Linear mechanisms can produce motion sharpening

Human observers are not normally conscious of blur from moving objects [Nature 284 (1980) 164]. Several recent reports have even shown that blurred images appear sharper when drifting than when stationary and have suggested different non-linear mechanisms to explain this phenomenon [Vision Res. 36 (1996) 2729; Vision Res. 38 (1998) 2099]. We demonstrate here that even though distortions of drifting narrow-band sine-wave gratings cannot be explained by linear mechanisms, these mechanisms may have an important role in sharpening of moving edges. We show first that the effective spatial filter for a moving object that is formed by a simple difference-of-Gaussians spatial filter and the typical biphasic temporal impulse response function can be approximated by a combination of Gaussian filters only. When this filter is applied to moving, Gaussian-blurred edges, regions of blurring and sharpening are found over the same ranges of blur widths and velocities where recent experimental findings have shown them to exist. In general, that means that the output of the filter shows blurring in response to small blur widths and sharpening in response to larger blur widths.