Masking Gratings Research Articles

Ageing and visual spatiotemporal processing.

Ageing affects many visual functions. Here, we investigated the effects of ageing on vernier acuity and backward masking using the shine-through paradigm. We divided healthy older adults (>60years) into two groups depending on whether vernier duration was comparable to younger adults (Older Adults 1) or not (Older Adults 2). Backward masking was deteriorated for Older Adults 2 but not for Older Adults 1. In addition, by using complex masking gratings, we found deficits in spatial and temporal vision in Older Adults 2, which cannot be explained by deteriorated visual acuity, pointing to cortical rather than retinal causes. Our results highlight the importance of taking into account individual differences in visual ageing research. In addition, our results have important implications for schizophrenia. Schizophrenia has been suggested to be a form of early brain ageing. Linking our current masking results in ageing to previous masking results in schizophrenia shows that schizophrenia is not a form of early ageing, at least not in the visual domain.

Schizophrenia and visual backward masking: a general deficit of target enhancement

The obvious symptoms of schizophrenia are of cognitive and psychopathological nature. However, schizophrenia affects also visual processing which becomes particularly evident when stimuli are presented for short durations and are followed by a masking stimulus. Visual deficits are of great interest because they might be related to the genetic variations underlying the disease (endophenotype concept). Visual masking deficits are usually attributed to specific dysfunctions of the visual system such as a hypo- or hyper-active magnocellular system. Here, we propose that visual deficits are a manifestation of a general deficit related to the enhancement of weak neural signals as occurring in all other sorts of information processing. We summarize previous findings with the shine-through masking paradigm where a shortly presented vernier target is followed by a masking grating. The mask deteriorates visual processing of schizophrenic patients by almost an order of magnitude compared to healthy controls. We propose that these deficits are caused by dysfunctions of attention and the cholinergic system leading to weak neural activity corresponding to the vernier. High density electrophysiological recordings (EEG) show that indeed neural activity is strongly reduced in schizophrenic patients which we attribute to the lack of vernier enhancement. When only the masking grating is presented, EEG responses are roughly comparable between patients and control. Our hypothesis is supported by findings relating visual masking to genetic deviants of the nicotinic α7 receptor (CHRNA7).

Visual masking deficits in healthy and schizophrenic women

Schizophrenic patients have serious visual masking deficits which are likely related to the genetic underpinnings of the disease because also unaffected relatives of the patients show masking deficits. We presented a left/right offset vernier followed by an ISI and a masking grating. Observers indicated the offset direction. We determined the SOA (vernier duration plus ISI), to reach 75% correct responses. Analyzing a new sample, we found that patients (n=239) needed SOAs of 127.6ms, relatives (n=125) of 62.4ms, and controls (n=145) of 32.2ms. In addition, we analyzed the data for the groups separately for men and women. We found main effects of Group and Gender but no significant interaction. Female observers needed SOAs of about 15ms longer than the male observers in the control group, 25ms in the relatives group and 37ms in the schizophrenia group. For the control group, hence, females performed by a factor of nearly 2 worse than males. No gender effect was found for executive functions as measured with the WCST. It seems that both gender and the susceptibility for schizophrenia are independent main effects affecting spatio-temporal vision. Our results show that a proper gender balance is crucial in experiments where signal to noise ratio is low.

Modelling contrast discrimination data suggest both the pedestal effect and stochastic resonance to be caused by the same mechanism

Computational models of spatial vision typically make use of a (rectified) linear filter, a nonlinearity and dominant late noise to account for human contrast discrimination data. Linear-nonlinear cascade models predict an improvement in observers' contrast detection performance when low, subthreshold levels of external noise are added (i.e., stochastic resonance). Here, we address the issue whether a single contrast gain-control model of early spatial vision can account for both the pedestal effect, i.e., the improved detectability of a grating in the presence of a low-contrast masking grating, and stochastic resonance. We measured contrast discrimination performance without noise and in both weak and moderate levels of noise. Making use of a full quantitative description of our data with few parameters combined with comprehensive model selection assessments, we show the pedestal effect to be more reduced in the presence of weak noise than in moderate noise. This reduction rules out independent, additive sources of performance improvement and, together with a simulation study, supports the parsimonious explanation that a single mechanism underlies the pedestal effect and stochastic resonance in contrast perception.

Collinear contextual suppression in schizophrenic patients

Schizophrenic patients show aberrancies of contextual processing over a broad range. Of particular importance are low level contextual deficiencies since they might cause higher level processing deficits. It was previously found that schizophrenic patients reveal diminished contextual facilitation in visual detection tasks taken as an indication of a modified neural circuitry. Here, we show that contextual suppression is not affected. Sixteen schizophrenic patients and sixteen healthy controls participated in a backward masking task in which a vernier target was followed by a masking grating. In accordance with a previous publication, schizophrenic patients needed longer SOAs between the vernier and the grating onset to obtain a performance level comparable to healthy controls. To study contextual processing we added single collinear lines to the grating. These lines yielded a strong impairment of performance in patients and controls. This impairment is comparable between the two groups if SOAs were individually adjusted. Hence, whereas contextual facilitation is deficient, contextual suppression seems to be intact in schizophrenic patients.

Binocular contrast vision at and above threshold

A fundamental problem for any visual system with binocular overlap is the combination of information from the two eyes. Electrophysiology shows that binocular integration of luminance contrast occurs early in visual cortex, but a specific systems architecture has not been established for human vision. Here, we address this by performing binocular summation and monocular, binocular, and dichoptic masking experiments for horizontal 1 cycle per degree test and masking gratings. These data reject three previously published proposals, each of which predict too little binocular summation and insufficient dichoptic facilitation. However, a simple development of one of the rejected models (the twin summation model) and a completely new model (the two-stage model) provide very good fits to the data. Two features common to both models are gently accelerating (almost linear) contrast transduction prior to binocular summation and suppressive ocular interactions that contribute to contrast gain control. With all model parameters fixed, both models correctly predict (1) systematic variation in psychometric slopes, (2) dichoptic contrast matching, and (3) high levels of binocular summation for various levels of binocular pedestal contrast. A review of evidence from elsewhere leads us to favor the two-stage model.

Contrast dependence of spatial summation revealed by classification image analysis

Detection of low-contrast luminance-defined stimuli can involve spatial summation over a large portion of the visual field. However prior psychophysical results suggest that the summation region may shrink substantially in the presence of high-contrast masking gratings or noise (Legge & Foley, 1980; Kersten, 1984). This may be related to recent findings that the extent of spatial summation in V1 neurons depends upon contrast (Sceniak et al., 1999). Here we use a classification image technique to directly test whether the psychophysical receptive field for a simple stimulus (a vertical edge in noise) is dependent upon contrast. Classification images for yes/no edge detection and 2IFC contrast discrimination were estimated at signal (edge) contrasts ranging from 0.6%–50% and noise contrasts ranging from 4%–50%. Estimated receptive fields were found to be well approximated by elongated 2D Gaussian derivative or Gabor filters, and were much longer than V1 receptive fields. Both signal contrast and external noise contrast were found to strongly affect the extent of summation: a good predictor for receptive field size and shape was found to be signal-to-noise ratio (SNR). As SNR was increased from as low as 0.07 to as high as 2.0, receptive fields were found to shrink dramatically, by a factor of more than 5 in width and by roughly 60% in length. The dependence of summation on SNR may be due to ‘late’ noise sources and/or to nonlinear summation mechanisms.

Fusion of competing features is not serial

How features of an object are bound into a unique percept is one of the puzzling problems in the cognitive and neuro-sciences. In order to investigate the spatio-temporal mechanisms of feature binding, we serially present two verniers with opposite offset directions for very short durations. Only one vernier is perceived with its offset dominated by the vernier presented second. This dominance reverses if the two verniers are followed by masking gratings, i.e. the first presented vernier dominates performance. Therefore, feature fusion can neither be explained completely by spatially local mechanisms nor by the temporal order of appearance of elements.

Contrast discrimination with pulse trains in pink noise.

Detection performance was measured with sinusoidal and pulse-train gratings. Although the 2.09-cycles-per-degree pulse-train, or line, grating contained at least eight harmonics all at equal contrast, it was no more detectable than its most detectable component. The addition of broadband pink noise designed to equalize the detectability of the components of the pulse train made the pulse train approximately a factor of 4 more detectable than any of its components. However, in contrast-discrimination experiments, with a pedestal or masking grating of the same form and phase as the signal and with 15% contrast, the noise did not affect the discrimination performance of the pulse train relative to that obtained with its sinusoidal components. We discuss the implications of these observations for models of early vision, in particular the implications for possible sources of internal noise.

Contrast discrimination under temporally varying contrast conditions.

Psychophysical contrast discrimination of a 0.8-cpd vertical grating was tested using a paradigm that alternated test and masking gratings at 8 Hz. Masking contrasts were lower than, equal to, or higher than the test contrasts. Six test contrasts were combined factorially with six masking contrasts to generate a series of six contrast increment threshold versus test contrast curves (tvc curves). A particularly simple relationship existed between these curves. The curves could be brought into alignment by shifting them diagonally by the ratio of their masking contrasts. It is shown that this behavior is predicted by a model in which contrast gain is set by the average of the test and masking contrasts coupled with a simple model of contrast discrimination. Contrast gain control integrates contrast over a period of at least 125 msec, and contrast discrimination is a function of this time-averaged contrast.

Zero frequency masking and a model of contrast sensitivity

Stimulating the visual system tends to desensitize it to certain stimulus properties. Such desensitization is usually called adaptation or masking, but the distinction between the two is unclear. Nonspecific desensitization by light is usually regarded as adaptation, whereas pattern-specific desensitization is typically considered masking. Here we unify the treatment of such desensitizing phenomena by handling both in the spatial frequency domain. The amount of adapting light in a stimulus is represented in the spatial frequency domain by the component at zero frequency. To determine whether such adapting light acts like other components in the spatial frequency domain, we compared the effect of masking by the zero frequency component with the effects of masking by components at other frequencies. We show that the zero frequency component acts like other masking components, decreasing sensitivity to nearby test frequencies and thereby producing the insensitivity to low spatial frequencies that gives the contrast sensitivity curve its band-pass shape at high light levels. Treating light adaptation as masking by the zero frequency component leads to a general model that describes visual sensitivity to test gratings of varying spatial frequency at varying mean luminance, in the presence (or absence) of masking gratings of varying spatial frequency. Individual components of the model provide insight into visual processing at the system level.

Contrast dependence of the oscillatory motion threshold across the visual field.

Observer sensitivity to oscillatory step displacements of sine-wave gratings was investigated at various loci in the visual field (0-30 degrees) as a function of contrast. Detection thresholds at 10 Hz and high grating contrasts were approximately 11-15 arcsec in the fovea and 37-47 arcsec at 30 degrees eccentricity. At any given contrast, threshold displacement increases linearly with eccentricity. The data provide evidence against an interpretation based on cortical magnification, because the slope and the scale-free x intercept of the eccentricity function vary strongly with contrast. While foveal thresholds for high-contrast gratings are in the range of the hyperacuities, the oscillatory motion threshold falls off an order of magnitude more slowly than the traditional hyperacuities. Rather than conceiving of the oscillatory motion threshold as a spatial acuity limited by cortical magnification, we suggest an alternative approach that is based on a form of contrast discrimination. Oscillatory motion can be decomposed into the sum of a modulating counterphase grating and a static masking grating, both of which are in spatial quadrature (i.e., 90 degrees out of phase). At low grating contrast, oscillatory motion can be detected when the counterphase component exceeds a constant contrast value. Above a critical contrast value of the static component Cscrit, threshold rises as a power function of contrast with a slope near 1.0. The critical contrast value Cscrit increases linearly with eccentricity, indicating that oscillating gratings observed with the peripheral visual field are less easily masked compared with foveally fixated gratings.

Contrast adaptation and contrast masking in human vision

After a preliminary study of visual evoked potentials (VEPS) to a test grating seen in the presence of masks at different orientations, psychophysical data are presented showing the effects of adaptation and of masking on thresholds for detecting the same test grating. The test is a vertical grating of spatial frequency 2 cycles per degree; adapting and masking gratings differ from the test either in orientation or in spatial frequency. The effects of adaptation and masking are explained by a single mechanism model that assumes: (i) adaptation and masking both alter the contrast response (or transducer) function of the mechanism that detects the test; (ii) masks, but not adaptors, stimulate the mechanism that detects the test; and (iii) a test is detectable when it raises response level by a constant amount. The model incorporates two distinct tuning functions, a broad adaptive contrast function and a narrow effective contrast function. It accounts adequately for all the data, including the location and size of the facilitative dip found in some masking functions, the constant slopes of the threshold elevation segments of adaptation functions and the varying slopes of masking functions. It also predicts the sometimes surprising joint effects of adaptation followed by masking and of two masks operating simultaneously.

Facilitation and masking of briefly presented gratings: Time-course and contrast dependence

We measured two-alternative forced-choice contrast thresholds for briefly presented sinusoidal gratings in the presence of superimposed masking gratings of various contrasts, and at a range of onset asynchronies. Facilitation (lower thresholds) occurred when the mask was simultaneous, in-phase, and near-threshold, but was abolished at asynchronies of 50 msec or more and by presenting the test grating as a brief contrast reversal instead of a pulse. We argue that facilitation requires temporal summation of responses within the same neural channels, but our results do not distinguish between transducer and uncertainty models. Masking (threshold elevation) occurred over a broader range of asynchronies, and was not abolished by test contrast reversal. Masking and facilitation probably depend on different processes with different time-courses. The occurrence of masking at asynchronies outside the range of temporal summation suggests that a static, compressive transducer does not, in general, account for masking. Brief masking and prolonged contrast adaptation are very similar in magnitude, and as a function of contrast and relative spatial frequency. Masking and adaptation may have a common origin, but differ in speed of recovery.

Seeing objects in motion.

This paper reports estimates of the conjoint spatiotemporal tuning functions of the neural mechanisms of the human vision system which detect image motion. The functions were derived from measurements of the minimum contrast necessary to detect the direction of drift of a sinusoidal grating, in the presence of phase-reversed masking gratings of various spatial and temporal frequencies. A mask of similar spatial and temporal frequencies to the test grating reduces sensitivity considerably, whereas one differing greatly in spatial or temporal frequency has little or no effect. The results show that for test gratings drifting at 8 Hz, the tuning function is bandpass in both space and time, peaked at the temporal and spatial frequency (SF) of the test (SFs were 0.1, 1 or 5 c deg-1; c represents cycles throughout). For a grating of 5 c deg-1 drifting at 0.3 Hz, the function is bandpass in space but lowpass in time. Fourier transform of the frequency results yields a function in space-time which we term the 'spatiotemporal receptive field'. For movement detectors (bandpass in space and time) the fields comprise alternating ridges of opposing polarity, elongated in space-time along the preferred velocity axis of the detector. We suggest that this organization explains how detectors analyse form and motion concurrently and accounts, at least in part, for a variety of perceptual phenomena, including summation, reduction of motion smear, metacontrast, stroboscopic motion and spatiotemporal interpolation.

Simultaneous masking between two sinusoidal gratings measured in terms of temporal sensitivity.

Effects of high-contrast simultaneous masking gratings on the temporal sensitivity of signal gratings were measured as a function of masking spatial frequency, where temporal sensitivity was defined as the reciprocal of the minimum exposure duration required for a signal to be detected. The sensitivity of signal gratings was shown to be facilitated when signal frequencies and masking frequencies were separated by more than about 2 octaves but inhibited when the two frequencies were not separated this much. The results were quantitatively different from the results obtained in terms of contrast sensitivity. Psychometric functions on grating detection were shown to become steeper when the temporal sensitivity was enhanced and less steep when it was decreased.

Electrophysiological evidence for spatial frequency selective mechanisms in adults and infants

VEP in response to a grating reversed in phase at 7 Hz have been recorded in the presence of a masking grating reversed in phase at a different rate. In adults VEP responses are depressed by a masking grating, either optically superimposed or viewed dichoptically, and the masking effects are spatial frequency selective. Spatial frequency tuning of masking effects on VEP has also been found in infants of 6 and 14 weeks of age. tested at 0.3 and 1 c/deg, respectively. The infant tuning functions have a steeper low-frequency attenuation than adult's functions at the corresponding spatial frequencies and are more similar to those obtained in the adult at higher spatial frequencies.

Temporal determinants of spatial sine-wave masking

A temporal forced-choice procedure was used to measure the contrast threshold for a sinusoidal test grating (spatial frequency = f) superimposed upon a sinusoidal background or masking grating (spatial frequency = 3f). The spatial contrast of the background grating was varied, and threshold measurements were made at each of a number of background contrasts to describe a threshold versus masking contrast ( tvc) function. Tvc functions were obtained when the background and test grating contrasts were, independently of each other, held steady (0 Hz) or modulated at 5 Hz. The changes of temporal modulation frequency affected the slopes of the tvc functions. In some cases the tvc functions for steady and flickering test gratings crossed one another. The changes of slope suggest, and the crossovers imply, that some steady and flickering patterns are detected by separate visual mechanisms.

Spatial disinhibition of orientation analyzers.

Following the successive presentation of two masking gratings (M1 and M2), subjects were required to detect the presence or absence of a single vertical line (TS). When the orientations of the two masks were optimal, M1 was able to reduce the masking effect of M2 on the TS. For a vertical TS, disinhibition was maximal when the orientations of M1 and M2 were similar and was minimal when the orientational difference was greater than 15 deg. It is suggested that the spatial selectivity of the disinhibition function reflects the activity of neurons tuned to orientation, and that the disinhibition masking paradigm may be a useful psychophysical technique to measure tuning functions of other feature detectors.

Binocular interactions in normal and anomalous binocular vision.

Binocular interactions for grating patterns were investigated in humans with normal binocular vision and in humans with abnormal binocular visual experience due to strabismus and/or amblyopia via 1) comparison of monocular and binocular contrast thresholds; 2) interocular transfer of the threshold elevation aftereffect; and 3) dichoptic masking. Whereas the normal observers showed improved binocular over monocular contrast sensitivity (i.e., binocular summation) and substantial interocular transfer of the threshold elevation aftereffect, the abnormal observers showed an absence of binocular summation and no significant interocular transfer. The dichoptic masking experiments showed that a suprathreshold masking grating presented to one eye elevated the contrast threshold for gratings presented to the fellow eye, within a narrow range of spatial frequencies (about 1 octave wide at half height) and orientations, centered about the spatial frequency and orientation of the mask. The magnitude and bandwidth of this masking effect was similar in subjects with normal and abnormal binocular vision, occurring even when the masking grating was presented to the amblyopic eye. These effects depend upon the contrast of the masking grating. In individuals with normal binocular vision, a grating with subthreshold contrast presented to one eye reduces the contrast threshold for detection of gratings of similar spatial frequency and orientation presented to the fellow eye. No such subthreshold summation is evident in the amblyopic observers. We conclude that while strabismus and/or amblyopia disrupted the normal excitatory interactions between the two eyes, cortical inhibitory binocular connections were not disrupted.