Here’s another classic.  Even better, here’s David Hubel’s Nobel lecture from 1981, in which he explains the happy accident that led to their discovery.

Hubel and his partner had connected a microelectrode to a particular neuron in a cat’s visual cortex.  They were trying to get the neuron to fire reliably in response to visual stimulation (a black dot).  But nothing worked.  And then all of a sudden, it started firing like crazy.  Turns out this particular neuron was “turned on” by the shadow of the glass slide as it was inserted into their projector – a sharp black line moving across a light background.  What’s more, it would only fire when the black line was at certain orientations.  It had no interest in a black dot at all.

This was one of the first papers to show that some neurons in our cortex (our upper brain) are very specifically tuned to a very narrow set of stimuli.  And it spawned a huge course of “single unit” studies in which psychologists test individual neurons throughout the brain to see which environmental conditions will make them fire.  Still a huge part of brain research.