"I experience numbers in a very visual way, using, colours, texture, shape and form, sequences of numbers form, landscapes in my mind. It just happens. It's like having a fourth dimension."
That is a quote from the Brainman/The Boy with the Incredible Brain documentary, Daniel Tammet describing his number synaesthesia. One word in this quote makes me feel that it is not a genuine description of a synaesthete's own experiences. It is the word "using". I feel that a real synesthete wouldn't have used the word "using" here, because synaesthetes don't use or apply colours, textures, shapes or forms to their sensory experiences to transform our normal thought processes into synaesthetic thought processes. We simply experience these additional sensory experiences. We do not create our own synaesthesia. We do not perform our synaesthesia. Synaesthesia is natural to us. We do not use visual characteristics in our synaesthesia.
Two other elements of this quote strain credulity. The first is the unnatural rhythm of the way it was spoken, which is a fad that was once common in TV commercials. I think the advert people thought that if they got actors to speak in an unnatural rhythm it would subconsciously grab the TV viewer's attention. I just found it very annoying and contrived.
The other element that I find hard to believe is Tammet's claim that sequences of numbers (spontaneously) form landscapes in his mind. This could certainly be interpreted as a description of number forms (known also as a type of spatial-sequence synaesthesia), but the problem is that this type of synaesthesia involves spatially experienced lines of numbers or sequential items, not landscapes. People who have number forms generally describe it as lines, not landscapes, for a very good reason. The fantastical shapes that they describe often could never be the shapes of landscapes. An oval ring that hangs in the air could never be the shape of a landscape. This is only one of many aspects of Tammet's self-descriptions of his own supposed synaesthesia that differs from typical reports of synaesthetes, which even the most simple Google search can retrieve in scores. I am a synaesthete, and I believe that Tammet is not one of our type.