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PERCEPTION – VISION Approaches to the study of vision (Lec. 1 Fiona or p.8 in Goldstein). 1 : Experimental approach E.g. Psychophysics- this term was coined by Gustav Fechner (1860) and refers to quantitative methods for precise measurement of the relationship between stimuli and perception. This is used more widely now to include methods of measurement, which focus on this relationship. Includes; Absolute / difference thresholds ……stimuli must be at a certain level for the sensory system to detect it , the lowest stimulus level (physical intensity)which can still be detected is the “absolute threshold”. The smallest amount this physical intensity must be changed for a difference to be detected is known as “the just noticeable difference”. Absolute threshold = detection 50% of the time. In relation to vision the range of stimuli is restricted i.e. the visible spectrum & one example of the absolute sensory threshold is “candle flame at 30 miles on a dark clear night”. Weber’s Law …… ∆S = K S “The greater the magnitude of the stimulus the larger the difference you need to detect it”. In relation to vision Weber’s fraction is .08. “The smaller the fraction the more sensitive one is to the difference between stimuli in that modality, the larger the fraction the less sensitive”. Signal detection (theory)…. . different levels of a stimuli presented at intervals, participant must respond through a key press when stimuli is presented. Should ideally include trials with no stimuli present. Signal detection = measures participants ability to detect when stimuli is present against the absence of a stimuli. i.e. discriminate between Signal (stimuli) vs. Noise (absence of stimuli). Forced choice method …. . participant must respond same or different …..or match to sample is X like A or B …….or Alternative forced choice could have differing amounts of responses e.g. 2AFC = target/non target or 3AFC = target A/ or B/ or C Reaction time studies…. “ Relate response speed to processing” Preferential looking…. Infant vision studies often use preferential looking, infants have “spontaneous looking preferences” e.g. contours. 1 PSYCHO PERCEPTION PHYSICS STIMULUS PSYCHOPHYSICS
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2: Neuropsychological approach Focuses on the relationship between brain structures and psychological functions. Usually consists of patient studies e.g. Individuals who have suffered lesions/cortical damage due to carbon monoxide poisoning, stroke, head injuries etc. One example is visual form agnosia ; Dr.P a musician described by Oliver Sacks (1985, The Man Who Mistook His Wife for a Hat), began having difficulty recognising students and talking to furniture etc. Although his vision was fine he was unable to recognise objects (caused by a tumour). (Also page 115 Goldstein) Patient D.F. was a female who had suffered damage to the ventral stream
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