Phase-sensitive Response Research Articles

Estimating receptive fields of simple and complex cells in early visual cortex: A convolutional neural network model with parameterized rectification.

Neurons in the primary visual cortex respond selectively to simple features of visual stimuli, such as orientation and spatial frequency. Simple cells, which have phase-sensitive responses, can be modeled by a single receptive field filter in a linear-nonlinear model. However, it is challenging to analyze phase-invariant complex cells, which require more elaborate models having a combination of nonlinear subunits. Estimating parameters of these models is made additionally more difficult by cortical neurons' trial-to-trial response variability. We develop a simple convolutional neural network method to estimate receptive field models for both simple and complex visual cortex cells from their responses to natural images. The model consists of a spatiotemporal filter, a parameterized rectifier unit (PReLU), and a two-dimensional Gaussian "map" of the receptive field envelope. A single model parameter determines the simple vs. complex nature of the receptive field, capturing complex cell responses as a summation of homogeneous subunits, and collapsing to a linear-nonlinear model for simple type cells. The convolutional method predicts simple and complex cell responses to natural image stimuli as well as grating tuning curves. The fitted models yield a continuum of values for the PReLU parameter across the sampled neurons, showing that the simple/complex nature of cells can vary in a continuous manner. We demonstrate that complex-like cells respond less reliably than simple-like cells. However, compensation for this unreliability with noise ceiling analysis reveals predictive performance for complex cells proportionately closer to that for simple cells. Most spatial receptive field structures are well fit by Gabor functions, whose parameters confirm well-known properties of cat A17/18 receptive fields.

Long-term stabilization of carrier envelope phases of mid-infrared pulses for the precise detection of phase-sensitive responses to electromagnetic waves

We report a high performance mid-infrared pump visible probe measurement system, which can measure phase-sensitive responses to a mid-infrared pulse along the oscillating electromagnetic field. In this system, the pump light is a phase-locked mid-infrared pulse with a temporal width of 100 fs, which is produced via difference frequency generation (DFG) from two idler pulses of two optical parametric amplifiers (OPAs) that are excited by the same Ti:sapphire regenerative amplifier. The probe pulse is a visible pulse with a temporal width of 9 fs and is generated from a custom-built non-collinear OPA. By measuring the electric-field waveforms of mid-infrared pump pulses with electro-optic sampling and evaluating their carrier envelope phase (CEP) and the temporal positions of their envelopes relative to ultrashort visible probe pulses, we are able to perform double feedback corrections that eliminate both the following sources of drift. The CEP drift in mid-infrared pulses originating from fluctuations in the difference of optical-path lengths of the two idler pulses before the DFG is corrected by inserting a wedge plate in one idler path, and the drift in pump–probe delay times due to fluctuations in the difference of the overall optical-path lengths of the pump and probe pulses is corrected with mechanical delay lines. In this double feedback system, the absolute carrier phase of mid-infrared pulses can be fixed within 200 mrad and errors in the measurement of phase-sensitive responses can be reduced to within 1 fs over a few tens of hours.

Nonlinear interferometer: Design, implementation, and phase-sensitive sum frequency measurement.

Sum frequency generation (SFG) spectroscopy is a unique tool for probing the vibrational structure of numerous interfaces. Since SFG is a nonlinear spectroscopy, it has long been recognized that measuring only the intensity-the absolute square of the surface response-limits the potential of SFG for examining interfacial interactions and dynamics. The potential is unlocked by measuring the phase-sensitive or imaginary response. As with any phase, the phase-sensitive SFG response is measured relative to a reference; the spatial relationship between the phase reference and the sample modulates the observed interference intensity and impacts sensitivity and accuracy. We have designed and implemented a nonlinear interferometer to directly measure the phase-sensitive response. If the phase of the reference is known, then the interferometer produces an absolute phase of the surface. Compared to current configurations, phase accuracy and stability are greatly improved due to active stabilization of the sample-reference position. The design is versatile and thus can be used for any system that can be probed with SFG including buried interfaces and those with high vapor pressure. Feasibility and advantages of the interferometer are demonstrated using an octadecyltrichlorosilane film on fused silica.

3D plasmonic crystal metamaterials for ultra-sensitive biosensing.

We explore the excitation of plasmons in 3D plasmon crystal metamaterials and report the observation of a delocalized plasmon mode, which provides extremely high spectral sensitivity (>2600 nm per refractive index unit (RIU) change), outperforming all plasmonic counterparts excited in 2D nanoscale geometries, as well as a prominent phase-sensitive response (>3*104 deg. of phase per RIU). Combined with a large surface for bioimmobilization provided by the 3D matrix, the proposed sensor architecture promises a new important landmark in the advancement of plasmonic biosensing technology.

Phase sensitivity of complex cells in primary visual cortex

Neurons in the primary visual cortex are often classified as either simple or complex based on the linearity (or otherwise) of their response to spatial luminance contrast. In practice, classification is typically based on Fourier analysis of a cell’s response to an optimal drifting sine-wave grating. Simple cells are generally considered to be linear and produce responses modulated at the fundamental frequency of the stimulus grating. In contrast, complex cells exhibit significant nonlinearities that reduce the response at the fundamental frequency. Cells can therefore be easily and objectively classified based on the relative modulation of their responses – the ratio of the phase-sensitive response at the fundamental frequency of the stimulus (F1) to the phase-invariant sustained response (F0). Cells are classified as simple if F1/F0>1 and complex if F1/F0<1. This classification is broadly consistent with criteria based on the spatial organisation of cells’ receptive fields and is accordingly presumed to reflect disparate functional roles of simple and complex cells in coding visual information. However, Fourier analysis of spiking responses is sensitive to the number of spikes available –F1/F0 increases as the number of spikes is reduced, even for phase-invariant complex cells. Moreover, many complex cells encountered in the laboratory exhibit some phase sensitivity, evident as modulation of their responses at the fundamental frequency. There currently exists no objective quantitative means of assessing the significance or otherwise of these modulations. Here we derive a statistical basis for objectively assessing whether the modulation of neuronal responses is reliable, thereby adding a level of statistical certainty to measures of phase sensitivity. We apply our statistical analysis to neuronal responses to moving sine-wave gratings recorded from 367 cells in cat primary visual cortex. We find that approximately 60% of complex cells exhibit statistically significant (α<0.01) modulation of their responses to optimal moving gratings. These complex cells are phase sensitive and reliably encode spatial phase.

A Dissociation of Attention and Awareness in Phase-sensitive but Not Phase-insensitive Visual Channels

The elements most vivid in our conscious awareness are the ones to which we direct our attention. Scientific study confirms the impression of a close bond between selective attention and visual awareness, yet the nature of this association remains elusive. Using visual afterimages as an index, we investigate neural processing of stimuli as they enter awareness and as they become the object of attention. We find evidence of response enhancement accompanying both attention and awareness, both in the phase-sensitive neural channels characteristic of early processing stages and in the phase-insensitive channels typical of higher cortical areas. The effects of attention and awareness on phase-insensitive responses are positively correlated, but in the same experiments, we observe no correlation between the effects on phase-sensitive responses. This indicates independent signatures of attention and awareness in early visual areas yet a convergence of their effects at more advanced processing stages.

Silicon based total internal reflection bio and chemical sensing with spectral phase detection

Si-based total internal reflection (TIR) bio/chemical sensor presents an attractive alternative to Surface Plasmon Resonance (SPR) technology due to a relatively simple optical arrangement and technological implementation, as well as a relatively easy bio/chemical immobilization on Si/SiO(2) surface with a number of novel attractive applications. This sensor is based on the control of phase difference between p- and s-polarized components of light reflected from Si/air or Si/water interface in TIR geometry and a high sensitivity of the sensor is granted by a high refractive index of Si (3.56 at 1200 nm). We study properties of TIR sensors in a configuration of spectral phase detection and identify conditions of maximal phase sensitive response. We also experimentally show that the detection limit of Si-based TIR sensor can be lowered down to a level of detection of commercially available SPR devices (10(-6) Refractive Index Units, RIU) under the use of a proper low-noisy method of the phase control. The concept of Si-based TIR opens attractive prospects for the miniaturization of sensor devices, taking advantage of the advanced state of development of Si-based microfabrication technologies, while the proposed spectral phase detection scheme offers much easier packaging and calibration steps.

Microrheology of a sticking transition

The phenomenon of sticking of one object to another, which drastically reduces their relative motion, is ubiquitous in nature. We have studied the sticking process of a colloid, suspended in a fluid medium by an optical tweezer, to a rigid substrate. The evolution of the frictional coupling between the two as a function of their separation is detected by the diffusivity of the particle and also by its phase-sensitive response to an in-plane external oscillatory drive applied to the substrate. On contact, the coupling changes abruptly from viscous to elastic for a rigid silica particle, whereas it evolves slowly with time, similar to ageing in glassy systems, for a soft and deformable polystyrene particle. Depending on the relative strengths of the particle–substrate interaction, the tweezer potential and the external drive, three regimes of dynamics—stuck, ageing and non-stuck—are observed in the dynamical phase diagram. The tendency of small objects to stick together as they come into contact is a commonly observed phenomenon. Yet the interactions that govern this behaviour can be complex. A systematic study of the variation in the force between a particle and a solid surface as they are brought together finds many parallels with the characteristics of glassy and granular systems.

Complex Cells Increase Their Phase Sensitivity at Low Contrasts and Following Adaptation

One of the best-known dichotomies in neuroscience is the division of neurons in the mammalian primary visual cortex into simple and complex cells. Simple cells have receptive fields with separate on and off subregions and give phase-sensitive responses to moving gratings, whereas complex cells have uniform receptive fields and are phase invariant. The phase sensitivity of a cell is calculated as the ratio of the first Fourier coefficient (F1) to the mean time-average (Fo) of the response to moving sinusoidal gratings at 100% contrast. Cells are then classified as simple (F1/Fo >1) or complex (F1/Fo <1). We manipulated cell responses by changing the stimulus contrast or through adaptation. The F(1)/F(0) ratios of cells defined as complex at 100% contrast increased at low contrasts and following adaptation. Conversely, the F1/Fo ratios remained constant for cells defined as simple at 100% contrast. The latter cell type was primarily located in thalamorecipient layers 4 and 6. Many cells initially classified as complex exhibit F1/Fo >1 at low contrasts and after adaptation (particularly in layer 4). The results are consistent with the spike-threshold hypothesis, which suggests that the division of cells into two types arises from the nonlinear interaction of spike threshold with membrane potential responses.

Forward masking in the evoked responses of the guinea pig auditory cortex: Effects of variation of the interaural time and phase differences

In anaesthesized guinea pigs the evoked potentials of the auditory cortex were studied in a forward masking paradigm. In-phase and out-of-phase binaurally presented clicks with interaural time delay (ITD) were used as masker, in-phase click with ITD = 0 served as probe signal. Addition of the masking stimulus suppressed the probe-evoked response that followed the masker. The magnitude of the suppression correlated with the amount of the masker-evoked response: an increase in masker-evoked excitation caused a greater reduction in probe response magnitude. Amplitude of masker-evoked response was seen to be a monotonic or non-monotonic function of ITD. The non-monotonic response exhibited a sensitivity to the interaural phase differences when in-phase and out-of-phase maskers were presented, and showed the tendency to be periodic function of ITD in the expanded range of ITD values. Phase-sensitive responses differed in recovery time following the in-phase and out-of-phase masking stimuli. At near-threshold levels of a forward masker an enhancement of the probe-evoked response was observed.

Numerical computation of two-LO SIS mixer gain and noise

The authors present numerical calculations of the gain and noise of a two-LO (local oscillator) mixer based on an ideal SIS (superconductor-insulator-superconductor) IV curve. They demonstrate the phase-sensitive response of the mixer with two-LO pumping and present calculations of the noise for various DC bias conditions and embedding impedances. Preliminary results indicate that for certain DC bias conditions and signal terminations, the noise added to a signal by the two-LO mixer operated in the single-sideband mode can be a factor of 25 less than that added by a conventional single-sideband single-LO mixer. >

Spatial properties of cells in the rabbit's striate cortex.

The rabbit's striate cortex contains a variety of receptive-field types, including concentric, uniform, simple and complex types. The spatial summation of these receptive fields was studied. Two types of linearity tests were performed: null tests and drifting grating tests. For the null tests, stimuli were sine-wave gratings, temporally modulated by either a 1 Hz sine-wave or by a 1 Hz on-off square wave. For the drifting grating tests, the stimuli were sine-wave gratings moved across the cells' receptive fields at 1-3 Hz. Fourier analyses were performed on the averaged data from both the null tests (sine-wave modulation) and the drifting grating tests. Thirty-one concentric cells were studied. Of these, nine were classified as X and fourteen as Y based upon their responses to ratings. The X cells exhibited linear spatial summation; their responses were mainly at the fundamental modulation frequency. The Y cells exhibited non-linear spatial summation. Their responses contained, in addition to a fundamental component, a second harmonic component whose relative strength increased as the spatial frequency of the display was raised. Eight other concentric cells responded poorly to gratings and were classified as sluggish. Other properties of concentric cells were examined, particularly their responses to standing contrast. The X cells' responses were generally sustained, the Y cells' responses generally transient. Sluggish cells were either sustained or transient, but their responses to standing contrast could frequently be distinguished from those of X and Y cells. All five uniform cells studied exhibited non-linear spatial summation. Their behaviour on the linearity tests was indistinguishable from that of Y cells. Of the seventeen simple cells studied, seven behaved like X cells (linear simple) and ten like Y cells (non-linear simple). However, near their optimum spatial frequencies the responses of both linear and non-linear simple cells were dominated by the fundamental component. Two complex cells behaved like complex cells in the cat's striate cortex (Movshon, Thompson & Tolhurst, 1978b). Their responses were qualitatively like those of non-linear simple cells, but were quantitatively more non-linear. Near their optimum spatial frequencies, harmonic components dominated their responses. The behaviour of nine other complex cells was difficult to describe. Generally, they were unresponsive to gratings. When they could be made to respond to gratings, they gave phase-sensitive responses like those of simple cells.