Do normal people also experience synesthesia?
A form of synesthesia exists in all our brains. For instance, we speak of certain smells of particular liquids--like nail polish--as being sweet, even though we have never tasted them. This might involve the close neural links and cross activations--that is, when one area of the brain affects another--between the areas involved in smell and taste. This would make sense functionally--e.g., fruits are sweet and also smell sweet like acetone. But it also makes sense structurally, because the brain pathways for smell and taste are closely intermingled and they both project signals to the same parts of the frontal cortex during sensory processing.
Here's another example. Consider how, even as infants, we scrunch up our noses and raise our hands when we encounter disgusting smells and tastes. We also use these gestures, as well as the word "disgusting," to describe a person who is morally questionable. Why do we use the same word as for taste? Why not say he is "painful," for instance?
The reason, we suggest, is that there are underlying evolutionary and anatomical constraints at work. In lower vertebrates, certain regions of the frontal lobes have maps for smell and taste. But as mammals became more social, the same maps were later usurped by evolution for social functions such as territorial marking, aggression and sexuality, eventually culminating in mapping a whole new social dimension: morality. Hence, the interchangeable words and facial expressions for olfactory or gustatory disgust and moral disgust.
The article describes one colorblind subject who could experience certain colors only when making synesthetic associations; he could not see them with his normal vision. Could that effect only occur for a colorblind synesthete, or could synesthetes with normal vision also have the experience of seeing exotic colors?
The effect is most obvious and pronounced in the colorblind synesthetes, but occurs in "regular" synesthetes as well. The colors evoked by cross activation in the fusiform gyrus "bypass" earlier stages of color processing in the brain, which may confer an unusual tint to the colors evoked. This is important for understanding the phenomenon of synesthesia, because it suggests that the qualia label--that is, the subjective experience of the color sensation--depends not merely on the final stages of processing but on the total pattern of neural activity, including earlier stages.
If cross activation is the correct explanation for synesthesia, why is it that the condition often only works in one direction? That is, a synesthete may see numbers or letters as colored, but looking at colors doesn't evoke letters or numbers.
This may have something to do with the manner in which certain sensory dimensions like color are represented in brain maps, as opposed to the way in which numbers are mapped; this might confer an inherent bias toward unidirectional activation. If a number evokes a color then there's something in the visual image--the number--that the color can be ascribed to. Free-floating qualia not anchored to anything, however, may not be possible. That is, if a color evokes a number, where would the number be seen and how big would it be? It would have to be free floating, and that may not be possible. We also think of metaphors as arbitrary, but in fact they are constrained by evolution and by neural hardware (see main article). For example, we say "loud shirt" or "sharp" taste, but rarely "red" sound or "bitter" touch.
For synesthetes who associate a number or letter with a color, does the font, or typeface, matter?
The font that evokes the most optimally saturated color is usually the simplest--for example, the clean lines of the typeface Helvetica rather than the ornate Gothic. But we have seen rare examples in which an unusual font was more effective at invoking strong color. We suggest that such fonts might serve as ultranormal stimuli that evoke even higher responses from the neurons involved with graphemes (the physical appearance of letters or numbers) than more prototypical fonts.