Human cortical processing of colour and pattern

Neural Substrates of Blindsight After Hemispherectomy

Blindsight is a visual phenomenon whereby hemianopic patients are able to process visual information in their blind visual field without awareness. Previous research demonstrating the existence of blindsight in hemianopic patients has been criticized for the nature of the paradigms used, for the presence of methodological artifacts, and for the possibility that spared islands of visual cortex may have sustained the phenomenon because the patients generally had small circumscribed lesions. To respond to these criticisms, the authors have been investigating for several years now residual visual abilities in the blind field of hemispherectomized patients in whom a whole cerebral hemisphere has been removed or disconnected from the rest of the brain. These patients have offered a unique opportunity to establish the existence of blindsight and to investigate its underlying neuronal mechanisms because in these cases, spared islands of visual cortex cannot be evoked to explain the presence of visual abilities in the blind field. In addition, the authors have been using precise behavioral paradigms, strict control for potential methodological artifacts such as light scatter, fixation, criterion effects, and macular sparing, and they have utilized new neuroimaging techniques such as diffusion tensor imaging tractography to enhance their understanding of the phenomenon. The following article is a review of their research on the involvement of the superior colliculi in blindsight in hemispherectomized patients. NEUROSCIENTIST 13(5):506—518, 2007.

Brain areas involved in synaesthesia: a review

Despite a recent upsurge of research, much remains unknown about the neurobiological mechanisms underlying synaesthesia. By integrating results obtained so far in Magnetic Resonance Imaging (MRI) studies, this contribution sheds light on the role of particular brain regions in synaesthetic experiences. First, in accordance with its sensory nature, it seems that the sensory brain areas corresponding to the type of synaesthetic experience are activated. Synaesthetic colour experiences can activate colour regions in occipito-temporal cortex, but this is not necessarily restricted to V4. Furthermore, sensory and motor brain regions have been obtained that extend beyond the particular type of synaesthesia studied. Second, differences in experimental setup, number and type of synaesthetes tested, and method to delineate regions of interest may help explain inconsistent results obtained in the BOLD-MRI (Blood Oxygen Level Dependent functional MRI) studies. Third, an overview of obtained results shows that a network of brain areas rather than a single brain region underlies synaesthesia. Six brain regions of overlapping results emerge, these regions are in sensory and motor regions as well as 'higher level' regions in parietal and frontal lobe. We propose that these regions are related to three different cognitive processes inherently part of synaesthesia; the sensory processes, the (attentional) 'binding' processes, and cognitive control processes. Finally, we discuss how these functional and structural brain properties might relate to the development of synaesthesia. In particular, we believe this relationship is better understood by separating the question what underlies the presence of synaesthesia ('trait') from what determines particular synaesthetic associations ('type').

Blindsight mediated by an S-cone-independent collicular pathway: an fMRI study in hemispherectomized subjects

The purpose of our study was to investigate the ability to process achromatic and short-wavelength-sensitive cone (S-cone)-isolating (blue-yellow) stimuli in the blind visual field of hemispherectomized subjects and to demonstrate that blindsight is mediated by a collicular pathway that is independent of S-cone inputs. Blindsight has been described as the ability to respond to visual stimuli in the blind visual field without conscious awareness [Weiskrantz, L., Warrington, E. K., Sanders, M. D., & Marshall, J. Visual capacity in the hemianopic field following a restricted occipital ablation. Brain, 97, 709-728, 1974]. The roles of the subcortical neural structures in blindsight, such as the pulvinar and the superior colliculus, have been debated and an underlying neural correlate has yet to be confirmed. Using fMRI, we tested the ability to process visual stimuli that isolated the achromatic and short-wavelength-sensitive (S-)-cone pathways in three subjects: one control subject, one hemispherectomized subject with blindsight, and one hemispherectomized subject without blindsight. We demonstrated that (1) achromatic and S-cone-isolating stimuli presented to the normal visual hemifield of hemispherectomized subjects and to both visual hemifields of the control subject activated contralateral visual areas (V1/V2), as expected; (2) achromatic stimulus presentation but not S-cone-isolating stimulus presentation to the blind hemifield of the subject with blindsight activated visual areas FEF/V5; (3) whereas the cortical activation of the control subject was enhanced by an additional stimulus (achromatic and S-cone isolating) presented in the contralateral visual field, activation pattern of the subject with blindsight was enhanced by achromatic stimuli only. We conclude that the human superior colliculus is blind to the S-cone-isolating stimuli, and blindsight is mediated by an S-cone-independent collicular pathway.

Double dissociation between action-driven and perception-driven conflict resolution invoking anterior versus posterior brain systems

The ability to select and integrate relevant information in the presence of competing irrelevant information can be enhanced by advance information to direct attention and guide response selection. Attentional preparation can reduce perceptual and response conflict, yet little is known about the neural source of conflict resolution, whether it is resolved by modulating neural responses for perceptual selection to emphasize task-relevant information or for action selection to inhibit pre-potent responses to interfering information. We manipulated perceptual information that either matched or did not match the relevant color feature of an upcoming Stroop stimulus and recorded hemodynamic brain responses to these events. Longer reaction times to incongruent than congruent color-word Stroop stimuli indicated conflict; however, conflict was even greater when a color cue correctly predicted the Stroop target's color (match) than when it did not (nonmatch). A predominantly anterior network was activated for Stroop-match and a predominantly posterior network was activated for Stroop-nonmatch. Thus, when a stimulus feature did not match the expected feature, a perceptually-driven posterior attention system was engaged, whereas when interfering, automatically-processed semantic information required inhibition of pre-potent responses, an action-driven anterior control system was engaged. These findings show a double dissociation of anterior and posterior cortical systems engaging in different types of control for perceptually-driven and action-driven conflict resolution.

Childhood sexual abuse is associated with reduced gray matter volume in visual cortex of young women

BACKGROUND

Childhood sexual abuse (CSA) has been associated with alterations in brain morphology using region of interest analyses that have focused on stress-sensitive target regions. This study was designed to ascertain the effects on gray matter volume (GMV) of exposure to CSA in healthy young adult college students selected based on exposure history regardless of psychiatric outcome. Voxel-based morphometry (VBM) provided unbiased delineation of the most significantly affected brain regions.

METHODS

High-resolution T1-weighted magnetic resonance imaging (MRI) datasets were obtained for 23 unmedicated female subjects with CSA and 14 healthy female control subjects of equivalent age and socioeconomic status with no history of trauma. Cortical surface-based analysis (FreeSurfer) was performed to verify VBM results.

RESULTS

Gray matter volume was reduced by 12.6% and 18.1% in right and left primary visual (V1) and visual association cortices of abused subjects. This reduction was directly related to duration of CSA before age 12. Gray matter volume of left and right V1 correlated with measure of visual memory (r = .353, p = .032 and r = .448, p = .005). Cortical surface-based analysis indicated that GMV of abused subjects was reduced in the left fusiform (p = .004), left middle occipital (p = .04), and right lingual (p = .002) gyri.

CONCLUSIONS

Early visual experience exerts a strong influence on the developing mammalian visual cortex. Present findings indicate that exposure to CSA may also affect the development of this region and are apparent even in a population of subjects who are sufficiently healthy to matriculate.

Medial temporal lobe activity at recognition increases with the duration of mnemonic delay during an object working memory task

Object working memory (WM) engages a disseminated neural network, although the extent to which the length of time that data is held in WM influences regional activity within this network is unclear. We used functional magnetic resonance imaging to study a delayed matching to sample task in 14 healthy subjects, manipulating the duration of mnemonic delay. Across all lengths of delay, successful recognition was associated with the bilateral engagement of the inferior and middle frontal gyri and insula, the medial and inferior temporal, dorsal anterior cingulate and the posterior parietal cortices. As the length of time that data was held in WM increased, activation at recognition increased in the medial temporal, medial occipito-temporal, anterior cingulate and posterior parietal cortices. These results confirm the components of an object WM network required for successful recognition, and suggest that parts of this network, including the medial temporal cortex, are sensitive to the duration of mnemonic delay.

Telencephalon: Neocortex

The neocortex is an ultracomplex, six-layered structure that develops from the dorsal palliai sector of the telencephalic hemispheres (Figs. 2.24, 2.25, 11.1). All mammals, including monotremes and marsupials, possess a neocortex, but in reptiles, i.e. the ancestors of mammals, only a three-layered neocortical primordium is present [509, 511]. The term neocortex refers to its late phylogenetic appearance, in comparison to the “palaeocortical” olfactory cortex and the “archicortical” hippocampal cortex, both of which are present in all amniotes [509].

Is there continuity between categorical and coordinate spatial relations coding? Evidence from a grid/no-grid working memory paradigm

We ask the question whether the coding of categorical versus coordinate spatial relations depends on different neural networks showing hemispheric specialization or whether there is continuity between these two coding types. The 'continuous spatial coding' hypothesis would mean that the two coding types rely essentially on the same neural network consisting of more general-purpose processes, such as visuo-spatial attention, but with a different weighting of these general processes depending on exact task requirements. With event-related fMRI, we have studied right-handed male subjects performing a grid/no-grid visuo-spatial working memory task inducing categorical and coordinate spatial relations coding. Our data support the 'continuous spatial coding' hypothesis, indicating that, while based on the same fronto-parieto-occipital neural network than categorical spatial relations coding, the coding of coordinate spatial relations relies more heavily on attentional and executive processes, which could induce hemispheric differences similar to those described in the literature. The results also show that visuo-spatial working memory consists of a short-term posterior store with a capacity of up to three elements in the parietal and extrastriate cortices. This store depends on the presence of a visible space categorization and thus can be used for the coding of categorical spatial relations. When no visible space categorization is given or when more than three elements have to be coded, additional attentional and executive processes are recruited, mainly located in the dorso-lateral prefrontal cortex.

Contralateral visual hemifield representations in the human pulvinar nucleus

The pulvinar is a major nucleus of the thalamus. Macaque pulvinar includes two subregions that are connected to the visual cortex and are retinotopically organized, but the organizing principles of the visual portions of the human pulvinar are unknown. We employed two tasks to address the question of whether human pulvinar exhibits spatial organization using event-related functional magnetic resonance imaging. The first was a global-motion discrimination with a rich visual stimulus and the second a luminance-discrimination task of similar difficulty that used a minimal visual stimulus. Both tasks required central fixation and covert peripheral attention. A group analysis of blood-oxygen-level-dependent responses elicited in the motion-discrimination task revealed activity bilaterally in the ventral pulvinar (z = 2 in Talairach space). Clear position specificity was observed with activity elicited only by contralateral stimuli. Ipsilateral stimuli caused suppression. This locus of activity is distinct from the more dorsal (z = 10) region of the pulvinar that has previously been reported to be visually responsive but not retinotopic. In the luminance-discrimination task, similar activity was seen, but it was weaker and detectable only in the left pulvinar. In additional experiments with no task, passively viewed global-motion stimuli also activated the ventral pulvinar bilaterally. Our results show for the first time a distinct, bilateral visual representation in human inferior pulvinar that appears to be contralaterally organized.

Neural bases of asymmetric language switching in second-language learners: An ER-fMRI study

Using the ER-fMRI technique, the present study was designed to investigate the neural substrates of language switching among second-language learners. Twelve Chinese college students who were learning English were scanned when they performed language switching tasks (naming pictures in their first [L1, Chinese] and second [L2, English] languages according to response cues). Compared to non-switching conditions, language switching elicited greater activation in the right superior prefrontal cortex (BA9/10/32), left middle and superior frontal cortex (BA8/9/46), and right middle cingulum and caudate (BA11). When the direction of switching was considered, forward switching (from L1 to L2), but not backward switching (from L2 to L1), activated several brain regions related to executive functions (i.e., bilateral frontal cortices and left ACC) relative to non-switching conditions. These results suggest that neural correlates of language switching differ depending on the direction of the switch and that there does not seem to be a specific brain area acting as a "language switch".

Effects of memory load on hemispheric asymmetries of colour memory

Hemispheric asymmetries in colour perception have been a matter of debate for some time. Recent evidence suggests that lateralisation of colour processing may be largely task specific. Here we investigated hemispheric asymmetries during different types and phases of a delayed colour-matching (recognition) memory task. A total of 11 male and 12 female right-handed participants performed colour-memory tasks. The task involved presentation of a set of colour stimuli (encoding), and subsequent indication (forced choice) of which colours in a larger set had previously appeared at the retrieval or recognition phase. The effect of memory load (set size), and the effect of lateralisation at the encoding or retrieval phases were investigated. Overall, the results indicate a right hemisphere advantage in colour processing, which was particularly pronounced in high memory load conditions, and was seen in males rather than female participants. The results suggest that verbal (mnemonic) strategies can significantly affect the magnitude of hemispheric asymmetries in a non-verbal task.

Absence of S-cone input in human blindsight following hemispherectomy

Destruction of the occipital cortex presumably leads to permanent blindness in the contralateral visual field. Residual abilities to respond to visual stimuli in the blind field without consciously experiencing them have, however, been described in cortically blind patients and are termed 'blindsight'. Although the neuronal basis of blindsight remains unknown, possible neuronal correlates have been proposed based on the nature of the residual vision observed. The most prominent but still controversial hypothesis postulates the involvement of the superior colliculi in blindsight. Here we demonstrate, using a computer-based reaction time test in a group of hemispherectomized subjects, that human 'attention-blindsight' can be measured for achromatic stimuli but disappears for stimuli that solely activate S-cones. Given that primate data have shown that the superior colliculi lacks input from S-cones, our results lend strong support to the hypothesis that 'attention-blindsight' is mediated through a collicular pathway. The contribution of a direct geniculo-extrastriate-koniocellular projection was ruled out by testing hemispherectomized subjects in whom a whole hemisphere has been removed or disconnected for the treatment of epilepsy. A direct retino-pulvinar-cortical connection is also unlikely as the pulvinar nucleus is known to receive input from S-cones as well as from L/M-cone-driven colour-opponent ganglion cells.

Category-specific cortical mapping: color-naming areas

OBJECT

It has been hypothesized that a certain degree of specialization exists within language areas, depending on some specific lexical repertories or categories. To spare hypothetical category-specific cortical areas and to gain a better understanding of their organization, the authors studied patients who had undergone electrical stimulation mapping for brain tumors and they compared an object-naming task with a category-specific task (color naming).

METHODS

Thirty-six patients with no significant preoperative language deficit were prospectively studied during a 2-year period. Along with a reading task, both object- and color-naming tasks were used in brain mapping. During color naming, patients were asked to identify 11 visually presented basic colors. The modality specificity of the color-naming sites found was subsequently tested by asking patients to retrieve the color attributes of objects. High individual variability was observed in language organization among patients and in the tasks performed. Significant interferences in color naming were found in traditional language regions-that is, Broca (p < 0.003) and Wernicke centers (p = 0.05)--although some color-naming areas were occasionally situated outside of these regions. Color-naming interferences were exclusively localized in small cortical areas (< 1 cm2). Anatomical segregation of the different naming categories was apparent in 10 patients; in all, 13 color-specific naming areas (that is, sites evoking no object-naming interference) were detected in the dominant-hemisphere F3 and the supramarginal, angular, and posterior parts of the temporal gyri. Nevertheless, no specific brain region was found to be consistently involved in color naming (p > 0.05). At five sites, although visually presented color-naming tasks were impaired by stimulation, auditory color naming (for example, "What color is grass?") was performed with no difficulty, showing that modality-specific areas can be found during naming.

CONCLUSIONS

Within language areas, a relative specialization of cortical language areas for color naming can be found during electrical stimulation mapping.

Spatial information is processed even when it is task-irrelevant: implications for neuroimaging task design

Many neuroimaging studies have been designed to differentiate domain-specific processes in the brain. A common design constraint is to use identical stimuli for different domain-specific tasks. For example, an experiment investigating spatial versus identity processing would present compound spatial-identity stimuli in both spatial and identity tasks, and participants would be instructed to attend to, encode, maintain, or retrieve spatial information in the spatial task, and identity information in the identity task. An assumption in such studies is that spatial information will not be processed in the identity task, as it is irrelevant for that task. We report three experiments demonstrating violations of this assumption. Our results suggest that comparisons of spatial and identity tasks in existing neuroimaging studies have underestimated the amount of brain activation that is spatial-specific. For future neuroimaging studies, we recommend unique stimulus displays for each domain-specific task, and event-related measurement of post-stimulus processing.

Jumping the gun: is effective preparation contingent upon anticipatory activation in task-relevant neural circuitry?

Subjects switched between tasks that rely on separable "low-level" neural circuits, a motion and a color task. Using functional magnetic resonance imaging, we assessed anticipatory processes within these circuits during preparation to switch between tasks. Once the switch was made, we could then compare activation levels within the circuit associated with the newly relevant task to continuing activity in the circuit associated with the irrelevant task, allowing us to assess both the effectiveness of anticipatory switching mechanisms and the subsequent competition between alternative stimulus-response contingencies. Subjects prepared effectively for the color task, being equally fast and accurate on switch trials as on repeat trials, and this successful preparation was associated with robust preparatory activity within well-known color-processing regions. In contrast, subjects showed considerable behavioral costs when switching to the motion task, evincing a lack of effective preparation, borne out by the fact that motion circuits were silent during the preparatory period.

Individual differences among grapheme-color synesthetes: brain-behavior correlations

Grapheme-color synesthetes experience specific colors associated with specific number or letter characters. To determine the neural locus of this condition, we compared behavioral and fMRI responses in six grapheme-color synesthetes to control subjects. In our behavioral experiments, we found that a subject's synesthetic experience can aid in texture segregation (experiment 1) and reduce the effects of crowding (experiment 2). For synesthetes, graphemes produced larger fMRI responses in color-selective area human V4 than for control subjects (experiment 3). Importantly, we found a correlation within subjects between the behavioral and fMRI results; subjects with better performance on the behavioral experiments showed larger fMRI responses in early retinotopic visual areas (V1, V2, V3, and hV4). These results suggest that grapheme-color synesthesia is the result of cross-activation between grapheme-selective and color-selective brain areas. The correlation between the behavioral and fMRI results suggests that grapheme-color synesthetes may constitute a heterogeneous group.

Don't think of a white bear: an fMRI investigation of the effects of sequential instructional sets on cortical activity in a task-switching paradigm

Using functional magnetic resonance imaging (fMRI), we investigated processes involved in switching between two ongoing tasks, thought to be paradigmatic of executive control processes. Subjects were considerably slower and less accurate when switching between two tasks than when repeatedly carrying out one task, so-called "switch costs." Switch costs, however, generally occur only when more than one task is associated with each stimulus type. This has led to the surmise that switch costs may be due largely to ongoing interference from previously learned stimulus-response (S-R) associations, which are no longer relevant for the task at hand. We used a paradigm that specifically assessed this hypothesis and investigated three stages. In Stage 1, a single task was carried out with each stimulus type; in Stage 2, a second novel task was introduced for each stimulus type; and in Stage 3, subjects reverted to carrying out solely the original tasks (as in Stage 1). In Stage 1, only one task was associated with each stimulus type, whereas two tasks were associated with each stimulus type in Stages 2 and 3. We compared images obtained during Stage 3 to those obtained during Stage 1 and show that during Stage 3, there was robust activation in the network of areas associated with the Stage 2 tasks, even though these tasks were no longer relevant. Our data strongly suggest that switch costs may derive largely from continued activation of areas associated with carrying out the now-irrelevant task(s). We posit that a large component of executive control processes involves resolution of competition between potentially relevant tasks. Our data also revealed widespread activation of a frontoparietal network of areas, and we discuss how this network might be involved in mediating this competition.

Executive function and ADHD: a comparison of children's performance during neuropsychological testing and real-world activities

OBJECTIVE

Current understanding of executive function deficits in Attention-Deficit/Hyperactivity Disorder (ADHD) is derived almost exclusively from neuropsychological testing conducted in laboratory settings. This study compared children's performance on both neuropsychological and real-life measures of executive function and processing speed.

METHOD

The Stroop and Wisconsin Card Sorting Task (WCST) were selected as neuropsychological measures, whereas route tasks in a videogame and at the zoo were used to index real-life measures. Participants comprised a community sample of 22 unmedicated boys with ADHD individually matched on age and IQ with 22 normally developing control boys.

RESULTS

There were no group differences in executive function on the Stroop or zoo tasks, but the ADHD group exhibited deficits in set-shifting as assessed by the WCST (perseverative errors and responses) and videogame play (fewer challenges completed). Also, the ADHD group showed slowed processing speed on the Stroop (slower color naming) and zoo activity (longer time to complete task), as well as a slower rate of acquisition of the sorting rule on the WCST (more trials to complete first category). Efficient and flexible videogame play (number of challenges completed) was related positively to efficacy on the Stroop (number of items named correctly in the interference and two control conditions) and inversely related to set-shifting problems on the WCST (perseverative responses and errors). Also, problems in goal-directed behavior at the zoo (number of deviations from designated route) were related to problems in set-shifting on the WCST (perseverative responding).

CONCLUSIONS

Children with ADHD exhibit impairments in executive function and processing speed in real-world activities as well as in neuropsychological testing. Cognitive deficits detected by standardized neuropsychological testing are related to performance difficulties in real-world activities.

A multitrial analysis for revealing significant corticocortical networks in magnetoencephalography and electroencephalography

We present an MEG/EEG framework to reveal statistically significant brain areas engaged in the same cognitive process across trials without resort to averaging procedures. The variability of neuronal responses is assumed to take place only in the reconstructed time series of cortical sources and not in their positions. This hypothesis allows the use of the surrogate data method to detect recurrently active brain areas across trials adjusted with any cortically constrained focal MEEG inverse solution. Results obtained from synthetic data show that considering several trials enhances the accuracy of the source localisation. We apply this approach on MEG data recorded during a simple visual stimulation. The considered stimulus is frequency tagged in order to reveal the neural network correlated to its perception using phase synchronisation analysis. The results show consistent patterns of distributed synchronous networks centred on occipital areas.

Human brain regions required for the dividing and switching of attention between two features of a single object

This combined PET and ERP study was designed to identify the brain regions activated in switching and divided attention between different features of a single object using matched sensory stimuli and motor response. The ERP data have previously been reported in this journal [64]. We now present the corresponding PET data. We identified partially overlapping neural networks with paradigms requiring the switching or dividing of attention between the elements of complex visual stimuli. Regions of activation were found in the prefrontal and temporal cortices and cerebellum. Each task resulted in different prefrontal cortical regions of activation lending support to the functional subspecialisation of the prefrontal and temporal cortices being based on the cognitive operations required rather than the stimuli themselves.