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Psyc 303 Chapter 8 - Form. Comprehensive Exam Review

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PSYC 303
Thomas Spalek

Ch8 Contour Perception - Contours o Edges o Sudden changes in light intensity across space - Shapes o Regions of relatively uniform brightness in the visual system o Separated from other shapes or background by contours - First Order Contour o Region in the retinal image where light intensity changes abruptly - Emergent/Second-order contours/subjective contours o Appear to be contours but they are purely perceptual and made by the first-order contours that are present o looks like triangle - The visual system wired more for first order contours (V1) - Ganzfeld o A visual field used to illustrate that without contours we lose ability to see o A field containing no abrupt luminance changes (no contours). When looking into it it looks like a shapeless fog- colour fades to grey and won’t be seen even if picture is illuminated with coloured light. Some experience a blank out where they cannot see  Blank out can also happen naturally. Eg. Snow blindness. Contour interactions - Neural circuitry of brain exaggerates any edges - Hermann grid o Grey smudges. A receptive field that is center-excitatory and aligned with an intersection will have a larger net amount of inhibition in it’s surround than will a same- size receptive field centred btw two squares. Therefore, relatively, the intersections will be registered as containing less light than vertical and horizontal regions between squares. - Crowding o nearby contours reduce visual acuity for any one of the group of contours.  Explanation: both inhibitory and excitatory neural interactions that normally enhance the visibility of contours make it more difficult to perceive in certain conditions  Most likely to occur when spacing is such that neural response to one contour interferes with that of others nearby.  Actually seeing the interfering contour is not necessary, since a stimulus that is sub-threshold for perception may still be strong enough to excite a neural response o Amblyopia exanopsia  Diminished visual acuity- usually in one eye, but without visual pathology. Makes crowding effects greater Masking and Metacontrast - Spatial summation (neighboring stimuli) and lateral inhibition (distant stimuli) o General design principles of brain - Visual masking o Reduction in visibility of a contour or target caused by presentation of a second stimulus close to the target in space and/or time. o Crowding = simultaneous masking  Target stimuli and those that flank it are presented at the same time o How to mask: 1. Pattern masking: Present target briefly, immediately followed/replaced with masking stimulus that consists of some array of contours overlapping same position 2. Metacontrast: show target stimulus, then follow with masking stimulus that contains closely adjacent, but non-overlapping contours o What is seen when target and mask are separated in time depends on interstimulus interval (stimulus onset asynchrony)  Time btw presentation of target and presentation of mask  Since involves affecting visibility of a target presented earlier and is gone now, it is called backward masking  Strongest 50-100 ms btw target and mask o Masking is result of neural units competing by sending inhibitory signals to neighboring units proportional to their own activation level. o Researchers believe that presentation of mask interrupts processing of target stimulus  Two neural channels. One fast acting but short=lived, other slow and long lasting  Faster channel signals stimulus onset  Slower contains info about shape and colour  Appearance of mask overtakes slow processing of target Contour Detection - Contour map o Representation of original image with only the edges Feature Extraction - Feature o Size shape texture colour etc - Relevant features o Features that help differentiate object from others - Tasks o Visual Search Task  Observer looks for presence of single target item and
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