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MRIs of the three hemispherectomy patients. The left side of the image corresponds to the left side of the brain in all images. The complete right hemispherectomy of Patient D. R. is shown in the coronal and longitudinal planes, and likewise for Patient I. G. The partial left hemispherectomy of Patient J. B. is shown in the coronal and longitudinal planes. 

MRIs of the three hemispherectomy patients. The left side of the image corresponds to the left side of the brain in all images. The complete right hemispherectomy of Patient D. R. is shown in the coronal and longitudinal planes, and likewise for Patient I. G. The partial left hemispherectomy of Patient J. B. is shown in the coronal and longitudinal planes. 

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Individuals who have undergone hemispherectomy for treatment of intractable epilepsy offer a rare and valuable opportunity to examine the ability of a single cortical hemisphere to control oculomotor performance. We used peripheral auditory events to trigger saccades, thereby circumventing dense postsurgical hemianopia. In an antisaccade task, pati...

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... (Herter & Guitton, 2004; Hughes, Reuter- Lorenz, Fendrich, & Gazzaniga, 1992; Tusa, Zee, & Herdman, 1986; Sharpe, Lo, & Rabinovitch, 1979; Troost, Weber, & Daroff, 1972). However, the potential for a single hemicortex to acquire functional control of saccade suppression and the ability to modulate reflexive glances normally and bilaterally have not been explored. This investigation therefore aimed to characterize a single hemisphere ʼ s capacity for bilateral control of saccadic reflexes and to identify potential limits on the plasticity of lateralized saccadic control. Hemispherectomy involves the neurosurgical removal of an entire cortical hemisphere in some patients, or partial removal and complete disconnection of the remaining cortex in others (Ptito & Leh, 2007). Human autopsy and animal models indicate that the ipsilesional thalamus and other subcortical structures undergo extensive retrograde degen- eration (Theoret, Boire, Herbin, & Ptito, 2001; Ptito, Herbin, Boire, & Ptito, 1996; Ueke, 1966). The SC, however, is con- served bilaterally after hemispherectomy ( Theoret et al., 2001; Ptito et al., 1996; Ueke, 1966), suggesting that the capacity to generate reflexive contralesional saccades might also be preserved. The capacity for blindsight has been studied extensively following hemispherectomy (see Ptito & Leh, 2007 for a review), however, reflexive saccadic behavior has not been systematically studied in these patients primarily due to the dense postsurgical hemianopia that severely limits visually evoked contralesional saccades. To circumvent limitations caused by permanent hemianopia, we examined auditory-evoked saccades to left- and right- sided peripheral tones that these patients can easily localize (Zatorre, Ptito, & Villemure, 1995). The antisaccade task has proven to be an excellent tool for assessing the limits of saccadic control (Hallett, 1978; reviewed in Ramat, Leigh, Zee, & Optican, 2007; Leigh & Kennard, 2004; Munoz & Everling, 2004). Two key capacities that can be evaluated with this task are the ability to inhibit a prepotent, reflexive response to a stimulus onset (prosaccade) and the capability to perform voluntary saccades in the direction opposite to the sensory stimulus (antisaccade). Notably, studies of patients with focal lesions indicate that damage to dorsolateral prefrontal cortex (e.g., Pierrot- Deseilligny, Rivaud, Gaymard, & Agid, 1991; Guitton, Buchtel, & Douglas, 1985) and/or frontal eye fields (Machado & Rafal, 2004a; see also Henik, Rafal, & Rhodes, 1994) can im- pair the suppression of reflexive saccades especially in the contralesional direction (see Muri & Nyffeler, 2008, for a review), leading to release of the “ visual grasp reflex ” (Hess, Bürgi, & Bucher, 1946). Regions of posterior parietal cortex have been implicated in the vector inversion required to generate a saccade in the direction opposite to the visual stimulus (e.g., Nyffeler, Rivaud-Pechoux, Pierrot-Deseilligny, Diallo, & Gaymard, 2007). Chronic lesions affecting the in- traparietal sulcus have also been shown to reduce the grasp reflex toward contralesional stimuli and increase the latencies of antisaccades in the opposite direction (Rafal, 2006; Machado & Rafal, 2004a). It is unknown how the chronic absence of all oculomotor cortex unilaterally will affect the ability to perform antisaccades. Reflexive saccade behavior has also been fruitfully examined by varying the state of the fixation stimulus relative to the onset of the signal to saccade (reviewed in Leigh & Zee, 2006). Compared to the overlap condition in which the fixation point remains visible during the signal to saccade, extinguishing the fixation point several hundred milli- seconds before the onset of the saccade signal (the gap condition) enables shorter-latency saccades including express saccades with latencies ranging from 80 to 130 msec (Saslow, 1967). In humans, damage to posterior parietal cortex, especially in the right hemisphere, has been associated with increased visual saccade latency in the gap condition (Braun, Weber, Mergner, & Schulte-Mönting, 1992; Pierrot-Deseilligny et al., 1991; however, see Rafal, 2006). Here we use gap and overlap versions of the antisaccade task to examine whether hemispherectomized patients have the ability to volitionally inhibit stimulus-bound saccades. The prosaccade task is also examined under gap and overlap conditions to further assess reflexive responding and its modulation by fixation. We show that hemispherectomy leads to impaired control of reflexive saccadic behavior including the release of unintended contralesional saccades in the antisaccade task, and the attenuation of the gap effect for ipsilesional prosaccades. We posit that these effects are due to a limited ability of the intact hemisphere to exert top down control of the ipsilesional SC, and altered control dynamics affecting the SC in the intact hemisphere. Three hemispherectomized patients (D. R., I. G., J. B.; see Figure 1 for structural MR images) and four age-matched neurologically intact control subjects participated in this investigation. Detailed descriptions of these patients have been previously published (Zatorre et al., 1995 [D. R.: Case 1; I. G.: Case 5; J. B.: Case 3]; Leh, Johansen-Berg, & Ptito, 2006) and dense contralesional hemianopia has been es- tablished in all three patients (Herter & Guitton, 2004, 2007; Leh et al., 2006; Tomaiuolo, Ptito, Marzi, Paus, & Ptito, 1997). In brief, D. R. and I. G. are right-handed women, ages 25 and 47 years, respectively, at time of testing, both of whom underwent right hemispherectomy (Villemure & Mascott, 1995; Villemure & Rasmussen, 1990). D. R. suffered from Rasmussen ʼ s chronic encephalitis with seizure onset at age 5. At 17 years, she underwent modified right hemispherectomy that included removal of the temporal lobe, a frontal – parietal corticectomy. All remaining cortical tissue on the decorticate side was surgically disconnected from the rest of the brain, leaving her with a complete functional hemispherectomy. I. G. suffered a prenatal middle cerebral artery occlusion with seizure onset at age 7 and underwent complete anatomical hemispherectomy at age 13, removing her entire cortical hemisphere and homo- lateral basal ganglia. J. B. is a left-handed man, aged 34 at the time of testing, who underwent left hemispherectomy at age 20 for treatment of seizure disorder with onset at age 5 due to a porencephalic cyst. This included removal of the temporal, parietal, and occipital cortices, and disconnection of any remaining cortex from the rest of the brain. Preoperative testing determined that he was left- handed with language lateralized to the right cortical hemisphere, permitting left hemispherectomy. The frontal and occipital poles were left in place but were surgically disconnected from the rest of the brain, including the intact hemisphere and brainstem structures. All patients ʼ full-scale IQs fell in the low normal range. Four right-handed control subjects (2 men and 2 women, ranging from 26 to 33 years of age, similar to the patients) with no history of neurological or psychiatric disorders also participated in this study. All participants gave informed consent and all procedures were approved by the Institu- tional Review Boards of the Montreal Neurological Institute and the University of Michigan. Subjects were seated in a completely dark room with their heads stabilized by a chin rest and bite bar. They faced a black cylindrical screen located 55 cm from their eyes along the horizontal meridian. Bitemporal EOG was used to mea- sure horizontal eye position. To minimize drifts and noise, the skin was thoroughly cleaned at each point of electrode contact. Fluctuations in the DC offset were further reduced by a short adaptation period before calibration and recording. During recording, small drifts were corrected by automatically resetting the EOG output to zero as the subjects fixated at the start of each trial. Calibration checks occurred as needed and at least every 15 min by having the subjects fixate a target that jumped predictably from 0° → +20° → 0° → − 20° → 0°. This target displacement se- quence was repeated while the gain adjustments were made to assure a fixed output voltage for the 20° target offset. When properly calibrated and guarded against drifts, the EOG signal was accurate within ±1° over a range of ±30° for all subjects. This was well within our needs because, as explained below, we were interested in saccade latency and direction, not in endpoint accuracy. Auditory tones (2800 Hz, 90 dB; the signals to which subjects responded) were generated by two small speakers fixed to the front of the cylindrical screen, 45° to the left and right of the fixation light (LED, 0.5° diameter, 670 nm, 2 12.0 cd/m ). Because our aim was to study saccadic response times, we placed the speakers at relatively large eccentricities to promote rapid responses (e.g., Yao & Peck, 1997), albeit with decreased precision of localization (e.g., Zatorre et al., 1995). A small response box equipped with two horizontally aligned buttons was placed on the armrest of the subject ʼ s chair on the side of the dominant (nonparetic) hand. The subjects were required to saccade or to press the left or right button, depending on the task (see below). Four different tasks were run in a block design: antisaccades, antimanual (button-press), prosaccades, promanual. The stimulus conditions were the same in all four tasks; only the instructions varied. In the prosaccade task, participants were instructed to move their eyes “ toward the tone as fast as possible ” ; the promanual task required participants to use their dominant (or nonparetic) hand to press the response button “ on the same side as the auditory tone as fast as possible. ” In the antisaccade task, participants were told to move their eyes “ away from the tone as fast as possible. ” In the antimanual task, ...

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... The results for the visual modality support extensive work and reflect the time-consuming nature of evoking the constituent elements (i.e., response suppression and vector inversion) of the antisaccade task (for review, see Munoz and Everling 2004). In turn, results for the auditory modality support a less extensive body of research proposing that auditory antisaccades require the suppression of an auditory-evoked ocular grasp reflex (i.e., response suppression) and associated vector inversion (Reuter-Lorenz et al. 2011; see also Heath et al. 2015b). In other words, it appears that auditory antisaccades are governed via a two-component process similar to their visual counterparts. ...
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