Biology subjects

Williamson, R. S.

Publications and source records attributed to Williamson, R. S..

13 recordsLinked to original sources

bioRxiv2026

Layer 5 and 6b extratelencephalic neurons encode distinct sound features in auditory cortex

Extratelencephalic (ET) neurons in layers (L)5 and 6b of the auditory cortex (ACtx) provide major corticofugal outputs to subcortical structures and contribute to auditory learning and experience-dependent plasticity. Although both ET subtypes innervate overlapping down-stream targets, they differ in morphology, intrinsic physiology, and molecular identity, suggesting distinct functional roles. However, their in vivo response properties remain incompletely characterized. Here, we used a projection-defined viral strategy to express GCaMP8s selectively in L5 and L6b ET neurons in mouse ACtx and recorded calcium activity during presentation of diverse acoustic stimuli. L5 ET neurons predominantly exhibited sound-evoked excitation, higher response sparseness, and greater trial-to-trial reliability than L6b ET neurons; these differences were most pronounced for pure tones and sinusoidally amplitude-modulated (sAM) noise. In contrast, L6b ET neurons frequently exhibited sound-evoked suppression, particularly for more complex stimuli (including sAM noise and spectrotemporal ripples). Unsupervised clustering identified separable temporal response motifs and tuning profiles. L5 ET neurons favored single-peaked frequency tuning and monotonic intensity tuning, whereas L6b ET neurons exhibited multi-peaked and complex frequency tuning, more non-monotonic intensity dependence, and stronger pairwise functional coupling (noise correlations). Together, these results support complementary corticofugal processing streams, with L5 ET neurons conveying more selective acoustic feature representations and L6b ET neurons conveying more integrative corticofugal signals.

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bioRxiv2025

Canonical cortical architecture supports the emergence of noise-invariant auditory representations

Neurons in the auditory system must represent behaviorally relevant sounds in the presence of background noise (BN) to support noise-invariant perception and behavior. Although primary auditory cortex (ACtx) has been implicated in constructing noise-invariant representations, it remains unclear which excitatory subpopulations within ACtx carry out this transformation from noise-dependent to noise-invariant coding. To address this, we presented pure tones with and without continuous BN to head-fixed mice and used two-photon calcium imaging to record sound-evoked activity from three major excitatory subpopulations in ACtx: layer (L)2/3 intratelencephalic (IT) neurons, L5 IT neurons, and L5 extratelencephalic (ET) neurons. L2/3 IT neurons exhibited strong noise dependence at the level of single-neuron responses, pairwise interactions, and population representations. In contrast, deep-layer pathways showed greater noise invariance, with L5 IT neurons preserving stable representations most consistently and L5 ET neurons exhibiting more limited invariance at the population level. These findings reveal a functional division of labor in ACtx, in which superficial neurons remain noise-dependent and deep-layer broadcast pathways, particularly L5 IT, preferentially carry noise-invariant representations, suggesting that excitatory subpopulations contribute differentially to the construction and propagation of noise-invariant codes.

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bioRxiv2025

A cortical output channel for perceptual categorization

Perceptual categorization allows the brain to transform diverse sensory inputs into discrete representations that support flexible behavior [1-7]. Auditory cortex (ACtx) has been implicated in this process [8-14], but the cell-type-specific circuits that implement category learning remain unknown. We trained head-fixed mice to categorize the temporal rate of amplitude-modulated noise while performing longitudinal two-photon imaging of layer (L)5 extratelencephalic (L5 ET) neurons alongside comparison populations of L2/3 and L5 intratelencephalic (L5 IT) neurons. With learning, L5 ET neurons underwent pronounced tuning modifications and developed robust, categorical responses, whereas L2/3 and L5 IT neurons did not. This categorical code was task engagement-dependent: it was present during behavior and absent during passive listening in the same neurons on the same day, indicating context-gated expression. Using a generalized linear model to dissociate stimulusfrom choice-related signals, we confirmed that categorical selectivity in L5 ET neurons reflected sensory encoding rather than motor confounds. All three populations carried choice signals, but these were strongest in L5 ET neurons, suggesting a role in linking sensory categorization to action selection. These findings identify a projection-specific, deep-layer cortical output channel in which L5 ET neurons acquire categorical representations and selectively propagate behaviorally relevant signals to downstream targets.

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bioRxiv2025

Cell-type-specific plasticity in synaptic, intrinsic, and sound response properties of deep-layer auditory cortical neurons after noise trauma

Peripheral trauma, such as noise-induced hearing loss (NIHL), triggers compensatory plasticity in the auditory cortex (ACtx) to maintain auditory function. While cortical plasticity in superficial cortical layers has been relatively well studied, the plasticity mechanisms governing deep-layer excitatory projection neurons remain less understood. Here, we investigated the plasticity of layer (L)5 extratelencephalic (ETs) and L6 corticothalamic neurons (CTs) following NIHL. Using a combination of in vitro slice electrophysiology, optogenetics, and in vivo two-photon imaging in a mouse model of NIHL, we characterized changes in evoked thalamocortical (TC) synaptic input strength, intrinsic excitability, and sound response properties. We found that TC input was initially equivalent between ETs and CTs, then shifted to CT-dominant one day after noise exposure. This shift renormalized to equivalent seven days after noise exposure and was associated with a transient increase in both the quantal size (q) in TC[->]CT synapses and intrinsic CT suprathreshold excitability. ETs maintained stable intrinsic properties and showed minor changes in their TC input. In vivo imaging revealed that CTs displayed a persistent elevation in sound intensity thresholds, whereas ETs transiently shifted their best frequency representation and reduced their responsiveness to high-frequency tones one day after NIHL, followed by recovery at seven days. Together, our findings highlight cell-type-specific plasticity mechanisms in deep-layer cortical neurons, enhance our understanding of cortical adaptation to peripheral damage, and highlight targets for developing therapeutic strategies to mitigate hearing loss and related disorders such as tinnitus and hyperacusis. Short AbstractPeripheral damage drives auditory cortex (ACtx) plasticity, but the underlying synaptic and cellular mechanisms remain poorly understood. We used a combination of in vitro slice electrophysiology, optogenetics, and in vivo two-photon imaging to investigate layer 5 extratelencephalic (ET) and layer 6 corticothalamic (CT) neuronal plasticity in mice, following noise-induced hearing loss (NIHL). Thalamocortical (TC) input was initially balanced between CTs and ETs but shifted to CT-dominant one day post-NIHL and then normalized by day seven. This transient shift was accompanied by increased quantal size and suprathreshold excitability in CTs, with minimal changes in ETs. In vivo, CTs exhibited persistent elevation in sound intensity thresholds, while ETs showed a transient shift in frequency tuning and reduced high-frequency responsiveness that recovered within a week. These findings reveal distinct, cell-type-specific plasticity mechanisms in deep-layer ACtx neurons following peripheral damage and highlight potential targets for treating hearing loss-related disorders such as tinnitus and hyperacusis.

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bioRxiv2025

Pupil-linked arousal differentially modulates cell-type-specific sensory processing

Arousal is a ubiquitous influence on the brain that fluctuates during wakefulness and modulates sensation and perception. These fluctuations impact membrane potential, cortical state, and sensory encoding, yet prior studies report inconsistent effects, likely due to averaging across heterogeneous cell types. To resolve this, we combined two-photon calcium imaging and pupillometry in awake mice to examine arousal-related activity in excitatory subpopulations of the auditory cortex: intratelencephalic (IT), extratelencephalic (ET), and corticothalamic (CT) neurons. Pupil-linked arousal modulated all cell types through diverse linear and non-linear response motifs. ET neurons showed significant multiplicative and additive gain modulation, with enhanced response magnitude and encoding but reduced frequency selectivity. CT and L2/3 neurons exhibited inverted-U relationships between arousal and both response strength and decoding accuracy, while IT neurons were minimally affected. These patterns closely tracked changes in population-level reliability, revealing a mechanistic link between internal state and the stability of sensory representations.

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bioRxiv2025

Neurons in the inferior colliculus use multiplexing to encode features of frequency-modulated sweeps

Within the central auditory pathway, the inferior colliculus (IC) is a critical integration center for ascending sound information. While IC neurons have well-characterized receptive fields for individual sound features such as sound frequency, intensity, and location, growing evidence suggests that some neurons also use multiplexing to encode sound feature combinations. Here, we performed in vivo juxtacellular recordings in awake, head-fixed mice to examine how individual IC neurons and neuronal populations encode the speed, direction, and frequency range of frequency-modulated sweeps. To understand the strategies used by neurons to represent different sound features, we trained a support vector machine to decode sound features from different parameters of the spike train, including the firing rate, spike times relative to stimulus onset, distribution of inter-spike intervals, and first spike latency. We found that many IC neurons multiplex features of frequency-modulated (FM) sweeps using distinct temporal coding strategies rather than simple changes in mean firing rate, and that these feature representations are interdependent, yielding a combinatorial encoding of sound features within individual neurons. Accordingly, using static receptive fields for sweep frequency or direction alone yielded poor predictions of neuron responses to vocalizations that contain simple frequency changes. Lastly, we showed that encoding strategies varied across individual neurons, resulting in a highly informative population code for FM sweep parameters. Together, our results suggest that multiplexing is a common mechanism used by IC neurons to represent complex sound features.

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bioRxiv2024

Primary auditory cortex is necessary for the acquisition and expression of categorical behavior

The primary auditory cortex (ACtx) is critically involved in the association of sensory information with specific behavioral outcomes. Such sensory-guided behaviors are necessarily brain-wide endeavors, requiring a plethora of distinct brain areas, including those that are involved in aspects of decision making, motor planning, motor initiation, and reward prediction. ACtx comprises a number of distinct excitatory cell-types that allow for the brain-wide propagation of behaviorally-relevant sensory information. Exactly how ACtx involvement changes as a function of learning, as well as the functional role of distinct excitatory cell-types is unclear. Here, we addressed these questions by designing a two-choice auditory task in which water-restricted, head-fixed mice were trained to categorize the temporal rate of a sinusoidal amplitude modulated (sAM) noise burst and used transient cell-type specific optogenetics to probe ACtx necessity across the duration of learning. Our data demonstrate that ACtx is necessary for the ability to categorize the rate of sAM noise, and this necessity remains stable across learning. ACtx silencing substantially altered the behavioral strategies used to solve the task by introducing a fluctuating choice bias and increasing dependence on prior decisions. Furthermore, ACtx silencing did not impact the animals motor report, suggesting that ACtx is necessary for the conversion of sensation to action. Targeted inhibition of both intratelencephalic and extratelencephalic projections on just 20% of trials had a modest effect on task performance, but significantly degraded learning. Taken together, our data shows that ACtx plays a critical role in both the acquisition and expression of categorical behavior, and that distinct excitatory projections play important roles in learning and plasticity.

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bioRxiv2023

Symmetry in frontal but not motor and somatosensory corticocortical and corticostriatal circuitry

Neocortex and striatum are topographically organized for sensory and motor functions. While sensory and motor areas are lateralized for touch and motor control, respectively, frontal areas are involved in decision making, where lateralization of function may be less important. This study contrasted the topographic precision of cell type-specific ipsilateral and contralateral cortical projections while varying the injection site location in transgenic mice of both sexes. While sensory cortical areas had strongly topographic outputs to ipsilateral cortex and striatum, they were weaker and not as topographically precise to contralateral targets. Motor cortex had somewhat stronger projections, but still relatively weak contralateral topography. In contrast, frontal cortical areas had high degrees of topographic similarity for both ipsilateral and contralateral projections to cortex and striatum. Corticothalamic organization is mainly ipsilateral, with weaker, more medial contralateral projections. Corticostriatal computations might integrate input outside closed basal ganglia loops using contralateral projections, enabling the two hemispheres to act as a unit to converge on one result in motor planning and decision making. Significance StatementEach cerebral hemisphere is responsible for sensation and movement of the opposite side of the body. Many axonal projections cross the midline to target contralateral areas. Crossed corticocortical, corticostriatal, and corticothalamic projections originate from much of neocortex, but how these projections vary across cortical regions and cell types is unknown. We quantify differences in the strength and targeting of ipsilateral and contralateral projections from frontal, motor, and somatosensory areas. The contralateral corticocortical and corticostriatal projections are proposed to play a larger role in frontal areas than in sensory or motor ones as a circuit basis for unifying computation across hemispheres in motor planning, while contralateral connectivity plays a smaller role in sensory and motor processing.

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bioRxiv2020

Motor corollary discharge activates layer six circuits in the auditory cortex

Optogenetic activation of Ntsr1+ layer 6 corticothalamic (L6 CT) neurons modulates thalamocortical sensory processing and perception for hundreds of milliseconds following laser offset. Naturally occurring sources of extrasensory inputs that could recruit L6 CTs prior to upcoming sensory stimuli have not been identified. Here, we found that 100% of L6 CTs in mouse primary auditory cortex (A1) expressed FoxP2, a protein marker found in brain areas that coordinate sensory inputs with movement. To test the idea that motor preparatory inputs could be a natural extrasensory activator of L6 CTs, we combined quantitative videography, optogenetically targeted single unit recordings, and two-photon imaging during self-initiated behavior. We found that A1 L6 CTs were activated hundreds of milliseconds prior to orofacial movements, but not whole-body movements associated with locomotion. These findings identify new local circuit arrangements for routing motor corollary discharge into A1 and suggest new roles for CT neurons in active sensing.

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bioRxiv2019

Cellular and widefield imaging of sound frequency organization in primary and higher-order fields of the mouse auditory cortex

The mouse auditory cortex (ACtx) contains two core fields - A1 and AAF - arranged in a mirror reversal tonotopic gradient. The best frequency (BF) organization and naming scheme for additional higher-order fields remain a matter of debate, as does the correspondence between smoothly varying global tonotopy and heterogeneity in local cellular tuning. Here, we performed chronic widefield and 2-photon calcium imaging from the ACtx of awake Thy1-GCaMP6s reporter mice. Data-driven parcellation of widefield maps identified five fields, including a previously unidentified area at the ventral posterior extreme of the ACtx (VPAF) and a tonotopically organized suprarhinal auditory field (SRAF) that extended laterally as far as ectorhinal cortex. Widefield maps were stable over time, where single pixel BFs fluctuated by less than 0.5 octaves during a one-month imaging period. After accounting for neuropil signal and frequency tuning strength, BF organization in neighboring layer 2/3 neurons was intermediate to the heterogeneous salt and pepper organization and the highly precise local organization that have each been described in prior studies. Multiscale imaging data suggest there is no ultrasonic field or secondary auditory cortex in the mouse. Instead, VPAF and a dorsoposterior field (DP) emerged as the strongest candidates for higher-order auditory areas.

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bioRxiv2018

Optimizing optogenetic stimulation protocols in auditory corticofugal neurons based on closed-loop spike feedback

Optogenetics provides a means to probe functional connections between brain areas. By activating a set of presynaptic neurons and recording the activity from a downstream brain area, one can establish the sign and strength of a feedforward connection. One challenge is that there are virtually limitless patterns that can be used to stimulate a presynaptic brain area. Functional influences on downstream brain areas can depend not just on whether presynaptic neurons were activated, but how they were activated. Corticofugal axons from the auditory cortex (ACtx) heavily innervate the auditory tectum, the inferior colliculus (IC). Despite the anatomical weight of this connection, optogenetic activation of ACtx neurons produced only modest changes in the IC neuron firing rates. To determine whether different modes of cortical activation could more faithfully reveal the strength of feedforward connectivity, we employed a closed-loop evolutionary optimization procedure that tailored voltage command signals to the laser based on firing rate variations recorded from single units in the IC of awake male and female mice. Within minutes, the evolutionary search procedure converged on ACtx stimulation configurations that produced more effective and widespread enhancement of IC unit activity than generic activation parameters. Cortical modulation of midbrain spiking was bi-directional, as the evolutionary search procedure could be programmed to converge on activation patterns that suppressed or enhanced sound-evoked IC firing rate. These findings demonstrate that the feedforward influence between brain areas can vary both in sign and degree depending on how presynaptic neurons are activated in time.\n\nSignificance StatementNeurons in deep layers of the auditory cortex (ACtx) make extensive projections to subcortical auditory areas, yet little is known about how these descending projections modulate subcortical sound processing in real time. Here, we leveraged recent advances in multi-channel electrophysiology and optogenetics to record from multiple regions of the inferior colliculus (IC) while optogenetically stimulating cortical neurons expressing Chronos, an ultra-fast channelrhodopsin. To identify ACtx activation patterns associated with the strongest effects on IC firing rates, we applied a machine learning algorithm that utilized the firing rate of single IC neurons to iteratively tailor the voltage command signal sent to the laser. We show that the temporal patterning of ACtx spiking strongly impacts the cortical influence on midbrain sound processing.

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bioRxiv2018

Parallel systems for sound processing and functional connectivity among layer 5 and 6 auditory corticothalamic neurons

Cortical layers (L) 5 and 6 are populated by a spatially intermingled menagerie of neurons with distinct inputs and downstream targets. Here, we made optogenetically guided recordings from L5 and L6 corticothalamic (CT) neurons in the mouse auditory cortex to discern underlying patterns of functional connectivity and sensory processing in the largest sub-cerebral projection system. Whereas L5 CT neurons showed broad stimulus selectivity with sluggish response latencies and extended temporal non-linearities, L6 CTs exhibited sparse sound feature selectivity and rapid temporal processing. L5 CT spikes lagged behind neighboring units and imposed weak feedforward excitation within the local column. By contrast, L6 CT spikes drove robust and sustained activity in neighboring units. Our findings underscore a duality among CT projection neurons, where L5 CT units are canonical broadcast neurons that integrate sensory inputs for transmission to distributed downstream targets, while L6 CT neurons are positioned to regulate thalamocortical response gain and selectivity.

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bioRxiv2017

Homeostatic normalization of sensory gain in auditory corticofugal feedback neurons

Layer 5 (L5) cortical projection neurons innervate far-ranging brain areas to coordinate integrative sensory processing and adaptive behaviors. Here, we characterize a compensatory plasticity in L5 auditory cortex (ACtx) projection neurons with axons that innervate the inferior colliculus (IC), thalamus, lateral amygdala and striatum. We used widefield calcium imaging to monitor daily changes in sound processing from the dense plexus of corticocollicular (CCol) axon terminals in awake adult mice. CCol sound level growth functions were stable in control conditions but showed bi-phasic gain changes following damage to cochlear afferent synapses. Auditory nerve and CCol growth functions were sharply reduced hours after cochlear synaptopathy, but CCol response gain rebounded above baseline levels by the following day and remained elevated for 2 weeks despite a persistent reduction in auditory nerve input. Sustained potentiation of excitatory ACtx projection neurons that innervate multiple limbic and subcortical auditory centers may underlie hyperexcitability and aberrant functional coupling of distributed brain networks in tinnitus.

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