Coding and Representation

Neuronal activity does generally not in itself contain information about the stimulus. Spikes elicited by different stimuli look quite the same, but do generally occur in different cells. The information is contained in the specificity, or tuning curve of the cell generating the spike. This may be considered a modern account of the notion of the “specific sense energies” formulated by Johannes Müller already in 1826. In effect, any stimulation of the eye (or visual pathways) is perceived as visual and any stimulation of the ear (or auditory pathways) as auditory. The information encoded by each neuron is described by its tuning curve. Often, neurons are tuned simultaneously to different parameters such as position in visual space, edge orientation, and color, albeit to various extends (i.e., with sharper or coarser tuning curves). Tuning curves of different neurons overlap, leading to population coding where each stimulus is represented by the activities of a group, or population, of neurons. The first part of this chapter explores the consequences of population coding for neural information processing. In the second section, we study the fact that neighboring neurons in the cortical surface tend to have similar receptive fields and tuning curves. This similarity is defined for a combination of many parameters including position in the visual field as well as the well-known “perceptual dimensions” orientation, spatial frequency, color, motion, and depth. It is particularly evident for the tuning to visual field position, i.e. in retinotopic mapping of the visual field onto the cortical surface in the centimeter range.