Milner was a pioneer in the field of neuropsychology and in the study of memory and other cognitive functions in humankind. She studied the effects of damage to the medial temporal lobe on memory and systematically described the deficits in the most famous patient in cognitive neuroscience, Henry Molaison, formerly known as patient H.M. Though he was not able to remember new events he was able to learn new motor skills. Milner was invited to Hartford to study H.M., “who had undergone a bilateral temporal lobectomy that included removal of major portions of the hippocampus.”
In the early stages of her work with H.M., Milner wanted to completely understand his memory impairments. Dr. Milner showed that the medial temporal lobe amnestic syndrome is characterized by an inability to acquire new memories and an inability to recall established memories from a few years immediately before damage, while memories from the more remote past and other cognitive abilities, including language, perception and reasoning were intact. For example, Milner spent three days with H.M. as he learned a new perceptual-motor task in order to determine what type of learning and memory were intact in him. This task involved reproducing a drawing of a star by looking at it in a mirror. His performance improved over those three days. However, he retained absolutely no memory of any events that took place during those three days. This led Milner to speculate that there are different types of learning and memory, each dependent on a separate system of the brain. She was able to demonstrate two different memory systems- episodic memory and procedural memory.
Milner discovered from H.M. and other case studies that “bilateral medial temporal-lobe resection in man results in a persistent impairment of recent memory whenever the removal is carried far enough posteriorly to damage portions of the anterior hippocampus and hippocampal gyrus.” She showed that in patients with this syndrome the ability to learn certain motor skills remained normal. This finding introduced the concept of multiple memory systems within the brain and stimulated an enormous body of research. Milner stated in an interview with the McGill Journal of Medicine, “To see that HM had learned the task perfectly but with absolutely no awareness that he had done it before was an amazing dissociation. If you want to know what was an exciting moment of my life, that was one.”
She has made major contributions to the understanding of the role of the frontal lobes in memory processing, in the area of organizing information. “Dr. Milner’s seminal research has provided many landmark discoveries in the study of human memory and the brain’s temporal lobes, which play a key role in emotional responses, hearing, memory and speech.”
She demonstrated the critical role of the dorsolateral frontal cortex for the temporal organization of memory and her work showed that there is partial separability of the neural circuits subserving recognition memory from those mediating memory for temporal order. She described the inflexibility in problem solving that is now widely recognized as a common consequence of frontal-lobe injury. These refinements in the understanding of memory and exposition of the relevant brain regions revealed the diffuse nature of complex cognitive functions in the brain.
Milner helped describe the lateralization of function in the human brain and has shown how the representation of language in the cerebral hemispheres can vary in left-handed, right-handed and ambidextrous individuals (see handedness). These studies of the relationship between hand preference and speech lateralization led to an understanding of the effects of early unilateral brain lesions on the pattern of cerebral organization at maturity. Her studies were among the first to demonstrate convincingly that damage to the brain can lead to dramatic functional reorganization.