Visual Perception Week 9 Notes (Updated)
Visual Perception Week 9 Notes (Updated) PSYC 3124
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This 13 page Class Notes was uploaded by Freddi Marsillo on Thursday March 10, 2016. The Class Notes belongs to PSYC 3124 at George Washington University taught by Dr. John Philbeck in Spring 2016. Since its upload, it has received 39 views. For similar materials see Visual Perception in Psychlogy at George Washington University.
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Date Created: 03/10/16
Visual Perception Week 9 Notes 3/10/16 2:49 PM Paper: Terrain influences the accurate judgment of distance by Sinai et al. Introduction Main research question: does the brain select the most resource and time- efficient way to represent the three-dimensional location of objects? • More specifically, does the brain use a quasi-two-dimensional coordinate system (X, Y) with respect to the ground surface (s), rather than a three-dimensional coordinate system (x, y, z),in order to simplify and speed up computations? Experiment 1 Theory • When the ground surface is disrupted, the visual system can’t establish a reliable reference frame and fails to obtain correct absolute distance • If the texture of the ground surface is changed or there is a gap in the ground surface, the individual won’t be able to determine absolute distance Methods • Manipulation: target on other side of gap in the ground (0.5m d x 1.3m w) from a naïve observer Control • Target 3.66m away over a continuous surface • Experiment: o Blindfolded, turned 90 degrees, asked to walk distance equivalent to remembered absolute distance of the target o Asked to judge the absolute distance of a target, and then to perceptually set the distance of a matching target to be at an equal distance Results • Blindfolded walking manipulation: average distance walked was 4:60 + 0:12m • Blindfolded walking control: average distance walked was 3:69 + 0:12m • Perceptual matching manipulation: average matched distance was 4:24 + 0:6m • Perceptual matching control: average matched distance was 3:54 + 0:07m Experiment 2 Theory • When the ground surface is disrupted, the visual system can’t establish a reliable reference frame and fails to obtain correct absolute distance Methods • Manipulation: target on other side of a larger and deeper gap in the ground (2m d x 4.1m w) from a naïve observer • Control: target 3.66m away over a continuous surface • Experiment: o Blindfolded, turned 90 degrees, asked to walk distance equivalent to remembered absolute distance of the target o Asked to judge the absolute distance of a target, and then to perceptually set the distance of a matching target to be at an equal distance. Experiment 3 Theory • Distance/texture model: Z = (H x G)/3 • Z is perceived absolute distance • H is the observer’s eye height relative to the ground surface • G is the local texture gradient on the ground at the target’s location Methods • Observers stood on an elevated ground surface (2.0 m) and estimated horizontal distance of a target placed on a lower ground surface • 8 participants were tested using the blindfolded test and 5 were tested using the perceptual distance matching test Experiment 3 (part 2) Researchers believed this overestimation may be due to an exaggeration of the observer’s perception of H Methods • 5 observers stand on the higher surface and set the distance of a matching target to equal their perceived eye height above lower ground surface • Then they are asked to set the distance of a matching target to equal their perceived eye height with respect to feet • The participants were finally asked to measure perception of distance between feet and lower ground surface Experiment 4 Theory • Distance/texture model: Z = (H x G)/3 • H is the observer’s eye height relative to the ground surface • G is the local texture gradient on the ground at the target’s location Methods • Two distinct texture regions: grass, concrete. Observer stood on concrete, previewed target on grass, then did the blindfolded walking task. Width of grass field kept constant at 3.05 m while width of concrete was varied from 0.61-4.56 m • in control experiments, people were tested on homogeneous (grass only or concrete only) texture surface; blindfold walk accurate, also consistent with ground theory • The experiment was repeated with people standing on grass and viewing target placed 5.79 m away on concrete Most Important Take-Aways • The internal representation of space, which is based on the common ground surface, is used for both space perception and visually directed performance • Texture gradient of size + eye height influences judgment of absolute distance. Texture gradient on the ground acts as a depth cue for visual system to establish a reference frame. Meaning in Visual Search – Mary C. Potter Introduction Purpose of the paper: • Provide a preliminary analysis of the mind’s ability to scan the environment through brief glimpses and attach meaning to the images observed • To determine if people can detect an EXPECTED scene even when presented briefly or at the high rate allowing the 1/3 fixation rate • —To determine the form of information people require in anticipation of the scene in order to codifier the resulting information accurately • Put plainly: What kind of information informs the mind faster between words and visual information • The pace at which the human eye captures images for the brain to interpret is brief but the associated meaning from observations made do not always match or become translated appropriately due to the limited time of observation • Glances are too brief to ensure memory for what is observed Visual Search and Meaning • Average eye fixation lasts 1/3 of a second. Novel scenes or images do presented at this high rate do not look instantly familiar • In fact, research shows that some remain unfamiliar even moments after observing the novel scene or image • Hence, each glance is too brief to assure memory for what is observed • However, Potter believes that the briefest glance can be sufficient to trigger memory if the observer is aware of what to expect • This provides an observer with the power to either confirm or refute observations from glances • In the study, observers are exposed to a succession of rapid glances using photographs of various scenes • In some instances, they had foreknowledge of the observations to be made and in others they did not have any foreknowledge save the meanings of the observations to be made like boat and then the image of a boat or yacht instead to prepare the mind to receive such information thereby associate it accordingly • •Under rapid image succession, even with meanings provided pre- observations, memory formation remained relatively different Overview of Experiments Hypotheses: • H1: When an observer knew precisely the features of an anticipated scene, he makes a direct visual match • H2: When an observer only has general information regarding the scene, he needs to recognize and categorize the multiple information received in order to make a positive match • The reason for the hypotheses was to ensure the two parameters followed were consistently observed for accurate reproduction of the experiment and results • Visual matching is regarded as the ultimate form of quick memory formation and meaning association Experiment • One practice and 8 test sequences of 16 color pictures were shown on an L-W cine projector to two groups of 24 college students • A succession of rapid glances around the environment was stimulated by presenting observers with a sequence of various scenes and objects • Observer was instructed to look for a particular picture- target picture • Group 1: Observer was shown the target picture before viewing each sequence • Group 2: Observer was only given a name for the target picture. The names were brief descriptions of the main objects/events in the target photo. Colors and shapes were never specified • When the observer saw the picture, he/or she responded by pressing a lever that stopped the projector • Practice sequence rate was 250 msec per picture • Two of the eight test sequences had rates of: 125,167,250, or 333 msec per picture • Target picture was either the 9th, 10th, or 11th picture in the sequence. Four orders of rates, two different orders of pictures, and three orders of target positions were factorially combined Results • Exposure time of only 125 msec to the target stimuli was representative of 70 percent of target detection • Due to the small time of exposure, one may assume that the participant scanned only for pictures • Prior research has shown that pictures are very effective memory cues • However, although identification of pictures takes only a meager 125 msecs, retention of pictorial memory took upwards of 300 msecs • The visual pattern (picture) of a target stimuli is not necessary for detection • Very little difference between participants in the name target condition and participants in the picture target condition • Awareness of the meaning of a target stimuli is as important knowing the exact visual appearance of an object • Attention and selection to a target stimuli is guided by meaning detection Conclusion • Perceiving the emergence of a target did not help in knowing its meaning, which suggests people process what they see rapidly to an abstract level of meaning before choosing to either accept or reject the interpreted meaning • Without extended time to think on them unselected scenes are forgotten • Therefore, the 1/3- second length of an average glance is a delicate time in which an observer balances the need to scan his environment for desirable objects and the need to retain the useful information from the unwanted information • The hypotheses were accurately proven thereby indicating the processing rate of the mind from observations through the eye to the provocation of an idea is rapid and deserving further study under neurology Fast Extraction of Distance Information - Gajewski et al. Background/Intro – Speed of Extraction • The speed of extraction is the amount of time it takes for the visual system to extract (comprehend) information from a scene • To extract visual information from the environment, the eyes are directed from one location to another at a rate of 3x per second • Although gathering visual information about multiple objects requires multiple gaze shifts, a person can determine the gist of a scene on the basis of information within the time frame of a single fixation • Absolute distance (also called egocentric distance), the distance between an observer and an object, is one element of a scene for which the speed of extraction has not yet been determined The Speed of Extraction – Absolute Distance • Little is known about the time it takes to extract distance information from real-world environments • Information-processing speed can be affected by old age and brain injury, but even in individuals without brain damage, highly dynamic tasks such as driving can create conditions that limit one’s ability to fully extract distance information from the environment Experiment 1 • Experimenters compared blind-walking performance based on a viewing duration that was too short for quick eye movement (113 ms) with blind-walking performance based on an extended viewing time (5 s) • Blind-walking was used to demonstrate the visual system’s ability to extract spatial information • Participants: There were two groups of 14 naive observers • Stimuli and apparatus: o Participants viewed objects through a liquid crystal shutter window, which can transition between a translucent state and a transparent state very quickly o Timing accuracy of the shutter window was controlled by the experimenters o The window provided a field of view of 80 degrees horizontal x 60 degrees vertical o A masking stimulus was used to ensure performance was based on the glimpse through the shutter window during the experiment, rather than memory of the environment Procedure • The masking stimulus was projected onto a screen to the observer’s left, and this image reflected onto a beam splitter (which is like a two-way mirror) • The experiment environment was a slightly darkened room in a lab space that was 7.4 m away from participant’s viewing position • An orange cone was the target object presented at 11 distances ranging from 3m to 5 m • The two viewing conditions, brief (113 ms) and extended (5 s) were administered through 11 trials each • When the shutter window opened, the participant viewed the target object, then the shutter closed after either 113 ms or 5 s (depending on the condition) • The participants then walked through the darkened room to the distance at which they thought the cone was placed Results • Performance generally good • Differences between viewing duration depended on block order o When the 113 ms block was first, there was a greater bias toward underestimating the distance with brief viewing than with extended viewing o The bias largely disappeared with extended viewing, thus reflecting differences in what can be extracted during the two viewing times • In contrast, performance during the two viewing conditions was indistinguishable when the 5 s block was first • These results suggest that the information needed to support performance can be extracted in a single brief glance when longer viewing duration glances are provided beforehand • A control experiment was conducted to determine whether task familiarity bias existed, but yielded the same results Experiment 2 Aim • To determine the shortest viewing duration that would support a response sensitive to the distribution of target distances employed • Experiment 2 examined performance at an ultra-brief viewing duration (9ms) and moderately brief viewing duration (69ms) Method • Two groups of 12 naive observers • First group performed 9-ms block of trials first and second group performed 69-ms block first • A Styrofoam ball was placed on the floor as a target at nine distances in the 3 to 5m range Results • Participants exhibited nearly perfect response sensitivity • The outcomes did not depend on viewing duration or block order • The results confirmed that target detection in 9-ms viewing condition was highly reliable Experiment 3 Aim • Removed sources of distance information from the viewing situation o Removed angular declination that has shown to be potent cue within the distance range Method • One group of 12 naive observers • Target was presented at eye level with viewing durations of 9ms, 65 ms, and 187 ms at nine distances Results • Accuracy was generally high but performance did improve across the range of viewing duration • Although there was a modest drop in response sensitivity and a bias toward overestimation in 9ms, observers clearly extracted useful information about distance in this duration even in this reduced-cue setting Experiment 4 Aim • Objective is to examine performance with eye level objects: each with angular size rendered non informative o Done by systematically varying physical size of the target with distance so target always subtended to the same visual angle Method • 2 groups of 12 naive observers • Same as experiment 2 and 3, but angular size remained constant • Both groups of participants= 2 blocks of 15 trials • Floor level blocks vs eye level blocks • Target = five styrofoam balls o 9.8cm, 12.2cm, 14.6cm, 17.8cm and 19.8cm • Second group does eye level first • Distances of balls selected to appear equal in size • Floor level target= 2.06-4.87M • Eye level target= 2.55-5.10M Results • Clear performance difference between floor level and eye level • Accuracy did not depend on block order = bias for response sensitivity • Greater bias toward underestimation of floor target than with eye target • Effect of duration for floor and eye viewing conditions o No effect on bias or response sensitivity for the floor level targets and eye level targets • Responses were completely unbiased • Different biases for eye-level and floor-level targets might be attributed to differences in gaze direction in these scenarios • Useful information about distance can be extracted from glimpses as brief as 9 ms when relative angular size or angular declination is informative= one of more extraordinary outcomes • It is virtually impossible to rule out post perceptual influences in any distance judgment • Overall, Experiment 4 was not detrimental to fast extraction of distance to floor-level targets Discussion What duration of viewing is required for observers to determine the distances to objects in real-world environments ▯ Accurate response depended upon: • Visual cues • Stored information about the environment • Floor target-more than a brief glimpse • Bias of underestimation was eliminated only when performance was preceded by a block of extended-viewing trials • Particular interest-participants’ sensitivity to change in physical size • Visual angle of the target was completely non predictive • Greater visual angle, the greater physical size judgment • Target distance explained a significant amount of the variance in size judgments Implications for theories of scene perception and space perception • Domain=primarily on the directional and conceptual locations of objects as opposed to their absolute distance • Shown that the time needed to extract distance information varies widely depending on the kind of information available • Summation: absolute distance perception=dynamic even when static and stable head position 3/10/16 2:49 PM 3/10/16 2:49 PM