Aniruddha Das, Ph.D.
Our lab is interested in cortical mechanisms of visual processing. We have two broad areas of research – understanding task-related anticipation in visual cortex, and analyzing the cortical basis of visual form processing. We are also actively involved in developing new recording and analysis techniques for these two research directions.
Task-Related Anticipation in Visual Cortex By using a novel optical imaging technique developed by us for use in alert behaving monkeys, we recently discovered a robust anticipatory hemodynamic signal in primary visual cortex (V1). This signal is independent of visual stimulation or local neuronal activity. Given any regular, predictable visual task, this signal appears timed to bring fresh arterial blood to V1 in anticipation of the task onset. Our finding poses a serious challenge to the current understanding of fMRI which assumes that hemodynamic signals are uniformly predicted by underlying neural activity. It also likely indicates a novel task-related arousal mechanism in the brain – probably driven by distal neuromodulatory input – that prepares relevant cortical areas for upcoming task demands by bringing in fresh arterial blood in anticipation. A major focus of our lab, currently, is in understanding the neural basis and functional consequences of this signal. We are doing so with optical imaging and electrophysiology both in the alert monkey, and in an alert rat preparation that we are in the process of setting up.
Cortical Basis of Visual Form Processing It is now appreciated that even V1 has the sophisticated neural machinery to process reasonably complex visual items such as smooth contours, textures and surfaces. This is mediated through geometrically precise long-range connections within V1, combined with feedback from higher visual areas. Using optical imaging in alert monkey we find that the imaged response to a chain of short lines is suppressed relative to the linear sum of responses to individual lines, in a manner that is tuned for smoothness and collinearity; fMRI responses in human V1 are closely analogous. Similarly, illusory contours evoke robust imaging signals in monkey V1; linear discriminant analysis and fMRI shows an analogous response in humans. Combining human studies where we can ask our subjects complex perceptual questions with monkey optical imaging where we can carry out more invasive electrophysiology gives us a powerful tool to explore the cortical basis of early visual processing.
Developing New Brain Imaging and Analysis Techniques To pursue any question in neuroscience I believe it is very important to be able to develop new techniques as and when the question demands it. Our discovery of the anticipatory hemodynamic signal in V1 came about only because we had developed a dual-wavelength imaging technique that images cortical blood volume and oxygenation simultaneously with electrode recordings in the alert monkey. This allowed us to circumvent the limitations of neuoimaging signals such as fMRI that cannot disambiguate changes in blood volume from oxygenation. We are currently developing a flavoprotein imaging system to visualize neural activity at a spatial scale appropriate for studying visual processing. We are also developing imaging in the alert rodent, including two-photon imaging, to understand the neural basis of task-related anticipation.
M. M. B. Cardoso, Y. B. Sirotin, B. Lima, E. Glushenkova & A. Das: The Neuroimaging Signal is a Linear Sum of Neurally Distinct Stimulus- and Task-Related Components. Nat. Neurosci. 15(9): 1298-1306 (2012).
Y. B. Sirotin, M. Cardoso, B. Lima & A. Das: Spatial Homogeneity and Task-Synchrony of the Trial-Related Hemodynamic Signal. Neuroimage: 59: 2783-2797 (2012).
Y. B. Sirotin & A. Das. Spatial relationship between flavoprotein fluorescence and the hemodynamic response in the primary visual cortex of alert macaque monkeys. (Special invited article for Frontiers in Neuroenergetics: 2:6 June 2010).
Y. B. Sirotin, E. M. C. Hillman, C. Bordier and A. Das: Spatiotemporal precision and hemodynamic mechanism of optical point-spreads in alert primates. PNAS: 106(43): 18390-18395 (2009).
M. Kinoshita, C. D. Gilbert & A. Das: Optical imaging of contextual interactions in V1 of the behaving monkey. J. Neurophysiol: 102: 1930-1944 (2009).
Y. B. Sirotin and A. Das: Anticipatory haemodynamic signals in sensory cortex, not predicted by local neuronal activity. Nature: 457: 475–479 (2009).
A. Das: Contextual Interactions in Visual Processing: Elsevier Encyclopedia of Neuroscience: (2007).
A. Das: Receptive Fields: (chapter on Modeling Receptive Fields) Nature / Macmillan Encyclopedia of Cognitive Science: (2005).
D. Stettler, A. Das, J. Bennett and C. D. Gilbert: Lateral Connectivity and Contextual Interactions in Macaque Primary Visual Cortex. Neuron: 36: p 739-750. (2002).