DOI – Page 4 – 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).

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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).

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Olfactory communication and mate choice

From the regulation and reproduction in bacteria colonies (Bassler, 2002) to complex smell and taste systems of humans (Van Toller & Dodd, 1988), the ability of sensing chemical stimuli, known as chemosensation, is believed to be the most basic and ubiquitous of senses (Bhutta, 2007). One strain of thought places chemosensation as merely an evolved ability to detect dangerous and volatile substances – such as putrefied food (see Bhutta, 2007). Still, the notion that this ability to detect chemical stimuli, particularly in the domain of smell, serves a purpose in communication is not necessarily a contemporary concept (Wyatt, 2009).

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Continuity or Discontinuity: are our minds purely shaped by natural selection?

The debate concerning the origin of our minds stems back to the diverging opinions of Darwin (1871) and Wallace (1870). When Charles Darwin first discussed the evolution of our seemingly unique cognitive faculties, he proposed that there is “no fundamental difference between man and the higher mammals in their mental faculties.” (Darwin, 1871, pg. 66). Conversely, Wallace was suspicious of whether natural selection alone could have shaped the human mind, writing: “[…] that the same law which appears to have sufficed for the development of animals, has been alone the cause of man’s superior mental nature, […] will, I have no doubt, be overruled and explained away. But I venture to think they will nevertheless maintain their ground, and that they can only be met by the discovery of new facts or new laws, of a nature very different from any yet known to us.” In the intervening years, the debate surrounding the degree of continuity between animal and human minds still rages on in contemporary discussions (Bolhuis & Wynne, 2009; Penn, Holyoak & Povinelli, 2009).

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Orangutans – probably more interesting than you

In the past few years there has been a recent spate of articles concerning orangutan intelligence. So, as I’m fairly bored, and in need of a break from university work, I’ve decided to write a bit of an essay on some of these finds.

Orangutans… They’re orange, right?

Correct; but Pongo pygmaeus abelii are so much more than just some arboreal orange ape that eats a lot of fruit. In fact, these great apes, the last surviving members of the genus Pongo, are highly resourceful and intelligent creatures, as evident in their ability to make and use tools, perform calculated reciprocity and even whistle a tune.

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Schizophrenia and brain evolution (plus bold adjectives)

When exploring the etiology of schizophrenia, a feat that has mostly eluded understanding for over 100 years, a common denominator emerges in that associated deficiencies are rooted in cognitively demanding tasks. One suggestion is that, where schizophrenic individuals are involved, disorganised thoughts, abnormal speech, auditory hallucinations and paranoid delusions are symptomatic consequences of our haphazardly evolved brains. It might not seem revelatory, nor is it a particularly new thought on the matter, yet this disorder clearly has ties with human-specific, recently evolved behaviours, such as language and social relationships. And it is here in which our problem emerges: we don’t even know how language or social relationships evolved. In fact, the evolution of the human brain is still very much an enigma, despite the whole host of literature having you believe otherwise. As Darwin put it: “Ignorance more frequently begets confidence than does knowledge[…]”.

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