1.Does the superior colliculus control perceptual sensitivity or choice bias during attention? Evidence from a multialternative decision framework.

Sridharan D , Steinmetz NA, Moore T, Knudsen EI. Journal of Neuroscience.
[Abstract] [Pubmed] [Journal] [PDF] [SI]

2.Selective disinhibition: A robust mechanism for effecting target priority by selective attention.

Sridharan D ,Knudsen EI. Vision Research.Special Issue: Computational Models of Visual Attention.
[Abstract] [Pubmed] [Journal] [PDF] [SI] [Icon] [arXiV] [Code]

Under Review

3.Gamma oscillations in the midbrain spatial attention network: Linking circuits to function.

Sridharan D ,Knudsen EI. Current Opinion in Neurobiology.Special Issue: Brian Rhythms and Dynamic Coordination.
[Abstract] [Pubmed] [Journal] [PDF] [SI] [Icon] [arXiV] [Code]

Under Review

4.Distinguishing bias from sensitivity effects in multialternative detection tasks.

Sridharan D , Steinmetz NA, Moore T, Knudsen EI. Journal of Vision (2014) 14(9):16, 1-32.
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Studies investigating the neural bases of cognitive phenomena increasingly employ multialternative detection tasks that seek to measure the ability to detect a target stimulus or changes in some target feature (e.g., orientation or direction of motion) that could occur at one of many locations. In such tasks, it is essential to distinguish the behavioral and neural correlates of enhanced perceptual sensitivity from those of increased bias for a particular location or choice (choice bias).

5.Selective attention in birds.

Sridharan D , Schwarz JS, Knudsen EI. Current Biology(2014) 24(11): R510-513.
[Abstract] [Pubmed] [Journal] [PDF]

Over the past century, major strides have been made in characterizing the phenomenology of attention in humans. As a result of this research, a variety of attention disorders, such as attention deficit disorder, autism and schizophrenia, can now be reliably diagnosed. However, the etiologies of these disorders remain poorly understood. Developing targeted therapies for treating such disorders requires a mechanistic understanding of how attention works at the level of cells and circuits. Here we review recent evidence for remarkable similarities in the phenomenology of spatial selective attention in birds and primates. These studies open up new avenues for research into the neural mechanisms that control attention. The brains of birds and primates share many neuroanatomical and functional features. Like primates, birds (especially chickens) are readily trained to perform behavioral tasks that yield precise, quantitative measures of decision-making. In contrast to primates, they are readily available and tractable for developing and applying cutting-edge experimental techniques. We expect, therefore, that research on avian species will greatly accelerate the discovery of neural mechanisms that underlie attention.

6.Visuospatial selective attention in chickens.

Sridharan D , Ramamurthy DL, Schwarz JS, Knudsen EIProceedings of the National Academy of Sciences (2014) 111(19): E2056-2065.
[Abstract] [Pubmed] [Journal] [PDF] [SI]

Voluntary control of attention promotes intelligent, adaptive behaviors by enabling the selective processing of information that is most relevant for making decisions. Despite extensive research on attention in primates, the capacity for selective attention in nonprimate species has never been quantified. Here we demonstrate selective attention in chickens by applying protocols that have been used to characterize visual spatial attention in primates. Chickens were trained to localize and report the vertical position of a target in the presence of task-relevant distracters. A spatial cue, the location of which varied across individual trials, indicated the horizontal, but not vertical, position of the upcoming target. Spatial cueing improved localization performance: accuracy (d') increased and reaction times decreased in a space-specific manner. Distracters severely impaired perceptual performance, and this impairment was greatly reduced by spatial cueing. Signal detection analysis with an "indecision" model demonstrated that spatial cueing significantly increased choice certainty in localizing targets. By contrast, error-aversion certainty (certainty of not making an error) remained essentially constant across cueing protocols, target contrasts, and individuals. The results show that chickens shift spatial attention rapidly and dynamically, following principles of stimulus selection that closely parallel those documented in primates. The findings suggest that the mechanisms that control attention have been conserved through evolution, and establish chickens-a highly visual species that is easily trained and amenable to cutting-edge experimental technologies- as an attractive model for linking behavior to neural mechanisms of selective attention.

7.Magnetic tracking of eye position in freely behaving chickens.

Schwarz JS,Sridharan D , Knudsen EIFrontiers in Systems Neuroscience (2013) 7(91):1-8.
[Abstract] [Pubmed] [Journal] [PDF]

Research on the visual system of non-primates, such as birds and rodents, is increasing. Evidence that neural responses can differ dramatically between head-immobilized and freely behaving animals underlines the importance of studying visual processing in ethologically relevant contexts. In order to systematically study visual responses in freely behaving animals, an unobtrusive system for monitoring eye-in-orbit position in real time is essential. We describe a novel system for monitoring eye position that utilizes a head-mounted magnetic displacement sensor coupled with an eye-implanted magnet. This system is small, lightweight, and offers high temporal and spatial resolution in real time. We use the system to demonstrate the stability of the eye and the stereotypy of eye position during two different behavioral tasks in chickens. This approach offers a viable alternative to search coil and optical eye tracking techniques for high resolution tracking of eye-in-orbit position in behaving animals.

8.Spatial probability dynamically modulates visual target detection in chickens.

Sridharan D ,Ramamurthy DL, Knudsen EI.Plos One (2013) 8(5): e64136.
[Abstract] [Pubmed] [Journal] [SI]

The natural world contains a rich and ever-changing landscape of sensory information. To survive, an organism must be able to flexibly and rapidly locate the most relevant sources of information at any time. Humans and non-human primates exploit regularities in the spatial distribution of relevant stimuli (targets) to improve detection at locations of high target probability. Is the ability to flexibly modify behavior based on visual experience unique to primates? Chickens (Gallus domesticus) were trained on a multiple alternative Go/NoGo task to detect a small, briefly-flashed dot (target) in each of the quadrants of the visual field. When targets were presented with equal probability (25%) in each quadrant, chickens exhibited a distinct advantage for detecting targets at lower, relative to upper, hemifield locations. Increasing the probability of presentation in the upper hemifield locations (to 80%) dramatically improved detection performance at these locations to be on par with lower hemifield performance. Finally, detection performance in the upper hemifield changed on a rapid timescale, improving with successive target detections, and declining with successive detections at the diagonally opposite location in the lower hemifield. These data indicate the action of a process that in chickens, as in primates, flexibly and dynamically modulates detection performance based on the spatial probabilities of sensory stimuli as well as on recent performance history.

9.Gamma oscillations are generated locally in an attention-related midbrain network.

Goddard CA , Sridharan D , Huguenard JH, Knudsen EI. Neuron (2012) 73(3):567-80.
[Abstract] [Pubmed] [Journal] [SI]

Gamma-band (25-140 Hz) oscillations are a hallmark of sensory processing in the forebrain. The optic tectum (OT), a midbrain structure implicated in sensorimotor processing and attention, also exhibits gamma oscillations. However, the origin and mechanisms of these oscillations remain unknown. We discovered that in acute slices of the avian OT, persistent (~100 ms) epochs of large amplitude gamma oscillations can be evoked that closely resemble those recorded in vivo. We found that cholinergic, glutamatergic, and GABAergic mechanisms differentially regulate the structure of the oscillations at various timescales. These persistent oscillations originate in the multisensory layers of the OT and are broadcast to visual layers via the cholinergic nucleus Ipc, providing a potential mechanism for enhancing the processing of visual information within the OT. The finding that the midbrain contains an intrinsic gamma-generating circuit suggests that the OT could use its own oscillatory code to route signals to forebrain networks.

10.Space coding by gamma oscillations in the barn owl optic tectum.

Sridharan D,Boahen K, Knudsen EI.Journal of Neurophysiology (2011) 105: 2005-2017.
[Abstract] [Pubmed] [Journal] [PDF] [SI] [Cover]

Gamma-band (25–140 Hz) oscillations of the local field potential (LFP) are evoked by sensory stimuli in the mammalian forebrain and may be strongly modulated in amplitude when animals attend to these stimuli. The optic tectum (OT) is a midbrain structure known to contribute to multimodal sensory processing, gaze control, and attention. We found that presentation of spatially localized stimuli, either visual or auditory, evoked robust gamma oscillations with distinctive properties in the superficial (visual) layers and in the deep (multimodal) layers of the owl’s OT. Across layers, gamma power was tuned sharply for stimulus location and represented space topographically. In the superficial layers, induced LFP power peaked strongly in the low-gamma band (25–90 Hz) and increased gradually with visual contrast across a wide range of contrasts. Spikes recorded in these layers included presumptive axonal (input) spikes that encoded stimulus properties nearly identically with gamma oscillations and were tightly phase locked with the oscillations, suggesting that they contribute to the LFP oscillations. In the deep layers, induced LFP power was distributed across the low and high (90–140 Hz) gamma-bands and tended to reach its maximum value at relatively low visual contrasts. In these layers, gamma power was more sharply tuned for stimulus location, on average, than were somatic spike rates, and somatic spikes synchronized with gamma oscillations. Such gamma synchronized discharges of deep-layer neurons could provide a highresolution temporal code for signaling the location of salient sensory stimuli.

11.A critical role for the right fronto-insular cortex in switching between central-executive and default-mode networks.

Sridharan D ,Levitin DJ, Menon V.Proceedings of the National Academy of Sciences (2008) 105(34):12569-74.
[Abstract] [Pubmed] [Journal] [PDF] [SI]

Cognitively demanding tasks that evoke activation in the brain’s central-executive network (CEN) have been consistently shown to evoke decreased activation (deactivation) in the default-mode network (DMN). The neural mechanisms underlying this switch between activation and deactivation of large-scale brain networks remain completely unknown. Here, we use functional magnetic resonance imaging (fMRI) to investigate the mechanisms underlying switching of brain networks in three different experiments. We first examined this switching process in an auditory event segmentation task. We observed significant activation of the CEN and deactivation of the DMN, along with activation of a third network comprising the right fronto-insular cortex (rFIC) and anterior cingulate cortex (ACC), when participants perceived salient auditory event boundaries. Using chronometric techniques and Granger causality analysis, we show that the rFIC-ACC network, and the rFIC, in particular, plays a critical and causal role in switching between the CEN and the DMN. We replicated this causal connectivity pattern in two additional experiments: (i) a visual attention "oddball" task and (ii) a task-free resting state. These results indicate that the rFIC is likely to play a major role in switching between distinct brain networks across task paradigms and stimulus modalities. Our findings have important implications for a unified view of network mechanisms underlying both exogenous and endogenous cognitive control.

12.An in-silico model of dynamic routing through neuronal coherence.

Sridharan D ,Percival B , Arthur J, Boahen K.Advances in Neural Information Processing Systems(2008) 20:1401-1408.
[Abstract] [Proceedings] [PDF]

We describe a neurobiologically plausible model to implement dynamic routing using the concept of neuronal communication through neuronal coherence. The model has a three-tier architecture: a raw input tier, a routing control tier, and an invariant output tier. The correct mapping between input and output tiers is realized by an appropriate alignment of the phases of their respective background oscillations by the routing control units. We present an example architecture, implemented on a neuromorphic chip, that is able to achieve circular-shift invariance. A simple extension to our model can accomplish circular-shift dynamic routing with only O(N) connections, compared to O(N2) connections required by traditional models.

13.Neural dynamics of event segmentation in music: Converging evidence for dissociable ventral and dorsal networks.

Sridharan D ,Levitin DJ, Chafe CH, Berger J, Menon V.Neuron(2007) 55(3):521-32.
[Abstract] [Pubmed] [Journal] [PDF] [PDF] [News]

The real world presents our sensory systems with a continuous stream of undifferentiated information. Segmentation of this stream at event boundaries is necessary for object identification and feature extraction. Here, we investigate the neural dynamics of event segmentation in entire musical symphonies under natural listening conditions. We isolated time-dependent sequences of brain responses in a 10 s window surrounding transitions between movements of symphonic works. A strikingly right-lateralized network of brain regions showed peak response during the movement transitions when, paradoxically, there was no physical stimulus. Model-dependent and model-free analysis techniques provided converging evidence for activity in two distinct functional networks at the movement transition: a ventral fronto-temporal network associated with detecting salient events, followed in time by a dorsal fronto-parietal network associated with maintaining attention and updating working memory. Our study provides direct experimental evidence for dissociable and causally linked ventral and dorsal networks during event segmentation of ecologically valid auditory stimuli.

14.The role of the basal ganglia in exploration in a neural model based on reinforcement learning.

Sridharan D ,Prashanth PS, Chakravarthy VS.International Journal of Neural Systems (2006) 16(2):111-24.
[Abstract] [Pubmed] [Journal] [PDF] [Erratum]

We present a computational model of basal ganglia as a key player in exploratory behavior. The model describes exploration of a virtual rat in a simulated water pool experiment. The virtual rat is trained using a reward-based or reinforcement learning paradigm which requires units with stochastic behavior for exploration of the system’s state space. We model the Subthalamic Nucleus-Globus Pallidus externa (STN-GPe) segment of the basal ganglia as a pair of neuronal layers with oscillatory dynamics, exhibiting a variety of dynamic regimes such as chaos, traveling waves and clustering. Invoking the property of chaotic systems to explore state-space, we suggest that the complex exploratory dynamics of STN-GPe system in conjunction with dopamine-based reward signaling from the Substantia Nigra pars compacta (SNc) present the two key ingredients of a reinforcement learning system.