Our nine senses? Exactly what is a sense, anyway?

Mark Poesch asks on his blog, “Why is it that all children are taught that they have only “five senses”…. Doesn’t a sense of gravity (i.e., acceleration) count as a sixth? …could you live without it?” I responded in the comments that I tend to think that it falls under the sense of “touch” as there’s a physical mechanism in the inner ear–specifically, the macula utriculi and macula sacculi[1]–that is responsible for our sense of gravity or linear acceleration.

No surprise, the Wikipedia entry for “sense” points out that the definition of “sense” isn’t well defined. Given a certain definition of sense, there are nine human senses: vision (sight), audition (hearing), gustation (taste), olfaction (smell), tactition (touch), thermoception (heat, cold), nociception (pain), equilibrioception (balance, gravity), proprioception (body awareness). So, it seems Mark is right, conventional wisdom considers gravity its own separate, dedicated sense.

However, in the comments to his blog entry, I asked: “What about “sense of time”? Is that simply cognition (and not a sense)? Probably.” Of course, Wikipedia has an entry on “sense of time” too, which says: “Although the sense of time is not associated with a specific sensory system, the work of psychologists and neuroscientists indicates that our brains do have a system governing the perception of time.” Why isn’t it associated to a specific sensory system? Is the prerequisite for a sense that it must have a corresponding sense organ? This can’t be, since thermoception, nociception and proprioception fail to meet this criteria. If senses are the perception of stimuli, the passing of time definitely causes a stimulus which we perceive.

It appears that back in 2001, researchers identified that the basal ganglia and the parietal lobe are responsible for perceiving the passage of time. Perhaps this provides for a solid explanation for deja vu and other temporal sensation anomalies?

What do you think? Share your thoughts in the comments below.

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