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PubMed1983

Sine-wave gratings: scale invariance and spatial integration at suprathreshold contrast.

Thresholds for the detection of contrast differences and spatial frequency differences present in a sine-wave grating with suprathreshold contrast were measured as a function of the size of the grating and the number of cycles in the grating. For gratings with the same number of cycles, these thresholds are found to be independent of the size of the grating over the entire range of sizes tested (1.5 log unit): the thresholds do not change when the stimulus is enlarged by some scale factor. We call this phenomenon "scale invariance". The thresholds decrease with increasing number of grating cycles, at least up to the point where 16 cycles are present. This support the view that "spatial pooling" or "spatial integration" mechanisms act over a larger area than the "functional summation area" found in threshold contrast measurements (about 10 cycles high and wide).

Adult↗
PubMed1984

The effect of variability of background elements on the conspicuity of objects.

The effect of complex backgrounds on target visibility was studied by asking observers to locate a disc target presented on a background of randomly arranged discs. A previous paper (Vision Res. 22, 1241-1252, 1982) explored the effect of increasing the density (numerosity) of the background elements when the elements were all the same size and luminance. In this investigation the size or the luminance of the background elements varied although mean size and mean luminance was held constant. Variability of the size of the background elements had a substantial effect on the size contrast necessary to detect the target. The data are well described by a contrast metric CT which assumes that observers detect the target by comparing it to a weighted average of the larger elements. The larger elements that determine the weighted average are those that all appear the same size because, at the eccentricity of target, the observers are unable to distinguish their differences in size. The threshold values of CT for detection of the target at a given level of probability were dependent on eccentricity but independent of the degree of variability of the background elements. The threshold contrast was also the same as the necessary for the more elementary task of discriminating size differences of two discs. In a second experiment it is shown that variability in the luminance of the background elements did not affect target visibility. This result is explained by reference to the data in our earlier paper from which it can be shown that the observers were unable to perceive the differences in luminance in their peripheral visual field so that, within limits, backgrounds that vary in luminance can be validly characterised by mean luminance.

Adult↗
PubMed1985

Normative data for pigeon vision.

Normative data are reported for the visual acuity, luminance-difference threshold and size-difference threshold of pigeons (Columba livia). The mean visual acuity was 12.66 c/deg or 2.53 min (N = 54), the mean log luminance-difference threshold was 0.11 (N = 105) and the mean size-difference threshold was 0.94 mm (N = 20).

Animals↗
PubMed1988

Compound binocular rivalry.

Binocular rivalry was examined with random dot patterns consisting of three colours: red, green and grey. The microstructure of the patterns was defined by the individual dots, and the correspondence between the microstructures in the two eyes was manipulated. The macrostructures were defined by the distributions of red, green and grey dots over the displays, so that they consisted of orthogonally striped patterns. The degree of correspondence between the microstructures was varied in Expt 1, together with the spatial frequency of the microstructure. Rivalry periods of the macrostructures were briefer when the microstructures were in correspondence, In Expt 2 the spatial frequencies of the macrostructures were varied. The lower spatial frequency predominated for longer than the higher. The results are discussed in terms of independent pathways for corresponding and rivalry stimulation. In addition a stimulus pairing that produces clear dichoptic colour mixtures is presented.

Dominance, Cerebral↗
PubMed1989

Visually perceived eye level and perceived elevation of objects: linearly additive influences from visual field pitch and from gravity.

Observing a pitched visual field (i.e. tilted around a horizontal axis in the observer's frontal plane) results in large changes in the elevation visually perceived to correspond to eye level (VPEL) and in the perceived elevation and size of stationary objects viewed against the field. With topforward pitch (top toward observer) VPEL lies above true eye level and objects appear smaller and lower; with topbackward pitch VPEL lies below true eye level and objects appear larger and higher. Oscillation of the pitched field induces synchronous perceived oscillation of elevation of a stationary target viewed against the field. Typical VPEL settings deviated from true eye level by 20 degrees with the field pitched at 40 degrees, although some individuals mislocalized by as much as 40 degrees. VPEL varied linearly with visual field pitch with individual slopes for the relation between VPEL and visual field pitch ranging from +0.42 to +0.78 (avg = +0.56). The linear correlation (r) between VPEL in darkness and against an erect visual field was +0.91. The two relations--VPEL vs visual field pitch, VPEL in darkness vs VPEL in the erect illuminated visual field (slope approximately equal to 0.5)--are both accurately predicted by the linear model: VPEL = kvV + kbB; in which V is the influence of visual field structure and B is the influence of the body-referenced mechanism which combines information regarding the orientation of the head relative to gravity, the position of the eye in the orbit, and the vertical location of the image on the retina; kv and kb are the relative weights of V and B with kv + kb = 1. In an illuminated field kv = kb approximately equal to 0.5; in the dark kv = 0, kb = 1.

Gravitation↗
PubMed1989

Width discrimination for band-pass stimuli.

There is a disagreement in the literature about whether the discrimination function for width is a continuous or a segmented function of target size (Hirsch & Hylton, 1982; Mayer & Kim, 1986). We determined the Weber fraction for width for difference of Gaussian (DOG) luminance distributions. Data for seven different standard widths were collected simultaneously. Although the value of the Weber fraction varied with target size, there was no evidence for any discontinuities in the discrimination function, either in the fovea or at 2.0 deg eccentricity.

Humans↗
PubMed1990

Spatial interactions and models of adaptation.

Adaptation mechanisms can be divided into two classes: multiplicative mechanisms which reduce the gain and subtractive mechanisms which discount or filter out the background signal. This paper investigates the neural basis of subtractive adaptation in photopic vision. Specifically, can the spatial interactions revealed by Westheimer's effect be described as subtractive? The evidence presented here shows that they can. That is, small adapting fields raise threshold more than large ones because they produce more response compression, not because they reduce the gain. As the background is enlarged progressively more of the background signal is subtracted off, reducing the response compression at some later non-linear site. These results indicate that retinal center-surround antagonism is one of the mechanisms mediating subtractive adaptation.

Adaptation, Ocular↗
PubMed1990

Two mechanisms for localization? Evidence for separation-dependent and separation-independent processing of position information.

The Weber function for separation--i.e. delta s as a function of separation s--is typically measured using a pair of targets presented roughly symmetrically relative to the fovea. With this paradigm, as the separation increases, the eccentricity of the individual targets increases also. To disentangle the effects of separation and eccentricity on the Weber function for separation, we systematically examined each of these variables and also examined the effects of target size and exposure duration. Separation discrimination thresholds were measured for average separations from 3 to 6 deg across a wide range of eccentricities, and for eccentricities of 2.5-10 deg for a range of separations. The dependence of threshold on target size was measured by varying the length of the stimuli from 1 to 120 min arc; the dependence on exposure duration was measured using durations of 100 and 500 msec at 10 deg eccentricity for comparison with data collected previously at smaller eccentricities. We found that for separations less than the eccentricity of the targets, thresholds depend primarily on separation; for larger separations, thresholds depend solely on eccentricity. In general, unless the targets are very small or quite brief, the spatial and temporal characteristics of the targets are not major contributors to the slope of the Weber function. Two mechanisms are proposed to account for thresholds in the two regions, one separation-dependent and one separation-independent.

Differential Threshold↗
PubMed1992

Texture segregation in the cat: a parametric study.

We have investigated how different texture parameters affect texture segregation in the cat, and which strategies cats use to solve the segregation task. Five cats were presented with stimuli consisting of two adjacent panels. One side contained a square area of a particular texture embedded in a different background texture; the other side was filled with only the background texture. The animal's task was to detect at which side the texture difference was presented. Sensitivity for the texture difference was assessed by making one aspect of the texture (in most instances the size of the texture elements) dependent upon performance by means of a staircase procedure. Among the most prominent parametric effects are those of density and element position randomization. In general, segregation was optimal at intermediate densities and deteriorated at larger and smaller densities. Element position randomization caused a slight but systematic decrease in segregation performance. Furthermore, we found texture elements at the border between different textures to be of primary importance for segregation. Which strategy the animals used for solving the segregation task depended upon the presence of random figure/background reversals in subsequent stimulus presentations during training. The animals learned to detect texture differences if these reversals were present, and without reversals, they learned to identify the particular texture in the target square. Interestingly, parameter dependencies of segregation did not depend upon the detection strategy used. We have speculated that the two different strategies used by the cats to solve the segregation tasks are related to different hierarchical levels of texture segregation which can be traced back to different stages of texture processing in human models of segregation performance.

Animals↗
PubMed1992

On the scaling of size judgements by orientational cues.

Observers carried out multiple, concurrent size discriminations with a range of size standards. The task was to classify each stimulus as larger or smaller than the appropriate standard size for the set to which it belonged. The set to which each stimulus belonged was indicated by its orientation, or in different experiments, by its spatial location. Observers were able to maintain appropriate discrimination, both when there were four concurrent standards and when there were eight. Both angle and position functioned as effective cues. The size of the orientational cue appeared to make little difference to the efficiency of discrimination. However, when the relationship between standard size and orientation was random, rather than regular, performance got worse. The analogy between such discrimination and size constancy is pointed out, and the results are discussed in relation to Andrews, D. P.'s [(1964) Quarterly Journal of Experimental Psychology, 16, 104-115)] account of perceptual calibration.

Discrimination, Psychological↗
PubMed1993

Can we attend to large and small at the same time?

Evidence from several sources suggests that visual attention is tuned to stimulus scale. This tuning impairs performance when the observer must attend to more than one scale at a time. In experiments originally designed to measure the bandwidth of attention to stimulus scale, we have found tasks in which observers show no attentional tuning for scale. In separate blocks, observers located or identified targets in arrays of elements, either numbers in arrays of letters or static squares in arrays of flashing squares. Each display contained two arrays, either of the same scale or of different scales. The accuracy of locating the target element was lower in mixed-scale than in single-scale displays, but the accuracy of identifying the target was unaffected by a mixing of scales within the same display. This holds for both high-level discriminations--numbers vs letters--and low-level discriminations--static vs flashing. Thus, at least for identifying, one can attend to large and small at the same time. The difference in bandwidth between "what" and "where" implies that stimulus identification is not dependent on prior localization.

Attention↗
PubMed1995

An exponential pyramid model of the time-course of size processing.

Two prior approaches to size processing are discussed in this paper. The first approach is based on measurements of mental size transformations, the second on measurements of thresholds for size and separation. We first analyze these prior approaches and point out differences among prior models and similarities among prior results. This analysis led to new psychophysical experiments that tested the effect of size, relative precision, and eccentricity on the speed of perceptual processing. Speed was not affected by size, but was affected by relative precision and eccentricity. These new results, along with prior results, are then used to formulate a new model based on an exponential pyramid algorithm. This new model, which uses elements of both traditional approaches, can better account for prior, as well as our new results, on the time-course of size processing.

Algorithms↗