Kai Hamburger

Geometric-optical illusions at isoluminance

The idea of a largely segregated processing of color and form was initially supported by observations that geometric-optical illusions vanish under isoluminance. However, this finding is inconsistent with some psychophysical studies and also with physiological evidence showing that color and luminance are processed together by largely overlapping sets of neurons in the LGN, in V1, and in extrastriate areas. Here we examined the strength of nine geometric-optical illusions under isoluminance (Delboeuf, Ebbinghaus, Hering, Judd, Müller-Lyer, Poggendorff, Ponzo, Vertical, Zöllner). Subjects interactively manipulated computer-generated line drawings to counteract the illusory effect. In all cases, illusions presented under isoluminance (both for colors drawn from the cardinal L-M or S-(L+M) directions of DKL color space) were as effective as the luminance versions (both for high and low contrast). The magnitudes of the illusion effects were highly correlated across subjects for the different conditions. In two additional experiments we determined that the strong illusions observed under isoluminance were not due to individual deviations from the photometric point of isoluminance or due to chromatic aberrations. Our findings show that our conscious percept is affected similarly for both isoluminance and luminance conditions, suggesting that the joint processing for chromatic and luminance defined contours may extend well beyond early visual areas.

Perceptual filling-in from the edge of the blind spot.

Looking at the world with one eye, we do not notice a scotoma in the receptor-free area of the visual field where the optic nerve leaves the eye. Rather we perceive the brightness, color, and texture of the adjacent area as if they were actually there. The mechanisms underlying this kind of perceptual filling-in remain controversial. To better understand these processes, we determined the minimum region around the blind spot that needs to be stimulated for filling-in by carefully mapping the blind spot and presenting individually fitted stimulus frames of different width around it. Uniform filling-in was observed with frame widths as narrow as 0.05 degrees visual angle for color and 0.2 degrees for texture. Filling-in was incomplete, when the frame was no longer contiguous with the blind spot border due to an eye movement. These results are consistent with the idea that perceptual filling-in of the blind spot depends on local processes generated at the physiological edge of the cortical representation.

Reversal of apparent rotation in the Enigma-figure with and without motion adaptation and the effect of T-junctions.

We studied the time course of apparent rotation and directional reversal in Leviants Enigma figure. On average, periods of clockwise rotation lasted 5.0 s as opposed to 4.4 for counter-clockwise rotation, resulting in an average reversal frequency of 6.4 within 30 s. At the beginning of a trial, clockwise rotation was perceived almost twice as often as counter-clockwise rotation. This bias could be shifted by previous adaptation to a black-and-white rotating sector disk, suggesting a neural interaction between real motion and illusory motion. We further studied Enigma-type motion on a chromatic bar superimposed onto a black-and-white linear grating. Illusory motion was strongest when the bar was oriented at 90 deg to the grating lines and became progressively weaker with a decrease in angle. This suggests that T-junctions formed by the radial rays impinging onto the colored rings of the Enigma figure are instrumental for eliciting the rotary motion and may rule out a low-level sensory origin of the illusion.

The structural salience of landmarks: results from an on-line study and a virtual environment experiment

The term ‘structural salience’ refers to the characteristics of landmarks that are immediately related to navigation. One of the central aspects of this kind of salience is where a landmark is located at an intersection. Klippel and Winter (2005) developed a mathematical measure that describes the ideal position of a landmark at an intersection. In the first experiment (on-line study) we examined the four different landmark positions from a birds-eye perspective and in the second experiment (virtual environment) from an egocentric perspective. We compare our results with Klippel and Winters model and provide some evidence to support their assumptions empirically.

Filling-in with colour: Different modes of surface completion

We investigated the figural dynamics of filling-in processes in figures with more than one possible figure-ground organisation. Using a central disk and two concentric rings as well as similar stimuli consisting of three nested squares or parallel stripes, we tested for filling-in with different equiluminant colour combinations. We observed four modes of filling-in: First, in most of the cases, the inner ring assumed the colour of the central disk and outer ring (M1). Second, the central disk became filled-in with the colour of the inner ring, without any colour change on the outer ring (M2). Third, in a first step, the colour of the inner ring spread onto the central disk; then, in a second step, the colour of the outer ring spread over the whole stimulus (M3). This two step filling-in process has not been reported so far. Fourth, a mode (M4) was sometimes observed that was characterised by the central disk and outer ring assuming the colour of the inner ring. Thus, colour filling-in or colour spreading proceeded both in a centripetal (periphery to fovea) as well as a centrifugal direction. The colours red and yellow proved to be stronger inducers than blue and green. Conversely, the latter colours became filled-in more easily than the former. The filled-in colour was always that of the inducing stimulus, i.e., there was no colour mixture. This suggests a long-range, neural process underlying filling-in under these conditions.

Age effects on the perception of motion illusions.

Anomalous motion illusions represent a popular class of illusions and several studies have made an effort to explain their perception. However, understanding is still inconsistent. Age-related differences in susceptibility to illusory motion may contribute to further clarification of the underlying processing mechanisms. We investigated the effect of age on the perception of four different anomalous motion illusions. The Enigma illusion, the Rotating-Snakes illusion, the Pinna illusion, and the Rotating-Tilted-Lines illusion were tested on a total of one hundred and thirty-nine participants covering an age range from 3 to 82 years. In comparison with young adults, children showed a lower likelihood of perceiving motion in all illusions with the exception of the Rotating-Tilted-Lines illusion. For adult subjects, we found significant age effects in the Rotating-Snakes illusion and the Rotating-Tilted-Lines illusion: occurrence of the illusory effect decreased with age. The other two illusions turned out to be unaffected by aging. Finally, inter-correlations between different motion illusions revealed that only the Pinna illusion and the Rotating-Tilted-Lines illusion correlated significantly with each other. The results confirm that anomalous motion illusions should not be considered as a homogeneous group. Possible links between perceptual data and neurophysiological changes related to age are discussed. Perceptual differences due to age provide the opportunity to improve our understanding of illusory motion and point to specific underlying mechanisms.

Analysis of individual variations in the classical horizontal-vertical illusion

In the horizontal-vertical illusion (HVI), the length of the vertical line is overestimated, whereas in the bisection illusion (BI), the horizontal bisecting line is expected to be overestimated. Here, only half of our 22 observers showed the expected BI, whereas the other half underestimated the bisecting line. Observers also differed in their judgments of the strength of the HVI: The HVI was stronger for observers showing the classical bisection effect, and weaker or absent for those underestimating the bisecting line. To account for these results, we used a linear model to individually estimate the strength of two putative factors underlying both illusions. Whereas the strength of the HVI and BI were highly correlated, the estimated factors were uncorrelated. Therefore, in two control experiments, we then measured the pure horizontal-vertical (pHVI) and bisection (pBI) illusions. A significant correlation between the estimated factors and the measured illusion variants was found. Results were robust against variations of contrast, repetitive presentations, and choice of adjusted line. Thus, the classical HVI as an additive combination of two independent factors was confirmed, but we found considerable interindividual variations in the strength of the illusions. The results stress the importance of analyzing individual data rather than taking sample means for understanding these illusions.

The Giessen virtual environment laboratory: human wayfinding and landmark salience

In our virtual environment laboratory, we focus on different topics in human spatial cognition with projects on landmark salience, route knowledge, and survey knowledge. Within this laboratory note, we provide an overview of previous, current, and future work with our virtual environment Squareland.

Apparent rotation and jazzing in Leviant's Enigma illusion.

In 1981 Leviant devised Enigma, a figure that elicits perceived rotary motion in the absence of real motion. However, despite its striking appearance there is no good explanation for this motion illusion to date. Gregory (1993 Proceedings of the Royal Society of London B 253 123) pointed out a similarity to MacKays ‘complementary’ afterimage in his ray pattern and suggested accommodative fluctuations and small eye movements as a potential origin for these phenomena. Furthermore, Zeki et al (1993 Proceedings of the Royal Society of London B 252 215–222) found PET-activation in response to Enigma in visual area V5 and immediately surrounding areas (called V5 complex) suggesting that the illusory motion could be mediated by the same neurons as real motion. In the experiments reported here, I show that the rotary motion is perceived on coloured as well as achromatic annuli intercepting the radial lines. More importantly, the illusory streaming motion continues to be seen with a cycloplegic lens as well as through a pinhole (ie ruling out transient changes of accommodation), and in the positive afterimage (ie in the absence of eye movements). Apparent rotation is strongest with radial inducers impinging at right angles onto the annuli, but persist, although to a lesser degree, when the inducing lines are tilted in opposite directions, non-collinear, or replaced by dotted lines or lines with rounded terminators. For an explanation, the Enigma illusion requires a neural mechanism that uses lines abutting an empty annulus to elicit orthogonal streaming motion in one or the other direction.

Grouping by Contrast: Figure – Ground Segregation is Not Necessarily Fundamental

A central tenet of Gestalt psychology is that the visual scene can be separated into figure and ground. The two illusions we present demonstrate that Gestalt processes can group spatial contrast information that cuts across the figure/ground separation. This finding suggests that visual processes that organise the visual scene do not necessarily require structural segmentation as their primary input.