What the ‘Moonwalk’ Illusion Reveals about the Perception of Relative Depth from Motion

When one visual object moves behind another, the object farther from the viewer is progressively occluded and/or disoccluded by the nearer object. For nearly half a century, this dynamic occlusion cue has beenthought to be sufficient by itself for determining the relative depth of the two objects. This view is consistent with the self-evident geometric fact that the surface undergoing dynamic occlusion is always farther from the viewer than the occluding surface. Here we use a contextual manipulation ofa previously known motion illusion, which we refer to as the‘Moonwalk’ illusion, to demonstrate that the visual system cannot determine relative depth from dynamic occlusion alone. Indeed, in the Moonwalk illusion, human observers perceive a relative depth contrary to the dynamic occlusion cue. However, the perception of the expected relative depth is restored by contextual manipulations unrelated to dynamic occlusion. On the other hand, we show that an Ideal Observer can determine using dynamic occlusion alone in the same Moonwalk stimuli, indicating that the dynamic occlusion cue is, in principle, sufficient for determining relative depth. Our results indicate that in order to correctly perceive relative depth from dynamic occlusion, the human brain, unlike the Ideal Observer, needs additionalsegmentation information that delineate the occluder from the occluded object. Thus, neural mechanisms of object segmentation must, in addition to motion mechanisms that extract information about relative depth, play a crucial role in the perception of relative depth from motion

Purinergic regulation of vascular tone in the retrotrapezoid nucleus is specialized to support the drive to breathe

© Hawkins et al. Cerebral blood flow is highly sensitive to changes in CO2/H+ where an increase in CO2/H+ causes vasodilation and increased blood flow. Tissue CO2/H+ also functions as the main stimulus for breathing by activating chemosensitive neurons that control respiratory output. Considering that CO2/H+-induced vasodilation would accelerate removal of CO2/H+ and potentially counteract the drive to breathe, we hypothesize that chemosensitive brain regions have adapted a means of preventing vascular CO2/H+-reactivity. Here, we show in rat that purinergic signaling, possibly through P2Y2/4 receptors, in the retrotrapezoid nucleus (RTN) maintains arteriole tone during high CO2/H+ and disruption of this mechanism decreases the CO2ventilatory response. Our discovery that CO2/H+-dependent regulation of vascular tone in the RTN is the opposite to the rest of the cerebral vascular tree is novel and fundamentally important for understanding how regulation of vascular tone is tailored to support neural function and behavior, in this case the drive to breathe

Bilateral Giant Cavernous Carotid Artery Aneurysms in a Child With Juvenile Paget\u27s Disease

Background: Juvenile Paget disease (JPD) is a rare genetic bone disorder, also affecting the immune and vascular systems. We describe the first ever case of JPD associated with bilateral giant cavernous carotid artery aneurysms in a child. Case Description: A child with known JPD presented with left abducens nerve palsy and a computed tomographic angiogram revealed bilateral giant cavernous carotid artery aneurysms. He underwent a left-sided superficial temporal artery to middle cerebral artery bypass and endovascular carotid artery occlusion, followed by an identical procedure on the right side 3 months later and made an event-free recovery without any new neurological deficits. Conclusions: This previously unreported association poses the question of determining the optimal management strategy for such cases. The pathophysiology and clinical features of JPD are discussed, with special emphasis on the management of giant cavernous carotid aneurysms in this subgroup of individuals