The present-day view of the neural basis for the senses of muscle force and heaviness is that they are generated centrally, within the brain, from copies of motor commands. A corollary of the motor discharge generates a sense of effort which underlies these sensations. In recent experiments on force and heaviness sensations using thumb flexor muscles, a rather different explanation has been invoked: Subjects were proposed to rely predominantly on inputs of a peripheral origin, in particular, the signals of muscle spindles. The present experiments have been carried out at the elbow joint to determine whether these new ideas apply more widely. The effects of fatigue of elbow flexor muscles have been studied in force and heaviness matching tasks using three exercise regimes, a sustained maximum voluntary contraction (MVC), a maintained contraction of 35 % MVC, and a maintained contraction of 35 % MVC combined with muscle vibration at 80 Hz. In force-matching experiments, subjects were required to contract both arms and while the reference arm generated the target force under visual control, it was matched by the indicator arm without visual feedback. During the 100 % MVC exercise, force in the exercising reference arm fell rapidly to almost a half of its original value over 90 s while force in the indicator did not fall, leading to a significant overestimation of the reference force. During the 35 % MVC exercise, subjects also overestimated the reference force and this persisted at 5 and 10 min after the exercise. When 35 % MVC was combined with vibration, the amount by which the indicator arm overestimated the reference force was significantly reduced. In heaviness matching experiments, subjects could move their arms through a small range. The reference arm was loaded with a weight, and weights were added or removed from the indicator until heaviness felt the same in the two arms. There was a small, but significant fall in the matching weight used after 100 % MVC exercise, that is, the weight held by the fatigued arm felt lighter. The 35 % exercise did not alter heaviness sensation while 35 % MVC exercise with vibration led to a significant reduction in perceived heaviness. To conclude, while the results of these experiments on elbow flexors are not as clear cut as for thumb flexors, the central effort hypothesis falls short, in a number of respects in explaining the data which are able to be interpreted in terms of a peripheral afferent contribution to the senses of force and heaviness.

Weak electrocutaneous stimuli applied to the forearm are erroneously localized toward its middle (Steenbergen P, Buitenweg JR, Trojan J, Veltink PH. Exp Brain Res 232: 597–607, 2014). We asked whether mechanical touch stimuli exhibit a similar bias and whether the bias is toward the middle of the forearm or toward the middle of the recent stimulus distribution. In experiments 1 and 2, participants ( n = 12 and n = 10) localized by pointing von Frey filaments applied to four locations on the dorsal forearm. Individually adjusted weak and strong stimuli ( experiment 1) or two levels of strong stimuli ( experiment 2) were presented in single sessions in random order. Weaker stimuli were localized with greater variability than the strong, with compression toward the middle of the forearm. Responses to the two levels of strong stimuli did not differ. In experiment 3, participants ( n = 16) were presented two spatially offset stimulus distributions (8 cm center-to-center), each offset from the forearm middle, on 2 different days. Out of four target locations comprising each distribution, two were shared. Responses to weak stimuli were compressed compared with responses to strong stimuli. Importantly, biases for the common targets had opposite directions, each being toward the middle of the distribution within which targets were presented. Responses to strong stimuli also exhibited a distribution-dependent bias, a 2-cm overall shift across the forearm midpoint. We conclude that touch localization is subject to intensity-dependent biases determined by the recent history of stimulation and possibly also by the available or perceived response space. NEW & NOTEWORTHY Recent findings show that weak electrical stimuli applied to the forearm are mislocalized toward the forearm middle, relative to strong stimuli. We found that weak mechanical stimuli are similarly mislocalized. The bias changed if, as a group, stimuli were not centered on the forearm middle: weak stimuli gravitated toward the center of prior stimulation. Localization of strong stimuli was also biased, consistent with the tendency to center responses within the available response space.

Existing planet-finding spectrometers are limited by systematic errors that result from their seeing-limited design. Of particular concern is the use of multi-mode fibers (MMFs), which introduce modal noise and accept significant amounts of background radiation from the sky. We present the design of a single-mode fiber-based acquisition camera for a diffraction-limited spectrometer named "iLocater." By using the "extreme" adaptive optics (AO) system of the Large Binocular Telescope (LBT), iLocater will overcome the limitations that prevent Doppler instruments from reaching their full potential, allowing precise radial velocity (RV) measurements of terrestrial planets around nearby bright stars. The instrument presented in this paper, which we refer to as the acquisition camera "demonstration system," will measure on-sky single-mode fiber (SMF) coupling efficiency using one of the 8.4m primaries of the LBT in fall 2015.