Diego Contreras

Dissolvable films of silk fibroin for ultrathin conformal bio-integrated electronics

Electronics that are capable of intimate, non-invasive integration with the soft, curvilinear surfaces of biological tissues offer important opportunities for diagnosing and treating disease and for improving brain/machine interfaces. This article describes a material strategy for a type of bio-interfaced system that relies on ultrathin electronics supported by bioresorbable substrates of silk fibroin. Mounting such devices on tissue and then allowing the silk to dissolve and resorb initiates a spontaneous, conformal wrapping process driven by capillary forces at the biotic/abiotic interface. Specialized mesh designs and ultrathin forms for the electronics ensure minimal stresses on the tissue and highly conformal coverage, even for complex curvilinear surfaces, as confirmed by experimental and theoretical studies. In vivo, neural mapping experiments on feline animal models illustrate one mode of use for this class of technology. These concepts provide new capabilities for implantable and surgical devices.

Flexible, foldable, actively multiplexed, high-density electrode array for mapping brain activity in vivo

Arrays of electrodes for recording and stimulating the brain are used throughout clinical medicine and basic neuroscience research, yet are unable to sample large areas of the brain while maintaining high spatial resolution because of the need to individually wire each passive sensor at the electrode-tissue interface. To overcome this constraint, we developed new devices that integrate ultrathin and flexible silicon nanomembrane transistors into the electrode array, enabling new dense arrays of thousands of amplified and multiplexed sensors that are connected using fewer wires. We used this system to record spatial properties of cat brain activity in vivo, including sleep spindles, single-trial visual evoked responses and electrographic seizures. We found that seizures may manifest as recurrent spiral waves that propagate in the neocortex. The developments reported here herald a new generation of diagnostic and therapeutic brain-machine interface devices.

Dynamics of excitation and inhibition underlying stimulus selectivity in rat somatosensory cortex.

Neurons in sensory systems respond to stimuli within their receptive fields, but the magnitude of the response depends on specific stimulus features. In the rodent whisker system, the response magnitude to the deflection of a particular whisker is, in most cells, dependent on the direction of deflection. Here we use in vivo intracellular recordings from thalamorecipient neurons in layers 3 and 4 of the rat barrel cortex to elucidate the dynamics of the synaptic inputs underlying direction selectivity. We show that cells are direction selective despite a broadly tuned excitatory and inhibitory synaptic input. Selectivity emerges from a direction-dependent temporal shift of excitation relative to inhibition. For preferred direction deflections, excitation precedes inhibition, but as the direction diverges from the preferred, this separation decreases. Our results illustrate a mechanism by which the timing of the synaptic inputs, and not their relative peak amplitudes, primarily determine feature selectivity.

Balanced Excitation and Inhibition Determine Spike Timing during Frequency Adaptation

In layer 4 (L4) of the rat barrel cortex, a single whisker deflection evokes a stereotyped sequence of excitation followed by inhibition, hypothesized to result in a narrow temporal window for spike output. However, awake rats sweep their whiskers across objects, activating the cortex at frequencies known to induce short-term depression at both excitatory and inhibitory synapses within L4. Although periodic whisker deflection causes a frequency-dependent reduction of the cortical response magnitude, whether this adaptation involves changes in the relative balance of excitation and inhibition and how these changes might impact the proposed narrow window of spike timing in L4 is unknown. Here, we demonstrate for the first time that spike output in L4 is determined precisely by the dynamic interaction of excitatory and inhibitory conductances. Furthermore, we show that periodic whisker deflection results in balanced adaptation of the magnitude and timing of excitatory and inhibitory input to L4 neurons. This balanced adaptation mediates a reduction in spike output while preserving the narrow time window of spike generation, suggesting that L4 circuits are calibrated to maintain relative levels of excitation and inhibition across varying magnitudes of input.

NMDA/AMPA Ratio Impacts State Transitions and Entrainment to Oscillations in a Computational Model of the Nucleus Accumbens Medium Spiny Projection Neuron

We describe a computational model of the principal cell in the nucleus accumbens (NAcb), the medium spiny projection (MSP) neuron. The model neuron, constructed in NEURON, includes all of the known ionic currents in these cells and receives synaptic input from simulated spike trains via NMDA, AMPA, and GABAA receptors. After tuning the model by adjusting maximal current conductances in each compartment, the model cell closely matched whole-cell recordings from an adult rat NAcb slice preparation. Synaptic inputs in the range of 1000-1300 Hz are required to maintain an “up” state in the model. Cell firing in the model required concurrent depolarization of several dendritic branches, which responded independently to afferent input. Depolarization from action potentials traveled to the tips of the dendritic branches and increased Ca2+ influx through voltage-gated Ca2+ channels. As NMDA/AMPA current ratios were increased, the membrane showed an increase in hysteresis of “up” and “down” state dwell times, but intrinsic bistability was not observed. The number of oscillatory inputs required to entrain the model cell was determined to be ∼20% of the “up” state inputs. Altering the NMDA/AMPA ratio had a profound effect on processing of afferent input, including the ability to entrain to oscillations in afferent input in the theta range (4-12 Hz). These results suggest that afferent information integration by the NAcb MSP cell may be compromised by pathology in which the NMDA current is altered or modulated, as has been proposed in both schizophrenia and addiction.

Stimulus Feature Selectivity in Excitatory and Inhibitory Neurons in Primary Visual Cortex

Although several lines of evidence suggest that stimulus selectivity in somatosensory and visual cortices is critically dependent on unselective inhibition, particularly in the thalamorecipient layer 4, no comprehensive comparison of the responses of excitatory and inhibitory cells has been conducted. Here, we recorded intracellularly from a large population of regular spiking (RS; presumed excitatory) and fast spiking (FS; presumed inhibitory) cells in layers 2–6 of primary visual cortex. In layer 4, where selectivity for orientation and spatial frequency first emerges, we found no untuned FS cells. Instead, the tuning of the spike output of layer 4 FS cells was significantly but moderately broader than that of RS cells. However, the tuning of the underlying synaptic responses was not different, indicating that the difference in spike-output selectivity resulted from differences in the transformation of synaptic input into firing rate. Layer 4 FS cells exhibited significantly lower input resistance and faster time constants than layer 4 RS cells, leading to larger and faster membrane potential (Vm) fluctuations. FS cell Vm fluctuations were more broadly tuned than those of RS cells and matched spike-output tuning, suggesting that the broader spike tuning of these cells was driven by visually evoked synaptic noise. These differences were not observed outside of layer 4. Thus, cell type-specific differences in stimulus feature selectivity at the first level of cortical sensory processing may arise as a result of distinct biophysical properties that determine the dynamics of synaptic integration.

Synaptic Responses to Whisker Deflections in Rat Barrel Cortex as a Function of Cortical Layer and Stimulus Intensity

To study the synaptic and spike responses of barrel cortex neurons as a function of cortical layer and stimulus intensity, we recorded intracellularly in vivo from barbiturate anesthetized rats while increasing the velocity-acceleration of the whisker deflection. Granular (Gr; layer 4) cells had the EPSP with the shortest peak and onset latency, whereas supragranular (SGr; layers 2-3) cells had the EPSP with longest duration and slowest rate of rise. Infragranular (Igr; layers 5-6) cells had intermediate values, and thus each layer was unique. The spike response peak of Gr cells was followed by IGr and then by SGr cells. In all cells, depolarization reduced the duration and amplitude of the response, but only in Gr cells did it reveal an early IPSP that cut short the EPSP. This early IPSP was associated with a large decrease in input resistance and an apparent reversal potential below spike threshold; consequently, synaptic integration in Gr cells was limited to the initial 5-7 msec of the response. In contrast, in SGr and IGr cells, results suggest an overlap in time of the EPSP and IPSP, with a small drop in input resistance and an apparent reversal potential above spike threshold, facilitating input integration for up to 20 msec. Decreasing stimulus intensity (velocity-acceleration) reduced the amplitude and increased the peak latency of the response without altering its synaptic composition. We propose that layer 4 circuits are better suited to perform coincidence detection, whereas supra and infragranular circuits are better designed for input integration.

Stimulus-dependent changes in spike threshold enhance feature selectivity in rat barrel cortex neurons.

Feature selectivity is a fundamental property of sensory cortex neurons, yet the mechanisms underlying its genesis are not fully understood. Using intracellular recordings in vivo from layers 2-6 of rat barrel cortex, we studied the selectivity of neurons to the angular direction of whisker deflection. The spike output and the underlying synaptic response decreased exponentially in magnitude as the direction of deflection diverged from the preferred. However, the spike output was more sharply tuned for direction than the underlying synaptic response amplitude. This difference in selectivity was attributable to the rectification imposed by the spike threshold on the input-output function of cells. As in the visual system, spike threshold was not constant and showed trial-to-trial variability. However, here we show that the mean spike threshold was direction dependent and increased as the direction diverged from the preferred. Spike threshold was also related to the rate of rise of the synaptic response, which was direction dependent and steepest for the preferred direction. To assess the impact of the direction-dependent changes in spike threshold on direction selectivity, we applied a fixed threshold to the synaptic responses and calculated a predicted spike output. The predicted output was more broadly tuned than the obtained spike response, demonstrating for the first time that the regulation of the spike threshold by the properties of the synaptic response effectively enhances the selectivity of the spike output.

Electrophysiological classes of neocortical neurons

Neocortical network behavior and neocortical function emerge from synaptic interactions among neurons with specific electrophysiological and morphological characteristics. The intrinsic electrophysiological properties of neurons define their firing patterns and their input-output functions with critical consequences for their functional properties within the network. Understanding the role played by the active non-linear properties caused by ionic conductances distributed in the soma and the dendrites is a critical step towards understanding cortical function. Here I present a brief description of electrophysiological and morphological characteristics of neocortical cells that allow their classification in categories. I review some examples of differences in functional properties among different electrophysiological cell classes in the visual cortex, as well as the role played by specific ionic conductances in defining firing and accommodation properties of neocortical neurons.

Ketamine Disrupts Theta Modulation of Gamma in a Computer Model of Hippocampus

Abnormalities in oscillations have been suggested to play a role in schizophrenia. We studied theta-modulated gamma oscillations in a computer model of hippocampal CA3 in vivo with and without simulated application of ketamine, an NMDA receptor antagonist and psychotomimetic. Networks of 1200 multicompartment neurons [pyramidal, basket, and oriens-lacunosum moleculare (OLM) cells] generated theta and gamma oscillations from intrinsic network dynamics: basket cells primarily generated gamma and amplified theta, while OLM cells strongly contributed to theta. Extrinsic medial septal inputs paced theta and amplified both theta and gamma oscillations. Exploration of NMDA receptor reduction across all location combinations demonstrated that the experimentally observed ketamine effect occurred only with isolated reduction of NMDA receptors on OLMs. In the ketamine simulations, lower OLM activity reduced theta power and disinhibited pyramidal cells, resulting in increased basket cell activation and gamma power. Our simulations predict the following: (1) ketamine increases firing rates; (2) oscillations can be generated by intrinsic hippocampal circuits; (3) medial-septum inputs pace and augment oscillations; (4) pyramidal cells lead basket cells at the gamma peak but lag at trough; (5) basket cells amplify theta rhythms; (6) ketamine alters oscillations due to primary blockade at OLM NMDA receptors; (7) ketamine alters phase relationships of cell firing; (8) ketamine reduces network responsivity to the environment; (9) ketamine effect could be reversed by providing a continuous inward current to OLM cells. We suggest that this last prediction has implications for a possible novel treatment for cognitive deficits of schizophrenia by targeting OLM cells.