Over time, investigators have oscillated from one theory of encoding to another and back. During the past decade or so, however, a population-coding theory has returned. Many studies reveal that a brain does encode information as patterned activity in a population of neurons. In some species, brain maps in the superior colliculus, for example, encode the location of visual and auditory stimuli, as well as movement. On these maps, pinpointing the source of a sensory stimulus or directing a movement to a specific location involves many neurons.
Population coding exists in other areas as well.
Although the appeal of population coding may be justified on its own merit at this time, we must also recognize that our attention may be focused on this theory in part because of the present state of our laboratory techniques. When Sherrington supported population coding in the 1940s, a neuroscientist's conclusions depended largely on histology, leading to slides that showed many neurons or comparatively coarse electrophysiology that recorded the activity of many cells. In other words, the tools of Sherrington's era largely unveiled populations of neurons, so the accepted theories emphasized population coding. With the development of single-cell recording and histological markers for specific cell types, neuroscientists began unraveling the properties of individual cells. At that time, the single-neuron doctrine ruled.