So, where do those unusual connections come from? One theory is that we all have them when we're babies, when the brain makes millions of new synaptic connections a second. As we grow older, those connections get pruned back as the senses separate. But in synesthetes, more of that wiring just stays in place.
Linn and Kostenko can also connect colors and geometric shapes with food. Take Swiss cheese.
For Linn, it has a "very thin line going through it," while Kostenko sees "an olive color" that's "kind of wavy."
As for blue cheese, Linn said "it's pointy." Kostenko said she can see why her sister sees blue cheese as pointy, but for her "it's a lot of little balls."
When it comes to food, it can get even stranger for British pub owner James Wannerton, who tastes almost every word he hears, even the words of his customers' orders.
"Somebody will come in, they then order, say, a pint of [beer]. I get the bacon rind taste. They then order a packet of roasted nuts, and I don't get roasted nuts, I get some sort of peculiar burned meat taste. Then [I] have to give them their change -- change invariably tastes of processed cheese -- a cheesy taste," Wannerton said.
And some of the flavors can be very unpleasant. Take the name Derek.
"Ugh, it's horrible," Wannerton adds. "It's earwax."
There are other cases of synesthesia running in families. For as long as they can remember, sisters Trish Goodwin, 23, and Molly Altobelli, 25, have pictured numbers, months and days of the week as 3-D pictures in space, which they re-create in virtual reality at the University of Texas. "It was strange to find out that not everybody thinks of numbers this way and not everybody has a spatial layout for the days of the week," Goodwin said.
Theysaid it can help in keeping track of appointments, but the two sisters have very different ways of seeing the same thing.
"I sort of picture them out in front of me and they're kind of in an oval, and then I sort of zoom in on it and sort of mentally mark that day," Altobelli said.
"I do the same thing," Goodwin said, "but 'cause I'm picturing myself in a spot on the sequence, I can see that it's like two days away or it's almost right in front of me."
In addition to Goodwin and Altobelli, their younger sister, Kate, and mother, Elisa, both have synesthesia. Researchers have been studying the family, looking for a genetic clue. There are two ways to explain the trait, says David Eagleman, at the University of Texas Houston Medical School.
"One of them is that there's more wiring in a synesthete's brain. The other hypothesis is that the wiring is the same in everybody but something about this gene is causing there to be more cross talk among areas," Eagleman said.
While this sensory cross talk may be more hardwired in the synesthete's brain, perhaps there's a little remnant of it in all of us, said Cytowic.
"I see it as a gift, as a sort of spiritual god thing that enables more intuition, whether it's musical or even with people to be able to see some extra things out there," said Rosser. Without synethesia, she said she'd be living in a world that was just monochromatic.