GAIN CONTROLS IN EARLY VISUAL PATHWAYS
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. We are also developing fMRI of visual pathways in mouse to make systematic measurements across these parallel pathways.
Aspects of this work are currently funded by a Career Integration Grant from the Marie Curie scheme of the European Commission "PVPITM", a Wellcome Trust Strategic Infrastrcuture Award, and an International Collaboration Award from Research to Prevent Blindness and Stavros Niarchos Foundation with Adam Kohn.
SG Solomon, A Kohn (2014) Current Biology. Moving Sensory Adaptation beyond Suppressive Effects in Single Neurons.
J Larsson, SG Solomon, A Kohn (2016) Cortex. fMRI adaptation revisited.
A Niranjan, IN Christie, SG Solomon, JA Wells, MF Lythgoe (2016) Neuroimage. fMRI mapping of the visual system in the mouse brain with interleaved snapshot GE-EPI
BRAIN PATHWAYS FOR INSTINCTIVE BEHAVIOURS
Prey species including mice express rapid avoidance behaviours that appear to be innately triggered by appropriate sensory stimuli. We have been exploring these behaviours, and the neural mechanisms that may underlie them. In addition we are interested in using these unconditioned behaviours to understand basic aspects of visual processing (in healthy animals and in animals with retinal or central disease) as well as higher order cognitive behaviours such as choice, navigation and attention. Our hope is that these simple behaviours may be supported by tractable machinery, that allow deep insight into ancient brain circuits as well as more recently evolved circuits that may have built on them.
This work is currently supported by BBSRC Project Grant BB/R004765/1 to Sam Solomon, Kate Jeffery and Aman Saleem "Brain pathways for visually- guided defence behaviours"
G De Franceschi et al. (2016) Current Biology. Vision guides selection of freeze of flight defence strategies in mice.
WHAT DOES THE CORTEX SEE?
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.
This work is currently funded by a grant from the National Health and Medical Research Council of Australia (NHMRC; 2015-17; PI Paul Martin) "Brain pathways serving conscious and sub-conscious vision"
In the news: for a comment on Solomon et al. (1999) see Vivien Casagrande (1999) "The mystery of the visual system K pathway"; for a discussion of Cheong et al. (2013), among others, see Cris Niell (2013) "Vision: more than expected in the early visual system"; for a comment on Zeater et al. (2015) see Wallace et al. (2016) "Primate thalamus: more than meets the eye".
N Zeater, SK Cheong, SG Solomon, B Dreher, PR Martin (2015). Current Biology. Binocular visual responses in the primate lateral geniculate nucleus.
SK Cheong, C Tailby, SG Solomon, PR Martin (2013) Journal of Neuroscience. Cortical-like receptive fields in the lateral geniculate nucleus of marmoset monkeys.
SK Cheong , C Tailby, PR Martin, JB Levitt, SG Solomon (2011). Proceedings of the National Academy of Sciences of the USA. Slow intrinsic rhythm in the koniocellular visual pathway.
MOTION SIGNALS IN THE CEREBRAL CORTEX
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.
This work is currently funded by grants from the Australian Research Council (ARC; 2016-18) "Representational geometry: predicting behaviour from brain representations" (PI Tom Carlson) & "Propagating neural waves: combined experimental and modelling study" (PI Pulin Gong)
Solomon SS, Tailby C, Gharaei S, Camp AJ, Bourne JA, Solomon SG (2011). Journal of Physiology, Visual motion integration in the middle temporal area of a New World monkey, the marmoset.
JS McDonald, CWG Clifford, SS Solomon, SC Chen, SG Solomon (2014) Journal of neurophysiology . Integration and segregation of multiple motion signals by neurons in area MT of primate.
RG Townsend, SS Solomon, SC Chen, ANJ Pietersen, PR Martin, SG Solomon, P Gong (2017). Visual motion discrimination by propagating patterns in primate cerebral cortex
SS Solomon, SC Chen, JW Morley, SG Solomon (2015) Cerebral Cortex. Local and global correlations between neurons in the middle temporal area of primate visual cortex.
SS Solomon, JW Morley, SG Solomon (2017). Spectral signatures of feedforward and recurrent circuitry in monkey area MT.
E Goddard, SG Solomon, TA Carlson (2017). Dynamic Population Codes of Multiplexed Stimulus Features in Primate Area MT
Prolonged viewing of a simple pattern leads to changes in its appearances, and of similar patterns. It has long been thought that this reflects the habituation of nerve cells in the visual parts of the cerebral cortex. We have shown that habituation is not confined to cortex, but instead first arises in the eye, and is strongest in nerve cells that are part of the "magnocellular-pathway", thought to be important in motion vision. Our observations also provide a way in which to non-invasively 'knock-out' this population of nerve-cells in normal humans and animals.
S Di Marco, DA Protti, SG Solomon (2013) Journal of neurophysiology. Excitatory and inhibitory contributions to receptive fields of alpha-like retinal ganglion cells in mouse.
AJ Camp, C Tailby, SG Solomon (2009) Journal of Neuroscience. Adaptable mechanisms that regulate the contrast response of neurons in the primate lateral geniculate nucleus.
Solomon, S.G., Peirce, J.W., Dhruv, N. and Lennie, P. (2004) Neuron . Profound Contrast Adaptation Early in the Visual Pathway
In the news: for a comment on Solomon et al. (2004) see Baccus and Meister "Retina vs. cortex; contrast adaptation in parallel visual pathways"
Nerve cells early in the visual pathway are usually thought to be small and simple. We have shown that many of them are sensitive to a larger fraction of visual space than usually thought. This makes the signals of these nerve cells sensitive to the global spatial structure of images, helping make sensory processing more efficient.
DA Protti, S Di Marco, JY Huang, CR Vonhoff, V Nguyen, SG Solomon (2014) Journal of physiology. Inner retinal inhibition shapes the receptive field of retinal ganglion cells in primate.
Webb, B.S., Dhruv, N.T., Solomon, S.G., Tailby, C. and Lennie, P. (2005) Journal of Neuroscience. Early and late mechanisms of surround suppression in striate cortex of macaque
Solomon, S.G., White, A.J.R. and Martin, P.R. (2002) Journal of Neuroscience Extraclassical receptive field properties of parvocellular, magnocellular, and koniocellular cells in the primate lateral geniculate nucleus
In the news: for a comment on Webb et al. see Matthew Smith (2006) "Surround suppression in the early visual system"
In the news: for a comment on Martin et al. (2001) see Andrew Derrington (2001) Why do colours fade at the edges?
MACHINERY OF COLOUR VISION
Colour vision requires the presence of more than one light-sensitive receptor in the eye. But having more than one receptor is not sufficient for colour vision - the brain needs to compare the activity of different receptors to know the colour of an object. Where in the brain these comparison are made remains a matter of substantial debate even though it is now 200 years since Young, Hering and Helmholtz provided the framework in which we understand colour vision. With Peter Lennie, Paul Martin and Chris Tailby we helped show how colour signals are segregated in the eye and sent along parallel pathways to subsequent brain areas. In particular, blue colours are processed by specialised pathways in the eye, and are brought together with information about red and green colours in the cerebral cortex. Our experiments showed how these signals are combined in the cerebral cortex, and why there appears to be particular axes of colour vision that have 'special status'.
C Tailby, SG Solomon and P Lennie (2008) Journal of Neuroscience Functional Asymmetries in Visual Pathways Carrying S-Cone Signals in Macaque
C Tailby, SG Solomon, N Dhruv and P Lennie (2008) Journal of Neuroscience Habituation Reveals Fundamental Chromatic Mechanisms in Striate Cortex of Macaque.
SG Solomon and P Lennie (2007) Nature Reviews Neuroscience The Machinery of Colour Vision.
SG Solomon and P Lennie (2005) Journal of Neuroscience Chromatic Gain Controls in Visual Cortical Neurons
PR Martin, BB Lee, AJR White, SG Solomon and L Ruttiger (2001) Nature Chromatic sensitivity of ganglion cells in the peripheral primate retina