Distinct patterns of neural activity during memory formation of nonwords versus words

Research into the neural underpinnings of memory formation has focused on the encoding of familiar verbal information. Here, we address how the brain supports the encoding of novel information that does not have meaning. Electrical brain activity was recorded from the scalps of healthy young adults while they performed an incidental encoding task (syllable judgments) on separate series of words and "nonwords" (nonsense letter strings that are orthographically legal and pronounceable). Memory for the items was then probed with a recognition memory test. For words as well as nonwords, event-related potentials differed depending on whether an item would subsequently be remembered or forgotten. However, the polarity and timing of the effect varied across item type. For words, subsequently remembered items showed the Usually observed positive-going, frontally distributed modulation from around 600 msec after word onset. For nonwords, by contrast, a negative-going, spatially widespread modulation predicted encoding success from 1000 rnsec onward. Nonwords also showed a modulation shortly after item onset. These findings imply that the brain supports the encoding of familiar and unfamiliar letter strings in qualitatively different ways, including the engagement of distinct neural activity at different points in time. The processing of semantic attributes plays an important role in the encoding of words and the associated positive frontal modulation

Differentiating Phonotactic Probability and Neighborhood Density in Adult Word Learning

This is the author's accepted manuscript. The original is available at http://jslhr.pubs.asha.org/article.aspx?articleid=1762869&resultClick=3Purpose The purpose of this study was to differentiate effects of phonotactic probability, the likelihood of occurrence of a sound sequence, and neighborhood density, the number of words that sound similar to a given word, on adult word learning. A second purpose was to determine what aspect of word learning (viz., triggering learning, formation of an initial representation, or integration with existing representations) was influenced by each variable. Method Thirty-two adults were exposed to 16 nonwords paired with novel objects in a story context. The nonwords orthogonally varied in phonotactic probability and neighborhood density. Learning was measured following 1, 4, and 7 exposures in a picture-naming task. Partially correct (i.e., 2 of 3 phonemes correct) and completely correct responses (i.e., 3 of 3 phonemes correct) were analyzed together and independently to examine emerging and partial representations of new words versus complete and accurate representations of new words. ResultsAnalysis of partially correct and completely correct responses combined showed that adults learned a lower proportion of high-probability nonwords than low-probability nonwords (i.e., high-probability disadvantage) and learned a higher proportion of high-density nonwords than low-density nonwords (i.e., high-density advantage). Separate analysis of partially correct responses yielded an effect of phonotactic probability only, whereas analysis of completely correct responses yielded an effect of neighborhood density only. Conclusions These findings suggest that phonological and lexical processing influence different aspects of word learning. In particular, phonotactic probability may aid in triggering new learning, whereas neighborhood density may influence the integration of new lexical representations with existing representations

Why not model spoken word recognition instead of phoneme monitoring?

Norris, McQueen & Cutler present a detailed account of the decision stage of the phoneme monitoring task. However, we question whether this contributes to our understanding of the speech recognition process itself, and we fail to see why phonotactic knowledge is playing a role in phoneme recognition.

Dentate Gyrus Integrity is Necessary for Behavioral Pattern Separation but not Statistical Learning

Pattern separation, the creation of distinct representations of similar inputs, and statistical learning, the rapid extraction of regularities across multiple inputs, have both been linked to hippocampal processing. It has been proposed that there may be functional differentiation within the hippocampus, such that the trisynaptic pathway (entorhinal cortex \u3e dentate gyrus \u3e CA3 \u3e CA1) supports pattern separation, whereas the monosynaptic pathway (entorhinal cortex \u3e CA1) supports statistical learning. To test this hypothesis, we investigated the behavioral expression of these two processes in BL, an individual with highly selective bilateral lesions in the dentate gyrus that presumably disrupts the trisynaptic pathway. We tested pattern separation with two novel auditory versions of the continuous Mnemonic Similarity Task, requiring the discrimination of similar environmental sounds and trisyllabic words. For statistical learning, participants were exposed to a continuous speech stream made up of repeating trisyllabic words. They were then tested implicitly through a reaction time-based task and explicitly through a rating task and a forced-choice recognition task. BL showed significant deficits in pattern separation on the Mnemonic Similarity Tasks and on the explicit rating measure of statistical learning. In contrast, BL showed intact statistical learning on the implicit measure and the familiarity-based forced-choice recognition measure. Together, these results suggest that dentate gyrus integrity is critical for high-precision discrimination of similar inputs, but not the implicit expression of statistical regularities in behaviour. Our findings offer unique new support for the view that pattern separation and statistical learning rely on distinct neural mechanisms

“Thinking about Not-Thinking”: Neural Correlates of Conceptual Processing during Zen Meditation

Recent neuroimaging studies have identified a set of brain regions that are metabolically active during wakeful rest and consistently deactivate in a variety the performance of demanding tasks. This “default network” has been functionally linked to the stream of thoughts occurring automatically in the absence of goal-directed activity and which constitutes an aspect of mental behavior specifically addressed by many meditative practices. Zen meditation, in particular, is traditionally associated with a mental state of full awareness but reduced conceptual content, to be attained via a disciplined regulation of attention and bodily posture. Using fMRI and a simplified meditative condition interspersed with a lexical decision task, we investigated the neural correlates of conceptual processing during meditation in regular Zen practitioners and matched control subjects. While behavioral performance did not differ between groups, Zen practitioners displayed a reduced duration of the neural response linked to conceptual processing in regions of the default network, suggesting that meditative training may foster the ability to control the automatic cascade of semantic associations triggered by a stimulus and, by extension, to voluntarily regulate the flow of spontaneous mentation

Analyzing Reading Specialization Using fMRI, rs-fcMRI, and Development

Reading is an important, phylogenetically new skill. While neuroimaging studies have identified brain regions used in reading, it is unclear to what extent these regions become specialized for use predominantly in reading versus other tasks. The goal of this dissertation is to investigate the extent to which reading specialization exists at the region and network level, with a focus on orthography, the visual processing of words and letters. I used task-based and resting state functional connectivity: rs-fcMRI) studies to investigate the specialization of orthographic processing, purportedly localized to a left occipito-temporal fusiform cortex region. In Chapter 2, we find no visual region specialized for words or letter strings as compared to line drawn pictures and Amharic character strings: which compose the Ethiopian writing system). Rather, the region appears to be generally involved in visual processing with properties useful for reading, including the ability to process complex stimuli in groups. In Chapter 3, we use rs-fcMRI to demonstrate functional relationships between the left occipito-temporal fusiform cortex and spatial attention regions rather than regions consistently activated in reading tasks. In Chapter 4, we extend these findings by looking at the pattern of functional connectivity in a large network of reading-related regions found in a meta-analysis of reading studies. Using graph theoretic measures on resting state data, we did not find preferential functional connections between regions predominantly used in reading. Rather, we showed the network was basically composed of previously described, more general communities. Comparing the network structure of children and adults also shows few reading specific changes, but rather a change from local to distributed network structure, also seen previously. In Chapter 5, we describe a comparison of activity during matching and naming tasks, and show task-dependent processing differences in reading-related regions. Such differences also indicate a lack of specificity for reading, and suggest the need for careful task design. Together these results indicate a lack of neural specialization for reading at either the regional or network level, suggesting that fluent reading is instead performed by co-opting existing neural systems

Cardiovascular psychophysiological and behavioral evidence for an affective implicit priming mechanism

The mere exposure effect, positive affect elicited by exposure to a previously unfamiliar stimulus, is considered one of the most well established findings in the psychological literature. Yet its mechanism remains unknown. In Experiments 1 - 5, memory encoding was examined to determine whether the mere exposure effect was a form of conceptual or perceptual implicit priming, and, if not either, whether cardiovascular psychophysiology could reveal its nature. Experiment 1 examined the effects of study phase level of processing on recognition, the mere exposure effect, and word identification implicit priming. Deep relative to shallow processing improved recognition, but did not influence the mere exposure effect or word identification implicit priming. Experiments 2 and 3 examined the effect of study-test changes in font and orientation, respectively, on the mere exposure effect and word identification implicit priming. Different study-test font and orientation reduced word identification implicit priming, but had no influence on the mere exposure effect. The combined results from Experiments 1-3 suggested that conceptual and perceptual processing do not drive the mere exposure effect. Experiments 4 and 5 developed and used, respectively, an innovative cardiovascular psychophysiological implicit priming paradigm to examine whether stimulus-specific cardiovascular reactivity at study predicted the mere exposure effect at test. At encoding, stimulus-specific peripheral vasodilatation had predictive value for the mere exposure effect, but not for word identification implicit priming. Experiments 6 and 7 examined whether sustained or transitory anxiety (i.e., trait or state, respectively) would influence the mere exposure effect. Greater trait and state anxiety reduced the mere exposure effect. Together, the findings from these experiments (N = 362) identify a novel affective mechanism of implicit priming that is influenced by cardiovascular psychophysiology and variations in trait and state anxiety