Personal statement for single and joint honours degrees

You don't need to write about them separately - you can easily write about e.g. cellular and molecular neurobiology, systems neuroscience, and neuropsychology, using any relevant background you have, and is applicable to all of those courses. Cellular and molecular neurobiology necessarily is a biological science and to engage in at a higher level (i.e. research) requires some background in cellular and molecular biology. The others broadly correlate to the individual subject areas as well, while being centrally tied to the theme of brain and cognitive sciences.

Don't approach the PS as a tickbox exercise - write about what you are interested in, and either this will necessarily reveal its relation to the courses being applied to, or perhaps suggest that some of those are not areas of particular interest and to tweak your options slightly. For example, if you struggle to find interesting things to write of "basic science" biological aspects then it may be prudent to replace the natural sciences courses with neuroscience or psychology courses that are more aligned to your interests (and vice versa with the various other elements).