In a reciprocal fashion, the superior and inferior longitudinal and frontal occipital fasciculi convey information from the relatively distant cortical regions connected to this complex cortical-subcortical association [ 6 ]. A simplified schematic diagram showing the direct pathway of the dorsolateral circuitry is depicted in Figure 1. Basic schematic diagram of the direct pathway of the dorsolateral frontal circuitry adapted from . When considering the orbitofrontal circuit OFC , this performs the function of modulating adequate social behavior and is fundamental for maintaining empathy.
Disruptions in the pathways of this system lead to certain neuropsychiatric manifestations such as impulsivity, emotional lability, personality changes, explosive behavior, and lack of interpersonal sensitivity. Akin to the dorsolateral circuit, which contains neuronal cell bodies situated in Brodmann areas 9 and 10 dorsolateral portion , the orbitofrontal circuit starts and ends in Brodmann areas 10 inferomedial portion and The axonal projections from these areas run to the ventromedial portion of the caudate nucleus, where they diverge into the direct and indirect pathways.
The direct pathway enters the medial portion of the dorsomedial external and internal globus pallidi and the medial rostral region of the pars reticulata of the substantia nigra, where they continue to the anteroventral and dorsomedial portion of the thalamus, subsequently returning to the PFC Figure 2. The indirect pathway of the orbitofrontal circuit performs a modulatory function through its connection to the dorsal region of the external globus pallidus and to the lateral subthalamic nucleus, prior to projection to the loops of the direct pathway via internal globus pallidus and substantia nigra.
The indirect pathway of the OFC modulates the direct pathway through the connection to the dorsal region of the external globus pallidus and the lateral subthalamic nucleus. The indirect pathway of the orbitofrontal circuit is believed to run parallel to the indirect pathway of the dorsolateral circuit [ 9 , 11 ]. Externally to this circuitry, the lateral portion of the OFC receives afferences mainly from the temporal pole, cerebral amygdala, and ventral tegmental area.
In this case, the connections to the distal collateral areas are also reciprocal, as occurs with similar integrative pathways in the dorsolateral circuitry [ 9 , 12 ]. The circuitry involving the anterior cingulate regulates motivation by modulating inhibitory input in the supplemental motor area, through its own stimuli which maintain wakefulness and alertness states.
The most evident deficits of interruptions of any of the circuits situated in the prefrontal cortex are related with bilateral lesions of the anterior cingulate. Under these conditions, the principal clinical manifestations described are akinetic mutism, apathy, abulia, urinary incontinence, and lack of expressiveness to sensory stimuli. The subcortical connections of the anterior cingulate circuit are constituted by fibers that connect to the ventral striatum more specifically the ventromedial portions of the caudate nucleus and ventral portion of the putamen , the nucleus accumbens, and to the olfactory tubercle.
From these structures, the circuit projects to the ventral and rostromedial globus pallidus and to the dorsomedial thalamic nucleus, subsequently returning to the anterior cingulate cortex. The indirect pathway of the anterior cingulate circuit is thought to connect to the external globus pallidus and to the medial subthalamic nucleus before reentering the circuit of the direct pathway Figure 3 via the internal globus pallidus.
The reciprocal integration of the pathways with the structures situated externally to the circuit occurs through connections of the ventral striatum with the hippocampus, amygdala, and the entorhinal and perirhinal cortices [ 10 ]. Finally, it is noteworthy that besides the three main circuits of the frontal subcortical neuronal network, some authors have cited further two circuits, namely the inferior temporal cortical circuit ITCC and a circuit situated between the posterior parietal region Brodmann area 7 and the prefrontal region Brodmann area Reports associate lesions involving the ITCC with psychosis, deficits in visual discrimination, and visual hallucinations and cite that damage to the circuit between Brodmann areas 7 and 46 is associated with impaired interpretation of visuospatial stimuli [ 6 ].
Some of these changes include grouping, grasping, perseveration, snouting, imitation and utilization behavior, palmomental reflex, and persistent glabellar tap reflex [ 13 , 14 ]. The previous subsection of this chapter outlined the classic knowledge on the functional neuroanatomy of the neuronal circuits of the frontal lobe, along with the main neurological and psychiatric manifestations resulting from interruption in functioning of these specific pathways.
The aim of this section is to delve deeper into the connectionist theory of brain function, which is key to understanding the physiopathogeny of disconnection syndromes.
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This requires outlining the main pathways involved in the connection of structures encompassing the frontal lobe and beyond. As described earlier, disconnection syndrome is defined as the group of clinical manifestations secondary to lesions to white matter or to the association cortices, where the latter acts as a relay station between the primary motor cortex, sensory areas, and limbic system [ 1 ]. In recent years, publications have reported clinical cases with neuropsychiatric manifestations hitherto attributed to certain lesions to specific cortical topographies. However, complementary investigation using neuroimaging has disclosed changes in other brain sites [ 15 , 16 ].
A number of authors have stated that this phenomenon can be explained by disruption of the subcortical associative pathways involved in the neuronal network integrating the higher cortical functions [ 1 ]. Some of these fasciculi are described in more detail below Figure 4.
Frontal Lobes: Motor Cortex, Cognition, and Speech
Basic schematic diagram of the direct pathway of the anterior cingulate circuitry adapted from . Among the MF, the corpus callosum is important for its interhemispheric connective function, particularly via the fibers of the anterior commissure [ 17 ]. More specifically, it is also important to mention in detail the pathways involved in the intra and extralobar frontal integration, such as the fronto-orbitopolar tract, frontal aslant tract, and frontal superior and inferior longitudinal fasciculi [ 18 ].
The fronto-orbitopolar tract connects the posterior orbital gyrus to the anterior orbital gyrus and to the medial-ventral region of the frontal pole and has the function of associating the storage of memory with the senses, such as taste, smell, sight, and hearing. Damage to this tract can lead to impaired inhibitory response and speech initiation difficulties [ 18 ]. In , Catani et al.
The superior and inferior longitudinal tracts have the function of integrating, at different levels, the frontal regions involved in decision-making, i. In addition, the superior longitudinal tract SLT also integrates the neuronal network which extends beyond the frontal lobe, having, for example, involvement in the selection of sensory stimuly related to processing of attention, which occurs through the functioning of the frontoparietal circuitry. In this context, the authors suggest the subdivision of the SLT into three segments, namely the SLT I, SLT II, and SLT III, which connect, respectively, the superior parietal region to the dorsal prefrontal and dorsal premotor cortex; the inferior parietal region to the dorsolateral prefrontal and medial premotor cortex; and the supramarginal gyrus to the premotor ventral cortex [ 20 ].
Other examples connecting the frontal lobe to different cortical regions include the superior fronto-occipital fasciculus SFOF , which corresponds to the long association system of the dorsal visual pathways and appears to have a role in the interaction of the visuospatial function with superior integrative functions.
This tract has a hemispheric trajectory located medially, with projections located on the superior edge of the anterior branch of the internal capsule and along the length of the lateral portion of the caudate nucleus, laterally to the posteroinferior elongation of the lateral ventricle horn. Thus, the SFOF connects the mediodorsal parts of the occipital lobe, angular gyrus located in the inferior parietal lobe , Brodmann area 19, and the precuneus Brodmann area 7 to the dorsal and medial portions of the premotor and prefrontal region Brodmann areas 6 and 8.
The inferior fronto-occipital fasciculus IFOF has the primary function of connecting the inferolateral and dorsolateral frontal cortices with the posterior temporal and ventral occipital cortices, via a lateral hemispheric route, along the lateral portion of the lentiform nuclei, claustrum, and the external and extreme capsules. Studies show that this fasciculus connects the visual Brodmann areas 20 and 21 and auditory Brodmann area 22 associative areas, situated in the temporal lobe, with the prefrontal cortex, playing a role, together with other tracts, in complex visual integration and language and memory processing [ 17 ].
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Other important structures include the external capsule and the extreme capsule. The external capsule is situated between the putamen and the claustrum and has associative pathways coursing through it connecting the ventral and medial prefrontal cortices, ventral premotor cortex, precentral gyrus, rostral superior temporal, inferior temporal, and preoccipital regions. The extreme capsule is situated between the claustrum and caudal insular cortex and between the claustrum and orbitofrontal cortex in its rostral portion, representing the principle connective pathway between the ventrolateral prefrontal cortex and the caudal fronto-orbital cortex with the superior temporal region [ 17 ].
Some of the connecting pathways cited in this subsection are exposed in Figures 5 — MRI tractography in sagittal view showing anterior cingulate A. C fibers, fornix and uncinate fasciculus U. F fibers. MRI tractography in coronal view showing anterior cingulate A. In addition, with the advancement of knowledge on cognitive science made possible by technology, important examples of network models showing the broad cerebral neuronal connections have been demonstrated recently.
The DMN is basically a network associated with passive task conditions and with self-referring mental activity, whose main structures are the posterior cingulate and adjacent precuneus cortex, medial prefrontal cortex, and inferior parietal lobe [ 21 ]. For didactic purposes and practical applicability, several cases illustrating these clinical situations will be reported in the ensuing subsection.
Subsequently, the lesions were reconstructed using tractography based on the atlas of white matter obtained from the diffusion tension imagings DTIs of healthy adults [ 2 ]. In , Harlow described the case of Phineas Gage, a year-old man who sustained perforation of the left frontal part of his skull by an iron bar after an accident in the workplace.
According to the descriptions, after the event, Gage became unrecognizable to his friends; he became more flippant, used foul language, was more impatient when disagreed with, failed to display empathy and, although no neuropsychological description was made at the time, the clinical manifestations were believed to be linked to deficits in decision-making and emotion processing after sustaining the lesions to the frontal lobe.
In addition, there was partial disconnection of the frontal lobe in relation to the amygdala, thalamus, and striatum.
How Frontal Lobe Damage and Disease Affects Your Life
With regard to the fasciculi, the authors cited damage of the inferior longitudinal, superior frontal, and uncinate fasciculi. There were also lesions occurring in the orbitopolar and frontal aslant tracts. Other partially affected connective pathways were the frontostriatal, frontopontine and anterior thalamic projections [ 2 ].
At the time of hospitalization, Leborgne was divorced and had recently lost his father, which may have explained his long stay at this clinic. On the postmortem analysis of Leborgne, Broca identified damage to the posterior third of the left inferior frontal lobe.
The connecting lines indicate the evolutionary relationships among the species. Katerina Semendeferi and colleagues found that the frontal lobes in humans are not disproportionately larger than expected for a primate brain of its size. Figure and caption adapted from Passingham For years, scientists have attempted to pinpoint the bits of our brain that might help explain our uniquely human intelligence.
The frontal cortex, which resides toward the front of the brain, has frequently been singled out as large relative to other species. But the results have been mixed, with some researchers arguing for a disproportionate expansion , other researchers arguing for no expansion relative to great apes , and still other researchers arguing that it depends on which species are being analyzed.
Why the mixed findings? One potential reason is that some researchers based their conclusions on unscaled measurements, such as absolute brain size or total brain volume. By these measures, our frontal cortex does appear larger than other species. But if we take unscaled measurements seriously, then things get really absurd.
By this metric, chihuahuas and several species of fish have bigger brains proportional to their body than we do! Even just looking at the frontal cortex, sea lions have a larger frontal cortex than several "higher primates,"such as baboons and gibbons, whereas llamas exceed macaque monkeys. As Robert Barton and Chris Venditti so eloquently point out in their recent paper ,. Unless one is willing to take seriously the hypothesis that lemurs have more of the qualities bestowed by frontal cortices than do humans, or that llamas possess more than monkeys, it must be concluded that testing the hypothesis that any species is specialized for frontal cortex functions, as opposed to functions mediated by more extended networks, requires scaling to be taken into account.
To help us out of this absurd paradox, Barton and Venditti analyzed five independent data sets, taking into account two important facts about brain development:. The volumes of different brain regions develop at different rates as both brain and body size evolves. While it's true that the total volume of the neocortex-- the outer layer of the cerebral cortex-- increases with brain size faster than the cerebellum, the number of actual neurons in each of these two brain structures stays about the same.
Their result? The size of our frontal lobes, including specific frontal regions such as the prefrontal cortex, is nothing special relative to the size of our other brain structures. Even more humbling for humans, they also found that once we diverged from chimpanzees about 6 million years ago, the speed with which our frontal cortex volume increased relative to our other brain structures was "unremarkable". In fact, other species showed faster rates of change than us!
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So what gives? How come we can compose beautiful, lyrical poetry and play breathtaking cello sonatas, and gorillas can barely keep a tune? You use it to make decisions, such as what to eat or drink for breakfast in the morning, as well as for thinking or studying for a test. The frontal lobe is also where our personality is formed and where we can carry out higher mental processes such as planning.
In addition, the frontal lobe is necessary to being able to speak fluently without fault and meaningfully.