Nicola Bruno

When does action resist visual illusions

Over the past decade, many studies of non-pathological individuals have reported functional dissociations between perceptual judgments and motor responses. These results suggested an interpretation of the ventral and dorsal streams in the primate visual system as independent modules for visual awareness and the visual guidance of actions. However, recent comparisons of perception and grasping responses in size-contrast displays have been widely reported to show that apparent dissociations are actually experimental artifacts. An overview of the literature suggests that the issue of visuomotor dissociations in healthy individuals is far from settled. Some results suggest that immunity from visual illusions might be found when task requirements emphasize observer-relative reference frames. These results suggest that the functional specialization of the two visual subsystems might be less rigid than originally posited.

When is grasping affected by the Müller-Lyer illusion?: A quantitative review

Milner and Goodale (1995) [Milner, A. D., & Goodale, M. A. (1995). The visual brain in action. Oxford, UK: Oxford University Press] proposed a functional division of labor between vision-for-perception and vision-for-action. Their proposal is supported by neuropsychological, brain-imaging, and psychophysical evidence. However, it has remained controversial in the prediction that actions are not affected by visual illusions. Following up on a related review on pointing (see Bruno et al., 2008 [Bruno, N., Bernardis, P., & Gentilucci, M. (2008). Visually guided pointing, the Müller-Lyer illusion, and the functional interpretation of the dorsal-ventral split: Conclusions from 33 independent studies. Neuroscience and Biobehavioral Reviews, 32(3), 423-437]), here we re-analyze 18 studies on grasping objects embedded in the Müller-Lyer (ML) illusion. We find that median percent effects across studies are indeed larger for perceptual than for grasping measures. However, almost all grasping effects are larger than zero and the two distributions show substantial overlap and variability. A fine-grained analysis reveals that critical roles in accounting for this variability are played by the informational basis for guiding the action, by the number of trials per condition of the experiment, and by the angle of the illusion fins. When all these factors are considered together, the data support a difference between grasping and perception only when online visual feedback is available during movement. Thus, unlike pointing, grasping studies of the Müller-Lyer (ML) illusion suggest that the perceptual and motor effects of the illusion differ only because of online, feedback-driven corrections, and do not appear to support independent spatial representations for vision-for-action and vision-for-perception.

Visually guided pointing, the Muller-Lyer illusion, and the functional interpretation of the dorsal-ventral split: Conclusions from 33 independent studies

Models of the human vision propose a division of labor between vision-for-action (identified with the V1-PPT dorsal stream) and vision-for-perception (the V1-IT ventral stream). The idea has been successful in explaining a host of neuropsychological and behavioral data, but has remained controversial in predicting that visually guided actions should be immune from visual illusions. Here we evaluate this prediction by reanalyzing 33 independent studies of rapid pointing involving the Müller-Lyer or related illusions. We find that illusion effects vary widely across studies from around zero to comparable to perceptual effects. After examining several candidate factors both between and within participants, we show that almost 80% of this variability is explained well by two general concepts. The first is that the illusion has little effect when pointing is programmed from viewing the target rather than from memory. The second that the illusion effect is weakened when participants learn to selectively attend to target locations over repeated trials. These results are largely in accord with the vision-for-action vs. vision-for-perception distinction. However, they also suggest a potential involvement of learning and attentional processes during motor preparation. Whether these are specific to visuomotor mechanisms or shared with vision-for-perception remains to be established.

Amodal Completion of Partly Occluded Surfaces: Is There a Mosaic Stage?

Recent investigators have proposed that amodal completion is a sequential process requiring a preliminary mosaic stage. Results of 6 studies of the time course of completion processes show support for this mosaic-first view with pictorial displays, but not with displays involving occlusion specified by binocular parallax or when pictorial displays were observed monocularly. These results still do not rule out the mosaic-first view. A parallel model, however, can account more economically for the available data.

Lightness contrast in CRT and paper-and-illuminant displays

The increased use of CRT monitors for displaying and controlling stimuli in studies of surface color poses problems of comparability with data obtained with traditional paper-and-illuminant methods. A review of comparable studies using the two methodologies revealed that CRT studies tend to report larger contrast effects. To investigate factors that may be responsible for this difference, simultaneous lightness contrast was measured using both CRT and paper-and-illuminant presentations. The spatial distribution of luminance in the whole field of view and the visual angles subtended by the displays were controlled. The CRT presentation yielded contrast effects twice as big as those measured for a paper surface in a homogeneously illuminated room. However, a paper display under Gelb lighting yielded almost exactly the same effect size as that measured in the CRT presentation. These results demonstrate that contrast effects in both modes of presentation are affected by the spatial distribution of luminance beyond the basic experimental stimuli.

Amodal Completion in Visual Search: Preemption or Context Effects?

In a previous study, search for a notched-disk target abutting a square among complete-disk nontargets and squares was inefficient in 250-ms exposures, but relatively efficient in 100-ms exposures. This finding was interpreted as evidence that amodal completion proceeds through a mosaic and then a completion stage, with the latter preempting the former. We used the same target but changed its context: Nontargets were instead notched disks near squares. Task set was also different: Participants searched for a complete disk. Contrary to the prediction of the preemption model, search was efficient in the 100-ms condition and inefficient in the 250-ms condition. We propose that in both the present and the previous studies, the target was ambiguous, and task set and context affected how it was perceived. In both experiments, set effects were evident for 100-ms exposures; context effects were evident for 250-ms exposures.

Haptic perception after a change in hand size

When we actively explore with our hands (haptics), the brain must combine diverse sensory signals into a unified percept. Presumably, a key role is played by an internal model of the body, but the processes responsible for forming and maintaining such a model are poorly understood. We used a multisensory illusion to alter the internal model of ones hand by using fake hands smaller or larger than the participants hand. After such brief illusion we observed a change in haptically perceived object size. These results demonstrate that the brain rapidly recalibrates haptic signals using an internal model of the hand and that this model is multisensory rather than merely visual or memory-based.

A metanalysis of the effect of the Müller-Lyer illusion on saccadic eye movements: No general support for a dissociation of perception and oculomotor action

Milner and Goodales (1995) proposal of a functional division of labor between vision-for-perception and vision-for-action is supported by neuropsychological, brain-imaging, and psychophysical evidence. However, there remains considerable debate as to whether, as their proposal would predict, the effect of contextual illusions on vision-for-action can be dissociated from that on vision-for-perception. Meta-analytical efforts examining the effect of the Müller-Lyer (ML) illusion on pointing (Bruno, Bernardis, & Gentilucci, 2008) or grasping (Bruno & Franz, 2009) have been conducted to resolve the controversy. To complement this work, here we re-analyzed 17 papers detailing 21 independent studies investigating primary saccades to target locations that were perceptually biased by the ML illusion. Using a corrected percent illusion effect measure to compare across different studies and across experimental conditions within studies, we find that saccadic eye movements are always strongly biased by the illusion although the size of this effect can be reduced by factors such as display duration and between-trials variability in display length and orientation, possibly due to a process of saccadic adaptation. In contrast to some reports, we find no general support for differences between voluntary and reflexive saccades or between saccades performed in conjunction with a pointing movement and saccades performed without pointing. We conclude that studies on the effect of the Müller-Lyer illusion do not provide evidence for a functional dissociation between primary saccades and perception.

How does action resist visual illusion? Uncorrected oculomotor information does not account for accurate pointing in peripersonal space

Using spatially identical displays (variants of the Müller–Lyer illusion), we compared the accuracy of spatial verbal judgments with that of saccadic (eye) and pointing (hand) movements. Verbal judgments showed a clear effect of the illusion. The amplitude of the primary saccade from one endpoint of the pattern (at fixation) to the other also showed an effect of the illusion. Conversely, movement amplitudes when pointing from one endpoint (initial finger position) to the other were significantly more accurate than both saccades and verbal responses. In a control experiment in which the viewing conditions between the saccade and pointing experiments were equalized, saccade amplitude was again affected by the illusion. In several studies, systematic biases in conscious spatial judgments have been contrasted with accurate open-loop pointing in peripersonal space. It has been proposed that such seeming dissociations between vision-for-action and vision-for-consciousness might in fact be because of a simple oculomotor strategy: saccade to the target before it disappears, then use the efference copy of the (accurate) saccadic movement to drive pointing. The present data do not support the hypothesis in this simple form.

'Selfies' Reveal Systematic Deviations from Known Principles of Photographic Composition

We used ‘selfies’, self-portraits taken with a hand-held smartphone camera, to test three known principles of photographic composition: The rule of thirds, the golden ratio rule, and the eye centering principle. Although they are often taught and discussed, the origin of these principles remains unclear. It is possible that they stem from constraints on human perceptual processes. Alternatively, these principles might serve more practical purposes, such as forcing photographers to explore all quadrants of the image. Selfies provide an ideal test bed for these questions due to the control they give self-photographers when they compose the photograph. We used a database of images created by non-professional photographers (N = 388). After analysis, we conclude that there little support for any of the three principles, suggesting that none is strongly rooted in spontaneous perceptual