The adaptive base of the neural hierarchy: elementary motivational controls on network function

Abstract

This chapter has explored a hierarchical approach to motivation, based on the idea of a vertical integration among brainstem, limbic, paralimbic, and neocortical systems. When viewed in the traditional manner, this hierarchy has been seen in terms of top-down control, with the neocortical regions exerting higher-level, converging influences over the lower regions. When viewed from an evolutionary perspective, the hierarchy proves to be balanced by a complementary pattern of divergent projections ascending from the lower to the higher levels. These ascending systems respond first to the adaptively significant properties of a stimulus, including its novelty, conspicuousness, and need relatedness. By means of their increasingly specific projection patterns, the elementary motivational mechanisms support a progressive narrowing of activity within the cortex. Selection begins with activity in ascending brainstem systems, organizing the primordial states of tonic activation and phasic arousal. Although these states are relatively general, they bias information processing in specific ways by modulating both the spatial and temporal span of attention. As limbic circuits are recruited, certain classes of adaptive response and perceptual processes are potentiated, bringing into play postural orientations and global perceptions of the environment. This response and perceptual processing promotes activity within the interoceptive fields of the paralimbic cortices, leading to primitive hedonic and energetic representations. Back-projections from paralimbic to successive neocortical fields allow these affective representations to aid further in narrowing sensory and response options. Multiple levels of representation thus work together in gradually focusing and articulating the representations forming within the hierarchical networks of the neocortex. The recent evidence on neocortical architecture shows that cortical areas tend to be most closely connected with other regions of the same level of architectonic differentiation. Thus the functional units of the cortex may be not lobes or even hemispheres but the growth rings sharing a specific laminar architecture and dense horizontal interconnections. Functional binding--the integration of the distributed representations of the cortex--may take place across these horizontal lines. Furthermore, the density of horizontal interconnections decreases for the more recently evolved neocortical growth rings, such that the primary and sensory and motor cortices are relatively isolated from all but the adjacent architectonic ring. This suggests that the most important functional binding in the cortex occurs at the more primitive, paralimbic network levels.(ABSTRACT TRUNCATED AT 400 WORDS)