We have studied the IMG functions associated with visual detection of targets moving across a background structure (Barbur and Ruddock, 1980a) in four subjects, each possessing a different visual abnormality. Results for subject J.F., a tritanope, show that the low-frequency IMG response associated with the blue-sensitive (ZCl) mechanism (Barbur and Ruddock, 1980b) is absent in his case, thus providing confirmation that it is rh-activity which gives rise to the low-frequency response. We establish that, for an albino subject, D.C., the low visual acuity for resolution of gratings arises post-receptorally and show that the IMG function is restricted to the same low spatial frequency range as the visual acuity data. Visual responses to moving targets can be elicited from the blind hemi-field of a hemianopic subject, G.Y., but there is no IMG function associated with these responses, which also exhibit very low spatial resolution. Finally, IMG functions, measured with longwavelength background gratings, are grossly abnormal for a subject, M. W., who possesses a central colour vision defect in detection of long-wavelength targets. On the basis of these experimental data, we argue that the IMG functions involve central response mechanisms, and we conclude that these functions provide a new method for the investigation of defective visual systems.