What is spatial and temporal frequency?

For instance, the primate visual system is particularly tuned to spatial frequency information. Spatial frequency describes the periodic distributions of light and dark in an image.

The spatial frequency refers to how many complete periods the signal goes through for a given unit of distance (eg. cylcles/m) while the temporal frequency refers to how many complete periods the signal goes through for a given unit of time (eg. cyles/s or Hz).

For a periodic target of spatial period X, we define an angular period θ ≡ X/R, an angle over which the object waveform repeats itself. The angular period is in radians if X and R are in the same units. Inverting this angular period gives angular spatial frequency ξ ang= R/X .