significantly greater in the positive than in the negative priming group, while the late negative deflection corresponding to the parietal N400, was reduced in the positive priming group. In conclusion, inadequacy of specific processes at encoding, such as attention and/or meaning retrieval, could generate weak semantic representations, making words less accessible in subsequent implicit retrieval.

Implicit memory for object locations depends on reactivation of encoding-related brain regions

This study explored the correspondence between implicit memory and the reactivation of encoding-related brain regions. By using a classification method, we examined whether reactivation reflects only the similarities between study and test or voxels at the reactivated regions are diagnostic of facilitation in the implicit memory task. A simple detection task served as incidental encoding of object-location pairings. A subsequent visual search task served as the indirect (implicit) test of memory. Subjects did not know that their memory would be tested. Half of the subjects were unaware that some stimuli in the search task are the same as those that had appeared during the detection task. Another group of subjects was made aware of this relationship at the onset of the visual search task. Memory performance was superior for the study-test aware, compared to study-test unaware, subjects. Brain reactivation was calculated using a conjunction analysis implemented through overlaying the neural activity at encoding and testing. The conjunction analysis revealed that implicit memory in both groups of subjects was associated with reactivation of parietal and occipital brain regions. We were able to classify study-test aware and study-test unaware subjects based on the per-voxel reactivation values representing the neural dynamics between encoding and test. The classification results indicate that neural dynamics between encoding and test accounts for the differences in implicit memory. Overall, our study demonstrates that implicit memory performance requires and depends upon reactivation of encoding-related brain regions.

Priming effects from young-old to very old age on a word-stem completion task: minimizing explicit contamination

We investigated the size of repetition priming effects from young-old to very old age using a newly developed Word-Stem Completion (WSC) task. Retrospectively, we examined the role of explicit, intentional retrieval strategies in priming. We constructed our task by taking factors into account that were known to complicate the measurement of significant and valid priming effects. Within our sample of 170 cognitively healthy elderly persons of 55-94 years old, we found no effects on priming of age, gender, education, intelligence, cognitive status, memory complaints, or depressive symptoms. Participants that subsequent to task administration reported awareness of the study-test relationship obtained higher priming scores. However, analysis of stem-completion times showed that explicit contamination during the task was unlikely. The results suggest that WSC priming is age-invariant up to very old age. This task with increased validity might contribute to the differentiation with Alzheimer's disease by improving specificity of assessment.

Human hippocampal-dependent tasks: Is awareness necessary or sufficient?

The hippocampus has been shown to be required for the acquisition of declarative or explicit memory. Whether all hippocampal-dependent forms of learning and memory are explicit is an open question. Controversy has emerged about the existence of implicit hippocampal-dependent tasks. Two implicit tasks that may involve the hippocampusare a relational eye tracking task (Ryan et al. (2000) Psychol Sci 11:454-461) and transitive inference (Greene et al. (2006) J Cognit Neurosci 18:1156-1173; Greene et al. (2001) Mem Cognit 29:893-902). Recently, it was shown that both of these tasks may depend upon task awareness (Smith et al. (2006) J Neurosci 26:11304-11312; Smith and Squire (2005) J Neurosci 25:10138-10146). It is argued that in both cases, distinct, explicit versions of the tasks were created, which do not disprove the implicit nature of the original tasks.

The effect of encoding manipulation on word-stem cued recall: An event-related potential study

The purpose of the present study was to find out whether the neural correlates of explicit retrieval from episodic memory would vary according to conditions at encoding when the words were presented in separate study/test blocks. Event-related potentials (ERPs) were recorded while participants performed a word-stem cued-recall task. Deeply (semantically) studied words were associated with higher levels of recall and faster response times than shallowly (lexically) studied words. Robust ERP old/new effects were observed for each encoding condition. They varied in magnitude, being largest in the semantic condition. As expected, scalp distributions also differed: for deeply studied words, the old/new effect resembled that found in previous ERP studies of word-stem cued-recall tasks (parietal and right frontal effects, between 400-800 and 800-1100 ms post-stimulus), whereas for shallowly studied words, the parietal old/new effect was absent in the latter latency window. These results can be interpreted as reflecting access to different kinds of memory representation depending on the nature of the processing engaged during encoding. Furthermore, differences in the ERPs elicited by new items indicate that subjects adopted different processing strategies in the test blocks following each encoding condition.