Luminance Gratings Research Articles

Nearly instantaneous brightness induction

Brightness induction is the modulation of the perceived intensity of a region by the luminance of surrounding regions and reveals fundamental properties of neural organization in the visual system. Grating induction affords a unique opportunity to precisely measure the temporal properties of induction using a quadrature motion technique. Contrary to previous reports that induction is a sluggish process with temporal frequency cutoffs of 2-5 Hz (R. L. DeValois, M. A. Webster, K. K. DeValois, & B. Lingelbach, 1986; A. F. Rossi & M. A. Paradiso, 1996), we find that induction is nearly instantaneous. The temporal response of induced brightness differs from that of luminance gratings by a small time lag (<1 ms), or by a small temporal phase lag (<0.016 cycle), and remains relatively constant across wide variations in test field height. These data are not easily explained by an edge-dependent, homogeneous filling-in process (A. F. Rossi & M. A. Paradiso, 1996); however, they are consistent with an explanation of brightness induction based on spatial filtering by cortical simple cells (B. Blakeslee & M. E. McCourt, 1999).

Adaptation to Natural Binocular Disparities in Primate V1 Explained by a Generalized Energy Model

Sensory processing in the brain is thought to have evolved to encode naturally occurring stimuli efficiently. We report an adaptation in binocular cortical neurons that reflects the tight constraints imposed by the geometry of 3D vision. We show that the widely used binocular energy model predicts that neurons dedicate part of their dynamic range to impossible combinations of left and right images. Approximately 42% of the neurons we record from V1 of awake monkeys behave in this way (a powerful confirmation of the model), while about 58% deviate from the model in a manner that concentrates more of their dynamic range on stimuli that obey the constraints of binocular geometry. We propose a simple extension of the energy model, using multiple subunits, that explains the adaptation we observe, as well as other properties of binocular neurons that have been hard to account for, such as the response to anti-correlated stereograms.

Spatial Offset of Test Field Elements from Surround Elements Affects the Strength of Motion Aftereffects

Static movement aftereffects (MAEs) were measured after adaptation to vertical square-wave luminance gratings drifting horizontally within a central window in a surrounding stationary vertical grating. The relationship between the stationary test grating and the surround was manipulated by varying the alignment of the stationary stripes in the window and those in the surround, and the type of outline separating the window and the surround [no outline, black outline (invisible on black stripes), and red outline (visible throughout its length)]. Offsetting the stripes in the window significantly increased both the duration and ratings of the strength of MAEs. Manipulating the outline had no significant effect on either measure of MAE strength. In a second experiment, in which the stationary test fields alone were presented, participants judged how segregated the test field appeared from its surround. In contrast to the MAE measures, outline as well as offset contributed to judged segregation. In a third experiment, in which test-stripe offset was systematically manipulated, segregation ratings rose with offset. However, MAE strength was greater at medium than at either small or large (180 degrees phase shift) offsets. The effects of these manipulations on the MAE are interpreted in terms of a spatial mechanism which integrates motion signals along collinear contours of the test field and surround, and so causes a reduction of motion contrast at the edges of the test field.

Aging and the depth of binocular rivalry suppression.

Two experiments were designed to examine the effect of aging on the strength of binocular rivalry suppression. To produce rivalry, orthogonally oriented sine-wave luminance gratings were presented dichoptically. The observers were then required either to discriminate the spatial location of a probe spot presented to the dominant or suppressed eye's view or to detect the presence or absence of the probe. The observers in the younger and older age groups exhibited typical rivalry suppression for both tasks (i.e., the probe was more difficult to detect or discriminate when presented to the suppressed eye), but the magnitude of the suppression was significantly larger in the older observers. This increased suppression that accompanies aging can be explained by a reduction in the inhibition produced by the binocular matching circuitry of S. R. Lehky and R. Blake's (1991) model.

Spatial Luminance Contrast Sensitivity Measured with Transient VEP: Comparison with Psychophysics and Evidence of Multiple Mechanisms

To compare the spatial luminance contrast sensitivity function (CSF) obtained with transient visual evoked potentials (VEPs) with that obtained with psychophysical measurements. The stimuli consisted of horizontal luminance gratings. In the VEP experiments, 0.4, 0.8, 2, 4, 8, and 10 cpd of spatial frequency were used, at 1 Hz square-wave contrast-reversal mode. Eight to 10 Michelson contrasts were used at each spatial frequency. Contrast thresholds were estimated from extrapolation of contrast response functions. Psychophysical sensitivities were obtained with spatial gratings of 0.4, 0.8, 1, 2, 4, 6, 8, and 10 cpd and presented at 1 Hz square-wave contrast-reversal or stationary mode (dynamic and static presentation, respectively). CSF tuning was estimated by calculating the ratio between peak sensitivity and the sensitivity at 0.4 cpd. In all subjects tested (n = 6), VEP contrast-response functions showed nonlinearities-namely, amplitude saturation and double-slope amplitude functions that occurred at low and medium-to-high spatial frequencies, respectively. Mean electrophysiological and psychophysical CSFs peaked at 2 cpd. CSF tuning for electrophysiology and dynamic and static psychophysics were, respectively, 1.08, 1.11, and 1.31. Correlation coefficients (r(2)) between electrophysiological CSF and dynamic or static psychophysical CSF were, respectively, 0.81 and 0.45. Electrophysiological and psychophysical CSFs correlated more positively when temporal presentation was similar. Spatial frequencies higher than 2 cpd showed that at least two visual pathways sum their activities at high contrasts. At low contrast levels, the results suggest that the transient VEP is dominated by the magnocellular (M) pathway.

Colour contrast influences perceived shape in combined shading and texture patterns

The 'colour-shading effect' describes the phenomenon whereby a chromatic pattern influences perceived shape-from-shading in a luminance pattern. Specifically, the depth corrugations perceived in sinusoidal luminance gratings can be enhanced by spatially non-aligned, and suppressed by spatially aligned sinusoidal chromatic gratings. Here we examine whether colour contrast can influence perceived shape in patterns that combine shape-from-shading with shape-from-texture. Stimuli consisted of sinusoidal modulations of texture (defined by orientation), luminance and colour. When the texture and luminance modulations were suitably combined, one obtained a vivid impression of a corrugated depth surface. The addition of a colour grating to the texture-luminance combination was found to enhance the impression of depth when out-of-phase with the luminance modulation, and suppress the impression of depth when in-phase with the luminance modulation. The degree of depth enhancement and depth suppression was approximately constant across texture amplitude when measured linearly. In the absence of the luminance grating however, the colour grating had no phase-dependent affect on perceived depth. These results show that colour contrast modulates the contribution of shading to perceived shape in combined shading and texture patterns.

The perceived speed of chromatic and luminance gratings in the presence of adjacent modifier stimuli

The perceived speed of chromatic and luminance gratings in the presence of adjacent modifier stimuli

Psychophysical estimation of speed discrimination II Aging effects

We studied the effects of aging on a speed discrimination task using a pair of first-order drifting luminance gratings. Two reference speeds of 2 and 8 deg/s were presented at stimulus durations of 500 ms and 1000 ms. The choice of stimulus parameters, etc., was determined in preliminary experiments and described in Part I. Thresholds were estimated using a two-alternative-forced-choice staircase methodology. Data were collected from 16 younger subjects (mean age 24 years) and 17 older subjects (mean age 71 years). Results showed that thresholds for speed discrimination were higher for the older age group. This was especially true at stimulus duration of 500 ms for both slower and faster speeds. This could be attributed to differences in temporal integration of speed with age. Visual acuity and contrast sensitivity were not statistically observed to mediate age differences in the speed discrimination thresholds. Gender differences were observed in the older age group, with older women having higher thresholds.

A Multiscale Filtering Explanation of Gradient Induction and Remote Brightness Induction Effects: A Reply to Logvinenko (2003)

Grating induction is a brightness effect in which a counterphase spatial brightness variation (a grating) is induced in a homogeneous test strip that is surrounded by an inducing luminance grating (McCourt, 1982 Vision Research 22 119-134). Moulden and Kingdom (1991 Vision Research 31 1999-2008) introduced an interesting variant of grating induction (sometimes referred to as gradient induction) in which multiple strips of either a linear luminance ramp or a sine-wave grating were interlaced with strips of homogeneous luminance. We (Blakeslee and McCourt, 1999 Vision Research 39 4361-4377) demonstrated that a simple multiscale filtering explanation could account for grating induction. Recently, however, Logvinenko (2003 Perception 32 621-626) presented several arguments impugning the adequacy of spatial filtering approaches to understanding brightness induction in gradient induction stimuli. We propose that Logvinenko's arguments apply only to a limited class of filtering models, specifically those which employ only a single spatial filter. To test this hypothesis we modeled gradient induction stimuli as a function of inducing contrast, as well as Logvinenko's (2003) remote induction stimulus, using our multiscale oriented difference-of-Gaussians (ODOG) model (Blakeslee and McCourt 1999). The ODOG model successfully predicts the appearance of the inducing strips and the homogeneous test strips in the gradient induction stimuli and the appearance of the test patches in the remote induction stimuli. These results refute Logvinenko's (2003) claims, and we interpret them as providing strong evidence for a multiscale filtering approach to understanding both gradient induction and remote brightness induction effects.

Oscillatory brain activity and transcranial direct current stimulation in humans.

The aim of this study was to induce changes of the oscillatory activity in the visual cortex of healthy human subjects by modulation of neuronal excitability using weak transcranial direct current stimulation (tDCS). tDCS is a non-invasive stimulation method which induces prolonged, polarity-dependent increases or reductions in cortical excitability. An increase in high frequency oscillatory activity in the beta and gamma frequency ranges is closely related in time to the N70 peak of the primary visual evoked potential (VEP), which is an early sensory component of visual activation. Therefore this potential can be used to observe tDCS-induced changes related to oscillatory activity. VEPs were recorded using sinusoidal luminance gratings in an on/off mode before, immediately after and 10, 20, 30 min after the end of 10 min anodal or cathodal stimulation. Cathodal stimulation significantly decreased while anodal stimulation slightly increased the normalized beta and gamma frequency powers. We have shown here that tDCS transiently and reversibly changed the organized cortical activity elicited by visual stimulation. Since gamma activity is also related to a higher level of information processing, tDCS might be a suitable method to affect higher order cognitive processes.

A single mechanism for both luminance and chromatic grating vernier tasks: Evidence from temporal summation

Vernier thresholds are determined by luminance rather than chromatic contrast when both are present in vernier targets. The role of luminance and chromatic mechanisms in vernier performance under equiluminant conditions remains uncertain. Temporal summation functions for vernier thresholds with luminance and red-green equiluminant gratings were compared to those for detection thresholds with similar stimuli. Vernier thresholds showed similar temporal summation for luminance and chromatic gratings, which is consistent with a single mechanism underlying vernier performance in the two conditions. However, detection thresholds showed a shorter temporal summation duration for luminance gratings than for chromatic gratings, which suggests that two different mechanisms underlie detection thresholds. Analysis of physiological data supports the hypothesis that the frequency-doubled response of ganglion cells in the magnocellular pathway can provide accurate spatiotemporal information for vernier performance at equiluminance.

Changes in contrast thresholds with mean luminance for chromatic and luminance gratings: A reexamination of the transition from the DeVries–Rose to Weber regions

AbstractWe have examined the influence of the mean luminance level on the detection thresholds for luminance and red–green chromatic gratings for three different spatial frequencies. The changes in detection thresholds according to the mean luminance level reflect the two different regions, the DeVries–Rose and Weber ranges, found in previous studies. The results for luminance gratings suggest that the transition luminance is proportional to the spatial frequency of the grating. Predictions based on the constant‐flux hypothesis indicate, however, that the transition luminance is proportional to the square of the spatial frequency of the grating and so do not describe the distributions of luminance contrast thresholds adequately. For chromatic gratings, we obtained the same transition luminance for the two lowest spatial frequencies, showing that luminance and chromatic mechanisms behave differently as far as the dependence of the transition luminance on spatial frequency is concerned. Our results suggest that the transition luminance is related to the peak spatial frequency of visual mechanisms that respond to luminance and chromatic gratings. © 2004 Wiley Periodicals, Inc. Col Res Appl, 29, 177–182, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/col.20003

3D shape-contingent processing of luminance gratings

Studies on binocular contrast sensitivity have predominantly focused on flat, two-dimensional (2D) gratings. The underlying hypothesis of such studies is that contrast sensitivity is determined at the early stages of visual processing and is not influenced by the process of three-dimensional (3D) shape recovery. However, it can be argued that contrast detection involves identifying changes in albedo of a 3D surface rather than strictly determining the presence of 2D luminance changes. In support of this hypothesis, in three experiments we found that the relative salience of oriented luminance modulations was affected by the disparity content of the stimuli, when the same luminance distribution was assigned to a flat surface or to a surface modulated in depth. In the first experiment, in particular, we found that the relative salience of an oriented luminance grating decreased when it could be interpreted as the shading produced by a Lambertian illumination, rather than a change in reflectance. In the other two experiments, moreover, we found that this effect was reduced when the frequency of the luminance modulation did not match the frequency of the stereo corrugation. These results are consistent with the idea that the appearance of a luminance distribution is affected by the perceived 3D properties of the surface on which the luminance distribution is located.

Interaction between luminance gratings and disparity gratings

It was shown from geometry and photographic measurement that the shading pattern for a sinusoidal corrugated surface of frequency f approximates to a luminance-defined grating of frequency f, 2f or f + 2f in specific relative phase. It was confirmed that a luminance grating modifies the appearance of a suprathreshold stereoscopic corrugated surface, suggesting an interaction between shading and binocular disparity. Disparity thresholds for detecting random-dot, disparity-defined gratings of spatial frequency 0.2 or 0.4 c/deg were measured in the presence of luminance gratings of spatial frequency 0.4 c/deg with the same orientation. Phase-specific facilitation of disparity thresholds was greatest for a phase relationship inconsistent with shading of a corrugated surface, and was disrupted by positional uncertainty. The presence of texture-defined lines (which served to mark explicitly the successive spatial locations of salient depth features in the image) produced a similar pattern of facilitation, in the absence of shape-from-shading cues. The pattern of results indicates direct local interactions, including spatial cueing, rather than interaction of depth cues.

Color brings relief to human vision.

In natural scenes, chromatic variations, and the luminance variations that are aligned with them, mainly arise from surfaces such as flowers or painted objects. Pure or near-pure luminance variations, on the other hand, mainly arise from inhomogeneous illumination such as shadows or shading. Here, I provide evidence that knowledge of these color-luminance relationships is built into the machinery of the human visual system. When a pure-luminance grating is added to a differently oriented chromatic grating, the resulting 'plaid' appears to spring into three-dimensional relief, an example of 'shape-from-shading'. By psychophysical measurements, I found that the perception of shape-from-shading in the plaid was triggered when the chromatic and luminance gratings were not aligned, and suppressed when the gratings were aligned. This finding establishes a new role for color vision in determining the three-dimensional structure of an image: one that exploits the natural relationships that exist between color and luminance in the visual world.

Binocular recognition summation in the peripheral visual field: contrast and orientation dependence

Spatial frequency thresholds for recognition were measured for binocular and monocular viewing conditions at two contrast levels (95% and 7%). Measurements were obtained at the fovea and at four different eccentric retinal locations. Each eccentric retinal location was 8° from the fovea, one on the horizontal axis (180°), and the other three in the superior field on retinal axes of 90°, 45° and 135°. For the superior and horizontal retinal locations, the orientations of the gratings tested were horizontal (180°) and vertical (90°). For the retinal points on the oblique axes, the orientations of the gratings were 45° and 135°. Measurements were also obtained at the fovea for all four different grating orientations at both levels of contrast. Recognition threshold was defined as the highest spatial frequency at which luminance gratings were perceived veridically. At the fovea, binocular summation ratios (binocular spatial frequency/monocular spatial frequency) showed no significant differences for gratings of either contrast levels or for any orientation ( p>0.05). In the superior periphery, significantly higher summation ratios were shown by low contrast vertical gratings ( p<0.05), and in the horizontal periphery by low contrast horizontal gratings ( p<0.05). On the oblique axis, low contrast gratings that were parallel to the oblique meridian showed higher summation ratios compared to those at right angles to it. High contrast gratings, at least at 8° eccentricity, did not show this effect. Data suggest that meridional organisation of the retina (e.g. vertical gratings seen maximally in the superior field) occurs for resolution and that it is evidenced closer to the fovea than previously shown.

Color processing in macaque striate cortex: electrophysiological properties.

We have shown in the accompanying paper that optical imaging of macaque striate cortex reveals patches that are preferentially activated by equiluminant chromatic gratings compared with luminance gratings. These imaged color patches are highly correlated, although not always in one-to-one correspondence, with the cytochrome-oxidase (CO) blobs. In the present study, we have investigated the electrophysiological properties of neurons in the imaged color patches and the CO blobs. Our results indicate that individual blobs tend to contain cells of only one type of color opponency: either red/green or blue/yellow. Individual imaged color patches, however, can bridge blobs of similar opponency or differing opponency. When imaged color patches contain two blobs of differing opponency, the cells in the bridge region exhibit mixed color properties that are not opponent along the two cardinal color axes (either red/green or blue/yellow). Two blobs within a single imaged color patch receive input from the same eye or from different eyes. In the latter case, the bridge region between blobs contains binocular cells that are color selective. Because the cells recorded in imaged color patches were more color selective and unoriented than cells outside of color patches, color properties appear to be organized in a clustered and segregated fashion in primate V1.

Perceptual learning of luminance contrast detection: specific for spatial frequency and retinal location but not orientation

Performance of a wide range of simple visual tasks improves with practice. Here we ask whether such learning occurs for the fundamental visual task of luminance contrast detection. In two experiments we find that contrast sensitivity increases following extensive practice at detecting briefly presented sinusoidal luminance gratings and that learning is maintained after six months. Learning is spatial frequency tuned, specific to retinal location and can be specific to one eye, but is not selective for orientation. The selectivity of learning implies that it is based on plasticity in early visual, as opposed to central cognitive, processing mechanisms.

Range and Mechanism of Encoding of Horizontal Disparity in Macaque V1

The responses of single cortical neurons were measured as a function of the binocular disparity of dynamic random dot stereograms for a large sample of neurons (n = 787) from V1 of the awake macaque. From this sample, we selected 180 neurons whose tuning curves were strongly tuned for disparity, well sampled and well described by one-dimensional Gabor functions. The fitted parameters of the Gabor functions were used to resolve three outstanding issues in binocular stereopsis. First, we considered whether tuning curves can be meaningfully divided into discrete tuning types. Careful examination of the distributions of the Gabor parameters that determine tuning shape revealed no evidence for clustering. We conclude that a continuum of tuning types is present. Second, we investigated the mechanism of disparity encoding for V1 neurons. The shape of the disparity tuning function can be used to distinguish between position-encoding (in which disparity is encoded by an interocular shift in receptive field position) and phase-encoding (in which disparity is encoded by a difference in the receptive field profile in the 2 eyes). Both position and phase encoding were found to be common. This was confirmed by an independent assessment of disparity encoding based on the measurement of disparity sensitivity for sinusoidal luminance gratings of different spatial frequencies. The contributions of phase and position to disparity encoding were compared by estimating a population average of the rate of change in firing rate per degree of disparity. When this was calculated separately for the phase and position contributions, they were found to be closely similar. Third, we investigated the range of disparity tuning in V1 as a function of eccentricity in the parafoveal range. We find few cells which are selective for disparities greater than +/-1 degrees even at the largest eccentricity of approximately 5 degrees. The preferred disparity was correlated with the spatial scale of the tuning curve, and for most units lay within a +/-pi radians phase limit. Such a size-disparity correlation is potentially useful for the solution of the correspondence problem.

Dual-directional optokinetic nystagmus elicited by the intermittent display of gratings in primary open-angle glaucoma and normal eyes

Purpose. To compare optokinetic nystagmus (OKN) responses to the intermittent display of stimuli between normal subjects and patients with primary open-angle glaucoma (POAG). Methods. Optokinetic nystagmus (OKN) was recorded in 9 glaucomatous patients and 7 normal subjects. The computergenerated stimuli displayed sinusoidal luminance gratings (16 cd/m² mean luminance, 0.5 cyc/deg) with a p/2 phase shift between successive stimulus gratings. These stimulus gratings were separated by an interstimulus interval (ISI), during which a homogeneous luminance field of 16 cd/m 2 was presented. The ISI duration and the luminance contrast were set at different values. Results. For normal subjects, dual-directional alternating OKN could be evoked in the ISI range from 33 to 100 ms. The dual-directional alternating OKN was defined as that OKN slow phase alternatively tracked in the direction of p/2 shift (forward OKN) and against the p/2 shift (reverse OKN). By contrast, for most glaucomatous eyes, nearly no reverse OKN could be evoked at any ISI value. Conclusions. The lack of reverse OKN in POAG patients in the present experiments is a meaningful finding. The occurrence of reverse OKN during a certain range of ISI duration could be related to the biphasic characteristics of the temporal impulse response in normal subjects, whereas, the lack of reverse OKN might suggest the plausible damage of magnocellular cells in POAG.