Perceiving heading with different retinal regions and types of optic flow

We examined the ability to use optic flow to judge heading when different parts of the retina are stimulated and when the specified heading is in different directions relative to the display. To do so, we manipulated retinal eccentricity (the angle between the fovea and the center of the stimulus) and heading eccentricity (the angle between the specified heading and the center of the stimulus) independently. Observers viewed two sequences of moving dots that simulated translation through a random cloud of dots. They reported whether the direction of translation—the heading—in the second sequence was to the left or right of the direction in the first sequence. The results revealed a large and consistent effect of heading eccentricity: Judgments were much more accurate with radial flow fields (small heading eccentricities) than with lamellar fields (large heading eccentricities), regardless of the part of the retina being stimulated. The results also revealeda smaller and less consistent effect of retinal eccentricity: With radial flow (small heading eccentricities), judgments were more accurate when the stimulus was presented near the fovea. The variation of heading thresholds from radial to lamellar flow fields is predicted by a simple model of two-dimensional motion discrimination. The fact that the predictions are accurate implies that the human visual system is equally efficient at processing radial and lamellar flow fields. In addition, efficiency is reasonably constant no matter what part of the retina is being stimulated.     

New equally readable charts based on anisotropy of peripheral visual acuity1

Abstract: Anstis’ equally readable chart for visual acuity has been widely quoted in textbooks on visual perception. However, this chart does not reflect the anisotropy of peripheral visual acuity that has been reported by previous studies. Here, the authors reexamined peripheral visual acuity by measuring resolution thresholds for Landolt rings and recognition thresholds for hiragana letters as a function of retinal eccentricity across two principal retinal meridians: horizontal and vertical. Observers were required to identify the orientation of a gap in the Landolt ring or recognize a hiragana letter while the stimulus was moved slowly from the periphery toward the fovea across each of four meridians: nasal, temporal, superior, and inferior. The results revealed the horizontal‐vertical anisotropy of visual acuity in accordance with the results of previous studies. The mean thresholds obtained in the vertical meridian were approximately 1.51‐fold (for Landolt rings) and 1.42‐fold (for hiragana letters) higher than those obtained in the horizontal meridian at the same retinal eccentricity. The authors propose new equally readable charts for Landolt rings and hiragana letters.     

The effects of stimulus numerosity,retinal location,and rod contrast on perceived duration of brief visual stimuli

The effects of manipulating three stimulus parameters were determined on a standard perceived duration task using 40- and 70-msec visual stimuli. In Experiment 1, it was found that perceived duration increased with increasing number of target dots but decreased as the area that contained the dots also increased. Experiment 2 examined the effect of varying the retinal location (0, 2, or 4 deg from fovea) of the target dot and found perceived target duration to increase with increasing eccentricity. In Experiment 3, the background luminance for a constant red target was varied so as to alter the scotopic contrast of the target. Perceived duration was minimal for the scotopically matched target and background conditions. The results of all three experiments were discussed in terms of important retinal—and even rod—contributions to tasks of perceived duration.     

Contrast thresholds for identification of numeric characters in direct and eccentric view

Aubert and Foerster (1857) are frequently cited for having shown that the lower visual acuity of peripheral vision can be compensated for by increasing stimulus size. This result is seemingly consistent with the concept of cortical magnification, and it has been confirmed by many subsequent authors. Yet it is rarely noted that Aubert and Foerster also observed a loss of the "quality of form." We have studied the recognition of numeric characters in foveal and eccentric vision by determining the contrast required for 67% correct identification. At each eccentricity, the lowest contrast threshold is achieved with a specific stimulus size. But the contrast thresholds for these optimal stimuli are not independent of retinal eccentricity as cortical magnification scaling would predict. With high-contrast targets, however, threshold target sizes were consistent with cortical magnification out to 6 degrees eccentricity. Beyond 6 degrees, threshold target sizes were larger than cortical magnification predicted. We also investigated recognition performance in the presence of neighboring characters (crowding phenomenon). Target character size, distance of flanking characters, and precision of focusing of attention all affect recognition. The influence of these parameters is different in the fovea and in the periphery. Our findings confirm Aubert and Foester's original observation of a qualitative difference between foveal and peripheral vision.     

The relative benefit of word context is a constant proportion of letter identification time

Letter identification is reduced when the target letter is surrounded by other, flanking letters. This visual crowding is known to be impacted by physical changes to the target and flanks, such as spatial frequency content, polarity, and interletter spacing. There is also evidence that visual crowding is reduced when the flanking letters and the target letter form a word. The research reported here investigated whether these two phenomena are independent of each other or whether the degree of visual crowding impacts the benefit of word context. Stimulus duration thresholds for letters presented alone and for the middle letters of 3-letter words and nonwords were determined for stimuli presented at the fovea and at the periphery. In Experiment 1, the benefit of word context was found to be the same at the fovea, where visual crowding is minimal, and at the periphery, where visual crowding is substantial. In Experiment 2, visual crowding was manipulated by changing the interletter spacing. Here, too, the benefit of word context was fairly constant for the two retinal locations (fovea or periphery), as well as with changes in interletter spacing. These data call into question both the idea that the benefit of word context is greater when stimulus quality is reduced (as is the case with visual crowding) and the idea that words are processed more effectively when they are presented at the fovea.     

Interaction of stimulus size and retinal eccentricity in metacontrast masking

Metacontrast masking occurs both at the fovea and in the retinal periphery; foveally, the smallest stimulus elicited the strongest masking, whereas peripherally the reverse was the case. An analysis of variance showed a significant size effect, eccentricity effect, and size-eccentricity interaction. As stimulus size increased, the stimulus onset asynchrony of maximum masking shifted to greater values. Both foveal metacontrast and peak shifts contradicted predictions made by the hypothesis that metacontrast is mediated by an interaction of sustained and transient channels in the visual system. The data are consistent, however, with a lateral inhibitory model of metacontrast masking and stimulus coding.     

Spatial limits to the detection of transpositional symmetry in dynamic dot textures

If a set of random dots is translated along a uniform axis and superimposed on itself, then the resulting texture is seen to have three possible perceptual outcomes depending on the degree of translation: pairedness, striation, and randomness. Similar perceptual effects are obtained by plotting identical point-pairs randomly in space and uniformly over time. The threshold between striation and randomness in dynamic dot textures was investigated, since it represents a spatial limit to the detection of transpositional symmetry. This limit was found to vary as a function of three factors: stimulus field diameter, retinal eccentricity, and high frequency attenuation. The limit was found to be invariant over a fourfold range of texture density. It is proposed that the ability of the human visual system to detect correlations in dynamic transpositionally symmetric textures is the result of the activation of populations of orientationally selective cells identified in the mammalian visual cortex. It is also proposed that the limits to this ability could be related to the average size of the receptive fields of such cells within a given region of the retina.     

Task-irrelevant distractors in the delay period interfere selectively with visual short-term memory for spatial locations

Visual short-term memory (VSTM) enables the representation of information in a readily accessible state. VSTM is typically conceptualized as a form of “active” storage that is resistant to interference or disruption, yet several recent studies have shown that under some circumstances task-irrelevant distractors may indeed disrupt performance. Here, we investigated how task-irrelevant visual distractors affected VSTM by asking whether distractors induce a general loss of remembered information or selectively interfere with memory representations. In a VSTM task, participants recalled the spatial location of a target visual stimulus after a delay in which distractors were presented on 75% of trials. Notably, the distractor’s eccentricity always matched the eccentricity of the target, while in the critical conditions the distractor’s angular position was shifted either clockwise or counterclockwise relative to the target. We then computed estimates of recall error for both eccentricity and polar angle. A general interference model would predict an effect of distractors on both polar angle and eccentricity errors, while a selective interference model would predict effects of distractors on angle but not on eccentricity errors. Results showed that for stimulus angle there was an increase in the magnitude and variability of recall errors. However, distractors had no effect on estimates of stimulus eccentricity. Our results suggest that distractors selectively interfere with VSTM for spatial locations.     

Psychophysics of lateral tachistoscopic presentation

Human visual performance depends upon the retinal position to which a target is delivered. A general finding is that performance measured in a variety of psychophysical tasks deteriorates as a target is presented to more eccentric retinal regions. One purpose of this paper is to describe differences between foveal and peripheral vision in a number of psychophysical tasks. A second purpose is to review studies which have attempted to account for the fall off in visual performance between central and peripheral target presentations. A third purpose is to consider the contribution of the periphery to perception since targets which are sufficiently large project not only on receptors in the fovea but also on those in the periphery. In addition, stimuli presented to the peripheral retina can influence the processing of a target presented to the central retinal region. A fourth purpose is to review studies which have attempted to compensate for foveal and peripheral differences by scaling the target in size or some other attribute in proportion to the cortical magnification factor. A final purpose of this paper is to consider whether the fovea and the periphery are specialized for different functions.     

Perceived duration decreases with increasing eccentricity

Previous studies examining the influence of stimulus location on temporal perception yield inhomogeneous and contradicting results. Therefore, the aim of the present study is to soundly examine the effect of stimulus eccentricity. In a series of five experiments, subjects compared the duration of foveal disks to disks presented at different retinal eccentricities on the horizontal meridian. The results show that the perceived duration of a visual stimulus declines with increasing eccentricity. The effect was replicated with various stimulus orders (Experiments 1–3), as well as with cortically magnified stimuli (Experiments 4–5), ruling out that the effect was merely caused by different cortical representation sizes. The apparent decreasing duration of stimuli with increasing eccentricity is discussed with respect to current models of time perception, the possible influence of visual attention and respective underlying physiological characteristics of the visual system.     

Brightness as a function of retinal locus

Brightness functions were determined for the dark-adapted fovea and periphery. In one series of experiments, observers matched numbers to the brightness of a 1° white target at various intensities, presented half the time to the fovea, the other half to one of five peripheral loci: 5°, 12°, 20°, 35°, and 60°. In a second series, observers matched the brightness of a 1° white target in the fovea of one eye to the brightness of an identical target in the periphery of the other eye at various intensities. Thresholds were also determined for the fovea and for the five peripheral loci by a staircase procedure. The magnitude estimations and the interocular matches concur in showing that a stimulus of fixed luminance appears brighter in the periphery than in the fovea. The brightness was found to be maximal at 20°. Brightness grows as a similar power function of luminance at all six retinal positions.     

Brightness as a function of retinal locus

Brightness functions were determined for the dark-adapted fovea and periphery. In one series of experiments, observers matched numbers to the brightness of a 1° white target at various intensities, presented half the time to the fovea, the other half to one of five peripheral loci: 5°, 12°, 20°, 35°, and 60°. In a second series, observers matched the brightness of a 1° white target in the fovea of one eye to the brightness of an identical target in the periphery of the other eye at various intensities. Thresholds were also determined for the fovea and for the five peripheral loci by a staircase procedure. The magnitude estimations and the interocular matches concur in showing that a stimulus of fixed luminance appears brighter in the periphery than in the fovea. The brightness was found to be maximal at 20°. Brightness grows as a similar power function of luminance at all six retinal positions.     

Orientation sensitivity in the peripheral visual field

Orientation sensitivity in the peripheral visual field has been tested in two tasks: (a) setting horizontal the orientation of a grating at various retinal eccentricities, and (b) matching the orientation of a peripherally viewed grating as close as possible to an oblique reference viewed foveally. Both performances fall off with increasing retinal eccentricity. Magnification of the stimulus optimizes peripheral performance. Peripheral performance, optimized by magnification, varies with retinal eccentricity. It approaches, but does not reach, the foveal value (tested by the same method) at 10 deg of eccentricity, and is much lower at 20 and 30 deg of eccentricity.     

Effects of perceptual quality on the processing of human faces presented to the left and right cerebral hemispheres

Three experiments examined the effects of stimulus duration, retinal eccentricity, and visual noise on the processing of human faces presented to the left visual field/right hemisphere (LVF-RH) and right visual field/left hemisphere (RVF-LH). In Experiment 1 observers identified which of 10 similar male faces was presented on a screen. The single face was presented for 10, 55, or 100 ms at 1 degree, 4 degrees, or 9 degrees of visual angle to the left or right of fixation. Decreasing stimulus duration and increasing retinal eccentricity lowered face recognition. The effect of duration was the same for LVF-RH and RVF-LH trials, but the detrimental effect of increasing retinal eccentricity was larger on LVF-RH trials than on RVF-LH trials. In Experiment 2 observers indicated whether a single face from this same set was a member of a memorized set of five positive faces. The probe face on each trial was presented alone or embedded in visual noise. Visual noise increased the error rate more on LVF-RH trials than on RVF-LH trials. This effect was replicated in Experiment 3, which also required observers to make a much easier discrimination between male and female faces. In the male/female task visual noise tended to impair performance more on RVF-LH trials than on LVF-RH trials, opposite the effect for the male/male task. These results are discussed in terms of hemispheric asymmetry for global versus local features of faces, the level of feature analysis demanded by a task, and the level of feature analysis most disrupted by perceptual degradation.     

Perceptual strength is different from sensorimotor strength: Evidence from the centre–periphery asymmetry in masked priming

Near-threshold prime stimuli can facilitate or hinder responses to target stimuli, creating either a positive compatibility effect (PCE) or a negative compatibility effect (NCE). An asymmetry has been reported between primes presented in near periphery, which produced a PCE, and foveal primes, which produced an NCE under comparable conditions. This asymmetry has been attributed to the difference in retinal sensitivity, but it remains unclear whether this means that equating discrimination performance for primes in fovea and periphery, in order to account for differences in perceptual sensitivity, would make the priming effects the same. Wider work indicates that perceptual ability can dissociate from visuomotor effects, predicting that equating perceptual ability for fovea and periphery would not equate priming. We tested these opposite possibilities by matching discrimination performance for masked Gabor patches in fovea and near periphery (6°) and using these as primes in a masked priming paradigm expected to elicit NCEs. We found the asymmetry remained: NCE for fovea and PCE for periphery. We replicated this with both blocked and randomized procedures to check for attentional effects. We conclude that equating perceptual strength (discriminability) of stimuli does not equate their sensorimotor impact due to differences in the relative importance of different visual pathways and differing temporal dynamics in perceptual and sensorimotor processes.     

A central performance drop with luminance stimuli requiring spatial integration

This study reports four experiments that analyzed detection performance for luminance contrasts as a function of retinal eccentricity in order to find further support and explanations for the central performance drop (CPD) in the fovea. In the first experiment, 10 participants (16-37 years of age) had to detect a target patch in a stimulus consisting of bright and dark pixels. Luminance differences between target and context areas were achieved by placing a different number of bright (and dark) pixels in the target and the context area. Results showed a marked CPD; that is, performance did not peak in the fovea but in the parafovea. A spatial integration hypothesis was proposed to explain this CPD. Alternative explanations were tested in three further experiments with a total of 28 participants from 19-46 years of age (using the decision criterion effect, the selective masking effect, and inhibition by high frequencies). The findings did not contradict the spatial-integration hypothesis.     

The influence of texture on judgments of slant and relative distance in a picture with suggested depth

Six surfaces from natural environments with different visual textures were photographed at angles of 60, 65, and 70 deg from perpendicular. Measurements were taken of 24 Ss’ judgments of the inferred angles of slant and inferred midpoints of the six textured surfaces represented in the photographs which were viewed in the frontoparallel plane. Judgments of both slant and relative distance within the photographs were influenced by represented angle of slant and by variations in surface texture.     

A diffusion model of early visual search: Theoretical analysis and experimental results

Summary Meinecke (1989, Exp. 1, cond. HO) showed that the detectability of a visual target embedded in a linear noise array decreases with increasing retinal eccentricity, while the reaction time (RT) of the hits increases. One of the most interesting features of her results was that the RT of the correct rejections is consistently larger than the RT for signals presented near the fovea. This finding suggests that initially visual attention is concentrated near the fixation point and then diffuses across the stimulus array to perform a serial, exhaustive search. We present a diffusion model of early visual-search processes that quantitatively describes this evolution of attention in time and space; in contrast to most previous conceptions, it is based on a genuine relation between the spatial and temporal dimensions of the search processes performed. The model predicts quantitatively both detection performance and RT. We conducted an experiment similar to that of Meinecke (1989), but with an additional variation of the presentation time. All the main features of the 189 predictions could be explained by the model. The interpretation of the four model's parameters is discussed in some detail and compared with previous estimates of the microscopic search speed derived from alternative models. Finally, we consider some possible modifications related to results of Kehrer (1987, 1989), and some generalizations to multi target detection and two-dimensional stimulus arrays.     

Practice effects for visual resolution in the periphery

Resolution thresholds at 0° (fovea), 20°, 40°, and 60° along the horizontal meridian of the temporal visual field revealed a characteristic degradation in visual resolution with increasing stimulus eccentricity. However, substantial individual differences were found, particularly at 40° and 60° of eccentricity. Dramatic improvements in peripheral visual resolution occurred over a period of 11 practice sessions, with the time course of practice effects increasing for greater visual field eccentricities. Improvements with practice reduced, but did not eliminate, individual differences. The present visual resolution findings are compared to previous studies of peripheral motion detection and increment thresholds.     

Picturing peripheral acuity

The grain of the retina becomes progressively coarser from the fovea to the periphery. This is caused by the decreasing number of retinal receptive fields and decreasing amount of cortex devoted to each degree of visual field (= cortical magnification factor) as one goes into the periphery. We simulate this with a picture that is progressively blurred towards its edges; when strictly fixated at its centre looks equally sharp all over.