Neuronal tuning

Neuronal tuning refers to the property of brain cells by which they selectively represent a particular type of sensory, association, motor, or cognitive information. Neuronal responses are optimally tuned to specific patterns through experience. Neuronal tuning can be strong and sharp, as observed in primary visual cortex (area V1), or weak and broad, as observed in neural ensembles. Single neurons may be simultaneously tuned to several modalities, such as visual, auditory, and olfactory. Neurons that are tuned to different signals often integrate information from the different sources. In neural networks, such integration is the major principle of operation. The best examples of neuronal tuning can be seen in the visual, auditory, olfactory, somatosensory, and memory systems.

Accepted neuronal tuning models suggest that neurons respond to different degrees based on the similarity between the optimal stimulus of the neuron and the given stimulus. The first major evidence of neuronal tuning in the visual system was provided by Hubel and Wiesel in 1959. They discovered that oriented slits of light were the most effective stimuli for striate cortex “simple cell” neurons. Other neurons, “complex cells," responded best to lines of a certain orientation moving in a specific direction. Overall, the V1 neurons were found to be selectively tuned to certain orientations, sizes, positions, and forms. Hubel and Wiesel won the Nobel Prize in Physiology or Medicine in 1981 for their discoveries concerning information processing in the visual system.

While these simple cells in V1 respond to oriented bars through small receptive fields, the optimal visual stimulus becomes increasing complex as one moves toward the anterior of the brain. Neurons in area V4 are selectively tuned to different wavelengths, hues, and saturations of color. The middle temporal or area V5 is specifically tuned to the speed and direction of moving stimuli. At the apex of the ventral stream called the inferotemporal cortex, neurons became tuned to complex stimuli, such as faces. The specific tuning of intermediate neurons in the ventral stream is less clear, because the range of form variety that can be utilized for probing is nearly infinite.

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