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'''X(n)''', from lower levels in the processing hierarchy. '''X'''(n) is a field of bottom-up neuronal [[synapse | synaptic]] activations, coming from [[neurons]] at a lower level. Each [[neuron]] has a number of [[synapses]]; for generality, we describe each neuron activation as a set of numbers, '''X'''(n) = {X<sub>d</sub>(n), d = 1,... D}. Top-down, or priming signals to these [[neurons]] are sent by [[concept-models]], M<sub>m</sub>(S<sub>m</sub>,n); we enumerate [[concept-models]] by index m=1,2..M. Each model is characterized by its parameters, '''S<sub>m</sub>'''; in the [[neuron]] structure of the [[brain]] they are encoded by strength of synaptic connections, mathematically, we describe them as a set of numbers, '''S<sub>m</sub>''' = {S<sub>m</sub><sup>a</sup>, a = 1,... A}.
Models represent signals in the following way. Say, signal '''X(n)''', is coming from sensory [[neurons]] activated by object m, characterized by parameters '''S<sub>m</sub>'''. These parameters may include position, orientation, or lighting of an object m. [[Model (concept-models)|Model]] '''M<sub>m</sub>'''('''S<sub>m</sub>''',n) predicts a value '''X'''(n) of a signal at [[neuron]] n. For example, during visual perception, a [[neuron]] n in the visual cortex receives a signal '''X'''(n) from retina and a [[priming signal]] '''M<sub>m</sub>'''('''S<sub>m</sub>''',n) from an object-[[concept-model]] m. Neuron n is activated if both the bottom-up signal from lower-level-input and the top-down priming signal are strong. Various [[models (concept-models)|models]] compete for evidence in the [[bottom-up signals (bottom-up neural signals)|bottom-up signals]], while adapting their parameters for better match as described below. This is a simplified description of perception. The most benign everyday visual perception uses many levels from retina to object perception. The NMF premise is that the same laws describe the basic interaction dynamics at each level. Perception of minute features, or everyday objects, or cognition of complex abstract concepts is due to the same mechanism described below. Perception and cognition involve [[concept-models]] and learning. In perception, [[concept-models]] correspond to objects; in cognition [[models (concept-models)|models]] correspond to relationships and situations.
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