Vernier Stimulus Research Articles

Effect of contrast and gaps between Vernier stimulus elements on sweep visual evoked potential measurements of human cortical Vernier responses.

The present paper focuses on a classic hyperacuity, Vernier acuity-the ability to discriminate breaks in the collinearity of lines or edges on the order of only arcseconds of visual angle. We measured steady-state sweep visual evoked potentials (sVEPs) in response to 6 Hz periodic breaks in collinearity (Vernier offsets) in horizontal squarewave gratings. Vernier thresholds, estimated by extrapolating the amplitude of the first harmonic (1F) to 0 µV, were measured for gratings with 4%, 8%, 16%, 32%, 64%, and 80% contrast, with gaps of 0, 2, or 5 arcmin introduced between neighboring bar elements that formed the Vernier offsets. Thresholds for the 2F response component provided an estimate of motion thresholds. The data confirmed and extended evidence that the odd- and even-harmonic components reflect cortical activity of different neurons (i.e., neurons that respond asymmetrically to the periodic breaks in alignment and neurons that respond symmetrically to the local relative motion cue of the stimulus). Suprathreshold data (peak amplitude, response slope, and response phase at the peak amplitude) provided additional independent evidence of this notion. Vernier thresholds decreased linearly as contrast increased, with a slope of approximately -0.5 on log-log axes, similar to prior psychophysical results. The form of contrast dependence showed more similarity to measures of magnocellular ganglion cell spatial precision than measures from parvocellular ganglion cells. Our data thus support the hypothesis that magnocellular ganglion cell output from the retina has the requisite properties to support cortical calculation of Vernier offsets at a hyperacuity level.

The temporal dynamics of feature integration for color and form

When two visual stimuli are presented in rapid succession only one fused image is perceived. For example, a red disk followed by a green disk is perceived as a single yellow disk (e.g., Efron, Perc Psychophys, 1973). For two verniers that last 30ms each, it has been shown that the fusion process can be disrupted up to 420ms after stimulus onset by applying TMS over visual cortex (Scharnowski et al., J Vis, 2009). Here, we show that feature fusion can also be modulated with light masks. In Experiment 1, we used the same procedure as in the above TMS experiment: Two verniers were presented in rapid succession with opposite offset directions. Subjects had to indicate the offset direction of the fused vernier. In Experiment 2, a red and a green disk were presented in rapid succession and subjects had to indicate whether the fused disk appeared red or green. Verniers were presented for 30ms and disks for 10ms each. Light masks were presented from -100 to 240ms relative to the onset of the first stimulus. When masks were presented before the stimuli, the second stimulus dominated the percept and when masks were presented after the stimuli, the first stimulus dominated the percept. Feature fusion was modulated for a period of 240ms for both color and vernier stimuli. Our results indicate that for this duration individual memories exist for each vernier or color stimulus separately. Fusion is not completed beforehand. Taken together, our results show that light masks have similar effects on feature fusion as TMS and provide an adequate alternative to investigate feature integration. In addition, we showed that the process of unconscious feature fusion outlasts the physical duration of the stimuli by far.

Perceptual Learning in a Nonretinotopic Frame of Reference

Perceptual learning is the ability to improve perception through practice. Perceptual learning is usually specific for the task and features learned. For example, improvements in performance for a certain stimulus do not transfer if the stimulus is rotated by 90° or is presented at a different location. These findings are usually taken as evidence that orientation-specific, retinotopic encoding processes are changed during training. In this study, we used a novel masking paradigm in which the offset in an invisible, oblique vernier stimulus was perceived in an aligned vertical or horizontal flanking stimulus presented at a different location. Our results show that learning is specific for the perceived orientation of the vernier offset but not for its actual orientation and location. Specific encoding processes cannot be invoked to explain this improvement. We propose that perceptual learning involves changes in nonretinotopic, attentional readout processes.

Gestalt Factors Modulate Basic Spatial Vision

Human perception of a stimulus varies depending on the context in which the stimulus is presented. Such contextual modulation has often been explained by two basic neural mechanisms: lateral inhibition and spatial pooling. In the present study, we presented observers with a vernier stimulus flanked by single lines; observers’ ability to discriminate the offset direction of the vernier stimulus deteriorated in accordance with both explanations. However, when the flanking lines were part of a geometric shape (i.e., a good Gestalt), this deterioration strongly diminished. These findings cannot be explained by lateral inhibition or spatial pooling. It seems that Gestalt factors play an important role in contextual modulation. We propose that contextual modulation can be used as a quantitative measure to investigate the rules governing the grouping of elements into meaningful wholes.

Roving in perceptual learning: stimulus interference and overlapping neural populations

Volume 8, Number 6, Abstract 479, Page 479a doi:10.1167/8.6.479 http://journalofvision.org/8/6/479/ ISSN 1534-7362 Roving in perceptual learning: stimulus interference and overlapping neural populations Elisa M. Tartaglia Laboratory of Psychophysics, Brain Mind Institute, Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland [e-mail] Kristoffer C. Aaberg Laboratory of Psychophysics, Brain Mind Institute, Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland Michael H. Herzog Laboratory of Psychophysics, Brain Mind Institute, Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland Abstract Performance usually improves when observers train with one type of a visual stimulus. Roving denotes the situation when, instead of one, two or more types of stimuli are presented randomly interleaved (one per trial). For some stimulus types, performance improves also in roving situations whereas for others it does not. To understand when roving impedes perceptual learning, we conducted four experiments. In each experiment, the very same bisection stimulus was randomly interleaved with a different stimulus type. Performance improved when a bisection and a vernier stimulus were randomly interleaved. However, no learning was found when we randomly interleaved this bisection stimulus with another one being twice as long. Based on these results, we propose that roving impedes performance when two stimuli excite strongly overlapping neural populations

Distorting visual space with sound

Attentional repulsion is described as the perceived displacement of a vernier stimulus in a direction that is opposite to a brief peripheral visual cue (Suzuki & Cavanagh, 1997). Here, we demonstrate that visual repulsion can also be elicited using lateralized sounds. Given that repulsion is believed to be occurring in early retinotopic visual areas, these results raise the possibility that the location of a sound could directly influence the pattern of activity as early as primary visual cortex.

Vernier Acuity in Down Syndrome

Down syndrome (DS) is associated with reduced visual performance. Although poor optical quality has been implicated, no previous data are available regarding the contribution of cortical visual processes. The present study investigated Vernier performance for the first time in children with DS to evaluate the integrity of higher visual processing in this condition. Participants were 29 children aged 9 to 16 years who had DS and 68 age-matched developmentally normal children acting as controls. All wore best refractive correction, and none had clinically significant ocular abnormalities. An out-of-phase test-pedestal Vernier stimulus was used to facilitate short test distances and optimize compliance with testing. Testing was successfully completed by 86% (n=25) of the DS group and 96% (n=65) of the control group. Vernier thresholds were invariant with age in both groups. Mean Vernier acuities were 39.8 arc seconds (SD+/-13.3) and 14.6 arc seconds (SD+/-4.7) in DS and control groups, respectively. When compared with control data, mean Vernier acuity was reduced by a factor of 2.7 in DS. Vernier thresholds were successfully measured in children with DS and were found to be reduced, indicating that cortical visual function is compromised. Impairment in cortical function in DS may be implicit, relating to histologic reports of differences in the DS brain, or they may result from abnormal experience during visual development. The magnitude of the cortical deficit demonstrated in DS in the present study is significant and should be considered along with previously reported poor optical quality.

Orientation repulsion and attraction in alignment perception

Orientation masking induces changes of discrimination thresholds and perceived orientation. Studies on alignment discrimination of Vernier stimuli concentrated on masking induced changes of discrimination thresholds, without considering possible changes of perceived orientation and/or alignment of the two-line segments. Measuring both parameters in an orientation discrimination task, we confirmed a standard repulsion effect between a single line target and a mask grating that co-varied with elevated orientation discrimination thresholds. Masking a Vernier stimulus in an alignment discrimination task, we observed a strong misperception of alignment that was accompanied with elevated alignment discrimination thresholds. Orientation masking on perceived orientation and alignment of a Vernier stimulus revealed orientation repulsion and attraction that depended on the spatio-orientation configuration of the superimposed stimuli. Control of task-dependent effects confirmed that our observed pattern of results was independent of attentional or cognitive demands.

Vernier Acuity Is Normal in Migraine, whereas Global Form and Global Motion Perception Are Not

A recent study has demonstrated that some people with migraine display impairments of intermediate stages of motion and form processing. Deficits were identified by using tasks that required that local stimulus attributes be integrated into global percepts. Neurons capable of global processing of form and motion are known to be present in extrastriate cortical areas V4 and V5, respectively. It is not clear from the literature whether deficits of global processing in migraineurs are likely to arise from reduced input to extrastriate cortex from primary visual cortex (V1). The purpose of the study was to compare presumed measures of V1 performance (vernier acuity) to measures of global form and motion perception in migraineurs. Thirty migraineurs (17 with aura, and 13 without) and 20 age-matched nonheadache control subjects participated. Intermediate level motion and form perceptions were measured using global dot motion stimuli and Glass patterns, respectively. Vernier stimuli were broad vertical bars composed of small dot elements. Both a static luminance stimulus and a motion defined form vernier stimulus were used. Mean migraine and control group performance were not significantly different for either vernier task (static: t(48)=0.39, P=0.70; motion: t(48)=0.29, P=0.77). Mean migraine group performance was significantly worse than in control subjects for both the global form (t(48)=2.06, P=0.04) and global motion (t(48)=2.87, P<0.01) tasks. On average, migraineurs demonstrate abnormalities of intermediate stages of both motion and form processing. These abnormalities do not appear to arise from dropout of performance at V1, as vernier acuity was normal in the same individuals.

Distorting visual space with sound

Attentional repulsion is described as the perceived displacement of a vernier stimulus in a direction that is opposite to a brief peripheral visual cue ( Suzuki & Cavanagh, 1997). Here, we demonstrate that visual repulsion can also be elicited using lateralized sounds. Given that repulsion is believed to be occurring in early retinotopic visual areas, these results raise the possibility that the location of a sound could directly influence the pattern of activity as early as primary visual cortex.

A comparison of the effects of ageing upon vernier and bisection acuity

While most positional acuity tasks exhibit an age-related decline in performance, the effect of ageing upon vernier acuity continues to be the subject of some debate. In the present study we employed a stimulus design that enabled the simultaneous determination of bisection and vernier acuities in 36 subjects, aged between 22 and 84 years. This approach provided a means for directly testing the hypothesis that ageing affects bisection acuity but not vernier acuity by ensuring that differences in stimulus configuration and in the subject’s task were kept to an absolute minimum. Optimum thresholds increased as a function of age for both bisection and vernier tasks. Inter-subject threshold variability also increased with age. Issues surrounding the comparison of absolute vernier thresholds across different studies are discussed and two important methodological factors are identified: the precise statistical method used to estimate thresholds, and the magnitude, in angular terms, of the smallest spatial offset of the elements of the vernier stimulus which can be displayed. Comparison with previously published data indicates that the discrepancy between this study and most previous investigations with respect to the effect of age upon vernier performance can be at least partly accounted for by differences in the minimum displayable vernier offset. Vernier thresholds do increase with age. The increased variability of vernier thresholds in older subjects would appear to limit the diagnostic value of the test as a means of enabling normal ageing to be distinguished from visual loss due to pathology of the eye or visual system.

The attentional repulsion effect in perception and action.

The attentional repulsion effect refers to the perceived displacement of a Vernier stimulus in a direction that is opposite to a brief peripheral cue. The twofold purpose of the present study was to: (1) replicate the perceptual effect using a Vernier discrimination task, and (2) determine whether the effect would also affect action using a guided localization task. A perceptual attentional repulsion effect was found in experiment 1 and a similar effect was found in experiment 2, with a computer mouse localization task, and in experiment 3, with a guided limb localization task (in both cases pointing responses were biased in the direction opposite to that of the cue). These findings suggest that the attentional repulsion effect occurs early in visual processing, probably affecting the receptive fields of the position-coding units in primary visual cortex before "object-perception" and "object-action" information is segregated into separate pathways.

Mechanisms of perceptual learning for vernier acuity

Purpose. To understand the neural mechanisms underlying perceptual learning in Vernier acuity. Methods. Our training protocol was divided into three phases: namely pre-training, training and post-training. A three-alternative forced-choice interleaved staircase paradigm was used to measure vernier acuity in noise in ten normal adult observers. The Vernier stimulus was comprised of two segments with a 12 minute gap between the two segments. Each segment consisted of eight discrete Gabor patches (carrier SF, 10 cpd). Positional noise was produced by distributing each Gabor patch around the mean line position according to a Gaussian probability function. Observers were tested and trained at five noise levels (including zero). Vernier threshold was defined as the offset at which 66% correct responses were obtained. Trial-by-trial feedback was provided. Each observer had given more than 10000 responses at the end of the experiment. Results. Vernier acuity gradually improved across training sessions. With no noise, it improved, on average, by about 35%. Vernier acuity also gradually improved across training sessions at all spatial noise settings. We used several models (including the LAM model and the PTM model) to fit the data. All of the models were consistent in showing no statistically significant changes in additive internal noise across training sessions, but a significant increase in efficiency. Our observers showed no significant transfer of learning either across eyes or orientations. Conclusions. Vernier performance in noise can be improved after repetitive training. The improvement in Vernier acuity with practice can be mainly attributed to the improved efficiency. In our laboratory, we are currently studying the mechanisms of perceptual learning in amblyopes.

Potential vision testing--the relationship between visual acuity and Vernier acuity in the presence of simulated cataract.

To evaluate the utility of a computer controlled two-bar Vernier acuity measurement as a predictor of visual function in the presence of cataract we measured logMAR visual acuity and Vernier acuity in a group of 40 young normal observers under various levels of dioptric blur (0-3 D in dioptre steps). The Vernier thresholds were resistant to dioptric blur up to 2 D, but performance degraded with blur of 3 D for non-optimised Vernier stimulus parameters. The stimulus parameters, bar length and bar separation, were further investigated in two subjects under conditions of blur. By extending the Vernier bar length and increasing the bar separation the effect of blur could be further reduced even under the most blurred condition. The relationship between visual acuity and Vernier acuity was determined. Vernier acuity was measured in the presence of Vistech cataract simulating lenses and a prediction of visual acuity was made for three observers, two with no ocular abnormality and one with age-related maculopathy. The cataract simulating lenses affected the measured visual acuity in all three subjects, but had less effect on Vernier acuity. Predicted visual acuities were all within six letters (0.12 log units) of the visual acuity without the simulated cataract. As expected, the subject sufferng from age-related maculopathy, whilst showing similar levels of Vernier acuity to the two ocularly healthy subjects at 1.5 degrees of retinal eccentricity, showed much poorer Vernier acuity for stimuli presented at fixation.

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 properities of an invisible element: feature inheritance and shine-through. Proceedings of the Natlional 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.

Seeing properties of an invisible object: feature inheritance and shine-through.

We characterize a class of spatio-temporal illusions with two complementary properties. Firstly, if a vernier stimulus is flashed for a short time on a monitor and is followed immediately by a grating, the latter can express features of the vernier, such as its offset, its orientation, or its motion (feature inheritance). Yet the vernier stimulus itself remains perceptually invisible. Secondly, the vernier can be rendered visible by presenting gratings with a larger number of elements (shine-through). Under these conditions, subjects perceive two independent "objects" each carrying their own features. Transition between these two domains can be effected by subtle changes in the spatio-temporal layout of the grating. This should allow psychophysicists and electrophysiologists to investigate feature binding in a precise and quantitative manner.

Temporal modulation enhances the efficiency of spatial offset discriminations by 6-month-olds.

Fourteen- and 24-week-old infants were tested for sensitivity to small position differences under two conditions: (a) the Vernier stimulus was flashed on and off at 1.2 or 4.8 Hz (flash condition); and (b) the Vernier breaks were presented in apparent motion at 1.2 or 4.8 Hz (motion condition). The latter stimulus also contained local flicker cues. Each infant was tested at one temporal frequency with both stimuli. No benefit was shown by 14-week-olds at either temporal frequency from the additional motion and flicker cues. However, 24-week-olds required spatial offsets only one third as large in the motion condition as they required in the flash condition at 4.8 Hz. Ideal observer methodology was used to ensure that the spatial information for both of these discriminations was held strictly equivalent. Increasing sensitivity to flicker and/or motion or uncertainty reduction may underlie the enhancement in discrimination shown by 24-week-olds at 4.8 Hz.

The use of different orientation cues in vernier acuity.

The role of orientation cues in vernier acuity was investigated in five experiments that varied the salience of different possible reference systems: (1) retinal, (2) subjective (vestibular), and (3) stimulus-defined ("intrinsic"). We compared the eight possible combinations of presence and absence of the three types of information about vertical by changing the orientation of the vernier stimulus, by changing the angle between the two segments of the stimulus, by reducing the stimulus to two dots rather than two lines, and/or by tilting the subject's head or by having the subject lie in a supine position. The results show that vestibular vertical plays no important role in vernier discriminations, whereas retinal vertical and, to a lesser degree, the intrinsic reference do. A control experiment excluded the possible role of a fourth possible reference system, the frame of the monitor. Our results are compatible with a model of vernier acuity detection based on oriented receptive fields with inhibitory surrounds. They show further that observers can produce, in the case of two-dot stimuli, an internal standard for comparison and detect deviations from this standard with high precision.

Relevant Image Features for Vernier Acuity

The extraction of relevant image features is a key part of discrimination learning (E Gibson, 1969 Principles of Perceptual Learning and Development) and the identification of those features is necessary for the understanding of observer performance. Two mechanisms are thought to limit Vernier acuity judgments: orientation-selective and local-sign mechanisms (Waugh and Levi, 1993 Vision Research33 539 – 552; Beard et al, 1997 Vision Research37 325 – 346). The linear component of relevant image features can be determined for a Vernier task by adding external noise to the image and then averaging the noises separately for the four types of stimulus/response trials. The Vernier stimulus consisted of two short, dark, horizontal lines presented within low-contrast white noise. Two spatial separations were tested: nearly abutting and a wide horizontal separation. The task was to determine if the target lines were aligned or offset in the vertical direction. The noises were averaged separately for the groups of trials, corresponding to each of the four possible stimulus/response combinations (eg, stimulus=offset, response=aligned). The sum of the two ‘not aligned’ images was then subtracted from the sum of the ‘aligned’ images to obtain an overall image. We then computed a weighted average of adjacent pixels, smoothing the image and fostering visualisation of the relevant feature pattern. Our image analysis supports the view that as the vernier line separation narrows, oriented features play a larger role in the Vernier discrimination.