Associate Professor, Department of Psychology
B.S. Texas A&M University
Office: 331 LISB
Research Interests: Neural basis of sensation and perceptual awareness; sensorimotor integration
Overview: The general goal of our research is to understand the brain's ability to perceive and interact with objects in the environment. In one line of research, we are attempting to define the patterns of brain activity that correspond to a subject's awareness of visual stimuli. In other words, why are we aware of some objects and events in the world around us, but not others? For these studies, we use behavioral methods and functional magnetic resonance imaging (fMRI) to examine brain activation as human subjects observe illusory visual stimuli.
In a second line of research, we focus on the brain's ability to form spatial representations of the world using sensory cues. In particular, this research uses behavioral techniques to examine the many possible frames of reference used by the brain to map the location of an object in three-dimensional space. By assessing the performance of human subjects responding to sensory stimuli under various conditions, these experiments provide insights into the sensorimotor processes that allow us to accurately move the eyes or hands to the location of an object.
The Two-Wrongs model explains perception-action dissociations for illusions driven by distortions of the egocentric reference frame.
Front Hum Neurosci. 2015;9:140
Authors: Dassonville P, Reed SA
Several studies have demonstrated a dissociation of the effects of illusion on perception and action, with perception generally reported to be susceptible to illusions, while actions are seemingly immune. These findings have been interpreted to support Milner and Goodale's Two Visual Systems model, which proposes the existence of separate visual processing streams for perception and action. However, an alternative interpretation suggests that this type of behavioral dissociation will occur for any illusion that is caused by a distortion of the observer's egocentric reference frame, without requiring the existence of separate perception and action systems that are differently affected by the illusion. In this scenario, movements aimed at illusory targets will be accurate if they are guided within the same distorted reference frame used for target encoding, since the error of motor guidance will cancel with the error of encoding (hence, for actions, two wrongs do make a right). We further test this Two-Wrongs model by examining two illusions for which the hypothesis makes very different predictions: the rod-and-frame illusion (which affects perception but not actions) and the simultaneous-tilt illusion (which affects perception and actions equally). We demonstrate that the rod-and-frame illusion is caused by a distortion of the observer's egocentric reference frame suitable for the cancellation of errors predicted by the Two-Wrongs model. In contrast, the simultaneous-tilt illusion is caused by local interactions between stimulus elements within an undistorted reference frame, precluding the cancellation of errors associated with the Two-Wrongs model such that the illusion is reflected in both perception and actions. These results provide evidence for a class of illusions that lead to dissociations of perception and action through distortions of the observer's spatial reference frame, rather than through the actions of functionally separate visual processing streams.
PMID: 25852523 [PubMed]
An allocentric exception confirms an egocentric rule: a comment on Taghizadeh and Gail (2014).
Front Hum Neurosci. 2014;8:942
Authors: Dassonville P, Lester BD, Reed SA
PMID: 25520637 [PubMed]
The role of the right superior parietal lobule in processing visual context for the establishment of the egocentric reference frame.
J Cogn Neurosci. 2014 Oct;26(10):2201-9
Authors: Lester BD, Dassonville P
Visual cues contribute to the creation of an observer's egocentric reference frame, within which the locations and orientations of objects can be judged. However, these cues can also be misleading. In the rod-and-frame illusion, for example, a large tilted frame distorts the observer's sense of vertical, causing an enclosed rod to appear tilted in the opposite direction. To determine the brain region responsible for processing these spatial cues, we used TMS to suppress neural activity in the superior parietal lobule of healthy observers. Stimulation of the right hemisphere, but not the left, caused a significant reduction in rod-and-frame susceptibility. In contrast, a tilt illusion caused by a mechanism that does not involve a distortion of the observer's egocentric reference frame was unaffected. These results demonstrate that the right superior parietal lobule is actively involved in processing the contextual cues that contribute to our perception of egocentric space.
PMID: 24702459 [PubMed - indexed for MEDLINE]
Adaptation to leftward-shifting prisms enhances local processing in healthy individuals.
Neuropsychologia. 2014 Apr;56:418-27
Authors: Reed SA, Dassonville P
In healthy individuals, adaptation to left-shifting prisms has been shown to simulate the symptoms of hemispatial neglect, including a reduction in global processing that approximates the local bias observed in neglect patients. The current study tested whether leftward prism adaptation can more specifically enhance local processing abilities. In three experiments, the impact of local and global processing was assessed through tasks that measure susceptibility to illusions that are known to be driven by local or global contextual effects. Susceptibility to the rod-and-frame illusion - an illusion disproportionately driven by both local and global effects depending on frame size - was measured before and after adaptation to left- and right-shifting prisms. A significant increase in rod-and-frame susceptibility was found for the left-shifting prism group, suggesting that adaptation caused an increase in local processing effects. The results of a second experiment confirmed that leftward prism adaptation enhances local processing, as assessed with susceptibility to the simultaneous-tilt illusion. A final experiment employed a more specific measure of the global effect typically associated with the rod-and-frame illusion, and found that although the global effect was somewhat diminished after leftward prism adaptation, the trend failed to reach significance (p=.078). Rightward prism adaptation had no significant effects on performance in any of the experiments. Combined, these findings indicate that leftward prism adaptation in healthy individuals can simulate the local processing bias of neglect patients primarily through an increased sensitivity to local visual cues, and confirm that prism adaptation not only modulates lateral shifts of attention, but also prompts shifts from one level of processing to another.
PMID: 24560913 [PubMed - indexed for MEDLINE]
Shifts of visuospatial attention do not cause the spatial distortions of the Roelofs effect.
J Vis. 2013;13(12)
Authors: Lester BD, Dassonville P
When a visible frame is offset left or right of an observer's objective midline, subjective midline is pulled toward the frame's center, resulting in an illusion of perceived space known as the Roelofs effect. However, a large frame is not necessary to generate the effect-even a small peripheral stimulus is sufficient, raising the possibility that the effect would be brought about by any stimulus that draws attention away from the midline. To assess the relationship between attention and distortions of perceived space, we adopted a paradigm that included a spatial cue that attracted the participant's attention, and an occasional probe whose location was to be reported. If shifts of attention cause the Roelofs effect, the probe's perceived location should vary with the locus of attention. Exogenous attentional cues caused a Roelofs-like effect, but these cues created an asymmetry in the visual display that may have driven the effect directly. In contrast, there was no mislocation after endogenous cues that contained no asymmetry in the visual display. A final experiment used color-contingent attentional cues to eliminate the confound between cue location and asymmetry in the visual display, and provided a clear demonstration that the Roelofs effect is caused by an asymmetric visual display, independent of any shift of attention.
PMID: 24105425 [PubMed - indexed for MEDLINE]
Activation in a frontoparietal cortical network underlies individual differences in the performance of an embedded figures task.
PLoS One. 2011;6(7):e20742
Authors: Walter E, Dassonville P
The Embedded Figures Test (EFT) requires observers to search for a simple geometric shape hidden inside a more complex figure. Surprisingly, performance in the EFT is negatively correlated with susceptibility to illusions of spatial orientation, such as the Roelofs effect. Using fMRI, we previously demonstrated that regions in parietal cortex are involved in the contextual processing associated with the Roelofs task. In the present study, we found that similar parietal regions (superior parietal cortex and precuneus) were more active during the EFT than during a simple matching task. Importantly, these parietal activations overlapped with regions found to be involved during contextual processing in the Roelofs illusion. Additional parietal and frontal areas, in the right hemisphere, showed strong correlations between brain activity and behavioral performance during the search task. We propose that the posterior parietal regions are necessary for processing contextual information across many different, but related visuospatial tasks, with additional parietal and frontal regions serving to coordinate this processing in participants proficient in the task.
PMID: 21799729 [PubMed - indexed for MEDLINE]
Attentional control settings modulate susceptibility to the induced Roelofs effect.
Atten Percept Psychophys. 2011 Jul;73(5):1398-406
Authors: Lester BD, Dassonville P
When a visible frame is offset laterally from an observer's objective midline, the subjective midline is pulled toward the frame's center, causing the frame and any enclosed targets to be misperceived as being shifted somewhat in the opposite direction. This illusion, the Roelofs effect, is driven by environmental (bottom-up) visual cues, but whether it can be modulated by top-down (e.g., task-relevant) information is unknown. Here, we used an attentional manipulation (i.e., the color-contingency effect) to test whether attentional filtering can modulate the magnitude of the illusion. When observers were required to report the location of a colored target, presented within an array of differently colored distractors, there was a greater effect of the illusion when the Roelofs-inducing frame was the same color as the target. These results indicate that feature-based attentional processes can modulate the impact of contextual information on an observer's perception of space.
PMID: 21479725 [PubMed - indexed for MEDLINE]
Dissociations of perception and action do not automatically imply independent visual processing streams.
Cogn Neurosci. 2010 Mar;1(1):62-3
Authors: Dassonville P
Abstract Since the publication of Milner and Goodale's perception-action model of visual processing, there has been a general tendency to attribute any dissociation in the performance of perceptual and action tasks to a difference in the abilities and limitations of the ventral and dorsal visual streams. However, behavioral dissociations do not necessarily imply different underlying neural systems. In particular, there is a class of illusions, brought about by distortions of the observer's egocentric reference frame, that can cause perception-action dissociations without requiring or implying the existence of separate visual processing streams.
PMID: 24168246 [PubMed]