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In Old World primates, the three cone photoreceptor classes provide the starting point for trichromatic vision. These cone classes are characterized by differences in spectral sensitivity, in density with respect to distance from the fovea, and in relative abundance across individuals (Hofer et al., 2005). How neural signals originating from the cones lead to colour perception is still not understood. For example, there is controversy over the chromatic structure of the receptive fields of the midget class of retinal ganglion cells, which give rise to the parvocellular visual pathway. To distinguish colours, the responses of different cone types must be compared. It is uncertain if the centres and surrounds of midget ganglion cell receptive fields are composed of inputs from only one cone type, or from cone mixtures. These options lead to different colour-coding schemes in the retina, and thus dictate how colour must be processed downstream. To solve this and other problems concerning visual perception, it would be useful to have a way to directly map the cone fields that initially define every receptive field.The main impediments to achieving this goal have been the inability to resolve and stimulate individual cones in the living retina. To begin to overcome these difficulties, we used an adaptive optics scanning laser ophthalmoscope (AOSLO) to visualize and stimulate the cones in vivo in the macaque (Roorda et al., 2002; Arathorn et al., 2007). We made extracellular recordings in the lateral geniculate nucleus to map the cones providing excitatory input to single parvocellular neurons, and assessed the efficacy of each cone's input for eliciting responses (Sincich et al., 2009). We found that parvocellular neurons can be mapped reliably by this method, and that the probability of evoking a spike with each stimulus flash varied considerably from cone to cone. This variability appeared to have two sources: a high sensitivity to the position of stimuli relative to each cone, and more variation in the synaptic weighting of cones than would be predicted if each class had a characteristic weight. For receptive field centres comprised of multiple cones, we also found that stimulation of just one of these cones leads to geniculate transmission with high probability. This result suggests that activation of a single cone within a large receptive field is sufficient for perception.Although our studies are at an early stage, cone imaging with an AOSLO, performed in conjunction with traditional electrophysiology, is a promising way to study the inputs to the visual system at the elemental level of the single cone. Receptive fields recorded at any level of the visual system can then be mapped by what they are truly made of -cone fields.

Measurements of electric potentials from neural activity have played a key role in neuroscience for almost a century, and simulations of neural activity is an important tool for understanding such measurements. Volume conductor (VC) theory is used to compute extracellular electric potentials such as extracellular spikes, MUA, LFP, ECoG and EEG surrounding neurons, and also inversely, to reconstruct neuronal current source distributions from recorded potentials through current source density methods. In this book chapter, we show how VC theory can be derived from a detailed electrodiffusive theory for ion concentration dynamics in the extracellular medium, and show what assumptions that must be introduced to get the VC theory on the simplified form that is commonly used by neuroscientists. Furthermore, we provide examples of how the theory is applied to compute spikes, LFP signals and EEG signals generated by neurons and neuronal populations.

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