Calin I. Buia

Quantum transport properties of ultrathin silver nanowires

The quantum transport properties of ultrathin silver nanowires are investigated. For a perfect crystalline nanowire with four atoms per unit cell, three conduction channels are found, corresponding to three s bands crossing the Fermi level. One conductance channel is disrupted by a single-atom defect, either adding or removing one atom. The quantum interference effect leads to oscillation of conductance versus the inter-defect distance. In the presence of a multiple-atom defect, one conduction channel remains robust at the Fermi level regardless the details of defect configuration. The histogram of conductance calculated for a finite nanowire (seven atoms per cross section) with a large number of random defect configurations agrees well with recent experiments.

Role of Interneuron Diversity in the Cortical Microcircuit for Attention

Receptive fields of neurons in cortical area V4 are large enough to fit multiple stimuli, making V4 the ideal place to study the effects of selective attention at the single-neuron level. Experiments have revealed evidence for stimulus competition and have characterized the effect thereon of spatial and feature-based attention. We developed a biophysical model with spiking neurons and conductance-based synapses. To account for the comprehensive set of experimental results, it was necessary to include in the model, in addition to regular spiking excitatory (E) cells, two types of interneurons: feedforward interneurons (FFI) and top-down interneurons (TDI). Feature-based attention was mediated by a projection of the TDI to the FFI, stimulus competition was mediated by a cross-columnar excitatory connection to the FFI, whereas spatial attention was mediated by an increase in activity of the feedforward inputs from cortical area V2. The model predicts that spatial attention increases the FFI firing rate, whereas feature-based attention decreases the FFI firing rate and increases the TDI firing rate. During strong stimulus competition, the E cells were synchronous in the beta frequency range (15-35 Hz), but with feature-based attention, they became synchronous in the gamma frequency range (35-50 Hz). We propose that the FFI correspond to fast-spiking, parvalbumin-positive basket cells and that the TDI correspond to cells with a double-bouquet morphology that are immunoreactive to calbindin or calretinin. Taken together, the model results provide an experimentally testable hypothesis for the behavior of two interneuron types under attentional modulation.

Quantum interference effects in electronic transport through nanotube contacts

Quantum interference has dramatic effects on electronic transport through nanotube contacts. In optimal configuration the intertube conductance can approach that of a perfect nanotube (4e 2 /h). The maximum conductance increases rapidly with the contact length up to 10 nm, beyond which it exhibits long-wavelength oscillations. This is attributed to the resonant interference phenomena in the contact region. For two concentric nanotubes symmetry breaking can reduce the maximum intertube conductance from 4e 2 /h to 2e 2 /h. The phenomena discussed here can serve as a foundation for building nanotube electronic circuits and high-speed nanoscale electromechanical devices.

Rapid temporal modulation of synchrony in cortical interneuron networks with synaptic plasticity

The synchrony of neurons in the extrastriate visual cortex is modulated by selective attention even when there are only small changes in firing rate (Fries et al., Science 291 (2001) 1560). We used Hodgkin-Huxley type models of cortical neurons to investigate the mechanism by which the degree of synchrony can be modulated independently of changes in firing rate. The synchrony of local networks of model cortical interneurons interacting through inhibitory synapses with short-term synaptic plasticity was modulated on a fast timescale by selectively activating a fraction of the interneurons. We found that facilitation enhanced the synchrony of the system whereas depression reduced it. In systems with synaptic plasticity, the synchrony modulation was accompanied by larger changes in the firing rate.

2009 Special Issue: Spatial attention in area V4 is mediated by circuits in primary visual cortex

The ability to covertly select visual stimuli in our environment based on their behavioral relevance is an important skill. Stimulus selection has been studied experimentally, at the single neuron as well as at the population level, by recording from the visual cortex of subjects performing attention-demanding tasks, but studies at the local circuit level are lacking. We conducted simulations of a primary visual cortex (V1) model to provide insight into the local circuit computation underlying stimulus selection in V4. Two small oriented rectangular bars were placed at different locations in the 4 by 4 degree visual field represented by the V1 model, such that they activated different V1 neurons but such that they were both inside the classical receptive field (CRF) of the same V4 neuron. The biased competition framework [Desimone, R., & Duncan, J. (1995). Neural mechanisms of selective visual attention. Annual Review of Neuroscience, 18, 193-222] makes predictions for the response of V4 neurons and the modulation thereof by spatial and feature attention. In our simulation of the V1 network, we obtained results consistent with these predictions for V4 when the model had long-range excitatory projections targeting inhibitory neurons and when spatial attention was mediated by a spatially restricted projection that either inhibited the inhibitory neurons or excited the excitatory neurons. Although it is not clear whether attention effects measured in V4 neurons are generated mostly by local circuits within V4, our simulations suggest that spatial attention at a resolution less than the size of the CRF of a V4 neuron is inherited from upstream areas like V1 and relies on circuits mediating surround suppression at the single neuron level. Furthermore, the model displayed global oscillations in the alpha frequency range (around 10 Hz), whose coherence was highest in the absence of visual stimulation, which is consistent with electroencephalograms recorded in humans. By contrast, when a stimulus was presented the alpha oscillation sped up and became less coherent, whereas at the single column level (40-480 cells) transient beta/gamma oscillations were observed with a frequency between 25 and 50 Hz.

Dependence of V2 illusory contour response on V1 cell properties and topographic organization

An illusory contour is an image that is perceived as a contour in the absence of typical contour characteristics, such as a change in luminance or chromaticity across the stimulus. In cats and primates, cells that respond to illusory contours are sparse in cortical area V1, but are found in greater numbers in cortical area V2. We propose a model capable of illusory contour detection that is based on a realistic topographic organization of V1 cells, which reproduces the responses of individual cell types measured experimentally. The model allows us to explain several experimentally observed properties of V2 cells including variability in orientation tuning and inducer spacing preference. As a practical application, the model can be used to estimate the relationship between the severity of a cortical injury in the primary visual cortex and the deterioration of V2 cell responses to real and illusory contours.

A biophysical model of frequency-sweep selectivity in primary auditory cortex

Cells in primary auditory cortex respond preferably to frequency sweeps of a certain rate and direction. In a model by Fishbach et al. [J. Neurophysiol. 90 (2003) 3663-3678], direction selective cells emerged through patterned thalamocortical projections. The thalamic inputs to the auditory cortex were modeled as time-varying firing rates without explicit spikes. We investigate the biophysical constraints for the emergence of direction selectivity using a model that includes spiking thalamic neurons. We find that directional selectivity can still be achieved for a broad range of parameter values using an architecture similar to that proposed previously for visual cortex models. Furthermore, recurrent connections could improve the directional selectivity of the neurons.