Functional diagram of language areas

Despite numerous studies on functional mapping of language, characterization of language related areas is far from complete. Whether there is a map is a contentious issue in the first place. I don’t want to go into the philosophical debate, since I think it’s becoming clearer now that both sides (1) people who wants you to believe that brain is made of modules that can be labeled using terms defined in preexisting theories, and (2) people who think that that the cerebrum is just a homogeneous sheet of processing units are both wrong.

I do believe in the reality of linguistic units (phoneme, morpheme, phrases, etc.) and I am fond of learning various syntactic theories (government and binding, minimalism, HPSG, etc.), but I think it’s naive to assume that there are brain modules dedicated to levels of language representations and syntactic operations. I am very skeptical of Marr’s levels of design (computational / algorithmic / implementation) when it comes to the brain. Recent development in deep neural networks provides a good counter-example: we can design a processing system in which we cannot label the function of each component clearly, at least in terms of preexisting theories.

Having said that, I do not think that the cerebrum is just a homogeneous sheet. The most important structure obviously emerges from inputs and outputs as well as long and short range connecting fibers. Genes can modulate numerous parameters related to the formation of neo-cortex, although it does not look as heterogeneous as “old” organs (like the heart or subcortical areas) since there has not been nearly as much time historically to tweak these parameters.

So in short, I’ll start with inputs and outputs (for the nature of inputs and output see the appendix), and try to carefully characterize the working of local areas, paying full attention to the fact that traditional way of characterizing language areas may be totally wrong. Main source of information are the review papers by Friederici (2011), which summarizes numerous studies in the linguistic brain are summarized, and Hickok and Poeppel (2007), written by two of the powerhouse of knowledge and critical thinking in language.

Language related event related brain potentials (ERP) provide valuable insights, since ERP research has a rich history of critical works and also it provides timing information.

·      N100, associated with acoustic and phonological process, is localized around the auditory corterx.

·      ELAN (150mS), associated with syntactic category assignment, is localized  around anterior temporal / inferior frontal gyrus (IFG).

·      LAN (400mS), associated with morphosyntactic features for argument structures, is localized around anterior temporal gyrus/IFG.

·      N400, associated with lexical access load, is localized around mid-posterior superior temporal gyrus /IFG.

·      P600, associated with syntactic reanalysis (but can be semantically motivated), is localized around middle temporal gyrus/basal ganglia.

Results of MRI studies are harder to summarize. For one thing, stimulus conditions vary from one study to another. Also labels like “semantic”, “multimodal”, etc.  mean different things in different studies. For instance semantics as in thematic role assignment and word disambiguation are totally different.

So I focus focus on inputs, outputs, long-range connections, and the axes in the frontal and temporal areas along which the quality of information changes most dramatically.  The auditory input to the temporal lobe and the motor output to language related muscle control (see appendix) in the frontal lobe are clear. The output from the speech processing is less clear, seemingly go into higher area including the area 45. The input to the speech motor control is also unclear, which may consist of knowledge to be expressed and the motivation to speak. The main axis of qualitative variation, to me, appears to be the time-range. As the position in the language related cerebral cortex approaches the posterior end of the prefrontal cortex, or closer to the primary auditory cortex in the temporal lobe, the information seems to be more short term (e.g. a simple motor command or a short duration of certain speech spectrum). As the position approaches frontal end of the prefrontal cortex or furthest end from the primary auditory cortex, the information becomes long term (or even static), (e.g. an intended message content or a sentence or even a paragraph).

I will probably keep revising the diagram, but the figure below is the one I got for now.

Appendix: Subcortical language pathways

What kind of auditory signal does the primary auditory cortex (area 41) receive? The auditory pathway from the cochlea to the primary auditory cortex consists of Ventral and Dorsal Cochlear Nucleus, Superior Olivary Complex, Lateral Lemniscus, Inferior Colliculus, and Medial Geniculate Body. So by the time the signal reaches is area 41, it is already processed for directional information as well as for some speech related features such as complex spectrum and onset times. 

Speech motor control requires exquisite coordination of many muscles, so the output from the speech related primary motor cortex (inferior part of area 4, which in turn receives from area 6) goes out to many nerve tracts, including cranial (V:Trigmantal, VII: Facial, IX/X: Glosspharyngeal/Vagus, XII: Hypogrossal) and Laryngeal nerves. Besides these nerves from the primary motor cortex called pyramidal tracts, there are extrapyramidal speech tracts that goes from the cerebellum, premotor cortex via basal ganglia, and output to thalamus, which nevertheless do not directly innervate the lower motor neurons that control speech related muscles. Unlike the auditory signal pathway, coordination of these multiple output does not involve many lower level ganglions.

Caplan D. The neurobiological basis of language. Brain. 2007 May 1;130(5):1442-6.

Damasio AR, Geschwind N. The neural basis of language. Annual review of neuroscience. 1984 Mar;7(1):127-47.

Friederici AD. The brain basis of language processing: from structure to function. Physiological reviews. 2011 Oct 1;91(4):1357-92.

Hickok G, Poeppel D. The cortical organization of speech processing. Nature Reviews Neuroscience. 2007 May 1;8(5):393-402.

Stowe LA, Haverkort M, Zwarts F. Rethinking the neurological basis of language. Lingua. 2005 Jul 31;115(7):997-1042.

Tallal P, Miller S, Fitch RH. Neurobiological basis of speech: a case for the preeminence of temporal processing. Annals of the New York academy of sciences. 1993 Jun 1;682(1):27-47.