Semantic boost on episodic associations: an empirically-based computational model

Words become associated following repeated co-occurrence episodes. This process might be further determined by the semantic characteristics of the words. The present study focused on how semantic and episodic factors interact in incidental formation of word associations. First, we found that human participants associate semantically related words more easily than unrelated words; this advantage increased linearly with repeated co-occurrence. Second, we developed a computational model, SEMANT, suggesting a possible mechanism for this semantic-episodic interaction. In SEMANT, episodic associations are implemented through lateral connections between nodes in a pre-existent self-organized map of word semantics. These connections are strengthened at each instance of concomitant activation, proportionally with the amount of the overlapping activity waves of activated nodes. In computer simulations SEMANT replicated the dynamics of associative learning in humans and led to testable predictions concerning normal associative learning as well as impaired learning in a diffuse semantic system like that characteristic of schizophrenia.     

The assessment of recognition memory using the Remember/Know procedure in amnestic mild cognitive impairment and probable Alzheimer’s disease

This study used the Remember/Know (R/K) procedure combined with signal detection analyses to assess recognition memory in 20 elders with amnestic mild cognitive impairment (aMCI), 10 patients with probable Alzheimer’s disease (AD) as well as matched healthy older adults. Signal detection analyses first indicated that aMCI and control participants were comparable on general recognition performance. As regards AD patients, they were impaired relative to both aMCI and healthy elders. When assessing Remember and Know responses the aMCI group showed diminished sensitivity for Remember responses but intact Know responses compared to healthy elders. In contrast, AD patients showed decreased sensitivity for both Remember and Know responses compared to control and aMCI participants. The response bias index revealed that AD patients were more liberal than aMCI and control participants when providing Know responses. On the other measures, response bias was comparable between the groups. Overall, this study indicates that the R/K procedure can characterize different aspects of recognition memory performance in persons with aMCI or AD.     

The influence of instructions and terminology on the accuracy of remember–know judgments

The remember–know paradigm is one of the most widely used procedures to examine the subjective experience associated with memory retrieval. We examined how the terminology and instructions used to describe the experiences of remembering and knowing affected remember–know judgments. In Experiment 1 we found that using neutral terms, i.e., Type A memory and Type B memory, to describe the experiences of remembering and knowing reduced remember false alarms for younger and older adults as compared to using the terms Remember and Know, thereby increasing overall memory accuracy in the neutral terminology condition. In Experiment 2 we found that using what we call source-specific remember–know instructions, which were intended to constrain remember judgments to recollective experiences arising only from the study context, reduced remember hits and false alarms, and increased know hits and false alarms. Based on these data and other considerations, we conclude that researchers should use neutral terminology and source-specific instructions to collect the most accurate reports of the experiences of remembering and knowing arising from the study context.     

A model of episodic memory: Mental time travel along encoded trajectories using grid cells

The definition of episodic memory includes the concept of mental time travel: the ability to re-experience a previously experienced trajectory through continuous dimensions of space and time, and to recall specific events or stimuli along this trajectory. Lesions of the hippocampus and entorhinal cortex impair human episodic memory function and impair rat performance in tasks that could be solved by retrieval of trajectories. Recent physiological data suggests a novel model for encoding and retrieval of trajectories, and for associating specific stimuli with specific positions along the trajectory. During encoding in the model, external input drives the activity of head direction cells. Entorhinal grid cells integrate the head direction input to update an internal representation of location, and drive hippocampal place cells. Trajectories are encoded by Hebbian modification of excitatory synaptic connections between hippocampal place cells and head direction cells driven by external action. Associations are also formed between hippocampal cells and sensory stimuli. During retrieval, a sensory input cue activates hippocampal cells that drive head direction activity via previously modified synapses. Persistent spiking of head direction cells maintains the direction and speed of the action, updating the activity of entorhinal grid cells that thereby further update place cell activity. Additional cells, termed arc length cells, provide coding of trajectory segments based on the one-dimensional arc length from the context of prior actions or states, overcoming ambiguity where the overlap of trajectory segments causes multiple head directions to be associated with one place. These mechanisms allow retrieval of complex, self-crossing trajectories as continuous curves through space and time.     

The Influence of Episodic and Thematic Frames on Policy and Group Attitudes: Mediational Analysis

An experiment examined how episodic and thematic political message frames affect attitudes toward older adults and Social Security. When exposed to messages about abolishing Social Security, participants exposed to episodic frames were significantly more likely to endorse message‐consistent attitudes than participants exposed to a thematic frame. In mediation analyses, an episodic frame featuring a counterstereotypical exemplar increased endorsement of individual responsibility for retirement planning, which then led to more negative attitudes toward Social Security. These effects did not occur with a stereotypical exemplar in an episodic frame. The same mediated pathway influenced attitudes toward older adults in a more complex manner. Results provide support for individual responsibility attributions as a mediating mechanism underlying the effects of certain episodic frames.     

Formation of semantic associations between subliminally presented face-word pairs

Recent evidence suggests that consciousness of encoding is not necessary for the rapid formation of new semantic associations. We investigated whether unconsciously formed associations are as semantically precise as would be expected for associations formed with consciousness of encoding during episodic memory formation. Pairs of faces and written occupations were presented subliminally for unconscious associative encoding. Five minutes later, the same faces were presented suprathreshold for the cued unconscious retrieval of face-occupation associations. Retrieval instructions required participants to classify the presented individuals according to their putative (1) regularity of income, (2) length of education, and (3) creativity value of occupational activity. The three instructions yielded more classifications consistent with a person’s occupation if the person had been subliminally presented with his written occupation versus a meaningless word (control condition). This suggests that consciousness is not necessary to encode, long-term store, and retrieve semantically precise associations between primarily unrelated items.     

Applying an exemplar model to the artificial-grammar task: Inferring grammaticality from similarity

We present three artificial-grammar experiments. The first used position constraints, and the second used sequential constraints. The third varied both the amount of training and the degree of sequential constraint. Increasing both the amount of training and the redundancy of the grammar benefited participants' ability to infer grammatical status; nevertheless, they were unable to describe the grammar. We applied a multitrace model of memory to the task. The model used a global measure of similarity to assess the grammatical status of the probe and captured performance both in our experiments and in three classic studies from the literature. The model shows that retrieval is sensitive to structure in memory, even when individual exemplars are encoded sparsely. The work ties an understanding of performance in the artificial-grammar task to the principles used to understand performance in episodic-memory tasks.     

Across-task long-term priming: Interaction of task readiness and automatic retrieval

When humans carry out actions in response to external stimulation, they acquire associations between the stimulus and the action it triggered. When the same stimulus is used in two different tasks, the retrieval of associations compiled in the competing task hampers current performance. Previous research suggests that this across-task priming depends on the task set for the preceding task remaining active across the switch of tasks and, thus, competing with the activations needed for the new task. We present two experiments investigating this notion. Participants switched between two semantic classification tasks. In Experiment 1, participants switched between short runs of the two tasks. Across-task priming was observed on switch and repeat trials. In Experiment 2, participants switched between longer runs of the two tasks. Across-task priming was markedly reduced on repeat trials. The data suggest that whether or not across-task priming affects behaviour after the switch trial depends, amongst others, on whether the task set necessary for the previous task spills into the repeat trials. The implications of these findings for mechanisms of cognitive and mnemonic control are discussed.