The speculation that humor may be a uniquely human cognitive trait (Bergson 1924; Caron 2002) prompted our third hypothesis: humor will activate both anterior cingulate cortex (ACC) and frontoinsula cortex (FI), the 2 regions in which an evolutionarily recent neuron type, the Von Economo cells (previously termed ”spindle neurons”), are present. A review of the functional imaging literature reveals that the Von Economo cell regions, particularly FI, are active while reversal learning (O’Doherty and others 2001), decision making under uncertain conditions (Critchley and others 2001), and observing bizarre images of animal/object chimeras (Michelon and others 2003). Like humor, these paradigms involve incongruity detection and reappraisal and provided the impetus to formally test the hypothesis that humor activates the Von Economo regions ACC and FI.
Mirror neurons get all the glory, but I think spindle cells just might be even cooler. Unlike the rest of our neurons, which are generally short and bushy, spindle brain cells are long and gangly. They are also found only in humans and great apes (and perhaps whales), which suggests that their evolution was intertwined with higher cognition. (Humans have about a thousand times more spindle cells than any other primate.)
So what do spindle cells do? Why did human evolution devote so much attention to this particular kind of neuron? The strange structure of spindle cells their unique function: their antenna-like cell body is able to convey our social emotions across the entire brain. After your cortex receives an emotional input, spindle cells use their cellular velocity – they transmit electrical signals faster than any other neuron – to make sure that the rest of the cortex is also saturated in that specific feeling. In other words, your spindle cells hear the punchline, and then make sure that your other brain areas also get the joke.
The developmental timing of spindle cells hints at their larger purpose. While spindle cells begin to form around the age of four months, they don’t start to work until the second year of life, which is precisely when our social emotions–like shame, anger and guilt–also begin to appear. Spindle cells are concentrated in an area of the orbitofrontal cortex that is exquisitely sensitive to the emotional reactions of others. (This is why laughter is so contagious.) Spindle cells are also abundant in the ACC, the same brain region that is activated by difficult moral dilemmas, funny jokes and moments of self-control. I could go on and on, but it’s better for you to just sit down and read the collected works of John Allman, the world authority on spindle cells.