Chapter 4 Summary

The primary auditory pathway begins with receptors (hair cells) in the basilar membrane that respond to local pressure changes. This information is processed in the cochlear nuclei, additional nuclei in the brainstem, the inferior colliculus in the midbrain, and the medial geniculate nuclei of the thalamus before being sent on to the primary auditory cortex (A1).
The major perceptual qualities in audition are loudness, pitch, and timbre. Loudness is the perception elicited by sound intensity; pitch is the perception elicited by periodic variations in the power of sound stimuli; and timbre is the perception of differences in the qualities of sound sources when their intensities and frequencies are the same.
The higher-order processing of sound stimuli that gives rise to auditory percepts occurs in the belt areas of auditory cortex adjacent to A1, which are generally analogous to the extrastriate cortical areas in the visual system. These areas are especially responsive to auditory stimuli that have biological significance, such as speech.
The responses to mechanical forces acting on the body are generated by the somatic sensory system. The cutaneous/subcutaneous component of this system generates the perceptions of touch, pressure, and vibration that arise from mechanical stimulation of receptors in the cutaneous and subcutaneous tissues.
A distinct pain system generates noxious percepts that alert us to mechanical or thermal stimuli that are potentially harmful.
Responses to chemicals in the environment are generated by the olfactory system, the gustatory system, and the trigeminal chemosensory system.
In olfaction, responses to airborne molecules that interact with receptors in the nasal cavity lead to the perception of odors. Gustation results when soluble molecules interact with receptors primarily on the tongue, leading to the perception of tastes. The trigeminal system responds to noxious chemicals that find their way into the nose and mouth, generating percepts usually described as irritating; as such, the trigeminal system is closely related to the pain system.
All three chemosensory systems ultimately provide information to regions of the brain that are closely tied to autonomic and emotional responses, including the reticular formation, the entorhinal cortex, the insula, and the amygdala.
Although we are inclined to equate sensory perceptions with conscious awareness, the processing elicited by most sensory stimuli and the responses to them never reach this level.