bluuebunny/crossref_metadata_2025 huggingface dataset:
Contrast and Contextual Effects: Detection of Subthreshold Lines
Authors: Wehrhahn C , Dresp B
Published: December 2015

Contextual effects of collinear lines and features on the processing of line targets, finding expression in detection facilitation, have been described recently in the literature (Dresp and Bonnet, 1995 Vision Research35 1071 – 1078; Kapadia, Ito, Gilbert, and Westheimer, 1995 Neuron15 843 – 856). We are reporting a study of the effects of contrast intensity and contrast polarity on these contextual effects. Thin subthreshold lines, not detectable on a plain background, were flashed at a position collinear with a context line of varying luminance and polarity. A temporal 2AFC procedure was used to determine detection thresholds. The mere presence of a collinear context line of weak contrast, regardless of its polarity, was found to make the subthreshold lines detectable. For target lines and context lines of the same contrast polarity, increasing context contrast first enhances, then diminishes, target detectability. For target lines and context lines of opposite contrast polarity, increasing context contrast enhances target detectability. These preliminary results indicate that contrast intensity and relative contrast polarity interact in the genesis of grouping effects or contextual effects at early stages of visual processing.
Functional Aspects of Border-Signalling Mechanisms
Authors: Laurinen P I , Olzak L A
Published: December 2015

Multiple border-signalling mechanisms have been proposed in some models of object segregation. We investigated whether distinct functions of different border mechanisms could be isolated in human psychophysical tasks. The display was a low-spatial-frequency (1.5 cycles deg−1) sinusoidal grating. Two types of centre-surround stimuli were produced by (i) increasing or decreasing the mean luminance of a central patch of the grating to create a signed luminance border (LB), or (ii) by shifting the grating within the central patch 180° to create a sign-reversing border (PSB) without a change in mean luminance. Grating contrast was kept constant at 20%. Observers performed a spatial 2AFC task in each condition. The PSB stimulus served as the comparison stimulus. Test stimuli were either LB or PSB+LB. The mean luminance of the central test patches varied over trials to create border contrasts of −15% to 15% Michelson contrast. In separate sessions, subjects made four types of comparisons: which of the two central areas appeared (1) to differ more in depth from its surround; (2) to have greater modulation contrast of the sinusoid; (3) lighter/darker, (4) to have higher border contrast. Results depended strongly upon test border type and suggested that the mechanism responding to each has a distinct and different perceptual function, and the two may interact. Our results support the hypothesis that surface lightness is calculated by a sign-preserving mechanism, whereas a phase-insensitive mechanism is involved in percepts relating to segregation in depth.
Are Special Mechanisms Chosen in Short Experience-Based Identification of Degraded Geometric Forms?
Authors: Blum B , Barnes O
Published: December 2015

Perceptual performance schema are consistently prepared by human beings. Minimal ‘experience’ with pertinent stimulus attributes seem necessary. We presume that with new stimuli, a ‘linkage’ is formed with inner representations on the basis of commonly shared stimulus components. To investigate this hypothesis, we used ‘confusion stimuli’, ie degradations of five-corner and six-corner stars. These were presented within each of four paradigms: upright (u), rotated (10°; r), foveal/local viewing (A) or eccentric/global viewing (B), that is uA, uB, rA, and rB paradigms. Random presentation of stimuli prototypes preceded the stimulus so that in comparing the presented stimulus to one and the other prototype, the subject faced a 2AFC task. Series (4 to 6) each of 32 randomised paradigms were run on each of six subjects. The subjects keyed 5 or 6 on identifying one or the other stimulus. On the assumption that exposure time and/or acuity grades impose stimulus intensities, psychometric curves were derived by plotting probability of correct choice against either parameter. The results indicate that in hybrid paradigms such as the present uA and uB, or rA and rB subjects may use shared component strategy: ‘u strategy’ with uA and uB, ‘r strategy’ with rA and rB, shared A and B in generating viewpoint invariance. Similar log-linear psychometric curves and slopes for pairs of paradigms were obtained, and ‘transfer’ of learning presumably also based on shared stimulus properties was observed.
A Perplexing Puzzle Involving Perception of Straight Ahead
Authors: Enright J T
Published: December 2015

Perception of visual direction was investigated by requiring subjects repeatedly to adjust a single small light, in an otherwise darkened room, to perceived ‘straight ahead’. This task presumably requires comparing concurrent extra-retinal information (either proprioception or an efference copy) with an internally stored ‘standard’ of comparison. Moment-to-moment precision in that performance is remarkably good, with median threshold (standard deviation) of 47 arc min.

Nevertheless, the responses often involved a monotonic shift of direction over a few minutes during a test session in this reduced visual environment. These trends led to final settings that were immediately recognised as grossly erroneous when the room was relit, implying that the presumptive internal standard of comparison, while unstable, can be rapidly updated in a full visual environment. There are clear similarities between this phenomenon and the sudden ‘visual capture’ that occurs in a re-illuminated room, following distortions of visual direction that arose in a similarly reduced setting for subjects whose extraocular muscles were partially paralysed (Matin et al, 1982 Science216 198 – 201). In both cases, the visual stimuli that underlie rapid recalibration are unknown. Among the several possibilities that can be imagined, the strongest candidate hypothesis for this calibration of the straight-ahead direction is that, during fixation in a lit room, one utilises the directional distribution of image motion that arises because of microscale drift of the eye, as it moves toward its equilibrium orientation, much as a moving observer can use optic flow to evaluate ‘heading’ (the dynamic analogue of ‘straight ahead’).
Velocity Perception in 3-D Environments
Authors: Distler H , Bülthoff H H
Published: December 2015

Velocity perception has been investigated in many experiments with stimuli moving in the picture plane (2-D). For example, experiments with sine-wave gratings have shown that high-frequency patterns are perceived as moving faster than low-frequency patterns, and that high-contrast patterns are perceived as moving faster than low-contrast patterns. We investigated the influence of contrast and spatial frequency on perceived velocity in an open-loop driving simulation to determine whether contrast and spatial frequency account for differences in perceived velocity in complex 3-D environments. The simulated scene consisted of a textured road flanked by two meadows. We used road surface textures with different contrast and spatial frequency contents. In a 2AFC paradigm participants were simultaneously presented two driving simulation sequences depicting vehicles moving at different velocities on roads with different surface textures. Participants judged which vehicle was moving faster. Using an adaptive staircase procedure we determined the point of subjective equality for roads with different surface textures. The results show that perceived velocity in a driving simulation does depend on contrast and spatial frequency of the surface texture. Perceived velocity can be increased by increasing the contrast or the relative amount of high spatial frequencies in the surface texture. The relevance of these results for the design of driving simulators is discussed.
Hyperacuity of Binocular Depth Perception in Ordinary and Trained Subjects
Authors: Shevelev I A , Kolosova S A
Published: December 2015

Differential thresholds for binocular depth perception (BDT) were studied in 542 selected and non-selected adult healthy subjects with the help of depth-meters based on the vertical nonius principle. The BDTs fluctuated from 1.4 to 36 s arc and were lower for selected subjects with professional flight training. The highest statistical limit of normal BDT is 11.7 s arc, while for the trained group it is 10 s arc. BDT and the speed of perception are positively correlated. BDT decreased 4-fold BDT and the age of the subjects was also found. Under high background luminance (150 000 lx) the BDT increased by a factor of 3. For the upper lilmit of normal performance a background luminance of 120 000 lx can be accepted. The mechanism of the detected hyperacuity (vertical nonius principle of measurements based solely on the innate ability of binocular parallax differentiation) is discussed.
Estimation of 3-D Figures Induced by 2-D Mobile Constellations of Dots
Authors: Zdravkovic S
Published: December 2015

The depth effect induced by motion of two dots was investigated in two experiments. Subjects were asked to estimate the length and the angle of inclination of a ‘perceived rod’ that moves in a 3-D space as if the two stimulus dots were its end points. In the first experiment the dots moved on parallel straight and elliptical paths. For the elliptical paths two positions were of special interest: when both dots were on the inner halves of paths and when they both were on the outer halves. The results are set against predictions derived from the frontoparallel principle (Johansson and Jansson, 1968 Perception & Psychophysics4 165 – 170). This principle was confirmed for the inner halves, the results were inconclusive for the straight paths, and the principle was not confirmed for the outer halves of elliptical paths. This implies that the frontoparallel principle might be a special case of some more general principle, suggesting that subjects used some additional cues to estimate length in 3-D space. The results also indicate some compensatory strategies—while the length of the 3-D rod tends to be overestimated, its angle of inclination is underestimated. With these outcomes in mind a geometrical model of the percept is proposed. In the second experiment, the angle and the length of the “perceived rod” were varied. The dots moved on semi-elliptical paths (inner and outer halves). The results revealed that the angle of inclination that the perceived rod ought to have in the 3-D space is a factor in the estimation of its length. The geometrical model also provides an insight into the type of motion of the rod; specifically, rotation for the inner halves and oscillation for the outer halves of the elliptical paths.
Stochastic Properties of Segmentation-Rivalry Alternations
Authors: Arrington K F
Published: December 2015

Segmentation rivalry is the alternation of perceived depth that occurs when depth information from, eg, occlusion and stereo disparity cues are inconsistent (Arrington and Held, 1995 ARVO Abstracts). It was conjectured that the rivalry is based on reciprocal inhibition between end-stopped cell populations that code the same spatial location but opposite direction of surface segmentation in depth. Here the stochastic properties of these depth alternations are analysed and compared to prior analysis [Blake, Fox, and McIntyre, 1971 Journal of Experimental Psychology88 (3)] of binocular rivalry alternations, which suggested that successive phase durations are approximated by a theoretical gamma distribution: f( x)= b a/ Elog \Gamma( a) x a−1exp(− bx). Phenomenally in-front (F) versus phenomenally behind (B) phase durations during segmentation rivalry were collected at 2 min arc stereo disparity that produces a maximal alternation rate and ∼50% duty cycle. Least-square error fit to the two duration data sets was very good (F: a=3.69, b=0.514; B: a=6.54, b=0.780) and the chi-square goodness-of-fit showed insignificant ( p<0.01) departures from the theoretical gamma distribution when the histogram was smoothed and in some cases when it was not smoothed.

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Exocentric Directions in Egocentric Space
Authors: Koenderink J J , van Doorn A J , Lappin J S
Published: December 2015

Observers had to direct a pointer (using a radio link for remote control) at some location towards a beacon at another location such that the pointer appeared to point straight at the beacon. Experiments were done in the natural landscape under broad daylight with the subjects using natural (binocular) vision. Distances were in the range of 1 – 24 m. The location of the vantage point was prescribed, but the observers were allowed (indeed needed) to make eye, head, and body movements, including placement of the feet. Only one or two beacons were visible at any time, but positions were taken from configurations composed of many points. In this way samples of the nexus of pregeodesics of visual space were obtained. The results are interpreted in terms of a curvature of optical space, and in terms of a nonlinear range — depth relation. The results are in conflict with constant-curvature models (such as Luneburg's) of optical space: the curvature is elliptical in near space and changes to hyperbolical in far space. The nexus of pregeodesics does not necessarily involve a metric at all and is thus a more primitive structure of optical space than conventionally considered in the literature.
The Contribution of Provisual Actions to Judgments of form in Depth
Authors: Cowie R , Graham J , Gielen S
Published: December 2015

An action is ‘provisual’ if it is meant to improve the information that can be obtained through vision. Eye and head movements are the most studied examples, but it is more a general phenomenon, including for example manoeuvring for position. The effect of allowing provisual manipulation was compared with more traditional ways of facilitating perception of form in depth. Stimuli were parallel projections of a square and a rectangle (whose proportions ranged from 1 : 1.1 to 1 : 1.4), both slanted in depth, presented side by side on a computer screen. Nine subjects judged which was which. Information varied along three dimensions: (1) surface type—outline, shaded, and textured; (2) dimensionality—structures were either 2-D, or had a ‘leg’ at each corner (normal to the surface); (3) movement—static, passive (rotary) motion, and provisual (where subjects manoeuvred the objects using a mouse). Surface effects were weak ( p=0.047). Dimensionality had a stronger effect ( p=0.0003). The effect of passive movement was comparable, but provisual manipulation gave much better discrimination, irrespective of surface or dimensionality, reaching a ceiling with rectangle proportions around 1 : 1.3. The subjects' usual strategy was to manoeuvre structures into a roughly frontoparallel position. The judgment is almost trivial in that position, but finding it is an interesting theoretical problem—particularly given the demonstration that it can be achieved from parallel projections of a few coplanar points. A model is being developed based on neural networks capable of learning the probabilities associated with viewpoints and trajectories.
Do Monocular Tilt and Spatial Frequency Aftereffects Induce the Binocular Perception of Inclination and Slant?
Authors: Bradshaw M F , Rogers B J
Published: December 2015

Prolonged viewing of a set of tilted lines can affect the perceived orientation of a second set of lines with a different physical orientation (tilt aftereffect). Similarly, prolonged viewing of a set of lines of a particular spatial periodicity can affect the perceived periodicity of a second set of lines with a different physical periodicity (spatial frequency shift aftereffect). We investigated whether a binocular difference resulting from monocular tilt or spatial frequency aftereffects could induce the perception of 3-D inclination or slant respectively. Observers adapted to monocular patterns (5 deg in diameter) arranged in a vertical ‘dumbbell’ configuration in dichoptic alternation. The adapting patterns differed in either orientation (±6.25° or ±11.25°) or in spatial frequency (±0.5 or ±0.75 octaves) from a test surface comprising vertical lines at 4 cycles deg−1. The period of adaptation was 3 min. Observers judged whether the test surfaces appeared to (i) form a convex or concave hinge in depth (after adaptation to tilt), or (ii) to slant in opposite directions about a vertical axis (after adaptation to periodicity). Using a relative slant/inclination judgment may be more sensitive than depth matching or nulling of a single surface (eg Sloane and Blake, 1987 Perception & Psychophysics42 569 – 575). Our results suggest that (i) differences in perceived periodicity in separate monocular images do not induce the impression of stereoscopic slant (confirming the results of Sloane and Blake) and (ii) differences in perceived orientation in separate monocular images do not induce an impression of stereoscopic inclination.
Illusory Optic Flow Transformation with Binocular Vision
Authors: Grigo A , Lappe M
Published: December 2015

We investigated the influence of stereoscopic vision on the perception of optic flow fields in psychophysical experiments based on the effect of an illusory transformation found by Duffy and Wurtz (1993 Vision Research33 1481 – 1490). Human subjects are not able to determine the centre of an expanding optic flow field correctly if the expansion is transparently superimposed on a unidirectional motion pattern. Its location is rather perceived shifted in the direction of the translational movement. Duffy and Wurtz proposed that this illusory shift is caused by the visual system taking the presented flow pattern as a flow field composed of linear self-motion and an eye rotation. As a consequence, the centre of the expansional movement is determined by compensating for the simulated eye rotation, like determining one's direction of heading (Lappe and Rauschecker, 1994 Vision Research35 1619 – 1631).

In our experiments we examined the dependence of the illusory transformation on differences in depth between the superimposed movements. We presented the expansional and translational stimuli with different relative binocular disparities. In the case of zero disparity, we could confirm the results of Duffy and Wurtz. For uncrossed disparities (ie translation behind expansion) we found a small and nonsignificant decrease of the illusory shift. In contrast, there was a strong decrease up to 80% in the case of crossed disparity (ie translation in front of expansion). These findings confirm the assumption that the motion pattern is interpreted as a self-motion flow field: only in the unrealistic case of a large rotational component present in front of an expansion are the superimposed movements interpreted separately by the visual system.
Horizontal Disparity Pooling
Authors: Backus B T , van Ee R , Banks M S
Published: December 2015

Vertical magnification in one eye causes a frontoparallel surface to appear slanted about a vertical axis. Stenton, Frisby, and Mayhew [1984 Nature (London)309 622 – 623] showed using a dot display that, for mixtures of dots drawn from two populations of vertical magnification, the amount of horizontal magnification needed to null the perceive slant varies monotonically and nearly linearly with the mixture. We find a similar, but weaker, relationship for mixtures of horizontal magnification. In one experiment, a horizontal row of dots (containing no vertical disparity) was used to measure the local horizontal magnification needed to null the slant-biasing effect of a background which consisted of a pair of transparent planes made up of dots from two populations of horizontal magnification. The nulling magnification for the dot row was generally a monotonic function of the ratio of the number of dots from the two populations. In a second experiment, observers added horizontal disparity to the biplanar display itself until one plane appeared frontoparallel. The nulling horizontal magnification was again a monotonic function of the ratio of the number of dots from the two populations. We conclude that stereoscopic slant perception is influenced by pooling both of vertical and of horizontal magnifications within a stereoscopic image.
Hyperglobal Interactions in Perception of Stereoscopic Transparency
Authors: Gepshtein S , Cooperman A
Published: December 2015

A method has been developed (1) to measure limitations of human observers in perception of visual scenes containing a transparent surface, and (2) to study computational stages underlying stereoscopic transparency perception. Observers viewed random-dot stereograms of overlapping plane and cylindrical surfaces and had to distinguish between two orientations of the cylindrical surface under conditions of strict depth fixation control. Dot density of the transparent plane was increased at various intersurface depth separations until identification of cylinder orientation became random (limiting density). Limiting density dramatically decreased as depth separation between the surfaces grew, and this basic relationship could not be accounted for either by higher severity of matching with larger dot densities or by the ability of a denser surface to pull vergence to its depth. The basic relationship persisted with opposite contrast polarities of plane and cylinder dots, which suggests that competitive interactions between the surfaces occur as a separate process from binocular matching at each surface [Harris and Parker, 1995 Nature (London)374 808 – 811]. Thus, successive computational stages of matching and intersurface interaction characterising stereoscopic hyperglobality may be distinguished. We also found a facilitating role of intersurface brightness difference and a near — far asymmetry of the basic limiting density versus depth separation relationship with reversed surface positioning. We suggest a scheme of cortical inhibitory connections that agrees with the concept of disparity gradient limit and predicts performance similar to that reported. In the scheme, each activated unit produces a suppression zone in the disparity projection field so that lateral extension of the inhibition grows with binocular disparity.
Binocularly Perceived Direction and Outline Shape
Authors: van Ee R , Erkelens C J
Published: December 2015

Suppose there is a square Julesz random-dot pattern located in front of a background pattern and suppose there is a binocularly visible slider in the background. If subjects align the slider with the rims of the square, the outline of the occlusion in the background pattern has a vertically elongated rectangular shape (Erkelens, Muijs, and van Ee Vision Research in press). The cyclopean-eye concept predicts the outline shape of occluder and occlusion to be the same. However, subjects had the impression that the occluder had the shape of a square. We investigated this impression psychophysically. We measured perceived outline shape of stereoscopically defined rectangles presented in front of a random-dot pattern. In a series of 2AFC tasks subjects decided whether they perceived a horizontally or a vertically elongated rectangle. The horizontal side subtended 8 deg and the vertical side varied between 86% and 114% of 8 deg. The disparity between foreground and background was varied and all trials were presented either in the presence or the absence of monocularly visible contours. We repeated the experiment also during monocular viewing with the left and the right eye. We used exposure durations of 0.5 and 3.0 s. In all circumstances we found that the outline shape of the occluder was veridically perceived; it was not vertically elongated in accordance with the elongation found in the alignment tasks. Apparently information from the binocular alignment of objects in different depth planes is not used for extracting the outline shape of an object.
Stereoscopic Transparency and Segregation in Depth
Authors: Lankheet M J M , Palmen M
Published: December 2015

We previously described the spatiotemporal requirements for binocular correlation in stereopsis using sinusoidal gratings-in-depth (Lankheet and Lennie, 1996 Vision Research36 527 – 538). The use of smooth sinusoidal surfaces emphasised the effects of spatial and temporal integration. Binocular correlation, however, depends not only on integration, but also on segregation at depth discontinuities. In the present experiments we therefore investigated segregation-in-depth, using random dot stereograms that depicted two transparent frontoparallel planes positioned symmetrically on either side of a binocular fixation point. Sensitivity for segregating the two planes was established by adding Gaussian distributed disparity noise to the disparities specifying the planes, and finding the noise amplitude that rendered transparency just detectable. The stimuli consisted of dynamic random-dot displays (dot lifetime 4 frames, at a frame rate of 67 Hz), generated in real time by a Macintosh computer, displayed on a television monitor, and viewed through a stereoscope. We used a method of constant stimuli and a 2AFC procedure. Two transparent planes were presented in one interval, and a single plane, with Gaussian distributed disparity values spanning the same range, was presented in the other.

Segregation of stationary patterns was optimal for disparity differences of about ±9 min arc. Differences smaller than ±3 min arc and larger than about ±18 min arc could not be resolved. Motion contrast between the two patterns greatly facilitated segregation in depth. The facilitating effect increased with the difference in motion directions. The optimal speed varied with the difference in disparity.
Overt Visual Attention in Parkinson's Disease
Authors: Lee A C , Harris J P , Calvert J E
Published: December 2015

The ability of Parkinsonian (PD) patients to control overt visual attention was investigated, by measuring reaction time to a visual stimulus presented at different distances (1.5 deg, 6 deg, and 12 deg) and directions (left or right) from a central fixation point. Prior to the onset of the target stimulus (a square), a cue stimulus appeared just above the fixation point. With equal probability, the arrow pointed to the left, or to the right, or was ambiguous (with two heads). On 20% of their presentations, the left and right arrows pointed in the direction opposite to where the target was to appear. Subjects were informed that 20% of cues would be misleading, and correcting lenses were used to optimise their visual acuity. In previous work with a similar paradigm, only one target eccentricity was used, and subjects were not refracted, leaving open the possibility that PD subjects had more difficulty in seeing the cues and targets.

The eight PD subjects had longer reaction times than age-matched normal controls (and were relatively slower for the more eccentric targets), but made fewer errors in all conditions. In particular, they were more accurate than the controls on the presentations when the cue was misleading or ambiguous, suggesting that the PD group were ignoring the cue. It seems unlikely that the subjects discriminate the direction of the cues, given the use of optical correction, and they reported seeing the cues. Our data are consistent with those of other workers who have described a similar ‘disengagement of attention’ in PD (Clark et al, 1989 Neuropsychologia27 131 – 140) and attributed it to decreased catecholaminergic activity following destruction of midbrain structures (Wright et al, 1990 Neuropsychologia28 151 – 159).
Photic Environment, Visual Pigments, and the Limits of the Visible Spectrum
Authors: Govardovskii V I
Published: December 2015

The limits of the visible spectrum are set by the light available for vision, and by the visual pigment absorbance. The hundreds of visual pigments studied to the present day have absorbance maxima spread within the range from 350 to 620 nm. Yet this diversity is used for vision quite nonuniformly: rod and cone visual pigments are tightly clustered around a few preferred positions in the spectrum, eg near 500 nm in the rods of land animals. The so-called ‘sensitivity hypothesis’ assumes that the clustering is to maximise the number of absorbed photons available in the animals' light environment. In most cases, however, visual pigments are substantially more short-wave (blue-shifted) than is necessary for maximum quantal absorption. Examples of the ‘blue shift’ are the Purkinje shift during cone - rod transition in dark adaptation, the hypsochromic shift of rod visual pigments in deep-water fish, and a similar shift in the cone pigments of geckos and some snakes as a result of evolutionary adaptation to nocturnal habits. It is argued that an important limiting factor in vision is the dark noise produced by thermal isomerisation of the chromophore. Measurements of the dark noise in rods with different visual pigments show that the noise increases steeply when the absorbance maximum is shifted to longer wavelengths, thus precluding the use of long-wave pigments for vision at low intensities. The optimum spectral position of a pigment may be that which ensures a maximum light-to-noise ratio in a particular photic environment.
Head-Mounted Video Camera System in Testing Multihandicapped Children with Low Vision
Authors: Leinonen M T , Koskinen L
Published: December 2015

Vision testing with multihandicapped children is more difficult than with normally developed children: the child's ability to concentrate in the test situation is often markedly reduced, or the child is not capable of communicating with the examiner in the normal way owing to deficiencies in motor skills, hearing, or speech. Often observation of the eyes and face is difficult because of bent-down sitting position caused by poor control of head and neck.

By recording the test situation on a video tape it is possible to analyse the child's reactions afterwards more accurately. Our video system consists of a miniature video camera attached to the head of the child with a small mirror in front of the camera to provide a picture of the eyes. Simultaneous recording of the eye movements and the fixation target is possible by adjusting the position of the mirror so that it covers the view of the camera only partially. With the aid of a second, conventional video camera, we get an overview picture of the test situation. This picture is combined with the image of the miniature camera on the child's head and recorded on a video tape.

With our video system it is possible to see the eye movements even when the child's head is bent-down or the child is moving in the examination room. It also allows the use of full-field stimuli covering the eyes as in examination of the optokinetic nystagmus. The eye - hand coordination can also be monitored on the video tape.
Colour Perception 1978–1997
Authors: Mollon J D
Published: December 2015

In the past twenty years, the spectral sensitivities of the three types of cone have been established with some certainty: direct measurements by microspectrophotometry and electrophysiology are in fair agreement with psychophysical estimates. Particularly significant was the publication of DNA sequences for the four opsins of the human eye, by Jeremy Nathans and colleagues in 1986. This work was soon to transform the understanding of retinitis pigmentosa and other retinal dystrophies, and it has given many insights into the evolution of colour vision; but, curiously, the explanations of dichromacy and anomalous trichromacy have not proved as straightforward as we all expected in 1986. What is clear, however, is that normal colour vision exhibits a genetic polymorphism: much of the intersubject variance in colour matches can be traced to differences in the amino-acid sequence of the opsins for the long-wave and middle-wave cone pigments.

The last two decades have seen a major change in the status of opponent processes. In the 1970s it was still common for professors to tell undergraduates that the Young - Helmholtz theory of colour vision held at the receptor level and the Hering theory at the level of the retinal ganglion cells. It is now clear that the chromatically antagonistic processes revealed electrophysiologically and psychophysically in the early visual system do not correspond to the red - green and yellow - blue processes that Hering postulated on the basis of phenomenological observations. The existence of four unique hues is today one of the unexplained mysteries of colour science.

In one salient respect, research in colour vision has been changed by instrumental advances. Computer-controlled monitors (though offering splendid pitfalls to the unwary) have allowed the study of spatially and temporally complex chromatic displays, notably in the field of colour constancy. Most recently there has been interest in the chromatic statistics of natural scenes: how well is the visual system matched to the statistics of the world and can it adapt to the gamut of chromaticities present in a given scene?
Countercurrents in Matter and Mind
Authors: Hochstein S , Ahissar M
Published: December 2015

An especially efficient manner of transmission of matter or energy, employed by numerous biological systems, is the countercurrent mechanism. Transfer is effected between two closely aligned streaming currents where the currents flow in opposite directions. Final transfer can be 100% rather than the 50% ceiling of concurrent streams. We now report that perceptual systems may employ a similar mechanism. Information derived from the external world by the senses is transferred to the perceptual system in a hierarchy of processing areas. Simultaneously, this information is intermixed with previously stored internal information. The degree of mixture of previously existing information, with new, unprocessed information is titrated along the hierarchy. The brain may tap various points along the countercurrents to obtain the mixtures required for different tasks. Perceptual learning affects first the inner levels of this cortical hierarchy and only later descends to their input levels to achieve better performance with more difficult task conditions. Learning effects discussed at ECVP over the last two decades are reviewed in the light of this cortical scheme. Many seemingly contradictory findings are reconciled when put in the framework of countercurrent streams which respectively process sensory information and guide perceptual learning.
The Accuracy of Saccadic and Perceptual Decisions in Visual Search
Authors: Eckstein M P , Beutter B B , Stone L S
Published: December 2015

Saccadic eye movements during search for a target embedded in noise are suboptimally guided by information about target location (Eckstein et al, 1997, paper presented at ARVO). Our goal was to compare the spatial information used to guide the saccades with that used for the perceptual decision.

Three observers were asked to determine the location of a bright disk (diameter 21 min arc) in white noise (signal-to-noise ratio 4.2) from among ten possible locations evenly spaced at 5.9 deg eccentricity. In the first of four conditions, observers used natural eye movements. In the three remaining conditions, observers fixated a central cross at all times. The fixation conditions consisted of three different presentation times (100, 200, 300 ms), each followed by a mask. Eye-position data were collected with a resolution of ∼0.2 deg. In the natural viewing condition, we measured the accuracy with respect to the target and the latency of the first saccade. In the fixation conditions, we discarded trials in which observers broke fixation. Perceptual performance was computed for all conditions.

Averaged across observers, the first saccade was correct (closest to the target location) for 56±7 (SD) % of trials (chance 10%) and occurred after a latency of 313±56 ms. Perceptual performance averaged 53±4%, 63±4%, 65±2% correct at 100, 200, and 300 ms, respectively. We conclude that, for the signal-to-noise ratio used, at the time of initiation of the first saccade there is little difference between the amount of information about target location available to the perceptual and saccadic systems.
In Congenital Nystagmats, Reading Text with Rapid Serial Visual Presentation (RSVP) Reduces Mean Eye Velocity
Authors: Aquilante K , Wyatt H , Yager D
Published: December 2015

Most low-vision subjects read text faster from an RSVP display than they do with full PAGE reading. This result has been confirmed with a group of subjects with congenital nystagmus (Plass and Yager, 1995, paper presented at ARVO). The present experiment was intended to determine whether reading with RSVP also reduced the severity of the nystagmus, which may have contributed to the faster reading rates.

Eye position and velocity of congenital nystagmats were recorded with an infrared reflection technique, sampled at 40 s−1. Recordings were made in three conditions: (1) passively viewing a blank computer monitor; (2) silently reading full PAGE text at a size of approximately four times Snellen acuity; (3) silently reading RSVP text at the same size, at about 200 words min−1. We found that: (1) in frequency histograms of instantaneous eye velocities at 25 ms intervals, the full width at half height was an average of 60% smaller for RSVP reading compared to PAGE; (2) with a window of ±5 to 20 deg s−1, depending on the subject's baseline eye velocity, the number of epochs that were at least 75 ms in length, and the total time in these epochs, were significantly greater during RSVP reading than during PAGE reading or viewing a blank screen. We conclude that reading with RSVP reduces nystagmus, and may contribute to greater reading speed.
Even Experienced Drivers Have the Wrong Concept about How to Change Lanes
Authors: Wallis G M , Chatziastros A , Bülthoff H H
Published: December 2015

Imagine changing lanes on a motorway, and in particular the series of angles through which the steering wheel passes in completing the manoeuvre. The vast majority of us describe turning the wheel once and then returning the wheel to the middle position. This is wrong, because we have failed to describe the appropriate symmetrical motion of the steering wheel in the opposite direction required to straighten the car. To investigate this misconcept and so as to determine the type of motor programs used during steering, we tested the ability of subjects to change lanes with and without visual feedback. Five licensed drivers drove in a simulated environment projected on a 180° screen using a force-feedback steering wheel. A straight four-lane motorway appeared alternately with normal (daylight) or zero visibility (dark tunnel). In both conditions subjects were asked to change lanes—either to the left or to the right. In the condition without visual feedback no other feedback about performance was given.

Without visual feedback the results showed a predictable increase in variance in the final heading but also suggested a systematic deviation of heading towards the direction of the lane change ( p<0.001), due to the subjects' failure to complete the lane changing manoeuvre. Hence, we conclude that despite the highly practised nature of the task of lane changing, it is not stored as a complete motor program, but is instead achieved by the interplay of a simple steering program and visual feedback about current heading and location on the road.
Scaling of Frontoparallel Surfaces by Vertical Disparities: Effects of Field Size, Location, and Eccentricity
Authors: Rogers B J , Ledgeway T
Published: December 2015

Since the pattern of horizontal disparities created by a frontal surface depends on the distance of the surface from the observer, additional information about distance is needed in order to judge whether a surface lies in a frontal plane. Rogers and Bradshaw (1995 Perception24 155 – 179) showed that both vertical disparities and vergence angle can be used to scale the curvature of surfaces in a horizontal direction. In the present experiments, we measured the extent of frontal plane scaling as a function of the location and eccentricity of the vertical disparity information. Observers were presented with a series of random-textured stereoscopic surfaces with vertical disparity information appropriate for surfaces located at distances between 28 cm and infinity. The observer's task was to vary the pattern of horizontal disparities until the surface appeared to lie in a frontal plane. The stereoscopic images were masked to reveal either a circular, or an annular, or a rectangular patch with dimensions between 10 deg and 70 deg. Maximum scaling was found for the 70 deg diameter circular patch; it decreased by 50% when the patch was masked down to 20 deg in diameter. Scaling remained at over 80% when the central 60,deg of the display were masked. Scaling was reduced more when the horizontal width of the rectangle was made smaller than when the vertical height was made smaller. Vertical disparities are most effective for frontal plane scaling when they are present in more eccentric regions of the visual field, especially in a horizontal direction.
Effect of Motion on the Recognition of Area by Rhesus Monkeys in a Pseudo-Matching Task
Authors: Malecki H , Rosolen S G , Bonnier R
Published: December 2015

We examined the effect of target motion on the visual recognition of target area in rhesus monkeys. We used a pseudo-matching visual task, where ten adult monkeys were trained to recognise and point out the bigger one of two achromatic squares of different areas but having the same luminance and presented on the same background. The video screen was placed 0.5 m in front of their faces. The two areas were randomly sampled out of five areas (49, 72.25, 100, 132.25, and 169 mm2). The speed of the targets was varied in this paradigm (0, 6, 11, 16, 21, or 26 deg s−1). Performance in terms of area recognition thresholds was calculated for each monkey on the basis of 100 trials in standardised environmental conditions. Statistical analysis showed that performance with a target speed of 16 deg s−1 was significantly better than in the other conditions ( p<0.01). We conclude that this pseudo-matching task, based on a cognitive paradigm, reveals a significant effect of motion on the visual recognition of area in rhesus monkeys. The activities of specific cortical areas (V4 and V5) should be studied by other techniques in order to characterise those involved in remembering an object's qualities and those responding to motion. The links between V4, V5, and inferior temporal cortex could be tested with the aid of this pseudo-matching task.
Influence of Peripheral on Central Stereopsis: An Induced Depth Tilt Effect
Authors: Müller S , Wist E R
Published: December 2015

A large rotating black/white sectored disk (58 deg diameter) viewed with a neutral density filter over one eye is perceived as tilted in depth according to the Pulfrich phenomenon. But with fixation on a centrally located vertical bar (7 deg in length), the disk is perceived as vertical while the central bar is perceived as tilted in the opposite direction. This effect remains even if the central 38 deg portion of the disk is occluded leaving a peripheral annulus 10 deg in width. At an optimal rotation speed of 45° s−1 and a filter of 2 log units, the inter-individual perceived tilt of the bar ranges between 5° and 10° as measured by nulling out the illusory tilt by adjustment with a joystick. Variable errors were extremely small and corresponded well with central stereoscopic resolution. The amount of illusory tilt depends on the speed of disk rotation and filter density, and its direction on the relation between the direction of motion and the filter-covered eye. The effect is not limited to Pulfrich-induced stereotilt: When the disk was stationary but physically tilted in depth, the induced tilt on the central bar corresponded to about 50% of the physical tilt. This effect, in turn, could be cancelled or enhanced by rotating the tilted disk and inducing an appropriate Pulfrich effect. With monocular viewing no induced depth tilt occurs. The results are interpreted in terms of a stereoscopic induced effect operating beyond the known peripheral limits of stereopsis.
Text Font Affects Reading Speed at a Low Luminance but Not at a High Luminance
Authors: Yager D , Aquilante K , Plass R
Published: December 2015

We are beginning a series of studies in which we shall investigate which characteristics of text fonts may be successful in allowing text to be read with different kinds of degradations that may be experienced by low-vision patients. As a first attempt to simulate one of the problems that certain patients might have, we measured reading speeds with two proportionally spaced fonts at a high photopic luminance (146 cd m−2) and at a very low luminance (0.146 cd m−2). We used the RSVP (rapid serial visual presentation) reading method, which eliminates the need for scanning eye movements. The two fonts were ‘Swiss’, a simple sans-serif font, and ‘Dutch’, a serif font similar to Times Roman, both presented at the 20/80 size; letters were white on black, with a contrast of 0.94. Subjects were young, normally sighted high-school and optometry-college students. The reading speed results from 47 subjects were as follows, for four conditions: high luminance, Swiss 531 and Dutch 540 words min−1; low luminance, Swiss 479 and Dutch 429 words min−1.

At the high luminance, there is no significant difference between reading rates. There is a significant advantage for the Swiss font at the low luminance: p=0.005.

There may be a significant difference in reading speed with different fonts when the patient's perceptions are degraded by disease and/or aging processes. Other parameters to be investigated with this method include contrast, size, blur, and visual field position. The simulation results will suggest parameter values to test on low-vision patients.
Chromatic Response Curves of Surface Colours
Authors: Indow T
Published: December 2015

Basic data in colour vision such as chromatic response curves were obtained with aperture colours. It is not possible to apply the cancellation technique to define redness or yellowness components etc in a surface colour. A method was developed to obtain such information (Indow, 1987 Die Farbe34 253 – 260), which was based on the expanded form of multidimensional scaling. The results by an improved method are presented. First, it is shown how multiattribute perceptual differences between two Munsell colours, P j, P k, are predicted by distances ^ djk in the current Munsell solid. Colour differences djk are defined by matching with lightness differences in Munsell gray scale and root-mean-squares of ( djk - ^ djk) is 0.2∼0.3 in the unit of Munsell Chroma C. Second, principal hue vectors falpha, alpha= R, Y, G, B, or P in addition, are defined in the current Munsell solid. As shown before, fB is shifted from 5 R in the direction of 5 P, irrespective of whether P is included or not. Overall root-mean-squares of (scaled values of principal hue a in a colour j-coordinates of P j on falpha) is about 0.6 in the unit of C. On the basis of these findings, it becomes possible to define principal hue components for any Munsell colours and hence chromatic response curves are obtained as a function of H, V, and C.