Experience exerts a profound influence on the brain and, therefore, on behavior. When the effect of experience on the brain is particularly strong during a limited period in development, this period is referred to as a sensitive period. Such periods allow experience to instruct neural circuits to process or represent information in a way that is adaptive for the individual. When experience provides information that is essential for normal development and alters performance permanently, such sensitive periods are referred to as critical periods. Although sensitive periods are reflected in behavior, they are actually a property of neural circuits. Mechanisms of plasticity at the circuit level are discussed that have been shown to operate during sensitive periods. A hypothesis is proposed that experience during a sensitive period modifies the architecture of a circuit in fundamental ways, causing certain patterns of connectivity to become highly stable and, therefore, energetically preferred. Plasticity that occurs beyond the end of a sensitive period, which is substantial in many circuits, alters connectivity patterns within the architectural constraints established during the sensitive period. Preferences in a circuit that result from experience during sensitive periods are illustrated graphically as changes in a ''stability landscape,'' a metaphor that represents the relative contributions of genetic and experiential influences in shaping the information processing capabilities of a neural circuit. By understanding sensitive periods at the circuit level, as well as understanding the relationship between circuit properties and behavior, we gain a deeper insight into the critical role that experience plays in shaping the development of the brain and behavior.

A mechanistic understanding of attention is necessary for the elucidation of the neurobiological basis of conscious experience. This chapter presents a framework for thinking about attention that facilitates the analysis of this cognitive process in terms of underlying neural mechanisms. Four processes are fundamental to attention: working memory, top-down sensitivity control, competitive selection, and automatic bottom-up filtering for salient stimuli. Each process makes a distinct and essential contribution to attention. Voluntary control of attention involves the first three processes (working memory, top-down sensitivity control, and competitive selection) operating in a recurrent loop. Recent results from neurobiological research on attention are discussed within this framework.

A growing proportion of the U.S. workforce will have been raised in disadvantaged environments that are associated with relatively high proportions of individuals with diminished cognitive and social skills. A cross-disciplinary examination of research in economics, developmental psychology, and neurobiology reveals a striking convergence on a set of common principles that account for the potent effects of early environment on the capacity for human skill development. Central to these principles are the findings that early experiences have a uniquely powerful influence on the development of cognitive and social skills and on brain architecture and neurochemistry, that both skill development and brain maturation are hierarchical processes in which higher level functions depend on, and build on, lower level functions, and that the capacity for change in the foundations of human skill development and neural circuitry is highest earlier in life and decreases over time. These findings lead to the conclusion that the most efficient strategy for strengthening the future workforce, both economically and neurobiologically, and improving its quality of life is to invest in the environments of disadvantaged children during the early childhood years.child development ͉ early experience ͉ economic productivity ͉ critical and sensitive periods ͉ brain development

The receptive field properties and functional organization of visual and auditory responses were studied in the optic tectum of the barn owl (Tyto alba). Most units throughout the depth of the tectum responded to both visual and auditory stimuli. The entire visual field of each eye was represented topographically in the contralateral tectum. In the portion of the tectal map representing the zone of binocular vision, 50% of the superficial layer units and 100% of the deep layer units were driven binocularly. The representation of the frontal binocular region of space was greatly expanded in the map; the average magnification factor was 3 times greater for the frontal binocular zone than for the monocular zone.The responses of the superficial and deep tectal units to auditory stimuli were space specific; they responded only when a sound source was located in a particular region of space, or receptive field, regardless of the intensity or type of sound used. Most auditory receptive fields contained a distinct "best area" where a sound source was most effective in driving the unit. Auditory space, as defined by receptive fields and best areas, was represented topographically in the tectum.The auditory and visual maps of space had the same orientations, positions, magnification factors, and termination coordinates at the anterior and dorsal edges of the tectum. Yet the maps lost their registry near the posterior and ventral margins where the most peripheral regions of space were represented. These characteristics suggest that the spatiotopic organization in the tectum is a compromise between a tendency for the space representations of different modalities to align and for the representation of each modality to fill the entire tectum.

Auditory units that responded to sound only when it originated from a limited area of space were found in the lateral and anterior portions of the midbrain auditory nucleus of the owl (Tyto alba). The areas of space to which these units responded (their receptive fields) were largely independent of the nature and intensity of the sound stimulus. The units were arranged systematically within the midbrain auditory nucleus according to the relative locations of their receptive fields, thus creating a physiological map of auditory space.