On the Measurement of Movement Difficulty in the Standard Approach to Fitts' Law

Fitts' law is an empirical rule of thumb which predicts the time it takes people, under time pressure, to reach with some pointer a target of width W located at a distance D. It has been traditionally assumed that the predictor of movement time must be some mathematical transform of the quotient of D/W, called the index of difficulty (ID) of the movement task. We ask about the scale of measurement involved in this independent variable. We show that because there is no such thing as a zero-difficulty movement, the IDs of the literature run on non-ratio scales of measurement. One notable consequence is that, contrary to a widespread belief, the value of the y-intercept of Fitts' law is uninterpretable. To improve the traditional Fitts paradigm, we suggest grounding difficulty on relative target tolerance W/D, which has a physical zero, unlike relative target distance D/W. If no one can explain what is meant by a zero-difficulty movement task, everyone can understand what is meant by a target layout whose relative tolerance W/D is zero, and hence whose relative intolerance 1–W/D is 1 or 100%. We use the data of Fitts' famous tapping experiment to illustrate these points. Beyond the scale of measurement issue, there is reason to doubt that task difficulty is the right object to try to measure in basic research on Fitts' law, target layout manipulations having never provided users of the traditional Fitts paradigm with satisfactory control over the variations of the speed and accuracy of movements. We advocate the trade-off paradigm, a recently proposed alternative, which is immune to this criticism

Defining the Substrate Spectrum of the TIM22 Complex Identifies Pyruvate Carrier Subunits as Unconventional Cargos

Mitochondria carrier proteins, which possess six transmembrane spans (TM), are transported by the TIM22 complex. By using a quantitative proteomic approach, Gomkale and Cruz-Zaragoza et al. reveal subunits of the mitochondrial pyruvate carrier (MPC) that possess two or three TMs as unexpected substrates of the carrier pathway

Glucose-Induced Regulation of Protein Import Receptor Tom22 by Cytosolic and Mitochondria-Bound Kinases

SummaryMost mitochondrial proteins are imported by the translocase of the outer mitochondrial membrane (TOM). Tom22 functions as central receptor and transfers preproteins to the import pore. Casein kinase 2 (CK2) constitutively phosphorylates the cytosolic precursor of Tom22 at Ser44 and Ser46 and, thus, promotes its import. It is unknown whether Tom22 is regulated under different metabolic conditions. We report that CK1, which is involved in glucose-induced signal transduction, is bound to mitochondria. CK1 phosphorylates Tom22 at Thr57 and stimulates the assembly of Tom22 and Tom20. In contrast, protein kinase A (PKA), which is also activated by the addition of glucose, phosphorylates the precursor of Tom22 at Thr76 and impairs its import. Thus, PKA functions in an opposite manner to CK1 and CK2. Our results reveal that three kinases regulate the import and assembly of Tom22, demonstrating that the central receptor is a major target for the posttranslational regulation of mitochondrial protein import

Symmetric and asymmetric action integration during cooperative object manipulation in virtual environments

Cooperation between multiple users in a virtual environment (VE) can take place at one of three levels. These are defined as where users can perceive each other (Level 1), individually change the scene (Level 2), or simultaneously act on and manipulate the same object (Level 3). Despite representing the highest level of cooperation, multi-user object manipulation has rarely been studied. This paper describes a behavioral experiment in which the piano movers' problem (maneuvering a large object through a restricted space) was used to investigate object manipulation by pairs of participants in a VE. Participants' interactions with the object were integrated together either symmetrically or asymmetrically. The former only allowed the common component of participants' actions to take place, but the latter used the mean. Symmetric action integration was superior for sections of the task when both participants had to perform similar actions, but if participants had to move in different ways (e.g., one maneuvering themselves through a narrow opening while the other traveled down a wide corridor) then asymmetric integration was superior. With both forms of integration, the extent to which participants coordinated their actions was poor and this led to a substantial cooperation overhead (the reduction in performance caused by having to cooperate with another person)

Molecular phylogeny of Rotaliida (Foraminifera) based on complete small subunit rDNA sequences

The traditional morphology-based classification of Rotaliida was recently challenged by molecular phylogenetic studies based on partial small subunit (SSU) rDNA sequences. These studies revealed some unexpected groupings of rotaliid genera. However, the support for the new clades was rather weak, mainly because of the limited length of the analysed fragment. In order to improve the resolution of the phylogeny of the rotaliids, 26 new complete SSU rDNA sequences have been obtained. Phylogenetic analyses of these data, together with seven sequences obtained previously, confirm with stronger statistical support the presence of three major clades among the Rotaliida. The first clade comprises members of the families Uvigerinidae, Cassidulinidae and Bolivinidae. The second clade includes all analysed Discorbidae, Rosalinidae, Planulinidae, Planorbulinidae, Rotaliidae, Elphidiidae, Nummulitidae and one of the Nonionidae. Finally, the third clade comprises the Cibicididae, Pseudoparreliidae, Oridorsalidae, Stainforthiidae, Buliminidae and part of the Nonionidae. The clades 1 and 3 are strongly supported by analyses of the complete SSU rDNA, while the monophyly of clade 2 is less certain, probably due to the rapid evolutionary rates of some lineages included in this clade. These results clearly contradict the classical separation of rotaliid foraminifera into two orders: Rotaliida and Buliminida. Relatively good agreement has been found between molecular data and the morphological definition of the families for which more than one genus was sequenced. However, larger taxon sampling will be necessary for a better definition of the three major clade

Mitochondrial precursor proteins are imported through a hydrophilic membrane environment

We have analyzed how translocation intermediates of imported mitochondrial precursor proteins, which span contact sites, interact with the mitochondrial membranes. F1-ATPase subunit β(F1β) was trapped at contact sites by importing it into Neurospora mitochondria in the presence of low levels of nucleoside triphosphates. This F1β translocation intermediate could be extracted from the membranes by treatment with protein denaturants such as alkaline pH or urea. By performing import at low temperatures, the ADP/ATP carrier was accumulated in contact sites of Neurospora mitochondria and cytochrome b2 in contact sites of yeast mitochondria. These translocation intermediates were also extractable from the membranes at alkaline pH. Thus, translocation of precursor proteins across mitochondrial membranes seems to occur through an environment which is accessible to aqueous perturbants. We propose that proteinaceous structures are essential components of a translocation apparatus present in contact sites