Hemispheric Specialization and Interaction

Milner argues that by the early 1970s, research had firmly established that the left and right cerebral hemispheres are not functionally equivalent, but that the field had almost completely neglected the complementary question of how they interact. She notes that already by the 1860s, observations on patients with unilateral brain disease pointed to left-hemisphere dominance for speech, leading to an overgeneralized view of left-sided cognitive prepotency. It was not until World War II that studies on patients with well-lateralized lesions revealed the importance of the right hemisphere for spatial functions. The most direct evidence for dual asymmetry came from Roger Sperry's work with commissurotomy patients, whose hemispheres could be tested independently. Milner insists that understanding normal behavior requires tackling hemispheric interaction, not just specialization.

This is not a single sustained argument but a curated section of conference proceedings, originally published as part of The Neurosciences: Third Study Program (1974) and reprinted as a standalone paperback in 1975. The section consists of Milner's introduction, followed by contributions from R.W. Sperry, Michel Cuénod, Donald Broadbent, A.M. Liberman, C.J. Darwin, G. Berlucchi, Hans-Lukas Teuber, and Milner herself. Each chapter addresses a different facet of hemispheric specialization or interaction. Milner's editorial role was to frame the section and solicit contributions that would advance the discussion beyond simple cataloging of asymmetries, making the volume a collaborative snapshot of a field in transition.

In her introduction, Milner states that at the time of writing, the field had 'almost complete ignorance about the second half of our topic, that of hemispheric interaction, in so far as it pertains to man.' While reaction-time studies in normal subjects could make a few inferences about interhemispheric transfer, purely behavioral measures could not reveal the form in which information is transmitted between hemispheres or which pathways are essential for specific tasks. Milner argues that animal experiments are necessary to identify elementary mechanisms of commissural transfer, citing Michel Cuénod's work on pigeons as an example. This framing of interaction as the neglected half of the problem would guide Milner's own research for decades, including her later Balzan Prize-funded project on hemispheric interaction in cognitive processes.

The 1974/1975 section served as a foundation for Milner's subsequent decades of work on hemispheric interaction. When she received the 2009 Balzan Prize for Cognitive Neurosciences, she devoted the second half of the prize money to a research project explicitly titled 'Hemispheric Interaction in Cognitive Processes,' recruiting postdoctoral fellows to use functional imaging to explore how the two hemispheres work together in healthy volunteers and patients. The Balzan documentation cites her 1974 chapter as the origin of this line of inquiry. The project investigated how individual differences in hemispheric connectivity relate to memory capability, finding that stronger interhemispheric connectivity in the posterior hippocampus correlates with higher memory test performance.

Commissurotomy patients—individuals who had undergone surgical severing of the corpus callosum and other interhemispheric commissures for intractable epilepsy—provided the most direct evidence for dual functional asymmetry. In Milner's introduction, she highlights Roger Sperry's work with these patients, noting that by confining sensory input and motor output to one hemisphere, Sperry and colleagues were able to 'bring out the contrasting specializations of the two sides of the adult human brain.' Sperry's contribution to the volume details findings indicating 'two distinct modes of central processing' in the disconnected hemispheres. However, Milner also observes that comparing commissurotomized patients with matched patients with intact commissures revealed that 'each hemisphere acting alone was severely handicapped on memory tasks,' which excited her interest in how the hemispheres normally cooperate.

Several contributions address this problem. Donald Broadbent's chapter on the division of function and integration of behavior argues that if the two hemispheres were truly different in their modes of action, it should be relatively easy to find tasks that can be performed in parallel, one by each hemisphere. The fact that interference is the rule rather than the exception suggests that at some critical stage of decision-making, humans behave as a 'single-channel organism.' This implies that researchers cannot be content with cataloging contrasting specializations but must address how the hemispheres interact in normal behavior. Giovanni Berlucchi's chapter describes asymmetries in normal vision using brief lateralized presentations, showing greater accuracy for nonverbal patterns on the left and letters on the right. These findings, Milner argues, reflect the contrasting specializations that must somehow be integrated.