n experiments the researchers directly observed the convergent neurons where learning is suspected of taking place. These neurons responded to both a conditioned stimulus, in this case a novel saccharine solution, and an unconditioned stimulus, in the form of lithium chloride that made rats sick. Convergent activation is considered a key event for subsequent plasticity, according to Bernstein. Until now, however, there has been scant direct evidence of this activation during learning in the mammalian brain.