Lesions to right posterior parietal cortex impair visual depth perception from disparity but not motion cues

The posterior parietal cortex (PPC) is understood to be active when observers perceive three-dimensional (3D) structure. However, it is not clear how central this activity is in the construction of 3D spatial representations. Here, we examine whether PPC is essential for two aspects of visual depth perception by testing patients with lesions affecting this region. First, we measured subjects' ability to discriminate depth structure in various 3D surfaces and objects using binocular disparity. Patients with lesions to right PPC (N = 3) exhibited marked perceptual deficits on these tasks, whereas those with left hemisphere lesions (N = 2) were able to reliably discriminate depth as accurately as control subjects. Second, we presented an ambiguous 3D stimulus defined by structure from motion to determine whether PPC lesions influence the rate of bistable perceptual alternations. Patients' percept durations for the 3D stimulus were generally within a normal range, although the two patients with bilateral PPC lesions showed the fastest perceptual alternation rates in our sample. Intermittent stimulus presentation reduced the reversal rate similarly across subjects. Together, the results suggest that PPC plays a causal role in both inferring and maintaining the perception of 3D structure with stereopsis supported primarily by the right hemisphere, but do not lend support to the view that PPC is a critical contributor to bistable perceptual alternations. This article is part of the themed issue ‘Vision in our three-dimensional world’.

An SI value of 1 therefore indicates that the reduction in alternations reported during intermittent viewing is equal to that predicted by the reduced duration of the stimulus appearance. Index values above 1 indicate a greater stabilizing effect of intermittent presentation compared to continuous viewing.

Inferring perceptual transitions from eye movements
Eye position was recorded binocularly for the majority of bistable task trials, while a central fixation marker was presented on the remaining trials for the rotating sphere and binocular rivalry tasks. When observers were not instructed to fixate, these stimuli naturally elicited optokinetic nystagmyus (OKN) eye movements, which provide a physiological indicator of perception (7)(8)(9). To achieve this, eye position data were first low-pass filtered (25Hz cut-off frequency), and saccades and fast phases of

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OKN eye movements were then removed by applying a velocity threshold of ±20°s -1 in the horizontal direction. The remaining slow phase OKN velocity data were linearly interpolated and low-pass filtered (1Hz), and changes in velocity sign were then identified as perceptual alternations (Supplementary Figure S1). Comparison of the extracted versus reported perceptual time courses revealed that extracted perceptual transitions tended to precede reported transitions. Crosscorrelations showed that the maximum correlation coefficients (patient group mean r = 0.78 ± 0.04) occurred at a mean lag time of 1.10 ± 0.21s. Inspection of the eye movement data revealed no obvious abnormalities in any of the patients' eye movements in relation to the motion stimuli. The eye movement data therefore suggest that observers were actively engaged in the perceptual task and that abnormal eye movements did not influence their perceptual experience. All further analyses of perceptual alternations were therefore based on participants' manually reported percepts, since these data were available for all trials.

Anatomical description of lesion locations
Patient RH suffered a left-hemisphere stroke that affected the inferior parietal lobe, superior temporal lobe and angular gyrus. Behaviourally, RH exhibits severe extinction of visual events in the right hemifield during bilateral simultaneous stimulation and made left-sided line bisections (10)(11)(12). Consistent with the extensive damage to his left hemisphere, RH also shows deep dysphasia, dyslexia and dysgraphia. Patient MH's lesions are the result of an anoxic accident (carbon monoxide intoxication). MRI reveals diffuse and widespread bilateral sulcal widening that is most pronounced in the left parietal lobe, including the occipito-parietal junction, intraparietal sulcus and angular and supramarginal gyri. Subcortically, MH displays ventricular widening and bilateral lesions of the lentiform nuclei and claustrum. Consistent with the left parietal focus of his lesions, MH exhibits unilateral spatial attention bias with extinction in the right hemifield (11). MH also exhibits optic ataxia, but only when reaching to targets in the right visual hemifield with his right arm (11,13,14). Functional MRI has previously revealed that MH's motion-selective cortical area V5/hMT+ is intact and responsive (15). Patient PM shows right superior temporal, left angular and supramarginal lesions. There is also a small lesion in the left postcentral gyrus, bilateral putamen and caudate. Patient PF has bilateral lesions to the superior and inferior parietal gyri. Specifically, the left hemisphere lesion extends into the left angular gyrus and the right hemisphere lesion extends slightly into the right postcentral gyrus. There are small lesions in both the left and right putamen which extend into the caudate and another small lesion in the left thalamus. PF exhibits right hemifield extinction and dysgraphia (16). Patient MP has lesions to the right hemisphere affecting the inferior frontal gyrus, the superior temporal gyrus, the supramarginal and angular gyri and the post-central gyrus. The damage caused mild hemiparesis of his previously dominant left arm, although he retained normal sensation. Perceptually, he shows extinction during bilateral simultaneous stimulation (10,11). Further details regarding the performance of these patients on a range of behavioural tasks are available from previous studies (10,(12)(13)(14)(15)(16)(17)(18)(19).

Relationship between perceptual alternations and lesion locations
Previous evidence suggests that specific loci within PPC causally influence perceptual dynamics for bistable stimuli. In order to quantify the extent to which different regions of PPC are affected by lesions in each patient, we used spherical regions of interest (ROIs) centered on previously published MNI coordinates for anterior and posterior superior parietal areas that appear to influence perceptual alternation rates (Supplementary Table S3). We calculated the proportion of voxels in each ROI that overlapped with each patient's lesion mask (Supplementary Table S4

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A specific hypothesis postulated by previous studies conducted in healthy observers is that anterior and posterior SPL exert opposing influences on mean percept duration, with disruption of aSPL reducing it and disruption of pSPL increasing it (17,20). For the ROIs based on these studies, patients MP and PM showed slightly more damage to anterior ROIs than posterior ones (Supplementary Table S4). In agreement with the hypothesis, one of these patients (PM) showed shorter mean percept durations on both the rotating sphere and apparent motion dot quartet stimuli. Patient MP's mean percept duration was in the normal range, although there were limited data available for this observer (number of percept durations on the rotating sphere = 17), resulting in a larger standard error. In contrast, patients MH and PF's lesions overlapped both anterior and posterior ROIs, for which the predicted outcome is unclear. MH showed shorter percept durations than average, while PF's were variable across stimuli (close to average for the rotating sphere but shorter than average for apparent motion).   Figure S3: Scatter plot of observer age against mean rotating sphere percept duration. While ages were bimodally distributed, there was no clear correlation between age and percept duration. Figure S4: A) Perceptual reports for patient PF during one block of the dynamic disparity task. On this task, inter-trial intervals were always 1 second (grey bars), but stimulus duration was self-paced (i.e. variable) and averaged 1.03 ± 0.04s. The stimulus rotated clockwise on half of the trials in each block, and this result therefore shows a strong bias. B) Perceptual reports for the same patient during intermittent presentation of the bistable rotating sphere stimulus (i.e. similar to the dynamic disparity task, but without disparity).  Figure S5: Region-of-interest (ROI) analysis. A) Four ROIs were created in the PPC of each hemisphere, based on a spherical volume (10mm radius) centred on MNI coordinates reported by previous studies (Supplementary Table 3). B) Overlap between each of the 8 ROIs and the lesion mask of each patient was quantified. C) The mean percept duration on the bistable rotating sphere task for each patient was plotted against the proportion of lesioned voxels for each ROI (note the legend colours correspond to left and right ROIs separately).