Daniel J. Weintraub

The Poggendorff illusion: Amputations, rotations, and other perturbations·

Studies of the Poggendorff illusion (a transversal interrupted by parallel lines) showed that illusory effects increased linearly with increasing separation between the parallels, increased in inverse proportion to the tangent of the angle of intersection between transversal and parallels, decreased whenever line segments (other than a transversal segment) were omitted, decreasing to zero when the segment of a parallel forming the obtuse angle with the transversal was omitted, and varied systematically with the tilt of the whole display, approaching zero when the transversal was oriented in a horizontal or vertical position. Hypothesis: The Poggendorff illusion involves at least three kinds of effects on the perceived orientation of a segment: distortion by other segments (especially a segment intersecting at an obtuse angle), stability of vertical and Horizontal orientations, and assimilation towards vertical or horizontal.

The parallel-less Poggendorff: Virtual contours put the illusion down but not out

Four versions of Poggendorff displays were compared: (1) the conventional display of a transversal interrupted by parallel lines, (2) a transversals-only display, (3) a virtual-contour display where the parallels were suggested (but not drawn) via “good continuation” and “c1osure, ” and (4) the profile display, a degraded version of the virtual-contour display that retained many of its borders but did not suggest parallels. Each display was drawn with transversal tilted at 45 and 51 deg from vertical. All displays produced significant illusions at both tilts. By comparison with the illusion magnitudes of the conventional display, the effects for the other displays were small. The outcome supports the existence of two salient illusion-producing factors: the necessity for actual contours to form the subtended angle and assimilation of the transversal toward O’s horizontal.

Head-up display (HUD) utility. II - Runway to HUD transitions monitoring eye focus and decision times

An experiment conducted using a head-up display (HUD) suggests that the demonstrated superiority of the HUD over a conventional instrument panel stems from its superior layout of information. A HUD display presents instrument-panel information to pilots in such a way that the symbols appear as a virtual image at optical infinity superimposed on the landscape. In the experiment conducted, the luminance of the display symbology and its angle subtended at the eye remained fixed, while optical distance and gaze angle were varied. Concomitant measures of eye movements, eye accommodative state, and decision-making time concerning airspeed, altitude and runway condition were obtained. It is found that, while looking straight ahead, at zero diopters, the HUD shortens decision time by 80 to 90 msec, not statistically significant at the 0.05 (slope of diopter/gaze interaction) level. The question of a cognitive overload induced by the luminous symbols of the HUD is subsequently addressed.

Kinesthetic Aftereffects Day by Day: Trends, Task Features, Reliable Individual Differences

The kinesthetic aftereffect was measured across 15 days. Pre- and postinduction scores decreased across sessions for group I>T (inducing block wider than test block) and increased for groups I T and the positive one for group I T), another group rubbed an inducing block narrower than the test block (group I<T), and the members of the control group kept both hands motionless during the induction period (group no-I). Each subject was tested seven times throughout an interval of

Ebbinghaus illusion: context, contour, and age influence the judged size of a circle amidst circles.

Subjects (groups of age 6, 8, 10, 12, and 21 years, with 60 or more persons per group) made comparative size judgments between a single circle and a standard circle that was flanked by four context circles. The context circles varied in diameter, proximity to the standard circle, and amount of circumference displayed. (When circumferences are complete, the display is the usual Ebbinghaus configuration. A variant consisted of the Ebbinghaus display with the outer three quarters of each context-circle circumference removed.) A contour-plus-context theory accounts for the data: Contours attract at every age level, and context (size of flanking circles relative to the standard circle) leads to constrast in judged size beyond age 6. Contour and context effects decreased with decreasing proximity between context circles and the standard circle.

Fragments of Delboeuf and Ebbinghaus illusions: Contour/context explorations of misjudged circle size

Variants of the Delboeuf (concentric circle) display and the Ebbinghaus display (four large nonconcentric inducing circles) were produced by systematically eliminating fractions of the circumferences of the outer inducing circles. For the Ebbinghaus display, various amounts of the inner arcs and/or outer arcs were preserved; a series of dots was sometimes substituted for missing contours. Results are described by the following statements: Contours attract. Larger nonconcentric circles induce size contrast. Rotating an Ebbinghaus variant by 45° alters size misjudgments. The larger the horizontal (or vertical) extent of the inducing configuration, the smaller the judged size of the test (center) circle. A figurai aftereffect of the inducing configuration acts to reduce the perceptual size of the comparison circle when eye movements are permitted between test and comparison circles.

Contour displacements and tracking errors: Probing ’twixt Poggendorff parallels

Explanations of the Poggendorff effect were tested by varying the separation between outer parallels and by adding interior parallels. Error decreased with the addition of interior parallels, which can be explained by repulsion of parallels. A strong linear trend existed for judgmental error in millimeters plotted against separation between outer parallels. The nonzero intercept of a best-fit line and the slight nonlinearity of the data suggest a hypothesis of contour repulsion between parallels at moderate separations coupled with mistracking of the transversal across the region between parallels. Since the Poggendorff effect was independent of viewing distance, perceptual errors cannot be explained by purely peripheral mechanisms. A true intersection between transversal and parallel was the most critical feature of a display. Inverting a display increased the mean error.

Assessing Poggendorff effects via collinearity, perpendicularity, parallelism, and Oppel (distance) experiments*

By adjusting the orientation of, and separation between, two free-standing dots, Ss indicated directions and distances associated with the Poggendorff display (a transversal interrupted by parallel lines). Judged distance between parallels (with transversal absent) increased slightly when additional interior parallels were added; this Oppel effect can be interpreted as contour repulsion. Errors in judging the orientation of an actual transversal segment were too small to account for the Poggendorff effect. The usual large errors occurred for estimates of the orientation of the missing transversal segment between the parallel lines. Cognitive mistracking adequately describes the Poggendorff effect. Mistracking is a function of the angle subtended between transversal and parallels, and of the orientation of the entire display.

The utility of Head-Up Displays - Eye-focus vs decision times

Instrument-panel information is presented by a Head-Up Display (HUD) to pilots in such a way that the symbols appear far away (at optical infinity) superimposed upon the landscape. The use of a HUD makes it, therefore, unnecessary for the pilot to take his eyes off the changing scene in order to look down and refocus on the instrument panel. It is pointed out that the HUD has proved superior to the conventional instrument-panel display. An experiment was conducted with the objective to help to determine which of the differences between HUD and conventional instrument-panel display are mainly responsible for the superiority of the HUD. Important features of this experiment are that the format of each virtual-image display, its luminance, and its angle subtended at the eye, remained the same as the optical distance and location were varied. It was found that the HUD symbology at optical infinity does reduce decision times compared to the same format at the location of a conventional instrument panel.

The misperception of angles: Estimating the vertex of converging line segments

Estimates of the point of intersection of converging line segments depended upon the angle between lines and the orientation of the display. Main conclusion: The tilt of a line is perceptually altered to appear more nearly parallel to the more closely aligned axis, either horizontal or vertical, of an O’s visual field.