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McGill University
PSYC 213
Jelena Ristic

When people pay attention to some specific piece of information, they fail to pay attention to other elements of the environment, greatly limiting our perception. From the phenomenon of change blindness, we can realize that we don't see the world in detail, apart for our foveal vision.Attention limits vision, which shows quite clearly that vision is an active and constructive process. Perceptual mechanisms have shortcuts; they often estimate quickly what the environment should be like, constructing our perception. Our visual systems make many assumptions about how the world is. For instance, our brain assumes that the world is lit from above (the sun) and that the world is stable (things don't change size as they move away or toward us, for example). We have dominant representations of the world: for instance, stairs go upwards. In an illusion in the notes, it is possible to reverse our perception of a staircase, making the side that is usually seen as away from us seem close to us and the staircase be upside down, but this is “difficult” because our brains assume that that is not the way things are/should be. Size constancy (the fact that objects/people are of the same size no matter their distance from us) allows us to judge distance, and not get confused on people's sizes through their relative size on the retina as they move towards or away from us. There are two main approaches to vision theories: Bottom-up and top-down. Bottom-up: we build our perception from basic features that are available in our environment. We go from simple to complex. Ex: Ecological optics and pattern recognition accounts. Top-down: vision is perceived on the basis of our experience; we have preconceived notions of how the world should be. Ecological optics (J.J. Gibson): the visual system is passive and perception is a direct function of the environment. There is an “optical array” (the light that arrives to our eyes at any given moment) which is directly perceived as it is by our eyes; the environment “affords” us (grants us) our perceptions. Pattern-recognition theory: information is in the environment, but we (humans) actively construct perceptions; we recognize certain configurations of simple features in our environment, because they are stored in memory, which in turn interacts with our perception at any give moment. Vision is not a standalone passive system. We have the capacity to recognize an event (object, person, etc.) as an instance (example) of a particular category (of events). One interesting idea is that we have templates (pattern that is stored in memory that is representative of a certain category of objects); our percepts are compared to these templates; matches between what we perceive and the templates we have in memory activates traces in our memory, which explains recognition. Hinzman proposed the multiple trace memory model. He proposed the idea that we have two memories: primary and secondary. Primary memory has our immediate perceptions, whereas our secondary memory has our stored perceptions. He argues that every time we see something, a “trace” is left in our memory and is compared with all of our previous percepts. Every new percept acts as a “probe” into our memory, which “echoes” back recognition. However, this seems unlikely for many reasons: first of all, the whole recognition process would logically be much slower than it is with such a model and it is hard to imagine that our brain stores a representation of every single instant we have every lived through. Feature detection theory claims that our perceptions are built from individual features. Various tests show that, at least to a limited extent, this is true. For instance, if we put a lot of elements into a group and are asked to find a particular one, it is much easier to do this if the one we are asked to recognize has distinctive features. This shows that low-level features are processed first. Pandemonium is another account for perception that proposes the idea of “demons” that detect features in our environment and “shout” to other demons when they recognize some simple feature. Features gradually grow in
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