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.