There is an interesting and suggestive experiment which has been carried out by Hubel and Wiesel, Hirsch and Spinelli, and extended by Blakemore. See the Bibliography for some sources, particularly the Blakemore paper. In the experiment, a cat has its head clamped and is kept fully conscious looking at a white disk free to rotate about an axis which is approximately the same as the optical axis of the cat's eye. A bar is drawn in black across a diameter of the disk. Its orientation may therefore be altered to any value.
A microelectrode is inserted into a carefully selected area of the visual cortex of the cat, an electrode fine enough to record the response of a single neuron. The neuron is monitored via the electrode as the disk is rotated. It is found that the firing rate of the neuron will suddenly increase at some particular orientation, going swiftly with angle from the idling rate to a value substantially higher.
We can plausibly argue that the neuron has somehow got a particular sensitivity to edges of the orientation of the bar. Random selection of neurons obtained by simply pushing the microelectrode through, gives a random set of orientations to which the neurons are tuned.
Are the neurons in a ready trained state from the time when the cat is born, or do they acquire a particular orientation at some later date? And what makes them pick one orientation rather than another? These questions have been addressed, the `cat in the hat-box' experiment described by Blakemore in the paper cited below, shows that when a cat opens its eyes in an environent of vertical stripes, neurons in this area become tuned to vertical edges to a large extent, when the cat is brought up in a horizontally striped environment, it has large numbers of neurons dedicated to responding to horizontal edges, and when it is swapped between boxes in its early life, it acquires neurons which are tuned either to horizontal edges or to vertical edges, but not both.
