Other forms of synesthesia have been reported, but little has been done to analyze them scientifically. There are at least 80 different types of synesthesia.

Synesthesia is found in at least 4.4% of the population, as a high estimate, which is equivalent to 1 in 23 people. This study had also concluded that one common form of synesthesia—grapheme-color synesthesia (colored letters and numbers) – is found in more than one percent of the population, and this latter prevalence of graphemes-color synesthesia has now been independently verified in a yet larger sample. Earlier estimates of the prevalence of synesthesia were based on "best-guess" estimations only ("e.g." 1 in 250,000) or had limitations in their methodologies because they required synesthetes to refer themselves for study ("e.g." 1 in 2000) and for this reason the authors of those studies had been moderate in their claims. Also, some individuals will not self-classify as synesthetes because they do not realize that their perceptions are different from those of everyone else.

The most common forms of synesthesia are those that trigger colors, and the most prevalent of all is day-color. Also relatively common is grapheme-color synesthesia. We can think of "prevalence" both in terms of how common is synesthesia (or different forms of synesthesia) within the population, or how common are different forms of synesthesia within synesthetes. So within synesthetes, forms of synesthesia that trigger color also appear to be the most common forms of synesthesia with a prevalence rate of 86% within synesthetes. In another study, music-color is also prevalent at 18–41%. Some of the rarest are reported to be auditory-tactile, mirror-touch, and lexical-gustatory.

There is research to suggest that the likelihood of having synesthesia is greater in people with autism.

Grapheme-color synaesthesia or colored grapheme synesthesia is a form of synesthesia in which an individual's perception of numerals and letters is associated with the experience of colors. Like all forms of synesthesia, Grapheme-color synesthesia is involuntary, consistent, and memorable. Grapheme-color synesthesia is one of the most common forms of synesthesia, and because of the extensive knowledge of the visual system, one of the most studied.

While it is extremely unlikely that any two synesthetes will report the same colors for all letters and numbers, studies of large numbers of synesthetes find that there are some commonalities across letters (e.g., "A" is likely to be red). Early studies argued that grapheme-color synesthesia was not due to associative learning, such as from playing with colored refrigerator magnets. However, one recent study has documented a case of synesthesia in which synesthetic associations could be traced back to colored refrigerator magnets. Despite the existence of this individual case, the majority of synesthetic associations do not seem to be driven by learning of this sort. Rather, it seems that more frequent letters are paired with more frequent colors, and some meaning-based rules, such as ‘b’ being blue, drive most synesthetic associations.

There has been a lot more research as to why and how synesthesia occurs with more recent technology and as synesthesia has become more well known. It has been found that grapheme-color synesthetes have more grey matter in their brain. There is evidence of an increased grey matter volume in the left caudal intra- parietal sulcus (IPS). There was also found to be an increased grey matter volume in the right fusiform gyrus. These results are consistent with another study on the brain functioning of grapheme-color synesthetes. Grapheme-color synesthetes tend to have an increased thickness, volume, and surface area of the fusiform gyrus. Furthermore, the area of the brain where word, letter, and color processing are located, V4a, is where the most significant difference in make-up was found. Though not certain, these differences are thought to be part of the reasoning for the presence of grapheme-color synesthesia.

Individuals with grapheme-color synesthesia rarely claim that their sensations are problematic or unwanted. In some cases, individuals report useful effects, such as aid in memory or spelling of difficult words.

These experiences have led to the development of technologies intended to improve the retention and memory of graphemes by individuals without synesthesia. Computers, for instance, could use "artificial synesthesia" to color words and numbers to improve usability. A somewhat related example of "computer-aided synesthesia" is using letter coloring in a web browser to prevent IDN homograph attacks. (Someone with synesthesia can sometimes distinguish between barely different looking characters in a similar way.)

Hyperthymesia is the condition of possessing an extremely detailed autobiographical memory. People with hyperthymesia remember an abnormally vast number of their life experiences.

American neurobiologists Elizabeth Parker, Larry Cahill, and James McGaugh (2006) identified two defining characteristics of hyperthymesia: spending an excessive amount of time thinking about one's past, and displaying an extraordinary ability to recall specific events from one's past. The word "hyperthymesia" derives from Ancient Greek: "hyper-" ("excessive") and "thymesis" ("remembering").

Individuals with hyperthymesia can recall almost every day of their lives in near perfect detail, as well as public events that hold some personal significance to them. Those affected describe their memories as uncontrollable associations; when they encounter a date, they "see" a vivid depiction of that day in their heads. Recollection occurs without hesitation or conscious effort.

It is important to draw a distinction between those with hyperthymesia and those with other forms of exceptional memory, who generally use mnemonic or similar rehearsal strategies to memorize long strings of subjective information. Memories recalled by hyperthymestic individuals tend to be personal, autobiographical accounts of both significant and mundane events in their lives. This extensive and highly unusual memory does not derive from the use of mnemonic strategies; it is encoded involuntarily and retrieved automatically. Despite being able to remember the day of the week on which a particular date fell, hyperthymestics are not calendrical calculators like some people with autism or savant syndrome. Rather, hyperthymestic recall tends to be constrained to a person's lifetime and is believed to be a subconscious process.

Although hyperthymestics are not necessarily autistic, and likewise savants do not necessarily memorize autobiographical information, certain similarities exist between the two conditions. Like autistic savants, some individuals with hyperthymesia may also have an unusual and obsessive interest in dates. Russian psychologist Alexander Luria documented the famous case of mnemonist Solomon Shereshevsky, who was quite different from the first documented hyperthymestic known as AJ (real name Jill Price) in that Shereshevskii could memorize virtually unlimited amounts of information deliberately, while AJ could not – she could only remember autobiographical information (and events she had personally seen on the news or read about). In fact, she was not very good at memorizing anything at all, according to the study published in "Neurocase". Hyperthymestic individuals appear to have poorer than average memory for arbitrary information. Another striking parallel drawn between the two cases was that Shereshevsky exemplified an interesting case of synesthesia and it has been suggested that superior autobiographical memory is intimately tied to time-space synaesthesia.