Psychology of Ne/Si
Ne/Si is a function pair characterized by incomplete data and patchwork into missing data. The way Si/Ne flows through life is by setting "anchor-points" at certain locations, and then exploring a bit further and setting another anchor point. For this reason, when Si/Ne users talk they'll hop from one anchor to another to explain their perspective using specific instances that demonstrate the reality they wish to communicate.
When external reality is observed and it registers a recall of Si's memory, that anchor point is highlighted. The surrounding anchors then become visible tangents. If the individual has Ne above Si, there will be more tendency to start from that anchor-point and explore forward into unknown territory and chart a little bit more, then set another anchor via Si registering the sensation brought upon by that item. The direction Ne decides to explore doesn't have to have any rationality to it, and as such an Si map can be filled with trivia that is connected to each other, but which may not serve any practical purpose.
If Si is above Ne, the tendency will be to glide from one anchor-point to a nearby anchor-point. This is why Si-leads have a tendency to ramble on about their experiences. Once their mind goes on a certain trail, it will explain all the aspects of truth it knows about one anchor, then move on to the next and explain that one. This can potentially go on for ages until they've explained their entire worldview one impression at a time.
The map of Si is never complete, and unlike the Ni map which compensates for its incompletion by using an algorithm to project a pattern further into the unknown, Si is genuinely clueless about what it's never encountered. Ne/Si is a pair that toggles between being very optimistic and very paranoid. Si can be prone to paranoia of what is unknown because the map has nothing in that terrain, and so Si is left only to speculate based on the past. If the experiences of the past were unpalatable, Ne may generate a dozen negative scenarios which it may or may not be reasonable. This is why Si may be paranoid about change. The optimism is more common when Ne is above Si, and is due to the very same tendency. In the event of an unknown, Ne can generate dozens of positive outcomes where the tactile data may not indicate any justification for such an assumption. The possibilities themselves hold a level of reality in their mind, until data indicates otherwise, or rationalization proves it impossible.
When confronted with an unknown situation, the Ne/Si pairing has no choice but to resort to Ne. But Ne's speculations are based almost entirely on things being related to one another in the abstract - and not on the causality of reality. In such a case, the omission of a possibility is generally done via Ti/Fi instead of Si, since Si has no data to counter an Ne possibility. This means that in general Ne/Si users have poor predictive abilities, but strong creative abilities because their imagination is not limited by a particular direction of looking at things.
Psychology of Ni/Se
The psychology of Ni/Se is less compartmentalized. Aspects within the Ni map will generally have a link to most any other aspect in the map, in some form or another. The points of literal data which Se brings into the map alter the tapestry of the map and bend/curve it into new shapes, giving rise to concepts that it had no direct exposure to but which emerged naturally from the causal direction the map was heading.
When Se is above Ni, the craving for sensory stimulation is great. Unlike Ne which fails to capture the complete richness of a tactile object, Se experiences the fine crevices, dips, edges, sparkles and nuances which are the very thing that they find fascinating. (Ne's fascination with an object revolves around how it can be manipulated and combined with other concepts.) The attention-span of Se can be strong if the object has a lot of richness to absorb, or very short if it doesn't.
For both Ne and Se, the interest in exploring something will persist so long as it has something unexplored. But for Ne, what is unexplored is the implications of how an object can be manipulated - and so long as it can think up novel ways to use the same basic object (uninteresting as it may be to Se) it will remain engaged. For Se, it's engagement will not be based on "how many things can you think up from this" but on the quality of the object itself. It will seek out richer environments and bask in the overwhelm of stimuli it gets naturally from the objects around it - as opposed to it's head's fabrications.
All the while, Ni stores these sensations and creates a map of associations they have both in time & placement. These associations have long-reaching effects, as the formulas that Ni extracts from those events are generalized and apply somewhat to all other aspects. Ni does not have many specific anchor-points nor does it speak referencing those particulars but instead explains things within the context of their whole paradigm and how it all relates to the rest.
Although there is a benefit in Ni/Se's map intuiting and compensating for missing data, the formulas it builds, when too general, create erroneous perspectives formed from connecting elements in certain ways that have no justification other than the direction their map bent to form them. While Ne/Si can be prone to irrational optimism and paranoia, Ni/Se can be prone to irrational superstitions. It may seems as though a certain event lead, causally, to another, when no tactile trace exists for the connection. When taken more extremely, it may also generate answers to realms where everything is unknown using nothing but patterns and symmetries.
This is the origin of the majority of esoteric & mythical constructs throughout history, when Ni is applied to the unknown realm of meta-physics. Concepts of symmetry, reciprocity natural-law and alignments are basic patterns which Ni sees and fills the unknowns with. For this reason strong Ni users can have a "mystical" sense about them and "knowing" aura of certainty about the questions that baffle the rest of us. It may not mean that they have any more knowledge of a realm than we do, but by the nature of their psychic process they formulate an answer to it where many others remain clueless or adapt a hypothesis of this sort.