Final answer:
Audition begins to show "center-surround" response fields at the level of the auditory cortex, where complex processing of sound occurs allowing for selective auditory focus. This mechanism is critical for distinguishing sounds, such as conversations, in noisy environments and can be affected by hearing loss, where hearing thresholds for different frequencies are highlighted by audiograms.
Step-by-step explanation:
The concept of "center-surround" response fields primarily relates to how sensory neurons, particularly those involved in the sense of audition (hearing), process stimuli. In the auditory system, these center-surround receptive fields are observed starting at the level of the auditory cortex. The auditory cortex is where complex processing of auditory information occurs, allowing us to detect and interpret various sounds. This region of the brain can emphasize certain sounds while suppressing others, a process akin to how visual neurons enhance visual contrast through center-surround inhibition. In the context of hearing, this allows individuals to focus on specific sounds, like a conversation in a noisy environment.
For someone experiencing hearing loss, this center-surround mechanism can be compromised. As outlined by the audiograms presented in figures like 17.35 and 17.39, hearing losses measured in phons can illustrate how significantly certain frequencies are affected. Persons with a 40-phon hearing loss may still perceive conversations, albeit with difficulty, while those with a 60-phon loss may fail to understand speech without additional volume due to their limited frequency range.
Prior to reaching the auditory cortex, the basilar membrane in the cochlea performs the initial frequency analysis of sound, but it does not create center-surround fields. These fields are patterns of neuronal activity characterized by an opposing response to stimuli by the center and the surrounding areas of the receptive field and become evident only in higher cortical processing centers.