2 How do we distinguish objects from their background? Overview of QuestionsWhy do some perceptual psychologists say “the whole differs from the sum of its parts”?How do “rules of thumb” help us in arriving at a perception of the environment?How do we distinguish objects from their background?Why are even the most sophisticated computers unable to match a person’s ability to perceive objects?The instructor can use the last question to frame the entire chapter, if he or she desires to do so. The question can be posed at the end of the lecture material to see how the students have processed the material.
3 The Challenge of Object Perception The stimulus on the receptors is ambiguousInverse projection problem: an image on the retina can be caused by an infinite number of objectsObjects can be hidden or blurredOcclusions are common in the environment
5 The Challenge of Object Perception - continued Objects look different from different viewpointsViewpoint invariance: the ability to recognize an object regardless of the viewpointThe reasons for changes in lightness and darkness in the environment can be unclear
7 The Structuralist Approach Approach established by Wundt (late 1800s)States that perceptions are created by combining elements called sensationsStructuralism could not explain apparent movementStimulated the founding of Gestalt psychology in the 1920s by Wertheimer, Koffka, and Kohler
9 Gestalt Psychology Wertheimer, Koffka, and Kohler (1912) Perceptions are often greater (or at least different) than the sum of the sensationsStructuralism: Purely Bottom-Up processingMany perceptions undermine Structuralism
10 Illusory contours are another phenomenon that the structural approach could not explain since the observer sees things that are not present, such as the sides of the Necker cube.
11 The Gestalt ApproachThe whole differs from the sum of its partsPerception is not built up from sensations but is a result of perceptual organizationPrinciples of perceptual organizationPragnanz - every stimulus is seen as simply as possibleSimilarity - similar things are grouped together
15 Principles of Perceptual Organization - continued Good continuation - connected points resulting in straight or smooth curves belong togetherLines are seen as following the smoothest pathProximity - things that are near to each other are grouped togetherCommon fate - things moving in same direction are grouped together
17 (a) Nearness and (b) Nearness competing with Similarity.
18 Gestalt Laws of Perceptual Organization Common Fate – things that move together appear to be grouped together
19 Principles of Perceptual Organization - continued Meaningfulness or familiarity - things form groups if they appear familiar or meaningfulCommon region - elements in the same region tend to be grouped togetherUniform connectedness - connected region of visual properties are perceived as single unitSynchrony - elements occurring at the same time are seen as belonging together
20 Figure 5. 20 The Forest Has Eyes by Bev Doolittle (1985) Figure 5.20 The Forest Has Eyes by Bev Doolittle (1985). Can you find the 13 faces in this picture?
21 Figure 5.21 Grouping by (a) common region; (b) proximity; (c) connectedness; and (d) synchrony. The yellow lights blink on and off together.
22 The Gestalt Approach - continued Researchers have found neurons that respond maximally to displays that reflect:Good continuationSimilarityGestalt principles do not make strong enough predictions to qualify as “laws”They are better understood as heuristics - “best guess rules”The instructor can compare heuristics with algorithms and note that heuristics are not always correct, but they are quick and provide right answers enough of the time that they are effective in most cases.
23 Perceptual Segregation Figure-ground segregation - determining what part of environment is the figure so that it “stands out” from the backgroundProperties of figure and groundThe figure is more “thinglike” and more memorable than groundThe figure is seen in front of the groundThe ground is more uniform and extends behind figureThe contour separating figure from ground belongs to the figure
24 Figure 5.24 A version of Rubin’s reversible face-vase figure. This slide and the next one explain reversible figure-ground objects. These can be used as an example for how the viewer actually determines the figure rather than this being something that is dictated by the environment. The instructor can make sure the students understand the term reversible figure-ground by using these two slides.Figure 5.24 A version of Rubin’s reversible face-vase figure.
25 Figure 5.25 (a) When the vase is perceived as figure, it is seen in front of a homogeneous dark background. (b) When the faces are seen as figure, they are seen in front of a homogeneous light background.
26 Figure-Ground Segregation - continued Factors that determine which area is figure:Elements located in the lower part of displaysUnits that are symmetricalElements that are smallUnits that are oriented verticallyElements that have meaning
27 This slide provides the stimuli and results from an experiment on factors that affect which part of a display is seen as the figure. There was a preference for the lower part of the display over the upper, but there was not preference for left versus right.Figure 5.27 (a) Stimuli from Vecera et al. (2002). (b) Percentage of trials on which lower or left areas were seen as figure.
28 Figure 5.28 Examples of how displays that are (a) symmetrical; (b) small in size; c) oriented vertically or horizontally; or meaningful and more likely to be seen as figure.
29 Figure-Ground Segregation - Neural Evidence Recordings from V1 in the monkey cortex show:Response to area that is figureNo response to area that is groundThis result is important because:V1 neurons are early in the nervous systemIt reveals both a “feedforward” and “feedbackward” in the systemIt demonstrates contextual modulationThis slide contains a number of very important concepts the students should understand that are related to the last three points on the slide.Since V1 neurons are early in the system, they would not be assumed to be relevant for the upper level function of perception. However, since the system is a loop that communicates both forward (from the receptors to the further levels for the brain) and backward (from the cortex to the receptors), even a neuron like V1 with a small receptive field can play a key role in the perception of something as complex as figure-ground segregation. Contextual modulation (neurons outside of a given neuron’s receptive field having an impact on it firing rate) also shows the interconnectedness of the system and that the feedback runs in a number of directions.
30 Responses from V1 Cells (Adapted from Lamme et al., 1995.)
31 Questions Used in Modern Object Perception Research Why does the visual system respond best to specific types of stimuli?Must a figure be separated from ground before we can recognize objects?How do we recognize objects from different viewpoints?How does the brain process information about objects?This slide is the first of series that includes the major questions asked by modern researchers in object perception. The instructor may want to keep referring to this as he or she works through this section.
32 Why Does the Visual System Respond Best to Specific Types of Stimuli? Regularities in the environmentThere is a preponderance of verticals and horizontalsOblique effect - people are more sensitive to these orientationsOccurs due to biology and experienceGestalt heuristics are reflected in environmental objectsThe Gestalt heuristics that may work the best to discuss are good continuation or uniform contentedness. This would be a good place for students to review the Gestalt heuristics and see how they could find examples of them in their everyday environment.
33 Figure Left: Photographs like the ones taken by the participants in Coppola et al.’s (1998) experiment as they walked around the Duke University campus. The results of a computer analysis of the orientation in each type of scene (indoor campus, outdoor campus, and in the forest).This slide can be used to explain that is it not simply due to our carpentered environment that we have a preference for horizontal and vertical orientations.
34 Must a Figure Be Separated from Ground Before We Can Recognize Objects? Research has shown that objects may be recognized before or during the separation of figure from groundStimuli with a standing woman and a less meaningful shape were usedThe meaningful stimulus (the woman) was recognized more often than the otherWhen the picture of the woman was turned upside down, this effect disappeared
35 How Do We Recognize Objects From Different Viewpoints? Structural-description models3-D objects are based on 3-D volumes called volumetric features that are combined for a given shapeMarr’s model proposed a sequence of events using simple geometrical featuresThe sequence begins with identifying edges and proceeds to recognition of the object
36 Structural-Description Models (Think 3D) Recognition-by-components theory by Irving BiedermanVolumetric features are called geonsTheory proposes there are 36 geons that combine to make all 3-D objectsGeons include cylinders, rectangular solids, and pyramids
37 Structural-Description Models - continued Properties of geonsView-invariant properties - aspects of the object that remain visible from different viewpointsAccidental property - a property that appears rarely and from certain viewpointsDiscriminability - the ability to distinguish geons from one anotherPrinciple of componential recovery - the ability to recognize an object if we can identify its geonsThe instructor can ask the students if they can come up with objects that could NOT be identified from geons alone. The book mentions horses and zebras, but the students should be able to come up with examples of their own. This illustrates one of the criticisms of the theory.
38 Can you Recover the Components? The instructor can ask the students to think about exactly what is different between the two figures. They should notice that (a) has no vertices so the viewer cannot identify the constituent parts. It should also be noted that both images have the same amount of material missing - it is what is missing that matters, not how much.Can you Recover the Components?
39 Image-Description Models Ability to identify 3-D objects comes from stored 2-D viewpoints from different perspectivesFor a familiar object, view invariance occursFor a novel object, view invariance does not occurThis shows that an observer needs to have the different viewpoints encoded (stored) before recognition can occur from all viewpoints
40 How Does the Brain Process Information About Objects? Perceiving an object - sunburst or butterfly?Experiment by Sheinberg & LogothetisMonkey was trained to pull a lever for a sunburst or a butterflyBinocular rivalry was used - each picture shown to one eyeNeuron in the IT cortex was monitoredFiring was vigorous for only the butterflyIT (interotemporal) cortexImportant to note that the firing of the single neuron was directly related to the perception of the butterfly, not to the two physical stimuli that were always present. This suggests that there would be another neuron that would fire preferentially to the sunburst stimulus perception. It is also important to note the way the monkey pulled the levers indicated that the perception of the objects switched from one to the other.
42 Identifying an Object: Is That Harrison Ford? Grill-Spector experimentRegion-of-interest approach: the FFA for each person was determined first by:Showing participants faces and non-facesFinding the area that responded preferentially to facesInstructor can note that the FFA (fusiform face areas) are not in the exact same place for each person. Since this region responds best when perceiving a face, it makes sense that it also may respond best when someone is identifying a face.
43 Grill-Spector Experiment FFA in each participant was monitoredOn each trial, participants were shown either:A picture of Harrison Ford’s faceA picture of another person’s faceA random textureAll stimuli were shown for 50 ms followed by a random-pattern maskParticipants were to indicate what they saw60 pictures of each type were presentedThe instructor can explain the purpose of the masking stimulus at the end of the trial.
45 Grill-Spector Experiment - continued For trials that only included Harrison Ford’s face, results showed that FFA activation:Was greatest when picture was correctly identified as FordWas less when picture was identified as other objectShowed little response when there was no identification of a faceNeural processing is associated with both the presentation of the stimulus and with the response (recognition) to the stimulus
46 Activity in FFA is not passively processing info, It is influenced by attention and the task at hand.
47 Perceptual Intelligence Theory of unconscious inferenceCreated by Helmholtz (1866/1911) to explain why stimuli can be interpreted in more than one wayMain Principle - perceptions are result of unconscious assumptions about the environmentLikelihood principle - objects are perceived based on what is most likely to have caused the patternYour visual system is “making decisions” about what you see before you know what you are looking at!Note that this means that what we learn about our environment becomes part of the problem solving we do when we encounter an object. This idea is related to the Gestalt heuristic because we are giving our best guess based on experience.
48 Figure 5.44 The display in (a) looks like (b) -- a blue rectangle in front of a red rectangle -- but it could be (c), a blue rectangle and an appropriately positioned 6-sided red figure.
49 Modern Ideas on Perceptual Intelligence Palmer experimentObservers saw a context scene flashed briefly followed by a target pictureResults showed that:Targets congruent with the context were identified 80% of the timeTargets that were incongruent were only identified 40% of the timeWhen we are not sure, we “decide” to see things that fit the scene and “decide” not to see things that are not
50 Stimuli used in Steven Palmer’s (1975) experiment Stimuli used in Steven Palmer’s (1975) experiment. The scene at the left is presented first, then one of the objects on the right is presented. The observer is then asked to identify if one of the objects on the right was presented.
51 Modern Ideas about Perceptual Intelligence - continued Light-from-above-heuristicObjects are generally perceived with the assumption that illumination comes from aboveThis is consistent with our experiences from the environment
52 Figure 5.47 Why does (a) look like indentations in the sand and (b) look like mounds of sand? See text for explanation.
53 Figure 5.46 (a) Some of these discs are perceived as jutting out, and some are perceived as indentations. (b) Light coming from above will illuminate the top of a shape that is jutting out, and (c) the bottom of an indentation.