A simple cell in early visual cortex is generally orientation selective, has linear summation properties, and often has a receptive field that looks like a Gabor function.
You can listen to several varieties of simple cells with non-direction selective receptive fields.
Odd-type RF
Even-type RF
Odd or Even refers to the symmetry of the receptive field.
Our simple cells use a push-pull model. The cells is synaptically excited by on ganglion cells where the plot shows white, and by off ganglion cells where the plot shows dark.
At the same time, the simple cell is synaptically inhibited by ganglion cells of the opposite polarity at each location.
For example, the odd simple cell has a full set of inputs that looks like this:
Push-pull inputs to odd simple cell
We have not modeled any of the interesting color selective responses of the retina or cortex in our simulation.