Abstract
The perception of moving objects (real motion) is a critical function for interacting with a dynamic environment. Motion perception can be also induced by particular structural features of static images (illusory motion) or by photographic images of subjects in motion (implied motion, IM). Many cortical areas are involved in motion processing, particularly the medial temporal cortical area (MT), dedicated to the processing of real, illusory, and implied motion. Recently, there has been a growing interest in the influence of high-level visual processes on pupillary responses. However, just a few studies have measured the effect of motion processing on the pupil, and not always with consistent results. Here we systematically investigate the effects of real, illusory, and implied motion on the pupil diameter for the first time, by showing different types of stimuli (movies, illusions, and photos) with the same average luminance to the same observers. We find different pupillary responses depending on the nature of motion. Real motion elicits a larger pupillary dilation than IM, which in turn induces more dilation than control photos representing static subjects (No-IM). The pupil response is sensitive even to the strength of IM, as photos with enhanced IM (blur, motion streaks, speed lines) induce larger dilation than simple freezed IM (subjects captured in the instant they are moving). Also, the subject represented in the stimulus matters: human figures are interpreted as more dynamic and induce larger dilation than objects/animals. Interestingly, illusory motion induces much less dilation than all the other motion categories, despite being seen as moving. Overall, pupil responses depend on the individual perception of dynamicity, confirming that the pupil is modulated by the subjective interpretation of complex stimuli. We argue that the different pupillary responses to real, illusory, and implied motion reflect the top-down modulations of different cortical areas involved in their processing.
Highlights
Motion perception, defined as the perception of change of position of an object over time, is a crucial function for successfully interacting with a dynamic environment and is fundamental for survival [1, 2]
Pupil dilation was found in response to illusory motion in peripheral drift images and this result was attributed to the physiologically arousing nature of illusory motion [89]. These findings suggest that cortical mechanisms involved in motion processing may influence pupil size independently on luminance changes, but they do not allow to reach exhaustive conclusions on how the pupil responds to motion, especially in connection to the different types of motion
We systematically investigate the effect of motion perception and interpretation on the pupillary response, by showing different types of stimuli with the same average luminance to the same group of observers
Summary
Motion perception, defined as the perception of change of position of an object over time, is a crucial function for successfully interacting with a dynamic environment and is fundamental for survival [1, 2]. Illusory, and implied motion moving visual stimuli. This seems to be true for humans as well, as demonstrated by anatomical [8,9,10], PET [11,12,13], and fMRI studies [14,15,16,17]. Visual motion areas dedicated to real motion processing have been shown to be active in the presence of illusory motion as well [23,24,25]. Perception of illusory motion seems to depend on the activity of specialized visual motion areas and on the activity of other brain regions that are not involved in the processing of real motion [25]
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