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The endogenous cannabinoid system plays important roles in the retina of mice and monkeys via their classic CB1 and CB2 receptors. We have previously reported that the G protein-coupled receptor 55 (GPR55), a putative cannabinoid receptor, is exclusively expressed in rod photoreceptors in the monkey retina, suggesting its possible role in scotopic vision. To test this hypothesis, we recorded full-field electroretinograms (ERGs) after the intravitreal injection of the GPR55 agonist lysophosphatidylglucoside (LPG) or the selective GPR55 antagonist CID16020046 (CID), under light- and dark-adapted conditions. Thirteen vervet monkeys (Chlorocebus sabaeus) were used in this study: four controls (injected with the vehicle dimethyl sulfoxide, DMSO), four injected with LPG and five with CID. We analyzed amplitudes and latencies of the a-wave (photoreceptor responses) and the b-wave (rod and cone system responses) of the ERG. Our results showed that after injection of LPG, the amplitude of the scotopic b-wave was significantly higher, whereas after the injection of CID, it was significantly decreased, compared to the vehicle (DMSO). On the other hand, the a-wave amplitude, and the a-wave and b-wave latencies, of the scotopic ERG responses were not significantly affected by the injection of either compound. Furthermore, the photopic ERG waveforms were not affected by either drug. These results support the hypothesis that GPR55 plays an instrumental role in mediating scotopic vision.
There is a delay before sensory information arising from a given event reaches the central nervous system. This delay may be different for information carried by different senses. It will also vary depending on how far the event is from the observer and stimulus properties such as intensity. However, it seems that at least some of these processing time differences can be compensated for by a mechanism that resynchronizes asynchronous signals and enables us to perceive simultaneity correctly. This chapter explores how effectively simultaneity constancy can be achieved, both intramodally within the visual and tactile systems and cross-modally between combinations of auditory, visual, and tactile stimuli. We propose and provide support for a three-stage model of simultaneity constancy in which (1) signals within temporal and spatial windows are identified as corresponding to a single event, (2) a crude resynchronization is applied based on simple rules corresponding to the average processing speed differences between the individual sensory systems, and (3) fine-tuning adjustments are applied based on previous experience with particular combinations of stimuli.
Although time is essential for the perception of the outside world, there is no energy that carries duration information, and consequently there can be no sensory system for time. Time needs to be constructed by the brain, and because this process itself takes time, it follows that the perception of when an event occurs must necessarily lag behind the occurrence of the event itself.
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