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Chapter 3

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PSYC 221
Thomas Spalek

Chapter 3 Visual Perception -light waves eye focused and inverted by lens retina (rods and cones bipolar cells ganglion cells) optic nerve exits eye -retina composed of 3 basic layers: 1. rods and cones 2. bipolar cells 3. ganglion cells -retina: layer of the eye covered with rods and cones that initiate the process of visual sensation and perception -compression occurs:  the message that reaches the visual cortex is already a summarized and processed record of the original stimulus only a fraction of the original light is registered on the retina (the rest is absorbed/scattered by fluid and structures within the eye) -~120 million rods -~7 million cones -most cones lie in fovea (or foveal pit) -fovea: • highly sensitive region • responsible for precise, focused vision • composed largely of cones -many cones synapsed to one bipolar cell= loss of info=>peripheral visual - ~ 1 million ganglion cells combine to form optic nerve - visual sensation: initial encoding of stimulation from environment - visual perception: process of interpreting and understanding sensory info. Gathering visual information • saccades: voluntary sweeping of the eyes from one fixation point to another • fixation: in visual perception, the pause of the eye almost stationary o taking in visual info • change blindness: failure to notice changes in visual stimuli (ex. Photo) when those changes occur during a saccade • inattentional blindness: failure to see an object we are looking at directly because our attention is directed elsewhere Visual sensory memory -visual persistence: visual stimulus seems to be present event after its termination ex. Lightning consists of several 0.2s individual flashes, but to us, it seems like one flash of about 1sec 1. Something that looks like physical stimulus continues to be present briefly after 2. info. can be acquired from the stimulus for brief period after -visual sensory memory (aka iconic memory): the short-duration memory system that holds visual info. for brief periods of time (less than 500ms) -George Sperling: used tachistoscope to research visual sensory memory -tachistoschope (aka T-scope): apparatus used to research visual sensory memory for a short period of time tachisto “very swift or rapid”  scope instrument for seeing/observing -span of apprehension (aka span of attention): the number of items recallable after any short display -whole report condition: entire visual display reported -partial report condition: only one of the rows were reported -icon: the brief-duration visual image or record of a visual stimulus held in visual sensory memory -dark pre- and postexposure fields: -increased accuracy of info -longer persistence of info (compared to light pre- and postexp.) loss of info from iconic memory -decay: loss of info across timefading process (esp. sensory memory) nd st -interference: when 2 stimulus is presstted to subject right after 1 recent info cause forgetting of 1 info Experiment with Averbach and Coriell bar and circle marker -backwards masking: later visual stimulus can affect the perception of an earlier one ex. Circle marker made it seem like there was no letter in that position -masking: an effect, which a mask is presented very shortly after a stimulus and disrupts/prevents perception of earlier stimulus -erasure: masking or loss of info. caused by presentation of another stimulus after -usually in sensory memory -a specific kind of interference Argument about iconic memory -Ralph Haber: argued that static icon is irrelevant to understanding normal visual perception -ecological validity: debated principle that research must resemble the situations and task demands characteristic of the real world, rather than rely on artificial lab settings and tasks, so that results will generalize to the real world will have ecological validity -Haber argued: • T-scope is not similar to any ordinary visual experience • Although iconic memory and visual persistence are real, they are irrelevant to the task of perceiving continuous visual info -Geoffrey Loftus’ counterargument: cog. Psyc’s task is not to throw out concept of iconic memory but instead to determine how it contributes to normal visual perception -2ndcriticism of Haber: why build a theory of visual perception based on brief, discrete flashes of stimulation, when the normal visual environment is continuously present? **the first 50 milliseconds of fixation -subjects fixated on a word for 50ms, then it was replaced by irrelevant stimulus which remained in view for 175ms. -Subjects did not notice change … -focal attention: • Ulric Neisser • Mental process of visual attention • Responsible for transferring that info into short-term memory -templates: stored models of all categorical patterns ex. Diff. fonts of “G”  we know it is still a G visual feature detection -feature analysis/ feature detection: stimuli (patterns) are identified by breaking them up into their constituent features -ex. Visual feature single straight, horizontal line in capital letters A, G, H, L -pandemonium: -one ex. Of feature detection model -Selfridge’s early model of letter identification  demons: the mechanisms that process a visual stimulus DECISION DEMON -decide which letter is present -picks the loudest cognitive demon COGNITIVE DEMONS -try to match whole letter patterns Aa Bb COMPUTATIONAL DEMONS -try to match simple features DATA OR IMAGE DEMONS -encode the pattern -2 important ideas: 1. At basefeature detection model a. Evidence for feature detection in visual pattern recognition b. There are visual cortex cells for various visual features c. Ex. Neurons in cats’ brains that respond only to vertical lines, other neurons to diagonal lines 2. Simultaneous/ parallel processing computational demons all work at same time  NOT one-after-the-other process -problem with pandemonium model: it is all bottom-up processing (aka data-driven processing system) when mental processing of a stimulus is guided by features and elements in the pattern missing context -top-down processing (aka conceptually driven processing): guided by context/ experience/ knowledge  CN YOU RD THS -context: influence of surrounding info and your own knowledge Conceptually driven pattern recognition cont’d -repetition blindness: the tendency to not see a pattern (word, or picture etc) when it is quickly repeated -RSVP method (Rapid Serial Visual Presentation): when the sentence is presented at a rapid rate ex. When she spilled the ink there was ink all over we fail to see the 2 ndoccurrence of ‘ink’ b/c the cognitive system has just identified the stimulus so it ‘expects’ not to see the same thing again -misreading effect: tendency to read a word that SHOULD be in a sentence ex. I broke a wine class in my class yesterday Connectionist modeling • Connectionist models (aka neural net
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