The role of the hippocampus in recollection and familiarity remains debated. Using functional magnetic resonance imaging (fMRI), we explored whether hippocampal activity is modulated by increasing recollection confidence, increasing amount of recalled information, or both. We also investigated whether any hippocampal differences between recollection and familiarity relate to processing differences or amount of information in memory. Across two fMRI tasks, we separately compared brain responses to levels of confidence for cued word recall and word familiarity, respectively. Contrary to previous beliefs, increasing confidence/accuracy of cued recall of studied words did not increase hippocampal activity, when unconfounded by amount recollected. In contrast, additional recollection (i.e., recollecting more information than the word alone) increased hippocampal activity, although its accuracy matched that of word recall alone. Unlike cued word recall, increasing word familiarity accuracy did increase hippocampal activity linearly, although at an uncorrected level. This finding occurred although cued word recall and familiarity memory seemed matched with respect to information in memory. The detailed characteristics of these effects do not prove that word familiarity is exceptional in having hippocampal neural correlates. They suggest instead that participants fail to identify some aspects of recollection, misreporting it as familiarity, a problem with word‐like items that have strong and recallable semantic associates.
Word stem completion tasks involve showing participants a number of words and then later asking them to complete word stems to make a full word. If the stem is completed with one of the studied words, it indicates memory. It is a test widely used to assess both implicit and explicit forms of memory. An important aspect of stimulus selection is that target words should not frequently be generated spontaneously from the word stem, to ensure that production of the word really represents memory. In this article, we present a database of spontaneous stem completion rates for 395 stems from a group of 80 British undergraduate psychology students. It includes information on other characteristics of the words (word frequency, concreteness, imageability, age of acquisition, common part of speech, and number of letters) and, as such, can be used to select suitable words to include in a stem completion task. Supplemental materials for this article may be downloaded from http://brm .psychonomic-journals.org/content/supplemental.Word stem completion (WSC) tasks have a long history of use in psychology, where their initial introduction indicated that implicit memory can be preserved in amnesic patients (Warrington & Weiskrantz, 1970). Participants are shown lists of words, unaware of a later memory task. Subsequently, they are presented with the first few letters of a word and are asked to complete this stem with the first word that comes to mind (see Roediger, Weldon, Stadler, & Riegler, 1992, for a review). Explicit memory versions ask for completions that are remembered from the study list (Greene, 1986). An increase in stem completion levels due to preexposure of the words indicates memory. This methodology is still widely used in tasks ranging from cerebral blood flow measures (Sorond, Schnyer, Serrador, Milberg, & Lipsitz, 2008) to behavioral tasks in healthy participants (Benjamin Clarke & Butler, 2008) and patients (e.g., Alzheimer's disease, LaVoie & Faulkner, 2008; schizophrenia, Pope & Kern, 2006).The selection of words to use in WSC tasks is critical. The stem must be able to form multiple words when completed. Words should not be proper nouns and should have sufficient frequency for participants to know them and their meanings. They should have relatively matched imageability and concreteness. Information on these characteristics is available through established databases (e.g., M. Wilson, 1988).One key criterion, particularly important for implicit tasks, is that the words used should not commonly be produced in a spontaneous WSC task. Since participants must generate the first completion that comes to mind, a frequent spontaneous completion reduces the distance between chance and ceiling performance. In imaging studies, "correct" responses would be more likely to be guesses, adding unwanted noise to the neural signal. Matching low levels of spontaneous WSC across word lists-and ideally, all words-ensures that tasks are sensitive and reliable.Spontaneous WSC rates are not easily available, and studies have run p...
Neurosteroids have been implicated in the pathophysiology of post-traumatic stress disorder (PTSD). Allopregnanolone is reduced in subsets of individuals with PTSD and has been explored as a novel treatment strategy. Both direct trauma exposure and witnessed trauma are risk factors for PTSD; however, the role of neurosteroids in the behavioral outcomes of these unique experiences has not been explored. Here we investigate whether observational fear is associated with a reduced capacity for endogenous neurosteroidogenesis and the relationship with behavioral outcomes. We demonstrate that both mice directly subjected to the threat (foot shocks) and those witnessing the threat have decreased plasma levels of allopregnanolone. The expression of a key enzyme involved in endogenous neurosteroid synthesis, 5α-reductase type 2, is decreased in the basolateral amygdala (BLA), which is a major emotional processing hub implicated in PTSD. We demonstrate that knockdown of 5α-reductase type 2 exaggerates the behavioral expression of fear in response to witnessed trauma, whereas treatment with an exogenous, synthetic neuroactive steroid GABAA receptor PAM with molecular pharmacology similar to allopregnanolone (SGE-516 [tool compound]) decreases the behavioral response to observational fear. These data implicate impaired endogenous neurosteroidogenesis in the pathophysiology of threat exposure, both direct and witnessed. Further, these data suggest that treatment with exogenous 5α-reduced neurosteroids or targeting endogenous neurosteroidogenesis may be beneficial for the treatment of individuals with PTSD, whether resulting from direct or witnessed trauma.
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