Jackson also did research on aphasia, noting that some aphasic children were able to sing, even though they had lost the power of voluntary speech.

[8][9] He also studied what types of language loss was found in patients with left-brain injury, including set phrases, such as "Good bye" and "Oh, dear.

"[10][11] In his youth Jackson had been interested in conceptual issues and it is believed that in 1859 he contemplated the idea of abandoning medicine for philosophy.

[12] Thus, an important part of his work concerned the evolutionary organization of the nervous system for which he proposed three levels: a lower, a middle,[13] and a higher.

[15] Continental psychiatrists and psychologists (e.g. Théodule Ribot, Pierre Janet, Sigmund Freud, Henri Ey) have been more influenced by Jackson's theoretical ideas than their British counterparts.

Jackson could not use modern sophisticated neuro-investigative technology (it had not been invented), but had to rely upon his own powers of clinical observation, deductive logic and autopsy data.

[19] Some of his eminent successors in the field of British neurology have been critical of many of his theories and concepts; but as Sir Francis Walshe remarked of his work in 1943, " ... when all that is obsolete or irrelevant is discarded there remains a rich treasure of physiological insight we cannot afford to ignore."