Biomedical subjects
Publications and source records attributed to T E Parks.
As reported before, sighting-down one of the diagonal lines on a typical Poggendorff pattern will reduce the illusion even if that situation is only pictorial. But here it is also argued that sighting-down itself requires further understanding.
Illusory-figure patterns which are composed of pictorial elements representing three-dimensional partial disks are sometimes effective and sometimes not. When they are not effective, it is apparently because they contain information that contradicts the presence of an occluding figure and/or because all of the edges within the pattern are seen as edges of various pictured surfaces of the elements.
When elements in a pattern which would otherwise produce an illusory figure are modified by outlining or by increasing their regularity (in accordance with a proposed measure of the latter quality), the illusory effect is much reduced. Both results reflect the principle that 'coincidences' in a display play little or no role in the process of segmentation when other circumstances tend to force critical edges to 'belong' to individual elements of the display.
Physical darkness gradients were introduced into an illusory-figure pattern which were sufficient to eliminate the illusory lightness effect otherwise seen there. Next, such a gradient was left in place along one edge of that figure while the other edges were returned to physical homogeneity with the surround. Rather surprisingly, the physical gradient still present along the unaltered edge was now overwhelmed by illusory lightness. It is argued that this anomaly suggests that illusory-figure lightness may be composed of two additive components, one of which is figure-ground contrast enhancement and the other of which is contingent upon that first component.
An informal demonstration is offered, which strongly supports previous contentions that, when the elements of a Poggendorff display appear to be arranged in pictorial space such that the two critical line segments are at different heights, an illusory impression of misalignment may occur. A second pair of demonstrations shows, however, that such a height difference is neither a necessary nor a sufficient cause of the illusion. In addition, the harmful effect of adding certain pictorial elements to the standard Poggendorff pattern requires a new understanding.
Double-letter memory and test stimuli were used in two experiments on a speek comparison task. Faster decision times were found when memory and test stimuli were physically identical than when they were the same in name only. This finding was true even with retention intervals as long as 12 sec and even when difficult tasks filled the retention intervals. However, the decision-time advantage of physically identical comparisons was greatest when the interval was not filled with a task likely to interfere with rehearsals. High verbal subjects had a smaller advantage for the physically identical comparisons than did low verbal subjects but were affected in the same way as low verbal subjects in terms of which conditions raised the overall correct comparison times, raised the error rates, and reduced the advantage of physically identical comparisons.
Memory trigrams were presented by one of three methods: visual-concurrent (all three letters appeared simultaneously), visual-successive, and auditory-successive. During the 12-sec retention interval, subjects shadowed and reported their rehearsals and mnemonic associations via switches. On trials without associations, recall performance was interpreted as support for the hypothesis that the form of rehearsal is related to presentation modality. However, the frequency and temporal patterning of the rehearsals over the retention interval were virtually identical for all three presentation conditions, suggesting that the "control processes" were relatively independent of both method of presentation and modality of rehearsal. Most importantly, these data in combination with earlier data suggest that the efficiency of each rehearsal was also independent of those same factors, in each case quite comparable to that of a concurrent visual stimulus.
The ability to decide rapidly that two visual stimuli are nominally the same when they are also visually the same (the Posner effect) was examined for stimuli of increasing visual complexity (Experiment 1) and when a greater variety of visual differences between the two stimuli was employed (Experiment 2). When the two stimuli each consisted of a pair of letters or when to single-letter stimuli sometimes differed in both case and style, the Posner effect occurred even though subjects made overt verbal rehearsals of the first stimulus. The results suggest that losses of the Posner effect found under simpler circumstances are not attributable to a switch from visual to exclusively verbal coding.