Hierarchical processing

Figure 2.19: The stimulus captured by receptors works its way through a hierarchical network of neurons. In the early stages, signals are combined from multiple receptors and propagated upward. At later stages, information flows bidirectionally.

Upon leaving the sense-organ receptors, signals propagate among the neurons to eventually reach the cerebral cortex. Along the way, hierarchical processing is performed; see Figure 2.19. Through selectivity, each receptor responds to a narrow range of stimuli, across time, space, frequency, and so on. After passing through several neurons, signals from numerous receptors are simultaneously taken into account. This allows increasingly complex patterns to be detected in the stimulus. In the case of vision, feature detectors appear in the early hierarchical stages, enabling us to detect features such as edges, corners, and motion. Once in the cerebral cortex, the signals from sensors are combined with anything else from our life experiences that may become relevant for making an interpretation of the stimuli. Various perceptual phenomena occur, such as recognizing a face or identifying a song. Information or concepts that appear in the cerebral cortex tend to represent a global picture of the world around us. Surprisingly, topographic mapping methods reveal that spatial relationships among receptors are maintained in some cases among the distribution of neurons. Also, recall from Section 1.1 that place cells and grid cells encode spatial maps of familiar environments.