Hopefully, this review will provide a comprehensive summary of anatomo-functional relationships in the cortical gyro-sulcal system together with a consideration of how these contribute to brain function, cognition, and behavior, as well as to mental disorders. Cortical folding is a hallmark of brain topography whose variability across individuals and groups is strongly coupled with inter-individual and inter-group differences in brain function, cognition, and behavior.
Gyri and sulci are the standard nomenclature Fig. The anatomical and functional relationships between gyri and sulci have not been clearly elucidated.
An increasing number of studies over the last decade have therefore investigated, at a finer resolution, whether these two elementary anatomical units could serve as building blocks of cortical morphology and brain function, and whether they could be functionally and cognitively differentiated e. Uddin et al. Gyri has larger average convexity than sulci.
It has been demonstrated repeatedly that the local gyro-sulcal morphology of many cortical regions could predict the location of functional activity in adult human brains Amiez et al. For instance, the locations of visuomotor eye and hand conditional activity are localized in different sub-regions within the superior precentral sulcus Amiez et al.
The retinotopic organization of striate cortex can be predicted by surface topology Benson et al. The functional activities of different action controls are localized in different sulci in the posterior lateral frontal cortex Amiez and Petrides, and different value signals are localized in different sulcal regions of the orbitofrontal cortex Li et al. Associations between gyro-sulcal patterns and human behavior have also been demonstrated to investigate brain-behavior relationships as well as neuropsychological correlates of inter-participant cortical folding variations Fornito et al.
For instance, inter-participant differences in paracingulate sulcus asymmetry in healthy males have been shown to be related to individual differences in executive abilities in cognitively demanding tasks Fornito et al.
The leftward anterior cingulate cortex asymmetry pattern of male early adolescents is associated with individual differences in temperament Whittle et al. Adults whose left lateral occipito-temporal sulcus were interrupted by the gyrus in the posterior portion showed better reading fluency than those who had a continuous one, and the greater the interruption, the better was their performance Cachia et al. Another study reported more gyral than sulcal regions within the middle frontal cortex, inferior parietal lobe, and visual cortex, for which rs-fMRI signal temporal variability was positively correlated with fluid intelligence Yang et al.
In addition to studies on adults, one on newborns has reported that there were specific morphological characteristic differences between twins and singletons, as well as between normally developed newborns and those with intrauterine growth restriction. To measure brain functional activity, functional magnetic resonance imaging fMRI , rather than other techniques including electroencephalography EEG , magnetoencephalography MEG , and functional near infrared spectroscopy fNIRS , has been predominantly adopted for gyro-sulcal functional characteristic studies.
Gyro-sulcal functional characteristics have been extensively studied during performance of different tasks based on task-based fMRI t-fMRI Logothetis, ; Friston, ; Barch et al. The HCP grayordinate data consists of both cortical surface vertices and subcortical voxels of gray matter in the standard MNI space, which correspond closely across different individual brains to facilitate group-wise analysis Glasser et al.
In this review, our focus will be on fMRI-based studies since reported findings are substantially more extensive and better established. In previous studies, gyri and sulci are usually treated as morphological and anatomical nomenclature, and usually defined based on morphological features Li G. For example, in FreeSurfer Fischl, , a mid-surface is defined such that the displacements of all vertices from their original locations to it are summed to zero.
Usually, gyral vertices have negative values larger average convexity and sulcal vertices have positive values Fig. An alternative approach is labeling sulci and gyri via matching individual images to a gyro-sulcal atlas. Although this anatomy—function relation is less pronounced in secondary and tertiary cortices across participants due to great inter-individual variabilities in folding geometry Fischl et al.
It is worth noting that biological definition and delineation of gyral and sulcal regions, as well as the transition area of gyral wall, warrants extensive studies in the future. Most of these registration methods were traditionally designed to achieve automatic volumetric alignment but do not constrain cortices to align Talairach and Szikla, ; Woods et al.
With an increasing interest in cortex, many recent registration approaches have been performed on the entire cortical surface, rather than volumes, to align shape metrics such as curvature and sulcal depth Fischl et al.
These methods have thus demonstrated achievement of direct improvement of the cross-subject correspondence for landmarks Desai et al. However, such a relationship is variable in more developed higher-order cortices Ono et al.
Therefore, a fundamental improvement could possibly be achieved by exploring a more precise and higher-resolution mapping between folding patterns and brain cytoarchitecture or brain functions. The developing brain undergoes a number of complicated and inter-related neuronal processes Smart et al. In particular, the abundance of basal radial glia cells bRGCs in the outer subventricular zone oSVZ is suggested to be tangentially heterogeneous Borrell and Gotz, , such that an increase in bRGC number in local areas gives rise to the fanning-out of a radial fiber scaffold and dispersion of radially migrating neurons, leading to a gyrus-like structure in the corresponding locations in the cortical plate while the germinal layers remain unfolded Fietz et al.
The regulation of basal progenitor heterogeneity in the fetal phase persists in the mature stage. However, it is still unclear whether the timing of abundance of progenitors in oSVZ is concordant with that of gyrogenesis Kroenke and Bayly, , because the cerebrum is still lissencephalic when neurogenesis is complete around embryonic day E 90 Rash et al. In this regard, other processes, such as gliogenesis, axonogenesis, and myelinogenesis are also correlated with the development and growth of cortical folding Chi et al.
Nevertheless, the contribution of gliogenesis alone may be insufficient to induce cortical folding and may additionally need to be considered jointly considered with axonogenesis Rash et al. Axonogenesis has been relatively better studied and is more closely related to cortical folding, not only due to its concomitant occurrence with gyrification Huang et al. The heterogeneity in neuron distribution Hiletag and Barbas, and axon distribution Huang et al.
Within the cortex, somata and arbors of pyramidal neurons are stretched in gyri and compressed in sulci due to the heterogeneity in neuron numbers Hiletag and Barbas, , resulting in different lengths of apical segments. Identical neurons with different lengths of apical segments have been demonstrated to produce different types of action potentials Fig. Thus, differences in dendrite morphology between sulci and gyri result in putative functional differences in attenuation of excitatory postsynaptic potentials, and possibly provide a fundamental explanation for gyro-sulcal differences in fMRI signal time-frequency characteristics outlined in the section of 'Gyro-sulcal functional activity time-frequency differences'.
A A low magnification overview of coronal sections through 8AD area 8 of cortex, anterodorsal part and 8B area 8B of cortex in the rhesus monkey prefrontal cortex. Arrowheads highlight the neurons of typical morphologies.
Elongated dendritic arbors and cell bodies are observed in A1 and A2 while tangentially oriented dendrites and flattened cell bodies are observed in A3. B A low magnification overview of coronal section through 8AD and 8B.
Weil's stain was applied to the tissue, where the myelin sheaths are stained dark blue. B1 — B3 High magnification views of boundaries between white matters and gray matters at gyral crest and gyral wall in 8AD, and sulcal fundus in 8B. Arrowheads in B1 and B2 highlight the myelin sheaths that radially oriented across the boundaries. Such myelin sheaths are barely seen in B3 , where myelin sheaths tangentially course along the sulcal fundus.
C White matter axonal wiring patterns between gyri and sulci. Left column: maximum principal curvature CUR and diffusion tensor imaging DTI derived white matter fiber density FD were mapped to the cortical surface as references. Line thickness indicates the connection strength. Lines between ROI as well as the colors indicate the presence of connections and their strength.
These figures in the middle and the right panels were adapted from Deng et al. Right: comparison between gyri and sulci regarding the metrics in C and D. These figures are adapted from Zhang et al. Axonal transmission of action potentials is a key component of communication between neurons and a variety of theories have been proposed to interpret the contribution of axons to gyro-sulcal patterns and their related functional architecture Van Essen, ; Xu et al.
In Van Essen, , axons were found to be under a form of tension and assumed to pull cortical regions closer to each other. The cortical regions being pulled became gyral walls or sulci, leaving the other interleaved cortical regions with weaker axonal connections as gyri. Although tension along axons has been demonstrated by microdissection, the axons, as shown in Fig. Such a spatial relation between axons and gyro-sulcal patterns, which has gained support from both microscale Budde and Annese, and macroscale observations Nie et al.
Despite the spatial relation argument, there might nevertheless be a consensus that axonal wiring patterns are different between gyri and sulci. Also, axonogenesis is unlikely to be the only driving force for cortical folding since other neuronal maturation and mechanical processes could make significant contributions Bayly et al.
More importantly, the different axonal wiring patterns between gyri and sulci suggest that they may play different functional roles Van Essen, ; Deng et al. According to parsimonious principles of brain development Chklovskii et al. However, the cortico-cortical axon wiring diagram in this theory seems to consist of repeated units: cortical pairs connected by axons. If the axon-tension solution for wiring optimization is applied to all units, however, there would only be short-range connections over the entire cortex.
In fact, many studies have reported the presence of considerable number of long-range axons, and that the nature of the axon wiring diagram is a "small-world" or "rich-world" network rather than a composition of repeated units linked by local short-range axons Watts and Strogatz, ; He et al.
Brain networks allow the presence of long-range connections between gyri Xu et al. Regarding cortical folding patterns, long-range gyro-gyral white matter connections have been demonstrated to be stronger than gyro-sulcal and sulco-sulcal ones Fig. These long-range gyro-gyral connections introduce a higher wiring cost but this can be offset by their higher capacity as shortcuts in reducing communication steps and upgrading global integration in brain networks Kaise and Hilgetag, In this sense, denser gyro-gyral long-range connections might endow gyri with a plausible role as "hubs" of a structural network, which has been demonstrated in Zhang et al.
The complexity of cortical convolution increases generally along the phylogenetic tree and is usually related to species intelligence Roth and Dicke, Thus, identifying species-preserved and -specific anatomy and function is a unique and important approach to understanding the development of cortical folding, as well as its relation to structural and functional architectures Orban et al.
By limiting our interests to gyro-sulcal patterns, we have found that denser long-range axons between gyri in the section of 'Neuronal processes and gyro-sulcal functional differentiation' are preserved across humans and other primate species such as chimpanzees, macaques, and marmosets Nie et al. In addition, cross-species comparative studies have revealed that U-shaped short-range fibers coursing along sulcal fundi comprise a larger portion of the human brain relative to that of chimpanzees and macaques Zhang et al.
In Chen et al. These observations have led to a hypothesis that gyral folding and fiber connection patterns "co-evolve" in primate brainsl Chen et al. This co-evolution of gyral folding and fiber connection patterns is also related to the evolution of brain function. For example, de Schotten et al.
A graph analysis in Zhang et al. The human brain is considered to be a multi-frequency oscillation system Deco et al. While the conventional temporal resolution of fMRI with a repetition time TR of 2 s corresponding to a sampling frequency of 0. Investigating the potential time-frequency differences between gyro-sulcal fMRI signals has therefore received increasing interest.
The power spectrum distribution characteristics across different frequency bands, as well as other measures e. With the rapid development of deep learning techniques LeCun et al. Liu et al. In another study Zhang et al. Macaque rs-fMRI data was also used for cross-species studies Zhang et al. In a recent study Ge et al.
Finally, a more recent study Jiang et al. Moreover, as shown in Fig. A recent study Jiang et al. Summary of gyro-sulcal functional differences.
A Gyro-sulcal functional activity time-frequency difference. The average power spectrum of gyral and sulcal temporal patterns is shown in the seven t-fMRI data sets from the HCP, which is adapted from Liu et al.
The left sub-figure shows the gyro-sulcal functional connectivity black edges within the primary motor and somatosensory system.
Thicker edges represent stronger connections. The middle sub-figure shows the gyro-sulcal functional connectivity patterns of one example participant. The edges are colored based on the functional connectivity strength. The left and middle sub-figures are adapted from Deng et al. The right sub-figure shows the graph-theory-based property difference of gyro-sulcal functional connectivity, which is adapted from Liu et al.
C Gyro-sulcal functional network spatial overlap pattern difference. The static gyro-sulcal overlap pattern difference across multiple tasks is adapted from Jiang et al. The temporal dynamics of gyro-sulcal overlap pattern difference in Emotion task from the HCP is adapted from Jiang et al. D Gyro-sulcal functional signal representation accuracy difference. Functional connectivity FC is used to characterize functional integration and is defined as statistical dependencies among remote brain regions Friston, A larger coefficient value represents stronger FC, and vice versa.
Deng et al. They first focused on the primary motor and somatosensory systems as a test bed example, since these systems are proven to be structurally and functionally connected Kandel et al. It has been found that within the primary motor and somatosensory systems, including four gyral regions bilateral precentral gyrus and postcentral gyrus and four sulcal regions bilateral central sulcus and postcentral sulcus , the rs-FC strength is strong between gyro-gyral regions, moderate between gyro-sulcal regions, and weak between sulco-sulcal regions Fig.
In another study Liu et al. The FC dynamic temporal variability within or between brain regions has been extensively demonstrated Gilbert and Sigman, ; Chang and Glover, ; Smith et al. In Yang et al. It has been found that the dynamic temporal variance of the rs-fMRI signal was larger in gyral than in sulcal regions in the whole brain Yang et al.
Instead of assessing the FC between gyro-sulcal regions directly Deng et al. It has been found that the angular gyrus and the intraparietal sulcus, which are a part of the inferior parietal lobule, have differential rs-FCs with other brain regions Uddin et al.
Other measures have also been used to characterize functional interaction by quantifying the temporal correlation of fMRI signals. Li et al. The functional interaction strength was defined as the locations where both signals shared high co-power in the time-frequency domain, and the gyro-sulcal difference was then obtained Li et al. Second, certain graph theory-based properties such as edge degree, global efficiency, modularity, small-worldness, etc. It has been found that the resting state functional interaction network composed of gyral regions has higher global and local economical properties, and stronger small-worldness and functional interaction strength than the sulcal network Li et al.
Within the rs-FC network, the graph edge degree of the gyral regions is significantly higher than that of the sulcal regions Deng et al. In terms of modularity property, the sulcal network is significantly higher than the gyral one in t-fMRI but not rs-fMRI data Liu et al.
The functional network spatial overlap pattern is defined as the set of brain regions that are spatially overlapped and involved in multiple brain networks. Previous studies have demonstrated extensively that there are multiple spatially overlapped brain functional networks that are interacting with each other Bullmore and Sporns, ; Lv et al. The functional network spatial overlap pattern thus represents the higher-order brain regions that are involved in multiple cognitive tasks.
Using sparse coding approaches, Jiang et al. It has been found that both task-evoked and intrinsic connectivity functional networks can be effectively and robustly identified across the seven different t-fMRI of HCP grayordinate data Jiang et al.
Moreover, the THFRs are located more on gyral than on sulcal regions within the bilateral parietal lobe, frontal lobe, and visual association cortices across different t-fMRI data Fig. To further investigate the temporal dynamics of THFRs [i. It has been shown that the SOPFNs are also located more on gyral than on sulcal regions within bilateral parietal lobe, frontal lobe, and visual association cortices within each time window, and it is of interest that there is variability in the SOPFNs distribution percentage between gyri and sulci across different time windows Fig.
Specifically, the SOPFNs distribution percentage value of gyral regions was increased and achieved a peak value during the task blocks of block design t-fMRI, while that of sulcal regions was decreased and reached a trough value during the task blocks.
When the task paradigm was changed from a task block to a resting state block, the SOPFNs distribution percentage was decreased from the peak for gyral regions and increased from the trough for sulcal regions, and achieved relatively the same percentage before starting the next task block Jiang et al. Since sparse coding approaches can compactly represent the original fMRI BOLD signals using the identified small number of basis functional temporal patterns and corresponding functional network patterns, a larger PCC value indicates better representation of x i as well as better participation of x i in the neural function during a specific task.
It has been found that during the emotion processing, gyral regions have better signal representation accuracy than sulcal regions within the nine intrinsic functional networks. In another recent study Zhao et al. Inversely, a smaller value of the residual indicates better representation of x i. The signal representation accuracy was further assessed and compared between gyral and sulcal regions within the nine intrinsic functional networks under both the resting state and seven different assessment tasks including assessment of emotions, gambling, language, motor skills, relational skills, social skills, and working memory from the HCP.
It has been found that gyral regions consistently have better signal representation accuracy than sulcal regions within all nine intrinsic functional networks across all seven different t-fMRI of HCP grayordinate data. Specifically, in the resting state and "low-demand" tasks such as motor tasks, the gyro-sulcal signal representation accuracy contrast values are lowest and more consistent within the nine intrinsic networks as well as the whole-brain level across different participants.
Moreover, the gyro-sulcal signal representation accuracy contrast is more pronounced within the default mode and executive control networks compared to the other intrinsic functional networks in all seven tasks, while it is the lowest within the sensorimotor network across all tasks Zhao et al. We have previously proposed a functional model of gyri and sulci at the whole-brain level Deng et al.
Proposed functional models of gyri and sulci in previous studies. Each ROI consists of gyri and sulci. D The proposed functional model of gyri and sulci within functional networks. However, this model does not explicitly explain how the interplay between gyri and sulci or their resulting dynamic interactions occurs. In this review, we would therefore like to propose a dynamic functional model of gyri and sulci as illustrated in Fig.
The gyro-sulcal functional differences summarized in the section of 'Gyro-Sulcal Functional Differences from Various Perspectives' and Figs 3 and 4 from various perspectives are the foundation of this dynamic model. In the dynamic model in Fig. The SOPFNs distribution percentage of gyri begins to increase and achieves a peak value analogous to an activator since gyri are hypothesized to be the global functional center, while that of sulci begins to be inhibited to decrease and reach a trough value Fig.
When the task load is removed and the next resting state block starts, the excitatory and inhibitory effects vanish. The SOPFNs distribution percentage of gyri begins to drop from the peak point, while that of sulci begins to increase from the trough, until achieving relatively the same SOPFNs distribution percentage by the end of the resting state block Fig.
Such a gyro-sulcal dynamic balancing procedure then repeats when the next task block starts Jiang et al. The excitatory and inhibitory effects can be regulated by strong gyro-gyral functional connections and weak gyro-sulcal ones, analogous to an activator-inhibitor regulation system Turing, ; Meinhardt and Gierer, ; Nakamasu et al.
A and B An illustration of the dynamic functional model of gyri and sulci, at resting state A and during task performance B. Two ROI as well as their connections are used to represent the whole-brain network.
Areas of the gyri and sulci within each ROI illustrate the degree of involvement in global networks. The thickness of the double-head arrows illustrate the excitatory thick and inhibitory thin effects, putatively regulated by functional connections. C The degrees of involvement in global networks of gyri red and sulci blue during a task performance Jiang et al.
As a substrate for the dynamic model, long-range connections and short-range connections are needed to coincide in the axonal system. We have discussed the presence of long-range association projections in the section of 'Neuronal processes and gyro-sulcal functional differentiation', which are usually radial to the cortical surface of two remote gyri. The presence of a short-range axonal system, the connections of which are between two neighboring gyri and superficial to and parallel with the cortical surface, has also been demonstrated Bullmore and Sporns, ; Sepulcre et al.
Although the regulation mechanisms of long- and short-range axons in the dynamic model are still far from being clear, some studies have shown some progress toward achieving this goal.
The functional network segregation is suggested to be driven by the abundance of short-range axons Cao et al. A precise balance between the decreased short-range axon numbers and increased long-range axon numbers from 2 to 7 years in humans Ouyang et al. A disturbance of this balance could result in brain disorders, such as hyperconnectivity for ASD and hypoconnectivity for schizophrenia SZ Ouyang et al. It is noteworthy that the excitatory and inhibitory effects might be implemented by the diffusion of functional gradients, as suggested in activator-inhibitor models Turing, ; Meinhardt and Gierer, ; Nakamasu et al.
In fact, a global gradient has been observed in the cortex from a variety of perspectives ranging from genes to behavior Huntenburg et al. Similarly, a gyro-sulcal gradient could also exist, although it might be overwhelmed by the global one. Indeed, a gradient between gyro-sulcal functional differences has been evidenced. But with this article, then you would not be in the dark anymore whenever you encounter these terms being used. This is just to give a little background on the subject matter.
The brain is considered as the command center of our body. It handles all of our movements, senses, emotions, and bodily processes. It is true though that there are still lot of mysteries about our brain that needs to be unraveled, but what is already a common fact is that almost all the parts of our brain have already been extensively studied and named. Simply speaking, almost every part of the brain is now widely researched, every lobe identified for its general function, and even its shape explained.
Now I ask you this, have you ever seen a brain close hand, or even, just a well-illustrated picture of it? For those who are in line with the medical profession, then I am quite confident that they already are oriented to the brain and its anatomy. Furthermore, even those who are not in the medical profession already have a vague idea of how the brain looks like.
The very least that you can discern about its feature is that it has a lot of curves and ridges, with prominent depressions. And, these are what you call as gyrus and sulcus. But there is a difference between the two. Gyrus, or its plural term gyri, is the term used for the prominent raise or outward fold in the brain. Picture the raised ridges in your mind. The convolutions on the surface of the cerebral cortex are called gyri singular term is gyrus.
Each gyrus of the cerebral cortex has a name and is often used in neurolinguistic descriptions. They are named mainly according to their location. For example, gyri located in the frontal lobe are named as superior frontal gyrus, middle frontal gyrus and inferior frontal gyrus, whereas the gyri found in temporal lobe are denoted as superior and middle temporal gyrus.
Occipital lobe has superior and inferior occipital gyrus. Sulci singular term is sulcus are often referred to as fissures on the cortex surface. They are found in between gyri. Superior temporal sulcus is found in between the superior and middle temporal gyri. Inferior temporal sulcus separates the middle and inferior temporal gyri.
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