circuitry in the human cerebral cortex contains several dozen distinct retinotopic maps, each devoted to analyzing the visual input stream in a particular way. [ citation needed ] The primary visual cortex (Brodmann area 17), which is the main recipient of direct input from the visual part of the thalamus, contains many neurons that are most easily activated by edges with a particular orientation moving across a particular point in the visual field. Visual areas farther downstream extract features such as color, motion, and shape. In auditory areas, the primary map is tonotopic . Sounds are parsed according to frequency (i.e., high pitch vs. low pitch) by subcortical auditory areas, and this parsing is reflected by the primary auditory zone of the cortex. As with the visual system, there are a number of tonotopic cortical maps, each devoted to analyzing sound in a particular way. Within a topographic map there can sometimes be finer levels of spatial structure. In the primary visual cortex, for example, where the main organization is retinotopic and the main responses are to moving edges, cells that respond to different edge-orientations are spatially segregated from one another. [ citation needed ] Understanding the relationship between the brain and the mind is a great challenge. [ 16 ] It is very difficult to imagine how mental entities such as thoughts and emotions could be implemented by physical entities such as neurons and synapses , or by any other type of mechanism. The difficulty was expressed by Gottfried