Size Constancy

954 words 4 pages
Visual Perception Lab: Size Constancy


This investigation is designed to illustrate the ability of the visual system to compensate for changes in retinal image size with viewing distance,i.e the mechanism of size constancy. The accuracy of this mechanism will be evaluated under a number of different viewing conditions and thereby investigating the various cues for judging distance.

‘Size constancy’ is the perception of an object as having a fixed size, regardless of the change in size of the retinal image & the visual angle which accompanies changes in distances (i.e. as an objects moves into the distance, the retinal image decreases yet the object size remains the same). It is thus possible to accurately
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When our head or eyes move, objects closer to us move more across our retina thus appearing to move faster, whereas things further away from us sweep across less of our retina therefore appearing to move slower (i.e. looking out of a train window).
The amount of accommodation required to focus on an object is related to the distance of the object.
By removing these cues in turn & subsequently measuring the accuracy of the ‘size constancy’, you are able to determine the relative importance of each cue for judgement of depth, which is the purpose of this experiment.

Method Firstly the size of the reference diamond was set to 7.5cm (constant value- fixed variable) . The Subject then placed their head on the chin rest (to avoid head movements). The room lights were turned off and all obstacles were in place (occluder, reduction tube, pinhole). The targets were then positioned so they appear side by side & are the same height but they are at a different distance, the reference diamoond is placed 2m away while the test is placed closer at 1m. The subject then slowly decreased the size of the test diamond, from maximum to minimum, until both diamonds appeared to be the same size, & this is repeated from minimum to maximum. This procedure was repeated once more to obtain 4 values from which a mean was taken.
The whole procedure was repeated by removing each obstacle in


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