Extraction Of Global Motion Research Articles

Binocular global motion perception is improved by dichoptic segregation when stimuli have high contrast and high speed.

The brain combines information from the two eyes during vision. This combination is obligatory to a remarkable extent: In random-dot kinematograms (RDKs), randomly moving noise dots were similarly effective at preventing observers from seeing the motion of coherently moving signals dots, independent of whether the signal and noise were presented to the same eye or segregated to different eyes. However, motion detectors have varied binocularity: Neurons in visual brain area V1 that encode high contrast, high speed stimuli may be less completely binocular than neurons that encode low contrast, low speed stimuli. Also, neurons in MT often have unbalanced inputs from the two eyes. We predicted that for high contrast, high speed stimuli only, there would be a benefit to segregating the signal and noise of the RDK into different eyes. We found this benefit, both when performance was measured by percent coherence thresholds and when it was measured by luminance contrast ratio (signal-dot-contrast to noise-dot-contrast) thresholds. Thus, for high contrast, high speed stimuli, binocular fusion of local motion is not complete before the extraction of global motion. We also replicated a cross-over interaction: At high speed, global motion extraction was generally more efficient when dot contrast was high, but at low speed it was more efficient when dot contrast was low. We provide a schematic model of binocular global motion perception, to show how the contrast-speed interaction can be predicted from neurophysiology and why it should be exaggerated for segregated viewing. Our data bore out these predictions. We conclude that different neural populations limit performance during binocular global motion perception, depending on stimulus contrast and speed.

Behavioral measures of cortical hyperexcitability assessed in people who experience visual snow.

To determine whether visual perceptual measures in people who experience visual snow are consistent with an imbalance between inhibition and excitation in visual cortex. Sixteen patients with visual snow and 18 controls participated. Four visual tasks were included: center-surround contrast matching, luminance increment detection in noise, and global form and global motion coherence thresholds. Neuronal architecture capable of encoding the luminance and contrast stimuli is present within primary visual cortex, whereas the extraction of global motion and form signals requires extrastriate processing. All these tasks have been used previously to investigate the balance between inhibition and excitation within the visual system in both healthy and diseased states. The visual snow group demonstrated reduced center-surround contrast suppression (p = 0.03) and elevated luminance increment thresholds in noise (p = 0.02). Groups did not differ on the global form or global motion task. Our study demonstrates that visual perceptual measures involving the suprathreshold processing of contrast and luminance are abnormal in a group of individuals with visual snow. Our data are consistent with elevated excitability in primary visual cortex; however, further research is required to provide more direct evidence for this proposed mechanism. The ability to measure perceptual differences in visual snow reveals promise for the future development of clinical tests to assist in visual snow diagnosis and possibly a method for quantitatively assaying any benefits of treatments.

Human V6: Functional Characterisation and Localisation

Human visual area V6, in the parieto-occipital sulcus, is thought to have an important role in the extraction of optic flow for the monitoring and guidance of self-motion (egomotion) because it responds differentially to egomotion-compatible optic flow when compared to: (a) coherent but egomotion-incompatible flow (Cardin & Smith, 2010), and (b) incoherent motion (Pitzalis et al., 2010). It is not clear, however, whether V6 responds more strongly to egomotion-incompatible global motion than to incoherent motion. This is relevant not only for determining the functional properties of V6, but also in order to choose optimal stimuli for localising V6 accurately with fMRI. Localisation with retinotopic mapping is difficult and there is a need for a simple, reliable method. We conducted an event-related 3T fMRI experiment in which participants viewed a display of dots which either: a) followed a time-varying optic flow trajectory in a single, egomotion-compatible (EC) display; b) formed an egomotion-incompatible (EI) 3×3 array of optic flow patches; or c) moved randomly (RM). Results from V6 show an ordering of response magnitudes: EC > EI > RM. Neighbouring areas V3A and V7 responded more strongly to EC than to RM, but about equally to EC and EI. Our results suggest that although V6 may have a general role in the extraction of global motion, in clear contrast to neighbouring motion areas it is especially concerned with encoding EC stimuli. They suggest two strategies for localising V6: (1) contrasting EC and EI; or (2) contrasting EC and RM, which is more sensitive but carries a risk of including voxels from neighbouring regions that also show a EC > RM preference.

Interaction of first- and second-order signals in the extraction of global-motion and optic-flow

The intention of this series of experiments was to determine the extent to which the pathways sensitive to first-order and second-order motion are independent of one another at, and above, the level of global motion integration. We used translational, radial and rotational motion stimuli containing luminance-modulated dots, contrast-modulated dots, or a mixture of both. Our results show that the two classes of motion stimuli interact perceptually in a global motion coherence task, and the extent of this interaction is governed by whether the two varieties of local motion signal produce an equivalent response in the pathways that encode each type of motion. This provides strong psychophysical evidence that global motion and optic flow processing are cue-invariant. The fidelity of the first-order motion signal was moderated by either reducing the luminance of the dots or by increasing the displacement of the dots on each positional update. The experiments were carried out with two different types of second-order elements (contrast-modulated dots and flicker-modulated dots) and the results were comparable, suggesting that these findings are generalisable to a variety of second-order stimuli. In addition, the interaction between the two different types of second-order stimuli was investigated and we found that the relative modulation depth was also crucial to whether the two populations interacted. We conclude that the relative output of local motion sensors sensitive to either first-order or second-order motion dictates their weight in subsequent cue-invariant global motion computations.

Reponses of human V6 to random motion, egomotion_incompatible and egomotion-compatible optic flow

The extraction of optic flow cues from the visual scene is fundamental for guidance of egomotion. Area V6 is thought to be important in this role because of the properties of macaque V6 (Galletti et al., 1999), and because human V6 (hV6) responds differentially to egomotion-compatible optic flow compared to: (a) egomotion-incompatible flow (Cardin & Smith, 2010), and (b) incoherent motion (Pitzalis et al., 2010). It is not clear, however, whether hV6 responds more strongly to egomotion-incompatible global motion than to incoherent motion. This is relevant not only for determining the functional properties of hV6, but also in order to choose optimal stimuli for localising hV6 accurately with fMRI. Localization with retinotopic mapping is difficult and there is a need for a simple, reliable method. We conducted an event-related 3T fMRI experiment in which participants viewed a display of dots that moved randomly (RM), followed a time-varying optic flow trajectory in a single, egomotion-compatible (EC) display, or formed an egomotion-incompatible (EI) 3 × 3 array of optic flow patches as used in our previous work. Analysis was conducted by fitting a GLM. Beta values were extracted from retinotopically defined regions and averaged across hemispheres. Results from hV6 show an ordering of response magnitudes: EC > EI > RM. Neighbouring areas V3A and V7 responded more strongly to EC than to RM, but about equally to EC and EI. Our results suggest that hV6 may have a general role in the extraction of global motion but, in clear contrast to other neighbouring motion areas, is especially concerned with encoding EC stimuli. They suggest two strategies for localising hV6: (1) contrasting EC and EI; or (2) contrasting EC and RM, which will give a larger differential response, but requires use of standard retinotopic mapping in order to exclude voxels from neighbouring regions that also show a EC > RM preference.

No interaction of first- and second-order signals in the extraction of global-motion and optic-flow

Edwards and Badcock ( Vision Research 35, 2589, 1995) argued for independent first-order (FO) and second-order (SO) motion systems up to and including the global-motion level. That study used luminance (which they called FO) and contrast (SO) modulated dots. They found that SO noise dots did not mask signal extraction with luminance increment dots while luminance increment dots did mask SO signal extraction. However, they argued this asymmetry was not due to a combined FO–SO pathway, but rather due to the fact that the luminance-modulated dots, being also local variations in contrast, are both FO and SO stimuli. We test their claim of FO and SO independence by using a stimulus that can generate pure FO and SO signals, specifically one consisting of multiple Gabors (the global-Gabor stimulus) in which the Gaussian envelopes are static and the carriers drift. The carrier can either be luminance-modulated (FO) or contrast-modulated (SO) and motion signals from the randomly-oriented local Gabors must be combined to detect the global-motion vector. Results show no cross-masking of FO and SO signals, thus supporting the hypothesis of independent FO and SO systems up to and including the level extracting optic-flow.

When S-cones contribute to chromatic global motion processing

There is common consensus now that color-defined motion can be perceived by the human visual system. For global motion integration tasks based on isoluminant random dot kinematograms conflicting evidence exists, whether observers can (Ruppertsberg et al., 2003) or cannot (Bilodeau & Faubert, 1999) extract a common motion direction for stimuli modulated along the isoluminant red-green axis. Here we report conditions, in which S-cones contribute to chromatic global motion processing. When the display included extra-foveal regions, the individual elements were large ( approximately 0.3 degrees ) and the displacement was large ( approximately 1 degrees ), stimuli modulated along the yellowish-violet axis proved to be effective in a global motion task. The color contrast thresholds for detection for both color axes were well below the contrasts required for global motion integration, and therefore the discrimination-to-detection ratio was >1. We conclude that there is significant S-cone input to chromatic global motion processing and the extraction of global motion is not mediated by the same mechanism as simple detection. Whether the koniocellular or the magnocellular pathway is involved in transmitting S-cone signals is a topic of current debate (Chatterjee & Callaway, 2002).

The direction aftereffect is driven by adaptation of local motion detectors

The processing of motion information by the visual system can be decomposed into two general stages; point-by-point local motion extraction, followed by global motion extraction through the pooling of the local motion signals. The direction aftereffect (DAE) is a well known phenomenon in which prior adaptation to a unidirectional moving pattern results in an exaggerated perceived direction difference between the adapted direction and a subsequently viewed stimulus moving in a different direction. The experiments in this paper sought to identify where the adaptation underlying the DAE occurs within the motion processing hierarchy. We found that the DAE exhibits interocular transfer, thus demonstrating that the underlying adapted neural mechanisms are binocularly driven and must, therefore, reside in the visual cortex. The remaining experiments measured the speed tuning of the DAE, and used the derived function to test a number of local and global models of the phenomenon. Our data provide compelling evidence that the DAE is driven by the adaptation of motion-sensitive neurons at the local-processing stage of motion encoding. This is in contrast to earlier research showing that direction repulsion, which can be viewed as a simultaneous presentation counterpart to the DAE, is a global motion process. This leads us to conclude that the DAE and direction repulsion reflect interactions between motion-sensitive neural mechanisms at different levels of the motion-processing hierarchy.