This defines two stimulus series: a sum of two sinusoids (plaid)
drifting in different directions at 30 deg increments, and a single
sinusoid drifting in different directions at 30 deg increments. The
two component sinusoids of the plaid are offset in orientation by 120
deg. This is a classical stimulus for classifying a direction
selective neuron in terms of its response to pattern vs. component
motion. A background gray control stimulus is also included, thus
25 unique stimuli are defined here.