Research Items (10)
In support of the visual stream dissociation hypothesis, which states that distinct visual streams serve vision-for-perception and vision-for-action, visual size illusions were reported over 20 years ago to ‘deceive the eye but not the hand’. Ever since, inconclusive results and contradictory interpretations have accumulated. Therefore, we investigated the effects of the Ebbinghaus figure on repetitive aiming movements with distinct dynamics. Participants performed a Fitts’ task in which Ebbinghaus figures served as targets. We systematically varied the three parameters which have been shown to influence the perceived size of the Ebbinghaus figure’s target circle, namely the size of the target, its distance to the context circles and the size of the context circles. This paper shows that movement is significantly affected by the context size, but, in contrast to perception, not by the other two parameters. This is especially prominent in the approach phase of the movement towards the target, regardless of the dynamics. To reconcile the findings, we argue that different informational variables are used for size perception and the visual control of movements irrespective of whether certain variables induce (perceptual) illusions.
Over the last 20 years, visual illusions, like the Ebbinghaus figure, have become widespread to investigate functional segregation of the visual system. This segregation reveals itself, so it is claimed, in the insensitivity of movement to optical illusions. This claim, however, faces contradictory results (and interpretations) in the literature. These contradictions may be due to methodological weaknesses in, and differences across studies, some of which may hide a lack of perceptual illusion effects. Indeed, despite the long history of research with the Ebbinghaus figure, standardized configurations to predict the illusion effect are missing. Here, we present a complete geometrical description of the Ebbinghaus figure with three target sizes compatible with Fitts’ task. Each trial consisted of a stimulus and an isolated probe. The probe was controlled by the participant’s response through a staircase procedure. The participant was asked whether the probe or target appeared bigger. The factors target size, context size, target-context distance, and a control condition resulted in a 3×3×3+3 factorial design. The results indicate that the illusion magnitude, the perceptual distinctiveness, and the response time depend on the context size, distance, and especially, target size. In 33% of the factor combinations there was no illusion effect. The illusion magnitude ranged from zero to (exceptionally) ten percent of the target size. The small (or absent) illusion effects on perception and its possible influence on motor tasks might have been overlooked or misinterpreted in previous studies. Our results provide a basis for the application of the Ebbinghaus figure in psychophysical and motor control studies.
Background The inverse relationship between movement speed and accuracy in goal-directed aiming is mostly investigated using the classic Fitts’ paradigm. According to Fitts’ law, movement time scales linearly with a single quantity, the index of difficulty (ID), which quantifies task difficulty through the quotient of target width and distance. Fitts’ law remains silent, however, on how ID affects the dynamic and kinematic patterns (i.e., perceptual-motor system’s organization) in goal-directed aiming, a question that is still partially answered only. Methods Therefore, we here investigated the Fitts’ task performed in a discrete as well as a cyclic task under seven IDs obtained either by scaling target width under constant amplitude or by scaling target distance under constant target width. Results Under all experimental conditions Fitts’ law approximately held. However, qualitative and quantitative dynamic as well as kinematic differences for a given ID were found in how the different task variants were performed. That is, while ID predicted movement time, its value in predicting movement organization appeared to be limited. Conclusion We conclude that a complete description of Fitts’ law has yet to be achieved and speculate that the pertinence of the index of difficulty in studying the dynamics underlying goal-directed aiming may have to be reconsidered.
The influential two-visual streams hypothesis ascribes specific functional roles to the ventral and the dorsal network of the visual system. The ventral system has been hypothesized to process information for conscious perception (vision-forperception), whereas the dorsal stream processes information for action (vision-foraction).The idea of two separate visual networks in the human brain inspired an enormous amount of research over the past 20 or so years. The results are conflicting and divisive about the idea, causing a seemingly insurmountable gap between supporters and opponents. This thesis aims to unravel a part of the jigsaw puzzle of how perception and action are functioning.The Ebbinghaus figure consists of an object embedded in a specific context (e.g., centre circle surrounded by smaller or bigger circles). The perceived object size often deviates from its physical size, which is the so-called Ebbinghaus illusion. The Ebbinghaus figure has been used to distinguish vision-for-perception that is susceptible to visual illusions (i.e., relative size) from vision-for-action that remain unaffected by perceptions of relative sizes (i.e., absolute physical size). Albeit several papers report that the Ebbinghaus illusion affects solely perception, a growing number of studies demonstrate that action is similarly affected by this illusion. A rule to control the size perception of the centre object in the Ebbinghaus figure to ‘appear smaller’ or to ‘appear bigger’ does not exist so that predicting illusion magnitudes remains guesswork. Therefore, it remains also questionable whether all Ebbinghaus figures evoke an illusion, and which factors are key for illusion effects. We quantified the Ebbinghaus figure based on its geometry and systematically assessed its size illusion. One third of all Ebbinghaus configurations did not result in significant illusion effects. For the other part, the illusion effects were due to all geometrical parameters of the Ebbinghaus figure.After the quantification of Ebbinghaus figures, a visuomotor task was implemented in which precision and speed of the voluntary movement were investigated. The visuomotor behaviour was quantitatively and qualitatively described for discrete v and reciprocal sliding movements in terms of kinematics and the underlying dynamics. The description of the visuomotor task and of the perception of Ebbinghaus figures lead to combine both visuomotor task and Ebbinghaus figures.The latter study demonstrates that the Ebbinghaus figure influences the movement.The Ebbinghaus factors that affected perception, however, did not all appear to significantly influence the movement.Due to its systematic approach and the methodological contributions, this work can serve as a basis for future studies on the perception and action mechanisms. This dissertation demonstrated that the ventral stream and dorsal stream are not strictly functionally distinct, and that potentially different informational variables are used for ‘vision for perception’ and ‘vision for action’ irrespective of whether certain variables cause (perceptual) illusions.
Question - Are there probable reasons for the failure of the up–and–down transformed response method to estimate psychophysical threshold ?
And also see Miguel A. Garcı́a-Pérez his 1998 paper:
Forced-choice staircases with fixed step sizes: asymptotic and small-sample properties (doi:10.1016/S0042-6989(97)00340-4)
Over the last 20 years, visual illusions, like the Ebbinghaus figure, have become widespread to investigate functional segregation of the visual system. This segregation reveals itself, so it is claimed, in the insensitivity of movement to optical illusions. This claim, however, faces contradictory results (and interpretations) in the literature. While these contradictions may be explained along various lines, they may simply be due to a lack of illusion effects, which is frequently not verified. Indeed, despite the long history of psychophysics research with the Ebbinghaus figure, standardized configurations and rules to predict the illusion effect are missing. Here, we present a complete geometrical description of the Ebbinghaus figure with three stimulus sizes compatible with Fitts' task. Each trial consisted of a stimulus and an isolated probe. The probe was controlled by the participant's response through a staircase procedure. The participant was asked whether the probe or stimulus appeared bigger. The factors stimulus size, context size, target-context distance, and a control condition resulted in a 3×3×3+3 factorial design. The results indicate that the illusion magnitude, the perceptual distinctiveness, and the response time depend on the context size, distance, and especially, stimulus size. In 33% of the cases there was no illusion effect. The illusion magnitude ranged from zero to (exceptionally) ten percent of the stimulus size. This range suggests that the (reported) absence of motor effects may be due to absent (or small) illusion effects. Alternatively, the absence or presence of motor effects may be task specific, and, in particular, dependent on the motor classes involved. To investigate this issue we are currently running an experiment in which the same group of participants performs an aiming precision (Fitts') task. The participants perform cyclic movements between Ebbinghaus stimuli (as targets) for an index of difficulty (ID) of 4, 5 and 6 bits. Pilot data suggest that the dynamics underlying the movements are locally adjusted in some conditions. The results of both the perceptual and motor experiment will be discussed at the conference. Our perceptual results provide a basis for the application of the Ebbinghaus figure in psychophysical and motor control studies. The results from the presently conducted sensorimotor study will provide insight in the influence of optical illusions on motor control mechanisms. In combination, these experiments allow us to quantify the relation between perception and action as a function of the task layout.