However, this left us with something of a puzzle, as the shifts in disparity tuning which occur with delay (in the cat) had been proposed as a neuronal substrate for the Pulfrich effect. This is a visual illusion which occurs when the image from one eye is delayed: moving objects appear to shift in depth. This had been explained on the basis of cells which are sensitive to both direction of motion and to disparity: i.e. which jointly encode motion and depth. However, in our previous paper we had found that such cells were quite rare in monkey V1 (~10%). It seemed surprising to us that these few cells would cause such an illusion, when the vast majority of cells would not be subject to the illusion on this basis. So, we looked again at whether joint encoding of motion and depth really was necessary for the Pulfrich effect. We found that it wasn't. As previous workers had noted, the Pulfrich effect arises because of cells' finite temporal integration; but finite temporal integration does not necessarily imply direction selectivity. Our results suggested that all cells in V1, even the ones which show no shift in disparity tuning with delay, could potentially support the Pulfrich illusion.