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Saccades to spatially extended targets: the role of eccentricity.
Size and eccentricity of visual targets are known to modulate saccade parameters. Here we asked for a possible interaction between these target properties. We investigated latency and amplitude of saccades to targets of varying diameter presented at various eccentricities in the visual field. Effects of target size on saccadic eye movements highly depended on eccentricity of saccade targets. For large saccade targets, latencies increased and mean amplitudes decreased mainly at parafoveal eccentricities. By contrast, scatter of saccade amplitudes increased nearly linearly with target size and eccentricity. These effects are consistent with the known functional anatomy of the superior colliculus. Size- and eccentricity-related changes in saccade parameters may depend on distinct subpopulations of collicular neurons.
An evaluation of stereopsis tests.
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Man's visual perception in a space navigation task.
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Distortion by ophthalmic lenses calculated at the farpoint sphere.
Distortion by ophthalmic lenses has generally been calculated with respect to an ideal flat image surface. This results in improper distortion values as the ideal image surface for the mobile eye is the farpoint sphere. Results of distortion calculations for lenses ranging from +20 to -24.65 D are given for point focal lenses (corrected for oblique astigmatism), Percival lenses (designed to place the circle of least confusion on the farpoint sphere), minimum astigmatism and minimum distortion lens forms. Distortion of point focal Ostwalt lenses of powers +6.60 to -24.65 D range from 4.45 to -28.31% when computed at the flat image surface. Referred to the farpoint sphere the corresponding distortion values are +0.85 to -13.54%. Similarly, the distortion of the minimum astigmatic forms of lenses ranging in power from +20 to +7 D are +27.31 to 4.64% at the flat image surface but decreased to +16.38 to 0.84%, respectively, at the farpoint sphere. If the rotations of the unaided ametropic eye that are required to fixate two points in the field are compared with the necessary rotations with a spectacle correction (+20 to -24 D), it is found that the corrected eye rotation errors at the flat image surface range from 14.87 to -4.53%, but drop to +0.87 to -1.88% when referred to the farpoint sphere. It is concluded that distortion at the farpoint sphere is a more valid measure of distortion for the mobile eye than the classical measurement of distortion. This has implications for the study of space perception through lenses.
Analogue models of motion perception.
An object moving in discrete spatial jumps is difficult to distinguish from a continuously moving object, provided the time between jumps is not too great. The extent of this perceived continuity may be measured by probing the perceived spatial location at times between the target jumps, by either a vernier alignment or a stereoscopic technique. As the time between jumps increase the accuracy of spatial interpolation falls, until finally the object is seen only at its actual spatial locations. These results can be analysed in the frequency domain by treating the signal for apparent motion as the analogue of a periodic waveform containing relatively low frequencies (the continuous motion) and higher frequencies giving rise to the discreteness of the motion. If such an input has the higher frequencies progressively removed by physical filtering, it is perceived ads increasingly continuous. The fact that such filtering is not necessary for perceived continuity when the discrete jumps occur at rates greater than about 30 Hz suggests that frequencies greaster than that limit are removed by the visual system itself.
Efficient coding of natural time varying images in the early visual system.
We investigate the hypothesis that the early visual system efficiently codes natural time varying images, first by tracking part of the image, then by matching the spatiotemporal properties of the neural pathway to those of the tracked image. A representation for the time varying image is formulated which consists of two spatiotemporal components, a velocity field component and a stationary component. We show, using digitized sequences of natural images, that the spatiotemporal spectrum and other attributes of the image markedly differ before and after tracking. The temporal frequency bandwidth and velocity distribution of the velocity field component are diminished in the region of tracking and broaden with increasing eccentricity from this region. On the other hand, the spectrum of the stationary component is unaffected by tracking. Comparison of the properties of the tracked image to those of the M and P pathways suggests that each pathway transmits different attributes of the tracked image. A retinal architecture which varies with eccentricity also matches the properties of the tracked image.
Mathematical modelling of animate and intentional motion.
Our aim is to enable a machine to observe and interpret the behaviour of others. Mathematical models are employed to describe certain biological motions. The main challenge is to design models that are both tractable and meaningful. In the first part we will describe how computer vision techniques, in particular visual tracking, can be applied to recognize a small vocabulary of human actions in a constrained scenario. Mainly the problems of viewpoint and scale invariance need to be overcome to formalize a general framework. Hence the second part of the article is devoted to the question whether a particular human action should be captured in a single complex model or whether it is more promising to make extensive use of semantic knowledge and a collection of low-level models that encode certain motion primitives. Scene context plays a crucial role if we intend to give a higher-level interpretation rather than a low-level physical description of the observed motion. A semantic knowledge base is used to establish the scene context. This approach consists of three main components: visual analysis, the mapping from vision to language and the search of the semantic database. A small number of robust visual detectors is used to generate a higher-level description of the scene. The approach together with a number of results is presented in the third part of this article.
Dynamic object individuation in rhesus macaques: a study of the tunnel effect.
A manual-search experiment with rhesus monkeys (Macaca mulatta) explored dynamic object individuation in the tunnel effect: Subjects watched as a lemon rolled down a ramp and came to rest behind a tunnel (Occluder 1) and then as a kiwifruit emerged and became occluded at the end of its path behind a screen (Occluder 2). When the kiwifruit emerged at about the time that the lemon should have (had it continued its motion), subjects searched for food only behind Occluder 2-apparently perceiving the lemon to have transformed into a kiwifruit on the basis of spatiotemporally continuous motion. In contrast, when a brief pause interrupted the occlusion of the lemon and the emergence of the kiwifruit, monkeys searched for food behind both occluders. With further control conditions, this experiment demonstrates a spatiotemporal bias-similar to a bias found in adult visual perception-in the computation of object persistence in the context of a dynamic correspondence problem.
Motion capture and the 'Ouchi' effect.
PURPOSE: The role of motion contrast and motion capture in the 'Ouchi' effect was investigated in a simplified stimulus. METHODS: Sinewave gratings were rapidly oscillated vertically within two concentric stationary 'apertures', an inner disc surrounded by an annulus. RESULTS: A strong illusion of relative motion between the disc and annulus is observed in the horizontal direction for 8 and 12 cycles/deg given small orientation differences between the two gratings. CONCLUSION: These parameters coincide with those of the 'Ouchi' effect, suggesting a sharing of the same causal mechanism.
Logical operations in area and length measurement: age and training effects.
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Parental guidance in preschoolers' understanding of spatial-graphic representations.
This research was designed to observe whether parents guide their children's understanding of spatial-graphic representations and, if so, to describe the quality of the strategies they use. Parents read a picture book to their preschoolers (3 or 5 years, N=31) and children completed spatial-graphic comprehension tasks. Observational data revealed a range of creative behaviors used to address the book's spatial-graphic challenges. The incidence and quality of parental spatial-graphic behaviors were significantly related to 5-year-old children's performance on spatial-graphic measures. These findings, as well as the paucity of parent attention to aesthetics or graphic production techniques, are discussed in relation to representational development and educational practice.
Conditions for young infants' perception of object trajectories.
When an object moves behind an occluder and re-emerges, 4-month-old infants perceive trajectory continuity only when the occluder is narrow, raising the question of whether time or distance out of sight is the important constraining variable. One hundred and forty 4-month-olds were tested in five experiments aimed to disambiguate time and distance out of sight. Manipulating the object's visible speed had no effect on infants' responses, but reducing occlusion time by increasing object speed while occluded induced perception of trajectory continuity. In contrast, slowing the ball while it was behind a narrow or intermediate screen did not modify performance. It is concluded that 4-month-olds perceive trajectory continuity when time or distance out of sight is short.
Apparent size as a determiner of figural after-effects. I.
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Apparent size as a determiner of figural after-effects. II.
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Common and relative components of reflected light as information about the illumination, colour, and three-dimensional form of objects.
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Peripheral and central handicap and encoding.
A linear model of information processing led to many experiments on learning difficulties in the subnormal and severely subnormal. It assumes learning can result from a break in the chain of information flow at any point but overlooks compensatory mechanisms common to a developing organism. To compensate for the model's difficulties, we compared blind and deaf children with subnormal and subnormal autistic children (i.e. localized with general cognitive incapacities). Absence of a modality led to alternative encoding strategies, but in certain circumstances they also occurred in the centrally handicapped. Reasons for the similarities and differences are discussed.
Perceptual judgments of normal and clumsy children.
The performance of normal and clumsy children was compared on a range of measures of visuo-spatial perception. It was found that the clumsy children performed more poorly than the normal children on size-constancy judgements and area, slope, spatial position and linear length discriminations. The differences could not be explained by variations in visual acuity. These results are discussed in terms of information-processing models of motor control, and their possible implications for the assessment and treatment of clumsy children are briefly outlined.