The ventral pathway

Introduction

Three routes through the model

For the last few weeks we have been tracing out the route of speech repetition through the model in Fig. 100.

Comprehension of the phrase “my cat” traces the following route through the model:

_images/t16-VentralComprehension.png

Fig. 105 Comprehension of the phrase “my cat” in my extended version of Hickok & Poeppel’s dual pathway model.

Author’s image.

The blue arrow

_images/t16-VentralInteraction.png

Fig. 106 Answer to the question “What’s your name” in my extended version of Hickok & Poeppel’s dual pathway model.

Author’s image.

Linguistic notation in the ventral pathway

A review of linguistic notation

Linguistics deals with three kinds of information: (i) the description of speech sounds in phonetics and phonology, (ii) the description of meaning in semantics, and (iii) the description of morphemes and word order in morphology and syntax. For ease of representation on the printed page, a notation has been developed to label each kind of information so that it is always clear what is being talked about. As an example, take the English word ‘cat’, whose single quotes annotate it as belonging to a fourth kind of description, that of English spelling.

_images/t16-LingNotation.png

Fig. 107 Three ways of representing the English word spelled ‘cat’.

Author’s image.

The goal of the dorsal pathway is to deal with phonetics, so I will concentrate on the other two.

The second format, labels the left square bracket with a subscript capital letter that abbreviates a part of speech, here N for Noun. In fact, [kæt] is a singular noun, but there is no need to make the notation more complex than it already is.

The third notation encloses the word in a pair of brackets that you may have never seen, called white square brackets. They indicate the denotation or meaning of a word. A meaning is not a linguistic object, though it can be approximated linguistically, such as Dictionary.com’s definition of ‘cat’ as “a small domesticated carnivore, Felis domestica or F. catus, bred in a number of varieties.” I have tried to illustrate the non-linguistic status of ⟦kæt⟧ by appending a photo of a cat beneath it, but a photo is not a meaning, either, just another sort of approximation.

Processing in the ventral pathway as revealed by linguistic notation

Having refreshed your memory, I turn to using the notation as a means of explaining how the ventral pathway works:

Table 31 Way stations on the ventral pathway
Phase Output
ear \([mai̯kæt]\)
dSTG \([m][a][i̯][k][æ][t]\)
STS \([mai̯][kæt]\)
p(MTG+ITS) \(⟦mai̯⟧⟦kæt⟧\)
a(MTG+ITS) \(⟦⟦mai̯⟧⟦kæt⟧⟧\)
aIFG \([_{NP} [_{Poss} mai̯] [_N kæt]]\)

The ear transduces sound waves into the unanalyzed auditory object \([mai̯kæt]\), and the subcortical auditory pathway relays it to auditory cortex in the dorsal superior temporal gyrus, where it is segmented into its constituent phones. At the superior temporal sulcus, the segmentation is recognized as phonetic words. At the first stop on the ventral stream, the lexical interface at the posterior middle temporal gyrus and the posterior inferior temporal sulcus, which I have lumped into the abbreviation “p(MTG+ITS)”, converts the auditory words into semantic words. The first combinatorial net at the anterior portion of the middle temporal lobe combines the words into a semantic phrase, which is converted into a syntactic noun phrase at the second combinatorial net in the anterior inferior temporal gyrus.

Two or four sorts of semantics

You navigate through meaning so effortlessly that it may be difficult for you to imagine how fraught with problems it is. In a recent review article, {Meteyard et al., 2012} summarize a contentious debate in linguistics and cognitive science over two competing kinds of meaning, which are known as symbolic and embodied or sensorimotor. You have already been exposed to this distinction, though I didn’t make a fuss about it. Let us now make a small fuss.

Symbolic vs. sensorimotor representation in the dorsal pathway

As you know, we have conceptualized the dorsal pathway as speech repetition, in which its input is an auditory object and its output is a motor object, the result of the vocal tract’s articulation of the input. This is a quintessential sensorimotor process, which is to say that it invokes both sensory and motor functions. Linguistics fudges the notation for the two phases of the process, since it uses has square brackets like [kæat] for both. For the sake of perspicuity, I will mark motor objects in bold-face square brackets and reserve regular square brackets for auditory objects. Thus the sensorimotor input and output of the dorsal pathway can be illustrated as in:

(4)\[[kæt] \Longleftrightarrow \pmb{[kæt]}\]

But I have left out a crucial part: these are just the two edges of the pathway; there is a middle part in the Spt in which auditory objects are transformed into motor objects, and vice versa. Is that sensory, motor, both, or neither?

Now I have to confess that I have been holding something back from you, hoping that you would remember. In The cognition of speech: phonology, I introduced another notation – yes, a fifth one! – designed for dealing with exactly this situation in which we do not want to commit to qualifying an object as either sensory or motor. It is to enclose a sequence in the International Phonetic Alphabet between in slashes. Inserting such a sequence into the middle of (4) produces:

(5)\[[kæt] \Longleftrightarrow /kæt/ \Longleftrightarrow \pmb{[kæt]}\]

In The cognition of speech: phonology I called this a phonemic representation. I used it to distinguish between vowel nasalization in English, which depends on the context of the vowel and does not change the meaning of the word that it appears to, vowel nasalization in French, which does not depend on the context of the vowel and does change the meaning of the word that it appears in. You may want to review this material.

The fact that a phoneme represents information that does not depend on – or is invariant to – sensorimotor context suggests that it plays a unique role in the ventral pathway. Not coincidentally, the phoneme is also the level of processing that makes contact with meaning, though exactly how Spt could network with the lexical interface in p(MTG+ITS) is a mystery that we will not unravel this semester. But a more pressing concern …

References

  • Meteyard, L., Cuadrado, S. R., Bahrami, B., & Vigliocco, G. (2012). Coming of age: A review of embodiment and the neuroscience of semantics. Cortex, 48(7), 788-804.
  • Pulvermüller, F. (2013). How neurons make meaning: brain mechanisms for embodied and abstract-symbolic semantics. Trends in Cognitive Sciences, 17(9), 458-470.

The next topic

The next topic is The lexical interface.


Last edited Aug 26, 2019