7.1 Acoustic Cues and Signals
Hearing is a critical sense that allows animals to detect objects and events in their environments. It is based on the perception of sound, acoustic pressure waves that travel through the air. Understanding the different ways sounds are generated and how sound waves interact with the physical environment lays the groundwork for examining how the ear senses sound and how the brain processes acoustic information.
7.2 How Does Acoustic Information Enter the Brain?
In terrestrial vertebrates, acoustic signals are sensed by the ear. The external ear funnels sound waves into the ear canal and contributes to localizing sounds. The middle ear transforms oscillations in pressure into mechanical displacements of the fluid in the cochlea. The cochlea contains the basilar membrane, which separates complex sounds into their component frequencies, and the organ of Corti, which contains mechanosensitive hair cells that transduce movements of the cochlear endolymph into neural impulses. These neural impulses are processed in a series of interconnected nuclei called the ascending auditory pathway, eventually reaching the cerebral cortex.
7.3 How Does the Brain Process Acoustic Information?
The neural circuits of the auditory system process incoming stimuli to determine where sounds are coming from and what is making them. The locations of sound sources are determined early in the auditory pathway, primarily by comparing the relative timing and level of sounds arriving at the two ears. Neural circuits later in the auditory pathway are responsible for decoding information about what is being heard. Many complex sounds are perceived in terms of basic features of pitch, loudness, and amplitude, while other sounds like speech are perceived as discrete categories. In humans, the perception of phonetic speech categories emerges early in life as a consequence of experience.
7.4 Balance: A Sense of Where You Are
The vestibular system senses linear and angular movements of the head and its tilt relative to gravity. These forces are transduced to neural signals by hair cells within the semicircular canals and the otolith organs of the vestibular labyrinth. The vestibular nuclei are responsible for generating reflexes in the eye muscles that maintain gaze and for reflexes in the neck and body muscles that maintain head position and balance.