Resumo

Introduction: Under naturalistic viewing conditions, acquisition of visual information is constantly disrupted by transient events such as eye blinks and saccadic gaze shifts.Given that such events may be regarded as temporal scotoma, their inclusion into any realistic account of visual processing is of paramount importance. In the present study, we used an ethological approach to examine the temporal dynamics of blinks, head movements and gaze stabilization epochs in a diurnally active owl species, namely the burrowing owl.To better understand the relevance of this endeavour, it is important to keep in mind that owls have negligible eye movements such that head positionin this birdcan be taken as a reliable indicator of gaze. Method: Our results are based on 20 hours of video footage collectedwith a GR-D350UB JVC camcorder, between 0900 and 1600 hours,from a total of ten adult owls housed in spacious outdoor aviaries. Data werestored to hard discat a frame rate of 25 Hz. Digital material was coded through frame-by-frame repeated playback using the QuickTime Player software. Four exhaustive behavioural categories were considered for analysis: ¡§Fixation¡¨ (instance where the eyes did not move for at least 80 ms i.e. two video frames);¡§blink¡¨ (complete transient closure of the eyelids); ¡§gaze-shift¡¨ (head in motion); and ¡§out of view¡¨ (when neither eye was visible to the observer).Animals were observed while resting oractively engaged in a two-alternative force choice discrimination task.For each behavioural category and state, we merged the data obtained from all animals. Results: Visibility of subjects¡¦ eyes reached an average of 81% of the total observation time and was not significantly different across animals. The distribution of fixation durationswas broad, ranging from80 to 25,600ms,and negatively skewed with median values of1840 ms (mode: 480ms) for resting owls (RO) and 780 ms (mode: 240ms) for discriminating owls (DO). Blink episodes lasted on average 235 „b 45 ms (arithmetic mean „b SD; RO: 290 „b 64 ms; DO: 180 „b25 ms). Mean saccadic gaze shift duration was 161„b 144 ms (RO: 260„b160 ms; DO: 150„b130 ms). RO blinked significantly more often than DO (median: 7.45/min versus3.05/min, Wilcoxon rank sum, P < 0.001). Independently of the behavioural state of the animal, we also found that around 95% of blinks were concomitant with head movements. However, the fact that most head movements exhibited by DOwere not accompanied by blinks as they were seen to be in RO suggests that blinks were not simply triggered by reflexive mechanisms involving corneal stimulation. Conclusion: Our results provide evidence fora task-dependent interactionbetween blink and head movement control mechanisms, a phenomenon that has also been reported in primates.