Chart Characteristics
Instead of always having, as other tests, the entire region correspond between the left and the right eye (100% correspondence) and change the disparity (stereoacuity) this chart will have fixed disparity values and the percentage of correspondence will change. The chart will be used just like a standard acuity chart. The chart has six rows of four squares each. Each row has a fixed correspondence value. For example, the first row has 100% correspondence, the second row 85% etc. The task is to correctly determine whether the squares are in front (crossed disparity) or behind (uncrossed disparity) the background. To obtain a correct score for a given level of correspondence (row) the subject must identify the direction of the depth correctly for all four squares on the same row. The chance of getting a single row correct by chance alone is about 6% and two consecutive rows is about 0.4%. Of course this could be changed by increasing the number of squares present in a row but the goal is to make this a reasonably quick test. Because we are testing only one disparity level at a time for a given chart, a number of charts are possible. However, it is safe to say that there are in the literature two broad stereoscopic mechanisms identified which are associated with fine (small disparities) and coarse (large disparities) stereopsis. For this reason I have developed charts for only two levels of stereopsis, fine and coarse. Further, I have generated five charts (each with a different response key) for each of the two disparity conditions mentioned because I wanted Jami to be able to change charts at random so that the observers do not use the same response patterns over and over again. These charts were developed using anaglyph-type stereograms i.e. the left eye information is filtered through one colour filter and the right eye information is filtered with another coloured filter. In this particular case the left eye will look through a green filter and the right eye will look through a red filter. This is the least expensive way of viewing stereograms and this way Jami can just obtain red green glasses for her tests.
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Jocelyn Faubert, Ph.D.
e-mail: jocelyn.faubert@umontreal.ca