I seem to remember reading something Jung himself wrote in "Psychological Types". It was along the lines of saying that the 8 type functions by no means represented the whole of human cognition, and excluded other facilities such as imagination, willpower and memory.
I would hazard a guess that all four of the introverted functions draw heavily from our personal memories (in different ways). The extraverted functions are more associated with external, real-time events and information.
I actually have an excellent memory, even though as an INFJ I supposedly have Si as my weakest function in the 8-function type model. However, my memory is based on general impressions and vague hints which lead me to the required piece of knowledge.
In contrast, Si, as I understand it, excels at storing and recalling sensory information in amazing clarity and detail. It is also great for noticing subtle details in the immediate environment when they differ from how things are "supposed" to be.
Even though my memories start at a very early age (2 or 3 years old) they are very sparse on detail. I find it hard to remember people's faces or names, for example. It takes quite a lot of time and effort for me to pull out the full details of a specific memory.
I had a very good friend at my old workplace who shared my interest in type and who was a confirmed ISFJ. We used to enjoy sharing our different internal perceptions, as we were superficially quite similar (both being IFJs), but our internal experiences were very different. Talking with my friend gave me a much better understanding of how Si works as a dominant function. She had an extraordinary ability to recall precise and accurate details from her memory; especially relating to the people in her life. In contrast to me, she always remembered people's names, as well as a host of personal information relating to each person. She was the go-to person for workplace gossip, as she somehow knew exactly what was going on with everyone. She could tell who was having affairs and who was under stress, merely by observing subtle clues and hearing snippets from other people. However, she didn't tittle-tattle - as an introvert she only shared her information with people she could trust to be discreet, such as myself.
She in turn was amazed by my introverted intuition and rich imagination. She once told me she had very little imagination, but I had already observed that when she was recalling a memory, her eyes became unfocussed and she "tuned out" as she formed her vivid mental reconstructions. I do the same thing when using my Ni to conjure imaginary scenarios. I guessed that she probably used her imagination in service to her memory, whereas I do the opposite, my imagination is dominant and my memory subordinate to it.
This makes a lot of sense of the idea that Ni and Si are truly opposite functions. If they work the way I have conjectured, then they must compete with each other for the same resources in the brain. Specialising in Ni as a function would push Si deep into the unconscious, and vice versa.
Thoughts, anyone?