Single Unit publications

Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation technique that is widely used to stimulate the sensorimotor cortex, and yet the mechanism by which it influences the natural activity of cortical networks is still under debate. Here, we characterize the effects of anodal and cathodal tDCS on underlying neurons in active macaque sensorimotor cortex across a range of doses. We find changes in spike rates that are sensitive to both current intensity and polarity, behavioral state, and that are cell-type specific. At high currents, effects persist after the offset of stimulation, and the spatiotemporal activity associated with motor activity of the contralateral limb, measured by dynamics of neural ensembles, are altered. These data suggest that tDCS induces reproducible and noticeable changes in cortical neuron activity and support the theory that it affects brain activity through a combination of single neuron polarization and network interactions.

In rodents, the progression of extrastriate areas located laterally to primary visual cortex (V1) has been assigned to a putative object-processing pathway (homologous to the primate ventral stream), based on anatomical considerations. Recently, we found functional support for such attribution (Tafazoli et al., 2017), by showing that this cortical progression is specialized for coding object identity despite view changes, the hallmark property of a ventral-like pathway. Here, we sought to clarify what computations are at the base of such specialization. To this aim, we performed multielectrode recordings from V1 and laterolateral area LL (at the apex of the putative ventral-like hierarchy) of male adult rats, during the presentation of drifting gratings and noise movies. We found that the extent to which neuronal responses were entrained to the phase of the gratings sharply dropped from V1 to LL, along with the quality of the receptive fields inferred through reverse correlation. Concomitantly, the tendency of neurons to respond to different oriented gratings increased, whereas the sharpness of orientation tuning declined. Critically, these trends are consistent with the nonlinear summation of visual inputs that is expected to take place along the ventral stream, according to the predictions of hierarchical models of ventral computations and a meta-analysis of the monkey literature. This suggests an intriguing homology between the mechanisms responsible for building up shape selectivity and transformation tolerance in the visual cortex of primates and rodents, reasserting the potential of the latter as models to investigate ventral stream functions at the circuitry level.SIGNIFICANCE STATEMENT Despite the growing popularity of rodents as models of visual functions, it remains unclear whether their visual cortex contains specialized modules for processing shape information. To addresses this question, we compared how neuronal tuning evolves from rat primary visual cortex (V1) to a downstream visual cortical region (area LL) that previous work has implicated in shape processing. In our experiments, LL neurons displayed a stronger tendency to respond to drifting gratings with different orientations while maintaining a sustained response across the whole duration of the drift cycle. These trends match the increased complexity of pattern selectivity and the augmented tolerance to stimulus translation found in monkey visual temporal cortex, thus revealing a homology between shape processing in rodents and primates.

Mesoscale cortical activity can be defined as the organization of activity of large neuron populations into collective action, forming time-dependent patterns such as traveling waves. Although collective action may play an important role in the cross-scale integration of brain activity and in the emergence of cognitive behavior, a comprehensive formulation of the laws governing its dynamics is still lacking. Because collective action processes are macroscopic with respect to neuronal activity, these processes cannot be described directly with methods and models developed for the microscale (individual neurons).To identify the characteristic features of mesoscopic dynamics, and to lay the foundations for a theoretical description of mesoscopic activity in the hippocampus, we conduct a comprehensive examination of observational data of hippocampal local field potential (LFP) recordings. We use the strong correlation between rat running-speed and the LFP power to parameterize the energy input into the hippocampus, and show that both the power and non-linearity of collective action (e.g., theta and gamma rhythms) increase with increased speed. Our results show that collective-action dynamics are stochastic (the precise state of a single neuron is irrelevant), weakly non-linear, and weakly dissipative. These are the principles of the theory of weak turbulence. Therefore, we propose weak turbulence a theoretical framework for the description of mesoscopic activity in the hippocampus. The weak turbulence framework provides a complete description of the cross-scale energy exchange (the energy cascade). It uncovers the mechanism governing major features of LFP spectra and bispectra, such as the physical meaning of the exponent α of power-law LFP spectra (e.g., f -α, where f is the frequency), the strengthening of theta-gamma coupling with energy input into the hippocampus, as well as specific phase lags associated with their interaction. Remarkably, the weak turbulence framework is consistent with the theory of self organized criticality, which provides a simple explanation for the existence of the power-law background spectrum. Together with self-organized criticality, weak turbulence could provide a unifying approach to modeling the dynamics of mesoscopic activity.

The caudomedial nidopallium (NCM) is a higher level region of auditory cortex in songbirds that has been implicated in encoding learned vocalizations and mediating perception of complex sounds. We made cell-attached recordings in awake adult male zebra finches ( Taeniopygia guttata) to characterize responses of single NCM neurons to playback of tones and songs. Neurons fell into two broad classes: narrow fast-spiking cells and broad sparsely firing cells. Virtually all narrow-spiking cells responded to playback of pure tones, compared with approximately half of broad-spiking cells. In addition, narrow-spiking cells tended to have lower thresholds and faster, less variable spike onset latencies than did broad-spiking cells, as well as higher firing rates. Tonal responses of narrow-spiking cells also showed broader ranges for both frequency and amplitude compared with broad-spiking neurons and were more apt to have V-shaped tuning curves compared with broad-spiking neurons, which tended to have complex (discontinuous), columnar, or O-shaped frequency response areas. In response to playback of conspecific songs, narrow-spiking neurons showed high firing rates and low levels of selectivity whereas broad-spiking neurons responded sparsely and selectively. Broad-spiking neurons in which tones failed to evoke a response showed greater song selectivity compared with those with a clear tuning curve. These results are consistent with the idea that narrow-spiking neurons represent putative fast-spiking interneurons, which may provide a source of intrinsic inhibition that contributes to the more selective tuning in broad-spiking cells. NEW & NOTEWORTHY The response properties of neurons in higher level regions of auditory cortex in songbirds are of fundamental interest because processing in such regions is essential for vocal learning and plasticity and for auditory perception of complex sounds. Within a region of secondary auditory cortex, neurons with narrow spikes exhibited high firing rates to playback of both tones and multiple conspecific songs, whereas broad-spiking neurons responded sparsely and selectively to both tones and songs.

Hearing impairment in older people is thought to arise from impaired temporal processing in auditory circuits. We used a systems-level (scalp recordings) and a microcircuit-level (extracellular recordings) approach to investigate how aging affects the sensitivity to temporal envelopes of speech-like sounds in rats. Scalp-recorded potentials suggest an age-related increase in sensitivity to temporal regularity along the ascending auditory pathway. The underlying cellular changes in the midbrain were examined using extracellular recordings from inferior colliculus neurons. We observed an age-related increase in sensitivity to the sound's onset and temporal regularity (i.e., periodicity envelope) in the spiking output of inferior colliculus neurons, relative to their synaptic inputs (local field potentials). This relative enhancement for aged animals was most prominent for multi-unit (relative to single-unit) spiking activity. Spontaneous multi-unit, but not single-unit, activity was also enhanced in aged compared with young animals. Our results suggest that aging is associated with altered sensitivity to a sound's temporal regularities, and that these effects may be due to increased gain of neural network activity in the midbrain.

A 'pattern alternation paradigm' has been previously used in human ERP recordings to investigate the brain encoding of complex auditory regularities, but prior studies on regularity encoding in animal models to examine mechanisms of adaptation of auditory neuronal responses have used primarily oddball stimulus sequences to study stimulus-specific adaptation alone. In order to examine the sensitivity of neuronal adaptation to expected and unexpected events embedded in a complex sound sequence, we used a similar patterned sequence of sounds. We recorded single unit activity and compared neuronal responses in the rat inferior colliculus (IC) to sound stimuli conforming to pattern alternation regularity with those to stimuli in which occasional sound repetitions violated that alternation. Results show that some neurons in the rat inferior colliculus are sensitive to the history of patterned stimulation and to violations of patterned regularity, demonstrating that there is a population of subcortical neurons, located as early as the level of the midbrain, that can detect more complex stimulus regularities than previously supposed and that are as sensitive to complex statistics as some neurons in primary auditory cortex. Our findings indicate that these pattern-sensitive neurons can extract temporal and spectral regularities between successive acoustic stimuli. This is important because the extraction of regularities from the sound sequences will result in the development of expectancies for future sounds and hence, the present results are compatible with predictive coding models. Our results demonstrate that some collicular neurons, located as early as in the midbrain level, are involved in the generation and shaping of prediction errors in ways not previously considered and thus, the present findings challenge the prevailing view that perceptual organization of sound only emerges at the auditory cortex level.

Before primary motor cortex (M1) develops its motor functions, it functions like a somatosensory area. Here, by recording from neurons in the forelimb representation of M1 in postnatal day (P) 8–12 rats, we demonstrate a rapid shift in its sensory responses. At P8-10, M1 neurons respond overwhelmingly to feedback from sleep-related twitches of the forelimb, but the same neurons do not respond to wake-related movements. By P12, M1 neurons suddenly respond to wake movements, a transition that results from opening the sensory gate in the external cuneate nucleus. Also at P12, fewer M1 neurons respond to individual twitches, but the full complement of twitch-related feedback observed at P8 is unmasked through local disinhibition. Finally, through P12, M1 sensory responses originate in the deep thalamorecipient layers, not primary somatosensory cortex. These findings demonstrate that M1 initially establishes a sensory framework upon which its later-emerging role in motor control is built.

Cerebellar plasticity underlies motor learning. However, how the cerebellum operates to enable learned changes in motor output is largely unknown. We developed a sensory-driven adaptation protocol for reflexive whisker protraction and recorded Purkinje cell activity from crus 1 and 2 of awake mice. Before training, simple spikes of individual Purkinje cells correlated during reflexive protraction with the whisker position without lead or lag. After training, simple spikes and whisker protractions were both enhanced with the spiking activity now leading behavioral responses. Neuronal and behavioral changes did not occur in two cell-specific mouse models with impaired long-term potentiation at their parallel fiber to Purkinje cell synapses. Consistent with cerebellar plasticity rules, increased simple spike activity was prominent in cells with low complex spike response probability. Thus, potentiation at parallel fiber to Purkinje cell synapses may contribute to reflex adaptation and enable expression of cerebellar learning through increases in simple spike activity.

Figure-ground segregation is the process by which the visual system identifies image elements of figures and segregates them from the background. Previous studies examined figure-ground segregation in the visual cortex of monkeys where figures elicit stronger neuronal responses than backgrounds. It was demonstrated in anesthetized mice that neurons in the primary visual cortex (V1) of mice are sensitive to orientation contrast, but it is unknown whether mice can perceptually segregate figures from a background. Here, we examined figure-ground perception of mice and found that mice can detect figures defined by an orientation that differs from the background while the figure size, position or phase varied. Electrophysiological recordings in V1 of awake mice revealed that the responses elicited by figures were stronger than those elicited by the background and even stronger at the edge between figure and background. A figural response could even be evoked in the absence of a stimulus in the V1 receptive field. Current-source-density analysis suggested that the extra activity was caused by synaptic inputs into layer 2/3. We conclude that the neuronal mechanisms of figure-ground segregation in mice are similar to those in primates, enabling investigation with the powerful techniques for circuit analysis now available in mice.

Nitric oxide (NO) is a neurotransmitter synthesized in the brain by neuronal nitric oxide synthase (nNOS). Using immunohistochemistry and confocal imaging in the inferior colliculus (IC, auditory midbrain) of the guinea pig ( Cavia porcellus , male and female), we show that nNOS occurs in two distinct cellular distributions. We confirm that, in the cortices of the IC, a subset of neurons show cytoplasmic labeling for nNOS, whereas in the central nucleus (IC c ), such neurons are not present. However, we demonstrate that all neurons in the IC c do in fact express nNOS in the form of discrete puncta found at the cell membrane. Our multi-labeling studies reveal that nNOS puncta form multiprotein complexes with NMDA receptors, soluble guanylyl cyclase (sGC), and PSD95. These complexes are found apposed to glutamatergic terminals, which is indicative of synaptic function. Interestingly, these glutamatergic terminals express both vesicular glutamate transporters 1 and 2 denoting a specific source of brainstem inputs. With in vivo electrophysiological recordings of multiunit activity in the IC c , we found that local application of NMDA enhances sound-driven activity in a concentration-dependent and reversible fashion. This response is abolished by blockade of nNOS or sGC, indicating that the NMDA effect is mediated solely via the NO and cGMP signaling pathway. This discovery of a ubiquitous, but highly localized, expression of nNOS throughout the IC c and demonstration of the dramatic influence of the NMDA activated NO pathway on sound-driven neuronal activity imply a key role for NO signaling in auditory processing. SIGNIFICANCE STATEMENT We show that neuronal nitric oxide synthase (nNOS), the enzyme that synthesizes nitric oxide (NO), occurs as puncta in apparently all neurons in the central nucleus of the inferior colliculus (IC c ) in the auditory midbrain. Punctate nNOS appears at glutamatergic synapses in a complex with glutamate NMDA receptors (NMDA-Rs), soluble guanylyl cyclase (sGC, the NO receptor), and PSD95 (a protein that anchors receptors and enzymes at the postsynaptic density). We show that NMDA-R modulation of sound-driven activity in the IC c is solely mediated by activation of nNOS and sGC. The presence of nNOS throughout this sensory nucleus argues for a major role of NO in hearing. Furthermore, this punctate form of nNOS expression may exist and have gone unnoticed in other brain regions.

In week-old rats, somatosensory input arises predominantly from external stimuli or from sensory feedback (reafference) associated with myoclonic twitches during active sleep. A previous study suggested that the brainstem motor structures that produce twitches also send motor copies (or corollary discharge, CD) to the cerebellum. We tested this possibility by recording from two precerebellar nuclei—the inferior olive (IO) and lateral reticular nucleus (LRN). In most IO and LRN neurons, twitch-related activity peaked sharply around twitch onset, consistent with CD. Next, we identified twitch-production areas in the midbrain that project independently to the IO and LRN. Finally, we blocked calcium-activated slow potassium (SK) channels in the IO to explain how broadly tuned brainstem motor signals can be transformed into precise CD signals. We conclude that the precerebellar nuclei convey a diversity of sleep-related neural activity to the developing cerebellum to enable processing of convergent input from CD and reafferent signals.

Tinnitus and hyperacusis are life-disrupting perceptual abnormalities that are often preceded by acoustic overexposure. Animal models of overexposure have suggested a link between these phenomena and neural hyperactivity, i.e., elevated spontaneous rates (SRs) and sound-evoked responses. Prior work has focused on changes in central auditory responses, with less attention paid to the exact nature of the associated cochlear damage. The demonstration that acoustic overexposure can cause cochlear neuropathy without permanent threshold elevation suggests cochlear neuropathy per se may be a key elicitor of neural hyperactivity. We addressed this hypothesis by recording responses in the mouse inferior colliculus (IC) following a bilateral, neuropathic noise exposure. One to three weeks post-exposure, mean SRs were unchanged in mice recorded while awake, or under anesthesia. SRs were also unaffected by more intense, or unilateral exposures. These results suggest that neither neuropathy nor hair cell loss are sufficient to raise SRs in the IC, at least in 7-week-old mice, 1-3 weeks post exposure. However, it is not clear whether our mice had tinnitus. Tone-evoked rate-level functions at the CF were steeper following exposure, specifically in the region of maximal neuropathy. Furthermore, suppression driven by off-CF tones and by ipsilateral noise were reduced. Both changes were especially pronounced in neurons of awake mice. This neural hypersensitivity may manifest as behavioral hypersensitivity to sound - prior work reports that this same exposure causes elevated acoustic startle. Together, these results indicate that neuropathy may initiate a compensatory response in the central auditory system leading to the genesis of hyperacusis.

Under the predictive coding hypothesis, specific spatiotemporal patterns of cortical activation are postulated to occur during sensory processing as expectations generate feedback predictions and prediction errors generate feedforward signals. Establishing experimental evidence for this information flow within cortical hierarchy has been difficult, especially in humans, due to spatial and temporal limitations of non-invasive measures of cortical activity. This study investigated cortical responses to auditory novelty using the local/global deviant paradigm, which engages the hierarchical network underlying auditory predictive coding over short ('local deviance'; LD) and long ('global deviance'; GD) time scales. Electrocorticographic responses to auditory stimuli were obtained in neurosurgical patients from regions of interest (ROIs) including auditory, auditory-related and prefrontal cortex. LD and GD effects were assayed in averaged evoked potential (AEP) and high gamma (70-150 Hz) signals, the former likely dominated by local synaptic currents and the latter largely reflecting local spiking activity. AEP LD effects were distributed across all ROIs, with greatest percentage of significant sites in core and non-core auditory cortex. High gamma LD effects were localized primarily to auditory cortex in the superior temporal plane and on the lateral surface of the superior temporal gyrus (STG). LD effects exhibited progressively longer latencies in core, non-core, auditory-related and prefrontal cortices, consistent with feedforward signaling. The spatial distribution of AEP GD effects overlapped that of LD effects, but high gamma GD effects were more restricted to non-core areas. High gamma GD effects had shortest latencies in STG and preceded AEP GD effects in most ROIs. This latency profile, along with the paucity of high gamma GD effects in the superior temporal plane, suggest that the STG plays a prominent role in initiating novelty detection signals over long time scales. Thus, the data demonstrate distinct patterns of information flow in human cortex associated with auditory novelty detection over multiple time scales.

Multiple parallel neural pathways link sound-related signals to behavioral responses. For instance, the striatum, a brain structure involved in action selection and reward-related learning, receives neuronal projections from both the auditory thalamus and auditory cortex. It is not clear whether sound information that reaches the striatum through these two pathways is redundant or complementary. We used an optogenetic approach in awake mice of both sexes to identify thalamostriatal and corticostriatal neurons during extracellular recordings, and characterized neural responses evoked by sounds of different frequencies and amplitude modulation rates. We found that neurons in both pathways encode sound frequency with similar fidelity, but display different coding strategies for amplitude modulated noise. Whereas corticostriatal neurons provide a more accurate representation of amplitude modulation rate in their overall firing rate, thalamostriatal neurons convey information about the precise timing of acoustic events. These results demonstrate that auditory thalamus and auditory cortex neurons provide complementary information to the striatum, and suggest that these pathways could be differentially recruited depending on the requirements of a sound-driven behavior. SIGNIFICANCE STATEMENT Sensory signals from the cerebral cortex and the thalamus converge onto the striatum, a nucleus implicated in reward-related learning. It is not clear whether these two sensory inputs convey redundant or complementary information. By characterizing the sound-evoked responses of thalamostriatal and corticostriatal neurons, our work demonstrates that these neural pathways convey complementary information about the temporal features of sounds. This work opens new avenues for investigating how these pathways could be selectively recruited depending on task demands, and provides a framework for studying convergence of cortical and thalamic information onto the striatum in other sensory systems.

Interactions between the prefrontal cortex (PFC) and mediodorsal thalamus are critical for cognitive flexibility, yet the underlying computations are unknown. To investigate frontothalamic substrates of cognitive flexibility, we developed a behavioral task in which mice switched between different sets of learned cues that guided attention toward either visual or auditory targets. We found that PFC responses reflected both the individual cues and their meaning as task rules, indicating a hierarchical cue-to-rule transformation. Conversely, mediodorsal thalamus responses reflected the statistical regularity of cue presentation and were required for switching between such experimentally specified cueing contexts. A subset of these thalamic responses sustained context-relevant PFC representations, while another suppressed the context-irrelevant ones. Through modeling and experimental validation, we find that thalamic-mediated suppression may not only reduce PFC representational interference but could also preserve unused cortical traces for future use. Overall, our study provides a computational foundation for thalamic engagement in cognitive flexibility.

Perceptual constancy requires neural representations that are selective for object identity, but also tolerant across identity-preserving transformations. How such representations arise in the brain and support perception remains unclear. Here, we study tolerant representation of sound identity in the auditory system by recording neural activity in auditory cortex of ferrets during perceptual constancy. Ferrets generalize vowel identity across variations in fundamental frequency, sound level and location, while neurons represent sound identity robustly across acoustic variations. Stimulus features are encoded with distinct time-courses in all conditions, however encoding of sound identity is delayed when animals fail to generalize and during passive listening. Neurons also encode information about task-irrelevant sound features, as well as animals’ choices and accuracy, while population decoding out-performs animals’ behavior. Our results show that during perceptual constancy, sound identity is represented robustly in auditory cortex across widely varying conditions, and behavioral generalization requires conserved timing of identity information.

Mechanical, materials, and biological causes of intracortical probe failure have hampered their utility in basic science and clinical applications. By anticipating causes of failure, we can design a system that will prevent the known causes of failure. The neural probe design was centered around a bio-inspired, mechanically-softening polymer nanocomposite. The polymer nanocomposite was functionalized with recording microelectrodes using a microfabrication process designed for chemical and thermal process compatibility. A custom package based upon a ribbon cable, printed circuit board, and a 3D-printed housing was designed to enable connection to external electronics. Probes were implanted into the primary motor cortex of Sprague-Dawley rats for 16 weeks, during which regular recording and electrochemical impedance spectroscopy measurement sessions took place. The implanted mechanically-softening probes had stable electrochemical impedance spectra across the 16 weeks and single units were recorded out to 16 weeks. The demonstration of chronic neural recording with the mechanically-softening probe suggests that probe architecture, custom package, and general design strategy are appropriate for long-term studies in rodents.

Multisensory integration is a process by which signals from different sensory modalities are combined to facilitate detection and localization of external events. One substrate for multisensory integration is the midbrain superior colliculus (SC) which plays an important role in orienting behavior. In rodent SC, visual and somatosensory (whisker) representations are in approximate registration, but whether and how these signals interact is unclear. We measured spiking activity in SC of anesthetized hooded rats, during presentation of visual- and whisker stimuli that were tested simultaneously or in isolation. Visual responses were found in all layers, but were primarily located in superficial layers. Whisker responsive sites were primarily found in intermediate layers. In single- and multi-unit recording sites, spiking activity was usually only sensitive to one modality, when stimuli were presented in isolation. By contrast, we observed robust and primarily suppressive interactions when stimuli were presented simultaneously to both modalities. We conclude that while visual and whisker representations in SC of rat are partially overlapping, there is limited excitatory convergence onto individual sites. Multimodal integration may instead rely on suppressive interactions between modalities.