Subjective fixation disparity affected by dynamic asymmetry, resting vergence, and nonius bias.

PURPOSE: This study was undertaken to investigate how subjectively measured fixation disparity can be explained by (1) the convergent-divergent asymmetry of vergence dynamics (called dynamic asymmetry) for a disparity vergence step stimulus of 1° (60 arc min), (2) the dark vergence, and (3) the nonius bias. METHODS: Fixation disparity, dark vergence, and nonius bias were measured subjectively using nonius lines. Dynamic vergence step responses (both convergent and divergent) were measured objectively. RESULTS: In 20 subjects (mean age, 24.5 ± 4.3 years, visual acuity, ≥1.0; all emmetropic except for one with myopia, wearing contact lenses), multiple regression analyses showed that 39% of the variance in subjective fixation disparity was due to the characteristic factors of physiological vergence: dynamic asymmetry (calculated from convergent and divergent velocities), and dark vergence. An additional 23% of variance was due to the subjective nonius bias (i.e., the physical nonius offset required for perceived alignment of binocularly [nondichoptically] presented nonius lines). Together, these factors explained 62% of the interindividual differences in subjectively measured fixation disparity, demonstrating the influence of oculomotor and perceptual factors. CONCLUSIONS: Clinically relevant subjective fixation disparity originates from distinct physiological sources. Dynamic asymmetry in vergence dynamics, resting vergence, and nonius bias were found to affect fixation disparity directly, not only via changes in vergence dynamics.

Objective vs subjective measurements of dark vergence.

Dark vergence is a resting position of vergence (tonic vergence), measured in a dark visual field to eliminate fusional, accommodative, and proximal stimuli. The vergence resting position is relevant for measures of phoria and fixation disparity. Dark vergence differs reliably among subjects: the average subject converges at a viewing distance of about 1 m, while the inter-individual range is from infinity to about 40 cm. In previous research, dark vergence was measured subjectively, i.e. observers adjusted the horizontal offset of dichoptically presented nonius targets to perceived alignment. Results of such subjective vergence tests do not necessarily agree with those of the objective measurements of eye position with eye trackers. Therefore, we made simultaneous subjective and objective measurements of dark vergence and found similar results with both methods in repeated tests in two sessions. Thus, the nonius test is sufficient for a subjective estimation of dark vergence.

Magnetoneurographic registration of propagating magnetic fields in the lumbar spine after stimulation of the posterior tibial nerve.

Stimulation of the posterior tibial nerve has been associated with different somatosensory evoked potentials (SEP) recorded along the spine and thorax. The aim of this study was to register and describe the magnetic fields corresponding to different components of spinal SEP after stimulation of tibial nerves. In nine healthy subjects, right and left posterior tibial nerves were transcutaneously electrostimulated at the ankles. Neuromagnetic fields were registered over a circular 800 cm(2) area of the lumbosacral spine using a 61-channel biomagnetometer. Magnetic field maps were constructed and examined visually for dipolar patterns. Equivalent current dipoles (ECD) were calculated for each somatosensory evoked field (SEF) using a least-squares fit in a spherical model. In seven subjects dipolar SEF were detected over the lower back at two separate latencies and locations and propagating ECD could be localized. Both the first and second components found agreed anatomically and functionally with respect to propagation in the underlying nerve fibers. It was possible to record and identify SEF which correspond to the SEP described in the literature. Dipole localization based on an equivalent current dipole model allowed a basic evaluation of the plausibility of the measurements with respect to the processes being examined.

Measurement of refractive error and accommodation with the photorefractor PowerRef II.

The infrared photorefractor PowerRef II (PR II; PlusoptiX AG, Nurnberg, Germany) uses the principle of eccentric photorefraction. In eight subjects the mean non-cycloplegic refraction measured with the 'Full Scan' mode of the PR II at a far viewing distance (0.2 D) was significantly more hypermetropic by 0.6 D compared with subjective refraction. The mean accommodation differed by about this same amount between the PR II and the Canon R1 at three different viewing distances (3, 2 and 1 D). The PR II refraction at the 1 m reference distance was 0.25 D more hypermetropic compared with the subjective refraction at far (5 m); these measures were moderately correlated (r = 0.7). To determine temporal changes, the 'Dynamic Scan' mode was used over a 2-min period: the mean intraindividual standard deviation was 0.32 mm for pupil diameter and 0.29 D for accommodation, while the absolute measurement error of the 'Dynamic Scan' was found to be <0.12 D for the accommodation data. Interindividual reliabilities were satisfactory. However, the PR II did not provide a continuous stream of data and the specified sampling frequency of 25 Hz was rarely realized.

Effects of spatial attention on the brain stem frequency-following potential.

Can spatial attention or orienting affect human auditory information processing as peripheral as on the brain stem level? More specifically, is the reduction of the latency of the frequency-following potential (FFP; an evoked lower brain stem response) that we described in an earlier Neuroreport article really specifically attention-related? Here we demonstrate that, indeed, exogenous intramodal (auditory) spatial orienting, but not a transient modulation of general arousal, reduced the latency of the FFP by 27 micros; there were no effects on the FFP-amplitude. Although it might seem small, this reduction may be relevant in spatial hearing. We conclude that under certain conditions spatial attention can affect auditory information processing already on the brain stem level.

Short-term mobilization of processing resources is revealed in the event-related potential.

This study investigates whether an occasional effortful improvement of performance, as asked for by a precue, is reflected in event-related potential (ERP) changes. To estimate the limits of possible effort-induced behavioral and ERP changes, we manipulated the time between precue and imperative stimulus (IS; precue interval, PCI). The subjects could, in fact, improve their performance in the effort trials, with all but the shortest PCI. The postcue ERP revealed a fronto-central contingent negative variation (CNV), which was preceded by a frontal positive/occipital negative wave (P2/N2). Both the P2/N2 and the CNV were larger for effort than for standard trials for all PCIs. For the shortest PCI (300 ms), the CNV increase was seen after the IS. The CNV increase for PCIs 600 and 300 began at about 400 ms postcue. The results suggest that effortful performance improvement is associated with prior increase of a frontocentral CNV and a preceding P2/N2. The CNV increase is thought to reflect the activity of a frontal executive process by which additional processing resources can be mobilized on a trial-to-trial basis within less than 500 ms.