Do cerebral motivational asymmetries mediate the relationship between handedness and personality?

Handedness has long been tied to personality, but detailed explanations for the association are lacking. Importantly for purposes of theory development, measures of approach and withdrawal associated with Big Five personality traits have also been traced back to activity in brain areas that relate to handedness. Specifically, increased right-hemisphere frontal activity appears to be linked to both withdrawal motivation and left/inconsistent-handedness, while increased left-hemisphere frontal activity is associated with approach motivation and right/consistent-handedness. Cerebral motivational asymmetries therefore present one plausible mechanism by which approach and withdrawal motivation could mediate the relationship between handedness and personality. We tested this possibility in a large online study (N = 499) in which participants completed multiple survey measures. Results indicated that approach/withdrawal motivation partially accounts for relationships between handedness and personality.

Conflicting demands of abstract and specific visual object processing resolved by frontoparietal networks.

Object categorization and exemplar identification place conflicting demands on the visual system, yet humans easily perform these fundamentally contradictory tasks. Previous studies suggest the existence of dissociable visual processing subsystems to accomplish the two abilities-an abstract category (AC) subsystem that operates effectively in the left hemisphere and a specific exemplar (SE) subsystem that operates effectively in the right hemisphere. This multiple subsystems theory explains a range of visual abilities, but previous studies have not explored what mechanisms exist for coordinating the function of multiple subsystems and/or resolving the conflicts that would arise between them. We collected functional MRI data while participants performed two variants of a cue-probe working memory task that required AC or SE processing. During the maintenance phase of the task, the bilateral intraparietal sulcus (IPS) exhibited hemispheric asymmetries in functional connectivity consistent with exerting proactive control over the two visual subsystems: greater connectivity to the left hemisphere during the AC task, and greater connectivity to the right hemisphere during the SE task. Moreover, probe-evoked activation revealed activity in a broad frontoparietal network (containing IPS) associated with reactive control when the two visual subsystems were in conflict, and variations in this conflict signal across trials was related to the visual similarity of the cue-probe stimulus pairs. Although many studies have confirmed the existence of multiple visual processing subsystems, this study is the first to identify the mechanisms responsible for coordinating their operations.

Separability of abstract-category and specific-exemplar visual object subsystems: evidence from fMRI pattern analysis.

Previous research indicates that dissociable neural subsystems underlie abstract-category (AC) recognition and priming of objects (e.g., cat, piano) and specific-exemplar (SE) recognition and priming of objects (e.g., a calico cat, a different calico cat, a grand piano, etc.). However, the degree of separability between these subsystems is not known, despite the importance of this issue for assessing relevant theories. Visual object representations are widely distributed in visual cortex, thus a multivariate pattern analysis (MVPA) approach to analyzing functional magnetic resonance imaging (fMRI) data may be critical for assessing the separability of different kinds of visual object processing. Here we examined the neural representations of visual object categories and visual object exemplars using multi-voxel pattern analyses of brain activity elicited in visual object processing areas during a repetition-priming task. In the encoding phase, participants viewed visual objects and the printed names of other objects. In the subsequent test phase, participants identified objects that were either same-exemplar primed, different-exemplar primed, word-primed, or unprimed. In visual object processing areas, classifiers were trained to distinguish same-exemplar primed objects from word-primed objects. Then, the abilities of these classifiers to discriminate different-exemplar primed objects and word-primed objects (reflecting AC priming) and to discriminate same-exemplar primed objects and different-exemplar primed objects (reflecting SE priming) was assessed. Results indicated that (a) repetition priming in occipital-temporal regions is organized asymmetrically, such that AC priming is more prevalent in the left hemisphere and SE priming is more prevalent in the right hemisphere, and (b) AC and SE subsystems are weakly modular, not strongly modular or unified.

Can theories of visual representation help to explain asymmetries in amygdala function?

Emotional processing differs between the left and right hemispheres of the brain, and functional differences have been reported more specifically between the left and right amygdalae, subcortical structures heavily implicated in emotional processing. However, the empirical pattern of amygdalar asymmetries is inconsistent with extant theories of emotional asymmetries. Here we review this discrepancy, and we hypothesize that hemispheric differences in visual object processing help to explain the previously reported functional differences between the left and right amygdalae. The implication that perceptual factors play a large role in determining amygdalar asymmetries may help to explain amygdalar dysfunction in the development and maintenance of posttraumatic stress disorder.

Identifying objects impairs knowledge of other objects: a relearning explanation for the neural repetition effect.

Different items in long-term knowledge are stored in the neocortex as partially overlapping representations that can be altered slightly with usage. This encoding scheme affords well-documented benefits, but potential costs have not been well explored. Here we use functional magnetic resonance imaging (fMRI), neurocomputational modeling, and electrophysiological measures to show that strengthening some visual object representations not only enhances the subsequent ability to identify those (repeated) objects-an effect long known as repetition priming-but also impairs the ability to identify other (non-repeated) objects-a new effect labeled antipriming. As a result, the non-repeated objects elicit increased neural activity likely for the purpose of reestablishing their previously weakened representations. These results suggest a novel reevaluation of the ubiquitously observed repetition effect on neural activity, and they indicate that maintenance relearning may be a crucial aspect of preserving overlapping neural representations of visual objects in long-term memory.

What antipriming reveals about priming.

Across many domains of cognition, information processing is more effective when it has been performed recently than when it has not. This effect--termed repetition priming--is ubiquitous; however, its causes are not well understood. Here, I draw attention to the concept of antipriming and argue that it is crucial for understanding priming. Antipriming is a measurable impairment in processing information owing to recent processing of other information when the representations of information overlap and compete. Strengthening one representation after its usage causes priming for that item but also antipriming for some other, non-repeated items. Recent evidence demonstrates priming and antipriming within visual object identification systems. These findings might reflect a form of maintenance relearning of superimposed knowledge representations.

What form of memory underlies novelty preferences?

Novelty preferences (longer fixations on new stimuli than on previously presented stimuli) are widely used to assess memory in nonverbal populations, such as human infants and experimental animals, yet important questions remain about the nature of the processes that underlie them. We used a classical conditioning paradigm to test whether novelty preferences reflect (1) a stimulus-driven bias toward novelty in visual selective attention or (2) explicit memory for old stimuli. Results indicated that conditioning affected adults' looking behavior in the visual paired comparison, but not their recognition memory judgments. Furthermore, the typically observed novelty preference occurred only when a bias toward novelty had no competition from a bias toward salience due to conditioning. These results suggest that novelty preferences may reflect attentional processes and implicit memory to a greater degree than explicit memory, a finding with important implications for understanding memory in nonverbal populations and the development of memory in humans.

Hemispheric asymmetries in visual word-form processing: progress, conflict, and evaluating theories.

The ubiquitous left-hemisphere advantage in visual word processing can be accounted for in different ways. Competing theories have been tested recently using cAsE-aLtErNaTiNg words to investigate boundary conditions for the typical effect. We briefly summarize this research and examine the disagreements and commonalities across the theoretical perspectives. This work may provide a good example why a multi-level, multi-method, and multi-paradigm approach holds the greatest promise for rapid theoretical progress.

Initial storage of unfamiliar objects: examining memory stores with signal detection analyses.

The memory source(s) underlying performance of the object decision task were examined. Application of signal detection theory with independent indices of sensitivity and bias indicated that greater object decision sensitivity (possible vs. impossible object discrimination) is observed for primed than for unprimed objects and a greater bias to respond "possible" is observed for primed than for unprimed objects. To test whether a single memory store underlies the two effects, we assessed whether the two effects would be influenced in similar ways by a particular experimental manipulation. Sensitivity did not differ between processing of 180 degrees -rotated primed objects vs. un-rotated primed objects, but bias to respond "possible" differed between those conditions in predictable ways depending on the cerebral hemisphere of direct test presentations. Results support a conciliatory resolution to the debate concerning memory in this task and help to clarify properties of memory stores underlying the initial storage of unfamiliar objects.

A critical boundary to the left-hemisphere advantage in visual-word processing.

Two experiments explored boundary conditions for the ubiquitous left-hemisphere advantage in visual-word recognition. Subjects perceptually identified words presented directly to the left or right hemisphere. Strong left-hemisphere advantages were observed for UPPERCASE and lowercase words. However, only a weak effect was observed for AlTeRnAtInG-cAsE words, and a numerical reversal of the typical left-hemisphere advantage was observed for words in a visual prototype font (a very unfamiliar word format). Results support the theory that dissociable abstract and specific neural subsystems underlie visual-form recognition and fail to support the theory that a visual lexicon operates in the left hemisphere.

Evidence for dissociable neural mechanisms underlying inference generation in familiar and less-familiar scenarios.

In this study, we investigated whether the left and right hemispheres are differentially involved in causal inference generation. Participants read short inference-promoting texts that described either familiar or less-familiar scenarios. After each text, they performed a lexical decision on a letter string (which sometimes constituted an inference-related word) presented directly to the left or right hemisphere. Response-time results indicated that hemisphere of direct presentation interacted with type of inference scenario. When test stimuli were presented directly to the left hemisphere, lexical decisions were facilitated following familiar but not following less-familiar inference scenarios, whereas when test stimuli were presented directly to the right hemisphere, facilitation was observed in both familiar and less-familiar conditions. Thus, inferences may be generated in different ways depending on which of two dissociable neural subsystems underlies the activation of background information.

Interactive visual and postvisual processes and their roles in form-specific memory.

Effects of depth of encoding on form-specific memory were examined. After viewing words (e.g., "bear") presented centrally during initial encoding, participants completed word stems (e.g., "BEA") presented laterally and pattern masked during subsequent test. When the encoding task was perceptual, letter-case specific memory was not observed, unlike in previous experiments without pattern masking. However, when the encoding task required both perceptual and conceptual processing, letter-case specific memory was observed in direct right-hemisphere, but not in direct left-hemisphere, test presentations, like in previous studies without pattern masking. Results were not influenced by whether stems were completed to form the first words that came to mind or words explicitly retrieved from encoding. Depth of encoding may influence form-specific memory through interactive processing of visual and postvisual information.

Interhemispheric communication of abstract and specific visual-form information.

Pairs of letters were compared after being viewed in different visual fields (i.e. across-hemispheres, AH) or in the same visual field (i.e. within-hemisphere, WH). In an abstract-category comparison task, participants decided whether two letter exemplars belonged to the same abstract category (e.g. "k" and "K") or not (e.g. "k" and "P") and performed more accurately in AH trials than in WH trials. In a specific-exemplar comparison task, they decided whether two letters within the same abstract category were the same specific exemplars (e.g. "k" and "k") or not (e.g. "k" and "K") and performed more accurately in WH trials than in AH trials. This pattern of results was observed when the exemplars in a category were visually similar (e.g. "k" and "K", "a" and "a") but not when they were visually dissimilar (e.g. "a" and "A"). The reversed association technique was used to confirm the independence of subsystems underlying abstract category and specific-exemplar comparisons. Most important, the results support the theory that a specific-exemplar subsystem is more detrimentally affected by interhemispheric transfer of information than an abstract category subsystem.

Serotonin levels influence patterns of repetition priming.

Repetition priming in a word-stem completion task was examined in a group of control subjects and in a group of experimental subjects under conditions of acute tryptophan depletion (T-) and tryptophan augmentation (T+). Experimental subjects ingested amino acid compounds that depleted or loaded the body with tryptophan, and word-stem completion priming performance was measured. Results indicate differential effects of T- and T+ manipulations on word-stem completion priming. In the control group, both specific-visual and amodal priming were observed. Conversely, in the T+ condition, specific-visual priming, but no amodal priming, was observed, whereas in the T- condition, amodal priming, but no specific-visual priming, was observed. The authors conclude that serotonin (5-hydroxytryptamine) plays a critical role in repetition priming by helping to modulate which neural systems contribute to priming effects.

Hemispheric asymmetries in memory processes as measured in a false recognition paradigm.

Although memory differs in important ways between the left and right cerebral hemispheres, the nature of these differences remains controversial. We examined this issue in two experiments using a false memory paradigm that allowed novel tests of two theories that have not been assessed in a common paradigm previously. Lists of semantically related words (e.g., bed, rest, wake...), all highly associated to one "critical" word (e.g., sleep), were presented auditorily during a study phase. Memory for both the related words and the critical words was measured in a subsequent old/new recognition test using divided-visual-field word presentations. The most important results were that the ability to correctly reject previously unpresented words was greater when test items were presented to the right visual field/left hemisphere (RVF/LH) than to the left visual field/right hemisphere (LVF/RH) and that participants were more confident in correctly rejecting unpresented words when test items were presented to the RVF/LH than to the LVF/RH. Results were in line with the theory that associative activation of semantic information is restricted in the left hemisphere but diffuse in the right; however, these results contrasted with the theory that memory traces are interpretive in the left hemisphere but veridical in the right. A potential resolution to the seemingly contradictory theories of asymmetries in memory processing is briefly discussed.

Sensitivity reductions in false recognition: a measure of false memories with stronger theoretical implications.

Signal detection analyses of recognition memory indicate that a bias to respond "old" is large for critical words that are centrally related with previously encoded word lists, is small for words that are less related, and is not observed for unrelated words. Also, recognition sensitivity has not been previously shown to differ between those conditions, which has focused debate over how to explain false recognition on the bias differences. In 3 experiments, critical-word sensitivity was lower than sensitivity for other word types, but related-word sensitivity was not lower than sensitivity for unrelated words. Extant models that predict reduced critical-word sensitivity also predict lower sensitivity for related words than for unrelated words. These results provide crucial new constraints on theoretical explanations of false memories.

Implicit memory influences on metamemory during verbal learning after traumatic brain injury.

PURPOSE: Prior research has shown that individuals with traumatic brain injury (TBI) may be overconfident in their judgments of learning (JOLs; online measures of self-monitoring of learning and memory). JOLs had been presumed to be driven by explicit processes, but recent research has also revealed implicit memory involvement. Given that implicit learning mechanisms are often intact in those with TBI, the purpose of this study was to investigate whether priming and antipriming of immediate and delayed JOLs in individuals with TBI might affect their overconfidence. METHOD: A standard 3-field masked priming paradigm was combined with a paired-associate learning task with JOLs and administered to individuals with TBI and matched controls (18 per group). In each trial, a subliminal masked stimulus was immediately followed by supraliminal presentation of a word pair for study; participants also made immediate and delayed JOLs, with cued-recall testing 10 min after study and judgment. RESULTS: Antipriming significantly lowered JOLs and overconfidence for both groups, whereas delaying JOLs significantly improved recall for both groups. CONCLUSIONS: The results suggest that JOLs may be influenced by subliminal implicit memory. Clinical implications include the possible use of antipriming to reduce overconfidence after brain injury and delaying JOLs to improve recall.

The Diagnosticity of Color for Emotional Objects.

Object classification can be facilitated if simple diagnostic features can be used to determine class membership. Previous studies have found that simple shapes may be diagnostic for emotional content and automatically alter the allocation of visual attention. In the present study, we analyzed whether color is diagnostic of emotional content and tested whether emotionally diagnostic hues alter the allocation of visual attention. Reddish-yellow hues are more common in (i.e., diagnostic of) emotional images, particularly images with positive emotional content. An exogenous cueing paradigm was employed to test whether these diagnostic hues orient attention differently from other hues due to the emotional diagnosticity. In two experiments, we found that participants allocated attention differently to diagnostic hues than to non-diagnostic hues, in a pattern indicating a broadening of spatial attention when cued with diagnostic hues. Moreover, the attentional broadening effect was predicted by self-reported measures of affective style, linking the behavioral effect to emotional processes. These results confirm the existence and use of diagnostic features for the rapid detection of emotional content.

Hemispheric asymmetries in motivation neurally dissociate self-description processes.

When people describe themselves by responding to personality questionnaires, typically they endorse some items and reject others. Further, most people endorse likable traits and reject unlikeable traits. In one case, people use previously stored information about themselves to judge that a particular trait--usually likable--describes them well, and in the other, they use previously stored information to judge that a particular trait--usually unlikeable--does not describe them well. Here we report evidence that these processes are neurally dissociable, with the former benefiting from engaging left-hemisphere processes and the latter benefiting from engaging right-hemisphere processes. Hemispheric asymmetries in self-description are not unidirectional and do not differ across different personality traits, but do differ between endorsing likable items and rejecting unlikeable items. Thus, we are not dispassionate observers of ourselves. Dissociable, basic motivational processes are involved in the neural architecture underlying self-description.

Separable processes before, during, and after the N400 elicited by previously inferred and new information: evidence from time-frequency decompositions.

Successful comprehension during reading often requires inferring information not explicitly presented. This information is readily accessible when subsequently encountered, and a neural correlate of this is an attenuation of the N400 event-related potential (ERP). We used ERPs and time-frequency (TF) analysis to investigate neural correlates of processing inferred information after a causal coherence inference had been generated during text comprehension. Participants read short texts, some of which promoted inference generation. After each text, they performed lexical decisions to target words that were unrelated or inference-related to the preceding text. Consistent with previous findings, inference-related words elicited an attenuated N400 relative to unrelated words. TF analyses revealed unique contributions to the N400 from activity occurring at 1-6 Hz (theta) and 0-2 Hz (delta), supporting the view that multiple, sequential processes underlie the N400.