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In our daily life, we often need to selectively remember information related to the same retrieval cue in a consecutive manner (e.g., ingredients from a recipe). To investigate such selection processes during cued long-term memory (LTM) retrieval, we used a paradigm in which the retrieval demands were systematically varied from trial to trial and analyzed, by means of behavior and slow cortical EEG potentials (SCPs), the retrieval processes in the current trial depending on those of the previous trial. We varied whether the retrieval cue, the type of to-be-retrieved association (feature), or retrieval load was repeated or changed from trial to trial. The behavioral data revealed a benefit of feature repetition, probably due to trial-by-trial feature priming. SCPs further showed an effect of cue change with a mid-frontal maximum, suggesting increased control demands when the cue was repeated, as well as a parietal effect of retrieval-load change, indicating increased activation of posterior neural resources when focusing on a single association after all learned associations had been activated previously, compared to staying with single associations across trials. These effects suggest the existence of two distinct types of dynamic (trial-by-trial) control processes during LTM retrieval: (1) medial frontal processes that monitor or regulate interference within a set of activated associations, and (2) posterior processes regulating attention to LTM representations. The present study demonstrates that processes mediating selective LTM retrieval can be successfully studied by manipulating the history of processing demands in trial sequences.
Remembering is more than an activation of a memory trace. As retrieval cues are often not uniquely related to one specific memory, cognitive control should come into play to guide selective memory retrieval by focusing on relevant while ignoring irrelevant information. Here, we investigated, by means of EEG and fMRI, how the memory system deals with retrieval interference arising when retrieval cues are associated with two material types (faces and spatial positions), but only one is task-relevant. The topography of slow EEG potentials and the fMRI BOLD signal in posterior storage areas indicated that in such situations not only the relevant but also the irrelevant material becomes activated. This results in retrieval interference that triggers control processes mediated by the medial and lateral PFC, which are presumably involved in biasing target representations by boosting the task-relevant material. Moreover, memory-based conflict was found to be dissociable from response conflict that arises when the relevant and irrelevant materials imply different responses. The two types of conflict show different activations in the medial frontal cortex, supporting the claim of domain-specific prefrontal control systems.
We investigated EEG-power and EEG-coherence changes in a unimodal and a crossmodal matching-to-sample working memory task with either visual or kinesthetic stimuli. Angle-shaped trajectories were used as stimuli presented either as a moving dot on a screen or as a passive movement of a haptic device. Effects were evaluated during the different phases of encoding, maintenance, and recognition. Alpha power was modulated during encoding by the stimulus modality, and in crossmodal conditions during encoding and maintenance by the expected modality of the upcoming test stimulus. These power modulations were observed over modality-specific cortex regions. Systematic changes of coherence for crossmodal compared to unimodal tasks were not observed during encoding and maintenance but only during recognition. There, coherence in the theta-band increased between electrode sites over left central and occipital cortex areas in the crossmodal compared to the unimodal conditions. The results underline the importance of modality-specific representations and processes in unimodal and crossmodal working memory tasks. Crossmodal recognition of visually and kinesthetically presented object features seems to be related to a direct interaction of somatosensory/motor and visual cortex regions by means of long-range synchronization in the theta-band and such interactions seem to take place at the beginning of the recognition phase, i.e. when crossmodal transfer is actually necessary.
Direction of gaze (eye angle + head angle) has been shown to be important for representing space for action, implying a crucial role of vision for spatial updating. However, blind people have no access to vision yet are able to perform goal-directed actions successfully. Here, we investigated the role of visual experience for localizing and updating targets as a function of intervening gaze shifts in humans. People who differed in visual experience (late blind, congenitally blind, or sighted) were briefly presented with a proprioceptive reach target while facing it. Before they reached to the target's remembered location, they turned their head toward an eccentric direction that also induced corresponding eye movements in sighted and late blind individuals. We found that reaching errors varied systematically as a function of shift in gaze direction only in participants with early visual experience (sighted and late blind). In the late blind, this effect was solely present in people with moveable eyes but not in people with at least one glass eye. Our results suggest that the effect of gaze shifts on spatial updating develops on the basis of visual experience early in life and remains even after loss of vision as long as feedback from the eyes and head is available.