Long-Term Effects of Serial Anodal tDCS on Motion Perception in Subjects with Occipital Stroke Measured in the Unaffected Visual Hemifield.

Transcranial direct current stimulation (tDCS) is a novel neuromodulatory tool that has seen early transition to clinical trials, although the high variability of these findings necessitates further studies in clinically relevant populations. The majority of evidence into effects of repeated tDCS is based on research in the human motor system, but it is unclear whether the long-term effects of serial tDCS are motor-specific or transferable to other brain areas. This study aimed to examine whether serial anodal tDCS over the visual cortex can exogenously induce long-term neuroplastic changes in the visual cortex. However, when the visual cortex is affected by a cortical lesion, up-regulated endogenous neuroplastic adaptation processes may alter the susceptibility to tDCS. To this end, motion perception was investigated in the unaffected hemifield of subjects with unilateral visual cortex lesions. Twelve subjects with occipital ischemic lesions participated in a within-subject, sham-controlled, double-blind study. MRI-registered sham or anodal tDCS (1.5 mA, 20 min) was applied on five consecutive days over the visual cortex. Motion perception was tested before and after stimulation sessions and at 14- and 28-day follow-up. After a 16-day interval an identical study block with the other stimulation condition (anodal or sham tDCS) followed. Serial anodal tDCS over the visual cortex resulted in an improvement in motion perception, a function attributed to MT/V5. This effect was still measurable at 14- and 28-day follow-up measurements. Thus, this may represent evidence for long-term tDCS-induced plasticity and has implications for the design of studies examining the time course of tDCS effects in both the visual and motor systems.

Orientation specificity of learning vernier discriminations.

Orientation selective neurons in the primary visual cortex typically respond to a range of orientations that covers 20 degrees or more, while in psychophysical experiments, orientation bandwidth is often clearly narrower. Here, we measure the orientation specificity of perceptual learning for vernier discriminations. More than 70 observers, in separate groups, practiced a vernier discrimination task with a constant stimulus orientation. After a 1h session of training, the vernier was rotated by 2 degrees, 4 degrees, 10 degrees, 20 degrees, 45 degrees or 90 degrees. Improvement through training in the first session transferred to the second session (tested on the next day) up to 10 degrees of stimulus rotation. We found no transfer for rotations of 20 degrees, 45 degrees and 90 degrees. Hence, the orientation half-bandwidth of perceptual learning is around 15 degrees, leading to a bandwidth of 30 degrees and corresponding to that of single neurons in early visual cortices, while being narrower than that in higher cortical areas.

[Functional vision training after MIOL implantation]. / Funktionstraining nach MIOL-Implantation.

In a prospective and intraindividually comparative study, we investigated visual performance after implantation of two multifocal intraocular lenses. Eight patients received the ReSTOR (Alcon) bilaterally, and eight patients received the Tecnis ZM900 (AMO). The nondominant eye of each patient was trained based on the concept of perceptual learning using orientation discrimination of near-vertical bars. The fellow eye served as a control. After 3 h of training distributed over 2 weeks, the mean improvement of orientation discrimination in the trained eyes was 44%, which was significantly higher compared with the control eyes (9%). Moreover, contrast sensitivity and near vision under different contrast levels showed a significant benefit of training. These results support the concept of improving visual function by training after multifocal lens implantation. The superior function of the trained eyes was still present at the 6-month follow-up.

[Colour vision and acquired colour vision disturbances. Part I: basic aspects]. / Farbensehen und erworbene Farbsinnstörungen. Teil I: Grundlagen.

Colour vision is the most sensitive sensory ability of the human eye, making it possible to distinguish several million nuances of colour. The physiology of colour vision has meanwhile been researched in depth, including the genetic and biochemical principles. This knowledge has facilitated a better understanding of the results of clinical tests on colour vision. These clinical tests provide useful information on the aetiology of very different clinical pictures in ophthalmology and as such are important for the diagnosis of these diseases. Acquired colour vision deficiencies in patients with systemic vascular disease are early signs of dysfunctional microcirculation and play a role in the early diagnostic work-up. Part I of this review summarizes the basic principles of colour vision and its disturbances. Congenital and acquired colour vision disturbances are distinguished. The second part then describes the most commonly employed examination procedures to assess colour vision.

[Central vision]. / Die zentrale Sehfunktion.

The clinical assessment of vision by means of optotypes does by no means test just two-point resolution, since a correct naming of the letters or digits requires a preceding visual object recognition. Cortical lesions can massively deteriorate vision up to a "Seelenblindheit" in spite of intact optics and retina. There are different processing levels involved in the analysis which can be individually defective, leading to disorders from visual indiscrimination to agnosia or anomia.

Grasp effects of the Ebbinghaus illusion: obstacle avoidance is not the explanation.

The perception-versus-action hypothesis states that visual information is processed in two different streams, one for visual awareness (or perception) and one for motor performance. Previous reports that the Ebbinghaus illusion deceives perception but not grasping seemed to indicate that this dichotomy between perception and action was fundamental enough to be reflected in the overt behavior of non-neurological, healthy humans. Contrary to this view we show that the Ebbinghaus illusion affects grasping to the same extent as perception. We also show that the grasp effects cannot be accounted for by non-perceptual obstacle avoidance mechanisms as has recently been suggested. Instead, even subtle variations of the Ebbinghaus illusion affect grasping in the same way as they affect perception. Our results suggest that the same signals are responsible for the perceptual effects and for the motor effects of the Ebbinghaus illusion. This casts doubt on one line of evidence, which used to strongly favor the perception-versus-action hypothesis.

Feature-specific electrophysiological correlates of texture segregation.

Discrimination between a figure and its surround is an important first step of pattern recognition. This discrimination usually relies, as a first step, on the detection of borders between a figure and its surround, for example based on spatial gradients in luminance, colour, or texture. There is evidence that neurones in the visual cortex are specifically activated by segregation between textures, but the relation between segregation based on different types of features such as colour, luminance, and motion is unclear. Evoked EEG potentials specific to texture segregation were investigated in 17 observers in two separate experiments and by means of functional magnetic resonance imaging in a separate study (Fahle et al., in preparation). Differences in either luminance, colour, line orientation, motion, or stereoscopic depth defined a checkerboard pattern. Patterns defined by each of these features elicited segregation-specific potentials. In contrast to earlier reports (Vision Research 37 (1997) 1409), however, we find pronounced differences between the segregation-specific potentials evoked through different features, especially regarding their peak latencies. The topographical distribution of the activity evoked reveals different polarities and partly specific locations for different stimulus features, indicating the existence of different processors for texture segregation based on different features.

Discriminating contrast discontinuities: asymmetries, dipper functions, and perceptual learning.

In a visual search task, a target has to be found among distractors. For two given elements A and B, the search difficulty can depend on which of the two elements is defined as the target, a phenomenon called search asymmetry. Here, we study to what degree an element's ability to 'win' in a search asymmetry depends on its absolute contrast (first-stage signal) and to what degree it depends on its contrast difference from the background (second-stage signal). One quadrant contained a target texture (2 x 2 Gabor patches of contrast c(tg)), and the other three quadrants contained distractor textures (2 x 2 Gabor patches of contrast c(dt)). These four 'foreground textures' were embedded in a background texture consisting of patches with contrast c(bg). The task was to identify which quadrant contained the target. Quadrants are referred to as increments (foreground contrast c(fg)>c(bg)), or decrements (c(fg)

Spatiotemporal interpolation and quality of apparent motion.

We investigate the conditions under which stimuli in apparent (sampled) motion are indistinguishable from those in smooth motion and compare this discrimination with the precision achieved by the visual system in interpolating apparent motion. In an initial experiment, observers were required to discriminate smooth from apparent motion, at variable step sizes, contrasts, velocities, and stimulus types (broadband line or bar stimuli and grating patches of different spatial frequency). Thresholds for discriminating smooth from sampled motion were approximately 40 arc min under optimal conditions, corresponding to the diameter of foveal photoreceptors. The tolerated step size between stations increased with velocity, more so for low- than for high-spatial-frequency stimuli. Tolerated step size decreased with presentation duration and with stimulus contrast. A separate experiment examined precision of interpolation. Vernier offsets were produced through temporal delays along the trajectory of an apparent motion, and thresholds for the discrimination of direction of offset were measured as a function of speed of motion and of distance between stations of apparent motion. Perfect interpolation was achieved for distances between stations of approximately 2 arc min. A model based on spatiotemporal filtering at an early stage of processing accounts well for the results of both types of experiments.

Effects of visual illusions on grasping.

In 2 experiments, the Muller-Lyer illusion (F. C. Muller-Lyer, 1889; N = 16) and the parallel-lines illusion (W. Wundt, 1898; N = 26) clearly affected maximum preshape aperture in grasping (both ps < .001). The grasping effects were similar but not perfectly equal to the perceptual effects. Control experiments show that these differences can be attributed to problems in matching the perceptual task and the grasping task. A model is described stating the assumptions that are needed to compare the grasping effects and the perceptual effects of visual illusions. Further studies on the relationship between perception and grasping are reviewed. These studies provide no clear evidence for a dissociation between perception and grasping and therefore do not support the action versus perception hypothesis (A. D. Milner & M. A. Goodale, 1995).

Spatial aspects of object formation revealed by a new illusion, shine-through.

When a vernier stimulus is presented for a short time and followed by a grating comprising five straight lines, the vernier remains invisible but may bequeath its offset to the grating (feature inheritance). For more than seven grating elements, the vernier is rendered visible as a shine-through element. However, shine-through depends strongly on the spatio-temporal layout of the grating. Here, we show that spatially inhomogeneous gratings diminish shine-through and vernier discrimination. Even subtle deviations, in the range of a few minutes of arc, matter. However, longer presentation times of the vernier regenerate shine-through. Feature inheritance and shine-through may become a useful tool in investigating such different topics as time course of information processing, feature binding, attention, and masking.

Shine-through: temporal aspects.

If a vernier stimulus precedes a grating for a very short time, the vernier either remains invisible, but may bequeath some of its properties to the grating (feature inheritance), or might shine through keeping its features - depending on the number of grating elements [Herzog, M. H. & Koch, C., 2001. Seeing properties of an invisible element: feature inheritance and shine-through. Proceedings of the National Academy of Science USA 98, 4271-4275]. Feature inheritance and shine-through represent two different states of feature binding [Herzog, M. H., Koch, C., & Fahle, M., Switching binding states. Visual Cognition (in press)], whereas shine-through depends in subtle ways on the spatial layout of the grating [Herzog, M. H., Fahle, M., & Koch, C., (2001). Spatial aspects of object formation revealed by a new illusion, shine-through Vision Research]. Here, we show that also temporal parameters of the grating influence shine-through. For example, a delayed presentation of certain grating elements can deteriorate performance dramatically.

Perceptual deformation induced by visual motion.

The perceived position of a moving object can be misleading because the object has advanced while its previous retinal image has been transmitted through the visual stream, leading to a mismatch between actual location and its neural representation. It has been suggested that the human visual system compensates for neural processing delays to retrieve instantaneous position. However, such a mechanism would require a precise measure of the actual delay in order to provide a reliable position estimate. A novel illusory deformation of moving contours demonstrates that humans misjudge the spatial relationship between parts of coherently moving targets, and therefore do not perfectly account for neural delays. The size of this deformation increases with growing speed. In some subjects this illusion can be reversed by varying the luminance of individual dots; a manipulation that affects the neural delays. Our experiments agree with other evidence that the capacity of the visual system to compensate for processing delays is limited.

Purely temporal figure-ground segregation.

Visual figure-ground segregation is achieved by exploiting differences in features such as luminance, colour, motion or presentation time between a figure and its surround. Here we determine the shortest delay times required for figure-ground segregation based on purely temporal features. Previous studies usually employed stimulus onset asynchronies between figure- and ground-containing possible artefacts based on apparent motion cues or on luminance differences. Our stimuli systematically avoid these artefacts by constantly showing 20 x 20 'colons' that flip by 90 degrees around their midpoints at constant time intervals. Colons constituting the background flip in-phase whereas those constituting the target flip with a phase delay. We tested the impact of frequency modulation and phase reduction on target detection. Younger subjects performed well above chance even at temporal delays as short as 13 ms, whilst older subjects required up to three times longer delays in some conditions. Figure-ground segregation can rely on purely temporal delays down to around 10 ms even in the absence of luminance and motion artefacts, indicating a temporal precision of cortical information processing almost an order of magnitude lower than the one required for some models of feature binding in the visual cortex [e.g. Singer, W. (1999), Curr. Opin. Neurobiol., 9, 189-194]. Hence, in our experiment, observers are unable to use temporal stimulus features with the precision required for these models.

Perceptual learning: psychophysical thresholds and electrical brain topography.

We studied perceptual learning by determining psychophysical discrimination thresholds for visual hyper acuity targets (vernier stimuli) as a function of stimulus orientation. One aim was to relate perceptual improvements to changes of electrophysiological activity of the human brain. A group of 43 healthy adults participated in a psychophysical experiment where vernier thresholds for vertical and horizontal vernier targets were compared. In 16 subjects thresholds were measured for each orientation twice at an interval of 25 min. Between threshold estimations, evoked brain activity was recorded from 30 electrodes over the occipital brain areas while the subjects observed appearance and disappearance of supra-threshold vernier offsets. Mean evoked potentials were computed for the first and second 600 stimulus presentations, and the scalp topography of electrical brain activity was analyzed. Vertically oriented stimuli yielded significantly better performance than horizontal targets, and thresholds were significantly lower in the second half of the experiment, i.e. after prolonged viewing of stimuli. The improvements in discrimination performance were specific for stimulus orientation and did not generalize. Learning effects were also observed with electrical brain activity, and field strength of the potentials increased significantly as a function of time. Scalp topography of the evoked components was significantly affected indicating a shift of activation between different neuronal elements induced by perceptual learning.

Visual performance and recovery in recently detoxified alcoholics.

In order to assess the impact of chronic alcohol misuse on basic visual functions, we investigated motion perception, visual short-term memory, and visual divided attention in recently detoxified patients and matched controls by means of visual psychophysical tasks. Subjects were tested twice within the first 3 weeks of detoxification in order to assess the potential recovery of visual performance. Patients demonstrated significant impairments in visual perception of coherent motion for slow, but not faster, speeds, and in speed discrimination as assessed by random dot kinematograms. Visual short-term memory tested with a delayed vernier discrimination task, on the other hand, was not significantly affected in patients. When processing hierarchical letters, a divided attention task, detoxified patients showed neither impairments in overall attentional capacity nor attentional allocation, but slightly enhanced interference of global information on local target processing. The results of the visual divided attention task contradict the predictions of the 'right hemisphere' hypothesis of alcoholism: global target information - mediated by the right hemisphere - was not only accessible to detoxified patients, but seemed to exert an even greater influence on local processing during early detoxification, than in matched controls. Limited recovery within the first 3 weeks was seen only in visual speed discrimination. Recently detoxified patients revealed deficits similar to intoxicated social drinkers in identical tests of visual perception of motion, but not visual short-term memory.

Summation of texture segregation across orientation and spatial frequency: electrophysiological and psychophysical findings.

Objects are usually segregated from ground by several visual dimensions. We studied texture segregation in checkerboards defined by gradients in spatial frequency, orientation or both frequency and orientation, using Gabor-filtered noise patterns. Saliency was measured electrophysiologically using the visual evoked potential (VEP) associated with texture segregation ('tsVEP') (an associated component in the visual evoked potential), and psychophysically by a 2AFC task. Spatial frequency and orientation stimuli evoked percepts of texture segregation and tsVEPs in all 11 subjects. The tsVEPs to combined stimuli were larger than those to each dimension alone, but smaller (74%) than the algebraic sum of tsVEPs to both individual dimensions. Psychophysical detection rates differed significantly between all conditions (P < 0.001), with highest rates for the combined stimuli. The findings suggest that segregation based on a combination of 'orientation' and 'spatial frequency' is more salient than that based on either of these alone. The significant deviation from full additivity in the tsVEPs suggests that simultaneous contrasts in spatial frequency and orientation have a common processing stage.

Parallel detection of orientation differences in the presence of orientation gradients.

We measured thresholds and reaction times for detecting a target, defined by orientation contrast, as a function of the number of elements displayed simultaneously and of the linear orientation gradient present in the display. This test served to evaluate how well the human visual system is able to ignore smooth gradients in orientation - similar to what it does with gradients of luminance or wavelength. Smooth orientation gradients are common in natural environments as opposed to the usual laboratory (search) experiments. It turns out that targets defined by a discontinuity in the transition between line orientations can be processed in parallel, i. e. that 'search' times increase by between 0.5 and 6 ms, on average, per additional element displayed, irrespective of the number of elements. But thresholds of orientation difference for the detection of the target increase linearly with the orientation gradient present in the display, and tend to increase more strongly for small gradients, indicating a special bonus for (near) collinearity. The averaged data follow a Weber-law type while this is not true for the individual observers' data. These results show that the visual system is indeed able to detect targets based on orientation contrast, rather than on absolute orientation [cf. Nothdurft (1985). Vision Research, 25, 551-560], but that the orientation gradient cannot be ignored.

Component perimetry: a fast method to detect visual field defects caused by brain lesions.

PURPOSE: Noise field campimetry, performed according to Aulhorn and Köst, confronts patients with a large field of irregularly flickering dots, and many patients immediately perceive their visual field defects. The original method had a somewhat low specificity and sensitivity, especially for patients with visual field defects caused by cortical lesions. METHODS: The method was improved in two ways. First, the grain of the visual noise was increased toward the periphery of the visual field to accommodate the peripheral decrease in visual acuity. Second, the type of stimulus pattern was varied to include separate investigations of different visual components or functions (color, motion, temporal resolution, line orientation, stereoscopic depth, acuity, and figure-ground segmentation). To evaluate the reliability of the method, the visual fields were compared, as assessed by the new method, with those of conventional perimetry in 41 patients with neurologic disorders and 22 normal control subjects. RESULTS: The results were encouraging. All patients with suprageniculate lesions subjectively experienced visual field defects in component perimetry. Sizes of visual field defects obtained with both methods corresponded qualitatively with each other, with a highly significant correlation. The specificity of component perimetry was higher than that of the original noise field campimetry. CONCLUSIONS: This pilot study indicates that component perimetry is a subjective but relatively reliable method for detecting disorders of visual perception caused by lesions at different stages along the visual pathway, permitting fast screening of the visual field. In addition, this method seems to allow examination of the visual field, not only for defects in contrast sensitivity, as does conventional light perimetry, but also for the status of other components of vision such as color or motion perception. Further evaluation with larger patient cohorts is needed to allow exact assessment of the clinical usefulness of the method.

Performance asymmetries in visual search demonstrate failure of independent-processing models.

We report psychophysical data from orientation-popout experiments that are inconsistent with a rather general decision model. Stimuli consisted of 121 line segments arranged on an 11x11 grid. There were two tasks: in the 1-Singleton Task all lines except one had the same orientation, and observers had to report which quadrant contained the singleton. In the 3-Singleton Task three quadrants contained orientation singletons and observers had to identify the quadrant without singleton. These tasks can be viewed as asymmetric search tasks, in which either a singleton-quadrant has to be found among three homogeneous quadrants, or a homogeneous quadrant has to be found among three singleton-quadrants. Using tools from signal-detection theory we show that the large performance asymmetries between 1-Singleton and 3-Singleton Tasks are inconsistent with any model that makes two (very basic and common) assumptions: (1) independent processing of the four quadrants and (2) an ideal-observer decision. We conclude that at least one of the two assumptions is inadequate. As a plausible reason for the model failure we suggest a global competition between salient elements that reduces popout strength when more than one singleton is present.