Chapter 11 endnote 12, from Lisa Feldman Barrett.
Some context is:
A human brain’s sensory and motor neurons, however, communicate through intermediaries, called association neurons, and they endow your nervous system with a remarkable ability: decision-making.
Association neurons allow a behavior to be controlled by the nervous system of the acting creature as much as, or even more than, by the physical properties of the stimulating event. This is sometimes referred to as voluntary action or volition (not to be confused with the subjective experience of control or what is called "deliberate action"). Voluntary actions, which result from decision-making in the brain, need not (and typically do not) bear the hallmarks of experienced control. Variability and complexity are hallmarks of what association neurons do for a nervous system. Nature has continued to select for nervous systems with increasingly complex arrangements of association neurons.
From the perspective of the sensory and motor neurons in the peripheral nervous system, the entire brain can be thought of as a mass of association neurons. From the perspective of sensory and motor neurons in subcortical regions, the cerebral cortex and cerebellum are just layers of association neurons, an arrangement that’s efficient for sharing information. Whole regions of the cerebral cortex are also traditionally designated as input (sensory), output (motor), and association regions, and even layers of the cerebral cortex are (stereotypically) considered input (layer 4), output (layers 5 and 6) and association (layers 2 and 3).
I think the designation "association neuron" depends on your perspective and goals as a scientist. Any set of neurons can be considered association neurons, depending on the questions being asked. For example, neurons in the visual system can be considered association neurons if you are interested in the auditory system (and vice versa). An association region is not a place, but a flexible, functional designation.