An on-off temporal filter circuit for visual motion analysis

We describe a temporal filtering circuit, which when cascaded with a previously reported chip for spatial filtering, implements the spatio-temporal filters required to construct motion energy filters, which have been used to model the functional characteristics of direction and speed tuned neurons in the primary visual cortex. To facilitate the combination, the temporal filter circuit uses the same on-off signal representation used by the spatial filtering chip. We present measurements results from a filter fabricated in a 1.5μm CMOS n-well process that demonstrates that both the center frequency of the filter, which determines the tuned velocity bandwidth, can be tuned by adjusting external bias voltages. The filter can be tuned to bandwidths and center frequencies on the order of tens of Hertz, comparable to those measured in cortical neurons.