Neurons in the Prefrontal Cortex are Tuned to Categories of Information RIKEN-MIT Neuroscience Research Center Neural Mechanisms for the Top-Down Control of Visual Attention Categories are the meanings we assign to the things we experience. When we walk into a room, we instantly recognize that items are chairs, desks, coffee mugs, etc., even though "chairs" come in a vastly different shapes, colors, sizes, etc. This is critical for thought; our raw perceptions of the world would be useless without the meanings we attach to them. Yet, almost nothing is known about how the brain accomplishes this. Freedman et al (2001) trained monkeys to categorize a set of computer-generated images as "cats" and "dogs". Some images that were "morphs," or blends, of different cats and dogs. For example, one might be 60% cat and 40% dog. Another might be 40% cat and 60% dog. The monkeys learned to categorize them correctly as cats or dogs. Neural correlates of the category representation were found in the prefrontal cortex, a cortical area critical for complex thought and behavior. Surprisingly, category information was represented at the single neuron level. That is, single neurons responded similarly to stimuli from the same category regardless of their physical appearance. These neural representations of the categories resulted from training: the monkeys had no prior experience with cats and dogs. Indeed, it was found that after a monkey was retrained with the stimuli assigned to new categories, prefrontal neurons no longer reflected the previous cat and dog categories but instead coded the new categories. How these representations are rapidly formed and reconfigured will require further investigation, but it is clearly related to how we categorize our world into meaningful concepts. 　 This figure shows three prototype (100%) cats and three prototype (100%) dogs as well as morph stimuli made from different blends of the prototypes. Freedman, D. J., Riesenhuber, M., Poggio, T., Miller, E. K. Categorical representation of visual stimuli in the primate prefrontal cortex. Science, 291, pp312-316 (2001)