BA – Replicated Typo

The arcuate fasciculus within the dual stream model pt.2

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).

Continue reading “The arcuate fasciculus within the dual stream model pt.2”

Discerning the role of the arcuate fasciculus in speech processing pt.1

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).

Continue reading “Discerning the role of the arcuate fasciculus in speech processing pt.1”

Broca's area and the processing of hierarchically organised sequences pt.2

3. Neurological processing of hierarchically organised sequences in non-linguistic domains

A broader perspective sees grammar as just one of many hierarchically organised behaviours being processed in similar, prefrontal neurological regions (Greenfield, 1991; Givon, 1998). As Broca’s area is found to be functionally salient in grammatical processing, it is logical to assume that this is the place to search for activity in analogous hierarchical sequences. Such is the basis for studies into music (Maess et al., 2001), action planning (Koechlin and Jubault, 2006) and tool-production (Stout et al., 2008).

Continue reading “Broca's area and the processing of hierarchically organised sequences pt.2”

Broca's area and the processing of hierarchically organised sequences pt.1

Ever since its discovery in 1861, Broca’s area (named after its discoverer, Paul Broca) has been inextricably linked with language (Grodzinsky and Santi, 2008). Found in the left hemisphere of the Pre-Frontal Cortex (PFC), Broca’s region traditionally[1] comprises of Broadmann’s areas (BA) 44 and 45 (Hagoort, 2005). Despite being relegated in its status as the centre of language, this region is still believed to play a vital role in certain linguistic aspects.

Of particular emphasis is syntax. However, syntactic processing is not unequivocally confined to Broca’s area, with a vast body of evidence from “Studies investigating lesion deficit correlations point to a more distributed representation of syntactic processes in the left perisylvian region.” (Fiebach, 2005, pg. 80). A more constrained approach places Broca’s area as processing an important functional component of grammar (Grodzinsky and Santi, 2007). One of these suggestions points specifically to how humans are able to organise phrases in hierarchical structures[2].

In natural languages, “[…] the noun phrases and the verb phrase within a clause typically receive their grammatical role (e.g., subject or object) by means of hierarchical relations rather than through the bare linear order of the words in a string. [my emphasis]” (Musso et al., 2003, pg. 774). Furthermore, these phrases can be broken down into smaller segments, with noun phrases, for example, consisting of a determiner preceding a noun (Chomsky, 1957). According to Chomsky (1957) these rules exist without the need for interaction in other linguistic domains. Take for example his now famous phrase of “Colourless green ideas sleep furiously.” (ibid, pg. 15). Despite being syntactically correct, it is argued the sentence as a whole is semantically meaningless.

The relevant point to take away is a sentence is considered hierarchical if phrases are embedded within other phrases. Yet, examples of hierarchical organisation are found in many domains besides syntax. This includes other language phenomena, such as prosody. Also, non-linguistic behaviours – such as music (Givon, 1998), action sequences (Koechlin and Jubault, 2006), tool-use (cf. Scott-Frey, 2004) and tool-production (Stout et al., 2008) – are all cognitively demanding tasks, comparable with that of language. We can even see instances of non-human hierarchical representations: from the songs of humpback whales (Suzuki, Buck and Tyack, 2006) to various accounts of great apes (McGrew, 1992; Nakamichi, 2003) and crows (Hunt, 2000) using and manufacturing their own tools[3].

With this in mind, we can ask ourselves two questions corresponding to Broca’s area and hierarchical organisation: Does Broca’s area process hierarchically organised sequences in language? And if so, is this processing language-specific? The logic behind this two-part approach is to help focus in on the problem. For instance, it may be found hierarchical structures in sentences are processed by Broca’s area. But this belies the notion of other hierarchically organised behaviours also utilising the same cognitive abilities.

Continue reading “Broca's area and the processing of hierarchically organised sequences pt.1”