Psychophysiological Mechanisms of Writing Difficulties in Schoolchildren

The temporal and qualitative organization of writing movements was studied in six- to seven-year-old and nine- to ten-year-old right-handed and left-handed children with underdeveloped visuomotor coordination. The characteristic features of the temporal and qualitative organization of these movements at different stages of motor skill formation were found. Regarding the temporal structure of the series of writing movements, a pause was observed between individual movements in the series of motor task formation. The duration of this pause may serve as a criterion of the degree and efficiency of writing skill formation. The disturbance of the temporal structure of movements during the formation of handwriting speed at the initial stages of learning deteriorated the temporal organization of movements, drastically decreased the quality, and impeded the formation of the skill. The destructive processes were more pronounced in the case of accelerated handwriting in left-handed children and especially in children with underdeveloped visuomotor coordination.     

Two-year-old children copy more reliably and more often than nonhuman great apes in multiple observational learning tasks

Individuals observing a proficient model can potentially benefit by copying at least one of the following three elements: motor movements (i.e., actions), goals, and results. Although several studies have investigated this issue in human infants, there are still very few studies that have systematically examined great apes’ ability to spontaneously copy each of these three elements (particularly in comparison with human infants). We tested great apes and human children with eight two-target puzzle boxes—with varying levels of difficulty—to isolate the aspects that the various species may be more prone to copying. We found first trial evidence for observational learning of actions, goals, and results in children. Some copying was found for apes as well, but only if their performance was averaged across trials.     

Light and electron microscopy of rat kangaroo cells in mitosis

Chromosome orientation and behavior during prometaphase of mitosis in PtK₁ rat kangaroo cells were investigated by cinémicrography and electron microscopy. The first chromosome movements occur soon after the nuclear envelope begins to break down in the region near each pole. Initial chromosome behavior is primarily determined by the distance from the kinetochore region to the spindle poles. The predominant pattern is a movement to and/or association with the proximal pole. Movement to and association with the more distant pole, or direct alignment at or near the spindle equator (direct congression) are less frequent patterns. Except for rare cases, pole-associated chromosomes congress sooner or later and most congressed chromosomes oscillate about the equator. — Ultrastructural observations suggest that pole-associated chromosomes are oriented only to the proximal pole (monotelic or syntelic orientation) and they demonstrate that the sister-kinetochores of congressing or oscillating chromosomes are oriented to opposite poles (amphitelic orientation). — Based on the structure of the early prometaphase spindle and four assumptions concerning the formation of kinetochore fibers and their force-producing interaction with complementary elements, the different patterns of chromosome behavior observed can be explained as a result of synchronous or asynchronous formation of sister-kinetochore fibers. The few chromosomes whose kinetochore region is approximately equidistant from the poles amphi-orient immediately because their sister-kinetochores form fibers synchronously and they congress directly because of the bidirectional forces to which they are subjected. The kinetochore region of most chromosomes is not equidistant from the poles. Therefore, they form a functional fiber first to the nearer pole and move to, or associate with, it because of the unidirectional force. Eventually, however, these chromosomes achieve amphitelic orientation and congress. Once established, amphitelic orientation is stable. Re-orientations do not occur during congression or oscillatory movements.     

A theory for the use of visual orientation information which exploits the columnar structure of striate cortex

A neural model is constructed based on the structure of a visual orientation hypercolumn in mammalian striate cortex. It is then assumed that the perceived orientation of visual contours is determined by the pattern of neuronal activity across orientation columns. Using statistical estimation theory, limits on the precision of orientation estimation and discrimination are calculated. These limits are functions of single unit response properties such as orientation tuning width, response amplitude and response variability, as well as the degree of organization in the neural network. It is shown that a network of modest size, consisting of broadly orientation selective units, can reliably discriminate orientation with a precision equivalent to human performance. Of the various network parameters, the discrimination threshold depends most critically on the number of cells in the hypercolumn. The form of the dependence on cell number correctly predicts the results of psychophysical studies of orientation discrimination. The model system's performance is also consistent with psychophysical data in two situations in which human performance is not optimal. First, interference with orientation discrimination occurs when multiple stimuli activate cells in the same hypercolumn. Second, systematic errors in the estimation of orientation can occur when a stimulus is composed of intersecting lines. The results demonstrate that it is possible to relate neural activity to visual performance by an examination of the pattern of activity across orientation columns. This provides support for the hypothesis that perceived orientation is determined by the distributed pattern of neural activity. The results also encourage the view of neural activity. The results also are determined by the responses of many neurons rather than the sensitivity of individual cells.     

A recurrent neural network for partitioning of hand drawn characters into strokes of different orientations.

A neural network is described which is intended to extract orientation features that should be used for recognition of hand drawn characters. The network partitions the input hand drawn characters into separate line segments (strokes) according to their orientations. The network consists of several neural layers; each layer serves for extracting strokes of a certain orientation. Every neural layer has one-to-one correspondence with an input screen. The network uses an iterative update procedure which includes interactions of neurons inside each layer through oriented excitatory connections and inhibitory interrelations between the corresponding neurons of different layers. Computer simulation of the network was performed. Experiments showed that the network efficiently classifies all pixels of any hand drawn characters according to the orientations of the strokes constituting these characters and performs, as a result of that, a reasonable segmentation of characters.     

Eye-Hand Coordination during Visuomotor Adaptation with Different Rotation Angles

This study examined adaptive changes of eye-hand coordination during a visuomotor rotation task. Young adults made aiming movements to targets on a horizontal plane, while looking at the rotated feedback (cursor) of hand movements on a monitor. To vary the task difficulty, three rotation angles (30°, 75°, and 150°) were tested in three groups. All groups shortened hand movement time and trajectory length with practice. However, control strategies used were different among groups. The 30° group used proportionately more implicit adjustments of hand movements than other groups. The 75° group used more on-line feedback control, whereas the 150° group used explicit strategic adjustments. Regarding eye-hand coordination, timing of gaze shift to the target was gradually changed with practice from the late to early phase of hand movements in all groups, indicating an emerging gaze-anchoring behavior. Gaze locations prior to the gaze anchoring were also modified with practice from the cursor vicinity to an area between the starting position and the target. Reflecting various task difficulties, these changes occurred fastest in the 30° group, followed by the 75° group. The 150° group persisted in gazing at the cursor vicinity. These results suggest that the function of gaze control during visuomotor adaptation changes from a reactive control for exploring the relation between cursor and hand movements to a predictive control for guiding the hand to the task goal. That gaze-anchoring behavior emerged in all groups despite various control strategies indicates a generality of this adaptive pattern for eye-hand coordination in goal-directed actions.     

Anisotropies in peripheral vernier acuity

Vernier acuity for short horizontal, vertical and oblique target lines was measured in many locations in the periphery of the visual field in normal human observers. In the 10 deg periphery, the average alignment threshold with oblique vernier lines in eight locations for three observers was 2.29 times higher than that with vertical and horizontal target lines. This oblique effect was found everywhere in the visual field. Similar conclusions are drawn for configurations in which the lines were replaced by just their distal endpoints, but here, additionally, performance was distinctly better when the dot pair was collinear with the fixation point, i.e. oriented radially, than when it was oriented tangentially. Both for vernier lines and for dot pairs, in all observers, horizontal configurations showed somewhat better thresholds than vertical ones. These results suggest an inherent pattern of connectivity throughout the visual field favoring processing in the cardinal orientations over the obliques, the radial over the tangential and, to a limited extent, the horizontal over the vertical.     

Accuracy, stability, and corrective behavior in a visuomotor tracking task: a preliminary study

Visuomotor tracking tasks have been used to elucidate the underlying mechanisms that allow for the coordination of a movement to an environmental event. The main purpose of the present study was to examine the relationship between accuracy and stability of tracking performance and the amount of corrective movements that emerge for various coordination patterns in a unimanual visuomotor tracking task. Participants (N = 6) produced rhythmic elbow flexion-extension motions and were required to track an external sinusoidal signal at five different relative phases, 0°, 45°, 90°, 135°, and 180°. Differential accuracy and stability were found among the five tracking patterns with the 0° relative phase pattern being the most accurate and stable pattern. Corrective movements were correlated with changes in accuracy only for the 0° relative phase pattern, with more corrections emerging for less accurate performance. The amount of corrective movements decreased as the stability of tracking performance increased for the 0°, 45°, and 135° patterns. For the 90° and 180° tracking patterns, the amount of corrective movements was not correlated with pattern accuracy or pattern stability. The results demonstrate that corrective behaviors are an important motor process in maintaining the stability of stable perception-action coordination patterns, while offering little benefit for unstable perception-action patterns.     

Inverted sand dollars actively orient themselves in flow to increase likelihood of righting

The fact that sand dollars are often dislodged and inverted is an inescapable consequence of living at or slightly below the sediment–water interface. Once inverted, however, how do sand dollars effectively right themselves, given their small spines and stiff internal skeletons? Here, we examined the possibility that individuals of Mellita quinquiesperforata and Dendraster excentricus may take advantage of the interaction of their morphology and flow to increase the likelihood of righting. Based on flow tank observations, the critical velocity required to flip an inverted sand dollar varies with orientation and increases with test size. For both species, the critical velocity was lower when inverted sand dollars were oriented with the posterior margin facing directly downstream, compared with when the posterior margin was positioned in an upstream orientation. To test whether inverted sand dollars would actively rotate into a more advantageous position for flipping, we exposed inverted animals in three starting orientations – with their posterior edge directed upstream (the least favored position for flipping), perpendicular, and downstream to flow – to the minimum flow expected to induce flipping and compared their responses. Time‐lapse photography showed that regardless of initial orientation, within one hour, a majority of individuals of both species rotated into positions that were not statistically different from the downstream orientation (the most favored position for flipping). These results for D. excentricus were further confirmed in a field experiment. Taken together, these data suggest that inverted sand dollars are able to recognize flow direction and respond by modifying their orientation to maximize lift and drag for righting.     

Perception–action coupling model for human locomotor pointing

How do humans achieve the precise positioning of the feet during walking, for example, to reach the first step of a stairway? We addressed this question at the visuomotor integration level. Based on the optical specification of the required adaptation, a dynamical system model of the visuomotor control of human locomotor pointing was devised for the positioning of a foot on a visible target on the floor during walking. Visuomotor integration consists of directly linking optical information to a motor command that specifically modulates step length in accordance with the ongoing dynamics of locomotor pattern generation. The adaptation of locomotion emerges from a perception-action coupling type of control based on temporal information rather than on feedforward planning of movements. The proposed model reproduces experimental results obtained for human locomotor pointing.     

Human parietal cortex in action

Experiments using functional neuroimaging and transcranial magnetic stimulation in humans have revealed regions of the parietal lobes that are specialized for particular visuomotor actions, such as reaching, grasping and eye movements. In addition, the human parietal cortex is recruited by processing and perception of action-related information, even when no overt action occurs. Such information can include object shape and orientation, knowledge about how tools are employed and the understanding of actions made by other individuals. We review the known subregions of the human posterior parietal cortex and the principles behind their organization.     

A genome-wide approach to identifying novel-imprinted genes

Genomic imprinting is an epigenetic process in which the copy of a gene inherited from one parent (maternal or paternal) is consistently silenced or expressed at a significantly lower level than the copy from the other parent. In an effort to begin a systematic genome-wide screen for imprinted genes, we assayed differential allelic expression (DAE) at 3,877 bi-allelic protein-coding sites located in 2,625 human genes in 67 unrelated individuals using genotyping microarrays. We used the presence of both over- and under-expression of the reference allele compared to the alternate allele to identify candidate-imprinted genes. We found 61 genes with at least twofold DAE plus “flipping” of the more highly expressed allele between reference and alternate across heterozygous samples. Sixteen flipping genes were genotyped and assayed for DAE in an independent data set of lymphoblastoid cell lines from two CEPH pedigrees. We confirmed that PEG10 is paternally expressed, identified one gene (ZNF331) with multiple lines of data indicating it is imprinted, and predicted several additional imprinting candidate genes. Our findings suggest that there are at most several hundred genes in the human genome that are universally imprinted. With samples of mRNA from appropriate tissues and a collection of informative cSNPs, a genome-wide search using this methodology could expand the list of genes that undergo genomic imprinting in a tissue- or temporal-specific manner. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Motor Responses to Polarized Light and Gravity Sensing in Euglena gracilis*†

SYNOPSIS. Euglena gracilis strain Z has a motor response which results in orientation with respect to the polarization of a light stimulus. Cells swim preferentially in a direction perpendicular to the plane of polarization of the stimulus. If 2 polarized stimuli are given from opposite directions, the preferred direction is, under certain circumstances, at right angles to the directions of both stimuli. Euglena also preferentially assumes an orientation that is at right angles to the force of gravity. The relationships between these responses and phototactic movements oriented with respect to the direction of the stimulus are discussed.     

Interpreting the autopodia of tetrapods: interphalangeal lines hinge on too many assumptions

Recently Peters proposed the concept of ‘interphalangeal lines’, defined as sub-parallel lines that could supposedly be drawn across the joints of the digits of all tetrapods. The lines were viewed as potential axes of rotation, and it was suggested that they could be used to determine the resting position of the digits, reconstruct missing digital elements of fossil tetrapods, and provide information on systematic relationships. Evidence was adduced from the skeletons of recent and fossil vertebrates and from footprints. However, detailed analysis shows that these claims are largely unfounded. Linear alignments of joints on neighbouring digits are not consistently present in tetrapods, especially across locomotor cycles. Even if present, interphalangeal (IP) lines would rarely be in an appropriate orientation to facilitate joint movements during locomotion. There is no reason to believe that IP lines would be homologous across different taxa, so they cannot be used to infer systematic relationships. Finally, the alleged support from the ichnological record is undermined by the uncertain relationship between the joint structure of the skeleton and the form of the print. We conclude that IP lines cannot be consistently constructed on tetrapod extremities, and would have minimal functional relevance or predictive power in any case.     

The influence of cortical feature maps on the encoding of the orientation of a short line

The inhomogeneous distribution of the receptive fields of cortical neurons influences the cortical representation of the orientation of short lines seen in visual images. We construct a model of the response of populations of neurons in the human primary visual cortex by combining realistic response properties of individual neurons and cortical maps of orientation and location preferences. The encoding error, which characterizes the difference between the parameters of a visual stimulus and their cortical representation, is calculated using Fisher information as the square root of the variance of a statistically efficient estimator. The error of encoding orientation varies considerably with the location and orientation of the short line stimulus as modulated by the underlying orientation preference map. The average encoding error depends only weakly on the structure of the orientation preference map and is much smaller than the human error of estimating orientation measured psychophysically. From this comparison we conclude that the actual mechanism of orientation perception does not make efficient use of all the information available in the neuronal responses and that it is the decoding of visual information from neuronal responses that limits psychophysical performance. Action Editor: Terrence Sejnowski     

A model for learning human reaching movements

 Reaching movement is a fast movement towards a given target. The main characteristics of such a movement are straight path and a bell-shaped speed profile. In this work a mathematical model for the control of the human arm during ballistic reaching movements is presented. The model of the arm contains a 2 degrees of freedom planar manipulator, and a Hill-type, non-linear mechanical model of six muscles. The arm model is taken from the literature with minor changes. The nervous system is modeled as an adjustable pattern generator that creates the control signals to the muscles. The control signals in this model are rectangular pulses activated at various amplitudes and timings, that are determined according to the given target. These amplitudes and timings are the parameters that should be related to each target and initial conditions in the workspace. The model of the nervous system consists of an artificial neural net that maps any given target to the parameter space of the pattern generator. In order to train this net, the nervous system model includes a sensitivity model that transforms the error from the arm end-point coordinates to the parameter coordinates. The error is assessed only at the termination of the movement from knowledge of the results. The role of the non-linearity in the muscle model and the performance of the learning scheme are analysed, illustrated in simulations and discussed. The results of the present study demonstrate the central nervous system’s (CNS) ability to generate typical reaching movements with a simple feedforward controller that controls only the timing and amplitude of rectangular excitation pulses to the muscles and adjusts these parameters based on knowledge of the results. In this scheme, which is based on the adjustment of only a few parameters instead of the whole trajectory, the dimension of the control problem is reduced significantly. It is shown that the non-linear properties of the muscles are essential to achieve this simple control. This conclusion agrees with the general concept that motor control is the result of an interaction between the nervous system and the musculoskeletal dynamics. Received : 21 May 1996 / Accepted in revised form : 10 June 1997     

Reaction Times for Hand Movements Made in Response to Visual versus Vibratory Cues

Reaction times were determined for monkeys and humans who made wrist flexion and extension movements in response to vibratory and visual cues. Humans initiated movements approximately 50 msec sooner in response to vibratory as compared to visual cues. For monkeys, this difference was approximately 100 msec. Mean daily reaction times for monkeys and humans improved with practice until they reached a steady level of performance. Increased differences between vibratory and visual reaction times were weakly correlated with increased age of humans. The increase in the differences appeared to result from decreased reaction times by older subjects for vibratory-cued movements; reaction times for visually cued movements did not consistently vary across the age range of subjects tested (19-36 years). The results obtained using this novel paradigm suggest that it may be a useful tool for simultaneously testing behavioral performance or neurological function during somatosensorimotor and visuomotor tasks.     

Directional orientation in blue crabs, Callinectes sapidusRathbun: Escape responses and influence of wave direction

Blue crabs invade the upper intertidal at high tide and exhibit rapid, directed locomotory movements (i.e., escape response) which are significantly oriented offshore. The direction of wave surge and beach slope are conservative features of the intertidal, indicating offshore direction and can be used by blue crabs as orientation cues. Temporarily blinded crabs in the intertidal zone oriented down-slope (2–3°) and into the wave approach (up-wave) in experimental field test arenas. Up-wave orientation acuity increased in the presence of larger waves. Experiments in a wave test tank showed that orientation occurred over a wide range of surge velocities dependent upon threshold levels of surge displacement. We believe that the oriented response to wave surge serves as an important adaptation for movements by blue crabs in the nearshore zone.     

Reaction times for hand movements made in response to visual versus vibratory cues

Reaction times were determined for monkeys and humans who made wrist flexion and extension movements in response to vibratory and visual cues. Humans initiated movements approximately 50 msec sooner in response to vibratory as compared to visual cues. For monkeys, this difference was approximately 100 msec. Mean daily reaction times for monkeys and humans improved with practice until they reached a steady level of performance. Increased differences between vibratory and visual reaction times were weakly correlated with increased age of humans. The increase in the differences appeared to result from decreased reaction times by older subjects for vibratory-cued movements; reaction times for visually cued movements did not consistently vary across the age range of subjects tested (19-36 years). The results obtained using this novel paradigm suggest that it may be a useful tool for simultaneously testing behavioral performance or neurological function during somatosensorimotor and visuomotor tasks.