Low Spatial Frequency Patterns Research Articles

Motion onset VEPs can see through the blur

Motion-onset visual evoked potentials (MO VEPs) are robust to dioptric blur when low contrast and low spatial frequency patterns are used for stimulation. To reveal mechanisms of MO VEPs robustness, we studied whether the resistance to defocus persists even when using a high-contrast checkerboard using digital defocus in the emmetropic eyes of 13 subjects (males 20–60 years). We compared the dominant components of MO VEPs to pattern-reversal VEPs (PR VEP), which are sensitive to the blur. For stimulation, we used checkerboard patterns with 15´ and 60´ checks. To defocus the checkerboard, we rendered it with a second-order Zernike polynomial (Z20\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${Z}_{2}^{0}$$\\end{document}) with an equivalent defocus of 0, 2, or 4 D. For PR VEP, the checkerboards were reversed in terms of their contrast. To evoke MO VEP, the checkerboard of 60´ checks moved for 200 ms with a speed of 5 or 10 deg/s in the cardinal directions. The MO VEP did not change in peak time (P ≥ 0.0747) or interpeak amplitude (P > 0.0772) with digital blur. In contrast, for PR VEP, the results showed a decrease in interpeak amplitude (P ≤ 6.65ˑ10-4) and an increase in peak time (P ≤ 0.0385). Thus, we demonstrated that MO VEPs evoked by checkerboard, structure containing high spatial content, can be robust to defocus.

Melanopsin enhances image persistence

Contributions of the inner retinal photopigment melanopsin to human visual perception are incompletely understood. Here, we use a four-primary display to produce stimuli differing in melanopsin versus cone contrast in psychophysical paradigms in eight subjects with normal color vision. We address two predictions from electrophysiological recordings of the melanopsin system in non-human mammals: melanopsin influences color and/or supports image persistence under visual fixation. We first construct chromatic contrast sensitivity contours for stimuli differing in melanopsin excitation presented as a central annulus (10°) or peripheral (22.5°) spot. We find that although including melanopsin contrast produces modest changes in the average chromatic coordinates in both eccentricities, this occurs equally at low (0.5Hz) and higher (3.75Hz) temporal frequencies, arguing that it reflects divergence in cone spectral sensitivity in our participants from that captured in standardized cone fundamentals rather than a melanopsin contribution to color. We continue to ask whether the established ability of melanopsin to sustain firing of visual neurons under extended light exposure has a visual correlate, using the optical illusion of Troxler fading in which blurred spots in periphery disappear during visual fixation. We find that introducing additional melanopsin contrast (+28% Michelson contrast) to either bright or dark spots increases fading latency by 35%± 8.8% and 41%± 13.6%, respectively. Our data argue that the primary contribution of melanopsin to perception under these conditions isnot to provide a color percept but rather to enhance persistence of low spatial frequency patterns during visual fixation.

Spatial filtering, color constancy, and the color-changing dress.

The color-changing dress is a 2015 Internet phenomenon in which the colors in a picture of a dress are reported as blue-black by some observers and white-gold by others. The standard explanation is that observers make different inferences about the lighting (is the dress in shadow or bright yellow light?); based on these inferences, observers make a best guess about the reflectance of the dress. The assumption underlying this explanation is that reflectance is the key to color constancy because reflectance alone remains invariant under changes in lighting conditions. Here, we demonstrate an alternative type of invariance across illumination conditions: An object that appears to vary in color under blue, white, or yellow illumination does not change color in the high spatial frequency region. A first approximation to color constancy can therefore be accomplished by a high-pass filter that retains enough low spatial frequency content so as to not to completely desaturate the object. We demonstrate the implications of this idea on the Rubik's cube illusion; on a shirt placed under white, yellow, and blue illuminants; and on spatially filtered images of the dress. We hypothesize that observer perceptions of the dress's color vary because of individual differences in how the visual system extracts high and low spatial frequency color content from the environment, and we demonstrate cross-group differences in average sensitivity to low spatial frequency patterns.

Depth perception from dynamic occlusion in motion parallax: Roles of expansion-compression versus accretion-deletion

Motion parallax, or differential retinal image motion from observer movement, provides important information for depth perception. We previously measured the contribution of shear motion parallax to depth, which is only composed of relative motion information. Here, we examine the roles of relative motion and accretion-deletion information in dynamic occlusion motion parallax. Observers performed two-alternative forced choice depth-ordering tasks in response to low spatial frequency patterns of horizontal random dot motion that were synchronized to the observer's head movements. We examined conditions that isolated or combined expansion-compression and accretion-deletion across a range of simulated relative depths. At small depths, expansion-compression provided reliable depth perception while accretion-deletion had a minor contribution: When the two were in conflict, the perceived depth was dominated by expansion-compression. At larger depths in the cue-conflict experiment, accretion-deletion determined the depth-ordering performance. Accretion-deletion in isolation did not yield any percept of depth even though, in theory, it provided sufficient information for depth ordering. Thus, accretion-deletion can substantially enhance depth perception at larger depths but only in the presence of relative motion. The results indicate that expansion-compression contributes to depth from motion parallax across a broad range of depths whereas accretion-deletion contributes primarily at larger depths.

Independent coding of object motion and position revealed by distinct contingent aftereffects

Despite several findings of perceptual asynchronies between object features, it remains unclear whether independent neuronal populations necessarily code these perceptually unbound properties. To examine this, we investigated the binding between an object’s spatial frequency and its rotational motion using contingent motion aftereffects (MAE). Subjects adapted to an oscillating grating whose direction of rotation was paired with a high or low spatial frequency pattern. In separate adaptation conditions, we varied the moment when the spatial frequency change occurred relative to the direction reversal. After adapting to one stimulus, subjects made judgments of either the perceived MAE (rotational movement) or the position shift (instantaneous phase rotation) that accompanied the MAE. To null the spatial frequency-contingent MAE, motion reversals had to physically lag changes in spatial frequency during adaptation. To null the position shift that accompanied the MAE, however, no temporal lag between the attributes was required. This demonstrates that perceived motion and position can be perceptually misbound. Indeed, in certain conditions, subjects perceived the test pattern to drift in one direction while its position appeared shifted in the opposite direction. The dissociation between perceived motion and position of the same test pattern, following identical adaptation, demonstrates that distinguishable neural populations code for these object properties.

Illusions as neuro-signs

Almost the whole of visual science is concerned with illusions — if they are defined as images in which the physical description differs from the image perceived. One of the attractions of illusions, apart from their obvious appeal as geometrical abstract art, is that they direct us to an understanding of visual processing at many levels.But before the question of why an illusion is perceived can be addressed, we must have a physical description of the image, which is not always straightforward. After all, what is the physical description of a stick that is partially immersed in water? It is straight if the stick itself is measured, but not if a photograph is taken of it.For this reason (among others) psychologists make a distinction between the distal (or physical) stimulus and the proximal stimulus (its projection onto the retina). An illusion occurs when there is a mismatch between the proximal stimulus and perception.The distinction between distal and proximal stimuli can be appreciated with the image of black and white concentric circles below. The distal stimulus is circular and stable, but its projection onto the retina is distorted and dynamic. Spokes appear to radiate from the centre and to rotate; they are probably a consequence of slight astigmatic changes in the curvature of the lens, because they are reduced when viewed through a pinhole, they have similar temporal dynamics to measured fluctuations in the curvature of the lens, and they do not occur in those who have no crystalline lens (aphakia). Aphakic individuals lacking a lens in one eye see such patterns as stable using that eye, but see it with radiating spokes using the eye that has a lens.Figure 1View Large Image | View Hi-Res Image | Download PowerPoint SlideIn addition, viewing the pattern from a distance will reveal a portrait of Jan Evangelista Purkinje, who first described the radiating spokes phenomenon in 1825. The portrait is carried by low spatial frequency variations in the pattern, which correspond to the coarse grain over the whole pattern; these are normally inhibited by the higher spatial frequencies (the fine grain of the thin lines) when the pattern is close to the eye. When viewed from a distance, the high spatial frequency content (thin lines) is not sharply resolved and the low spatial frequency pattern emerges.Figure 1View Large Image | View Hi-Res Image | Download PowerPoint SlidePurkinje also demonstrated similar effects with radiating lines (see coloured image above). High-contrast coloured or black and white radiating lines are also visually dynamic — they appear to move.The motion consists of shimmering points which are perpendicular to the lines of the distal stimulus. Thus, with a radiating pattern they are circular, and can be seen more readily within the orange annuli. If a white surface is observed following fixation on the pattern for around 30 seconds then scintillating dots will seem to be streaming in a direction orthogonal to the previously presented lines.Regular geometrical patterns produce a range of visual effects; some are based on the optics of the eye (like the spokes) and others (such as the scintillations) could be caused by visual processing in the brain. For example, Purkinje's radiating lines illusion has been examined with the aid of neural imaging. Positron emission tomography (PET) scans indicate that a pattern of radiating lines causes the blood flow in V5 — the area of the brain that is involved in motion processing — to increase. Alternative interpretations, however, relate to eye movement instabilities during fixation. It is likely that several phenomena are induced by these patterns, with a different explanation for each one.When we know that an image is distorted before it reaches the retina — that the proximal stimulus differs from the distal stimulus — we consider that we have some interpretation of the illusion. Thus, the bent-stick distortion is due to differential refraction through air and water. But in general, explanations for the illusion are sought in higher levels of visual processing. Such illusions might provide an index of neural function, which I have called a ‘neuro-sign’.

Development of pattern visual evoked potentials: longitudinal measurements in human infants.

This experiment used longitudinal testing to trace the emergence of the major components of pattern visual evoked potentials (VEPs) in infants, using two paradigms: large-checkerboard pattern reversal and low spatial frequency pattern onset. Testing with both pattern-reversal and pattern-onset stimuli was performed on the same infants. Testing was conducted at weekly intervals during the first three postnatal months, and at intervals of 2 weeks to 1 month thereafter. The pattern-reversal and early pattern-onset responses recorded within individual subjects showed remarkably systematic developmental sequences. The broad, positive component seen at 200 to 250 ms in infants could be traced readily through the developmental sequence, to become the more sharply tuned positive component seen at about 100 ms in adults. Responses to low spatial frequency pattern onsets in infants were larger and more reliable than those in adults. The late components of the pattern-onset response, generally attributed to pattern offset, emerged later and with more complex changes. In all cases, response amplitude was much more variable than response latency, both within and between subjects. Frequent VEP recording in a longitudinal design can reveal systematic and detailed transitions of wave-form during development.

Development of the Temporal Properties of Visual Evoked Potentials to Luminance and Colour Contrast in Infants

We have studied the development of the temporal characteristics of the pattern visual evoked potentials (P-VEPs) in response to contrast reversal of patterns of low spatial frequency (0.1 c/deg) of either pure luminance contrast (yellow-black plaid patterns) or pure colour contrast (equiluminant red-green plaid patterns) in 15 infants between 6 and 30 weeks of age. High contrast patterns were modulated temporally either sinusoidally at various temporal frequencies to elicit steady-state responses, or abruptly at a low temporal frequency to elicit transient responses. Analysis of both the transient and steady-state responses suggests the existence of three different mechanisms contributing to the infant and adult P-VEP responses at low, medium and high temporal frequencies. The responses at the three different temporal frequency ranges have different time constants, and develop at different rates. The low frequency response predominates at 8 weeks, where it spans the range 1–6 Hz with an apparent latency of about 230 msec, for both colour and luminance stimulation. This response increases in bandwidth and decreases in latency progressively with age, at a similar rate for luminance and colour contrast, up to 14 weeks. After 14 weeks, the luminance response undergoes major changes, with the emergence of a new response with a shorter latency (about 100 msec) and a peak activity near 10 Hz. This mid-frequency response matures further with age, until it dominates the whole response of the adult P-VEP to luminance contrast. It also makes a contribution to the chromatic response at frequencies above 10 Hz, generating the characteristic double-peaked amplitude response in adults. However, its contribution is very limited below 10 Hz, where the response latency is 140 msec in adults, as it was at 14 weeks of age. A third component is evident at very high temporal frequencies of the luminance response as early as 6 weeks, extending up to 15 Hz in 8-week-olds and up to 25 Hz for older infants. It remains apparent up to 18 weeks, thereafter being swamped by the major mid-frequency response. The apparent latency of response over this frequency range is about 70 msec at all ages. The development of transient P-VEPs paralleled that of the steady-state P-VEPs. At all ages there was an early negative component (N70) at about 70 msec, corresponding to the fast steady-state response at high frequencies for luminance contrast. Before 14 weeks, the luminance and chromatic transient response had the same morphology, with a single major peak of similar latency to the apparent latency of the low temporal frequency response. After this age, the morphology of the luminance response changed, particularly in the first 100 msec, consistent with the emergence of the mid-frequency response. We discuss whether the high-frequency component may represent pre- or early post-synaptic cortical activity, already mature by 8 weeks, and how the different maturation rates of the mid and high-frequency components may reflect different intra-cortical circuitry for colour and luminance. Copyright © 1996 Elsevier Science Ltd.

Temporal impulse response functions for luminance and colour during saccades.

Previous work has shown that during saccadic eye movements, contrast sensitivity for low spatial frequency patterns modulated in luminance is selectively reduced by up to one logarithmic unit, while high spatial frequency patterns, and equiluminant patterns of all spatial frequencies are not suppressed at all [Burr et al. (1994). Nature, 371, 511-513]. Here we study the temporal characteristics for sensitivity to luminance and chromatic patterns during saccades, using the two-pulse summation technique. Sensitivity was measured for detecting two successive pulses as a function of stimulus-onset asynchrony, during normal viewing and during saccades. Impulse response functions were estimated from the summation data, for all conditions. For equiluminance, the functions were monophasic during normal viewing and saccades. For luminance modulation, the impulse response functions were di-phasic in both normal viewing and saccades. However, during saccades the impulse responses were faster in normal viewing. This result is consistent with the suggestion that saccadic suppression is mediated by contrast gain control mechanisms, known to occur in M-cells but not P-cells.

Loss of P retinal ganglion cell function in resolved optic neuritis

To determine the relative loss of P and M (X and Y) ganglion cell function in unilateral resolved optic neuritis, 10 patients with 20/20 or better Snellen acuity in both eyes had contrast sensitivity testing, color vision testing, and automated perimetry. We used contrast sensitivity gratings of 0.5 cycles per degree (cpd) with a rate of counterphase temporal modulation of 30 Hz and gratings of 11.4 cpd at 1 Hz. On 1 trial, patients responded when they detected the pattern of the grating and on the next trial when they 1st perceived movement. There was a significant difference in the 1 Hz high spatial frequency pattern and movement results suggesting loss of P cell function. Two patients were unable to perceive any movement with their involved eye with this target, but could detect the pattern. There was no significant difference between the involved and uninvolved eyes in the low spatial frequency pattern detection values. This is a function ascribed to the M cell. There was also loss of low spatial frequency movement detection. Although there was significant depression of the entire visual field in the involved eye, the probability plots showed the most significant loss in the cecocentral area. Farnsworth-Munsell 100 Hue color testing was also abnormal. Greater involvement of P than M ganglion cell axons may explain these contrast sensitivity abnormalities, central scotomata, and color vision loss.

Signs of early damage in glaucomatous monkey eyes: Low spatial frequency losses in the pattern ERG and VEP

Experimental glaucoma was created in one eye of three cynomolgus monkeys by argon laser application to the mid-trabecular meshwork. Simultaneous pattern electroretinograms (PERG) and pattern visual evoked potentials (PVEP) were measured in both control and glaucoma eyes to spatial frequencies of 0.5, 1.25, 2.5, and 3.5 cpd which were counterphase modulated at 6 Hz. The transient flash electroretinogram was also measured. While normal flash electroretinograms were recorded in all eyes both before and after the unilateral production of elevated intraocular pressure (IOP), reductions in PERG and PVEP amplitude were seen in the eyes with glaucoma as early as two weeks following a sustained increase of IOP, despite the absence of cupping of the optic nervehead judged by ophthalmoscopic examination and analysis of photograph by two observers. Optic nervehead abnormalities occurred subsequently. In glaucomatous monkey eyes, the earliest PERG and PVEP changes were most evident with lower spatial frequencies of stimulation. Our data suggest that the optimal stimulus parameters for the detection of early glaucoma are low spatial frequency patterns presented at a rapid rate of temporal modulation.

Adaptation, stimulus color, and the human electroretinogram.

(1) The human electroretinogram was recorded using two procedures that were designed to change the ratio of photopic to scotopic activity. (2) With the first procedure, responses were recorded with patterns that alternated with an asymmetrical time period to produce two average responses, one of which was more adapted than the other. (3) The second employed symmetrical alternation, but red and blue stimulation were used. (4) The electroretinogram was larger in responses to patterns of low spatial frequency for all conditions. The evoked potential, which was recorded simultaneously, was larger at intermediate frequencies. (5) The electroretinogram must be mediated before the antagonistic receptive field organization. (6) Sensitivity of electroretinogram with blue stimuli was higher than those with red at low spatial frequency. The possible scotopic contribution was discussed.

The effect of saccades on threshold perception--a model study.

The effect of saccadic eye movements on threshold perception is investigated theoretically. The proposed model considers eye movements by taking into account the shifting of the stimulus pattern on the retina during the occurrence of an eye movement. Saccades are characterized by high velocity and short duration. These motions cause overshoots in the response of linear filters to certain stimulus patterns. Therefore, the model predicts facilitation effects of saccades in the perception of low spatial frequency patterns and patterns flickering with high temporal frequencies. These results agree with experimentally obtained data presented in a subsequent paper. A simple approach is formulated which approximates the complex shifting function of a saccade by a switching of the pattern.