The detection of motion in chromatic stimuli: first-order and second-order spatial structure

A common framework for the analysis of complex motion? Standstill and capture illusions

A series of illusions was created by presenting stimuli, which consisted of two overlapping surfaces each defined by textures of independent visual features (i.e., modulation of luminance, color, depth, etc.). When presented concurrently with a stationary 2-D luminance texture, observers often fail to perceive the motion of an overlapping stereoscopically defined depth-texture. This illusory motion standstill arises due to a failure to represent two independent surfaces (one for luminance and one for depth textures) and motion transparency (the ability to perceive motion of both surfaces simultaneously). Instead the stimulus is represented as a single non-transparent surface taking on the stationary nature of the luminance-defined texture. By contrast, if it is the 2D-luminance defined texture that is in motion, observers often perceive the stationary depth texture as also moving. In this latter case, the failure to represent the motion transparency of the two textures gives rise to illusionary motion capture. Our past work demonstrated that the illusions of motion standstill and motion capture can occur for depth-textures that are rotating, or expanding / contracting, or else spiraling. Here I extend these findings to include stereo-shearing. More importantly, it is the motion (or lack thereof) of the luminance texture that determines how the motion of the depth will be perceived. This observation is strongly in favor of a single pathway for complex motion that operates on luminance-defines texture motion signals only. In addition, these complex motion illusions arise with chromatically-defined textures with smooth transitions between their colors. This suggests that in respect to color motion perception the complex motions' pathway is only able to accurately process signals from isoluminant colored textures with sharp transitions between colors, and/or moving at high speeds, which is conceivable if it relies on inputs from a hypothetical dual opponent color pathway.

The categorisation of non-categorical colours: a novel paradigm in colour perception

In this paper, we investigate a new paradigm for studying the development of the colour ‘signal’ by having observers discriminate and categorize the same set of controlled and calibrated cardinal coloured stimuli. Notably, in both tasks, each observer was free to decide whether two pairs of colors were the same or belonged to the same category. The use of the same stimulus set for both tasks provides, we argue, an incremental behavioural measure of colour processing from detection through discrimination to categorisation. The measured data spaces are different for the two tasks, and furthermore the categorisation data is unique to each observer. In addition, we develop a model which assumes that the principal difference between the tasks is the degree of similarity between the stimuli which has different constraints for the categorisation task compared to the discrimination task. This approach not only makes sense of the current (and associated) data but links the processes of discrimination and categorisation in a novel way and, by implication, expands upon the previous research linking categorisation to other tasks not limited to colour perception.

The detection of the motion of contrast modulation: a parametric study

Despite a long and productive history as a focus of research interest, the details of how humans detect motion in an image remain controversial. This debate has not been helped by the lack of a clear parametric description of motion discrimination for some of the more simple visual stimuli employed in the literature to date. With this in mind, in the present work, we examined a peculiarity observed in the perception of the motion of second-order (contrast-modulated) stimuli: Under certain stimulus conditions, there is a reversal in the perceived direction of motion of the pattern. The aim was to quantify this phenomenon, relate the reversal to forward (veridical) and ambiguous motion, and place the behavioral data in the context of the window of visibility model of spatiotemporal contrast sensitivity. The direction of motion of contrast-modulated patterns was measured as a function of temporal frequency and carrier contrast, under different critical stimulus conditions. The stimulus properties manipulated were spatial frequency, spatial-phase relationship of carrier and sidebands, color, duration, and, most critically, the retinal location of the stimulus. On a purely empirical basis, the data reconciled several conflicts in the recent literature. From a theoretical standpoint, the data were well explained by the window of visibility approach in the majority of conditions and were partially explained in the remaining conditions. The results raise some interesting questions about underlying motion detection mechanisms and the assumptions embodied in our approach to motion modeling and the visual system in general. Supplemental materials for this article may be downloaded from app.psychonomic-journals.org/content/supplemental.

The perception of speed based on L-M and S-(L+M) cone opponent processing

We have measured perceived speed and speed discrimination thresholds for stimuli that selectively activate the L-M, S-(L+M) cone opponent and L+M (luminance) post-receptoral pathways. For low speeds and low contrasts speed discrimination thresholds for L-M and S-(L+M) are similar but are higher and have a greater dependency upon contrast than those for luminance motion. These differences between chromatic and luminance speed perception can be eliminated when stimuli are equated with respect to their individual motion detection thresholds (MDTs). For fast moving gratings speed perception based upon L-M, S-(L+M) and L+M signals is similar in terms of threshold performance and contrast dependency. These results are consistent with the view that there are separate mechanisms for the analysis of chromatic and luminance motion, the relative contributions of which may change as a function of stimulus contrast and speed. The similarity in performance for S-(L+M) and L+M chromatic stimuli across a range of stimulus parameters suggests that signals derived from the two cone opponent pathways can be used equally well. Our results argue against the idea that speed perception is compromised when it is based upon information derived from the S-(L+M) cone opponent pathway.

Effects of spatial attention and salience cues on chromatic and achromatic motion processing

While several previous psychophysical and neurophysiological studies have demonstrated chromatic (red/green) input to motion processing, the nature of this input is still a matter of debate. In particular, there exists controversy as to whether chromatic motion processing is mediated by low-level motion mechanisms versus higher-level, attention- or salience-based mechanisms. To address the role of attention, in Experiment 1, we asked whether spatial attention exerts larger effects on chromatic (red/green), as compared to achromatic, motion. To this end, we employed a motion after-effect (MAE) paradigm, and measured attention effects by comparing MAE duration between conditions where subjects attended to the adapting moving grating stimulus versus ignored that stimulus because they were required to perform an attentionally demanding vowel detection task at the center of gaze. The results from these experiments revealed equal effects of spatial attention on chromatic and achromatic motion processing, which were essentially constant (roughly 1.4-fold) across a wide range of stimulus contrasts (3.2-25% cone contrast). These findings suggest that chromatic motion processing is not affected disproportionally by higher-level spatial attention mechanisms. To address the role of salience, in Experiment 2, we investigated the effects of bottom-up salience cues on the strength of chromatic and achromatic motion, as measured with the MAE. Salience was manipulated by varying the relationship between the moving gratings and the background color. The results of these experiments revealed small and insignificant effects of salience cues on chromatic and achromatic motion processing. These findings suggest that mechanisms sensitive to feature salience do not influence low-level chromatic motion mechanisms mediating the motion after-effect.

The perception of motion in chromatic stimuli

The issue of whether there is a motion mechanism sensitive to purely chromatic stimuli has been pertinent for the past 30 or more years. The aim of this review is to examine why such different conclusions have been drawn in the literature and to reach some reconciliation. The review critically examines the behavioral evidence and concludes that there is a purely chromatic motion mechanism but that it is limited to the fovea. Examination of motion performance for chromatic and luminance stimuli provides convincing evidence that there are at least two different mechanisms for the two kinds of stimuli. The authors further argue that the chromatic mechanism may be at a particular disadvantage when the integration of multiple local motion signals is required. Finally, the authors present a descriptive model that may go some way toward explaining the reasons for the differences in collected data outlined in this article.

The detection of motion in chromatic stimuli: pedestals and masks

This study seeks to clarify the reasons for some of the differences in the published data on chromatic motion perception, and to provide further support for the existence of a low-level motion mechanism sensitive to purely chromatic change. Observers discriminated the direction of motion of displaced sinusoidal gratings in the presence of a static grating mask (or pedestal). Each component of the stimulus was independently described in cardinal colour space and calibrated for subjective equiluminance using multiple methods. The motion structure, stimulus size, temporal frequency, contrast, relative phase and chromatic properties were all varied parametrically and the data cast in terms of predictions made by two different theoretical approaches to the test-mask combination. The vast majority of the data were well explained by a low-level motion mechanism sensitive to the motion of foveally-placed chromatic stimuli. Data consistent with either higher-level motion perception or a luminance-like signal were found outside the fovea and when the stimulus properties did not otherwise favour chromatic motion perception. There was some explanation of inconsistencies in previously published data and a strong suggestion that previous results showing pedestal-like behaviour for these stimulus combinations were a special case rather than a general result.