Synesthesia is loosely defined as a merging of the senses, the notion that one sensation can trigger a separate perception. This is most commonly manifested as individuals who perceive color with letters or numbers (known as grapheme-color synesthesia), however other variants include experiencing taste with words, texture with taste, or color with music. It is important to note that the secondary, or concurrent, sensation does not replace the primary, or inducer, perception, but is instead experienced in addition to it. There are currently 61 known variants of synesthesia, spanning the perceptual spectrum of color, taste, sound, touch, and even higher level cognitive conceptions, such as language. At first glance, these phenomena seem quite unique and binary (after all, you either taste rectangle or you don't), and the prevalence of synesthesia was originally thought to be roughly 1 in 1000. However, new research on sensory integration has revealed that varieties of synesthesia may be much more common than originally thought, and that there may be a sliding scale in the amount of sensory cross-talk among individuals.
I got to attend a talk by the dynamic neuroscientist Dr. David Eagleman at the Society for Neuroscience meeting this week, who spoke on temporal expansion and compression (i.e. the subjective speeding up and slowing down of time in our minds) and its possible neural underpinnings. The gist of Dr. Eagleman's research is that unexpected events seem to last longer (the "oddball effect"), whereas repetitive occurrences seem to speed up and go faster. The classic example of this is when an object is presented to you several times in sequence, you begin to think that the presentation has been sped up. However, if a novel object is introduced it seems to be presented for longer (13% longer to be exact) than the anticipated stimulus. The initial hypothesis for this phenomenon was that greater attention is paid to these surprising events than the expected ones. However if this were the case, strong salient or attention-grabbing images, such as violent or emotional pictures, should seem to last even longer than unexpected neutral images, as they garner even more attention. However this is not the case, with novel affective images subjectively lasting just as long as novel household items.
Instead the answer seems to lie in a perceptual compression rather than expansion of time, with the repeated images appearing to speed up as they become expected. This is thought to be rooted in an increase in neural efficiency for these events, the neurons and pathways becoming faster and more effective at firing for the expected event, leaving the brain greater capacity to process new information that cannot be predicted in this manner. This is paralleled in the EEG literature where conditioned anticipated events do not elicit the same amplitude of firing from cells that they once did. However, when a novel stimulus is introduced there is an expectation violation, which results in a greater magnitude of neuronal firing in the brain again. This then results in the event seeming longer, or rather no longer being temporally compressed as it had become.
Unfortunately no one is quite certain why increased or decreased cell firing results in this subjective expansion or compression of time, but perhaps we'll be closer to an answer by next year's meeting. In the meantime, at least it's comforting that repetitive (read: potentially boring) events actually seem to go faster than they really are.