Marieke K. van Vugt

Hippocampal Gamma Oscillations Increase with Memory Load

Although the hippocampus plays a crucial role in encoding and retrieval of contextually mediated episodic memories, considerable controversy surrounds the role of the hippocampus in short-term or working memory. To examine both hippocampal and neocortical contributions to working memory function, we recorded electrocorticographic activity from widespread cortical and subcortical sites as 20 neurosurgical patients performed working memory tasks. These recordings revealed significant increases in 48–90 Hz gamma oscillatory power with memory load for two classes of stimuli: letters and faces. Sites exhibiting gamma increases with memory load appeared primarily in the hippocampus and medial temporal lobe. These findings implicate gamma oscillatory activity in the maintenance of both letters and faces in working memory and provide the first direct evidence for modulation of hippocampal gamma oscillations as humans perform a working memory task.

Comparison of spectral analysis methods for characterizing brain oscillations

Spectral analysis methods are now routinely used in electrophysiological studies of human and animal cognition. Although a wide variety of spectral methods has been used, the ways in which these methods differ are not generally understood. Here we use simulation methods to characterize the similarities and differences between three spectral analysis methods: wavelets, multitapers and P(episode). P(episode) is a novel method that quantifies the fraction of time that oscillations exceed amplitude and duration thresholds. We show that wavelets and P(episode) used side-by-side helps to disentangle length and amplitude of a signal. P(episode) is especially sensitive to fluctuations around its thresholds, puts frequencies on a more equal footing, and is sensitive to long but low-amplitude signals. In contrast, multitaper methods are less sensitive to weak signals, but are very frequency-specific. If frequency specificity is not essential, then wavelets and P(episode) are recommended.

Investigating the impact of mindfulness meditation training on working memory: A mathematical modeling approach

We investigated whether mindfulness training (MT) influences information processing in a working memory task with complex visual stimuli. Participants were tested before (T1) and after (T2) participation in an intensive one-month MT retreat, and their performance was compared with that of an age- and education-matched control group. Accuracy did not differ across groups at either time point. Response times were faster and significantly less variable in the MT versus the control group at T2. Since these results could be due to changes in mnemonic processes, speed–accuracy trade-off, or nondecisional factors (e.g., motor execution), we used a mathematical modeling approach to disentangle these factors. The EZ-diffusion model (Wagenmakers, van der Maas, & Grasman, Psychonomic Bulletin & Review 14:(1), 3–22, 2007) suggested that MT leads to improved information quality and reduced response conservativeness, with no changes in nondecisional factors. The noisy exemplar model further suggested that the increase in information quality reflected a decrease in encoding noise and not an increase in forgetting. Thus, mathematical modeling may help clarify the mechanisms by which MT produces salutary effects on performance.

Mind the Hype: A Critical Evaluation and Prescriptive Agenda for Research on Mindfulness and Meditation

During the past two decades, mindfulness meditation has gone from being a fringe topic of scientific investigation to being an occasional replacement for psychotherapy, tool of corporate well-being, widely implemented educational practice, and “key to building more resilient soldiers.” Yet the mindfulness movement and empirical evidence supporting it have not gone without criticism. Misinformation and poor methodology associated with past studies of mindfulness may lead public consumers to be harmed, misled, and disappointed. Addressing such concerns, the present article discusses the difficulties of defining mindfulness, delineates the proper scope of research into mindfulness practices, and explicates crucial methodological issues for interpreting results from investigations of mindfulness. For doing so, the authors draw on their diverse areas of expertise to review the present state of mindfulness research, comprehensively summarizing what we do and do not know, while providing a prescriptive agenda for contemplative science, with a particular focus on assessment, mindfulness training, possible adverse effects, and intersection with brain imaging. Our goals are to inform interested scientists, the news media, and the public, to minimize harm, curb poor research practices, and staunch the flow of misinformation about the benefits, costs, and future prospects of mindfulness meditation.

Spatially distributed patterns of oscillatory coupling between high-frequency amplitudes and low-frequency phases in human iEEG.

Spatially distributed coherent oscillations provide temporal windows of excitability that allow for interactions between distinct neuronal groups. It has been hypothesized that this mechanism for neuronal communication is realized by bursts of high-frequency oscillations that are phase-coupled to a low-frequency spatially distributed coupling oscillation. This mechanism requires multiple physiologically different interacting sources, one generating the low-frequency coupling oscillation and the others generating phase-coupled high-frequency oscillations. Using human intracranial EEG (iEEG) data, we provide evidence for multiple oscillatory patterns, as characterized on the basis of their spatial maps (topographies) and their frequency spectra. In fact, we show that the spatial maps for the coupling oscillations are much more widespread than the ones for the associated phase-coupled bursts. Second, in the majority of the patterns of phase-amplitude coupling (PAC), phase-coupled bursts of high-frequency activity are synchronized across brain areas. Third and last, working memory operations affect the PAC strength in a heterogeneous way: in some PAC patterns, working memory operations increase their strength, whereas in others they decrease it.

The effects of mindfulness-based cognitive therapy on affective memory recall dynamics in depression: A mechanistic model of rumination

Objectives: converging research suggests that mindfulness training exerts its therapeutic effects on depression by reducing rumination. Theoretically, rumination is a multifaceted construct that aggregates multiple neurocognitive aspects of depression, including poor executive control, negative and overgeneral memory bias, and persistence or stickiness of negative mind states. Current measures of rumination, most-often self-reports, do not capture these different aspects of ruminative tendencies, and therefore are limited in providing detailed information about the mechanisms of mindfulness. Methods: we developed new insight into the potential mechanisms of rumination, based on three model-based metrics of free recall dynamics. These three measures reflect the patterns of memory retrieval of valenced information: the probability of first recall (Pstart) which represents initial affective bias, the probability of staying with the same valence category rather than switching, which indicates strength of positive or negative association networks (Pstay), and probability of stopping (Pstop) or ending recall within a given valence, which indicates persistence or stickiness of a mind state. We investigated the effects of Mindfulness-Based Cognitive Therapy (MBCT; N = 29) vs. wait-list control (N = 23) on these recall dynamics in a randomized controlled trial in individuals with recurrent depression. Participants completed a standard laboratory stressor, the Trier Social Stress Test, to induce negative mood and activate ruminative tendencies. Following that, participants completed a free recall task consisting of three word lists. This assessment was conducted both before and after treatment or wait-list. Results: while MBCT participant’s Pstart remained relatively stable, controls showed multiple indications of depression-related deterioration toward more negative and less positive bias. Following the intervention, MBCT participants decreased in their tendency to sustain trains of negative words and increased their tendency to sustain trains of positive words. Conversely, controls showed the opposite tendency: controls stayed in trains of negative words for longer, and stayed in trains of positive words for less time relative to pre-intervention scores. MBCT participants tended to stop recall less often with negative words, which indicates less persistence or stickiness of negatively valenced mental context. Conclusion: MBCT participants showed a decrease in patterns that may perpetuate rumination on all three types of recall dynamics (Pstart, Pstay, and Pstop), compared to controls. MBCT may weaken the strength of self-perpetuating negative associations networks that are responsible for the persistent and “sticky” negative mind states observed in depression, and increase the positive associations that are lacking in depression. This study also offers a novel, objective method of measuring several indices of ruminative tendencies indicative of the underlying mechanisms of rumination.

Intracranial electroencephalography reveals two distinct similarity effects during item recognition

Behavioral studies of visual recognition memory indicate that old/new decisions reflect both the similarity of the probe to the studied items (probe-item similarity) and the similarities among the studied items themselves (list homogeneity). Recording intracranial electroencephalography from 1,155 electrodes across 15 patients, we examined the oscillatory correlates of probe-item similarity and homogeneity effects in short-term recognition memory for synthetic faces. Frontal areas show increases in low-frequency oscillations with both probe-item and item-item similarity, whereas temporal lobe areas show distinct oscillatory correlates for probe-item similarity and homogeneity in the gamma band. We discuss these frontal low-frequency effects and the dissociation in the temporal lobe in terms of recent computational models of visual recognition memory.

Why are some people's names easier to learn than others? The effects of face similarity on memory for face-name associations

Using synthetic faces that varied along four perceptual dimensions (Wilson, Loffler, & Wilkinson, 2002), we examined the effects of face similarity on memory for face-name associations. The nature of these stimuli allowed us to go beyond the categorical similarity manipulations used in previous verbal associative memory studies to trace out the parametric relation between similarity and various performance measures. In Experiment 1, we found that recall performance diminished as a function of how many studied faces were in the vicinity of the cue face in similarity space. Also, incorrect recalls were more likely to come from nearby positions in face space. Experiments 2 and 3, respectively, demonstrated analogous effects with a set of more distinguishable, photorealistic faces, and in an associative recognition task. These results highlight the similarity between associative recall and associative recognition, and between face-name association and other domains of associative memory.

For whom the bell tolls: Periodic reactivation of sensory cortex in the gamma band as a substrate of visual working memory maintenance

Working memory (WM) is central to human cognition as it allows information to be kept online over brief periods of time and facilitates its usage in cognitive operations (Luck and Vogel, 2013). How this information maintenance actually is implemented is still a matter of debate. Several independent theories of WM, derived, respectively, from behavioral studies and neural considerations, advance the idea that items in WM decay over time and must be periodically reactivated. In this proposal, we show how recent data from intracranial EEG and attention research naturally leads to a simple model of such reactivation in the case of sensory memories. Specifically, in our model the amplitude of high-frequency activity (>50 Hz, in the gamma-band) underlies the representation of items in high-level visual areas. This activity decreases to noise-levels within 500 ms, unless it is reactivated. We propose that top-down attention, which targets multiple sensory items in a cyclical or rhythmic fashion at around 6–10 Hz, reactivates these decaying gamma-band representations. Therefore, working memory capacity is essentially the number of representations that can simultaneously be kept active by a rhythmically sampling attentional spotlight given the known decay rate. Since attention samples at 6–10 Hz, the predicted WM capacity is 3–5 items, in agreement with empirical findings.

Reiterated Concerns and Further Challenges for Mindfulness and Meditation Research : A Reply to Davidson and Dahl

In response to our article, Davidson and Dahl offer commentary and advice regarding additional topics crucial to a comprehensive prescriptive agenda for future research on mindfulness and meditation. Their commentary raises further challenges and provides an important complement to our article. More consideration of these issues is especially welcome because limited space precluded us from addressing all relevant topics. While we agree with many of Davidson and Dahl’s suggestions, the present reply (a) highlights reasons why the concerns we expressed are still especially germane to mindfulness and meditation research (even though those concerns may not be entirely unique) and (b) gives more context to other issues posed by them. We discuss special characteristics of individuals who participate in mindfulness and meditation research and focus on the vulnerability of this field inherent in its relative youthfulness compared to other more mature scientific disciplines. Moreover, our reply highlights the serious consequences of adverse experiences suffered by a significant subset of individuals during mindfulness and other contemplative practices. We also scrutinize common contemporary applications of mindfulness and meditation to illness, and some caveats are introduced regarding mobile technologies for guidance of contemplative practices.