Deg Of Visual Angle Research Articles

Depth from Binocular Rivalry without Spatial Disparity

Some new stereoscopic effects are reported that arise from dichoptic stimuli containing no binocular disparity. In one effect, identical arrays of small black discs are presented to the two eyes and slightly smaller white discs are superimposed on one of each pair of black discs. This creates the impression of a surface with holes in it, through which is seen a surface with fluctuating black and white areas. This is referred to as the 'sieve effect'. The white discs must subtend less than about 1 deg of visual angle. With larger discs the black and white areas no longer exhibit alternating rivalry but combine to produce binocular lustre. This destroys the sieve effect. The sieve effect is weak or nonexistent when the black and white discs are the same size, showing that well-defined binocular rims are required for the effect. When the monocular white discs are reduced to dots, the impression of a surface seen through holes gives way to the impression of an array of dots behind or standing out from the background. In this case the monocular dots permanently dominate the homogeneous backgrounds in the other eye and the impression of depth can be explained in terms of apparent parallax or of disparity due to the instability of vergence.

The relationship between time of arrival of nontargets and their spatial location: evidence for asymmetries in visual attentional processing.

A brief advance of the presentation of distant nontargets has previously been shown to secure them an analysis, suggesting that even stimuli beyond the putative 'spotlight' of attention are analysed. In the present experiment nontargets were presented near to fixation or peripherally (at 0.6 or 1.9 deg of visual angle) for a duration of 17, 33, or 67 ms before being masked. After a subsequent period (SOA) of 0, 20, 40, 60, or 80 ms the target was presented at fixation. The results from the shortest nontarget duration show a time lag in the development of the effect of far nontargets on response to the target. At durations of 33 and 67 ms the identity of far nontargets has been established before masking takes place, since their effect is observable at zero SOA. However, while this effect remains relatively constant over SOAs of 20 to 80 ms it increases with SOA in the near condition, suggesting that, although both near and far nontargets have been identified, it is only the near nontargets which are subject to further processing during the interval between nontarget and target presentations. The results favour a model of attention in which stimuli are passively selected and subject to additional processing on the basis of their spatial location rather than that the location or representation of distractor items are actively suppressed.

Angular induction as a function of the length and position of segments and gaps.

The perceptual distortions which are manifested in the Poggendorff illusion can be studied with the use of a more restricted set of stimulus elements. Experiments were designed in which angular induction effects between two line elements, known respectively as the test segment and induction segment, were evaluated. In some stimulus configurations the induction 'segment' consisted of a tandem pair of segments. Previous studies had shown that the induction segment will bias operant judgments of collinearity for a test segment, this effect being a function of the relative angle between the two. Six experiments are reported, in which the length and position of segments in relation to the tip of the test segment were varied. It was found that substantial induction is produced by a very short segment, and that this can bias judgment even when its displacement spans more than 10 deg of visual angle. Several aspects of the data suggest that the strength of effect is a log-linear function of segment position. However, the results from displacement of single or tandem segments do not conform to predictions based on length/response summation, and thus do not support a linear-systems approach. Neural substrates for these interactions are given brief attention.

Eye torsion elicited by oscillating gratings: Effects of orientation, wavelength and stationary contours

We studied binocular cyclorotatory (torsional) eye movements in response to gratings that oscillated sinusoidally in a frontal plane. The square-wave gratings viewed by the right and left eye were presented and controlled separately to induce cycloversion and cyclovergence by oscillation in phase and out of phase. Eye movements were recorded with scierai induction coils. Stimulus oscillation frequency ranged from 0.125 to 1 Hz and the wavelength of die gratings ranged from 0.92 to 25.75 deg of visual angle. Cycloversion and cyclovergence gain were, on average, comparable in magnitude and decreased with increasing oscillation frequency. There was no consistent effect of the wavelength on the magnitude of the responses. In general, responses were considerably higher to gratings that were oriented horizontally than to those oriented vertically. This anisotropy was present both in cycloversion and cyclovergence. It was enhanced in a larger sized stimulus and by presenting stationary, orthogonal contours (mimicking a “shear” movement), but it was not consistently influenced by wavelength. Cyclovergence showed a phase lag, which increased with oscillation frequency but which was independent of wavelength. In contrast, cycloversion showed a slight phase lead which was independent of both oscillation frequency and wavelength.

The role of frame size on vertical and horizontal observers in the rod-and-frame illusion

The rod-and-frame illusion was used to examine a proposed distinction between the mechanism responsible for frame effects on rod-adjustment errors with large displays and the mechanism responsible for errors with small displays. It was suggested that visual-vestibular mechanisms are involved only when the rod is surrounded by a large tilted frame. Errors in the perceived vertical with small frame would instead be due to purely visual mechanisms. To examine this dual process model, we compared errors at small and large frame when the body was vertical or horizontal. There is evidence to suggest that tilting the body affects visual-vestibular interactions, but there is no reason to expect that body tilt would affect intravisual interactions. Hence, we hypothesized that body tilt would increase errors for large frame, but not for small frame. Eight subjects were tested in four different conditions, corresponding to the combination of two body orientations (vertical versus horizontal) and two frame sizes (47.5 versus 10.5 deg of visual angle). Fourier analysis of data was performed. Repeated measures ANOVA tested the hypothesis about frame size and body orientation. The hypothesis was not confirmed. More specifically, we found that tilting the body increased errors for the small frame as well as for the large frame. The interaction between frame size and body orientation was not significant. Results are discussed in relation to the proposed dual-process model.

Receptive fields of neurons at the confluence of cerebral cortical areas 17, 18, 20 a, and 20 b in the cat

The activity of neurons was recorded extracellularly at the junction of visual cortical areas 17, 18, 20a, and 20b in the cat. The receptive fields of these neurons were striking for their size, which ranged from a diameter of more than 40 deg of visual angle to the complete visual field of the contralateral eye. It is speculated that these large receptive fields may be generated by perturbations in the individual maps as the four areas merge together.

The visible persistence of stimuli in stroboscopic motion

This paper reports an improved paradigm to measure visible persistence. The stimulus is a pair of lines stroboscopically displayed in successive positions moving in opposite directions. The subjects' judgement of simultaneous appearance of all the presented lines is used to estimate visible persistence. This paradigm permitted independent manipulation of spatial and temporal stimulus separations in linear motion. The resulting estimates of visible persistence increase with spatial separation up to 0.24 deg of visual angle and approaches a maximum value at larger spatial separations. The results are consistent with the existence of a hypothetical visual gain mechanism that operates over small retinal distances to effectively decrease persistence duration with decreasing spatial separation.

Pursuit-locked apparent motion

A vertical line, illuminated in n brief pulses at each position, was repositioned a distance D to the right every T msec. Under some conditions, the appearance of a single line jumping from one position to the next (standard apparent motion) gave way to the striking appearance of a bundle of n parallel vertical lines moving with constant velocity across the entire screen. This appearance implies the emergence of continuous pursuit eye movements, which have previously been reported for similarly discontinuous displays. Such “pursuit-locked” apparent motion spontaneously occurred when D was less than about 3 deg of visual angle, and when T was less than a visual persistence time of about 100 msec but greater than a velocity-limited tracking time of about 18 msec per deg of visual angle.

Parafoveal words are effective in both hemifields: preattentive processing of semantic and phonological codes.

What effect may an unattended word have during a single fixation? Attention was selectively directed to a word exposed for 50 ms by the demand to make a rapid lexical decision response, and during the same presentation a second word was displayed approximately 2.3 deg of visual angle away on the same horizontal axis. The second word was backwards and forwards masked by a random-dot display, and was described to subjects as a distractor which was to be ignored. Although no response was required to this word, it was found to influence response latency to the attended word according to the semantic relationship which existed between the two. They interfered with the lexical decision response when the two words were related in meaning and also when they sounded as if they were related in meaning. These effects argue for automatic processing of the meaning of printed words presented in either visual hemifield, and for their automatic phonological activation.

Temporal limits to the detection of correlation in transpositionally symmetric dynamic dot textures.

Adding a delay within spatially correlated point-pairs constituting a dynamic transpositionally symmetric texture has three distinct perceptual outcomes (the specific outcome depending on the size of the delay): global pattern, coherent motion, and apparent randomness. The temporal limits for global pattern and coherent motion were measured for eight different values of the extent of transposition (.11 to .70 deg of visual angle) and for four different stimulus sizes (3,6,7, and 9 deg square),with texture density constant at the retina. Temporal limits for coherent motion varied inversely as a function of increasing spatial separation (.11 deg ≤Δ≤.42 deg), but beyond this separation (.42 deg<Δd≤.70 deg), the limit was invariant as a function of Δd (∼60 msec). The temporal limit for global pattern was independent of the size of Δd and fell at ∼60 msec, exhibiting spatiotemporal characteristics different from those for coherent motion. On the basis of this difference, it is suggested that the human visual system processes the perceptual attributes global pattern and coherent motion independently, despite their common correlational basis. Further evidence to support this proposal is provided by the finding that variations in stimulus size have no effect on the temporal limit for global pattern and have differential effects on the temporal limit for coherent motion, depending on the size of △d.

On the Necessity of Correcting Peripheral Target Luminance for Pupillary Area

Despite the decrease in pupillary area for peripheral targets, retinal illuminance remains fairly constant to about 80 deg visual angle. Constant illuminance is maintained in the retinal periphery because the light that enters the pupil is concentrated into smaller retinal images. The correction of the peripheral target luminances for pupillary area is therefore unnecessary except under certain conditions.

An automated eye movement laboratory for on-line electrooculography

This paper describes an automated eye movement laboratory that uses electrooculography (EOG) to study people’s eye movements while they read. An on-line minicomputer processes bioelectric potentials that correspond to saccadic eye movements. Horizontal saccades larger than 1.5 deg of visual angle are detected and analyzed in real-time as they occur. The laboratory is designed for prolonged yet unobtrusive observation of human eye movements during sustained reading periods of minutes or hours. All important functions regarding data collection and data reduction are performed automatically, according to simple procedures that can be applied uniformly and without bias to nearly all subjects that we study. Results from three experiments are cited in order to quantify the performance of the laboratory with respect to four criteria: saccade detection accuracy, measurement accuracy, sensitivity, and the uniformity of these measures over different subjects.

Spatial properties of the visual detectability of moving spatial white noise.

We determined the spatial parameters that described the visual detection of spatio-temporal correlation in moving two-dimensional noise patterns. The target field (5.21 x 5.31 deg of visual angle) was divided into horizontal stripes of equal width D. Adjacent bars alternately contained noise patterns moving with velocity leads to V1 and leads to V2. We varied D, leads to V1 and leads to V2. Roughly three different percepts occurred. If the stripes were very broad the different movements in alternate stripes were perceived together with the division of the field into stripes. If the stripes were very narrow the division into stripes was not seen, but the moving noise patterns with velocities leads to V1 and leads to V2 were perceived as transparent sheets moving through each other. For intermediate stripe widths the target field looked incoherent and the subject was not clear about the percept. In this region the subject found it difficult and sometimes impossible to discriminate these patterns from a completely uncorrelated spatio-temporal white noise pattern (snow). To quantify the detectability, the patterns were masked with snow (spatio-temporal white noise). The r.m.s. contrast of the total stimulus was kept at a constant value, whereas the subject set the signal-to-noise ratio (SNR) to a threshold value. At certain bar width the thresholds reached a maximum value. Thse critical bar widths depended on the velocities leads to V1 and leads to V2. These critical bar widths were interpreted in terms of a simple general model for the detection of spatio-temporal correlation. In these terms the span of the elementary correlators rose monotonically with the velocity to which the correlator is most sensitive.

Processing of alphabetic information presented in the fovea or the periphery: functional visual field and cognitive load.

In a tachistoscopic reaction-time study that required subjects to respond ‘same’ or ‘different’ to vertically arrayed letter pairs, it was found that cognitive load (level of required match) and retinal eccentricity were both potent variables. It seems that subjects can process low-level information (physical matches) quite effectively out to at least 7 deg visual angle from fixation, in either direction. It was also demonstrated that higher-level processing may proceed fairly efficiently out to at least 4 deg from fixation. These estimates suggest that the abilities of the information processor to deal effectively with the periphery may have been underestimated in the past.

Saccade size in reading depends upon character spaces and not visual angle.

Recently there has been a considerable amount of research involving the use of eye movements to study the reading process (Rayner, 1978). This trend is similar to another large-scale research effort undertaken a number of years ago that also dealt with eye movements and reading (Huey, 1908; Tinker, 1958, 1963; Woodworth, 1938). One difference between the earlier research and the more current work is that more sophisticated equipment is now used and display changes contingent upon the position of the eye can be made (McConkie & Rayner, 1975; O'Regan, 1980; Rayner, 1975). Much of the research using dynamic display changes has focused on perceptual aspects of the reading process. Despite this widespread activity, it is still the case that a rather elementary aspect concerning visual factors has not been adequately resolved. That is, it remains unclear as to whether saccades are executed to traverse a certain amount of visual angle or a certain number of letters. For example, eye movements in reading might average around 2 deg of visual angle (Rayner, 1978) because they serve the purpose of bringing text into foveal vision for detailed analysis. If saccades are determined by a critical visual angle, then when viewing distance is decreased (or if the letters are larger) and fewer characters fall within the fovea, the number of characters per saccade will decrease. On the other hand, larger letters or closer viewing distances might allow the letters to be perceived farther out in extrafoveal vision. Is so, readers might execute saccades of a greater visual angle in order to cover a desired number of characters. If either of these alternatives is correct, measures of saccade length in one metric (either visual angle or character spaces) will remain constant as letter size or viewing distance is altered (both change the size of the retinal image, hence the number of letters falling within the fovea), while measures in the other metric will change drastically as retinal image size changes. Huey (1908) and O'Regan (1980) have addressed

Visual fatigue: The need for an integrated model

Two experiments were conducted to examine possible decrements in performance over time on two visual tracking tasks. In Experiment 1, six subjects tracked a horizontal, sinusoidally moving target in the picture plane for 6.5 min. In Experiment 2, six subjects tracked a target sinusoidally changing focus in the depth plane between 0 and 4 D over 6.5 min. Results indicated a linearly decreasing amplitude of both pursuit eye movements (.29 deg of visual angle per minute) and accommodation (.11 D/min). These visual fatigue effects are discussed in the context of several competing explanations.

Spatial selectivity in vision: Field size depends upon noise size

The size of the visual attentional field was estimated by a modified Stroop task that required the naming of centrally fixated color patches. Noise material consisted of incompatible color words located above and below the color patches, and the target-noise separation was either .5 or 2.5 deg of visual angle. In addition, the size of the noise material subtended either .24 (small) or .57 deg (large) of vertical visual angle. Relative to comparable neutral noise conditions, the large noise material slowed color-naming reaction times (RTs) at both target-noise separations, but the small noise material only affected RT when the tareget-noise separation was .5 deg of visual angle. Assuming that the effects of noise indicate the extent of the attentional field, these results suggest that any estimate of attentional field size is critically dependent upon the size of the noise material.

Stimulus size and acuity in information processing

Ten subjects served in a speeded-recognition task wherein letter targets were varied from.143 deg to 2.14 deg of visual angle in height. Identification latencies were found to decrease on the order of 40 msec for both blocked and randomized presentations of sizes as target size increased. Beyond a size of approximately.75 deg of visual angle, reaction time became asymptotic, and blocked presentation of target size was judged to be faster than randomized. These results were interpreted to support the view that a percept in the visual system develops gradually over time, permitting gross figure-ground differentiation very early in processing, but discrimination of fine detail only later. Implications of these results for human-factors applications are discussed.

Failure of spatial selectivity in vision

In a modified Stroop word-color paradigm, subjects were instructed to identify the color of a target patch in the center of a briefly exposed visual display and to ignore color names printed above and below the patch at distances of 1, 3, and 5 deg of visual angle. Earlier studies of selective attention to a specified location in the visual field have shown that the focus of attention subtends approximately 1 deg of angle. That is, when distractors associated with an incompatible response are spaced within about 1 deg of the target, reaction time to the target is slower than when the distractors are not associated with an incompatible response. Our results show interference with a 5-deg separation between target and noise elements. This finding casts doubt on the utility of the notion of attentional selectivity.

Effects of stimulus size, intensity, color, and eye strain on autokinetic movement: an error signal and noise analysis.

In two experiments, with a total of 116 male and female undergraduate S s, predictions were tested from the following hypothesized explanation of autokinetic movement (AKM): undetected eye drifts lead to movement of the AKM target across the retina; this movement or error signal leads to the initiation of a cancelling response, and the AKM is directly due to CNS monitoring of the efferent responses or signals sent to the eye muscles with subsequent detection of these signals against a background of efferent noise. The error signal variables studied were stimulus color (yellow or blue-green), stimulus intensity (dim or bright), and stimulus size (1.0 or .1 deg visual angle). Small, dim lights foveally viewed and yellow as opposed to blue-green lights led to significantly more error signals and hence more AKM as measured by mean number of perceived direction changes. Also, increasing retinal displacement by changing viewing angle (from zero to 60 deg) or increasing displacement by increasing diopter strength of prisms (from zero to 30) decreased AKM as measured by number of direction changes, and this effect is due at least in part to the increase in efferent noise caused by the increases in eye muscle tension involved. It was concluded that retinal displacement affects both error signal and noise variables in the AKM and, presumably, in the perception of real movement as well.