Research

Our lab is interested in how hearing circuits function in normal and pathological conditions. We are interested in the representation of sound at various levels - behavioral responses to sound, auditory evoked potentials from the entire auditory system, in vivo discharge patterns to sound from a single neuron or ensembles, intrinsic and synaptic currents produced by neurons, and the changes in protein expression and molecular organization that underlie functional changes.

Current Projects and Research Interests

_{See Publications for papers referred to in text}

I. Alterations in Hearing Function and Recovery from Noise Exposure

We are testing how different types of noise exposure differing in intensity and duration affect hearing and recover from exposure over time. Noise exposure affects millions of Americans, both civilians and military service members, and we are testing how brief noise exposures such as those due to blast (Race et al. 2017, Han et al. 2020) or gunshot exposure may differ from continuous noise such as that experienced in many workplaces (e.g. construction, manufacturing). These projects are funded or have been funded by the Department of Defense and the Indiana CTSI.

II. Infrared Neural Stimulation and Closed-Loop Control of Auditory Thalamocortical Circuits

Infrared neural stimulation (INS) offers a novel mode of neuroprosthetic substitution (Coventry et. al 2020) that may be more selective than electrical stimulation and that does not require genetic modification of neurons, making INS more easily suitable and a powerful technology for humans. Moreover, we have developed a powerful algorithm called Spiker-Net for learning and modifying neural stimulation parameters in real-time. These projects are funded or have been funded by the National Institutes of Health and the Purdue Institute for Integrative Neuroscience.

III. Changes in Hearing Function and Assessment with Aging

Age-related hearing loss (ARHL) affects tens of millions of people in the United States alone, and left untreated, it is a major risk factor for poor outcomes in aging (Helfer et al. 2020). We are currently working on diagnostics of auditory system function, both behavioral and electrophysiological, that are non-invasive and can be used in rodents and humans (Parthasarathy and Bartlett 2012, Parthasarathy et al. 2014, 2016, 2019, Lai et al. 2017, 2018). In addition, we record from neurons in the inferior colliculus (IC), auditory thalamus, and auditory cortex in order to track age-related changes in neural representations of sound features (Rabang et al. 2012, Herrmann et al. 2017). These projects are funded or have been funded by the National Institutes of Health and the American Federation for Aging Research.

IV. Dynamic interactions between the Medial Geniculate Body (Auditory Thalamus) and Auditory Cortex

The auditory thalamus is the information bottleneck of the auditory system, where all information for conscious hearing is routed to cortex within a small volume. However, it is not a homogeneous structure,comprised of the ventral, dorsal and medial divisions (Bartlett 2013). We have demonstrated that the auditory thalamus differ significantly in organization and neural processing compared to other thalamic regions. In particular, we have demonstrated that there is a rapid, feed-forward GABAergic input from the inferior colliculus (Peruzzi et al. 1997, Bartlett and Smith 1999, Venkataraman and Bartlett 2013). Furthermore, the excitatory synaptic dynamics form a continuum, with large excitatory inputs exhibiting synaptic depression and small excitatory inputs exhibiting synaptic facilitation, both of which drastically alter responses to repetitive inputs from IC or auditory cortex (Bartlett and Smith 2002, Smith et al. 2007, Venkataraman and Bartlett 2013). These properties shape neural representations of temporal modulation that are differentially distributed within subdivisions of the auditory thalamus (Bartlett and Wang 2007, 2011). Finally, collaboration with Donald Caspary's group, we are working on how predictability and corticothalamic feedback interact (Kommajosyula et al. 2019, 2021).

Table below from Bartlett (2013).

V. Coding Properties of the Inferior Colliculus

The inferior colliculus is a major integrative center of the central auditory system, integrating excitatory and inhibitory inputs from lower brain stem nuclei. To understand coding mechanisms of these neurons, we utilize chronic and acute recordings of single unit neural activity to spectral and temporal varying stimuli. Furthermore, synaptic mechanisms of response generation are further elucidated using single and multicompartmental computational models. These projects are funded or have been funded by the National Institutes of Health and the American Federation for Aging Research.

VI. In-Vitro Slice Electrophysiology

Along with extracellular single unit recordings, we also utilize brain-slice recordings to understand cellular properties of auditory neurons (Venkataraman and Bartlett 2013, 2014). These studies encompass both electrical and optical stimulation modalities. In addition, we have collaborated with Dr. Amy Brewster for understanding hippocampal circuits in epilepsy. These projects are funded or have been funded by the National Institutes of Health and the Hearing Health Foundation (formerly the Deafness Research Foundation).

VII. Neuroanatomy

Our lab also explores anatomical connections within the central auditory system to complement our electrophysiological measurements and to understand mechanisms of change in aging, development or following acoustic trauma. Moreover, neuroanatomy is a critical assessment for neuroprosthetic devices and their long-term efficacy.

In the image below, the different subdivisions of the auditory thalamus (MGB) have been stained for the vesicular glutamate transporter 2 (VGLUT2) which labels presynaptic terminals from IC and from layer 5 of cortex, but not from layer 6 of cortex. MGV terminals are significantly larger, especially in the lateral portion of MGV, consistent with electrophysiological measurements of excitatory postsynaptic potentials/currents (Bartlett and Smith 1999, 2002, Venkataraman and Bartlett 2013).

In the image below, the MGB has been immuno-labeled for neurons with NeuN (blue), activated glia with GFAP (yellow) and microglia (red). One can see a cluster of activated glia surrounding the track of the stimulating electrode in the MGB, which was used to stimulate auditory cortex to drive changes in behavior and neural activity.