A new species of Thomasomys Coues, 1884 (Rodentia: Sigmodontinae) from the montane forests of northern Peru with comments on the “aureus” group

We describe a new species of the cricetid rodent Thomasomys (Sigmodontinae) of the "aureus" group based on four specimens collected from Carmen de la Frontera, Piura Department, Peru. This new species has a very long and white tail, very long mystacial vibrissae that extend posteriorly beyond the pinnae, and a distinctive cranial and dental morphology that differ from any other known species of Thomasomys. It is also one of the largest species of Thomasomys, exceeded in size only by T. apeco. A phylogenetic analysis using sequences of Cytb recovered this species within a non-monophyletic "aureus" group with a genetic distance between 5.47% (with T. auricularis) to 10.17% (with Thomasomys sp. 1). In addition to this finding, the phylogenetic position of T. apeco, T. praetor, and T. pyrrhonotus are presented for the first time, prompting a discussion on the nature of the "aureus" group.

The Euler spiral of rat whiskers

This paper reports on an analytical study of the intrinsic shapes of 523 whiskers from 15 rats. We show that the variety of whiskers on a rat’s cheek, each of which has different lengths and shapes, can be described by a simple mathematical equation such that each whisker is represented as an interval on the Euler spiral. When all the representative curves of mystacial vibrissae for a single rat are assembled together, they span an interval extending from one coiled domain of the Euler spiral to the other. We additionally find that each whisker makes nearly the same angle of 47∘ with the normal to the spherical virtual surface formed by the tips of whiskers, which constitutes the rat’s tactile sensory shroud or “search space.” The implications of the linear curvature model for gaining insight into relationships between growth, form, and function are discussed.

Effect of whisker geometry on contact force produced by vibrissae moving at different velocities

Rats and mice are able to perform a variety of subtle tactile discriminations with their mystacial vibrissae. Increasingly, the design and interpretation of neurophysiological and behavioral studies are inspired by and linked to a more precise understanding of the detailed physical properties of the whiskers and their associated hair follicles. Here we used a piezoelectric sensor (bimorph) to examine how contact forces are influenced by the geometry of individual whisker hairs. For a given point along a whisker, bimorph signals are linearly related to whisker movement velocity. The slope of this linear function, called velocity sensitivity (VS), diminishes nonlinearly as whisker diameter decreases. Whiskers differ in overall length, thickness, and proximal-distal taper. Thus VS varies along an individual whisker and among different whiskers on the mystacial pad. Thinner, shorter whiskers, such as those located rostrally in rats and those in mice, have lower overall VSs, rendering them potentially less effective for mediating discriminations that rely on subtle velocity cues. The nonlinear effect of diameter combined with the linear effect of arc length produces radial distance tuning curves wherein small differences in the proximal-distal location of impacts yields larger differences in signal magnitude. Such position-dependent cues could contribute to the localization of objects near the face. Proximal-to-distal changes in contact location during whisking sweeps could also provide signals that aid texture discrimination. NEW & NOTEWORTHY This study describes the geometry of facial whiskers distributed across the mystacial pad with emphasis on velocity encoding of object strikes. Findings indicate how the shapes, lengths, and thicknesses of individual hairs can contribute to sophisticated vibrissa-based tactile discrimination.

Development of a Bio-Inspired Tactile Magnetostrictive Whisker Sensor Using Alfenol

Alfenol (FexAl100−x) is an alloy similar to Galfenol (Fe-Ga alloys) in crystal lattice structure and magnetostriction trend (peaking at ∼20% composition). Although single-crystal Fe80Al20 exhibits lower magnetostriction (∼184 ppm, about half of Fe80Ga20), its magneto-elastic coupling coefficient is on par with that of Fe-Ga. In addition, characteristics such as machinability and rollability are superior to that of Galfenol, making it possible to achieve textured sheets (thickness∼200 μm) which, while having a high elastic modulus, are very flexible. Furthermore, Aluminum is non toxic, cheap (∼1% the cost of Ga) and is available in abundance. These attributes make Alfenol an ideal candidate for a bio-inspired whisker-like tactile sensor (mimicking mystacial vibrissae of cats, sea lions, etc.). This work deals with the design and development of an accurate, cost efficient, real-time, and non-invasive sensor prototype that tracks displacements, vibrations and scour on bridge piers with minimal signal conditioning. Making such a sensor is possible thanks to Alfenol’s linear response to strain in the presence of appropriate bias magnets. The change in its magnetic state due to inverse magnetostriction from applied bending stresses will be observed using Hall Effect sensors to derive deflection information. A protocol to manufacture rolled and textured Alfenol whisker samples will be presented in this research. The effect of bias conditions on sensor performance will be studied empirically and by using multi-physics simulations. Optimization of the sensor by varying the dimensions of the whisker, and its correlation to flux leakage will also be examined followed by an effort to understand the micro-magnetic response of Alfenol to mechanical stimulation. Finally, results from using this biomimetic sensor to measure displacements and vibrations, and its viability to be used as a flow sensor will be discussed. The robustness of this sensor has been exploited to develop a novel real-life application to provide an early warning system for bridge pier scour due to soil transportation during a weather event. The effectiveness of these sensors for scour detection in riverbeds will subsequently be simulated in a water flume and analyzed.

Novel two-alternative forced choice paradigm for bilateral vibrotactile whisker frequency discrimination in head-fixed mice and rats

Rats and mice receive a constant bilateral stream of tactile information with their large mystacial vibrissae when navigating in their environment. In a two-alternative forced choice paradigm (2-AFC), head-fixed rats and mice learned to discriminate vibrotactile frequencies applied simultaneously to individual whiskers on the left and right sides of the snout. Mice and rats discriminated 90-Hz pulsatile stimuli from pulsatile stimuli with lower repetition frequencies (10–80 Hz) but with identical kinematic properties in each pulse. Psychometric curves displayed an average perceptual threshold of 50.6-Hz and 53.0-Hz frequency difference corresponding to Weber fractions of 0.56 and 0.58 in mice and rats, respectively. Both species performed >400 trials a day (>200 trials per session, 2 sessions/day), with a peak performance of >90% correct responses. In general, rats and mice trained in the identical task showed comparable psychometric curves. Behavioral readouts, such as reaction times, learning rates, trial omissions, and impulsivity, were also very similar in the two species. Furthermore, whisking of the animals before stimulus presentation reduced task performance. This behavioral paradigm, combined with whisker position tracking, allows precise stimulus control in the 2-AFC task for head-fixed rodents. It is compatible with state-of-the-art neurophysiological recording techniques, such as electrophysiology and two-photon imaging, and therefore represents a valuable framework for neurophysiological investigations of perceptual decision-making.

Mechanical signals at the base of a rat vibrissa: the effect of intrinsic vibrissa curvature and implications for tactile exploration

Rats actively tap and sweep their large mystacial vibrissae (whiskers) against objects to tactually explore their surroundings. When a vibrissa makes contact with an object, it bends, and this bending generates forces and bending moments at the vibrissa base. Researchers have only recently begun to quantify these mechanical variables. The present study quantifies the forces and bending moments at the vibrissa base with a quasi-static model of vibrissa deflection. The model was validated with experiments on real vibrissae. Initial simulations demonstrated that almost all vibrissa-object collisions during natural behavior will occur with the concave side of the vibrissa facing the object, and we therefore paid particular attention to the role of the vibrissa's intrinsic curvature in shaping the forces at the base. Both simulations and experiments showed that vibrissae with larger intrinsic curvatures will generate larger axial forces. Simulations also demonstrated that the range of forces and moments at the vibrissal base vary over approximately three orders of magnitude, depending on the location along the vibrissa at which object contact is made. Both simulations and experiments demonstrated that collisions in which the concave side of the vibrissa faces the object generate longer-duration contacts and larger net forces than collisions with the convex side. These results suggest that the orientation of the vibrissa's intrinsic curvature on the mystacial pad may increase forces during object contact and provide increased sensitivity to detailed surface features.

Active vibrissal sensing in rodents and marsupials

In rats, the long facial whiskers (mystacial macrovibrissae) are repetitively and rapidly swept back and forth during exploration in a behaviour known as ‘whisking’. In this paper, we summarize previous evidence from rats, and present new data for rat, mouse and the marsupial grey short-tailed opossum ( Monodelphis domestica ) showing that whisking in all three species is actively controlled both with respect to movement of the animal's body and relative to environmental structure. Using automatic whisker tracking, and Fourier analysis, we first show that the whisking motion of the mystacial vibrissae, in the horizontal plane, can be approximated as a blend of two sinusoids at the fundamental frequency (mean 8.5, 11.3 and 7.3 Hz in rat, mouse and opossum, respectively) and its second harmonic. The oscillation at the second harmonic is particularly strong in mouse (around 22 Hz) consistent with previous reports of fast whisking in that species. In all three species, we found evidence of asymmetric whisking during head turning and following unilateral object contacts consistent with active control of whisker movement. We propose that the presence of active vibrissal touch in both rodents and marsupials suggests that this behavioural capacity emerged at an early stage in the evolution of therian mammals.

Superior Colliculus Control of Vibrissa Movements

This study tested the role of the superior colliculus in generating movements of the mystacial vibrissae—whisking. First, we compared the kinematics of whisking generated by the superior colliculus with those generated by the motor cortex. We found that in anesthetized rats, microstimulation of the colliculus evoked a sustained vibrissa protraction, whereas stimulation of motor cortex produced rhythmic protractions. Movements generated by the superior colliculus are independent of motor cortex and can be evoked at lower thresholds and shorter latencies than those generated by the motor cortex. Next we tested the hypothesis that the colliculus is acting as a simple reflex loop with the neurons that drive vibrissa movement receiving sensory input evoked by vibrissa contacts. We found that most tecto-facial neurons do not receive sensory input. Not only did these neurons not spike in response to sensory stimulation, but field potential analysis revealed that subthreshold sensory inputs do not overlap spatially with tecto-facial neurons. Together these findings suggest that the superior colliculus plays a pivotal role in vibrissa movement—regulating vibrissa set point and whisk amplitude—but does not function as a simple reflex loop. With the motor cortex controlling the whisking frequency, the superior colliculus control of set point and amplitude would account for the main parameters of voluntary whisking.