Academic journal article Attention, Perception and Psychophysics

The Detection of the Motion of Contrast Modulation: A Parametric Study

Academic journal article Attention, Perception and Psychophysics

The Detection of the Motion of Contrast Modulation: A Parametric Study

Article excerpt

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 ability to accurately identify motion in an image is a critical property of the visual system, and one that has attracted a great deal of research interest over the past 30 years or so. However, despite the wealth of data collected and the extent of the confluence of that data, there is still uncertainty regarding how we detect the motion of the simplest luminance edge, let alone the more complex patterns employed in much of the recent research in motion psychophysics. It certainly appears that the motion detection system is a strongly hierarchical process and that the initial signal specific to the motion subsystem is related to the direction of motion of an edge (Lennie & Movshon, 2005; Marr, 1982). The edge is usually coded by a firstorder modulation of the image statistics (Cavanagh & Mather, 1989; Julesz, 1971), and the directional signal relating to any 1-D edge is accurate only to within 690?, a property known as the aperture problem (Marr & Ullman, 1981). From this relatively simple starting point, the signal is progressively built into a neural representation that is a remarkably complex and powerful contributor to the overall percept of the visual scene, revealing not only the motion in the input, but also the depth and, in some cases, the form (Warren, 2004). Furthermore, visual motion in an image is strongly linked to the change in apparent position of a perceptually consistent auditory signal, indicating its importance as a key signal facilitating the integration of the internal representation of the external environment as a whole (e.g., Kitagawa & Ichigara, 2002). The hierarchy of signal construction is seen in both the behavioral data (Wilson, Ferrera, & Yo, 1992) and the neurophysiological data (Duffy, 2004), evidence that, in turn, shapes the way in which we define and describe the stimuli that we use to further explore the issue.

An example of the influence of the apparent hierarchy is the description of a visual stimulus in terms of its firstorder and second-order spatial statistics and of the consideration of each spatial dimension (x and y) independently. Consistent with this, there has been an argument presented within the motion literature for independent pathways in the system that deal with the first-order and second-order components of the pattern separately (e. …

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