Network

The web of interconnected neurons in the brain is known as a network, and there are a number of important emergent phenomena that arise in these networks, associated with different patterns of connectivity present in the neocortex. We can only really understand brain function in terms of these networks and populations of neurons.

The feedforward flow of neural activity from lower layers of neurons that are closer to the sensory inputs up to higher layers can be understood in terms of categorization, where detailed sensory patterns are systematically transformed into more abstract categories that provide a more efficient basis space for behavior.

Thus, the network level of analysis is focused on the nature of representations, i.e., the properties of the patterns of neuron firing in a given area of the network. Distributed representations are critical for efficient encoding of high-dimensional information, as elaborated in the discussion of combinatorial-vs-conjunctive encodings. These different types of representations have important implications for generalization performance: how well the system can process novel inputs and situations.

Bidirectional connectivity enables additional emergent phenomena to arise, including attractor dynamics (e.g., top-down imagery and ambiguity resolution), and multiple constraint satisfaction, which involves finding a representation that best satisfies constraints from the current inputs and prior learned knowledge encoded in synaptic weights. The constraint satisfaction process is an efficient, dedicated-weight-gradient-search through representational space, providing optimized representations relative to purely feedforward activation flow.

Bidirectional excitatory connectivity requires inhibition to control the potential runaway positive feedback loops, consistent with the extensive networks of inhibitory interneurons in the neocortex. Inhibition is also beneficial for imposing a form of competition between different representations, which is an essential feature of attention.

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Next in Intro Book: categorization