Affect and the structure of the human cortex

Chapter 4 endnote 39, from How Emotions are Made: The Secret Life of the Brain by Lisa Feldman Barrett.
Some context is:

...interoception is not a mechanism dedicated to manufacturing affect. Interoception is a fundamental feature of the human nervous system, and why you experience these sensations as affect is one of the great mysteries of science.

Why are interoceptive perceptions usually experienced as affect? The reasons are not well understood. In my lab, we hypothesize that it has something to do with the structure of the brain. Interoceptive predictions cross only one synapse from body-budgeting (visceromotor) neurons (in the cingulate cortices and anterior insula) to primary interoceptive cortex (in mid and posterior insula). Compare this to visual, auditory and somatosensory predictions, which flow across many synaptic connections before arrive to the primary exteroceptive cortices; this provides many opportunities for the prediction to become elaborated with details. As a consequence, visual, auditory and somatosensory predictions are highly processed in high dimensional, specific detail in comparison to interoceptive predictions. Also, primary visual, auditory, and somatosensory cortices have a very well developed layer IV in the primate brain, which might mean that we are better able to compute prediction error and learn from those sensory inputs in a more detailed way; by contrast, layer IV in primary interoceptive cortex is not as well developed. The representation in primary visual, auditory and somatosensory cortices might also be more separable than in primary interoceptive cortex to begin with. This might explain why people have more vivid, precise experiences of what is going on outside the body (or on the body) than inside the body; you can see, hear, and feel with your skin in more detail than you can feel internal bodily sensations, which you routinely access as low dimensional features of affective feelings of valence and arousal.