The early human visual system laid bare by novel techniques and models: psychophysical dissection using complex flicker

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The retina is made up of an array of a large number of light-sensitive cells: three types of cones responsible for daytime vision, and a rod responsible for night-time vision. Light that falls on the retina activates these cells, producing electrical responses. These responses are then combined in various ways within different pathways that encode colour and/or brightness and how they change over time and/or space. How these pathways process visual information and how this information is recombined in the brain to produce our perceptions of colour and brightness is not fully understood. Much of the specialization of the visual pathways occurs in the retina itself and cannot be directly examined in humans. Indeed, one of the most important functions of the retina is to process and reduce the amount of information in the visual image before sending it to the brain.An important feature of all the different pathways is the presence of ON and OFF channels, which carry information about visual attributes that are greater than and smaller than, respectively, the prevailing mean level. The visual signals are split into ON and OFF signals at the first junction between the light-sensitive cells and the subsequent neural components that send the signals to the brain. The visual attributes carried by the ON and OFF channels can be luminance; i.e. brighter or dimmer, or dimensions of colour; i.e., redder or greener, and bluer or yellower. The splitting into ON and OFF channels is often understood to be an efficient and powerful way to process signals that are larger or smaller than the mean, given that neurons in the visual pathway have a resting level (which should be low to save energy) and can only increase their response above this level by a factor of a few hundreds. Whether this is the reason we have ON and OFF pathways is an open question, but the focus of our study is what effects these pathways have on vision, and how these effects can be used to examine the visual system in more detail. We have found that the bifurcation of signals into ON and OFF channels has important implications for vision beyond improving the efficiency of processing and encoding the visual image. And, remarkable, we have found that we can manipulate the signals in the ON and OFF pathways using lights that flicker in complex temporal patterns, and that these manipulations can have profound effects on our perception of colour. Using complex flicker, and a new model that we have developed, we can break down the visual process into its various constituent stages and study each one separately, thereby linking perception to the underlying physiological processes. We can also compare between pathways to see how our percept of red- or greenness differs from our percept of brightness or blue- or yellowness. In this work, we aim to measure and model these kinds of effects and provide new insights into the inner workings of the visual system, leading to a general model of visual processing.