Priming for novel object associations: Neural differences from object item priming and equivalent forms of recognition

Is implicit memory associated with the hippocampus?

According to the traditional memory-systems view, the hippocampus is critical during explicit (conscious) long-term memory, whereas other brain regions support implicit (nonconscious) memory. In the last two decades, some fMRI studies have reported hippocampal activity during implicit memory tasks. The aim of the present discussion paper was to identify whether any implicit memory fMRI studies have provided convincing evidence that the hippocampus is associated with nonconscious processes without being confounded by conscious processes. Experimental protocol and analysis parameters included the stimulus type(s), task(s), measures of subjective awareness, explicit memory accuracy, the relevant fMRI contrast(s) or analysis, and confound(s). A systematic review was conducted to identify implicit memory studies that reported fMRI activity in the hippocampus. After applying exclusion criteria, 13 articles remained for analysis. We found that there were no implicit memory fMRI studies where subjective awareness was absent, explicit memory performance was at chance, and there were no confounds that could have driven the observed hippocampal activity. The confounds included explicit memory (including false memory), imbalanced attentional states between conditions (yielding activation of the default-mode network), imbalanced stimuli between conditions, and differential novelty. As such, not a single fMRI study provided convincing evidence that implicit memory was associated with the hippocampus. Neuropsychological evidence was also considered, and implicit memory deficits were caused by factors known to disrupt brain regions beyond the hippocampus, such that the behavioral effects could not be attributed to this region. The present results indicate that implicit memory is not associated with the hippocampus.

The dead salmon strikes again: Reports of unconscious processing in the hippocampus may reflect Type-I error

Steinkrauss and Slotnick (2024) reviewed neuroimaging studies linking the hippocampus with implicit memory. They conclude that there is no convincing evidence that the hippocampus is associated with implicit memory because prior studies are confounded by explicit memory (among other factors). Here, we ask a different yet equally important question: do reports of unconscious hippocampal activity reflect a Type-I error (i.e. a false positive)? We find that 39% of studies linking the hippocampus with implicit memory (7 of 18) do not report correcting for multiple comparisons. These results indicate that many unconscious hippocampal effects may reflect a Type-I error.

Conscious awareness and memory systems in the brain

The term "memory" typically refers to conscious retrieval of events and experiences from our past, but experience can also change our behaviour without corresponding awareness of the learning process or the associated outcome. Based primarily on early neuropsychological work, theoretical perspectives have distinguished between conscious memory, said to depend critically on structures in the medial temporal lobe (MTL), and a collection of performance-based memories that do not. The most influential of these memory systems perspectives, the declarative memory theory, continues to be a mainstay of scientific work today despite mounting evidence suggesting that contributions of MTL structures go beyond the kinds or types of memory that can be explicitly reported. Consistent with these reports, more recent perspectives have focused increasingly on the processing operations supported by particular brain regions and the qualities or characteristics of resulting representations whether memory is expressed with or without awareness. These alternatives to the standard model generally converge on two key points. First, the hippocampus is critical for relational memory binding and representation even without awareness and, second, there may be little difference between some types of priming and explicit, familiarity-based recognition. Here, we examine the evolution of memory systems perspectives and critically evaluate scientific evidence that has challenged the status quo. Along the way, we highlight some of the challenges that researchers encounter in the context of this work, which can be contentious, and describe innovative methods that have been used to examine unconscious memory in the lab. This article is categorized under: Psychology > Memory Psychology > Theory and Methods Philosophy > Consciousness.

STING mediates neuroinflammatory response by activating NLRP3-related pyroptosis in severe traumatic brain injury

Long-term neurological deficits after severe traumatic brain injury (TBI), including cognitive dysfunction and emotional impairments, can significantly impair rehabilitation. Glial activation induced by inflammatory response is involved in the neurological deficits post-TBI. This study aimed to investigate the role of the stimulator of interferon genes (STING)-nucleotide-binding oligomerization domain-like receptor pyrin domain-containing-3 (NLRP3) signaling in a rodent model of severe TBI. Severe TBI models were established using weight-drop plus blood loss-reinfusion. Selective STING agonist ADU-S100 or antagonist C-176 was given as a single dose after modeling. Further, NLRP3 inhibitor MCC950 or activator nigericin, or caspase-1 inhibitor VX765, was given as an intracerebroventricular injection 30 min before modeling. After that, a novel object recognition test, open field test, force swimming test, western blot, and immunofluorescence assays were used to assess behavioral and pathological changes in severe TBI. Administration of C-176 alleviated TBI-induced cognitive dysfunction and emotional impairments, neuronal loss, and inflammatory activation of glia cells. However, the administration of STING agonist ADU-S100 exacerbated TBI-induced behavioral and pathological changes. In addition, STING activation exacerbated pyroptosis-associated neuroinflammation via promoting glial activation, as evidenced by increased cleaved caspase-1 and GSDMD N-terminal expression. In contrast, the administration of C-176 showed anti-pyroptotic effects. The neuroprotective effects of C-176 were partially reversed by the NLRP3 activator, nigericin. Collectively, glial STING is responsible for neuroinflammation post-TBI. However, pharmacologic inhibition of STING led to a remarkable improvement of neuroinflammation partly through suppressing NLRP3 signaling. The STING-NLRP3 signaling is a potential therapeutic target in TBI-induced neurological dysfunction.

Study-test congruence of response levels in item stimulus-response priming

We investigated stimulus-response (S-R) memory links during object priming using a binary associative size judgement paradigm. At study, participants decided which of two objects was bigger in real life and, at test, made the same or the reverse judgement. We examined the effects of response congruence on item S-R priming in the associative paradigm. In Experiment 1, a task reversal manipulation had minimal impact on RT priming when classifications were congruent for both recombined objects between study and test. Experiment 2 found that RT priming was more disrupted by classification incongruence of the selected than of the nonselected item alone, with incongruence of the nonselected object having no effect on RTs. Experiment 3, however, found that classification incongruence of both items eliminated RT priming, indicating that a significant effect of classification incongruence for the nonselected item is only evident if both items are classification-incongruent. Finally, across all experiments, we found that accuracy was more sensitive than RTs to decision/action incongruence. We interpret these findings in light of a two-stream account of S-R priming, and suggest a few extensions to account for interactions between S-R links of recombined items.

Can pupillometry distinguish accurate from inaccurate familiarity?

Pupillometry, the measurement of pupil diameter, has become increasingly popular as a tool to investigate human memory. It has long been accepted that the pupil is able to distinguish familiar from completely novel items, a phenomenon known as "pupil old/new effect". Surprisingly, most pupillometric studies on the pupil old/new effect tend to disregard the possibility that the pupillary response to familiarity memory may not be entirely exclusive. Here, we investigated whether the pupillary response to old items correctly judged familiar (hits; accurate familiarity) can be differentiated from the pupillary response to new items wrongly judged familiar (false alarms; inaccurate familiarity). We found no evidence that the two processes could be isolated, as both accurate and inaccurate familiarity showed nearly identical mean and across-time pupillary responses. However, both familiarity hits and false alarms showed pupillary responses unequivocally distinct from those observed during either recollection or novelty detection, which suggests that the pupil measure of familiarity hits and/or false alarms was sufficiently sensitive. The pupillary response to false alarms may have been partially driven by perceptual fluency, since novel objects incorrectly judged to be old (i.e., false alarms) showed a higher degree of similarity to studied images than items correctly judged as novel (i.e., correct rejections). Thus, our results suggest that pupil dilation may not be able to distinguish accurate from inaccurate familiarity using standard recognition memory paradigms, and they also suggest that the pupillary response during familiarity feelings may also partly reflect perceptual fluency.

Neural Correlates of Repetition Priming: A Coordinate-Based Meta-Analysis of fMRI Studies

Repetition priming is a form of implicit memory, whereby classification or identification of a stimulus is improved by prior presentation of the same stimulus. Repetition priming is accompanied with a deceased fMRI signal for primed vs. unprimed stimuli in various brain regions, often called "repetition suppression," or RS. Previous studies proposed that RS in posterior regions is associated with priming of perceptual processes, whereas RS in more anterior (prefrontal) regions is associated with priming of conceptual processes. To clarify which regions exhibit reliable RS associated with perceptual and conceptual priming, we conducted a quantitative meta-analysis using coordinate-based activation likelihood estimation. This analysis included 65 fMRI studies that (i) employed visual repetition priming during either perceptual or conceptual tasks, (ii) demonstrated behavioral priming, and (iii) reported the results from whole-brain analyses. Our results showed that repetition priming was mainly associated with RS in left inferior frontal gyrus and fusiform gyrus. Importantly, RS in these regions was found for both perceptual and conceptual tasks, and no regions show RS that was selective to one of these tasks. These results question the simple distinction between conceptual and perceptual priming, and suggest consideration of other factors such as stimulus-response bindings.

Interactions with the integrative memory model

The integrative memory model formalizes a new conceptualization of memory in which interactions between representations and cognitive operations within large-scale cerebral networks generate subjective memory feelings. Such interactions allow to explain the complexity of memory expressions, such as the existence of multiples sources for familiarity and recollection feelings and the fact that expectations determine how one recognizes previously encountered information.

The neural mechanism of fluency-based memory illusions: the role of fluency context

Recognition memory judgments can be influenced by a variety of signals including fluency. Here, we investigated whether the neural correlates of memory illusions (i.e., misattribution of fluency to prior study) can be modulated by fluency context. Using a masked priming/recognition memory paradigm, we found memory illusions for low confidence decisions. When fluency varied randomly across trials, we found reductions in perirhinal cortex (PrC) activity for primed trials, as well as a (pre)cuneus-PrC (BA 35) connectivity. When the fluency context was unchanging, there was increased PrC activity for primed trials, with the (pre)cuneus showing greater connectivity with PrC (BA 36). Thus, our results tentatively suggest two neural mechanisms via which fluency can lead to memory illusions.

Alteration of FDG uptake by performing novel object recognition task in a rat model of Traumatic Brain Injury

Traumatic Brain Injury (TBI) affects approximately 2.5 million people in the United States, of which 80% are considered to be mild (mTBI). Previous studies have shown that cerebral glucose uptake and metabolism are altered after brain trauma and functional metabolic deficits observed following mTBI are associated with changes in cognitive performance. Imaging of glucose uptake using [¹⁸F] Fluorodeoxyglucose (FDG) based Positron Emission Tomography (PET) with anesthesia during the uptake period demonstrated limited variability in results, but may have depressed uptake. Anesthesia has been found to interfere with blood glucose levels, and hence, FDG uptake. Conversely, forced cognitive testing during uptake may increase glucose demand in targeted regions, such as hippocampus, allowing for better differentiation of outcomes. Therefore, the objective of this study was to investigate the influence of a directed cognitive function task during the FDG uptake period on uptake measurements both in naïve rats and at 2 days after mild lateral fluid percussion (mLFP) TBI. Adult male Sprague Dawley rats underwent FDG uptake with either cognitive testing with the Novel Object Recognition (NOR) test or No Novel Object (NNO), followed by PET scans at baseline (prior to injury) and at 2days post mLFP. At baseline, FDG uptake in the right hippocampus was elevated in rats completing the NOR in comparison to the NNO (control group). Further, the NNO group rats demonstrated a greater fold change in the FDG uptake between baseline and post injury scans than the NOR group. Overall, these data suggest that cognitive activity during FDG uptake affects the regional uptake pattern in the brain, increasing uptake at baseline and suppressing the effects of injury.

Repetition priming in amnesia: Distinguishing associative learning at different levels of abstraction

Learned associations between stimuli and responses make important contributions to priming. The current study aimed to determine whether medial temporal lobe (MTL) binding mechanisms mediate this learning. Prior studies implicating the MTL in stimulus-response (S-R) learning have not isolated associative learning at the response level from associative learning at other levels of representation (e.g., task sets or decisions). The current study investigated whether the MTL is specifically involved in associative learning at the response level by testing a group of amnesic patients with MTL damage on a priming paradigm that isolates associative learning at the response level. Patients demonstrated intact priming when associative learning was isolated to the stimulus-response level. In contrast, their priming was reduced when associations between stimuli and more abstract representations (e.g., stimulus-task or stimulus-decision associations) could contribute to performance. These results provide novel neuropsychological evidence that S-R contributions to priming can be supported by regions outside the MTL, and suggest that the MTL may play a critical role in linking stimuli to more abstract tasks or decisions during priming.