Dr Samuel Solomon
Reader in Visual Neuroscience
Div of Psychology & Lang Sciences
- Joined UCL
- 3rd Jan 2012
I am interested in the work done by the eye and the brain to analyse the visual world and support visual perception. I'm particularly interested in how visual perception reflects the basic properties of networks of nerve cells at each level of the visual pathway. My main interests are two-fold. First, I want to understand, at the level of individual neurons and groups of them, how sensory signals interact with internal representations. Second, I want to use our substantial knowledge of the visual system to develop model systems with which we can gain insight into more complex brain functions.
The responses of neurons in the visual pathways of the brain depends on both the spatial and temporal context in which a stimulus (eg. an edge) is presented. The mechanisms that provide this context sensitivity are often called “gain controls”. They are thought to be ubiquitous in visual and other sensory systems, and may be analagous to other brain mechanisms that help learning and decision making. Abnormal gain controls may underly several important brain disorders, and also influence the capacity of clinical tests to detect changes in visual sensitivity that accompany, for example, macular degeneration. Despite their ubiquity, we do not know whether these gain controls are expressed all of the parallel visual pathways, or whether their impact on processing is the same in each case. In this work, we are investigating the prevalence and signatures of gain controls in the visual thalamus and mid-brain (an area called the superior colliculus), both of which are direct recipients of the output of the eye, but are involved in very different forms of visual analysis. To understand the properties of gain controls, we are recording the activity of nerve cells in each of these areas, during anaesthesia and wakefullness. We are also developing fMRI of visual pathways in mouse to make systematic measurements across these parallel pathways, and developing tests of visual spatial attention in mice so that we can understand whether attention influences the action of these gain controls.
- University of Sydney
- PhD, Neuroscience | 2002
1. What signals are passed form the retina to primary visual cortex? Textbooks will tell you that there are two main pathways - the "magnocellular", and "parvocellular" - and that these provide signals necessary for motion and form/colour vision respectively. We have explored the signals provided by a third pathway, the evolutionarily older "koniocellular" pathway, and find that neurons there can show remarkable properties, including selectivity to oriented edges, and capacity to be excited by inputs from either eye. Both these properties were thought to emerge in visual cortex, so their existence in the areas of the brain that provide input to cortex raises many questions about how they contribute to subsequent processing. In addition we have found that some nerve cells in the early visual pathway show shared activity - this is manifest at long time scales (slow rhythms), yoked to fluctuations in the electroencephalogram over visual cortex, and is confined to the koniocellular pathway.
2. In primates including humans a small area of the cerebral cortex - area MT - is thought to hold a special role in motion vision and the control of eye movements. We have been making measurements of the motion signals carried by single nerve cells in area MT, and in populations of nerve cells there. We are particularly interested in 1) how the functional connectivity of neurons constrains the signals that they provide, and 2) how signals for surface segmentation (eg. transparency) and surface integration (eg. 'pattern motion') are carried by individual nerve cells.