Black points and blue squares are the same as in Fig. 5, with other points from the experiments described in Sect. 4. Specifically, open green squares describe the control test of Sect. 4.1, which is run as in the fiducial case but at a precise S/N_int value of 1.945. The filled green circles show the corresponding tests at the same S/N_int value but with random numbers of sources injected, so that the observer had no knowledge of the probability a source was present. The filled red squares are as in Sect. 4.2, in which the observer had no knowledge of source line width or peak S/N and cubelets were inspected in random order, with no information given as to whether identifications were correct until the whole set of cubes was inspected. The filled orange squares are from the test described in Sect. 4.3, where numbers, line width, and peak S/N were all randomised at different S/N_int levels and sources were injected into a single large data cube rather than the standard set of 100 cubelets.