How to Build Temporal Lobes

In this chapter we must create another sense organ; our half-formed brain can see and smell, but cannot hear, and relatedly has no language or speech. The difficult task of crafting auditory and language apparatus lies before us as we continue in our brain-building exercise. We must shape the outer (lateral) surface of the lobe into its distinct gyri and sulci, which allows us to pack more brain tissue into the skull. The basic external structure of this lobe includes superior, middle, and inferior gyri BA20, 21, and 22, respectively, after Brodmann (1909) based on distinctive cellular architecture, and including BA37/38 for the temporal pole and B41/42 for the auditory cortex, also called Heschl’s gyrus, and embedded deep within the Sylvian fissure. Here, we need functional structures capable of processing epochs of sound waves at specific frequencies—not all sound waves—just those relevant to behaviour on a human scale to provide sensory input to this lobe. In simple terms, sounds are oscillations (more commonly termed vibrations) within gaseous and viscous mediums that cause a neural stimulus to excite nerve cells in the auditory system, producing an auditory percept. Our ears need to perform a few other functions too: the perception and the location of sound, and control postural equilibrium, or balance, and contribute to the co-ordination of head and eye. Hence, we need two auditory systems—one for balance and one for hearing.