Norma Graham

Probability summation and regional variation in contrast sensitivity across the visual field

Abstract Contrast sensitivity at different positions in the visual field has been measured at various spatial frequencies using a patch of grating suitably vignetted to give a stimulus localized in both space and spatial frequency. While contrast sensitivity along a vertical line through the fixation point falls off steadily from a maximum at the centre, sensitivity along a horizontal line displaced 42 periods of the grating above the fixation point is approximately constant, at least out to 32 periods from the mid-line. The way in which detectability increases with increasing number of cycles (2 up to 64) has been measured for gratings with short horizontal bars centred on the fixation point and for gratings with short vertical bars centred on the mid-line 42 periods above it. The relation between sensitivity and number of cycles can in each case be explained exactly assuming probability summation across space as long as the variation in sensitivity across the visual field is taken into account.

Detection of grating patterns containing two spatial frequencies: A comparison of single-channel and multiple-channels models

Abstract Contrast thresholds were measured for gratings containing two superimposed sinusoidal components. The frequency of one component was always three times that of the other, but the phase between components and the ratio of their contrasts took on several values. Two models of pattern vision were tested (1) a single-channel model in which pattern vision is a function of a single neural network and (2) a multiple-channels model in which the stimulus information is processed by many channels, each sensitive to a narrow range of spatial frequencies. Results support the multiple-channels and reject the single-channel model.

Grating summation in fovea and periphery

Abstract Results from previous studies measuring the detectability of sinusoidal gratings have been interpreted by models postulating several sizes of receptive fields. It has not been clear, however, whether or not these several sizes coexist at a single position in the visual field. Perhaps there is only one size centered at each position, but the size varies as a function of eccentricity. In this study, the detectability of compound gratings containing two sinusoidal components was compared to that of each component alone. Measurements were made in the fovea and 7.5° into the periphery. Stimuli were localized in a small region of the visual field and sharp spatial and temporal transients eliminated by weighting grating contrast with Gaussian functions of space and time. To reduce possible effects of expectation, bias and frequency uncertainty, a temporal, forced-choice, interlaced staircase procedure was used. The results are consistent with models postulating several sizes of receptive fields at each position in the visual field but not with models postulating only one size at each position, even when the size varies as a function of eccentricity to account for the differences in spatial interaction characteristic of different parts of the visual field.

Visual detection of aperiodic spatial stimuli by probability summation among narrowband channels.

Abstract Recent psychophysical results of Shapley and Tolhurst and of Kulikowski and King-Smith have suggested that the visual system contains broadband channels like “edge detectors” and “line detectors” as well as relatively narrowband “spatial frequency” channels. These recent results (including thresholds for aperiodic stimuli) can be completely explained using only relatively narrowband channels with probability summation among them. This explanation requires many fewer free parameters than the original explanation based on both broadband and narrowband channels. The bandwidths of the individual narrowband channels can be estimated and are similar to those previously estimated from sine-wave summation experiments.

Contrast and spatial variables in texture segregation: Testing a simple spatial-frequency channels model

Observers were shown patterns composed of two textures in which each texture contained two types of elements. The elements were arranged in a striped pattern in the top and bottom regions and in a checked pattern in the center region. Observers rated the degree to which the three regions were seen as distinct. When the elements were squares or lines, perceived segregation resulting from differences in element size could be canceled by differences in element contrast. Minimal perceived segregation occurred when the products of the area and the contrast (areal contrasts) of the elements were equal. This dependence of perceived segregation on the areal contrasts of the elements is consistent with a simple model based on the hypothesis that the perceived segregation of the regions is a function of their differential stimulation of spatial-frequency channels. Two aspects of the data were not consistent with quantitative predictions of the model. First, as the size difference between the large and small elements increased, the ratings at the point of minimum perceived segregation increased. Second, some effects of changing the fundamental frequency of the textures were not predicted by the model. These discrepancies may be explained by a more complex model in which a rectification or similar nonlinearity occurs between two stages of orientation- and spatial-frequency-selective linear filters.

Spatial frequency uncertainty effects in the detection of sinusoidal gratings

Abstract A sinusoidal grating is less detectable when it is randomly intermixed in a block of trials with gratings of other spatial frequencies than when it is the only grating presented in a block of trials. This spatial-frequency uncertainty effect is expected if observers have attentional control over multiple spatial-frequency channels. When intermixed blocks contain a preponderance of one spatial frequency, the primary , the uncertainty effect is smaller for frequencies near the primary than for frequencies further away. Both the tuning of the uncertainty effect and the analysis of sequential conditional probabilities suggest that observers can employ different attention strategies.

Spatial frequency channels in the human visual system: Effects of luminance and pattern drift rate

Abstract Recent evidence indicates that the human visual system contains multiple channels, with each channel sensitive to a different narrow range of spatial frequency. In this study the sensitivity of these channels for patterns at low mean luminance or high drift rate is measured by the effect of adaptation to sinusoidal gratings on the contrast thresholds for sinusoidal gratings. The channels do not behave in the way expected from retinal ganglion cell physiology; rather, they remain selectively sensitive to narrow ranges of spatial frequency even when the luminance is low or the drift rate high.

Modeling the dynamics of light adaptation: the merging of two traditions

Light adaptation has been studied using both aperiodic and periodic stimuli. Two well-documented phenomena are described: the background-onset effect (from an aperiodic-stimulus tradition) and high-temporal-frequency linearity (from the periodic-stimulus tradition). These phenomena have been explained within two different theoretical frameworks. Here we briefly review those frameworks. We then show that the models developed to predict the phenomenon from one tradition cannot predict the phenomenon from the other tradition, but that the models from the two traditions can be merged into a class of models that predicts both phenomena.

Beyond multiple pattern analyzers modeled as linear filters (as classical V1 simple cells): useful additions of the last 25 years.

This review briefly discusses processes that have been suggested in the last 25 years as important to the intermediate stages of visual processing of patterns. Five categories of processes are presented: (1) Higher-order processes including FRF structures; (2) Divisive contrast nonlinearities including contrast normalization; (3) Subtractive contrast nonlinearities including contrast comparison; (4) Non-classical receptive fields (surround suppression, cross-orientation inhibition); (5) Contour integration.

Investigating simple and complex mechanisms in texture segregation using the speed-accuracy tradeoff method.

Several recent models of texture segregation have proposed two mechanisms: simple, linear channels (first-order, Fourier mechanisms) and complex channels (second-order, non-Fourier mechanisms). We used the speed-accuracy tradeoff (SAT) method to examine the time course of texture segregation processing in simple and complex channels. The stimuli were texture patterns designed to segregate primarily as a result of activity in one set of channels or the other. We presented subjects with textures that were checked or striped arrangements of either Gaussian-blob or Gabor-patch elements. Subjects were required to identify the orientation of a rectangular texture region embedded in a background field of a different texture. A range of contrasts and a control task were used to equate visibility of the Gabor and Gaussian textures. SAT functions were obtained by requiring subjects to respond within 200 msec after an auditory cue. We found that when segregation depended primarily on simple channels, performance was faster than when it depended primarily on complex channels: the 75% correct level was reached 100-200 msec sooner and this extra speed was reflected both in smaller delay and higher rate parameters.