I've been considering what the basic difference between introversion and extroversion in a given trait are. One telling characteristic is the amount of delay before changes become apparent, and how hard they are to get to change direction. So as an alternative to Augusta's description of a field/body dichotomy, and Jung's subjective/objective one, I'm thinking the difference could be described as an analogy to high-mass/low-mass inertia in physics.
Someone with Fi tends to give time for emotional tension to build up before making a statement, and will try to make sure it is percieved as important. This seems to correspond to high mass. In a like manner, Ti will try to create many mutually supporting logical correlations before voicing an opinion. Ni will attempt to locate several different coherent sources of prediction, and Si will try to find similarities in many different sensation sources. Another way to put it is that the introverted trait processes in parallel, searching out and using many similar and related pieces of information.
Extroverted traits are more of a low-mass sort of thing. They have advantages in being able to operate with speed and agility, with greater maneuverability. But they are also easily blocked and side-tracked. Fe will try and make an immediate emotional impression, but easily be affected as well. Te will argue an opinion in a manner intended to immediately convince, but will be easily dissuaded as well if the particular argument produces an invalid result. Se will try and immediately accomplish a physical task or get a specific person to do so, but may report it as a complete failure when it doesn't happen right away. Ne tries to find a path to create a temporal prediction, but if a given prediction doesn't "pan out" in a short amount of time, it is soon dismissed as probably being simply invalid/valueless speculation. Extroverted traits are more like sequential processing, which is able to happen rapidly and on many unrelated information sources.
To relate it back to the fields/bodies, sequential data processing has the flexibility to switch to many distinctly different information objects. The distinct difference and ability to be seen as completely unrelated to each other is what makes them bodies rather than fields. The parallel form of processing has to use more related sorts of information because the difference in timing is not there, so distinguishing aspects as widely different objects is not as easy in the short term. Large amounts of highly interrelated information tends to not have clear-cut distinctions, so fields is a more appropriate description than bodies here.
This presents a picture of human consciousness as a sort of computer which processes data both in parallel and in sequence. When one form of processing is more active, it generally causes the person to behave in an introverted or extroverted fashion. I doubt they exist in isolation at any time -- more likely they all come into play subconsciously whenever one is consciously dominant.