1. Organs of Hearing
 
 

a) Outer ear	b) Middle ear
c) Inner ear

Auditory signals are transmitted along the cochlear nerve, up through the brain stem, via the Medial Geniculate Body (in the middle of the brain) and then outwards on each side to the primary auditory area of the cerebral cortex, on the upper surface of the superior temporal gyrus (STG). Deacon (1997: 236) suggests that processing of some stimuli by the midbrain may explain primal and involuntary responses that we have in some cases, the "infectiousness" of our own species-specific non-linguistic calls, laughter and crying.

Sounds presented to the left ear mainly activate auditory cortex on the right temporal lobe, but also the left. And vice-versa for the right ear. Note that men tend to have a right ear advantage, that is, they make fewer errors in speech perception when sounds are presented to their right ear only, compared to left ear presentation. This asymmetry is particularly strong in consonant perception. Women, however, are more balanced ...
 

2. Primary auditory cortex

Tonotopic representation: different parts of primary auditory cortex are activated by sounds of different pitches, giving rise to a tonotopic map. There are several such tonotopic maps in auditory cortex, in fact. Using MEG (magnetoencephalography), Poeppel (1997) and co-workers have shown that vowels activate locations in primary auditory cortex.

Interestingly, the visual experience of watching a talking face also activates primary auditory cortex, showing that visual information may be used in speech understanding and is integrated with auditory speech at a very early stage in processing. Variations in individual speakers' voice characteristics seem to be reflected in activations of different regions of the superior temporal sulcus, the deep ridge just below the STG.

Penfield and Roberts showed in the 1940's that stimulation of the surface of the STG with mild electric currents induced various kinds of auditory hallucinations. Researchers at the Institute of Psychiatry in London studying patients suffering from schizophrenia have shown that "during continuous auditory hallucinations, described by the patient as spirits talking to him, there was a marked attenuation of activation in response to [an] auditory stimulus ... We can therefore conclude that hallucinations involve primary, sensory cortical areas which seem to 'compete' with exogenous sensory stimuli".
 

3. Speech production

In many vertebrate species, both vocalisation and swallowing employ coordinated control of the oral, vocal and respiratory muscles, under the overall control of structures in the midbrain (Deacon ch. 8). In humans, involuntary calls (crying, laughing) may also be produced by midbrain activity. For spoken language, however, a voluntary, intentional take-over of the respiratory system (including the larynx) and the oral muscles is required. Such voluntary movements are under the control of the motor areas (primary and supplementary).

Penfield's homunculus shows the organisation of the primary motor cortex. Stimulation of particular areas (either electrical stimulation of the surface of the exposed cortex, or magnetic stimulation from the exterior) can cause or inhibit voluntary movements of the corresponding articulator (tongue, leg, hand etc.).

Dogil et al. (2002) have shown that more and more complex articulation employs more and more focussed activation in primary motor cortex.