While exploring tide pools with her husband, Carol Steen jumped from a rock and tore a ligament in her knee. “At the point of impact, everything I saw suddenly became orange,” she recalls. “My husband was orange. The waves were dark orange, and the sand was light orange.”

For Steen, orange is the color of pain. She has synesthesia, a condition in which the senses are cross-wired. She perceives colors and shapes in her mind’s eye when she processes certain sounds, smells, or tactile sensations. She also associates specific colors with numbers and letters. To Steen, the letter “K” is green. Ammonia smells blue. The sound of her apartment’s elevator bell is a flashing magenta triangle.

Synesthesia comes in many forms. Some synesthetes experience vivid tastes or scents when they hear certain sounds. Others strongly associate days or months with specific points in space; July might be elevated and to the left, while October is located to the far right, near the floor. Colored letters and digits, or color-grapheme synesthesia, is one of the more common (and best studied) forms of the condition.

In the past decade, synesthesia research has really come of age, says Michael Dixon, a psychologist at the University of Waterloo in Ontario. “When I started doing this research in late 1990s, the big push was just to prove that it was real,” he says. Studies using functional magnetic resonance imaging (fMRI) and diffusion tensor imaging (DTI) handily settled that debate. When individuals with color-grapheme synesthesia listened to numbers being spoken aloud, for instance, their brain activity increased in regions involved in processing colors.

Now that synesthesia is a proven phenomenon, researchers have moved on to asking more penetrating questions. For starters: How common is it? Estimating prevalence is tricky since it depends on what you define as synesthesia, Dixon says. Dozens of forms have been recognized, and new types are frequently added to the list. But the best current estimates suggest 1.1 to 4.4 percent of the population — as many as 1 in 23 people — are synesthetic.

Synesthesia is known to run in families, though the gene for the condition has yet to be unearthed. And despite successful imaging studies, researchers are still sorting out exactly what goes on in the brains of synesthetes like Steen. Some neuroscientists suggest they might have extra wiring between adjacent brain regions. Others propose they simply have extra activity in the existing wiring.

Some recent research even suggests we might all be hardwired for synesthesia. As a graduate student at McMaster University, Ferrine Spector studied sensory associations in people with normal perception. She asked volunteers to sniff various odors and describe the colors and textures they called to mind. Many of the associations were clearly a product of experience; lemon was typically described as yellow, for example. But other associations weren’t so easy to peg. The scent of mushrooms was often described as blue. The odor of lavender was frequently described as green and sticky.