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.

Neural adaptation across viewpoint and exemplar in fusiform cortex.

The visual system has the remarkable ability to generalize across different viewpoints and exemplars to recognize abstract categories of objects, and to discriminate between different viewpoints and exemplars to recognize specific instances of particular objects. Behavioral experiments indicate the critical role of the right hemisphere in specific-viewpoint and -exemplar visual form processing and the left hemisphere in abstract-viewpoint and -exemplar visual form processing. Neuroimaging studies indicate the role of fusiform cortex in these processes, however results conflict in their support of the behavioral findings. We investigated this inconsistency in the present study by examining adaptation across viewpoint and exemplar changes in the functionally defined fusiform face area (FFA) and in fusiform regions exhibiting adaptation. Subjects were adapted to particular views of common objects and then tested with objects appearing in four critical conditions: same-exemplar, same-viewpoint adapted, same-exemplar, different-viewpoint adapted, different-exemplar adapted, and not adapted. In line with previous results, the FFA demonstrated a release from neural adaptation for repeated different viewpoints and exemplars of an object. In contrast to previous work, a (non-FFA) right medial fusiform area also demonstrated a release from neural adaptation for repeated different viewpoints and exemplars of an object. Finally, a left lateral fusiform area demonstrated neural adaptation for repeated different viewpoints, but not exemplars, of an object. Test-phase task demands did not affect adaptation in these regions. Together, results suggest that dissociable neural subsystems in fusiform cortex support the specific identification of a particular object and the abstract recognition of that object observed from a different viewpoint. In addition, results suggest that areas within fusiform cortex do not support abstract recognition of different exemplars of objects within a category.

Left Hemisphere Dominance for Negative Facial Expressions: The Influence of Task.

Major theories of hemisphere asymmetries in facial expression processing predict right hemisphere dominance for negative facial expressions of disgust, fear, and sadness, however, some studies observe left hemisphere dominance for one or more of these expressions. Research suggests that tasks requiring the identification of six basic emotional facial expressions (angry, disgusted, fearful, happy, sad, and surprised) are more likely to produce left hemisphere involvement than tasks that do not require expression identification. The present research investigated this possibility in two experiments that presented six basic emotional facial expressions to the right or left hemisphere using a divided-visual field paradigm. In Experiment 1, participants identified emotional expressions by pushing a key corresponding to one of six labels. In Experiment 2, participants detected emotional expressions by pushing a key corresponding to whether an expression was emotional or not. In line with predictions, fearful facial expressions exhibited a left hemisphere advantage during the identification task but not during the detection task. In contrast to predictions, sad expressions exhibited a left hemisphere advantage during both identification and detection tasks. In addition, happy facial expressions exhibited a left hemisphere advantage during the detection task but not during the identification task. Only angry facial expressions exhibited a right hemisphere advantage, and this was only observed when data from both experiments were combined. Together, results highlight the influence of task demands on hemisphere asymmetries in facial expression processing and suggest a greater role for the left hemisphere in negative expressions than predicted by previous theories.

Task-domain and hemisphere-asymmetry effects in cisgender and transmale individuals.

The present research examined the extent to which transmale individuals' functional brain organization resembles that of their assigned sex or gender identity. Cisgender-female, cisgender-male, and transmale participants, who were assigned female sex but did not have a female gender identity, were compared in terms of effects that have been observed in cisgender individuals: task-domain effects, in which males perform better than females on spatial tasks and females perform better than males on verbal tasks; and hemisphere-asymmetry effects, in which males show larger differences between the left and right hemispheres than females. In addition, the present research measured participants' intelligence in order to control for potential moderating effects. Participants performed spatial (mental rotation) and verbal (lexical decision) tasks presented to each hemisphere using a divided-visual field paradigm, and then completed an intelligence assessment. In the mental-rotation task, cismale and transmale participants performed better than cisfemale participants, however this group difference was explained by intelligence scores, with higher scores predicting better performance. In the lexical-decision task, cismale and transmale participants exhibited a greater left-hemisphere advantage than cisfemales, and this difference was not affected by intelligence scores. Taken together, results do not support task-domain effects when intelligence is accounted for; however, they do demonstrate a hemisphere-asymmetry effect in the verbal domain that is moderated by gender identity and not assigned sex.

A core system for the implementation of task sets.

When performing tasks, humans are thought to adopt task sets that configure moment-to-moment data processing. Recently developed mixed blocked/event-related designs allow task set-related signals to be extracted in fMRI experiments, including activity related to cues that signal the beginning of a task block, "set-maintenance" activity sustained for the duration of a task block, and event-related signals for different trial types. Data were conjointly analyzed from mixed design experiments using ten different tasks and 183 subjects. Dorsal anterior cingulate cortex/medial superior frontal cortex (dACC/msFC) and bilateral anterior insula/frontal operculum (aI/fO) showed reliable start-cue and sustained activations across all or nearly all tasks. These regions also carried the most reliable error-related signals in a subset of tasks, suggesting that the regions form a "core" task-set system. Prefrontal regions commonly related to task control carried task-set signals in a smaller subset of tasks and lacked convergence across signal types.

Priming for letters and pseudoletters in mid-fusiform cortex: examining letter selectivity and case invariance.

A large body of research indicates a critical role for the left mid-fusiform cortex in reading, however, the extent to which this area is dedicated exclusively to the processing of words and letters has been debated. Two questions regarding left mid-fusiform function are critical to this debate: (1) Are letters stored preferentially compared to visually equivalent non-letters (letter selectivity)? (2) Are letter representations abstract with respect to changes in letter case (e.g., A/a; case invariance)? The present study addressed these questions by comparing priming for letters and pseudoletters in left and right mid-fusiform regions using functional magnetic resonance imaging while subjects performed a same/different matching task. Results revealed priming for letters but not pseudoletters in the left mid-fusiform region, suggesting that representations are letter selective. However, no priming for different-case-primed letters was observed in this region, indicating that representations are not case invariant. In addition, priming for pseudoletters but not letters was observed in the homologous right mid-fusiform region. Overall, findings contradict strongly modular theories of letter/word processing and suggest that left and right mid-fusiform regions support generic object processes that are differentially effective for representing disparate types of visual stimuli.

Identity versus similarity priming for letters in left mid-fusiform cortex.

Using functional magnetic resonance imaging, this study examined neural responses in the Visual Word Form Area to prime - test letter pairs in which visual similarity and identity were manipulated. Results revealed the greatest priming for pairs with high visual similarity, less priming for pairs with medium similarity, and the least priming for pairs with low similarity. Moreover, when visual similarity was equated, priming magnitude did not differ for pairs with the same letter identity compared with those with different letter identities. As such, results contrast with views of the Visual Word Form Area as supporting arbitrary (nonsimilarity based) mapping requirements of reading, and suggest a less modular perspective on visual form recognition.

Letter-specific processing in children and adults matched for reading level.

Expert readers perform faster and more accurately during tasks that involve letters from the known language compared to tasks that involve unfamiliar letter-like forms (e.g., pseudoletters). Previous work with typically developing participants suggests that this letter-specific processing emerges as a consequence of increased reading ability, rather than increased age. In contrast, others have suggested that adults rely on visual information to a greater extent than children when reading, despite reading at similar less-than-expert levels, implying that adults may exhibit greater letter specificity than children. The present study aimed to discriminate between these possibilities by comparing the advantage for letters over pseudoletters in children and adults reading at the same less-than-expert (fourth grade) level. Results revealed greater letter specificity in adults than in children in both error rate and response time measures. Moreover, the magnitude of letter specificity did not vary with reading ability. Thus, results suggest that adults are more sensitive than children to the visual forms of letters, and that differences in letter specificity are not necessarily dependent on reading skill.

Development of letter-specific processing: the effect of reading ability.

During development, perceptual processing is tuned to inputs in the environment such that certain frequently encountered classes of stimuli are processed more effectively than similar comparison stimuli. Letters represent a class of stimuli that are encountered frequently in the environment, at least in literate cultures. Thus, the present study examined the development of letter-specific processing in children 6-19 years old by comparing the difference between performance on a letter-matching task and an unfamiliar non-letter-matching task in different subject groups. Results revealed an increase in letter-specific processing with development. Moreover, comparisons of letter-specific processing in groups of subjects matched either in age or reading ability indicate that the emergence of letter-specific processing is linked to increased reading skill, rather than increased age per se. Findings support theories of perceptual expertise, which suggest that skilled processing drives the specialization of perceptual mechanisms for certain classes of stimuli.

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.

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.

Task effects in the mid-fusiform gyrus: a comparison of orthographic, phonological, and semantic processing of Chinese characters.

The left mid-fusiform gyrus is repeatedly reported to be involved in visual word processing. Nevertheless, it is controversial whether this area responds to orthographic processing of reading. To examine this idea, neural activity was measured using functional magnetic resonance imaging in the present study while subjects performed phonological, semantic, and orthographic tasks with Chinese characters under equivalent task difficulties. One region in the left mid-fusiform gyrus exhibited greater activity during the orthographic task than during the phonological and semantic tasks, which did not differ, suggesting that this region is involved in orthographic processing to a greater extent than phonological or semantic processing. In addition, a region in the right mid-fusiform gyrus exhibited a similar effect. This right mid-fusiform activity may relate to the use of pictorial Chinese characters.

Dissociable neural subsystems underlie visual working memory for abstract categories and specific exemplars.

An ongoing debate concerns whether visual object representations are relatively abstract, relatively specific, both abstract and specific within a unified system, or abstract and specific in separate and dissociable neural subsystems. Most of the evidence for the dissociable subsystems theory has come from experiments that used familiar shapes, and the usage of familiar shapes has allowed for alternative explanations for the results. Thus, we examined abstract and specific visual working memory when the stimuli were novel objects viewed for the first and only time. When participants judged whether cues and probes belonged to the same abstract visual category, they performed more accurately when the probes were presented directly to the left hemisphere than when they were presented directly to the right hemisphere. In contrast, when participants judged whether or not cues and probes were the same specific visual exemplar, they performed more accurately when the probes were presented directly to the right hemisphere than when they were presented directly to the left hemisphere. For the first time, results from experiments using visual working memory tasks support the dissociable subsystems theory.

The development of sustained and transient neural activity.

Sustained and transient signals were compared in a group of 7-8-year-old children and a group of adults performing the same cognitive task using functional magnetic resonance imaging (fMRI) in conjunction with a mixed blocked/event-related design. Results revealed several regions, including a region in the right lateral inferior frontal gyrus, that exhibited opposing developmental trajectories in sustained and transient signals--in particular, decreased sustained signals and increased transient signals with age. Re-analysis of the data assuming "blocked" and "event-related" designs, as opposed to a mixed design, produced different results. In combination, these results may help to explain contradictory findings in the literature regarding the direction of neural development in frontal cortex. Moreover, these results underscore the value of separating sustained and transient signals in fMRI studies of development.

Sustained and transient activity during an object-naming task: a mixed blocked and event-related fMRI study.

Cognitive tasks often involve at least two types of processes-sustained processes potentially related to ongoing task demands and transient processes related to the processing of individual items within the task. Using functional magnetic resonance imaging, in conjunction with a mixed-blocked and event-related design, we examined sustained and transient patterns of neural activity during an object-naming task. Subjects were imaged during runs that alternated between control blocks and task blocks. During task blocks, primed and unprimed objects were intermixed and jittered in time. Regions of interest based on separate analyses of sustained and transient activities were tested independently for sustained and transient responses. Three general patterns of results were observed. (1) Some regions exhibited transient responses but little or no sustained response. These regions were widely distributed across the brain. (2) Other regions clearly exhibited both transient and sustained responses. These regions were found primarily in lateral and medial frontal lobes. (3) A few regions exhibited a sustained response but little or no transient responses. These regions were found in the basal ganglia, orbitofrontal lobe, and right lateral frontal lobe. Furthermore, two homotopic regional pairs in the right and left inferior frontal lobe (frontal operculum and inferior frontal cortex) showed a crossover of sustained and transient effects, with greater transient activity in the left and greater sustained activity in the right hemisphere. The asymmetric relationship between sustained and transient responses in prefrontal regions may be an example of task-specific biasing at work.

Comparison of functional activation foci in children and adults using a common stereotactic space.

The development of methods allowing direct comparisons between child and adult neuroimaging data is an important prerequisite for studying the neural bases of cognitive development. Several issues arise when attempting to make such direct comparisons, including the comparability of anatomical localization of functional responses and the magnitude and time course of the hemodynamic responses themselves. Previous results suggest that, after transformation into a common stereotactic space, anatomical differences between children (ages 7 and 8) and adults are small relative to the resolution of fMRI data. Here, we investigate whether time courses (BOLD responses) and locations of functional activation foci show similarities as well. Event-related fMRI was performed on 16 children (ages 7 and 8) and 16 adults, who pressed buttons in response to a visual stimulus. After transforming images into Talairach space, the coordinates of four consistent activations in each hemisphere were determined for each subject: two foci in the sensorimotor cortex, one focus in the visual cortex, and one focus in the supplementary motor area (eight activations in total). In seven foci, time courses were similar between children and adults, and peak amplitudes of time courses were comparable in all eight foci. There were negligible between-group differences in location of all foci. Variability of activation location was statistically similar in the two groups. In voxelwise group comparison images, minimal differences were found between children and adults in visual and motor cortex regions. The small differences in time courses and locations of activation foci between child and adult brains validate the feasibility of direct statistical comparison of these groups within a common space.

The feasibility of a common stereotactic space for children and adults in fMRI studies of development.

The question of whether pediatric and adult neuroimaging data can be analyzed in a common stereotactic space is a critical issue for developmental neuroscience. Two studies were performed to address this question. In Study 1, high-resolution structural MR brain images of 20 children (7-8 years of age) and 20 young adults (18-30 years of age) were transformed to a common space. Overall brain shape was assessed by tracing the outer boundaries of the brains in three orientations, and more local anatomy was assessed by analysis of portions of 10 selected sulci. Small, but consistent, differences in location and variability were observed in specific locations of the sulcal tracings and outer-boundary sections. In Study 2, a computer simulation was used to assess the extent to which the small anatomical differences observed in Study 1 would produce spurious effects in functional imaging data. Results indicate that, assuming a functional resolution of 5 mm in images averaged across subjects, anatomical differences in either variability or location between children and adults of the magnitude obperved in Study 1 would not negatively affect functional image comparisons. We conclude that atlas-transformed brain morphology is relatively consistent between 7- and 8-year-old children and adults at a resolution appropriate to current functional imaging and that the small anatomical differences present do not limit the usefulness of comparing child and adult functional images within a common stereotactic space.