SUMMARY

559

15-1 The Nature of Thought

The complex processes we call thinking, or cognition, are products of both human and nonhuman brain activity. We use such words as language and memory to describe cognitive operations, but these concepts are abstract psychological constructs—merely inferred and not found in discrete places in the brain. They exist but have no physical form.

The brain carries out multiple cognitive operations—perception, action for perception, imagery, planning, spatial cognition, and attention. Each requires widespread activity in many cortical areas. The unit of cognition, however, is the neuron.

15-2 Cognition and the Association Cortex

The brain’s association cortex includes medial, dorsal, and orbital subdivisions of the prefrontal cortex, the posterior parietal cortex, and anterior regions of the temporal lobe. Cell assemblies in the association cortex specifically take part in most forms of cognition.

The frontal lobes not only plan, organize, and initiate movements but also organize our behavior over time (temporally). As a general rule, the temporal lobes generate knowledge about objects, whereas the parietal lobes produce varied forms of spatial cognition. Neurons in both the temporal and the parietal lobes contribute to our ability to selectively attend to sensory information.

Regions in the frontal and parietal lobes contain mirror neurons that represent actions—one’s own or those of others. Such neural representations could be used both for imitating others’ actions and for moving faster and more accurately. A significant area of the cortex is multisensory, allowing the brain to combine characteristics of stimuli across sensory modalities, whether we encounter the stimuli together or separately.

15-3 Expanding Frontiers of Cognitive Neuroscience

Neuropsychological studies that began in the late 1800s to examine the behavioral capacities of people and laboratory animals with localized brain injuries did not allow investigators to study “normal” brains. Today, noninvasive brain recording systems and imaging techniques further the field of cognitive neuroscience, which studies the neural basis of cognition by measuring brain activity while healthy participants engage in cognitive tasks.

An important step in identifying the neural bases of cognition is mapping cortical connections for the brain connectome. Two promising imaging tools are functional connectivity magnetic resonance imaging (fcMRI) and tractography using diffusion tensor imaging.

The cerebellum, which houses 80 percent of the human brain’s neurons, was long believed primarily to have motor functions. Emerging data, however, reveal cerebellar involvement in a wide range of cognitive functions as well.

Social neuroscience, a field that combines cognitive neuroscience with social psychology, explores how we understand others’ intentions by constructing a theory of mind. Social neuroscience also investigates how we develop attitudes, beliefs, and a sense of self. Using noninvasive imaging techniques such as fMRI, researchers have shown that social cognition primarily involves activity in the prefrontal cortex.

Neuroeconomics combines psychology, neuroscience, and economics in seeking to understand human decision making. fMRI studies reveal two decision-making pathways. One is slow and reflective, involving diffuse regions of association cortex. The other is quick and reflexive, involving the dopaminergic reward system.

15-4 Cerebral Asymmetry in Thinking

Cognitive operations are organized asymmetrically in the left and right cerebral hemispheres: each carries out complementary functions. The most obvious functional difference is language, typically housed in the left hemisphere.

Cerebral asymmetry, manifested in anatomical differences between the two hemispheres, can be inferred from the differential effects of injury to opposite sides of the brain. Asymmetry can also be seen in the healthy brain and in the brain that is surgically split to relieve intractable epilepsy. Various syndromes result from association cortex injury, among them agnosia, apraxia, aphasia, and amnesia. Each includes the loss or disturbance of some cognitive function.

15-5 Variations in Cognitive Organization

Unique brains produce unique thought patterns. Marked variations in brain organization among individuals are exhibited in idiosyncratic capacities such as synesthesia. Systematic differences in cognition exist as well, manifested in the performance of females and males on cognitive tests, especially on tests of spatial and verbal behavior.

Sex differences in cognition result from gonadal hormones’ actions on cortical organization, possibly on the architecture of cortical neurons, and ultimately on neural networks. Female and male cerebral hemispheres exhibit marked differences in anatomical organization.

Right- and left-handers also differ in hemispheric organization. Left-handers constitute at least three distinct groups. In one, speech appears to reside in the left hemisphere, as it does in right-handers. The other two groups have anomalous speech representation, either in the right hemisphere or in both hemispheres. The reasons for these organizational differences remain unknown.

15-6 Intelligence

Intelligence is easy to spot but difficult to define. Obvious differences exist across, as well as within, species, and we find varied forms of intelligence among humans within our own culture and in other cultures. Intelligence is unrelated to differences in brain size within a species or to any obvious gross structural differences among members of the species. Intelligence may be related to synaptic organization and processing efficiency.

15-7 Consciousness

The larger a species’ brain is relative to its body size, the more knowledge the brain acquires. Consciousness, the mind’s level of responsiveness to impressions made by the senses, emerges from the nervous system’s complexity.