Personally, I have excellent long Term Memory. Facts, pictures, conversations, diagrams, experiences, music, ideas, knowledge, I have no problem remembering it once it is in my LTM. Short Term Memory, not so much. My long term memory is almost exclusively in pictures. There may be a few exceptions (eg SOHCAHTOA and the Seabus announcement) are embedded with words, but my prime mode of memory is pictures. I have tried to remember Pi through reciting words to failure, but I do have long term memory of the first 35 digits of pi by simple recitation. My memory of pictures and speeches is also excellent, as I can recite entire speeches I gave months ago word for word with no review.
Of course, I have just finished Temple Grandin's Thinking in Pictures and have only very recently (while reading her book) realized to the extent at which my "visual thinking" and "visual memory" and "ability to Connect" to others' feelings differs from so many of my peers, and is so similar to those that people with autism possess. No wonder the majority of my elementary friends possessed autism, because I was able to relate to them.
Like Temple, I can design facilities or systems in my head and mentally "run" them without actually operating the system. An example is one I use frequently, especially when designing transit networks, is to mentally be on the bus, to mentally see the sight that I would see on a regular trip (again, I think in pictures so I can actually reproduce images of the moving streetscape in my mind), reproduce the sounds and feeling and even smell and what I would be doing if I were on the bus mentally. This way, actually being on the bus, I would tell any errors in my schedule, the traffic on that route and estimate the time recovery should be, and count the traffic lights that would delay the trip.
Though I have to admit it, I can think in words, which is the main discrepancy between me and autistics, that they cannot in any way think in words, but what differs me from NT peers is that I can, again, I CAN think in pictures if it is easier.
So to clarify, I think in both words and pictures, but only remember in pictures and feelings. What is new, is that this has been my main language since... Forever. I, until reading her book thought that everyone thought that way (I had suspension last year but she verified it), that everyone could actually mentally "see" or visualize their bed in front of their mind at any time. But it is crude reality that they can't. Similarly I encountered crude reality again here: I thought that people got C+ and lower grades because they weren't trying, but this was only partially correct. What I found is that people, even if they tried, couldn't get anything better than a C+; that they really were that stupid. The reason for this is, to repeat, they lacked visual thinking, visual memory, and with that, an adequate LTI. What has been my main mode of thinking for all my life, I thought existed in everybody, and it does, though just more naturally in me than almost all others.
Note that as I am typing this, I am not forming sentences before I type, I start typing what I am seeing in my mental picture, then typing each word as it fits the picture; I think in pictures and instantaneously translate it into words, I have no idea of the words in the sentence I am about to type before I start typing, but I already mentally with pictures or feelings know what I