I've been told that the point spread function of a pixel is its distribution of intensity, while the sampling aperture of a pixel is its distribution of sensitivity. I'm a little unsure of what the difference between intensity and sensitivity is. It sounds like the former deals with how a pixel is displayed, while the latter deals with how it is captured. Is that correct?
Sounds correct to me. The point-spread function indicates how the intensity of a point source is spread among multiple pixels in an image. In an imaging sensor, the sampling aperture of a particular pixel indicates how it responds to intensity from points in space as a function of the spatial offset from the pixel's center.