Segmentation of subcomponents within the superior longitudinal fascicle in humans: a quantitative, in vivo, DT-MRI study

Previous research in non-human primates has shown that the superior longitudinal fascicle (SLF), a major intrahemispheric fiber tract, is actually composed of four separate components. In humans, only post-mortem investigations have been available to examine the trajectory of this tract. This study evaluates the hypothesis that the four subcomponents observed in non-human primates can also be found in the human brain using in vivo diffusion tensor magnetic resonance imaging (DT-MRI). The results of our study demonstrated that the four subdivisions could indeed be identified and segmented in humans. SLF I is located in the white matter of the superior parietal and superior frontal lobes and extends to the dorsal premotor and dorsolateral prefrontal regions. SLF II occupies the central core of the white matter above the insula. It extends from the angular gyrus to the caudal-lateral prefrontal regions. SLF III is situated in the white matter of the parietal and frontal opercula and extends from the supramarginal gyrus to the ventral premotor and prefrontal regions. The fourth subdivision of the SLF, the arcuate fascicle, stems from the caudal part of the superior temporal gyrus arches around the caudal end of the Sylvian fissure and extends to the lateral prefrontal cortex along with the SLF II fibers. Since DT-MRI allows the precise definition of only the stem portion of each fiber pathway, the origin and termination of the subdivisions of SLF are extrapolated from the available data in experimental material from non-human primates.     

Cortical connections of the macaque anterior intraparietal (AIP) area

We traced the cortical connections of the anterior intraparietal (AIP) area, which is known to play a crucial role in visuomotor transformations for grasping. AIP displayed major connections with 1) areas of the inferior parietal lobule convexity, the rostral part of the lateral intraparietal area and the SII region; 2) ventral visual stream areas of the lower bank of the superior temporal sulcus and the middle temporal gyrus; and 3) the premotor area F5 and prefrontal areas 46 and 12. Additional connections were observed with the caudal intraparietal area and the ventral part of the frontal eye field. This study suggests that visuomotor transformations for object-oriented actions, processed in AIP, rely not only on dorsal visual stream information related to the object's physical properties but also on ventral visual stream information related to object identity. The identification of direct anatomical connections with the inferotemporal cortex suggests that AIP also has a unique role in linking the parietofrontal network of areas involved in sensorimotor transformations for grasping with areas involved in object recognition. Thus, AIP could represent a crucial node in a cortical circuit in which hand-related sensory and motor signals gain access to representations of object identity for tactile object recognition.     

高定义角度纤维追踪技术重建人脑颞顶枕区短纤维束连接

目的观察人脑颞顶枕区短纤维束连接的空间走行、皮质连接及三维空间关系。方法对10名健康志愿者进行MR扫描。下载90名健康志愿者的弥散谱成像(DSI)综合数据模板(NTU-90)。应用高定义角度纤维束追踪技术(HDFT)对所收集数据进行人脑颞顶枕区3条短纤维束连接[上纵束的垂直部(SLF-V)、枕纵束(VOF)和颞顶束(TP)]重建,并分析其纤维走行、皮质连接、空间三维关系及左右侧别差异。结果 3条纤维束在10名志愿者和NTU-90中均重建成功。SLF-V纤维束起源于颞中回和颞下回后部,终止于角回和缘上回;VOF纤维束起源于下枕叶和梭状回,少量纤维来源于颞下回的后部和枕极前部,终止点位于角回和上枕叶外侧部;TP纤维束主要起源于颞中回、颞下回、梭状回和下颞-下枕结合部,连接于顶上小叶;左右侧SLF-V、VOF和TP的纤维束体积差异无统计学意义(P均0.05)。结论应用HDFT纤维束成像技术成功重建了SLF-V、VOF和TP,可为脑功能研究和神经外科手术提供解剖学依据。     

The role of the inferior parietal cortex in linking the tactile perception and manual construction of object shapes

We employed functional magnetic resonance imaging (fMRI) in 12 healthy subjects to measure cerebral activation related to a set of higher order manual sensorimotor tasks performed in the absence of visual guidance. Purposeless manipulation of meaningless plasticine lumps served as a reference against which we contrasted two tasks where manual manipulation served a meaningful purpose, either the perception and recognition of three-dimensional shapes or the construction of such shapes out of an amorphous plasticine lump. These tasks were compared with the corresponding mental imagery of the modelling process which evokes the constructive concept but lacks concomitant sensorimotor input and output. Neural overlap was found in a bilateral activity increase in the posterior and anterior intraparietal sulcus area (IPS and AIP). Differential activation was seen in the supplementary and cingulate motor areas, the left M1 and the superior parietal lobe for modelling and in the left angular and ventral premotor cortex for imagery. Our data thus point to a congruent neural substrate for both perceptive and constructive object-oriented sensorimotor cognition in the AIP and posterior IPS. The leftward asymmetry of the inferior parietal activations, including the angular gyrus, during imagery of modelling along with the ventral premotor activations emphasize the close vicinity of the circuitry for cognitive manipulative motor behaviour and language.     

Cortical connections of the inferior parietal cortical convexity of the macaque monkey

We traced the cortical connections of the 4 cytoarchitectonic fields--Opt, PG, PFG, PF--forming the cortical convexity of the macaque inferior parietal lobule (IPL). Each of these fields displayed markedly distinct sets of connections. Although Opt and PG are both targets of dorsal visual stream and temporal visual areas, PG is also target of somatosensory and auditory areas. Primary parietal and frontal connections of Opt include area PGm and eye-related areas. In contrast, major parietal and frontal connections of PG include IPL, caudal superior parietal lobule (SPL), and agranular frontal arm-related areas. PFG is target of somatosensory areas and also of the medial superior temporal area (MST) and temporal visual areas and is connected with IPL, rostral SPL, and ventral premotor arm- and face-related areas. Finally, PF is primarily connected with somatosensory areas and with parietal and frontal face- and arm-related areas. The present data challenge the bipartite subdivision of the IPL convexity into a caudal and a rostral area (7a and 7b, respectively) and provide a new anatomical frame of reference of the macaque IPL convexity that advances our present knowledge on the functional organization of this cortical sector, giving new insight into its possible role in space perception and motor control.     

Cortical input to the frontal pole of the marmoset monkey

We used fluorescent tracers to map the pattern of cortical afferents to frontal area 10 in marmosets. Dense projections originated in several subdivisions of orbitofrontal cortex, in the medial frontal cortex (particularly areas 14 and 32), and in the dorsolateral frontal cortex (particularly areas 8Ad and 9). Major projections also stemmed, in variable proportions depending on location of the injection site, from both the inferior and superior temporal sensory association areas, suggesting a degree of audiovisual convergence. Other temporal projections included the superior temporal polysensory cortex, temporal pole, and parabelt auditory cortex. Medial area 10 received additional projections from retrosplenial, rostral calcarine, and parahippocampal areas, while lateral area 10 received small projections from the ventral somatosensory and premotor areas. There were no afferents from posterior parietal or occipital areas. Most frontal connections were balanced in terms of laminar origin, giving few indications of an anatomical hierarchy. The pattern of frontopolar afferents suggests an interface between high-order representations of the sensory world and internally generated states, including working memory, which may subserve ongoing evaluation of the consequences of decisions as well as other cognitive functions. The results also suggest the existence of functional differences between subregions of area 10.     

Cortical connections of area V4 in the macaque

To determine the locus, full extent, and topographic organization of cortical connections of area V4 (visual area 4), we injected anterograde and retrograde tracers under electrophysiological guidance into 21 sites in 9 macaques. Injection sites included representations ranging from central to far peripheral eccentricities in the upper and lower fields. Our results indicated that all parts of V4 are connected with occipital areas V2 (visual area 2), V3 (visual area 3), and V3A (visual complex V3, part A), superior temporal areas V4t (V4 transition zone), MT (medial temporal area), and FST (fundus of the superior temporal sulcus [STS] area), inferior temporal areas TEO (cytoarchitectonic area TEO in posterior inferior temporal cortex) and TE (cytoarchitectonic area TE in anterior temporal cortex), and the frontal eye field (FEF). By contrast, mainly peripheral field representations of V4 are connected with occipitoparietal areas DP (dorsal prelunate area), VIP (ventral intraparietal area), LIP (lateral intraparietal area), PIP (posterior intraparietal area), parieto-occipital area, and MST (medial STS area), and parahippocampal area TF (cytoarchitectonic area TF on the parahippocampal gyrus). Based on the distribution of labeled cells and terminals, projections from V4 to V2 and V3 are feedback, those to V3A, V4t, MT, DP, VIP, PIP, and FEF are the intermediate type, and those to FST, MST, LIP, TEO, TE, and TF are feedforward. Peripheral field projections from V4 to parietal areas could provide a direct route for rapid activation of circuits serving spatial vision and spatial attention. By contrast, the predominance of central field projections from V4 to inferior temporal areas is consistent with the need for detailed form analysis for object vision.     

The organization and connections of anterior and posterior parietal cortex in titi monkeys: do New World monkeys have an area 2?

We used multiunit electrophysiological recording techniques to examine the topographic organization of somatosensory area 3b and cortex posterior to area 3b, including area 1 and the presumptive area 5, in the New World titi monkey, Callicebus moloch. We also examined the ipsilateral and contralateral connections of these fields, as well as those in a region of cortex that appeared to be similar to both area 7b and the anterior intraparietal area (7b/AIP) described in macaque monkeys. All data were combined with architectonic analysis to generate comprehensive reconstructions. These studies led to several observations. First, area 1 in titi monkeys is not as precisely organized in terms of topographic order and receptive field size as is area 1 in macaque monkeys and a few New World monkeys. Second, cortex caudal to area 1 in titi monkeys is dominated by the representation of the hand and forelimb, and contains neurons that are often responsive to visual stimulation as well as somatic stimulation. This organization is more like area 5 described in macaque monkeys than like area 2. Third, ipsilateral and contralateral cortical connections become more broadly distributed away from area 3b towards the posterior parietal cortex. Specifically, area 3b has a relatively restricted pattern of connectivity with adjacent somatosensory fields 3a, 1, S2 and PV; area 1 has more broadly distributed connections than area 3b; and the presumptive areas 5 and 7b/AIP have highly diverse connections, including connections with motor and premotor cortex, extrastriate visual areas, auditory areas and somatosensory areas of the lateral sulcus. Fourth, the hand representation of the presumptive area 5 has dense callosal connections. Our results, together with previous studies in other primates, suggest that anterior parietal cortex has expanded in some primate lineages, perhaps in relation to manual abilities, and that the region of cortex we term area 5 is involved in integrating somatic inputs with the motor system and across hemispheres. Such connections could form the substrate for intentional reaching, grasping and intermanual transfer of information necessary for bilateral coordination of the hands.     

A genetic model for understanding higher order visual processing: functional interactions of the ventral visual stream in Williams syndrome

Williams syndrome (WS) is a rare neurodevelopmental disorder caused by a 1.6 Mb microdeletion on chromosome 7q11.23 and characterized by hypersocial personality and prominent visuospatial construction impairments. Previous WS studies have identified functional and structural abnormalities in the hippocampal formation, prefrontal regions crucial for amygdala regulation and social cognition, and the dorsal visual stream, notably the intraparietal sulcus (IPS). Although aberrant ventral stream activation has not been found in WS, object-related visual information that is processed in the ventral stream is a critical source of input into these abnormal regions. The present study, therefore, examined neural interactions of ventral stream areas in WS. Using a passive face- and house-viewing paradigm, activation and functional connectivity of stimulus-selective regions in fusiform and parahippocampal gyri, respectively, were investigated. During house viewing, significant activation differences were observed between participants with WS and a matched control group in IPS. Abnormal functional connectivity was found between parahippocampal gyrus and parietal cortex and between fusiform gyrus and a network of brain regions including amygdala and portions of prefrontal cortex. These results indicate that abnormal upstream visual object processing may contribute to the complex cognitive/behavioral phenotype in WS and provide a systems-level characterization of genetically mediated abnormalities of neural interactions.     

Neural correlates of the spontaneous phase transition during bimanual coordination

Repetitive bimanual finger-tapping movements tend toward mirror symmetry: There is a spontaneous transition from less stable asymmetrical movement patterns to more stable symmetrical ones under frequency stress but not vice versa. During this phase transition, the interaction between the signals controlling each hand (cross talk) is expected to be prominent. To depict the regions of the brain in which cortical cross talk occurs during bimanual coordination, we conducted event-related functional magnetic resonance imaging using a bimanual repetitive-tapping task. Transition-related activity was found in the following areas: the bilateral ventral premotor cortex, inferior frontal gyrus, middle frontal gyrus, inferior parietal lobule, insula, and thalamus; the right rostral portion of the dorsal premotor cortex and midbrain; the left cerebellum; and the presupplementary motor area, rostral cingulate zone, and corpus callosum. These regions were discrete from those activated by bimanual movement execution. The phase-transition-related activation was right lateralized in the prefrontal, premotor, and parietal regions. These findings suggest that the cortical neural cross talk occurs in the distributed networks upstream of the primary motor cortex through asymmetric interhemispheric interaction.     

A distributed left hemisphere network active during planning of everyday tool use skills

Determining the relationship between mechanisms involved in action planning and/or execution is critical to understanding the neural bases of skilled behaviors, including tool use. Here we report findings from two fMRI studies of healthy, right-handed adults in which an event-related design was used to distinguish regions involved in planning (i.e. identifying, retrieving and preparing actions associated with a familiar tools' uses) versus executing tool use gestures with the dominant right (experiment 1) and non-dominant left (experiment 2) hands. For either limb, planning tool use actions activates a distributed network in the left cerebral hemisphere consisting of: (i) posterior superior temporal sulcus, along with proximal regions of the middle and superior temporal gyri; (ii) inferior frontal and ventral premotor cortices; (iii) two distinct parietal areas, one located in the anterior supramarginal gyrus (SMG) and another in posterior SMG and angular gyrus; and (iv) dorsolateral prefrontal cortex (DLFPC). With the exception of left DLFPC, adjacent and partially overlapping sub-regions of left parietal, frontal and temporal cortex are also engaged during action execution. We suggest that this left lateralized network constitutes a neural substrate for the interaction of semantic and motoric representations upon which meaningful skills depend.     

Kinesthetic working memory and action control within the dorsal stream

There is wide agreement that the "dorsal (action) stream" processes visual information for movement control. However, movements depend not only on vision but also on tactile and kinesthetic information (=haptics). Using functional magnetic resonance imaging, the present study investigates to what extent networks within the dorsal stream are also utilized for kinesthetic action control and whether they are also involved in kinesthetic working memory. Fourteen blindfolded participants performed a delayed-recognition task in which right-handed movements had to be encoded, maintained, and later recognized without any visual feedback. Encoding of hand movements activated somatosensory areas, superior parietal lobe (dorsodorsal stream), anterior intraparietal sulcus (aIPS) and adjoining areas (ventrodorsal stream), premotor cortex, and occipitotemporal cortex (ventral stream). Short-term maintenance of kinesthetic information elicited load-dependent activity in the aIPS and adjacent anterior portion of the superior parietal lobe (ventrodorsal stream) of the left hemisphere. We propose that the action representation system of the dorsodorsal and ventrodorsal stream is utilized not only for visual but also for kinesthetic action control. Moreover, the present findings demonstrate that networks within the ventrodorsal stream, in particular the left aIPS and closely adjacent areas, are also engaged in working memory maintenance of kinesthetic information.     

Functional architecture of retinotopy in visual association cortex of behaving monkey

While the receptive field properties of single neurons in the inferior parietal cortex have been quantitatively described from numerous electrical measurements, the visual topography of area 7a and the adjacent dorsal prelunate area (DP) remains unknown. This lacuna may be a technical byproduct of the difficulty of reconstructing tens to hundreds of penetrations, or may be the result of varying functional retinotopic architectures. Intrinsic optical imaging, performed in behaving monkey for extended periods of time, was used to evaluate retinotopy simultaneously at multiple positions across the cortical surface. As electrical recordings through an implanted artificial dura are difficult, the measurement and quantification of retinotopy with long-term recordings was validated by imaging early visual cortex (areas V1 and V2). Retinotopic topography was found in each of the three other areas studied within a single day's experiment. However, the ventral portion of DP (DPv) had a retinotopic topography that varied from day to day, while the more dorsal aspects (DPd) exhibited consistent retinotopy. This suggests that the dorsal prelunate gyrus may consist of more than one visual area. The retinotopy of area 7a also varied from day to day. Possible mechanisms for this variability across days are discussed as well as its impact upon our understanding of the representation of extrapersonal space in the inferior parietal cortex.     

Distribution of cortical activation during visuospatial n-back tasks as revealed by functional magnetic resonance imaging

Human neuroimaging studies conducted during visuospatial working memory tasks have inconsistently detected activation in the prefrontal cortical areas depending presumably on the type of memory and control tasks employed. We used functional magnetic resonance imaging to study brain activation related to the performance of a visuospatial n-back task with different memory loads (0-back, 1-back and 2-back tasks). Comparison of the 2-back versus 0-back tasks revealed consistent, bilateral activation in the medial frontal gyrus (MFG), superior frontal sulcus and adjacent cortical tissue (SFS/SFG) in all subjects and in six out of seven subjects in the intraparietal sulcus (IPS). Activation was also detected in the inferior frontal gyrus, medially in the superior frontal gyrus, precentral gyrus, superior and inferior parietal lobuli, occipital visual association areas, anterior and posterior cingulate areas and in the insula. Comparison between the 1- back versus 0-back tasks revealed activation only in a few brain areas. Activation in the MFG, SFS/SFG and IPS appeared dependent on memory load. The results suggest that the performance of a visuospatial working memory task engages a network of distributed brain areas and that areas in the dorsal visual pathway are engaged in mnemonic processing of visuospatial information.      

Cortical Networks for Visual Reaching: Physiological and Anatomical Organization of Frontal and Parietal Lobe Arm Regions

The functional and structural properties of the dorsolateralfrontal lobe and posterior parietal proximal arm representationswere studied in macaque monkeys. Physiological mapping of primarymotor (MI), dorsal premotor (PMd), and posterior parietal (area5) cortices was performed in behaving monkeys trained in aninstructed-delay reaching task. The parietofrontal corticocorticatconnectivities of these same areas were subsequently examinedanatomically by means of retrograde tracing techniques. Signal-, set-, movement-, and position-related directional neuronalactivities were distributed nonuniformly within the task-relatedareas in both frontal and parietal cortices. Within the frontallobe, moving caudally from PMd to the MI, the activity thatsignals for the visuospatial events leading to target localizationdecreased, while the activity more directly linked to movementgeneration increased. Physiological recordings in the superior parietal lobule revealeda gradient-like distribution of functional properties similarto that observed in the frontal lobe. Signal- and set-relatedactivities were encountered more frequently in the intermediateand ventral part of the medial bank of the intraparietal sulcus(IPS), in area MIP. Movement- and position-related activitieswere distributed more uniformly within the superior parietallobule (SPL), in both dorsal area 5 and in MIP. Frontal and parietal regions sharing similar functional propertieswere preferentially connected through their association pathways.As a result of this study, area MIP, and possibly areas MDPand 7m as well, emerge as the parietal nodes by which visualinformation may be relayed to the frontal lobe arm region. Theseparietal and frontal areas, along with their association connections,represent a potential cortical network for visual reaching.The architecture of this network is ideal for coding reachingas the result of a combination between visual and somatic information.     

Idiom comprehension: a prefrontal task?

We investigated the neural correlates of idiomatic sentence processing using event-related functional magnetic resonance imaging. Twenty-two healthy subjects were presented with 62 literal sentences and 62 idiomatic sentences, each followed by a picture and were required to judge whether the sentence matched the picture or not. A common network of cortical activity was engaged by both conditions, with the nonliteral task eliciting overall greater activation, both in terms of magnitude and spatial extent. The network that was specifically activated by the nonliteral condition involved the left temporal cortex, the left superior medial frontal gyrus (Brodmann area 9), and the left inferior frontal gyrus (IFG). Activations were also present in the right superior and middle temporal gyri and temporal pole and in the right IFG. In contrast, literal sentences selectively activated the left inferior parietal lobule and the right supramarginal gyrus. An analysis of effective connectivity indicated that the medial prefrontal area significantly increased the connection between frontotemporal areas bilaterally during idiomatic processing. Overall, the present findings indicate a crucial role of the prefrontal cortex in idiom comprehension, which could reflect the selection between alternative sentence meanings.     

Delineation of the middle longitudinal fascicle in humans: a quantitative, in vivo, DT-MRI study

Experimental and imaging studies in monkeys have outlined various long association fiber bundles within the temporoparietal region. In the present study the trajectory of the middle longitudinal fascicle (MdLF) has been delineated in 4 human subjects using diffusion tensor magnetic resonance imaging segmentation and tractography. The MdLF seems to extend from the inferior parietal lobule (IPL), specifically the angular gyrus, to the temporal pole remaining within the white matter of the superior temporal gyrus (STG). Comparison of the superior longitudinal fascicle II-arcuate fascicle (SLF II-AF) with the MdLF in the same subjects revealed that MdLF is located in a medial and caudal position relative to SLF II-AF and that it extends more rostrally. Given the location of MdLF between the IPL (angular gyrus) and the STG, it is suggested that MdLF could have a role in language and attention functions.     

Cortical projections of area V2 in the macaque

To determine the locus, extent and topograhic organization of cortical projections of area V2, we injected tritiated amino acids under electrophysiological control into 15 V2 sites in 14 macaques. The injection sites included the foveal representation and representations ranging from central to far peripheral eccentricities in both the upper and lower visual fields. The results indicated that all V2 sites project topographically back to V1 and forward to V3, V4 and MT. There is also a topographically organized projection from V2 to V4t, but this projection is limited to the lower visual field representation. V2 thus appears to project to virtually all the visual cortex within the occipital lobe. In addition to these projections to occipital visual areas, V2 sites representing eccentricities of approximately 30 degrees and greater project to three visual areas in parietal cortex-the medial superior temporal (MST), parieto-occipital (PO) and ventral intraparietal (VIP) areas. This peripheral field representation of V2 also projects to area VTF, a visual area located in area TF on the posterior parahippocampal gyrus. Projections from the peripheral field representation of V2 of parietal areas could provide a direct route for rapid activation of circuits serving spatial vision and spatial attention.      

Selective aging of the human cerebral cortex observed in vivo: differential vulnerability of the prefrontal gray matter

In a prospective cross-sectional study, we used computerized volumetry of magnetic resonance images to examine the patterns of brain aging in 148 healthy volunteers. The most substantial age-related decline was found in the volume of the prefrontal gray matter. Smaller age-related differences were observed in the volume of the fusiform, inferior temporal and superior parietal cortices. The effects of age on the hippocampal formation, the postcentral gyrus, prefrontal white matter and superior parietal white matter were even weaker. No significant age- related differences were observed in the parahippocampal and anterior cingulate gyri, inferior parietal lobule, pericalcarine gray matter, the precentral gray and white matter, postcentral white matter and inferior parietal white matter. The volume of the total brain volume and the hippocampal formation was larger in men than in women even after adjustment for height. Inferior temporal cortex showed steeper aging trend in men. Small but consistent rightward asymmetry was found in the whole cerebral hemispheres, superior parietal, fusiform and orbito-frontal cortices, postcentral and prefrontal white matter. The left side was larger than the right in the dorsolateral prefrontal, parahippocampal, inferior parietal and pericalcarine cortices, and in the parietal white matter. However, there were no significant differences in age trends between the hemispheres.      

Cross-modal binding and activated attentional networks during audio-visual speech integration: a functional MRI study

We evaluated the neural substrates of cross-modal binding and divided attention during audio-visual speech integration using functional magnetic resonance imaging. The subjects (n = 17) were exposed to phonemically concordant or discordant auditory and visual speech stimuli. Three different matching tasks were performed: auditory-auditory (AA), visual-visual (VV) and auditory-visual (AV). Subjects were asked whether the prompted pair were congruent or not. We defined the neural substrates for the within-modal matching tasks by VV-AA and AA-VV. We defined the cross-modal area as the intersection of the loci defined by AV-AA and AV-VV. The auditory task activated the bilateral anterior superior temporal gyrus and superior temporal sulcus, the left planum temporale and left lingual gyrus. The visual task activated the bilateral middle and inferior frontal gyrus, right occipito-temporal junction, intraparietal sulcus and left cerebellum. The bilateral dorsal premotor cortex, posterior parietal cortex (including the bilateral superior parietal lobule and the left intraparietal sulcus) and right cerebellum showed more prominent activation during AV compared with AA and VV. Within these areas, the posterior parietal cortex showed more activation during concordant than discordant stimuli, and hence was related to cross-modal binding. Our results indicate a close relationship between cross-modal attentional control and cross-modal binding during speech reading.