Aarlenne Z KhanUniversité de Montréal | UdeM · School of Optometry
Aarlenne Z Khan
PhD Cognitive Neuroscience, PhD Psychology
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Publications (54)
Anti-saccades are eye movements in which the saccade is executed in the opposite direction of a visual target and are often hypometric. Because the visual target and saccade goal are decoupled, it has been suggested that competition between the two locations occurs and needs to be resolved. It has been hypothesized that the hypometria of anti-sacca...
During covert and pre-saccadic attentional shifts, it is unclear how facilitation and suppression processes interact for target selection. A recent countermanding task pointed to greater suppression at unattended locations during trials with saccades compared to trials without saccades (i.e., fixation and successful stop trials), whereas target fac...
Anti-saccades are eye movements that require inhibition to stop the automatic saccade to the visual target and to perform instead a saccade in the opposite direction. The inhibitory processes underlying anti-saccades have been primarily associated with frontal cortex areas for their role in executive control. Impaired performance in anti-saccades h...
When vision is removed, limb position has been shown to progressively drift during repetitive arm movements. The posterior parietal cortex (PPC) is known to be involved in the processing of multisensory information, the formation of internal hand estimate, and online motor control. Here, we compared hand position drift between healthy controls and...
The premotor theory of attention and the visual attention model make different predictions about the temporal and spatial allocation of presaccadic attentional facilitation. The current experiment investigated the spatial and temporal dynamics of presaccadic attentional facilitation during pro-and antisaccade planning; we investigated whether atten...
Previous studies have shown that eye and arm movements tend to be intrinsically coupled in their behavior. There is, however, no consensus on whether planning of eye and arm movements is based on shared or independent representations. One way to gain insight into these processes is to compare how exogenous attentional modulation influences the temp...
Transsaccadic memory is a process by which remembered object information is updated across a saccade. To date, studies on transsaccadic memory have used simple stimuli-that is, a single dot or feature of an object. It remains unknown how transsaccadic memory occurs for more realistic, complex objects with multiple features. An object's location is...
Optic ataxia is a consequence of brain damage to the posterior parietal cortex (PPC) resulting in errors during visually-guided arm movements. Because the PPC is known to be involved in multisensory integration processes, optic ataxia has been proposed to originate from a deficit of transforming sensory information into an accurate reaching movemen...
Simultanagnosia is a deficit in which patients are unable to perceive multiple objects simultaneously. To date, it remains disputed whether this deficit results from disrupted object or space perception. We asked both healthy participants as well as a patient with simultanagnosia to perform different visual search tasks (both pop-out and serial) of...
Previous studies have shown that attention shifts to the saccade goal before movement onset (e.g. Deubel et al., 2008), with a peak approximately 50 ms before saccade onset. The current experiment investigated the timing and dynamics of this attention shift during anti-saccade planning; we investigated whether attention shifts only to the anti-sacc...
It is relatively easy to adapt visually-guided saccades because the visual vector and the saccade vector match. The retinal error at the saccade landing position is compared to the prediction error, based on target location and efference copy. If these errors do not match, planning processes at the level(s) of the visual and/or motor vector process...
Previous studies have shown that the influence of a behaviorally irrelevant distractor on saccade reaction times (SRTs) varies depending on the temporal and spatial relationship between the distractor and the saccade target. We measured distractor influence on SRTs to a subsequently presented target, varying the spatial location and the timing betw...
Previous studies have shown that attention shifts to the saccade goal before movement onset (e.g. Deubel et al., 2008), with a peak approximately 50 ms before saccade onset. The current experiment investigated the timing and dynamics of this attention shift during anti-saccade planning; we investigated whether attention shifts only to the anti-sacc...
When planning to reach, information about both the target location as well as the initial hand position yield the movement vector. The initial hand position can be determined by proprioceptive information as well as visual information, if available. Bayesian integration posits that we utilize all information available, with greater weighting on the...
Simultanagnosia is a deficit in which patients are unable to perceive multiple objects simultaneously. To date, it remains disputed whether this deficit results from disrupted object or space perception. We asked both healthy participants as well as a patient with simultanagnosia to perform different visual search tasks of variable difficulty. We a...
Because we have limited processing abilities with respect to the plethora of visual information entering our brain, spatial selection mechanisms are crucial. These mechanisms result in both enhancing processing a location of interest and in suppressing processing at other locations; together, they enable successful further processing of locations o...
When confronted with a complex moving stimulus, the brain can integrate local element velocities to obtain a single motion signal, or segregate the elements to maintain awareness of their identities. The integrated motion signal can drive smooth-pursuit eye movements (Heinen and Watamaniuk, 1998), whereas the segregated signal guides attentive trac...
Optic ataxia is a component of Balint's syndrome and is a disorder that results from damage to the posterior parietal cortex (PPC) leading to deficits in reaching and grasping objects presented in the visual field opposite to the damaged hemisphere. It is also often the case that Balint's syndrome is accompanied by visual field defects due to the p...
In order to efficiently interact with our environment we need to constantly to update the spatial representation of visual targets for movement. This is required not only when we move our eyes but also when we want to reach toward a location different from the actual physical target (for example symmetrical). These two types of remapping are very d...
Prior to the onset of a saccade or a reach, attention is directed to the goal of the upcoming movement. However, it remains unknown whether attentional resources are shared across effectors for simultaneous eye and hand movements. Using a 4-AFC shape discrimination task, we investigated attentional allocation during the planning of a saccade alone,...
When moving through our environment, it is vital to preferentially process positions on our future path in order to react quickly to critical situations. During smooth pursuit, attention may be directed ahead with either a focused locus or a broad bias. We examined the 2D spatial extent of attention during a smooth pursuit task using both saccade (...
Previous studies have shown that normal subjects update visual space in eye-centered coordinates (Henriques et al. 1998). Furthermore, this remapping takes place in the posterior parietal cortex (Medendorp et al. 2002; Batista et al. 1999). Here we explore how updated information is retained in patients with bilateral posterior parietal lesions (op...
The saccade generator accounts for previous saccades in order to ‘update’ internal representations for subsequent saccades. Here we tested the role of limb proprioception on updating the locations of hand-held objects for the saccade generator. We measured radial saccades (8 directions, approximately 7 and 13 degrees eccentricity) from a central ta...
Previous studies have observed that similar brain areas are activated during covert shifts of attention and during the execution of saccades, leading to the suggestion that the brain systems controlling these functions share similar neural substrates. In the present study we tested the extent of the functional overlapping between the two systems.
I...
Numerous studies have suggested that saccades to a location trigger an automatic attentional shift to the saccade goal, which enhances perceptual processing at that location. Here, we argue for a functional dissociation between pre-saccadic perceptual enhancement and saccade planning. A patient with a lesion in the right posterior parietal cortex p...
Numerous studies on visuomotor control (reaching and pointing) have implicitly assumed that there is a single eye-centered representation of target location for planning movements. However, visual information about target location enters our brains through two eyes which are horizontally separated and therefore provide disparate location informatio...
To interact with an object in three-dimensional (3 D) space, the brain must construct the object's egocentric spatial location using binocular vision; both retinal images are merged to provide angular eccentricity (horizontal and vertical position) and retinal disparity provides information about distance. We have demonstrated theoretically that th...
The aim of the current work was to investigate the spatial extent of attentional facilitation and inhibition of return (IOR) in response to an exogenous cue over time. We used saccade latencies as a behavioral correlate of attentional allocation. Two humans and two monkeys made saccades to visual targets at 136 locations spread across the visual fi...
Presenting a behaviorally irrelevant cue shortly before a target at the same location decreases the latencies of saccades to the target, a phenomenon known as exogenous attention facilitation. It remains unclear whether exogenous attention interacts with early, sensory stages or later, motor planning stages of saccade production. To distinguish bet...
The pre-motor theory of attention suggests that the mechanisms involved in target selection for eye movements are the same as those for spatial attention shifts. The pre-saccadic facilitation of perceptual discrimination at the location of a saccadic goal (paradigm of Deubel and Schneider, 1996) has been considered as an argument for this theory. W...
A salient peripheral cue can capture attention, influencing subsequent responses to a target. Attentional cueing effects have been studied for head-restrained saccades; however, under natural conditions, the head contributes to gaze shifts. We asked whether attention influences head movements in combined eye-head gaze shifts and, if so, whether thi...
A well-known theory in the field of attention today is the premotor theory of attention which suggests that the mechanisms involved in eye movements are the same as those for spatial attention shifts. We tested a parietal damaged patient with unilateral optic ataxia and 4 controls on a dual saccade/attentional task and show a dissociation between s...
To reach for an object, one needs to know its egocentric distance (absolute depth). It remains an unresolved issue which signals are required by the brain to calculate this absolute depth information. We devised a geometric model of binocular 3D eye orientation and investigated the signals necessary to uniquely determine the depth of a non-foveated...
Recent neurophysiological studies suggest that reach planning areas in the posterior parietal cortex encode both target and initial hand position in gaze-centered coordinates, which could be used to calculate a desired movement vector. We tested how varying gaze, target position, and initial hand position affected reach errors in two left unilatera...
The saccade generator updates memorized target representations for saccades during eye and head movements. Here, we tested if proprioceptive feedback from the arm can also update handheld object locations for saccades, and what intrinsic coordinate system(s) is used in this transformation. We measured radial saccades beginning from a central light-...
Recent studies have suggested that internal updating of visuospatial targets in humans occurs in gaze-centered coordinates and takes place in the parietal and extrastriate cortices. We explored how information for reaching is updated in two patients with bilateral lesions in these areas. Subjects performed two visuomotor tasks: (i) a fixation reach...
Optic ataxia is a disorder associated with posterior parietal lobe lesions, in which visually guided reaching errors typically occur for peripheral targets. It has been assumed that these errors are related to a faulty sensorimotor transformation of inputs from the 'ataxic visual field'. However, we show here that the errors observed in the contral...
Eye-hand coordination is complicated by the fact that the eyes are constantly in motion relative to the head. This poses problems in interpreting the spatial information gathered from the retinas and using this to guide hand motion. In particular, eye-centered visual information must somehow be spatially updated across eye movements to be useful fo...
We previously found that subjects switched 'ocular dominance' as a function of horizontal gaze direction in a reaching task [Vision Res. 41 (14) (2001) 1743]. Here we extend these findings to show that when subjects pointed to targets across the horizontal binocular field, they aligned the fingertip with a vertical plane located between the eyes an...
In recent years the scientific community has come to appreciate that the early cortical representations for visually guided arm movements are probably coded in a visual frame, i.e. relative to retinal landmarks. While this scheme accounts for many behavioral and neurophysiological observations, it also poses certain problems for manual control. For...
Ocular dominance is the tendency to prefer visual input from one eye to the other [e.g. Porac, C. & Coren, S. (1976). The dominant eye. Psychological Bulletin 83(5), 880-897]. In standard sighting tests, most people consistently fall into either the left- or right eye-dominant category [Miles, W. R. (1930). Ocular dominance in human adults. Journal...