PERCEPTION LECTURE - Alan Gilchrist
Theoretical approaches to lightness and perception
Theories of lightness, like theories of perception in general, can be categorized as high-level, low-level and mid-level. However I will argue that in practice there are only two categories: one-stage mid-level theories, and two-stage low-high theories. Low-level theories usually include a high-level component and high-level theories include a low-level component, the distinction being mainly one of emphasis. Two stage theories are the modern incarnation of the persistent sensation/perception dichotomy according to which an early experience of raw sensations, faithful to the proximal stimulus, is following by a process of cognitive interpretation, typically based on past experience. In my view, the doctrine of raw sensations is like that of phlogiston or the ether; everyone believes it must be true but there is no evidence for it. When visual angle matches can be made at all, they are not read off an early sensory stage, but are post-perceptual, achieved by a cognitive process of flattening the visual world. Likewise, brightness (luminance) matches depend on a cognitive process of flattening the illumination. Brightness is not the input to lightness. As for cognitive influences on perception, the many claims tend to fall apart upon close inspection of the evidence. Much of the evidence for the current revival of the new look is probably better explained by (1) a natural desire of subjects to please the experimenter, and (2) the ease of intuiting an experimental hypothesis. High level theories of lightness are overkill. The visual system doesn't need to know the amount of illumination, merely which surfaces share the same illumination. This leaves mid-level theories derived from the gestalt school. Here the debate seems to revolve around layer models and framework models, the strengths and weaknesses of which will be reviewed.
THE RANK PRIZE LECTURE - Karl Gegenfurtner
Vision and eye movements
The existence of a central fovea, the small retinal region with high analytical performance, is arguably the most prominent design feature of the primate visual system. This centralization comes along with the corresponding capability to move the eyes to reposition the fovea continuously. Past research on perception was mainly concerned with foveal vision while the eyes were stationary. Research on the role of eye movements in visual perception emphasized their negative aspects, for example the active suppression of vision before and during the execution of saccades. But is the only benefit of our precise eye movement system to provide high acuity of small regions at the cost of retinal blur during their execution? In my talk I will compare human visual perception with and without eye movements to emphasize different aspects and functions of eye movements. I will argue that our visual system has evolved to optimize the interaction between perception and the active sampling of information.
For orientation and interaction in our environment we tend to make repeated fixations within a single object or, when the object moves, we track it for extended periods of time. When our eyes are fixating a stationary target, we can perceive and later memorize even complex natural images at presentation durations of only 100 ms. This is about a third of a typical fixation duration. Our motion system is able to obtain an excellent estimate of the speed and direction of moving objects within a similar time frame. What is then the added benefit of moving our eyes?
Recently we have shown that lightness judgments are significantly determined by where on an object we fixate (Toscan et al., 2013a, b). When we look at regions that are darker due to illumination effects, the whole uniformly colored object appears darker, and vice versa for brighter regions. Under free viewing conditions, fixations are not chosen randomly. Observers prefer those points that are maximally informative about the object's lightness.
For pursuit eye movements, we have shown that our sensitivity to visual stimuli is dynamically adjusted when pursuit is initiated. As a consequence of these adjustments, colored stimuli are actually seen better during pursuit than during fixation (Schütz et al, 2008) and small changes in the speed and direction of the object are more easily detected (Braun et al, 2010), enabling a better tracking of moving objects. Pursuit itself increases our ability to predict the future path of motion (Spering et al., 2011), lending empirical support to the widespread belief that in sports it's a good idea to keep your eyes on the ball.
These results demonstrate that the movements of our eyes and visual information uptake are intricately intertwined. The two processes interact to enable an optimal vision of the world, one that we cannot fully grasp while fixating a small spot on a display.
Braun, D.I., Schütz, A.C. & Gegenfurtner, K.R. (2010) Localization of speed differences of context stimuli during fixation and smooth pursuit eye movements. Vision Research, 50, 2740-2749.
Schütz, A.C., Braun, D.I., Kerzel, D. & Gegenfurtner, K.R. (2008) Improved visual sensitivity during smooth pursuit eye movements. Nature Neuroscience, 11, 1211-1216.
Spering, M., Schütz, A.C., Braun, D.I. & Gegenfurtner, K.R. (2011) Keep your eyes on the ball: Smooth pursuit eye movements enhance prediction of visual motion. Journal of Neurophysiology, 105, 1756-1767.
Toscani, M., Valsecchi, M. & Gegenfurtner, K.R. (2013a) Optimal sampling of visual information for lightness judgments. Proceedings of the National Academy of Sciences USA, 110(27), 11163-11168.
Toscani, M., Valsecchi, M. & Gegenfurtner, K.R. (2013b) Selection of visual information for lightness judgments by eye movements. Philosophical Transactions of the Royal Society B: Biological Sciences, 368, 20130056.