Sridharan Devarajan
Attention: Tectum

Personal Background

Hailing from a spiritually inclined family in Chennai (India), I have always been intrigued by the workings of the human mind. As an undergraduate in Aerospace Engineering at the Indian Institute of Technology (IIT), Madras, I was fascinated by the mechanics of fluid flow. Four years of modeling and solving fluid flows gave me the background and confidence to tackle almost any physical problem. However, late in college, I realized that, among the many challenging questions awaiting solutions, understanding the mechanics of the human mind was the only one that would fulfil me emotionally as well as intellectually. Thus, I am now pursuing my dream of understanding how computations performed by networks of neurons in the brain give rise to mental phenomena.

Research Goals

I have always been fascinated by the hard problem of human conscious perception [1]. Decades of research in neuroscience has established that much of the information from the external world that enters our brain is, in fact, not perceived consciously [2]. To consciously perceive an object or event—to have cognitive access [3], in philosophy jargon—one must pay attention to it, and load it into working memory [3,4]. Intriguingly, these processes are accompanied by various kinds of oscillations (and synchrony) [5,6]. What are the computations performed by brain when one pays attention, and what is the role of oscillations in these computations?

To answer these questions, I study the optic tectum, a brain-stem structure that controls gaze in birds. Gaze control and attention are closely linked behaviorally and share a common neural substrate. Gaze is often directed to an overtly attended object. Also, microstimulation of the gaze control circuitry in birds is known to produce effects that mimic the neural effects of attention [7]. Hence, the optic tectum and associated brain structures (isthmic nuclei) are likely to be critically involved in attention.

In order to establish the precise role of the tectum and isthmic nucei in attention, I study the physiology of neurons in these regions using in vivo and in vitro extracellular recording of neural activity (in the lab of Prof. Eric Knudsen). Based on tectal anatomy and physiology, I plan to construct an in silico network level model of the tectal circuit. The model will mimic the neural effects of attention, and provide us with clues as to how oscillations arise, and what role they play, during attention.

Project Status

Over the past nine months, I have been recording spikes and local field potentials (LFP) from the optic tectum of the barn owl (Tyto Alba). These recordings indicate that LFP power in the gamma band (30-90Hz oscillations) increases when a stimulus is in the neuron's receptive field. This power diminishes when a second stimulus (a competitor) is presented outside the receptive field (in collaboration with Shreesh Mysore).

Synchrony in-vivo and in-silico. A stimulus evokes increased synchrony (at 50Hz) in the deep tectum (left). The in-silico model behaves similarly (right).

Based on in vitro findings of the facilitatory effects of acetylcholine on retinal excitatory synapses onto tectal excitatory and inhibitory neurons (courtesy: Alex Goddard), I have created a simple model of the tectal circuit in silico (on John Arthur's chip). The model posits an important role for acetylcholine in evoking both the neural (enhanced firing rate), and network (oscillations and synchrony) effects of attention.

More details on the data and the model can be found in a poster presented at the 2008 Society for Neuroscience (SfN) Annual Meeting in Washington DC. I am actively performing experiments (with several collaborators) to test and refine the model.

Other Projects

Music puts me on a different mental plane altogether. In a previous life, I tried to understand the neural basis of music perception with functional brain imaging (fMRI). The basic finding, that attentional brain circuits are involved in parsing transitions in the music, has been published, and received (somewhat unexpectedly) extensive press coverage.

In the transition to my current life, I worked with Brian Percival in implementing an elegant way to perform dynamic routing using neuronal oscillations on John Arthur's hippocampus chip.

Publications

ID Article Full Text

SJ3

D Sridharan, D J Levitin and V Menon. A Causal Role for the right Fronto-Insular Cortex in Switching between Executive-Control and Default-Mode Networks, Proceedings of the National Academy of Sciences, 105(34):12569-74, 2008.


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C35

D Sridharan, B Percival, J Arthur and K Boahen, An in-silico Neural Model of Dynamic Routing through Neuronal Coherence, Advances in Neural Information Processing Systems 20, D Koller, Y Singer and J Platt Eds., MIT Press, 2008.


Full Text
SJ2
D Sridharan, D J Levitin, C H Chafe, J Berger and V Menon. Neural Dynamics of Event Segmentation in Music: Converging Evidence for Dissociable Ventral and Dorsal Networks, Neuron, 55(3):521-32, 2007

Full Text
SJ1
D Sridharan, P S Prashanth and V S Chakravarthy. The Role of the Basal Ganglia in Exploration in a Neural Model based on Reinforcement Learning. International Journal of Neural Systems. 16(2):111-24, 2006.
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SC1
D Sridharan, P S Prashanth and V S Chakravarthy. The Role of the Basal Ganglia in Exploratory Behavior in a Model based on Reinforcement Learning. International Conference on Neural Information Processing, N R Pal, N Kasabov, R K Mudi, S Pal and S K Parui Eds., Springer, Lecture Notes in Computer Science (LNCS) 3316:70-77, 2004.
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Links

Visit my personal homepage to find out about other things that fulfil me emotionally!

My CV (with complete list of publications)

References

  1. Chalmers, D. Facing up to the problem of consciousness, Journal of Consciousness Studies, 2(3):200-219, 1995.
  2. Velmans, M. Is human information processing conscious?, Behavioral Brain Sciences, 14:651–669, 1991.
  3. Block, N. Consciousness, Accessibility and the mesh between psychology and neuroscience. Behavioral Brain Sciences, 30:481-548, 2007.
  4. Knudsen, E I. Fundamental components of attention. Annual Review of Neuroscience, 30(1):57–78., 2007.
  5. Engel, A K, Fries, P, Singer, W. Dynamic predictions: oscillations and synchrony in top-down processing. Nature Reviews Neuroscience, 2(10):704-716, 2001.
  6. John, E R. From synchronous neuronal discharges to subjective awareness? Progress in Brain Research, 150:143-171, 2005.
  7. Winkowski D E, Knudsen E I. Top-down control of multimodal sensitivity in the barn owl optic tectum. Journal of Neuroscience, 27(48):13279-91 , 2007.
  8. Wang Y, Luksch H, Brecha N C, Karten H J. Columnar projections from the cholinergic nucleus isthmi to the optic tectum in chicks (Gallus gallus): a possible substrate for synchronizing tectal channels. Journal of Comparative Neurology, 494(1):7-35, 2006. (Image courtesy: Phyllis Knudsen)