Phylogenetic differences in the morphology and shape of the central sulcus in great apes and humans: implications for the evolution of motor functions

The central sulcus divides the primary motor and somatosensory cortices in many anthropoid primate brains. Differences exist in the surface area and depth of the central sulcus along the dorso-ventral plane in great apes and humans compared to other primate species. Within hominid species, there are variations in the depth and aspect of their hand motor area, or knob, within the precentral gyrus. In this study, we used post-image analyses on magnetic resonance images to characterize the central sulcus shape of humans, chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla), and orangutans (Pongo pygmaeus and Pongo abelii). Using these data, we examined the morphological variability of central sulcus in hominids, focusing on the hand region, a significant change in human evolution. We show that the central sulcus shape differs between great ape species, but all show similar variations in the location of their hand knob. However, the prevalence of the knob location along the dorso-ventral plane and lateralization differs between species and the presence of a second ventral motor knob seems to be unique to humans. Humans and orangutans exhibit the most similar and complex central sulcus shapes. However, their similarities may reflect divergent evolutionary processes related to selection for different positional and habitual locomotor functions.

Morphometry of the Hand Knob Region and Motor Function Change in Eloquent Area Glioma Patients

PURPOSE

The hand knob area is the cortical representation of motor hand function. The current study aimed to investigate the effects of eloquent area gliomas on the morphometry of the hand motor cortex and preoperative hand motor function.

METHODS

A retrospective study of 320 glioma patients was conducted. Seventy-eight patients with gliomas involving motor functional area were finally enrolled. Using axial T2-weight magnetic resonance images, the width and height of the hand knob were measured in both hemispheres, and differences were compared between the affected and unaffected hemispheres. Receiver operating characteristic (ROC) curve and logistic regression analysis were used to estimate the degree of correlation between distance measurements and motor impairment.

RESULTS

The width and height of the hand knob in the affected and unaffected hemispheres were significantly different (p < 0.0001). The width, height and distance from the tumor to hand knob were reduced in the functionally impaired group compared to the unimpaired group (p = 0.0003, p < 0.0001, p = 0.0005, respectively). The three parameters were significantly correlated and remained significant in ROC and logistic regression analysis. The optimal cut-off value of width, height and distance for identifying preoperative hand muscle strength were 5.73 mm, 5.80 mm and 5.92 mm, respectively.

CONCLUSION

The morphometry of the hand knob is often changed by the infiltration or extrusion of the tumors that were located in or near the hand knob. The width, height of hand knob and the distance from tumor to hand knob could serve as anatomic biomarkers related to preoperative neurological motor deficits.

Motor skill for tool-use is associated with asymmetries in Broca's area and the motor hand area of the precentral gyrus in chimpanzees (Pan troglodytes)

Among nonhuman primates, chimpanzees are well known for their sophistication and diversity of tool use in both captivity and the wild. The evolution of tool manufacture and use has been proposed as a driving mechanism for the development of increasing brain size, complex cognition and motor skills, as well as the population-level handedness observed in modern humans. Notwithstanding, our understanding of the neurological correlates of tool use in chimpanzees and other primates remains poorly understood. Here, we assessed the hand preference and performance skill of chimpanzees on a tool use task and correlated these data with measures of neuroanatomical asymmetries in the inferior frontal gyrus (IFG) and the pli-de-passage fronto-parietal moyen (PPFM). The IFG is the homolog to Broca's area in the chimpanzee brain and the PPFM is a buried gyrus that connects the pre- and post-central gyri and corresponds to the motor-hand area of the precentral gyrus. We found that chimpanzees that performed the task better with their right compared to left hand showed greater leftward asymmetries in the IFG and PPFM. This association between hand performance and PPFM asymmetry was particularly robust for right-handed individuals. Based on these findings, we propose that the evolution of tool use was associated with increased left hemisphere specialization for motor skill. We further suggest that lateralization in motor planning, rather than hand preference per se, was selected for with increasing tool manufacture and use in Hominid evolution.

Localization of cortical primary motor area of the hand using navigated transcranial magnetic stimulation, BOLD and arterial spin labeling fMRI

BACKGROUND

Although the relationship between neuronavigated transcranial magnetic stimulation (nTMS) and functional magnetic resonance imaging (fMRI) has been widely studied in motor mapping, it is unknown how the motor response type or the choice of motor task affect this relationship.

NEW METHOD

Centers of gravity (CoGs) and response maxima were measured with blood-oxygen-level dependent (BOLD) and arterial spin labeling (ASL) fMRI during motor tasks against nTMS CoGs and response maxima, which were mapped with motor evoked potentials (MEPs) and silent periods (SPs).

RESULTS

No differences in motor representations (CoGs and response maxima) were observed in lateral-medial direction (p=0.265). fMRI methods localized the motor representation more posterior than nTMS (p<0.001). This was not affected by the BOLD fMRI motor task (p>0.999) nor nTMS response type (p>0.999). ASL fMRI maxima did not differ from the nTMS nor BOLD fMRI CoGs (p≥0.070), but the ASL CoG was deeper in comparison to other methods (p≤0.042). The BOLD fMRI motor task did not influence the depth of the motor representation (p≥0.745). The median Euclidean distances between the nTMS and fMRI motor representations varied between 7.7mm and 14.5mm and did not differ between the methods (F≤1.23, p≥0.318).

COMPARISON WITH EXISTING METHODS

The relationship between fMRI and nTMS mapped excitatory (MEP) and inhibitory (SP) responses, and whether the choice of motor task affects this relationship, have not been studied before.

CONCLUSIONS

The congruence between fMRI and nTMS is good. The choice of nTMS motor response type nor BOLD fMRI motor task had no effect on this relationship.

Functional neuronal anisotropy assessed with neuronavigated transcranial magnetic stimulation

BACKGROUND

Transcranial magnetic stimulation (TMS) can evaluate cortical excitability and integrity of motor pathways via TMS-induced responses. The responses are affected by the orientation of the stimulated neurons with respect to the direction of the TMS-induced electric field. Therefore, besides being a functional imaging tool, TMS may potentially assess the local structural properties. Yet, TMS has not been used for this purpose.

NEW METHOD

A novel principle to evaluate the relation between function and structure of the motor cortex is presented. This functional anisotropy is evaluated by an anisotropy index (AI), based on motor evoked potential amplitudes induced with different TMS coil orientations, i.e. different electric field directions at a cortical target. To compare the AI with anatomical anisotropy in an explorative manner, diffusion tensor imaging-derived fractional anisotropy (FA) was estimated at different depths near the stimulation site.

RESULTS

AI correlated inversely with cortical excitability through the TMS-induced electric field at motor threshold level. Further, there was a trend of negative correlation between AI and FA.

COMPARISON WITH EXISTING METHODS

None of the existing methods alone can detect the relationship between direct motor cortex activation and local neuronal structure.

CONCLUSIONS

The AI appears to provide information on the functional neuronal anisotropy of the motor cortex by coupling neurophysiology and neuroanatomy within the stimulated cortical region. The AI could prove useful in the evaluation of neurological disorders and traumas involving concurrent structural and functional changes in the motor cortex. Further studies on patients are needed to confirm the usability of AI.

Evolution of the central sulcus morphology in primates

The central sulcus (CS) divides the pre- and postcentral gyri along the dorsal-ventral plane of which all motor and sensory functions are topographically organized. The motor-hand area of the precentral gyrus or KNOB has been described as the anatomical substrate of the hand in humans. Given the importance of the hand in primate evolution, here we examine the evolution of the motor-hand area by comparing the relative size and pattern of cortical folding of the CS surface area from magnetic resonance images in 131 primates, including Old World monkeys, apes and humans. We found that humans and great apes have a well-formed motor-hand area that can be seen in the variation in depth of the CS along the dorsal-ventral plane. We further found that great apes have relatively large CS surface areas compared to Old World monkeys. However, relative to great apes, humans have a small motor-hand area in terms of both adjusted and absolute surface areas.

Distinction between hand dominance and hand preference in primates: a behavioral investigation of manual dexterity in nonhuman primates (macaques) and human subjects

Background The present study aimed to determine and confront hand preference (hand chosen in priority to perform a manual dexterity task) and hand dominance (hand with best motor performance) in eight macaques (Macaca fascicularis) and in 20 human subjects (10 left-handers and 10 right-handers). Methods Four manual dexterity tests have been executed by the monkeys, over several weeks during learning and stable performance phases (in controlled body position): the modified Brinkman board, the reach and grasp drawer, the tube and the bimanual board tasks. Three behavioral tests, adapted versions from the monkeys tasks (modified Brinkman board, tube and bimanual board tasks), as well as a handedness questionnaire, have been conducted in human subjects. Results In monkeys, there was a large disparity across individuals and motor tasks. For hand dominance, two monkeys were rather right lateralized, three monkeys rather left lateralized, whereas in three monkeys, the different parameters measured were not consistent. For hand preference, none of the eight monkeys exhibited a homogeneous lateralization across the four motor tasks. Macaca fascicularis do not exhibit a clear hand preference. Furthermore, hand preference often changed with task repetition, both during training and plateau phases. For human subjects, the hand preference mostly followed the self-assessment of lateralization by the subjects and the questionnaire (in the latter, right-handers were more lateralized than left-handers), except a few discrepancies based on the tube task. There was no hand dominance in seven right-handers (the other three performed better with the right hand) and in four left-handers. Five left-handers showed left-hand dominance, whereas surprisingly, one left-hander performed better with the right hand. In the modified Brinkman board task, females performed better than males, right-handers better than left-handers. Conclusions The present study argues for a distinction between hand preference and hand dominance, especially in macaque monkeys.

Cortical representation of lateralized grasping in chimpanzees (Pan troglodytes): a combined MRI and PET study

Functional imaging studies in humans have localized the motor-hand region to a neuroanatomical landmark call the KNOB within the precentral gyrus. It has also been reported that the KNOB is larger in the hemisphere contralateral to an individual's preferred hand, and therefore may represent the neural substrate for handedness. The KNOB has also been neuronatomically described in chimpanzees and other great apes and is similarly associated with handedness. However, whether the chimpanzee KNOB represents the hand region is unclear from the extant literature. Here, we used PET to quantify neural metabolic activity in chimpanzees when engaged in unilateral reach-and-grasping responses and found significantly lateralized activation of the KNOB region in the hemisphere contralateral to the hand used by the chimpanzees. We subsequently constructed a probabilistic map of the KNOB region in chimpanzees in order to assess the overlap in consistency in the anatomical landmarks of the KNOB with the functional maps generated from the PET analysis. We found significant overlap in the anatomical and functional voxels comprising the KNOB region, suggesting that the KNOB does correspond to the hand region in chimpanzees. Lastly, from the probabilistic maps, we compared right- and left-handed chimpanzees on lateralization in grey and white matter within the KNOB region and found that asymmetries in white matter of the KNOB region were larger in the hemisphere contralateral to the preferred hand. These results suggest that neuroanatomical asymmetries in the KNOB likely reflect changes in connectivity in primary motor cortex that are experience dependent in chimpanzees and possibly humans.

Observer-independent characterization of sulcal landmarks and depth asymmetry in the central sulcus of the chimpanzee brain

The central sulcus (CS) divides primary motor and sensory cortex in many mammalian brains. Recent studies have shown that experiential factors can influence the volume and lateralization of the CS in both human and nonhuman primates. In this study, we sought to define specific landmarks and the depth of the CS region corresponding to the motor-hand area of chimpanzees for comparison with humans using a novel, observer independent method applied to sample of 32 magnetic resonance images (MRI) scans. Our results showed that the dorsal-ventral location of the motor-hand region is comparable between humans and chimpanzees, though the depth of the CS was significantly greater in humans compared to chimpanzees. We further found that CS area corresponding to the motor-hand area was significantly larger in the hemisphere contralateral to the chimpanzees preferred hand. The methods employed here offer some potential advantages over traditional region-of-interest in the comparative study of cortical organization and gyrification in primates and are discussed.

Volumetric and lateralized differences in selected brain regions of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus)

The two species of Pan, bonobos and common chimpanzees, have been reported to have different social organization, cognitive and linguistic abilities and motor skill, despite their close biological relationship. Here, we examined whether bonobos and chimpanzee differ in selected brain regions that may map to these different social and cognitive abilities. Eight chimpanzees and eight bonobos matched on age, sex and rearing experiences were magnetic resonance images scanned and volumetric measures were obtained for the whole brain, cerebellum, striatum, motor-hand area, hippocampus, inferior frontal gyrus and planum temporale. Chimpanzees had significantly larger cerebellum and borderline significantly larger hippocampus and putamen, after adjusting for brain size, compared with bonobos. Bonobos showed greater leftward asymmetries in the striatum and motor-hand area compared with chimpanzees. No significant differences in either the volume or lateralization for the so-called language homologs were found between species. The results suggest that the two species of Pan are quite similar neurologically, though some volumetric and lateralized differences may reflect inherent differences in social organization, cognition and motor skills.

New morphologic variants of the hand motor cortex as seen with MR imaging in a large study population

BACKGROUND AND PURPOSE

The hand motor cortex (HMC) has been classically described as having an omega or epsilon shape in axial-plane images obtained with CT and MR imaging. The aim of this study was to use MR imaging and Talairach normalization in a large sample population that was homogeneous for age and handedness to evaluate in a sex model a new classification with 5 morphologic variants of the HMC in the axial plane (omega, medially asymmetric epsilon, epsilon, laterally asymmetric epsilon, and null).

MATERIALS AND METHODS

Structural brain MR images were obtained from 257 right-handed healthy subjects (143 men and 114 women; mean age, 23.1 +/- 1.1 years) via a Talairach space transformed 3D magnetization-prepared rapid acquisition of gradient echo sequence. The frequencies of the different HMC variants were reported for hemisphere and sex.

RESULTS

The new variants of the HMC (medially asymmetric epsilon, laterally asymmetric epsilon, and null) were observed in 2.9%, 7.0%, and 1.8% of the hemispheres, respectively. Statistically significant sex differences were observed: The epsilon variant was twice as frequent in men, and an interhemispheric concordance for morphologic variants was observed only for women.

CONCLUSION

The large study population permitted the description of a new morphologic classification that included 3 new variants of the HMC. This new morphologic classification should facilitate the identification of the precentral gyrus in subsequent studies and in everyday practice.

Histological asymmetries of primary motor cortex predict handedness in chimpanzees (Pan troglodytes)

Like humans, chimpanzees display robust and consistent hand preferences during the performance of certain tasks. Although correlations have been demonstrated between gross anatomic measures of primary motor cortex asymmetry and handedness in captive chimpanzees, the relationship between histological architecture and behavioral lateralization has not yet been investigated. Therefore, we examined interhemispheric asymmetry of several different microstructural characteristics of the primary motor cortex in the region of hand representation from 18 chimpanzees tested on a coordinated bimanual task before death. At the population level our data showed leftward bias for higher layer II/III neuron density. Of note, however, there was no population-level asymmetry in the areal fraction of Nissl-stained cell bodies, a finding that differs from previous studies of this cortical region in humans. Nonetheless, we found that asymmetry in the density of layer II/III parvalbumin-immunoreactive interneurons was the best predictor of individual hand preference. These results suggest that histological asymmetries are related to handedness in chimpanzees, while overall patterns of asymmetry at the population level might differ from humans.

Behavioral and neuroanatomical correlates of white matter asymmetries in chimpanzees (Pan troglodytes)

Although behavioral and brain asymmetries have been documented in non-human primates, lateralization in cortical connectivity as reflected in white matter has not been described in any species, despite the intrinsic theoretical interest in white matter expansion during primate brain evolution. Here we report evidence of population-level leftward asymmetries in the white matter of chimpanzees. We further report that lateralization in white matter correlates with their handedness as well as neuroanatomical asymmetries in the precentral gyrus. These findings suggest that chimpanzees show asymmetries in cortical connectivity and these may serve as the foundation for morphological and behavioral laterality in primates, including humans.

Correlated activity of sensorimotor cortex neurons in the left and right hemispheres of the rabbit brain in immobilization catatonia

Spike sequences extracted from multineuron activity from neurons in the sensorimotor cortex, and recorded simultaneously in the left and right hemispheres of the brains of rabbits in the state of immobilization catatonia ("animal hypnosis") and on recovery of animals from this state were analyzed. Cross-correlation analysis of spike flows revealed a temporal relationship between the appearance of neuron spikes in the left and right hemispheres; these were regarded as the mutual influences of these neurons on each other. The intensity of the influences of left hemisphere neurons on cells in the right brain was shown to change significantly in relation to baseline measures at all stages of the experiment and at all of the time points studied. The intensity of the influences of neurons in the right hemisphere on cells in the left hemisphere changed significantly only after animals recovered from the state of immobilization and over much more restricted time periods.

Further evidence of an association between handedness and neuroanatomical asymmetries in the primary motor cortex of chimpanzees (Pan troglodytes)

The neurobiology of handedness is still poorly understood in nonhuman primates. Recently, an association between hand preference and precentral gyrus morphology in chimpanzees was reported. The aim of this study was to further evaluate the association between handedness and asymmetries in the precentral gyrus of chimpanzees (Pan troglodytes) and to evaluate the association between hand preference and brain asymmetry using a different approach to the classification of handedness in chimpanzees. The overall results suggest that differences in handedness groups are specific to a region of the precentral gyrus commonly known as the "knob" and that subjects that show different hand preferences differ in brain asymmetries for specific regions of the primary motor cortex. Moreover, using a continuous scale of measurement rather than discrete classification of handedness, significant associations were found between hand use and asymmetries within the precentral gyrus.

Handedness in chimpanzees (Pan troglodytes) is associated with asymmetries of the primary motor cortex but not with homologous language areas

The neurobiology of hand preferences in nonhuman primates is poorly understood. In this study, the authors report the first evidence of an association between hand preference and precentral gyrus-morphology in chimpanzees (Pan troglodytes). Hand preferences did not significantly correlate with other asymmetric brain regions associated with language functions in humans including the planum temporale and frontal operculum. The overall results suggest that homologous regions of the motor cortex control hand preferences in humans and apes and that these functions evolved independently of left-hemisphere specialization for language and speech.

Neurochemical organization of chimpanzee inferior pulvinar complex

The pulvinar of primates, which connects with all visual areas, has been implicated in visual attention and in control of eye movements. Recently, five separate neurochemical subdivisions of a region termed the inferior pulvinar complex have been identified in monkeys (Gray et al. [1999] J Comp Neurol 409:452-468; Gutierrez et al. [1995] J Comp Neurol 363:545-562), and comparable subdivisions have been mapped in humans (Cola et al. [1999] NeuroReport 10:3733-3738). In the present study, we investigated the inferior pulvinar of the chimpanzee (Pan troglodytes), the closest evolutionary relative of humans, using cytochrome oxidase (CO) and acetylcholinesterase (AChE) histochemistry, and immunocytochemistry for calbindin. Each staining method demarcated five histochemical zones corresponding, from medial to lateral, to the posterior (PI(P)), medial (PI(M)), central PI(C)), lateral (PI(L)), and the lateral-shell (PI(L-S)) divisions in monkeys. The PI(P) division stained darkly for calbindin and lightly for CO and AChE. The PI(M) division was characterized by less neuropil staining for calbindin, and by distinct, intensely stained patches of CO and AChE. PI(C) appeared lighter than adjacent divisions with CO and AChE histochemistry and was moderately stained with calbindin. PI(L) was moderately to darkly stained with each method and was adjoined by a lighter staining shell, PI(L-S). Thus, in the aspects of organization we examined, the inferior pulvinar of chimpanzees closely resembles that of humans and monkeys. This investigation provides a foundation for more detailed studies of the thalamic relationships of extrastriate cortex in apes and humans.

Asymmetries in the hippocampus and amygdala of chimpanzees (Pan troglodytes)

Magnetic resonance imaging was used to measure the hippocampal and amygdalar volumes of 60 chimpanzees (Pan troglodytes). An asymmetry quotient (AQ) was then used to calculate the asymmetry for each of the structures. A one-sample t test indicated that there was a population-level right hemisphere asymmetry for the hippocampus. There was no significant population-level asymmetry for the amygdala. An analysis of variance using sex and rearing history as between-group variables showed no significant main effects or interaction effects on the AQ scores; however, males were more strongly lateralized than females. Several of these findings are consistent with results found in the human literature.

Further evidence of a right hand advantage in motor skill by chimpanzees (Pan troglodytes)

Asymmetries in motor skill when grasping small food items was evaluated in a sample of captive chimpanzees. In two experiments, error rates in grasping food were assessed when controlling for individual differences in grip morphology. In both experiments, chimpanzees were found to make more errors with the left compared to the right hand. Male chimpanzees were also found to make more errors compared to females. These results are discussed in the context of a presumed disassociation between hand preference and performance as indicators of cerebral dominance in motor functions.

Comparative assessment of handedness for a coordinated bimanual task in chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus)

Hand preferences for a coordinated bimanual task were assessed in a sample of 31 captive gorillas (Gorilla gorilla) and 19 captive orangutans (Pongo pygmaeus) and were compared with chimpanzee (Pan troglodytes) hand preferences in subjects that were matched on the basis of age, sex, and rearing history. The task required that the apes remove food from the inside edges of a symmetrical polyvinyl chloride pipe presented to them in their home cages. The results indicate significant species differences with chimpanzees showing population-level right-handedness and orangutans showing population-level left-handedness. The gorillas showed a nonsignificant trend toward right-handedness. The results are discussed in terms of possible ecological or biomechanical factors that may influence hand preferences in different ape species.

"Right-Trunkers" and "Left-Trunkers": Side Preferences of Trunk Movements in Wild Asian Elephants (Elephas maximus)

In this article, the side preferences of feeding-related trunk movements of free-ranging Asian elephants (Elephas maximus) were investigated for the first time. It is hypothesized that a functional asymmetry of the trunk is necessary to perform skillful feeding movements more efficiently. This might be connected with a corresponding hemispheric specialization. Video recordings of 41 wild elephants provided frequencies and durations of the following trunk-movement categories: object contact, retrieval, and reaching. In each category, individual side preferences were found. The strength of side preferences varied between the trunk-movement categories and the sexes. Mean durations of retrieval and reaching correlated negatively with the strength of side biases. Comparing the side preferences in the unpaired trunk with analogous phenomena in other unpaired grasping organs and in primate handedness. the authors discuss possible explanations for the evolution of asymmetries in unpaired grasping organs.

Grip morphology and hand use in chimpanzees (Pan troglodytes): evidence of a left hemisphere specialization in motor skill

Three experiments on grip morphology and hand use were conducted in a sample of chimpanzees. In Experiment 1, grip morphology when grasping food items was recorded, and it was found that subjects who adopted a precision grip were more right-handed than chimpanzees using other grips. In Experiment 2, the effect of food type on grasping was assessed. Smaller food items elicited significantly more precision grips for the right hand. In Experiment 3, error rates in grasping foods were compared between the left and right hands. Significantly more errors were made for the left compared with the right hand. The cumulative results indicate that chimpanzees show a left-hemisphere asymmetry in motor skill that is associated with the use of precision grips.