Intra-saccadic displacement sensitivity after a lesion to the posterior parietal cortex

More (corrective) consecutive saccades after a lesion to the posterior parietal cortex

To reach a target, primary saccades (S1s) are often followed by (corrective) consecutive saccades (S2, and potentially S3, S4, S5), which are based on retinal and extraretinal feedback. Processing these extraretinal signals was found to be significantly impaired by lesions to the posterior parietal cortex (PPC). Recent studies, however, added a more nuanced view to the role of the PPC, where patients with PPC lesions still used extraretinal signals for S2s and perceptual judgements (Fabius et al., 2020; Rath-Wilson & Guitton, 2015). Hence, it seems that a PPC lesion is not disrupting extraretinal processing per se. Yet, a lesion might still result in less reliable processing of extraretinal signals. Here, we investigated whether this lower reliability manifests as decreased or delayed S2 initiation. Patients with PPC lesions (n = 7) and controls (n = 26) performed a prosaccade task where the target either remained visible or was removed after S1 onset. When S1 is removed, accurate S2s (corrections of S1 error) rely solely on extraretinal signals. We analysed S2 quantity and timing using linear mixed-effects modelling and additive hazards analyses. Patients demonstrated slower S1 execution and lower S1 amplitudes than controls, but their S2s still compensated the S1 undershoot, also when they only relied on extraretinal information. Surprisingly, patients showed an increased amount of S2s. This deviation from control behaviour can be seen as suboptimal, but given the decreased accuracy of the primary saccade, it could be optimal for patients to employ more (corrective) consecutive saccades to overcome this inaccuracy.

The posterior parietal cortex processes visuo-spatial and extra-retinal information for saccadic remapping: A case study

Optimally collecting information and controlling behaviour require that we constantly scan our visual environment through eye movements. How the dynamic interaction between short-lived retinal images and extra-retinal signals of eye motion results in our subjective experience of visual stability remains a major issue in Cognitive Neuroscience. The present study aimed to assess and determine the nature of the contribution of the posterior parietal cortex (PPC) to the saccadic remapping mechanisms which contribute to such perceptual visual constancy. Perceptual responses in transsaccadic visual localization tasks were measured in a patient presenting with a PPC lesion and manifesting optic ataxia in the left hemifield with no neglect. Two perceptual localization tasks, each with versus without an intervening saccade, were used: the saccadic suppression of displacement (SSD) task (Ostendorf, Liebermann, & Ploner, 2010) and the peri-saccadic flash localization (LOC) task (Zimmerman & Lappe, 2010). Compared to a group of age-matched healthy subjects, the patient showed a specific pattern of perceptual deficits in the ataxic (left) hemifield. First, a significant impairment occurred in the stationary eye conditions, attesting for an alteration of visuo-spatial encoding. Second, in the saccade conditions, an additional perceptual deficit (an error of ~5° along the saccade direction) was observed in both tasks and mainly in conditions where extra-retinal signals are thought to be critically involved, revealing a constant underestimation by extra-retinal signals of the saccade size, despite preserved saccade accuracy. These findings highlight a crucial role of the PPC in saccadic remapping processes underlying perceptual visual constancy and provide empirical evidence for models such as Ziesche and Hamker's (2014).