Studies of the angular function of a Duncker-type induced motion illusion.

Duncker (1929/1955, Source Book of Gestalt Psychology, pp 161-172) demonstrated a laboratory version of induced motion. He showed that, when a stationary spot of light in a dark laboratory is enclosed in an oscillating rectangular frame, the frame is perceived as stationary and the dot appears to move in the direction opposite the true motion of the frame. Zivotofsky (2004, Investigative Ophthalmology & Visual Science 45 2867-2872) studied a more complex variant of the Duncker illusion, in which both the inducing and the test stimuli moved: a single red test dot moved horizontally left or right while a dense background set of black dots on a white background moved vertically up or down. When the background inducing dots moved up (down), the truly horizontally translating test dot appeared to drift at an angle down (up) from the horizontal. In experiment 1, we used two methods to measure the complete angular function of the Zivotofsky effect and found it to peak with an inducer-test direction separation of approximately 30 degrees, similar to the inducing angle that has been found to maximise other direction illusions. Experiment 2 tested and confirmed predictions regarding the effects of relative test and inducer speeds based on the vectorial subtraction of the inducing velocity from the test velocity.

The hierarchical order of processes underlying the direction illusion and the direction aftereffect.

Motion perception involves the processing of velocity signals through several hierarchical stages of the visual cortex. To better understand this process, a number of studies have sought to localise the neural substrates of two misperceptions of motion direction, the direction illusion (DI) and the direction aftereffect (DAE). These studies have produced contradictory evidence as to the hierarchical order of the processing stages from which the respective phenomena arise. We have used a simple stimulus configuration to further investigate the sequential order of processes giving rise to the DI and DAE. To this end, we measured the two phenomena invoked in combination, and also manually parsed this combined effect into its two constituents by measuring the two phenomena individually in both possible sequential orders. Comparing the predictions made from each order to the outcome from the combined effect allowed us to test the tenability of two models: the DAE-first model and the DI-first model. Our results indicate that DAE-invoking activity does not occur earlier in the motion processing hierarchy than DI-invoking activity. Although the DI-first model is not inconsistent with our data, the possible involvement of non-sequential processing may be better able to reconcile these results with those of previous studies.

Challenging the distribution shift: statically-induced direction illusion implicates differential processing of object-relative and non-object-relative motion.

The direction illusion is the phenomenal exaggeration of the angle between the drift directions, typically, of two superimposed sets of random dots. The direction illusion is commonly attributed to mutual inhibition between direction-selective cell populations (distribution-shift model). A second explanation attributes the direction illusion to the differential processing of relative and non-relative motion components (differential processing model). Our first experiment demonstrates that, as predicted by the differential processing model, a static line can invoke a misperception of direction in a single set of dots--a phenomenon we refer to as the statically-induced direction illusion. In a second experiment, we find that the orientation of a static line can also influence the size of the conventional direction illusion. A third experiment eliminates the possibility that these results can be explained by the presence of motion streaks. While the results of these experiments are in agreement with the predictions made by the differential processing model, they pose serious problems for the distribution-shift account of shifts in perceived direction.

Tapered dipoles in briefly flashed glass-pattern sequences disambiguate perceived motion direction.

In order to investigate the relationship between 'neural speedlines', form (shape), and fast motion-direction decisions, Glass patterns were constructed with dipoles assuming a tapered shape. The results of a 2-alternative forced-choice direction-discrimination task, for both concentric and translational Glass-pattern sequences, suggest that with short stimulus presentations (< 1 s) form can influence direction decisions. This result implies that neural speedlines may be analogous to tapered lines and further supports Geisler's (1999, Nature 400 65-69) model of form/motion interaction.

Dichoptic reduction of the direction illusion is not due to binocular rivalry.

Simultaneous direction repulsion (the direction illusion) occurs in bidirectional motion displays, typically transparent motion random dot kinematograms. Several laboratories have reported a greatly reduced illusion with dichoptic presentation of the two coherently translating stimuli as compared to monocular or binocular presentation. Some researchers have argued that those results might be due to a confounding factor, namely binocular rivalry occurring between test and inducing stimuli in the dichoptic condition, and so have attributed decisive weight to the results reported by Kim and Wilson (1997, Vision Research, 37, 991-1005) who used centre-surround grating stimuli and found large monocular as well as large dichoptic effects. Here we use centre-surround dot stimuli - with which no binocular rivalry occurs - to confirm a strong monocular contribution to the direction illusion. In addition, we fail to find evidence of a direction illusion with centre-surround grating stimuli, even when seeking to replicate the methods of Kim and Wilson (1997). In light of other evidence that a global motion-sensitive mechanism can determine the magnitude of the direction illusion, we propose that simultaneous direction repulsion can result from activity at multiple stages of the motion processing hierarchy.

Motion streaks in fast motion rivalry cause orientation-selective suppression.

We studied binocular rivalry between orthogonally translating arrays of random Gaussian blobs and measured the strength of rivalry suppression for static oriented probes. Suppression depth was quantified by expressing monocular probe thresholds during dominance relative to thresholds during suppression. Rivalry between two fast motions or two slow motions was compared in order to test the suggestion that fast-moving objects leave oriented "motion streaks" due to temporal integration (W. S. Geisler, 1999). If fast motions do produce motion streaks, then fast motion rivalry might also entail rivalry between the orthogonal streak orientations. We tested this using a static oriented probe that was aligned either parallel to the motion trajectory (hence collinear with the "streaks") or was orthogonal to the trajectory, predicting that rivalry suppression would be greater for parallel probes, and only for rivalry between fast motions. Results confirmed that suppression depth did depend on probe orientation for fast motion but not for slow motion. Further experiments showed that threshold elevations for the oriented probe during suppression exhibited clear orientation tuning. However, orientation-tuned elevations were also present during dominance, suggesting within-channel masking as the basis of the extra-deep suppression. In sum, the presence of orientation-dependent suppression in fast motion rivalry is consistent with the "motion streaks" hypothesis.

What is the reference in reference repulsion?

Reference repulsion is a mechanism posited to explain systematic biases of direction judgment of single drifting dot displays (Rauber and Treue, 1998 Perception 27 393-402). Rauber and Treue obtained systematic but, surprisingly, very different effects depending upon whether standard and comparison stimuli were presented simultaneously or successively. Successive effects were described as exhibiting repulsion from both vertical and horizontal cardinal axes, whereas simultaneous effects showed repulsion from horizontal only. We contend that the proposed mechanism makes no testable predictions because the so-called reference can only be specified a posteriori, a fact acknowledged by Rauber and Treue. We attempted to replicate Rauber and Treue's experiments, but we obtained no systematic biases of direction judgment. Comparisons across several studies suggest that errors in direction judgments of single drifting dot patterns vary widely in magnitude and direction, as might be expected with what are essentially baseline or pretest measures. In our view, reference repulsion describes neither a real perceptual mechanism nor a predictable pattern of direction misjudgments.

Orientation illusions vary in size and direction as a function of task-dependent attention.

Adding an upright inner square frame to an outer tilted square frame causes a central rod's perceived orientation to be directionally opposite the usual rod-and-frame illusion (RFI). Zoccolotti, Antonucci, Daini, Martelli, and Spinelli (1997) attributed this double RFI (DRFI) to Rock's (1990) hierarchical organization principle. In Experiment 1, this explanation predicted results for small (11 degrees ) but not larger (22 degrees and 33 degrees ) outer frame orientations. In two experiments with the DRFI, bottom-up, goal-driven attention was varied and direct and indirect measures of the framework's influence were compared. In Experiment 2, the RFI angular function was compared with two other DRFI conditions: a direct measure of perceived rod orientation and an indirect measure of the inner frame. These conditions induced directionally opposite effects. In Experiment 3, direct and indirect measures of the inner frame's perceived tilt were compared. Angular functions differing in size and direction were obtained. Experiment 4 replicated the previous results, using a different psychophysical procedure. All the results were consistent with the hierarchical organization mechanism but suggested different processing strategies due to different attentional weights. They were also consistent with other recent findings based on the Bayesian approach to accounts of illusory phenomena (e.g., Jazayeri & Movshon, 2006, 2007; Weiss, Simoncelli, & Adelson, 2002).

Retinotopic encoding of the direction aftereffect.

Kohn and Movshon [Kohn, A., & Movshon, J. (2003). Neuronal adaptation to visual motion in area MT of the macaque. Neuron, 39, 681-691; Kohn, A., & Movshon, J. A. (2004). Adaptation changes the direction tuning of macaque MT neurons. Nature Neuroscience, 7(7), 764-772] measured the contrast response functions of single neurons in MT (V5) before and after adaptation to high contrast gratings. They found that when gratings were smaller than the MT receptive field, so that adapting and test regions could be either co-localised or non-overlapping, adaptation was spatially specific. This led to the hypothesis that grating adaptation occurs in V1, where receptive fields are small and retinotopically organized, and that MT merely inherits this adaptation. We predicted that spatial specificity would be less for dot stimuli that probably adapt MT cells directly. Also, given recent contradictory claims that hMT primarily exhibits both spatiotopy [d'Avossa, G., Tosetti, M., Crespi, S., Biagi, L., Burr, D., & Morrone, M. (2006). Spatiotopic selectivity of BOLD responses to visual motion in human area MT. Nature Neuroscience, 10, 249-255] and retinotopy [Gardner, J. L., Merriam, E. P., Movshon, J. A., & Heeger, D. J. (2008). Maps of visual space in human occipital cortex are retinotopic, not spatiotopic. The Journal of Neuroscience, 28, 3988-3999], we were interested in producing relevant psychophysical evidence using the direction aftereffect. In three experiments, we measured direction aftereffects (DAEs) induced and tested either with drifting gratings or drifting dots when stimulus location was changed both retinotopically and spatiotopically between adaptation and test; when retinotopic location only was changed; and when spatiotopic location only was changed. We predicted and found that spatial specificity was greater for gratings than for dots. We also found very small spatiotopic effects that call into question some recent claims that area MT exhibits a high degree of spatiotopicity.

The different mechanisms of the motion direction illusion and aftereffect.

Direction repulsion is the illusory expansion of the angle between two directions of motion, and may occur when the two directions are presented simultaneously (an illusion) or successively (an aftereffect). Here we demonstrate that the motion direction illusion (DI) and aftereffect (DAE) have different mechanisms. Two experiments show that when the two interacting stimuli are presented to different eyes, the DI is greatly reduced but the DAE is obtained at near to full strength. These results suggest that different populations of cells within the visual pathway produce the DI and DAE.

Testing the tilt-constancy theory of visual illusions.

Prinzmetal and Beck (2001) Journal of Experimental Psychology: Human Perception and Performance 27 206 - 217) argued that a subset of visual illusions is caused by the same mechanisms that are responsible for the perception of vertical and horizontal a theory they referred to as the tilt-constancy theory of visual illusions. They argued that these illusions should increase if the observer's head or head and body are tilted because extra reliance would then be placed on the illusion-inducing local visual context. Exactly that result had previously been reported in the case of the tilted-room and the rod-and-frame illusions. Prinzmetal and Beck reported similar increases in the tilt illusion (TI), as well as the Zöllner, Poggendorff, and Ponzo illusions. In two experiments, we re-examined the effect of head tilt on the TI. In experiment 1, we used more conventional TI stimuli, more standard experimental methods, and a more complete experimental design than Prinzmetal and Beck, and additionally extended the investigation to attraction as well as repulsion effects. Experiment 2 more closely replicated the Prinzmetal and Beck stimuli. Although we found that head tilt did increase TIs in both experiments, the increases were of the order of 1 degrees -2 degrees, more modest than the 7 degrees reported by Prinzmetal and Beck. Significantly, the TI increase was larger when inducing tilts and head tilts were in the same direction than when they were in opposite directions, suggesting that the tilt-constancy theory may be oversimplified. In addition, because previous evidence renders unlikely the claim that the Poggendorff illusion can be explained simply in terms of misperceived orientation of the transversals, the question arises whether there might be some other explanation for the increase in the Zöllner, Poggendorff, and Ponzo illusions with body tilt that Prinzmetal and Beck reported.

Contrast configuration influences grouping in apparent motion.

We investigated whether the same principles that influence grouping in static displays also influence grouping in apparent motion. Using the Ternus display, we found that the proportion of group motion reports was influenced by changes in contrast configuration. Subjects made judgments of completion of these same configurations in a static display. Generally, contrast configurations that induced a high proportion of group motion responses were judged as more 'complete' in static displays. Using a stereo display, we then tested whether stereo information and T-junction information were critical for this increase in group motion. Perceived grouping was consistently higher for same contrast polarity configurations than for opposite contrast polarity configurations, regardless of the presence of stereo information or explicit T-junctions. Thus, while grouping in static and moving displays showed a similar dependence on contrast configuration, motion grouping showed little dependence on stereo or T-junction information.

Visual orientation illusions: global mechanisms involved in hierarchical effects and frames of reference.

Five experiments were conducted in order to determine which of two hypotheses, initially proposed by Rock (1990), accounts for interactions between oriented elements in a visual scene. We also explored the suggestion that two hypothetical processes--namely, frame of reference and hierarchical organization--describe phenomena arising from distinct mechanisms (Spinelli, Antonucci, Daini, Martelli, & Zoccolotti, 1999). Double inducing stimulus versions of one-dimensional and two-dimensional tilt illusions, the rod-and-frame illusion, and combinations of these were used. Our data suggest that both hypotheses can predict orientation interactions in conditions in which only one mechanism--namely, the global visual mechanism of symmetry axes extraction (Wenderoth & Beh, 1977)--is activated. Which hypothesis is appropriate to predict the perceived orientation depends on some physical features of the objects.