Channel Tuning Research Articles

Multi-wavelength random fiber laser with a spectral-flexible characteristic

In past decades, multi-wavelength lasers have attracted much attention due to their wide applications in many fields. In this paper, we demonstrate a multi-wavelength random fiber laser with customizable spectra enabled by an acousto–optic tunable filter. The operating wavelength range can be tuned from 1114.5 to 1132.5 nm with a maximal output power of 5.55 W, and spectral channel tuning can also be realized with a maximal number of five. The effect of gain competition and the interaction between Raman gain and insertion loss are also analyzed. Furthermore, the output spectra can be ordered by radiating appropriate radio frequency signals to the acousto–optic tunable filter. This work may provide a reference for agile shape spectrum generation and promote multi-wavelength random fiber laser practicability in sensing, telecommunications, and precise spectroscopy.

A Loading-Aware TDMA Sleep Scheme to Improve Power Consumption Performance on Medium to High Traffic for NG-EPON Networks

The inter-cycle sleep design previously proposed as a power-saving scheme for next-generation Ethernet passive optical networks (NG-EPONs) is able to effectively decrease the power consumption of heavy-load traffic. However, with a medium to high traffic load, these networks may still suffer from a high level of power consumption if no enhanced mechanisms are used. It was noted that the optical line terminal (OLT) often fully opens all communication channels at a medium to heavy load. Moreover, the number of opened channels was changed cyclically according to the traffic loading status. Accordingly, optical network units (ONUs) may be frequently allocated to different channels with changing loads. This may lead to inefficient operation and result in significant channel tuning delays. We thus propose a loading-aware time-division multiple access (TDMA) mechanism that allows the OLT to reserve a maximum bandwidth for each ONU when the network experiences heavy-load traffic. The performance results of our simulations reveal that the proposed scheme is able to reduce power consumption for targeted medium to high loads since ONUs under such loads can extend their sleep time because the cycle length is maximized. Moreover, the total delay is maintained at a relatively low level after applying the proposed scheme since the tuning delay is reduced significantly; however, the transmission delay is slightly increased due to the increased cycle length.

Spatially Guided Distractor Suppression during Visual Search.

Past work has demonstrated that active suppression of salient distractors is a critical part of visual selection. Evidence for goal-driven suppression includes below-baseline visual encoding at the position of salient distractors (Gaspelin and Luck, 2018) and neural signals such as the distractor positivity (Pd) that track how many distractors are presented in a given hemifield (Feldmann-Wüstefeld and Vogel, 2019). One basic question regarding distractor suppression is whether it is inherently spatial or nonspatial in character. Indeed, past work has shown that distractors evoke both spatial (Theeuwes, 1992) and nonspatial forms of interference (Folk and Remington, 1998), motivating a direct examination of whether space is integral to goal-driven distractor suppression. Here, we use behavioral and EEG data from adult humans (male and female) to provide clear evidence for a spatial gradient of suppression surrounding salient singleton distractors. Replicating past work, both reaction time and neural indices of target selection improved monotonically as the distance between target and distractor increased. Importantly, these target selection effects were paralleled by a monotonic decline in the amplitude of the Pd, an electrophysiological index of distractor suppression. Moreover, multivariate analyses revealed spatially selective activity in the θ-band that tracked the position of the target and, critically, revealed suppressed activity at spatial channels centered on distractor positions. Thus, goal-driven selection of relevant over irrelevant information benefits from a spatial gradient of suppression surrounding salient distractors.SIGNIFICANCE STATEMENT Past work has shown that distractor suppression is an important part of goal-driven attentional selection, but has not yet revealed whether suppression is spatially directed. Using behavioral data, event-related potentials (ERPs) of the EEG signal [N2pc and distractor positivity (Pd) component], as well as a multivariate model of EEG data [channel tuning functions (CTF)], we show that suppression-related neural activity increases monotonically as the distance between targets and distractors decreases, and that spatially-selective activity in the θ-band reveals depressed activity in spatial channels that index distractor positions. Thus, we provide robust evidence for spatially-guided distractor suppression, a result that has important implications for models of goal-driven attentional control.

Conceptualizing conduction as a pliant electrical response: impact of gap junctions and ion channels.

Vasomotor responses conduct among resistance arteries to coordinate blood flow delivery pursuant to energetic demand. Conduction is set by the electrical and mechanical properties of vascular cells, the former tied to how gap junctions and ion channels distribute and dissipate charge, respectively. These membrane proteins are subject to modulation; thus, conduction could be viewed as "pliant" to the current regulatory state. This study used in silico approaches to conceptualize electrical pliancy and to illustrate how gap junctional and ion channel properties distinctly impact conduction along a single skeletal muscle artery or a branching cerebrovascular network. Initial simulations revealed how vascular cells encoded with electrotonic properties best reproduced spreading behavior; the endothelium's importance as a charge source and a longitudinal conduit was readily observed. Alterations in gap junctional conductance produced unique electrical fingerprints: 1) decreased endothelial coupling impaired longitudinal but enhanced radial spread, and 2) reduced myoendothelial coupling limited radial but enhanced longitudinal spread. Subsequent simulations illustrated how tuning ion channel activity, e.g., inward rectifying- and voltage-gated K+ channels, modified charge dissipation, resting membrane potential, and the spread of the electrical phenomenon. Restricting ion channel tuning to a network subregion then revealed how electrical spread could be locally shaped in accordance with the aggregate changes in membrane resistance. In summary, our analysis frames and reimagines electrical conduction as a pliable process, with subtle regulatory changes to membrane proteins shaping network spread and tissue perfusion.NEW & NOTEWORTHY Conducted vasomotor responses depend on initiation and spread of electrical phenomena along arterial walls and their translation into contractile responses. Using computational approaches, we show how subtle but widespread regulation of gap junctions and ion channels can modulate the range and amplitude of electrical spread. Ion channels are regulated by endocrine and mechanical signals and may differ regionally in networks. Subregional electrical changes are not spatially confined but may affect electrical conduction in neighboring regions.

From ERPs to MVPA Using the Amsterdam Decoding and Modeling Toolbox (ADAM).

In recent years, time-resolved multivariate pattern analysis (MVPA) has gained much popularity in the analysis of electroencephalography (EEG) and magnetoencephalography (MEG) data. However, MVPA may appear daunting to those who have been applying traditional analyses using event-related potentials (ERPs) or event-related fields (ERFs). To ease this transition, we recently developed the Amsterdam Decoding and Modeling (ADAM) toolbox in MATLAB. ADAM is an entry-level toolbox that allows a direct comparison of ERP/ERF results to MVPA results using any dataset in standard EEGLAB or Fieldtrip format. The toolbox performs and visualizes multiple-comparison corrected group decoding and forward encoding results in a variety of ways, such as classifier performance across time, temporal generalization (time-by-time) matrices of classifier performance, channel tuning functions (CTFs) and topographical maps of (forward-transformed) classifier weights. All analyses can be performed directly on raw data or can be preceded by a time-frequency decomposition of the data in which case the analyses are performed separately on different frequency bands. The figures ADAM produces are publication-ready. In the current manuscript, we provide a cookbook in which we apply a decoding analysis to a publicly available MEG/EEG dataset involving the perception of famous, non-famous and scrambled faces. The manuscript covers the steps involved in single subject analysis and shows how to perform and visualize a subsequent group-level statistical analysis. The processing pipeline covers computation and visualization of group ERPs, ERP difference waves, as well as MVPA decoding results. It ends with a comparison of the differences and similarities between EEG and MEG decoding results. The manuscript has a level of description that allows application of these analyses to any dataset in EEGLAB or Fieldtrip format.

Multivariate EEG analyses support high-resolution tracking of feature-based attentional selection

The primary electrophysiological marker of feature-based selection is the N2pc, a lateralized posterior negativity emerging around 180–200 ms. As it relies on hemispheric differences, its ability to discriminate the locus of focal attention is severely limited. Here we demonstrate that multivariate analyses of raw EEG data provide a much more fine-grained spatial profile of feature-based target selection. When training a pattern classifier to determine target position from EEG, we were able to decode target positions on the vertical midline, which cannot be achieved using standard N2pc methodology. Next, we used a forward encoding model to construct a channel tuning function that describes the continuous relationship between target position and multivariate EEG in an eight-position display. This model can spatially discriminate individual target positions in these displays and is fully invertible, enabling us to construct hypothetical topographic activation maps for target positions that were never used. When tested against the real pattern of neural activity obtained from a different group of subjects, the constructed maps from the forward model turned out statistically indistinguishable, thus providing independent validation of our model. Our findings demonstrate the power of multivariate EEG analysis to track feature-based target selection with high spatial and temporal precision.

Recent Advances in Various Metal-Organic Channels for Photochemistry beyond Confined Spaces.

Tailor-made molecular channel architectures are a hot issue in the fields of nanotechnology, molecular sieves, ion sensors, recognition, confined space reactors, and fluidic transport systems. Carbon nanotubes have been a particular focus, though they cannot easily be synthesized to predefined structures and sizes. Rational design and construction of molecular channel structures based on coordination chemistry has been recognized as a useful approach. Metal-organic channel (MOC) structures can be generated by utilizing, at least in principle, molecular self-assembly of metal ions as angular units with designed ligands as spacers. Recent developments in molecular channel chemistry include exciting advances in photochemical applications and supramolecular material functionality, in addition to general applications such as transport, diffusion and exchange, separation, gas storage, catalysis, and simple encapsulation. In order to carry out the diverse channel functionalities, a large number of studies have been conducted on the synthesis of robust and stable 3D coordination polymers, which show permanent porosity without any guest molecules within the channels, in that the robustness of the channel structures after removal of the solvate/guest molecules is of interest because the structural integrity of the extended structures must be maintained during the reactions. These compounds can be regarded as analogous to zeolites. This Account highlights advances in the construction, from metal cations and multidentate pyridyl ligands, of various MOCs and useful molecular materials as photoreaction platforms. We begin by discussing the fact that detailed proof-of-concept construction of various systematic MOC structures has been introduced mainly in terms of the metal ions as angular components and the pyridyl ligands as spacers. This approach leads to structural complexity of assembled MOC motifs such as metallamacrocycles, helical and cylindrical coordination polymers, vertical arrays of 1D coordination polymers, interweaving and eclipsed stacking of 2D coordination polymers, and typical 3D coordination polymers. Notwithstanding the diversity of their skeletal structures, confined spaces of the channels are suitable for the study of photochemical performances including radical trapping, photocyclopropanation, dye inclusion and energy transfer, and guest-to-host structural transmission. The key aspect of their utility is not only the preparation of photoresistant MOCs but also channel tuning for inclusion of photoactive guests. Guest molecules, which are compatible with the size, shape, and polarity of the channels, can be incorporated into the crystals, instead of simple organic solvents, thus giving rise to host-guest complexes in the solid state. Such MOC studies could facilitate the development of chemical sensors, new photocatalytic systems, and useful molecular photochemical reactors.

Swelling of Bicontinuous Cubic Phases in Guerbet Glycolipid: Effects of Additives.

Inverse bicontinuous cubic phases of lyotropic liquid crystal self-assembly have received much attention in biomedical, biosensing, and nanotechnology applications. An Ia3d bicontinuous cubic based on the gyroid G-surface can be formed by the Guerbet synthetic glucolipid 2-hexyl-decyl-β-d-glucopyranoside (β-Glc-OC6C10) in excess water. The small water channel diameter of this cubic phase could provide nanoscale constraints in encapsulation of large molecules and crystallization of membrane proteins, hence stresses the importance of water channel tuning ability. This work investigates the swelling behavior of lyotropic self-assembly of β-Glc-OC6C10 which could be controlled and modulated by different surfactants as a hydration-modulating agent. Our results demonstrate that addition of nonionic glycolipid octyl-β-d-glucopyranoside (β-Glc-OC8) at 20 and 25 mol % gives the largest attainable cubic water channel diameter of ca. 62 Å, and formation of coacervates which may be attributed to a sponge phase were seen at 20 mol % octyl-β-d-maltopyranoside (β-Mal-OC8). Swelling of the cubic water channel can also be attained in charged surfactant-doped systems dioctyl sodium sulfosuccinate (AOT) and hexadecyltrimethylammonium bromide (CTAB), of which phase transition occurred from cubic to a lamellar phase. Destabilization of the cubic phase to an inverse hexagonal phase was observed when a high amount of charged lecithin (LEC) and stearylamine (SA) was added to the lipid self-assembly.

Polarimeters for the SPASCHARM Experiment

A beam channel of polarized protons and antiprotons produced from decays of [Formula: see text]- and anti-[Formula: see text]-hyperons for the SPASCHARM experiment is to be built at IHEP U-70 accelerator in Protvino, Russia. The methods for tagging and measuring polarization of the beam (anti)protons are discussed in this report. The fast on-line beam tagging exploits the correlations between polarization and kinematics of (anti)protons originated from (anti)[Formula: see text]-decays. In the intermediate focus of primary target, decay (anti)protons of different transverse polarizations are spatially dispersed transversely with respect to the beam axis. The tagging system, consisting of fast beam detectors with good spatial resolution, measures the momentum and trajectory of each beam particle, including its position at the intermediate focus, thus allowing instant (on-line) assignment of the transverse polarization value to each (anti)proton. This system is also extremely useful for the beam channel tuning. While being fast and convenient, the polarization tagging fully relies on computing of particle transportation in the beam channel. In order to verify the real beam polarization and operating of the tagging system and beam channel, the independent absolute beam polarimetry is to be used. It is based on measuring the spin asymmetries in elastic scattering of beam (anti)protons in Coulomb-Nuclear Interference (CNI) and diffractive kinematic regions. It is estimated that less than one week of data taking would allow measuring an absolute beam polarization at the statistical accuracy of [Formula: see text]4–5%.

The topography of alpha-band activity tracks the content of spatial working memory.

Working memory (WM) is a system for the online storage of information. An emerging view is that neuronal oscillations coordinate the cellular assemblies that code the content of WM. In line with this view, previous work has demonstrated that oscillatory activity in the alpha band (8-12 Hz) plays a role in WM maintenance, but the exact contributions of this activity have remained unclear. Here, we used an inverted spatial encoding model in combination with electroencephalography (EEG) to test whether the topographic distribution of alpha-band activity tracks spatial representations held in WM. Participants in three experiments performed spatial WM tasks that required them to remember the precise angular location of a sample stimulus for 1,000-1,750 ms. Across all three experiments, we found that the topographic distribution of alpha-band activity tracked the specific location that was held in WM. Evoked (i.e., activity phase-locked to stimulus onset) and total (i.e., activity regardless of phase) power across a range of low-frequency bands transiently tracked the location of the sample stimulus following stimulus onset. However, only total power in the alpha band tracked the content of spatial WM throughout the memory delay period, which enabled reconstruction of location-selective channel tuning functions (CTFs). These findings demonstrate that alpha-band activity is directly related to the coding of spatial representations held in WM and provide a promising method for tracking the content of this online memory system.

Performance sustaining intracortical neural prostheses

Objective. Neural prostheses, or brain–machine interfaces, aim to restore efficient communication and movement ability to those suffering from paralysis. A major challenge these systems face is robust performance, particularly with aging signal sources. The aim in this study was to develop a neural prosthesis that could sustain high performance in spite of signal instability while still minimizing retraining time. Approach. We trained two rhesus macaques implanted with intracortical microelectrode arrays 1–4 years prior to this study to acquire targets with a neurally-controlled cursor. We measured their performance via achieved bitrate (bits per second, bps). This task was repeated over contiguous days to evaluate the sustained performance across time. Main results. We found that in the monkey with a younger (i.e., two year old) implant and better signal quality, a fixed decoder could sustain performance for a month at a rate of 4 bps, the highest achieved communication rate reported to date. This fixed decoder was evaluated across 22 months and experienced a performance decline at a rate of 0.24 bps yr-1. In the monkey with the older (i.e., 3.5 year old) implant and poorer signal quality, a fixed decoder could not sustain performance for more than a few days. Nevertheless, performance in this monkey was maintained for two weeks without requiring additional online retraining time by utilizing prior days’ experimental data. Upon analysis of the changes in channel tuning, we found that this stability appeared partially attributable to the cancelling-out of neural tuning fluctuations when projected to two-dimensional cursor movements. Significance. The findings in this study (1) document the highest-performing communication neural prosthesis in monkeys, (2) confirm and extend prior reports of the stability of fixed decoders, and (3) demonstrate a protocol for system stability under conditions where fixed decoders would otherwise fail. These improvements to decoder stability are important for minimizing training time and should make neural prostheses more practical to use.

Towards an Effective QoS On Demand Multicast Routing Protocol for Multi-Channel Multi Interface WMNs

Wireless Mesh Networks (WMNs) is becoming an emerging paradigm, due to its simple and cost effective deployment, and increased growth in popularity for the next generation wireless Internet. Unlike the single channel, it is challenging to provision a robust multicasting by means of multi-channel and multi-interface WMNs. Therefore, in order to tackle the multicast issues, a high-quality path selection and channel assignment are essential. The proposed Quality of Service – On Demand Multichannel Multicast Routing Protocol (QoS- ODMMRP) includes merged path selection and top-down channel tuning mechanism to support multi-channel and multi-interface WMNs. Thus, the proposed work reduces the number of transmissions and increases the communication throughput efficiently.

Critical band masking reveals the effects of optical distortions on the channel mediating letter identification.

There is evidence that letter identification is mediated by only a narrow band of spatial frequencies and that the center frequency of the neural channel thought to underlie this selectivity is related to the size of the letters. When letters are spatially filtered (at a fixed size) the channel tuning characteristics change according to the properties of the spatial filter (Majaj et al., 2002). Optical aberrations in the eye act to spatially filter the image formed on the retina—their effect is generally to attenuate high frequencies more than low frequencies but often in a non-monotonic way. We might expect the change in the spatial frequency spectrum caused by the aberration to predict the shift in channel tuning observed for aberrated letters. We show that this is not the case. We used critical-band masking to estimate channel-tuning in the presence of three types of aberration—defocus, coma and secondary astigmatism. We found that the maximum masking was shifted to lower frequencies in the presence of an aberration and that this result was not simply predicted by the spatial-frequency-dependent degradation in image quality, assessed via metrics that have previously been shown to correlate well with performance loss in the presence of an aberration. We show that if image quality effects are taken into account (using visual Strehl metrics), the neural channel required to model the data is shifted to lower frequencies compared to the control (no-aberration) condition. Additionally, we show that when spurious resolution (caused by π phase shifts in the optical transfer function) in the image is masked, the channel tuning properties for aberrated letters are affected, suggesting that there may be interference between visual channels. Even in the presence of simulated aberrations, whose properties change from trial-to-trial, observers exhibit flexibility in selecting the spatial frequencies that support letter identification.

Induced α rhythms track the content and quality of visual working memory representations with high temporal precision.

Past work has suggested that neuronal oscillations coordinate the cellular assemblies that represent items in working memory (WM). In line with this hypothesis, we show that the spatial distribution of power in the alpha frequency band (8-12 Hz) can be used to decode the content and quality of the representations stored in visual WM. We acquired EEG data during an orientation WM task, and used a forward encoding model of orientation selectivity to reconstruct orientation-specific response profiles (termed channel tuning functions, or CTFs) that tracked the orientation of the memorandum during both encoding and delay periods of the trial. Critically, these EEG-based CTFs were robust predictors of both between- and within-subject differences in mnemonic precision, showing that EEG-based CTFs provide a sensitive measure of the quality of sensory population codes. Experiments 2 and 3 established that these EEG-based CTFs are contingent on the voluntary storage goals of the observer. When observers were given a postsample cue to store or drop the memorandum, the resulting CTF was sustained in the "store" condition and rapidly eliminated following the "drop" cue. When observers were instructed to store one of two simultaneously presented stimuli, only the stored item was represented in a sustained fashion throughout the delay period. These findings suggest that the oscillatory activity in the alpha frequency band plays a central role in the active storage of information in visual WM, and demonstrate a powerful approach for tracking the precision of on-line memories with high temporal resolution.

Intention estimation in brain–machine interfaces

Objective. The objective of this work was to quantitatively investigate the mechanisms underlying the performance gains of the recently reported ‘recalibrated feedback intention-trained Kalman Filter’ (ReFIT-KF). Approach. This was accomplished by designing variants of the ReFIT-KF algorithm and evaluating training and online data to understand the neural basis of this improvement. We focused on assessing the contribution of two training set innovations of the ReFIT-KF algorithm: intention estimation and the two-stage training paradigm. Main results. Within the two-stage training paradigm, we found that intention estimation independently increased target acquisition rates by 37% and 59%, respectively, across two monkeys implanted with multiunit intracortical arrays. Intention estimation improved performance by enhancing the tuning properties and the mutual information between the kinematic and neural training data. Furthermore, intention estimation led to fewer shifts in channel tuning between the training set and online control, suggesting that less adaptation was required during online control. Retraining the decoder with online BMI training data also reduced shifts in tuning, suggesting a benefit of training a decoder in the same behavioral context; however, retraining also led to slower online decode velocities. Finally, we demonstrated that one- and two-stage training paradigms performed comparably when intention estimation is applied. Significance. These findings highlight the utility of intention estimation in reducing the need for adaptive strategies and improving the online performance of BMIs, helping to guide future BMI design decisions.

A simpler structure for local spatial channels revealed by sustained perifoveal stimuli

A new evaluation of the local structure of spatial channels with sustained stimuli in perifoveal retina employs the masking sensitivity approach to minimize analytic assumptions. The stimuli were designed to address the range of channel tunings of the predominantly sustained response system in the near periphery. Under these conditions, the range of identifiable channels spanned a narrow range of spatial frequencies, from roughly 2-8 cpd at 2° eccentricity to 1-4 cpd at 8° eccentricity. The analysis showed that there are no sustained channels tuned below 2 cpd for the central visual field. This two-octave range of channel tuning is much narrower than is conventionally assumed. For local sustained stimuli, human peripheral spatial processing therefore appears to be based on a simpler channel structure than is often supposed.

Ferroelectric Gated Electrical Transport in CdS Nanotetrapods

Complex nanostructures such as branched semiconductor nanotetrapods are promising building blocks for next-generation nanoelectronics. Here we report on the electrical transport properties of individual CdS tetrapods in a field effect transistor (FET) configuration with a ferroelectric Ba(0.7)Sr(0.3)TiO(3) film as high-k, switchable gate dielectric. A cryogenic four-probe scanning tunneling microscopy (STM) is used to probe the electrical transport through individual nanotetrapods at different temperatures. A p-type field effect is observed at room temperature, owing to the enhanced gate capacitance coupling. And the reversible remnant polarization of the ferroelectric gate dielectric leads to a well-defined nonvolatile memory effect. The field effect is shown to originate from the channel tuning in the arm/core/arm junctions of nanotetrapods. At low temperature (8.5 K), the nanotetrapod devices exhibit a ferroelectric-modulated single-electron transistor (SET) behavior. The results illustrate how the characteristics of a ferroelectric such as switchable polarization and high dielectric constant can be exploited to control the functionality of individual three-dimensional nanoarchitectures.

Submicron Trenches Reduce the Pseudomonas fluorescens Colonization Rate on Solid Surfaces

Bacterial adhesion and spreading on biomaterials are considered key features of pathogenicity. Roughness and topography of the substrate have been reported to affect bacterial adhesion, but little is known about their effect on spreading. Submicron row and channel tuning with bacterial diameter (S2) were designed to test bacterial motility on these surfaces. Random nanometer-sized structures (S1) were used as controls. Optical microscopy and AFM were employed to detect biological and surface pattern details in the micro- and nanoscale, respectively. Results showed that motility strategies (flagella orientation, elongation, aggregation in rafts, formation of network structures, and development of a bacterial frontier) were affected by the presence of submicropatterns. Importantly, the rate of bacterial spreading on S2 was significantly reduced and influenced by the orientation of the submicropatterns. Consequently, submicroengineered substrates could be employed as a tool to downgrade bacterial colonization. Such patterns could impact on the design of proper engineered structures to control biofilm spreading on solid surfaces.

Optimal Scheduling for Minimum Delay in Passive Star Coupled WDM Optical Networks

In this paper, a local optimization framework is proposed, which is able to include system constraints including channel availability, receiver availability and tuning overhead by linear mathematical formulations so that it is sufficient to obtain the optimal performance in terms of message delay. A mixed integer linear programming (MILP) based scheme for passive star coupled WDM optical networks is presented. Based on the new solution, the wavelength assignment and message sequence that guarantee the delivery of the given traffic request, while minimizing the average delay can be achieved. Moreover, the negative effect of the tuning overhead has been incorporated into the new algorithm, which has been ignored in most of the previous work. Numerical results obtained suggest that the proposed scheme is a promising approach for optimizing the network performance in terms of average message delay.

Spatial frequency channels derived from individual differences

Contrast sensitivity functions differ from observer to observer. We propose that these differences arise because each observer has unique weights for the outputs of the neural channels that underlie the contrast sensitivity function. By applying principal components analysis to individual contrast sensitivity functions of 297 observers, estimates of the channel tuning curves were found. We find evidence for three broadly tuned bandpass channels with peaks at 4, 8, and 16 c/deg and bandwidth near 1.3 octaves. These channel tuning curves were reproduced in a cross-validation study of 56 observers.