Broad and narrow conceptual tuning in the human frontal lobes

Memory enhancement with stimulants: Differential neural effects of methylphenidate, modafinil, and caffeine. A pilot study

Human memory is susceptible to manipulation in many respects. While consolidation is well known to be prone to disruption, there is also growing evidence for the enhancement of memory function. Beside cognitive strategies and mnemonic training, the use of stimulants may improve memory processing in healthy adults. In this single-dose, double-blind, within-subject, randomized, placebo-controlled pilot study, 20 mg methylphenidate (N = 13) or 200 mg modafinil (N = 12) or 200 mg caffeine (N = 14) were administrated to in total 39 healthy participants while performing a declarative memory task. Each participant received only one substance and functional magnetic resonance imaging (fMRI) was used to assess drug-dependent memory effects of the substance for encoding and recognition compared to task-related activation under placebo. While methylphenidate showed some behavioral effect regarding memory recall performance, on the neural level, methylphenidate-dependent deactivations were found in fronto-parietal and temporal regions during recognition of previously learned words. No BOLD alterations were seen during encoding. Caffeine led to deactivations in the precentral gyrus during encoding whereas modafinil did not show any BOLD signal alterations at all. These results should be interpreted with caution since this a pilot study with several limitations, most importantly the small number of participants per group. However, our main finding of task-related deactivations may point to a drug-dependent increase of efficiency in physiological response to memory processing.

Viewing images of foods evokes taste quality-specific activity in gustatory insular cortex

Previous studies have shown that the conceptual representation of food involves brain regions associated with taste perception. The specificity of this response, however, is unknown. Does viewing pictures of food produce a general, nonspecific response in taste-sensitive regions of the brain? Or is the response specific for how a particular food tastes? Building on recent findings that specific tastes can be decoded from taste-sensitive regions of insular cortex, we asked whether viewing pictures of foods associated with a specific taste (e.g., sweet, salty, and sour) can also be decoded from these same regions, and if so, are the patterns of neural activity elicited by the pictures and their associated tastes similar? Using ultrahigh-resolution functional magnetic resonance imaging at high magnetic field strength (7-Tesla), we were able to decode specific tastes delivered during scanning, as well as the specific taste category associated with food pictures within the dorsal mid-insula, a primary taste responsive region of brain. Thus, merely viewing food pictures triggers an automatic retrieval of specific taste quality information associated with the depicted foods, within gustatory cortex. However, the patterns of activity elicited by pictures and their associated tastes were unrelated, thus suggesting a clear neural distinction between inferred and directly experienced sensory events. These data show how higher-order inferences derived from stimuli in one modality (i.e., vision) can be represented in brain regions typically thought to represent only low-level information about a different modality (i.e., taste).

Forced conceptual thought induced by electrical stimulation of the left prefrontal gyrus involves widespread neural networks

BACKGROUND

Early accounts of forced thought were reported at the onset of a focal seizure, and characterized as vague, repetitive, and involuntary intellectual auras distinct from perceptual or psychic hallucinations or illusions. Here, we examine the neural underpinnings involved in conceptual thought by presenting a series of 3 patients with epilepsy reporting intrusive thoughts during electrical stimulation of the left lateral prefrontal cortex (PFC) during invasive surgical evaluation. We illustrate the widespread networks involved through two independent brain imaging modalities: resting state functional magnetic resonance imaging (fMRI) (rs-fMRI) and task-based meta-analytic connectivity modeling (MACM).

METHODS

We report the clinical and stimulation characteristics of three patients with left hemispheric language dominance who demonstrate forced thought with functional mapping. To examine the brain networks underlying this phenomenon, we used the regions of interest (ROI) centered at the active electrode pairs. We modeled functional networks using two approaches: (1) rs-fMRI functional connectivity analysis, representing 81 healthy controls and (2) meta-analytic connectivity modeling (MACM), representing 8260 healthy subjects. We also determined the overlapping regions between these three subjects' rs-fMRI and MACM networks through a conjunction analysis.

RESULTS

We identified that left PFC was associated with a large-scale functional network including frontal, temporal, and parietal regions, a network that has been associated with multiple cognitive functions including semantics, speech, attention, working memory, and explicit memory.

CONCLUSIONS

We illustrate the neural networks involved in conceptual thought through a unique patient population and argue that PFC supports this function through activation of a widespread network.

Association Between Hippocampus, Thalamus, and Caudate in Mild Cognitive Impairment APOEε4 Carriers: A Structural Covariance MRI Study

Objective: Although, the apolipoprotein E (APOE) genotype is widely recognized as one of the most important risk factors for Alzheimer's disease (AD) development, the neural mechanisms by which the ε4 allele promotes the AD occurring remain under debate. The aim of this study was to evaluate neurobiological effects of the APOE-genotype on the pattern of the structural covariance in mild cognitive impairment (MCI) subjects. Methods: We enrolled 95 MCI subjects and 49 healthy controls. According to APOE-genotype, MCI subjects were divided into three groups: APOEε4 non-carriers (MCIε4-/-, n = 55), APOEε4 heterozygous carriers (MCIε4+/-, n = 31), and APOEε4 homozygous carriers (MCIε4+/+, n = 9) while all controls were APOEε4 non-carriers. In order to explore their brain structural pattern, T1-weighted anatomical brain 1.5-T MRI scans were collected. A whole-brain voxel-based morphometry analysis was performed, and all significant regions (p < 0.05 family-wise error, whole brain) were selected as a region of interest for the structural covariance analysis. Moreover, in order to evaluate the progression of the disease, a clinical follow-up was performed for 2 years. Results: The F-test showed in voxel-based morphometry analysis a strong overall difference among the groups in the middle frontal and temporal gyri and in the bilateral hippocampi, thalami, and parahippocampal gyri, with a grading in the atrophy in these latter three structures according to the following order: MCIε4+/+ > MCIε4+/- > MCIε4-/- > controls. Structural covariance analysis revealed a strong structural association between the left thalamus and the left caudate and between the right hippocampus and the left caudate (p < 0.05 family-wise error, whole brain) in the MCIε4 carrier groups (MCIε4+/+ > MCIε4+/-), whereas no significant associations were observed in MCIε4-/- subjects. Of note, the 38% of MCIs enrolled in this study developed AD within 2 years of follow-up. Conclusion: This study improves the knowledge on neurobiological effect of APOE ε4 in early pathophysiological phenomena underlying the MCI-to-AD evolution, as our results demonstrate changes in the structural association between hippocampal formation and thalamo-striatal connections occurring in MCI ε4 carriers. Our results strongly support the role of subcortical structures in MCI ε4 carriers and open a clinical window on the role of these structures as early disease markers.

Incremental learning of perceptual and conceptual representations and the puzzle of neural repetition suppression

Incremental learning models of long-term perceptual and conceptual knowledge hold that neural representations are gradually acquired over many individual experiences via Hebbian-like activity-dependent synaptic plasticity across cortical connections of the brain. In such models, variation in task relevance of information, anatomic constraints, and the statistics of sensory inputs and motor outputs lead to qualitative alterations in the nature of representations that are acquired. Here, the proposal that behavioral repetition priming and neural repetition suppression effects are empirical markers of incremental learning in the cortex is discussed, and research results that both support and challenge this position are reviewed. Discussion is focused on a recent fMRI-adaptation study from our laboratory that shows decoupling of experience-dependent changes in neural tuning, priming, and repetition suppression, with representational changes that appear to work counter to the explicit task demands. Finally, critical experiments that may help to clarify and resolve current challenges are outlined.

Object identification leads to a conceptual broadening of object representations in lateral prefrontal cortex

Recent experience identifying objects leads to later improvements in both speed and accuracy ("repetition priming"), along with simultaneous reductions of neural activity ("repetition suppression"). A popular interpretation of these joint behavioral and neural phenomena is that object representations become perceptually "sharper" with stimulus repetition, eliminating cells that are poorly stimulus-selective and responsive and reducing support for competing representations downstream. Here, we test this hypothesis in an fMRI-adaptation experiment using pictures of objects. Prior to fMRI, participants repeatedly named a set of object pictures. During fMRI, participants viewed adaptation sequences composed of rapidly repeated objects (3-6 repetitions over several seconds) that were either named previously or that were new for the fMRI session, followed by single "deviant" object pictures used to measure recovery from adaptation and that shared a relationship to the adapted picture (a different exemplar of the same object, a conceptual associate, or an unrelated picture). Effects of adaptation and recovery were found throughout visually responsive brain regions. Occipitotemporal cortical regions displayed repetition suppression to previously named relative to new adapters but failed to exhibit pronounced changes in neural tuning. In contrast, changes in the slope of the recovery curves were found in the left lateral prefrontal cortex: Greater residual adaptation was observed to exemplar stimuli and conceptual associates following previously named adapting stimuli, consistent with greater rather than reduced neural overlap among representations of conceptually related objects. Furthermore, this change in neural tuning was directly related to the proportion of conceptual errors made by participants in the naming sessions pre- and post-fMRI, establishing that the experience-dependent conceptual broadening of object representations seen in fMRI is also manifest in behavior. In a follow-up behavioral experiment, we further show that recent naming experience leads to greater semantic priming when using the previously named pictures as briefly presented primes. Taken together, our results fail to support perceptual sharpening as the primary mediator between repetition suppression and behavioral priming at durations typically used to study priming and instead highlight an experience-dependent broadening of conceptual representations. We suggest that alternative mechanisms, such as increases in neural synchronization, are more promising in explaining priming in the face of repetition suppression.

Auditory Perceptual Category Formation Does Not Require Perceptual Warping

Categorical perception occurs when a perceiver's stimulus classifications affect their ability to make fine perceptual discriminations and is the most intensively studied form of category learning. On the basis of categorical perception studies, it has been proposed that category learning proceeds by the deformation of an initially homogeneous perceptual space (“perceptual warping”), so that stimuli within the same category are perceived as more similar to each other (more difficult to tell apart) than stimuli that are the same physical distance apart but that belong to different categories. Here, we present a significant counterexample in which robust category learning occurs without these differential perceptual space deformations. Two artificial categories were defined along the dimension of pitch for a perceptually unfamiliar, multidimensional class of sounds. A group of participants (selected on the basis of their listening abilities) were trained to sort sounds into these two arbitrary categories. Category formation, verified empirically, was accompanied by a heightened sensitivity along the entire pitch range, as indicated by changes in an EEG index of implicit perceptual distance (mismatch negativity), with no significant resemblance to the local perceptual deformations predicted by categorical perception. This demonstrates that robust categories can be initially formed within a continuous perceptual dimension without perceptual warping. We suggest that perceptual category formation is a flexible, multistage process sequentially combining different types of learning mechanisms rather than a single process with a universal set of behavioral and neural correlates.

Time course of shape and category selectivity revealed by EEG rapid adaptation

A hallmark of human cognition is the ability to rapidly assign meaning to sensory stimuli. It has been suggested that this fast visual object categorization ability is accomplished by a feedforward processing hierarchy consisting of shape-selective neurons in occipito-temporal cortex that feed into task circuits in frontal cortex computing conceptual category membership. We performed an EEG rapid adaptation study to test this hypothesis. Participants were trained to categorize novel stimuli generated with a morphing system that precisely controlled both stimulus shape and category membership. We subsequently performed EEG recordings while participants performed a category matching task on pairs of successively presented stimuli. We used space-time cluster analysis to identify channels and latencies exhibiting selective neural responses. Neural signals before 200 msec on posterior channels demonstrated a release from adaptation for shape changes, irrespective of category membership, compatible with a shape- but not explicitly category-selective neural representation. A subsequent cluster with anterior topography appeared after 200 msec and exhibited release from adaptation consistent with explicit categorization. These signals were subsequently modulated by perceptual uncertainty starting around 300 msec. The degree of category selectivity of the anterior signals was strongly predictive of behavioral performance. We also observed a posterior category-selective signal after 300 msec exhibiting significant functional connectivity with the initial anterior category-selective signal. In summary, our study supports the proposition that perceptual categorization is accomplished by the brain within a quarter second through a largely feedforward process culminating in frontal areas, followed by later category-selective signals in posterior regions.

Semantic memory retrieval circuit: role of pre-SMA, caudate, and thalamus

We propose that pre-supplementary motor area (pre-SMA)-thalamic interactions govern processes fundamental to semantic retrieval of an integrated object memory. At the onset of semantic retrieval, pre-SMA initiates electrical interactions between multiple cortical regions associated with semantic memory subsystems encodings as indexed by an increase in theta-band EEG power. This starts between 100-150 ms after stimulus presentation and is sustained throughout the task. We posit that this activity represents initiation of the object memory search, which continues in searching for an object memory. When the correct memory is retrieved, there is a high beta-band EEG power increase, which reflects communication between pre-SMA and thalamus, designates the end of the search process and resultant in object retrieval from multiple semantic memory subsystems. This high beta signal is also detected in cortical regions. This circuit is modulated by the caudate nuclei to facilitate correct and suppress incorrect target memories.

Recurrent Processing during Object Recognition

How does the brain learn to recognize objects visually, and perform this difficult feat robustly in the face of many sources of ambiguity and variability? We present a computational model based on the biology of the relevant visual pathways that learns to reliably recognize 100 different object categories in the face of naturally occurring variability in location, rotation, size, and lighting. The model exhibits robustness to highly ambiguous, partially occluded inputs. Both the unified, biologically plausible learning mechanism and the robustness to occlusion derive from the role that recurrent connectivity and recurrent processing mechanisms play in the model. Furthermore, this interaction of recurrent connectivity and learning predicts that high-level visual representations should be shaped by error signals from nearby, associated brain areas over the course of visual learning. Consistent with this prediction, we show how semantic knowledge about object categories changes the nature of their learned visual representations, as well as how this representational shift supports the mapping between perceptual and conceptual knowledge. Altogether, these findings support the potential importance of ongoing recurrent processing throughout the brain's visual system and suggest ways in which object recognition can be understood in terms of interactions within and between processes over time.

Bridging the gap between neuroscientific and psychodynamic models in child and adolescent psychiatry

This article provides a selective review of the neuroscience and child-psychoanalytic literature, focusing on areas of significant overlap and emphasizing comprehensive theories in developmental neuroscience and child psychoanalysis with testable mechanisms of action. Topics include molecular biology and genetics findings relevant to psychotherapy research, neuroimaging findings relevant to psychotherapy, brain regions of interest for psychotherapy, neurobiologic changes caused by psychotherapy, use of neuroimaging to predict treatment outcome, and schemas as a bridging concept between psychodynamic and cognitive neuroscience models. The combined efforts of neuroscientists and psychodynamic clinicians and theorists are needed to unravel the mechanisms of human mental functioning.

Category-specific semantic memory: converging evidence from bold fMRI and Alzheimer's disease

Patients with Alzheimer's disease have category-specific semantic memory difficulty for natural relative to manufactured objects. We assessed the basis for this deficit by asking healthy adults and patients to judge whether pairs of words share a feature (e.g. "banana:lemon-COLOR"). In an fMRI study, healthy adults showed gray matter (GM) activation of temporal-occipital cortex (TOC) where visual-perceptual features may be represented, and prefrontal cortex (PFC) which may contribute to feature selection. Tractography revealed dorsal and ventral stream white matter (WM) projections between PFC and TOC. Patients had greater difficulty with natural than manufactured objects. This was associated with greater overlap between diseased GM areas correlated with natural kinds in patients and fMRI activation in healthy adults for natural kinds. The dorsal WM projection between PFC and TOC in patients correlated only with judgments of natural kinds. Patients thus remained dependent on the same neural network as controls during judgments of natural kinds, despite disease in these areas. For manufactured objects, patients' judgments showed limited correlations with PFC and TOC GM areas activated by controls, and did not correlate with the PFC-TOC dorsal WM tract. Regions outside of the PFC-TOC network thus may help support patients' judgments of manufactured objects. We conclude that a large-scale neural network for semantic memory implicates both feature knowledge representations in modality-specific association cortex and heteromodal regions important for accessing this knowledge, and that patients' relative deficit for natural kinds is due in part to their dependence on this network despite disease in these areas.

Repetition Priming and Repetition Suppression: A Case for Enhanced Efficiency Through Neural Synchronization

Stimulus repetition in identification tasks leads to improved behavioral performance ("repetition priming") but attenuated neural responses ("repetition suppression") throughout task-engaged cortical regions. While it's clear that this pervasive brain-behavior relationship reflects some form of improved processing efficiency, the exact form that it takes remains elusive. In this Discussion Paper, we review four different theoretical proposals that have the potential to link repetition suppression and priming, with a particular focus on a proposal that stimulus repetition affects improved efficiency through enhanced neural synchronization. We argue that despite exciting recent work on the role of neural synchronization in cognitive processes such as attention and perception, similar studies in the domain of learning and memory - and priming, in particular - have been lacking. We emphasize the need for new studies with adequate spatiotemporal resolution, formulate several novel predictions, and discuss our ongoing efforts to disentangle the current proposals.

Repetition priming and repetition suppression: Multiple mechanisms in need of testing

In our Discussion Paper, we reviewed four theoretical proposals that have the potential to link the neural and behavioral phenomena of Repetition Suppression and Repetition Priming. We argued that among these proposals, the Synchrony and Bayesian Explaining Away models appear to be the most promising in addressing existing data, and we articulated a series of predictions to distinguish between them. The commentaries have helped to clarify some of these predictions, have highlighted additional evidence supporting the Facilitation and Sharpening models, and have emphasized dissociations by repetition lag and brain location. Our reply addresses these issues in turn, and we argue that progress will require the testing of Repetition Suppression, changes in neural tuning, and changes in synchronization throughout the brain and over a variety of lags and task contexts.

Broader visual orientation tuning in patients with schizophrenia

Reduced gamma-aminobutyric acid (GABA) levels in cerebral cortex are thought to contribute to information processing deficits in patients with schizophrenia (SZ), and we have previously reported lower in vivo GABA levels in the visual cortex of patients with SZ. GABA-mediated inhibition plays a role in sharpening orientation tuning of visual cortical neurons. Therefore, we predicted that tuning for visual stimulus orientation would be wider in SZ. We measured orientation tuning with a psychophysical procedure in which subjects performed a target detection task of a low-contrast oriented grating, following adaptation to a high-contrast grating. Contrast detection thresholds were determined for a range of adapter-target orientation offsets. For both SZ and healthy controls, contrast thresholds decreased as orientation offset increased, suggesting that this tuning curve reflects the selectivity of visual cortical neurons for stimulus orientation. After accounting for generalized deficits in task performance in SZ, there was no difference between patients and controls for detection of target stimuli having either the same orientation as the adapter or orientations far from the adapter. However, patients' thresholds were significantly higher for intermediate adapter-target offsets. In addition, the mean width parameter of a Gaussian fit to the psychophysical orientation tuning curves was significantly larger for the patient group. We also present preliminary data relating visual cortical GABA levels, as measured with magnetic resonance spectroscopy, and orientation tuning width. These results suggest that our finding of broader orientation tuning in SZ may be due to diminished visual cortical GABA levels.

Hemispheric asymmetry of visual scene processing in the human brain: evidence from repetition priming and intrinsic activity

Asymmetrical specialization of cognitive processes across the cerebral hemispheres is a hallmark of healthy brain development and an important evolutionary trait underlying higher cognition in humans. While previous research, including studies of priming, divided visual field presentation, and split-brain patients, demonstrates a general pattern of right/left asymmetry of form-specific versus form-abstract visual processing, little is known about brain organization underlying this dissociation. Here, using repetition priming of complex visual scenes and high-resolution functional magnetic resonance imaging (MRI), we demonstrate asymmetrical form specificity of visual processing between the right and left hemispheres within a region known to be critical for processing of visual spatial scenes (parahippocampal place area [PPA]). Next, we use resting-state functional connectivity MRI analyses to demonstrate that this functional asymmetry is associated with differential intrinsic activity correlations of the right versus left PPA with regions critically involved in perceptual versus conceptual processing, respectively. Our results demonstrate that the PPA comprises lateralized subregions across the cerebral hemispheres that are engaged in functionally dissociable yet complementary components of visual scene analysis. Furthermore, this functional asymmetry is associated with differential intrinsic functional connectivity of the PPA with distinct brain areas known to mediate dissociable cognitive processes.