Perception… • Perception involves organising and interpreting sensations into meaningful patterns. • When we speak, we produce, on average, a dozen distinct units of sounds (phonemes) per second. We are capable of understanding up to 40 phonemes per second (Pinker, 1994). This requires organisation of sensations into units! • Once we organise these units, we then need to interpret the information organised.
Organising… • A scrawl on a piece of paper is not just a set of lines of particular orientation, but a series of letters and words. • We interpret the lines and perceive them as letters and words. Psych Club
Perceptual organisation • Perceptual organisation incorporates sensations into percepts(meaningful perceptual units, like images of objects), locates them in space and preserves their meaning as you look at them from different points of view. • If you put your textbook (percept) on the floor, it does not suddenly look like part of the floor (locate), and if you look at it from different points of view, you will still know that it is not part of the floor (points of view).
Form perception • Form perception refers to the organisation of sensations into meaningful shapes and patterns. • If you put your textbook on your lap, you don’t automatically perceive that it’s part of your leg, even though it might be resting on it! • If you cover a bit of information with your pen, you don’t perceive that a piece of information has disappeared, just because your pen is blocking the information.
Gestalt principles • The first psychologists to study form perception were the Gestalt psychologists in the early 1900s. • Gestalt is a German word that translates ‘loosely’ to ‘whole’ or ‘form’.
Ambiguous… • Consider the lady on the left. • What do you perceive? • Depending on the perceiver’s gestalt (or whole view of the picture), will depend if they see a young or old woman.
Principles… • Based on experiments from 1920s-1930s, the Gestalt psychologists offered a small number of perceptual rules that the brain automatically and unconsciously follows as it organises sensory input into meaningful information. • Figure-ground perception, similarity, proximity, good continuation, simplicity, and closure.
Figure-ground perception • Figure-ground perception: This is where we distinguish between figure and ground, such as words in black ink against a white page. • The figure is the object we are viewing (penguin) • The ground is the background or foreground the figure is in (penguin 1 or penguin 2).
Figure-ground perception See how this uses figure-ground reversal to create an ambiguous figure, where two interpretations are possible. M.C. Escher’s ‘Winged Lion’ 2004.
Figure-ground perception • Camouflage happens when the figure and the ground are not easily separated and they blend together. • Military uniforms are designed to use the colours of the surrounding environment so that the figure is difficult to separate from the ground. Pintos, 1979, by Bev Doolittle.
Similarity • The brain tends to group similar elements together, like people arching, actually form a tree…
Artists use a technique called pointillism where they create painting using very small coloured dots, the same size. We don’t perceive dots though, we group dots of similar colours and interpret whole shapes to make sense of it all.
Proximity • This is basically ‘nearness’. • The brain tends to group together objects that are close to one another. • The first 6 lines have no particular organisation, but the same 6 lines in the second part are perceived as 3 pairs.
Proximity • We perceive parts of a stimulus that are positioned close together, as belonging together in a group
Proximity • Series of letters located in close proximity to each other will be perceptually grouped and interpreted as words. What’s that on the road ahead? What’s that on the road a head? • When there is a subtle difference in the proximity of just one of the parts, it produces an entirely different interpretation of the sentence.
Good continuation • If possible, the brain organises stimuli into continuous lines or patterns, rather than disconnected elements. • The figure appears as an X on a circle, rather than pieces of a pie with lines extending beyond the perimeter.
Simplicity • People tend to perceive the simplest pattern possible. • Most people would see the figure as a heart with an arrow through it, because that is the simplest interpretation.
Closure • People tend to perceive incomplete figures as complete. If part of a shape is missing, perceptual processes complete the pattern, like the lines below, we actually just perceive it as a square.
Closure • Company logos and signs often require the use of closure to mentally complete an incomplete figure.
Closure • The helicopter pilot’s failure to use closure to interpret the stranded survivor’s message, could have cost him his life!! The Far Side, by Gary Larson. 1982 FarWorks, Inc.
Closure • Another part of closure is called illusory contour. People see three circles and a triangle, yet none of those shapes exist. • The brain simply fills in the gaps to perceive familiar patterns. • The brain treats illusory contours as if they were real, because illusory contours activate the same areas of early visual processing in the primary visual cortex as real contours.
Gestalt Principles • Figure-ground • Similarity • Proximity • Good continuation • Simplicity • Closure
Biederman • Irving Biedermanput forward a theory on how the brain forms and recognises images. • Consider this: you’re home, late at night, and you’re channel surfing. You are rapidly pressing the television remote control in search of something to watch and from less than one second’s glance, you can perceive what the show is about and whether you’ll want to watch it. • How does the brain recognise a visual array on a screen in less than a second, and then make two decisions?
Biederman • Biederman showed that we don’t even need a half a second to recognise most scenes! Typically a tenth of a second will do (100 milliseconds). • He called his theory the recognition-by-components theory, where we perceive and categorise objects in our environment by breaking them down into component parts and then matching the components and the way they are arranged against similar ‘sketches’ in our memory.
Recognition-by-components • According to this theory, the brain combines the simple features from the primary cortex (like lines and orientations) into a small number of geometrical forms (called geons – geometric icons). • From this geometrical alphabet of 20-30 geons, the outlines of virtually any object can be constructed, just like millions of words can be constructed from the 26-letter alphabet.
These are simple geons that can be used to create thousands of different objects, simply by altering the relationships among them, like their relative size and placement.
Recognition-by-components • Biederman argues that combining what your primary visual cortex gets, into geons, not only allows rapid identification of objects, but also explains why we can recognise objects, even when parts of them are blocked or missing. • The brain fills in gaps in a segment of a geon, like a blocked piece of a circle. However, if the lines where separate geons connect are missing or unclear, the brain can no longer tell where one component ends and another begins.
People can rapidly identify objects, even if many parts of them are missing, as long as the relationships among their geons remain clear. When they can no longer tell where one geon ends and another begins, you can’t identify them.
Perceptual illusions • Sometimes, the brain’s efforts to organise sensations into accurate percepts, fails. • This is what happens with perceptual illusions, where normal perceptual processes produce misinterpretations. • Impossible figures are one type – They provide conflicting cues for 3D organisation. Each portion the brain works out, but as soon as it organises sensations in one way, another part of the figure changes and then you’re back to square 1!
Perceptual illusions • Escher painted the first picture, the stairs, an impossible figure. He used perceptual research. • You’ll find that your brain tries to use Gestalt’s processes, like simplicity, but the data doesn’t allow it! The brain tries to bring order to the picture, but it is impossible! • At times the brain’s efforts to organise percepts can sometimes produce these illusions.
Perceptual Organisation • There are four forms of perceptual organisation. • Form perception (which is the Gestalt principles) • Depth perception (which we will discuss now) • Motion perception (which is perceiving the movement in objects – is it a meal, mate, friend or danger?) • Perceptual constancy (which we will discuss later)
Depth perception • The second aspect of perceptual organisation is depth or distance perception. This is where you perceive things in three dimensions. • You perceive your textbook as having height, width and breadth and being at a particular distance. We make 3D judgments, based on a two-dimensional retinal image. • We do all this so fast that we have no awareness of the computations our nervous system is making!
Depth perception • Other sensory systems (like sound and touch) give us valuable information for depth perception, but again we focus on the visual system. • There are 2 kinds of visual information that provide important information about depth and distance: • Binocularcues (visual input from the two eyes) • Monocular cues (visual input from one eye)
Depth perception - Binocular • Because our eyes are in different locations, all objects produce a different image on each retina, (apart from the most distant objects). • Retinal disparity (which means difference)is the greatest for close objects, but diminishes with distance. • Put your finger about 15cm from your nose and close your left eye, and then your right. You will see that each eye sees your finger in a slightly different position. If you do this with a distant object, there is only minimal differences.
Depth perception - Binocular • Most cells in the primary visual cortex are binocular cells; they receive information from both eyes. • When your brain sees a finger 15cm away, it uses both eyes to work out how far away the finger is; the depth/distance of the finger. • When your brain sees a house 5km away, and the retinal disparity is minimal, it knows that it is some distance away.
Depth perception - Monocular • Although binocular cues are really important for depth perception, you don’t automatically crash whenever you have to shut one eye, because an eyelash fell in there. You can rely on monocular cues, information received from one eye. • Look at the following photo of the Taj Mahal as it illustrates some important depth cues…
Depth perception - Monocular • Linear perspective: Parallel lines appear to ‘meet’ in the distance.
Depth perception – MonocularGiotto’s Flight into Egypt Artists working in 2D media rely on monocular depth cues to represent a 3D world. They use interposition and elevation to convey depth.
Depth perception – MonocularGallipoli landing – Australian War Memorial neg. ART02161 Since the late fifteenth century, artists have used monocular cues such as linear perspective to give the illusion of depth.
Depth perception This art, The annunciation by Carlo Crivelli is an example of Renaissance art that uses linear perspective, as it appears to have the depth of a three dimensional painting, even though it is still just a painting.