Neural mechanisms of long-range spatial vision: an investigation of perceptive, integrative and association fields across the lifespan

Incoming light is processed by retinal receptors. Light reflected from objects that are nearby each other will fall onto adjacent retinal locations, and will be fed to correspondingly located neurons in the visual cortex. In the visual cortex, units of different complexity will process this information further, ultimately giving rise to our everyday visual experience. The area of space on the retina to which a visual cortex unit responds is called a visual field. The concept of visual fields has played a crucial role in the study of visual perception since the pioneering studies of Hubel and Wiesel in the 1960s. Visual fields determine which information will be bound together and which will be individuated. Away from the centre of the retina, visual fields increase in size, but this size is flexible and, somewhat surprisingly, depends on the strength of the received signals (colour or brightness) as well as the presence or absence of nearby objects, which activate neighbouring neural units. There are also visual fields of different complexity: while neurons in the primary visual cortex simply integrate the amount of brightness or colour contrast that fall within their scope, neurons beyond it integrate or individuate more complex attributes of objects, e.g. orientation, to identify the object's shape. Therefore, visual fields determine information processing across space, especially processing of relatively long-range spatial information, which will be at some distance from each other once we move away from the centre of the retina.

The aim of this project is to build a unitary framework that would encompass neural processing within visual fields of different complexity across the lifespan. Despite years of research, such a unitary framework is still lacking: while we know a lot about "low-level" processing of colour and luminance supported by neurons in the primary visual cortex, the intermediate stages of visual processing are more difficult to probe and distinct aspects of such "mid-level" vision are often studied separately. The uniqueness of our approach is that we will probe visual fields of different complexity, covering both "low" and "mid-level" vision, using the same paradigm. This paradigm is simple yet powerful: it relies on the same stimulus, consisting of multiple oriented elements (black and white gratings known as Gabor patches) that can form different letters, to probe visual fields of different complexity by simply changing the task that the participants are performing: a) detecting the presence or absence of a target element, b) judging its properties, or c) judging the identity of the whole letter. We will establish how visual fields process stimuli defined by brightness, colour, or both, thus integrating all types of contrast visible to the human eye. Finally, we will conduct our experiments on a large sample that consists of participants that are between 20 and 80 years of age. Our experiments will combine behavioural and neuroscientific techniques, using new, state-of-the-art electroencephalographic (EEG) techniques to ascertain which neural mechanisms underlie age-related changes in visual processing. We will determine the degree to which age-related deficits are driven by an increase in neural noise, and relate it to more basic changes in contrast sensitivity. Such analyses of age-related differences will offer an exciting window into the neural mechanisms that sustain information processing in human vision. In this way, this project will not only provide a basis for theoretical developments in visual perception, but will also considerably affect our understanding of the aging of sensory mechanisms. Such information can be used to improve the quality of life for the elderly.