Normal Visual Conditions Research Articles

High Susceptibility to Experimental Myopia in a Mouse Model with a Retinal ON Pathway Defect

Nob mice share the same mutation in the Nyx gene that is found in humans with complete congenital stationary night blindness (CSNB1). Nob mutant mice were studied to determine whether this defect resulted in myopia, as it does in humans. Refractive development was measured in unmanipulated wild-type C57BL/6J (WT) and nob mice from 4 to 12 weeks of age by using an infrared photorefractor. The right eye was form deprived by means of a skull-mounted goggling apparatus at 4 weeks of age. Refractive errors were recorded every 2 weeks after goggling. The content of dopamine and the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were measured by HPLC with electrochemical detection (HPLC-ECD) in retinas of nob and WT mice under light- and dark-adapted conditions. The nob mice had greater hyperopic refractive errors than did the WT mice under normal visual conditions, until 12 weeks of age when both strains had similar refractions. At 6 weeks of age, refractions became less hyperopic in the nob mice but continued to become more hyperopic in the WT mice. After 2 weeks of form deprivation (6 weeks of age), the nob mice displayed a significant myopic shift (~4 D) in refractive error relative to the opposite and control eyes, whereas WT mice required 6 weeks of goggling to elicit a similar response. As expected with loss of ON pathway transmission, light exposure did not alter DOPAC levels in the nob mice. However, dopamine and DOPAC levels were significantly lower in the nob mice compared with WT. Under normal laboratory visual conditions, only minor differences in refractive development were observed between the nob and WT mice. The largest myopic shift in the nob mice resulted after form deprivation, suggesting that visual pathways dependent on nyctalopin and/or abnormally low dopaminergic activity play a role in regulating refractive development. These findings demonstrate an interaction of genetics and environment in refractive development.

Maintenance of upright stance in humans in normal and virtual visual environments

We recorded in 16 healthy subjects the sagittal and frontal components of the stabilogram when standing on a rigid motionless or movable (oscillating) platform under four conditions of visual control: (i) open eyes, OE; (ii) closed eyes, CE; (iii) central vision, CV, and (iv) virtual visual environment, VVE. Under the latter condition, subjects observed the 3D image of a room, which was generated by the computer; the image was adhered to head movements in such a manner that a peculiar connection for normal visual conditions between movements of the head and displacements of the visible visual environment was reproduced. Through a low-pass filtration of a trajectory of the center of pressure of feet (CPF), two elementary variables were received, horizontal motions of the center of gravity (CG) and the difference between the CPF and the CG (CPF-CG). Changes in these variables (CG and CPF-CG) were estimated using spectral analysis and subsequent calculation of the median frequency (MF) and root mean square value (RMS) of the spectra. The MFs of the spectra of the investigated variables were approximately identical under conditions of standing on oscillating and motionless supports and showed no clear dependence on various visual conditions. Unlike MFs, the RMSs of the spectra of body sways appeared more dependent on changes of conditions of standing and the mode of visual control (differing from each other in the higher sensitivity to modifications of conditions of standing and visual control). With standing on the motionless support, the RMSs of the spectra of both variables were the greatest under VVE and CE conditions and the smallest under OE condition. The body oscillations were considerably amplified under conditions of standing on a movable support, and a different pattern of visual influences on the RMS of both investigated variables was revealed. The RMSs had the greatest value under CE condition and were much smaller (50–40%) under other visual conditions, including the VVE condition. Therefore, our findings show that, under VVE conditions, visual sensory afferentation is ignored by cerebral structures controlling postural adjustments if standing occurs on a motionless support but is effectively used at the maintenance of upright stance on an oscillating support.

Basketball Jump Shooting Is Controlled Online by Vision

An experiment was conducted to examine whether basketball jump shooting relies on online visual (i.e., dorsal stream-mediated) control rather than motor preprogramming. Seventeen expert basketball players (eight males and nine females) performed jump shots under normal vision and in three conditions in which movement initiation was delayed by zero, one, or two seconds relative to viewing the basket. Shots were evaluated in terms of both outcome and execution measures. Even though most shots still landed near the basket in the absence of vision, end-point accuracy was significantly better under normal visual conditions than under the delay conditions, where players tended to undershoot the basket. In addition, an overall decrease of inter-joint coordination strength and stability was found as a function of visual condition. Although these results do not exclude a role of motor preprogramming, they demonstrate that visual sensory information plays an important role in the continuous guidance of the basketball jump shot.

The effect of visual transformation on bimanual circling movement

In order to examine the effect of visual transformation on bimanual movements of various difficulty, fourteen participants performed bimanual circling tasks in three asymmetric movement modes--90 degrees (the left hand precedes the right hand by 1/4 cycle), 180 degrees (the delay between two hands is 1/2 cycle), and 270 degrees (the left hand precedes the right hand by 3/4 cycle)--under the normal vision condition and the visual transformation condition. In the visual transformation condition, movement of the right hand was transformed so that the required bimanual movement was always presented visually as a symmetric pattern. Additionally, the participants also performed a 0 degrees mode (in-phase symmetric) movement. Results revealed that the visual transformation increased the movement accuracy, with the variability of the right-left difference unchanged. Thus, proper visual transformation can improve the accuracy of a movement task. The 0 degrees mode was performed with higher stability and accuracy than any other movement modes of the visual transformation condition and normal vision conditions. In addition, the constant error associated with the 90 degrees and 270 degrees modes indicated that, in the normal vision condition, the executed movement was shifted to the 180 degrees mode, whereas in the visual transformation condition it stayed around the required mode and was slightly shifted to the 0 degrees mode. This result suggests that visual transformation can change the relationship between the intention to realize the required mode and the intrinsic neuromuscular dynamics. The effect size of visual transformation was larger in the 90 degrees and 270 degrees modes than in the 180 degrees mode. It is thus concluded that the effect of visual transformation depends upon the difficulty of the movement task.

2.6 The elderly combat to strive for better physical health

Previous studies have demonstrated that stimulating the cutaneous plantar sensory receptors of the foot through textured insoles improves human balance and walking. This study investigated the effect of medial and lateral zoned textured insoles using tibialis anterior/peroneus longus surface electromyographic activity and Centre-of-Pressure as indicators of postural stability while walking.15 asymptomatic subjects were tested using a within-subject randomised repeated measures design. The effect of lateral and medial zoned insoles of varying heights (control, 2, 4 and 6 mm) on stability while walking under normal and impaired visual conditions was assessed.Impaired vision resulted in an increase in foot CoP variability while walking (p < 0.05). The laterally zoned insole was associated with a significant (repeated measures ANOVA p < 0.05) increase in the rate of medial–lateral CoP change.These findings suggest that the site of stimulation of the plantar foot cutaneous receptors may increase postural instability during walking. This should be considered in the design of insoles that aim to improve balance and reduce falls risk. The importance of vision in balance control has been highlighted and using impaired vision may serve as a way of trialling clinical products in the healthy population.

Effect of augmented visual feedback from a virtual reality simulation system on manual dexterity training

Little research has been published about the impact of simulation technology on the learning process of novel motor skills. Especially the role of augmented feedback (FB) on the quality of performance and the transfer of the acquired behaviour to a no-augmented FB condition require further investigation. Therefore, novice dental students were randomly assigned to one of three groups and given the task of drilling a geometrical class 1 cavity. The FB group trained under augmented visual FB conditions, provided by the virtual reality (VR) system (DentSim). The no-FB group practised under normal vision conditions, in the absence of augmented FB. A control group performed the test sessions without participating in any training programme. All preparations were evaluated by the VR grading system according to four traditional (outline shape, floor depth, floor smoothness and wall inclination), and two critical, criteria (pulp exposure and damage to adjacent teeth). Performance analyses revealed an overall trend towards significant improvement with training for the experimental groups. The FB group obtained the highest scores. It scored better for floor depth (P < 0.001), whilst the no-FB group was best for floor smoothness (P < 0.005). However, at the retention tests, the FB group demonstrated inferior performance in comparison with the no-FB group. The transfer test on a traditional unit revealed no significant differences between the training groups. Consequently, drilling experience on a VR system under the condition of frequently provided FB and lack of any tutorial input was considered to be not beneficial to learning. The present data are discussed in view of the guidance hypothesis of FB, which refers to the apprentice's dependence on FB.

Visual afferent information dominates other sources of afferent information during mixed practice of a video-aiming task

The goal of the present study was to determine whether learning of a manual video-aiming task that would have to be performed in a target-only condition (transfer test) would be improved, unaffected or hindered by alternating practice between a normal vision and a target-only condition. The results of the present study indicate that a mixed schedule of practice hindered learning of the task. Specifically, participants who practiced the task in the mixed schedule showed larger direction constant error in transfer than participants who practiced only in the target-only condition. The results suggest that participants in the mixed condition continued to rely on the rapidly decaying information that was extracted in the normal vision condition to plan and control their movement in the target-only condition, which ultimately led to biased movement endpoints.

Learning a new bimanual coordination pattern is influenced by existing attractors.

The present study investigates whether the acquisition of a rhythmical bimanual coordination pattern is influenced by existing intrinsic coordination tendencies. Participants were required to learn 1 of 5 new coordination patterns, whose relative phase phi was either 36, 60, or 90 degrees away from the 0 degree and 180 degree attractors, respectively. They performed 35 trials, each consisting of 2 conditions: In the augmented feedback condition, continuous visual guidance was provided, while in the normal feedback condition participants were required to rely on normal vision of their arms. We found that all to-be-learned patterns were performed with higher accuracy in the visually guided condition, whereas interference with pre-existing coordination tendencies was more pronounced in the normal vision condition. Comparing the learning progress of the 5 groups, we found for patterns close to anti-phase, a smaller improvement and significantly larger phase errors than for patterns close to in-phase. This indicates that the acquisition of a new phase relationship is influenced by existing attractors and that the 180 degree attractor interfered more strongly with the to-be-learned pattern than the 0 degree attractor.

Coordination of upper and lower limb segments: deficits on the ipsilesional side after unilateral stroke.

Coordination of the ipsilateral limbs was studied in unilateral stroke patients and a control group of healthy age-matched controls. Cyclical single-limb movements of the forearm and lower leg as well as their coordination, with the segments moving either in the same (isodirectional) or in different directions (nonisodirectional), were investigated under normal vision and blindfolded conditions. Findings revealed that stroke patients experienced difficulties with coordination of the limb segments on the ipsilesional side and this effect was more pronounced during nonisodirectional than during isodirectional coordination. In addition, cycle durations were larger and movement amplitudes shorter in stroke patients as compared to controls. Overall, the present findings clearly demonstrated motor control deficits in stroke patients on the so-called "unaffected side." The availability of normal vision did not alleviate these deficits. Therefore, the more general implication of the present findings appears to be that interlimb coordination is a complex function, requiring the integrity of both hemispheres. Comparison of the left- and right-hemispheric stroke groups revealed that patients with a left-hemisphere lesion tended to be more variable in performing the more difficult nonisodirectional pattern than patients with a right-hemisphere lesion. This possibly hints at a more pronounced involvement of the left hemisphere in the organization of ipsilateral coordination. The spatiotemporal features of movement (cycle duration, amplitude), however, did not differ between both stroke groups.

Development of Multiple Movement Representations With Practice: Specificity Versus Flexibility

The question addressed in the present experiment was whether an individual who practices a task under different conditions of afferent information develops different movement representations, each of which is based on the most accurate source of afferent information for movement control. In Experiment 1, participants (N = 23) performed a manual aiming movement in a target-only condition for 520 trials before performing in a normal vision condition for an equivalent amount of practice. Control groups performed all practice trials in either a normal vision or a target-only condition. The results revealed that the movement representation developed in the initial (target-only) practice phase remained accessible for movement planning and control. The results of Experiment 2 indicated, however, that participants did not maintain such a representation when their initial practice in the target-only condition was reduced (40 or 160 trials) before they had extensive practice in normal vision. Those results indicate that extensive practice in a target-only and then in a normal vision condition enables an individual to plan and control his or her movement on the basis of the most efficient source of available afferent information. Because visual afferent information provides optimal information for ensuring movement accuracy, however, if initial practice in the target-only condition is only modest or moderate it is likely that that information source will progressively dominate all other sources of afferent information for movement planning and control.

Motor learning and Parkinson’s disease: refinement of within-limb and between-limb coordination as a result of practice

Even though the basal ganglia have been assigned a role in motor learning, few studies have addressed motor learning capabilities in Parkinson patients. In the present experiment, improvement of bimanual figure drawing across practice was compared between Parkinson patients and normal age-matched controls. At regular intervals during acquisition, performance was assessed under normal vision and blindfolded conditions. At initiation of practice, the typical signs associated with Parkinson’s disease became evident, such as bradykinesia and hypometria. Moreover, reduced synchronization between the force–time specifications of both limbs was observed. When vision was withdrawn, Parkinson patients showed a larger drift of drawing performance across the workspace, indicative of a decline in proprioception. In spite of the aforementioned deficits, Parkinson patients made marked improvements in the speed of execution, the consistency of the spatial trajectories, and the synchronization between the limbs across practice, even though they never reached the performance levels obtained in elderly controls. The findings demonstrate that Parkinson patients do benefit from practice to refine their upper limb control and to alleviate their most basic motor deficits.

Practice Does Not Diminish the Role of Visual Information in On-Line Control of a Precision Walking Task: Support for the Specificity of Practice Hypothesis

It has been proposed that motor learning is specific to the sources of afferent information available during practice (Proteau, 1992). That hypothesis has obtained support from studies of aiming but not from studies of gross motor skills. The many procedural differences between the two sets of studies might have caused the conflicting results. In the present investigation of a precision walking task, the validity of the specificity of practice hypothesis was tested. Thirty-two participants were asked to walk for 20 m on a 2.5-cm-wide line. Subjects practiced the task for either 20 or 100 trials under normal visual conditions or while blindfolded. Following acquisition, all subjects performed the task for 20 additional trials while blindfolded and without knowledge of results. Practice improved the performance of blindfolded participants. Withdrawing vision in transfer resulted in a large and significant increase in error, however, a finding that supports the specificity of practice hypothesis.

Vestibulospinal adaptation to microgravity.

Human balance control is known to be transiently disrupted after spaceflight; however, the mechanisms responsible for postflight postural ataxia are still under investigation. In this report, we propose a conceptual model of vestibulospinal adaptation based on theoretical adaptive control concepts and supported by the results from a comprehensive study of balance control recovery after spaceflight. The conceptual model predicts that immediately after spaceflight the balance control system of a returning astronaut does not expect to receive gravity-induced afferent inputs and that descending vestibulospinal control of balance is disrupted until the central nervous system is able to cope with the newly available vestibular otolith information. Predictions of the model are tested using data from a study of the neurosensory control of balance in astronauts immediately after landing. In that study, the mechanisms of sensorimotor balance control were assessed under normal, reduced, and/or altered (sway-referenced) visual and somatosensory input conditions. We conclude that the adaptive control model accurately describes the neurobehavioral responses to spaceflight and that similar models of altered sensory, motor, or environmental constraints are needed clinically to predict responses that patients with sensorimotor pathologies may have to various visual-vestibular or changing stimulus environments.

Vestibulospinal adaptation to microgravity

Human balance control is known to be transiently disrupted after spaceflight; however, the mechanisms responsible for postflight postural ataxia are still under investigation. In this report, we propose a conceptual model of vestibulospinal adaptation based on theoretical adaptive control concepts and supported by the results from a comprehensive study of balance control recovery after spaceflight. The conceptual model predicts that immediately after spaceflight the balance control system of a returning astronaut does not expect to receive gravity-induced afferent inputs and that descending vestibulospinal control of balance is disrupted until the central nervous system is able to cope with the newly available vestibular otolith information. Predictions of the model are tested using data from a study of the neurosensory control of balance in astronauts immediately after landing. In that study, the mechanisms of sensorimotor balance control were assessed under normal, reduced, and/or altered (sway-referenced) visual and somatosensory input conditions. We conclude that the adaptive control model accurately describes the neurobehavioral responses to spaceflight and that similar models of altered sensory, motor, or environmental constraints are needed clinically to predict responses that patients with sensorimotor pathologies may have to various visual-vestibular or changing stimulus environments. (Otolaryngol Head Neck Surg 1998;118:S39-S44.)

The effect of task constraints on the manipulation of visual information and the implications for the specificity of learning hypothesis

The specificity of learning hypothesis predicts that the removal and addition of vision causes a deterioration in aiming performance, and further that this detrimental effect increases as a function of practice (Proteau, 1992). The purpose of this study was to determine the effects of the addition and removal of visual information from a task emphasising the directional component of the movement. Subjects practised the task for a moderate and extensive number of trials in a target-only or normal vision condition. Following each practice phase, subjects were transferred to the other condition. The data indicated that while subjects' directional aiming error increased following the removal of vision, there were no detrimental effects following its addition. In fact, directional error was reduced when transferring to the normal vision condition. These positive effects of the addition of visual information relating to the directional component are not consistent with the present version of the specificity of learning hypothesis.

Biomechanical and perceptual determinants of drawing angles

This study focuses on perceptual and biomechanical determinants of the kinematics of angular drawing movements. Two experiments are reported in which twelve righthanded adults were asked to draw geometrical patterns consisting of three segments comprising either two acute or two obtuse angles. In Experiment 1, a lower frequency of pauses was observed in acute patterns and their segment length tended to be overestimated. The former effect is attributed to the exploitation of elasticity (Guiard, 1993). In order to evaluate whether the latter effect was due to perceptual factors, a second experiment was conducted. Twelve subjects drew a a subset of the angular patterns under normal visual conditions and under conditions in which they could neither see their moving limb nor the resulting drawing trace. Again, subjects produced more pauses at obtuse than at acute angles and tended to overestimate the segment length in acute patterns. It is concluded that pauses are likely to occur between segments of discrete movement sequences when potential energy needs to be dissipated. When conditions arise that allow subjects to exploit elasticity, however, segment length tends to increase. The results of Experiment 2 confirm that these phenomena are independent from visual perception.

Modifications with aging in the role played by vision and proprioception for movement control.

Young and older adults performed manual aiming movements to a visible target for either 40 or 200 trials. Under each level of practice, half of the subjects practiced the task under normal visual conditions (proprioception + vision [PV] condition), whereas the other half were not permitted to see their ongoing movement toward the target (proprioception-only [P] condition). Each acquisition trial was followed with knowledge of results (KR). After the last acquisition trial, all subjects were transferred to a common task in which only the target to be reached was visually available, with no KR. During acquisition, the younger subjects were found to be spatially more accurate than their older counterparts, and this was so regardless of the number of acquisition trials. Withdrawing KR during the transfer test did not modify the spatial accuracy of the subjects who had trained under the P condition. This indicates that the subjects had a reliable reference of the movement to be realized. Withdrawing vision of the moving hand and KR in the transfer test caused a significant increase in spatial error for both the older and the younger subjects. However, the increase in error was less pronounced for the older than for the younger subjects. In fact, the older subjects performed as well in the transfer test as the subjects who had trained in the P condition. This pattern of results suggests that in the transfer test, the older subjects could still guide their movements with the proprioceptive information that was available during both acquisition and transfer. However, such was not the case for the younger subjects. This suggests that, unlike the younger subjects, the older subjects could still rely on the proprioceptive cues available during acquisition in the PV condition. These results are taken to indicate that practicing with numerous sources of afferent information, as was the case in the PV condition, resulted in an integrated reference store for the younger subjects. In contrast, while practicing the task in the PV condition, the older subjects appeared to process independently from each other the different sources of sensory information available.

Postural Sway Characteristics of the Elderly Under Normal and Altered Visual and Support Surface Conditions

One of the most pervasive findings in the literature on the aged is the general slowing of cognitive-motor responses with advancing age. Hence, an increased slowness in the processing of information from vestibular, visual, and somatosensory systems could contribute greatly to a decline in postural stability. To examine this question, in a cross-sectional investigation, postural sway behavior of elderly (n = 18) and young (n = 10) adults was examined under conditions that stressed the slower integrative mechanisms rather than the reflexive mechanisms of postural control. The postural sway behavior of young and elderly subjects was examined for a prolonged duration (80 s), under altered visual and/or support surface (5 cm thick foam surface) conditions, and contrasted with normal stance. Results showed that the exclusion or disruption of one of the sensory inputs, alone, was not consistently sufficient to differentiate between elderly and young adults, because of compensation by the remaining sensory sources. Both alterations together (i.e., visual and surface), however, had a substantially greater effect upon the elderly than the young.

Stabilometry in balance assessment of dizzy and normal subjects

Normal adults and patients referred to the Dizzy Clinic at the Medical College of Ohio had their standing balance assessed during combinations of normal and altered visual and somatosensory orientation conditions using a fixed-force platform to measure center-of-pressure translations. Significant differences were identified between normal subjects and dizzy patients, depending on the particular diagnostic category, the sensory condition tested, and the particular sway component being measured. Patients with central and peripheral vestibular dysfunctions had significantly greater sway than all other categories in most test conditions, especially with eyes closed and with a visual conflict dome while standing on a foam surface. The central vestibular dysfunction and peripheral vestibular dysfunction groups could be differentiated statistically under eyes-closed and visual conflict-foam conditions. The normal and psychogenic groups could not be differentiated statistically for any test conditions except one: there was significantly greater mean anterior/posterior sway displacement in the psychogenic group compared with all other diagnostic categories for the eyes-open foam test condition. Our results indicate that static stabilometry recordings of postural sway can be used to evaluate and quantify a dizzy patient's ability to receive and process vestibular, visual, and somatosensory-proprioceptive cues for postural stability. It can also be used to monitor patients with vestibular disorders and to document their responses to rehabilitation programs.

Receptive field sizes and responsiveness to light in area 18 of the adult cat after chiasmotomy. Postoperative evolution; role of visual experience.

Receptive field sizes to stimulation of the ipsilateral temporal retina were studied in area 18 of adult cats at different times after complete midsagittal section of the optic chiasm. A specific postoperative evolution could thus be noticed: immediately after section, the average area of the receptive fields was reduced, as compared to control preparations, owing to the disappearance of large fields located at more than 20 degrees of eccentricity. A progressive reappearance of these large fields occurred between 8 and 45 days after chiasmotomy, provided that the animal was placed in normal visual conditions during its postoperative period. No such recovery could be assessed after as long as 55 days, in animals kept in complete darkness after operation. Chiasmotomized cats also displayed a reduction of their percentage of light reactive cells with respect to controls, as expected from the suppression of the contralateral input. This percentage was at first very low and progressively increased, during postoperative recovery but again not when the animal had been kept in the dark. Finally, an increase of cells with "diffuse responses" was observed in the late postoperative recovery stage. This latter evolution also appeared to depend upon postoperative visual experience. On the other hand, no clear indication of an interhemispheric transfer could be obtained in these experiments, even at the 17-18 boundary.