Efeito do escotoma temporal causado pelo piscar na atividade dos neurônios do Wulst visual da coruja buraqueira (Athene cunicularia)

Abstract

Several times a minute, we rapidly close and reopen our eyelids. This behaviour, known as blinking, is essential, among other things, to keep the corneal surface healthy and free from potentially hazardous external agents. However, it also transiently disrupts patterned vision. Yet, intriguingly, we usually do not consciously experience such disruptions, even though comparable alterations of the retinal image due to external sources are readily noticeable. What are the neural mechanisms responsible for perceptual continuity during blinks? How does the brain distinguish visual stimuli produced by the own movement of the beholder from stimuli of similar physical characteristics but of exogenous origin? The aim of this thesis is to address these entirely opened issues by specifically examining how ongoing neuronal activity in the visual wulst of the awake burrowing owl (Athene cunicularia) is modulated by blinks. First, a camera-video acquisition system, originally developed for human pupillometry, was adapted to the experimental requirements of our animal model. A thorough pupilometric study was then undertaken to gain a better understanding of the retinal sensitivity to changes in light intensities. The results of this analysis show that the pupillary reflex is about seven times faster than that seen in primates and may cause an almost 5-fold reduction in retinal illuminance. Subsequently, a kinematic analysis of the owls reflex (evoked by an air-puff stimulation system) and spontaneous eyeblinks was performed, showing that the closing and opening phases of the former has shorter time duration than the latter. Moreover, a lack of complete closure of the pupil by the eyelid was found in both types of eyeblinks. Counterintuitively, a reduction in pupil size during the early phase of the eyelid closure was also observed. Finally, the effects of both types of eyeblinks were studied in 58 single neurons recorded from the visual wulst of the awake burrowing owl. In general, during sustained stimulation with sinusoidal gratings both reflex and spontaneous eyeblinks cause a reduction in the neuronal activity. However, an increase in spike discharge was also found in some cells during spontaneous blinks. In addition, simulating an eyeblink by instantly decreasing the monitor luminance for a brief period, diminishes the neuronal activity even further. When executed in the dark, only spontaneous eyeblinks are associated with a robust increase of neuronal activity, which starts before the onset of the eyelid movement. We conclude that our results are compatible with the idea that eyeblinks in part modulate the activity of neurons in the visual wulst trough an extra-retinal signal. This signal may be associated with a corollary discharge pathway incorporating the visual wuslt in owls.