3.1 What is the dual stream model?

Given these separate anatomical accounts, attributing a function(s) to the arcuate is not clear cut, and any current account is far from the authoritative statement on the matter. Nonetheless, a vast majority of literature does place the arcuate as part of the dual stream model[1] of speech processing, although its exact role within these neural networks is still being disputed – and largely depends on which anatomical account you prescribe to.

The basic assumption of dual stream accounts is that phonological networks interact with both conceptual-semantic and motor-articulatory systems, leading to a distinction between the neural networks that process this speech information. These separate interactions are summarised under two processing streams: the dorsal stream and the ventral stream (Hickok and Poeppel, 2007). Connecting phonological networks with conceptual-semantic systems, using structures in the superior and middle portions of the temporal lobe, is the ventral stream. Meanwhile, the dorsal stream is linked via structures in the posterior frontal lobe to the posterior temporal lobe and parietal operculum, which connects phonological networks with motor-articulatory systems (ibid).

Phonological processing has two major aspects: receptive processing of phonemes and expressive production of phonemes (Glasser and Rilling, 2008). Current accounts of Wernicke’s area (posterior BA 22 and BA 40) place it as a hub for processing incoming phonemes (cf. Martin, 2003), while the production of phonemes takes place in the posterior portion of Broca’s area (BA 44 and BA 6) (cf. Bookheimer, 2002). Arguably, this notion of phonological processing can be considered the ventral stream, as described by Hickok and Poeppel (2007), and contrasts with the lexical-semantic system. Here, middle and inferior temporal areas (BA 21 and BA 37) connect with “[…] Broca’s area and frontal areas more anterior and superior to it (BA 44, 45, 47, 9)” (Glasser and Rilling, 2008, pg. 1). Besides lexical-semantic systems, these areas are also implicated in other higher functioning language domains, such as syntax (cf. Grodzinsky and Santi, 2008). This is the dorsal stream. Importantly, these two streams are disconnected from one another and are also left-lateralised.

As Hickok and Poeppel (2007) point out in their review, the dual stream model is far from a new idea, with Wernicke himself offering a similar proposal as part of his language model. Also, similar organisational systems are found in visual and somatosensory domains, which “[…] suggests that the present dual-stream proposal for cortical speech processing might be a reflection of a more general principle of sensory system organization.” (Hickok and Poeppel, 2007, pg. 393). This leaves us with working out the role of the arcuate within these networks, a task that will be discussed in the next section.

3.2 The dual stream model, the arcuate fasciculus and conduction aphasia

The classical incarnation of the arcuate is largely accepted as being involved in a dorsal-like processing system. Differences emerge in the extent of the arcuate’s anatomy, subsequently leading to different hypotheses for its functional role in speech processing. Of the literature read, it is perhaps suitable to frame these accounts by examining their explanatory power in the previously mentioned condition, conduction aphasia.

Conduction aphasia’s hallmark characteristic is difficulty in repetition, often accompanied by paraphasic impairments (phonological errors in fluency) and lexical-semantic deficits (difficulties in naming) (cf. Glasser and Rilling, 2008). As such, limiting the arcuate to a direct phonetic pathway reduces its ability to explain these cases, meaning it is specifically implicated in repetition problems – and disassociated from lexical-semantic impairments.

Catani et al. (2004) reconcile these characteristics by attributing them to lesions at different points in their extended perisylvian network, located on either the direct (phonetic) or indirect (semantic) pathways. Even though damage to the traditional, direct route for the arcuate is reduced to just explaining repetition, the indirect pathways extend into areas associated with lexical-semantic functions. Lesions to these extended pathways explain the differences observed in some patients. Nonetheless, this does not explain the observations of Hickok and Poepell (2004), who argue conduction aphasia is more likely to result from cortical dysfunction, explained by damage to “[…] the sound-based speech processing systems in left STG and/or to the temporal–parietal system (Spt) which interfaces these systems with motor–articulatory networks.” (ibid, pg. 93).

Taking these considerations in mind, Saur et al. (2008) further relegate the arcuate and do not attribute it any special status within the dorsal stream. Instead, they speculate that phonological impairments of conduction aphasia are the result of a disruption in their proposed dorsal arcuate/SLF system. This offers a strong explanatory case for why conduction aphasia does not always result from an arcuate lesion. It also does not rely on extending the perisylvian network and fits in with additional DTI studies of the arcuate’s anatomy (Schmahmann et al., 2007; Frey et al., 2008). Here, lexical-semantic impairments are not explained, which is probably because they offer a different anatomical route for their ventral stream, which uses the extreme capsule (EmC) to connect temporal and ventrolateral prefrontal cortex; and therefore, it is presumed the authors likely ascribe to an account similar to those presented by Hickok and Poepell (2007), Dronkers et al. (2007) and Schmahmann et al. (2007).

Offering another position is Glasser and Rilling (2008), who see the arcuate as a critical substrate of both ventral and dorsal pathways, including right hemispheric language processing (see figure three). This version places it somewhat at odds with the previous studies mentioned, as they claim the left-lateralised arcuate is not a direct phonetic pathway but two functionally distinct neural networks. Importantly, Wernicke-like and Broca-like conduction aphasias are explained under an adapted version of Hickok and Poepell’s (2004) model, with lesions in the STG pathway causing phonological impairments, whilst damage to the MTG pathway is associated with lexical-semantic impairments.

Fig. 3: Hickok and Poeppel’s (2004) dual stream model, showing the functions of the STG and MTG pathways in the left hemisphere (top) and right hemisphere (bottom) (Glasser and Rilling, 2008).

Furthermore, they also acknowledge the EmC acting as a ventral pathway, which accounts for instances where the arcuate lesion does not cause conduction aphasia or where conduction aphasia arises from lesions in different regions (Glasser and Rilling, 2008). This leads them to conclude that:

“[…] conduction aphasia can be best explained by damage to the relatively superficial STG and extreme capsule pathways and/or the surrounding phonologic cortex, rather than a deeper lesion that damages mostly the more medial MTG pathway of the arcuate fasciculus, as argued by the classic model.” (ibid, pg. 7).

Still, this account does not necessarily negate alternative explanations for conduction aphasia, nor does it definitively place the arcuate as a critical component of the dual stream – processing both ventral and dorsal streams. Further work is needed before these pathways can be fully elucidated and subsequently attributed a functional role.  However, given evidence of recent selection pressures (Rilling et al., 2008) – something neither the SLF nor EmC show signs of – the arcuate seems likely to be subserving a critical role in processing of a human-related behaviour, language being a plausible candidate.

4. Conclusion

Discerning the role of the arcuate fasiculus in speech processing is a process yet to be completed. This understanding might evolve out of current models based on speech processing, especially the dual stream model discussed above, which seems to provide an intuitive, comprehensive understanding of the mechanisms involved. Based on DTI tractography, we can now begin to appreciate the complexity of the arcuate, and at the same time realise that these approaches are not all revealing — as demonstrated in the differences between studies, where some show pathway terminations in more frontal regions (Rilling et al., 2008), whilst others claim these connections are non-existant (Saur et al., 2008).

That these differing interpretations exist does not necessarily hinder all attempts to ascertain a functional role for the arcuate substrate. For instance, it is reasonable to assume that, under the dual-stream model, the arcuate is almost certainly involved in mapping phonological networks with motor-articulatory systems. What remains to be solved is whether or not this pathway also extends into conceptual semantic domains i.e. the ventral stream. To elucidate upon this, we need to not only enhance the range of DTI studies performed, but to also offer more comparative accounts between primates, other mammals and the neurological regions/networks themselves.

Still, the dual stream model, for all its explanatory capacity, is unsubstantiated. And, even if the dual stream account is largely accurate, the arcuate is not necessarily a critical component, and may just be part of several substrates connecting language-associated regions. This leads us to another viable criticism: the apparent connections between function and anatomy are not always clear-cut. As such, one might argue the regions described are involved in other cognitively demanding processing — see: tool use, action sequences and other complex behaviours. Despite these possible avenues, recent insights into the arcuate’s anatomical, functional and evolutionary underpinnings suggest a functional role within speech processing. Whether this role underpins both dorsal and ventral pathways remains an open question.

Main References
Hickok, G. (2004). Dorsal and ventral streams: a framework for understanding aspects of the functional anatomy of language Cognition, 92 (1-2), 67-99 DOI: 10.1016/j.cognition.2003.10.011

Glasser, M., & Rilling, J. (2008). DTI Tractography of the Human Brain’s Language Pathways Cerebral Cortex, 18 (11), 2471-2482 DOI: 10.1093/cercor/bhn011

Schmahmann, J., Pandya, D., Wang, R., Dai, G., D’Arceuil, H., de Crespigny, A., & Wedeen, V. (2007). Association fibre pathways of the brain: parallel observations from diffusion spectrum imaging and autoradiography Brain, 130 (3), 630-653 DOI: 10.1093/brain/awl359

Originally identified by Reil (1809) and subsequently named by Burdach (1819), the arcuate fasciculus is a white-matter, neural pathway that intersects with both the lateral temporal cortex and frontal cortex via a “dorsal projection that arches around the Sylvain fissure.” (Rilling et al., 2008, pg. 426). Classical hypotheses saw this pathway as a critical component in connecting two centres of language: Broca’s area (speech production) and Wernicke’s area (speech comprehension) (Catani and Mesulam, 2008).

Much of these assumptions were based on a tentative relationship between language-impairment and damaged portions of the brain. Notably, damage to the arcuate fasciculus is implicated in a syndrome known as conduction aphasia, where an individual has difficulty in speech repetition. Often characterised by errors in spontaneous speech, an individual with conduction aphasia will be fully aware of their mistake, retaining well-preserved auditory comprehension and speech production while also being syntactically and grammatically correct (ibid).

In addition, these lesions were in the left-hemisphere (LH), leading to further speculations on a left-lateralisation for language processing. Though as we now know: language is far more distributed, and not just confined to the left-hemisphere (LH). In fact, the right-hemisphere (RH) has been shown to process prosody, discourse comprehension, connotative meaning and many other language phenomena (cf. Bookheimer, 2002). Furthermore, recent studies (cf. Hickok and Poeppel, 2004; Dronkers et al., 2007) demonstrate the arcuate fasciculus may not implicitly involved in conduction aphasia.

Despite its diminished role in language, the arcuate fasciculus is part of a renaissance in neuroscientific investigation, largely due to recent advances in brain imaging techniques. With these new methods of analysis come new hypotheses for functional aspects of brain regions and their neural networks. In particular, the dual stream model for speech and language processing (Hickok and Poeppel, 2004) has become a key in discerning the arcuate fasciculus’ role (Glasser and Rilling, 2008; Saur et al., 2008). With this in mind, the essay will now discuss and review these accounts of the arcuate fasciculus, and attempt to place it within current accounts of speech processing.

2. The anatomy, function and evolution of the arcuate fasiculus

2.1 The flaws in classical accounts of anatomy and function of the arcuate fasciculus

Prior efforts in discerning the anatomical aspects of arcuate fasciculus[1] were largely based on observations by Constantin Von Monakow, Joseph Jules Dejerine and, later on, Norman Geschwin (cf. Catani and Mesulam, 2008). Much of these earlier studies relied on post-mortems in humans, employing relatively crude methods, such as blunt dissections and myelin stain techniques (ibid).

These methods did not reveal much in-depth information concerning the arcuate’s specific anatomical terminations, just a basic outline of its pathway – stretching from the temporal lobe to the frontal lobe. This putative anatomy was used as the basis for hypothesising the arcuate’s function: a language substrate that connected speech comprehension (Wernicke’s area) with speech production (Broca’s area). More importantly, confirmation of this hypothesis relied on accounts that disconnecting the arcuate would result in conduction aphasia (ibid).

Specifically, original accounts of conduction aphasia distinguished between two groups:“[…] the Broca-like syndrome in which the deficit in repetition is accompanied by a relative impairment in fluency and the Wernicke-like syndrome in which the deficit in repetition is accompanied by a relative impairment in comprehension. One explanation for this dichotomy is that more anterior lesions encroach on Broca’s area, whereas more posterior lesions encroach on Wernicke’s area” (Catani et al., 2004, pg. 13).

These disconnectionist approaches would later be challenged by a range of studies (including: Damasio and Damasio, 1980; Naeser et al., 1982), showing the classical arcuate cannot explain cases where Broca-like and Wernicke-like conduction aphasias emerge from subcortical lesions. Subsequent studies (Hickok et al., 2000; Dronkers et al., 2007) also argue conduction aphasia is not even associated with the arcuate. This is demonstrated in several ways, including: arcuate lesions not inducing conduction aphasia (Selnes et al., 2002); left auditory cortical lesions causing conduction aphasia (Graves et al., 2008); and, cortical stimulation eliciting conduction aphasia-like symptoms (ibid).

2.2 Arcuate Fasciculus as a direct phonetic pathway

More recently, diffusion tensor imaging (DTI) tractography, a method of reconstructing white matter pathways by tracing the diffusion of water, reveals a more complex picture of the arcuate fasciculus (Catani and Mesulam, 2008).

Here, one group of studies (Catani et al., 2004; Parker et al., 2005; Powell et al., 2006) not only confirm some findings from the classical accounts, which connects the temporal lobe with the frontal lobe, they also demonstrate additional indirect pathways outside of the typical perisylvian connectivity (see figure 1). Also, there appears to be a far more extensive branching of the arcuate’s cortical terminations “[…] beyond the classical limits of Broca’s and Wernicke’s areas to include part of the middle and precentral gyrus and the posterior middle temporal gyrus, respectively.” (Catani and Mesulam, 2008, pg. 957).

Tractography reconstruction of the arcuate fasciculus showing Broca’s and Wernicke’s areas being connected through three segments: a long, direct pathway (classical arcuate); an anterior, indirect pathway; and, a posterior, indirect pathway. Taken from Catani et al. (2004).

Another reaffirmation is the apparent hemispheric asymmetry in the arcuate fasciculus. Again using DTI, Catani et al. (2007) find differences in connection patterns between the right and left hemispheres, showing a leftward lateralisation in ~80% of tested subjects. Despite language functions showing a greater degree of hemispheric distribution than originally thought (Bookheimer, 2002), left-lateralised asymmetry is still considered to be a key aspect of language processing (Catani and Mesulam, 2008).

Makris et al. (2005) observe that the left-lateralised arcuate runs alongside neuronal pathways in the superior longitudinal fasciculus (SLF). They claim the SLF can be split in four distinct components, one of which being the arcuate, and claim (on the basis of previous work by Petrides and Pandya, 2002) these fibre pathways are bidirectional. They also present a detailed anatomical description of the arcuate, showing that neurons branching out from the caudal superior temporal gyrus (which includes wernicke’s area) and the superior temporal sulcus, curving around the Sylvian fissure (caudal portion), which then: “[…] runs along with the fibers of SLF II […] AF [arcuate fasciculus] fibers continue into the frontal lobe and terminate predominantly in the dorsal part of area 8 and in area 46. AF becomes distinct mainly at the parietal opercular level.” (ibid, pg. 863).

These accounts place the classical, left-lateralised arcuate as a direct phonetic pathway, where phonologically-rooted functions are sent directly to the frontal cortex. As Catani and Mesulam (2008) explain, in discovering two additional pathways (indirect) that connect to Geschwind’s territory (corresponding to areas BA 39 and 40), the expanded arcuate also assumes a role in processing semantic information. Although, attributing semantic processing to BA 40 may not necessarily hold true, and might just be limited to BA 39 (Price, 2000). Glasser and Rilling (2008) raise issue with this, also adding that BA 40 is “[…] more likely to be involved in phonetic working memory than semantic processing, and semantic processing is found far more often in the temporal lobe.” (pg. 9). Lastly, Johnson-Frey (2008) found that grasping actions and tool use planning are represented in the left inferior parietal lobe (BA 39), leading to speculations that Geschwind’s territory might primarily be involved in complex tool use (Glasser and Rilling, 2008).

However, these anatomical and functional accounts are far from unequivocal. Another group of papers (Schmahmann et al., 2007; Saur et al., 2008; Frey et al., 2008) also place the arcuate as a direct phonetic pathway; however, they find no evidence of arcuate pathway terminations in BA 45. The basis of this assertion rests on comparative studies between humans and macaques, arguing that anatomical analysis suggests it is the extreme capsule (EmC) and the MdLF are involved in language, not the arcuate (Schmahmann et al., 2007). Instead, the arcuate, SLF, MLF and ILF (inferior longitudinal fasciculus) form a composite fibre bundle that “[…] is mainly restricted to sensory-motor mapping of sound to articulation.” (Saur et al., 2008, pg. 18035).

2.3 Arcuate fasciculus as a phonological, lexical-semantic and prosodic processor

Running contrary to the notion of the arcuate as a direct phonetic pathway are Glasser and Rilling (2008), who hypothesise it is comprised of different routes and different functions. Specifically, they posit two pathways: one terminating in the posterior superior temporal gyrus (STG) and the other in the middle temporal gyrus (MTG), with these neural pathways involving phonetic and lexical-semantic processing, respectively. Interestingly, they also implicate the right-hemispheric arcuate in phonological processing (bilateral activation) and during prosodic processing (right-lateralised activation). Both of these findings being confirmatory of a wider distributed processing of language, particularly in the right-lateralisation of prosodic processing (Ethofer et al., 2006).

In a related study, Rilling et al. (2008) further delineate the arcuate, showing specific termination points, with the superior, middle and inferior temporal gyri making up the temporal projection, whilst the frontal projection consists of the ventral premotor cortex (BA 6), pars opercularis (BA 44), pars triangularis (BA 45) and the middle frontal gyrus (BA 9). Furthermore, they offer a comparative account between the arcuate connectivity in humans, chimpanzees and macaques.

In investigating the respective pathways of all three primates, Rilling et al. show how significant differences in these lineages are suggestive of gradual modifications to cortical terminations during human evolution (see figure two). As such, chimpanzees display more structural similarity to humans in their pathway trajectories, with extensive terminations in the frontal region; however, terminations in the middle and frontal gyri are less pronounced. Being of a phylogenetically older lineage, macaques lack any temporal lobe terminations.

A Schematic summary of tractography results for humans, chimpanzees and macaques (Rilling et al., 2008). Notice the more extensive projections in the human brain.

When combined with data from two additional fibre tracts (SLF and the extreme capsule), each predicted not to show any significant differences across all three of the primates, the study reveals a very strong case for the arcuate having being subjected to selective pressure on the human lineage (ibid) – somewhat countering the claims of Schmahmann et al. (2007). Even if we accept the possibility of selection, the arcuate might not have been selected because of its apparent role in language. As the authors note, there is the possibility for aspects of the arcuate being involved in other cognitively demanding behaviours, such as tool use (cf. Johnson-Frey, 2008). However, “[…] the correspondence between the structures modified in human evolution identified in this study and structures known to be involved in language function is notable.” (Rilling et al., 2008, pg. 428).

Main References

CATANI, M., & MESULAM, M. (2008). The arcuate fasciculus and the disconnection theme in language and aphasia: History and current state Cortex, 44 (8), 953-961 DOI: 10.1016/j.cortex.2008.04.002

Glasser, M., & Rilling, J. (2008). DTI Tractography of the Human Brain’s Language Pathways Cerebral Cortex, 18 (11), 2471-2482 DOI: 10.1093/cercor/bhn011

Choi J, Jeong B, Rohan ML, Polcari AM, Teicher MH.  Preliminary evidence for white matter tract abnormalities in young adults exposed to parental verbal abuse. Biol Psychiatry. 2009 Feb 1;65(3):227-34.

We have just published findings from my laboratory that are beginning to illuminate the neurobiological effects of exposure to parental verbal abuse. We had previously shown that exposure to high levels of parental verbal abuse had the same impact (based on symptom ratings) as witnessing domestic violence and extrafamilial sexual abuse. It had somewhat more impact than exposure to parental physical abuse, but less impact than familial sexual abuse. Hence, it appears to be a potent form of childhood adversity.

Teicher MH, Samson JA, Polcari A, McGreenery CE. Sticks stones and hurtful words: Relative effects of various forms of childhood maltreatment. Am J Psychiatry 2006; 163: 993-1000

White Matter Tract Abnormalities in Young Adults Exposed to Parental Verbal Abuse

Synopsis

In this study we used a new MRI technique called diffusion tensor imaging (DTI) to ascertain whether exposure to a high level of parental verbal abuse (PVA) was associated with abnormalities in brain white matter (WM) tract integrity. The brain consists of regions of gray matter that contains the cell bodies and dendritic branches of neurons, and white matter, which are the myelinated axonal fiber tracts providing communication between neurons in different gray matter regions. We screen 1271 healthy young adults for exposure to childhood adversity, and collected DTI (Siemens 3.0 T Trio Scanner) on 16 unmedicated subjects with a history of high-level exposure to PVA but no other form of maltreatment (4M/12F, mean age 21.9±2.4 yrs), and 16 healthy controls (5M/11F, 21.0±1.6 yrs). Group differences in fractional anisotropy (FA), covaried by parental education and income, were evaluated using tract-based spatial statistics (TBSS), and correlated with symptom ratings and verbal IQ. FA is an index of the integrity of the fiber pathway. Reduced FA may indicate a reduction in myelin, number of axons, or diameter of axons.

Three WM tracts had significantly reduced FA: (1) arcuate fasciculus in left superior temporal gyrus, (2) cingulum bundle in the fusiform gyrus by the posterior tail of the left hippocampus, and (3) the left body of fornix. FA values were strongly associated with the maximal PVA scores (r=-0.806, P<10-7; r=-0.658, P<.0001; r=-0.584, P<.0001, respectively).

Figure 1.  Three white matter tract regions (shown in red) that differed significantly in fractional anisotropy (FA) between subjects with history of exposure to high levels of parental verbal aggression and healthy controls. Region 1 contained fibers from arcuate fasciculus. Region 2 contained fibers from the cingulum bundle near the tail of the hippocampus. Region 3 is part of the left fornix (hippocampal efferents). Green shows the mean FA skeleton and background image is in MNI 152. Tractography from representative subjects show tracts passing through the region identified by TBSS.

FA in region 1 correlated with verbal IQ (r=0.405, P<.03).  The arcuate fasciculus is the fiber pathway that connects Wenicke’s area in the temporal lobe to Broca’s area in the frontal lobe.  It plays an important role in verbal comprehension and communication.  FA in region 2 was inversely associated with ratings of depression (r=–0.442), dissociation (r=–0.447), and limbic irritability(r=–0.483). The cingulum bundle is the most prominent tract of the limbic lobe, and connects the limbic lobe with the neocortex, particularly the cingulate gyrus.  FA in region 3 was inversely correlated with anxiety (r=–0.36) and somatization (r=–0.371).  The fornix is a pathway that interconnects hippocampus with the septal area and mammillary bodies, and is known to play a role in anxiety and memory. Interestingly, the hippocampus receives serotonin fibers from the midbrain raphe via two pathways: the cingulum bundle (which predominantly innervates dorsal hippocampus), and the fornix (which innervates all portions).  Hence, two of the fiber tracts with segments of reduced FA in PVA subjects, provide pathways for serotonin fibers to innervate the hippocampus.

This study provides the first evidence that high levels of parental verbal aggression may be a form of abuse or adversity that alters trajectories of brain development.  It supports our previous hypothesis that different forms of childhood maltreatment will exert some comparable an array of consistent neurobiological effects (particularly on limbic regions or connection) as they are all stressors.  However, different forms of abuse will also have some unique effects based on sensory systems activated that convey the aversive stimulus to specific parts of the brain that process and interpret the information.

Support

This work was supported, in part, by National Institute of Mental Health RO1 grants MH53636 and MH-66222, and National Institute of Drug Abuse RO1 grants DA-016934 and DA-017846 to MHT.

from the International Journal of Pediatric Otorhinolaryngology

Conclusions
In this study, we determined the sequence of myelination of language-correlated regions in infants and children by quantitative MRI assessment. The higher cortical areas matured later than the primary cortical areas, and the arcuate fasciculus matured last. The observation that myelination reaches maturity after 18 months suggests that myelination may be a reason for the acceleration in vocabulary acquisition observed in children from that age. The slow pace of myelination also suggested the possibility of language development’s continuation into early adult life. Myelination assessed by MRI was at least 1 month behind that assessed by histological staining. No gender or left-right hemisphere differences in myelination were noted.

from Cerebral Cortex

Diffusion Tensor Imaging (DTI) tractography has been used to detect leftward asymmetries in the arcuate fasciculus, a pathway that links temporal and inferior frontal language cortices. In this study, we more specifically define this asymmetry with respect to both anatomy and function. Twenty right-handed male subjects were scanned with DTI, and the arcuate fasciculus was reconstructed using deterministic tractography. The arcuate was divided into 2 segments with different hypothesized functions, one terminating in the posterior superior temporal gyrus (STG) and another terminating in the middle temporal gyrus (MTG). Tractography results were compared with peak activation coordinates from prior functional neuroimaging studies of phonology, lexical–semantic processing, and prosodic processing to assign putative functions to these pathways. STG terminations were strongly left lateralized and overlapped with phonological activations in the left but not the right hemisphere, suggesting that only the left hemisphere phonological cortex is directly connected with the frontal lobe via the arcuate fasciculus. MTG terminations were also strongly left lateralized, overlapping with left lateralized lexical–semantic activations. Smaller right hemisphere MTG terminations overlapped with right lateralized prosodic activations. We combine our findings with a recent model of brain language processing to explain 6 aphasia syndromes.

Despite the fact that I’ve seen some really impactful primate related research lately, I’ve completely neglected updating Primatology.net with it. I can’t believe it has been almost three months since I’ve posted there! I should really resume posting there. Actually, I was considering putting up this following blog post over there, since it has to do with differences in neuroanatomy of the primate brain… but because these comparative studies are in the context of identifying specific architectural differences in the human brain related to language, I think posting it here is more fitting.

If you’re a reader of Neurophilosophy, you may have an idea of what research I’m referring too, the new Nature Neuroscience paper from James Rilling and team. Before I jump into this paper, “The evolution of the arcuate fasciculus revealed with comparative DTI,” please let me share another recent paper that gives some introduction about what I’m gonna talk about.

See earlier this month, Current Biology published a paper, “Communicative Signaling Activates ‘Broca’s’ Homolog in Chimpanzees,” where researchers not only confirmed that the Broca’s area as an important area of the human brain for language comprehension, but also chimpanzees have similar activity in the homologous area of their brains when communicative signals are produced or heard. The Broca’s area has long been thought to be one of the specialized functional areas of the brain for language comprehension. In fact was discovered almost 150 years ago by a physician named Pierre Paul Broca, who conducted an autopsy of patient with a speech deficit. Broca was able to determine the patient had a syphilitic lesion in the left cerebral hemisphere and identified this area as his namesake.

If you’ve heard anything about Broca’s area, it larger in the left hemisphere of the brain. Comparing activity levels between the two hemisphere, during language-related tasks, have shown the left hemisphere Broca’s area is more active. That’s due to the lateralization of the brain, which I’m sure you’ve heard of.

Anyways, the results of this study have important implications in figuring out the functional and structural differences of the human and chimpanzee brain. Why? Well, for starters, the linguistic abilities of humans have been thought to be unique to us for a while. This is a really big misconception because research on signing apes and other communicating animals, have begun to show us that we’re not alone in our abilities to symbolize information and exchange it by way of complex sound and gesture.

In order to investigate the differences of the activity between Broca’s areas in humans and related structure in chimpanzees, Taglialatela et al., put three chimpanzee subjects in PET and fMRI machines and stimulated to vocalize by putting treats just out of their reach. They then recorded the activity of the subjects would vocalize in frustration. They were able to see the very same the neuroanatomical structures associated with the production of communicative behaviors in humans, fire in chimpanzees.

Now, of course that doesn’t mean chimpanzees are gonna be reciting Shakespeare anytime soon. This leads me to the first paper I mentioned today, the one from Rilling and crew. Rilling et al., did a comparative anatomical study on the structure of arcuate fasciculus, a large white matter tract, in humans, chimpanzees and macaques. The arcuate fasciculus functions as a linker between Broca’s area and another language associated area of the brain, Wernicke’s area. The researchers used diffusion tensor imaging (DTI), a type of noninvasive medical imaging that’s a lot like MRI but it compares and contrasts the local characteristics of water diffusion within tissues.

While the arcuate fasiculus of the rhesus macaque, the chimpanzee, and the human linked up to the frontal cortex — including with Broca’s area, it was observed that the human arcuate fasiculus is much larger. It more spreads deep into the middle temporal lobe, leaving the classical Wernicke’s area. In chimps, the arcuate fasciculus made very superficial connections to the temporal cortex regions homologous to Wernicke’s area. Macaques showed a much lower extend of this integration. Rilling commented,

“We know from previous functional imaging studies that the middle temporal lobe is involved with analyzing the meanings of words. In humans, it seems the brain not only evolved larger language regions but also a network of fibers to connect those regions, which supports humans’ superior language capabilities.”

This following diagram was published in Rilling et al.’s paper and illustrates their results:

So from these two papers, the evolution of specialized language areas maybe active in both chimpanzee and human brains but as the human brain diverged from other primate counterparts, major re-wiring at the arcuate fasciculus accompanied the massive expansion of brain size. Ultimately the area that is associated with understanding word meaning, Wernicke’s area, has been strongly connected with Broca’s area.

Rilling, J.K., Glasser, M.F., Preuss, T.M., Ma, X., Zhao, T., Hu, X., Behrens, T.E. (2008). The evolution of the arcuate fasciculus revealed with comparative DTI. Nature Neuroscience DOI: 10.1038/nn2072

TAGLIALATELA, J., RUSSELL, J., SCHAEFFER, J., HOPKINS, W. (2008). Communicative Signaling Activates ‘Broca’s’ Homolog in Chimpanzees. Current Biology, 18(5), 343-348. DOI: 10.1016/j.cub.2008.01.049