Vector Reflectometry in a Beam Waveguide

We present a one-port calibration technique for characterization of beam waveguide components with a vector network analyzer. This technique involves using a set of known delays to separate the responses of the instrument and the device under test. We demonstrate this technique by measuring the reflected performance of a millimeter-wave variable-delay polarization modulator

Intermodal attention shifts in multimodal working memory

Attention maintains task-relevant information in working memory (WM) in an active state. We investigated whether the attention-based maintenance of stimulus representations that were encoded through different modalities is flexibly controlled by top-down mechanisms that depend on behavioral goals. Distinct components of the ERP reflect the maintenance of tactile and visual information in WM. We concurrently measured tactile (tCDA) and visual contralateral delay activity (CDA) to track the attentional activation of tactile and visual information during multimodal WM. Participants simultaneously received tactile and visual sample stimuli on the left and right sides and memorized all stimuli on one task-relevant side. After 500 msec, an auditory retrocue indicated whether the sample set's tactile or visual content had to be compared with a subsequent test stimulus set. tCDA and CDA components that emerged simultaneously during the encoding phase were consistently reduced after retrocues that marked the corresponding (tactile or visual) modality as task-irrelevant. The absolute size of cue-dependent modulations was similar for the tCDA/CDA components and did not depend on the number of tactile/visual stimuli that were initially encoded into WM. Our results suggest that modality-specific maintenance processes in sensory brain regions are flexibly modulated by top-down influences that optimize multimodal WM representations for behavioral goals

The control of attentional target selection in a colour/colour conjunction task

To investigate the time course of attentional object selection processes in visual search tasks where targets are defined by a combination of features from the same dimension, we measured the N2pc component as an electrophysiological marker of attentional object selection during colour/colour conjunction search. In Experiment 1, participants searched for targets defined by a combination of two colours, while ignoring distractor objects that matched only one of these colours. Reliable N2pc components were triggered by targets and also by partially matching distractors, even when these distractors were accompanied by a target in the same display. The target N2pc was initially equal in size to the sum of the two N2pc components to the two different types of partially matching distractors, and became superadditive from about 250 ms after search display onset. Experiment 2 demonstrated that the superadditivity of the target N2pc was not due to a selective disengagement of attention from task-irrelevant partially matching distractors. These results indicate that attention was initially deployed separately and in parallel to all target-matching colours, before attentional allocation processes became sensitive to the presence of both matching colours within the same object. They suggest that attention can be controlled simultaneously and independently by multiple features from the same dimension, and that feature-guided attentional selection processes operate in parallel for different target-matching objects in the visual field

The N2cc component as an electrophysiological marker of space-based and feature-based attentional target selection processes in touch

An electrophysiological correlate of attentional target selection processes in touch (N2cc component) has recently been discovered in lateralized tactile working memory experiments. This tactile N2cc emerges at the same time as the visual N2pc component, but has a different modality-specific topography over central somatosensory areas. Here, we investigated links between N2cc components and the space-based versus feature-based attentional selection of task-relevant tactile stimuli. On each trial, a pair of tactile items was presented simultaneously to one finger on the left and right hand. Target stimuli were defined by their location (e.g., left index finger; Spatial Attention Task), by a non-spatial feature (continuous versus pulsed; Feature-based Attention Task), or by a combination of spatial and non-spatial features (Conjunction Task). Reliable N2cc components were observed in all three tasks. They emerged considerably earlier in the Spatial Attention Task than in the Feature-based Attention Task, suggesting that space-based selection mechanisms in touch operate faster than feature-guided mechanisms. The temporal pattern of N2cc components observed in the Conjunction Task revealed that space-based and feature-based attention both contributed to target selection, which was initially driven primarily by spatial location. Overall, these findings establish the N2cc component as a new electrophysiological marker of the selective attentional processing of task-relevant stimuli in touch

Variable-delay Polarization Modulators for Cryogenic Millimeter-wave Applications

We describe the design, construction, and initial validation of the variable-delay polarization modulator (VPM) designed for the PIPER cosmic microwave background polarimeter. The VPM modulates between linear and circular polarization by introducing a variable phase delay between orthogonal linear polarizations. Each VPM has a diameter of 39 cm and is engineered to operate in a cryogenic environment (1.5 K). We describe the mechanical design and performance of the kinematic double-blade flexure and drive mechanism along with the construction of the high precision wire grid polarizers.Comment: 8 pages, 10 Figures, Submitted to Review of Scientific Instrument

Recovery of Large Angular Scale CMB Polarization for Instruments Employing Variable-delay Polarization Modulators

Variable-delay Polarization Modulators (VPMs) are currently being implemented in experiments designed to measure the polarization of the cosmic microwave background on large angular scales because of their capability for providing rapid, front-end polarization modulation and control over systematic errors. Despite the advantages provided by the VPM, it is important to identify and mitigate any time-varying effects that leak into the synchronously modulated component of the signal. In this paper, the effect of emission from a $300$ K VPM on the system performance is considered and addressed. Though instrument design can greatly reduce the influence of modulated VPM emission, some residual modulated signal is expected. VPM emission is treated in the presence of rotational misalignments and temperature variation. Simulations of time-ordered data are used to evaluate the effect of these residual errors on the power spectrum. The analysis and modeling in this paper guides experimentalists on the critical aspects of observations using VPMs as front-end modulators. By implementing the characterizations and controls as described, front-end VPM modulation can be very powerful for mitigating $1/f$ noise in large angular scale polarimetric surveys. None of the systematic errors studied fundamentally limit the detection and characterization of B-modes on large scales for a tensor-to-scalar ratio of $r=0.01$. Indeed, $r<0.01$ is achievable with commensurately improved characterizations and controls.Comment: 13 pages, 13 figures, 1 table, matches published versio

Hands behind your back: effects of arm posture on tactile attention in the space behind the body

Previous research has shown that tactile-spatial information originating from the front of the body is remapped from an anatomical to an external-spatial coordinate system, guided by the availability of visual information early in development. Comparably little is known about regions of space for which visual information is not typically available, such as the space behind the body. This study tests for the first time the electrophysiological correlates of the effects of proprioceptive information on tactile-attentional mechanisms in the space behind the back. Observers were blindfolded and tactually cued to detect infrequent tactile targets on either their left or right hand and to respond to them either vocally or with index finger movements. We measured event-related potentials (ERPs) to tactile probes on the hands in order to explore tactile-spatial attention when the hands were either held close together or far apart behind the observer's back. Results show systematic effects of arm posture on tactile-spatial attention different from those previously found for front space. While attentional selection is typically more effective for hands placed far apart than close together in front space, we found that selection occurred more rapidly for close than far hands behind the back, during both covert attention and movement preparation tasks. This suggests that proprioceptive space may ‘wrap’ around the body, following the hands as they extend horizontally from the front body midline to the centre of the back

The Cosmology Large Angular Scale Surveyor (CLASS): 40 GHz Optical Design

The Cosmology Large Angular Scale Surveyor (CLASS) instrument will measure the polarization of the cosmic microwave background at 40, 90, and 150 GHz from Cerro Toco in the Atacama desert of northern Chile. In this paper, we describe the optical design of the 40 GHz telescope system. The telescope is a diffraction limited catadioptric design consisting of a front-end Variable-delay Polarization Modulator (VPM), two ambient temperature mirrors, two cryogenic dielectric lenses, thermal blocking filters, and an array of 36 smooth-wall scalar feedhorn antennas. The feed horns guide the signal to antenna-coupled transition-edge sensor (TES) bolometers. Polarization diplexing and bandpass definition are handled on the same microchip as the TES. The feed horn beams are truncated with 10 dB edge taper by a 4 K Lyot-stop to limit detector loading from stray light and control the edge illumination of the front-end VPM. The field-of-view is 19 deg x 14 deg with a resolution for each beam on the sky of 1.5 deg. FWHM