Chapter 3: What Do the Two Hemispheres Do?

The chapter provides a comprehensive examination of the functional and structural asymmetries distinguishing the two cerebral hemispheres, challenging the notion that neurological differences are merely random allocations of capabilities. It synthesizes evidence from diverse methodologies—including brain lesion analysis, temporary inactivations (such as the Wada procedure), studies of callosotomy or split-brain patients, and functional neuroimaging—to establish persistent, pervasive differences in how the right hemisphere (RH) and the left hemisphere (LH) process information and construct reality. Structurally, the RH is often described as larger and possessing greater white matter connectivity, favoring global integration, whereas the LH is optimized for local communication within regions. Functionally, the RH demonstrates dominance across the intensity axis of attention, covering alertness, vigilance, sustained attention, and broad, flexible focus; the LH specializes narrowly in highly focused attention. This divergence governs the processing of novelty: the RH is vigilant for new and unexpected stimuli, making it central to flexibility, frame shifts, creative association, and problem-solving through the exploration of multiple possibilities. By contrast, the LH primarily processes known, predictable information, focusing on singular, expected outcomes and prone to pathological inflexibility or perseveration if damaged. These approaches define hemispheric contributions to perception and cognition: the RH excels at holistic processing and Gestalt perception, integrating disparate sensory inputs to perceive context, spatial depth, and the undivided flow of time; the LH abstracts and divides the world into discrete parts, fixed categories, and static, schematic representations. Furthermore, the RH is the primary mediator of social and emotional understanding, showing dominance in recognizing facial expressions, vocal prosody, humor, and metaphor, and is intimately connected to the limbic system, supporting embodied self-awareness, empathy, and theory of mind. The RH prioritizes living individuals, while the LH exhibits an affinity for non-living objects, tools, and abstract categories. In terms of rational judgment, the RH is more realistic, accepts ambiguity and uncertainty, and underwrites our moral sense and capacity for altruism, whereas the LH, lacking contextual grounding, tends toward unrealistic optimism, denial of impairment (anosognosia), and confabulation, seeking inappropriate certainty through arbitrary categorization and rule-making. The chapter concludes that the right hemisphere fundamentally attends to the ‘Other’ and the world as a contextualized, living whole, grounding experience, while the left hemisphere attends to the abstract, functional, and self-made representation of that world.