Biomedical subjects

Anthony Hayes

Publications and source records attributed to Anthony Hayes.

6 recordsLinked to original sources

PubMed2006

The perceived position shift of a pattern that contains internal motion is accompanied by a change in the pattern's apparent size and shape.

When the sinusoidal grating of a "Gabor pattern" is drifted, the apparent position of the pattern shifts in the direction of motion [De Valois, R. L., & De Valois, K. K. (1991). Vernier acuity with stationary moving Gabors. Vision Research, 31, 1619-1626]. We investigated the underlying cause of this illusion by determining whether the effect is a consequence of the internal motion shifting the perceived position of the whole pattern, or a consequence of a shift in the perceived location of the centroid (centre of mass) of the Gabor envelope. While each of these two possible distortions can account for a perceived positional offset, they give different predictions for the apparent size of the stimulus. A simple shift in perceived position results in no change in apparent size, while a centroid shift will likely result in either a decrease or an increase in the pattern's apparent size, depending on whether the trailing or leading edge of the Gabor stimulus is most affected by motion. We examined whether there is a change in the apparent size of Gabor patterns containing a range of grating motion speeds. We found that the perceived size of the pattern increased in the presence of motion as a function of speed, and is thus consistent with a centroid-shift explanation. We verified that this size change is a consequence of an increase in contrast at the leading edge, since the leading edge appears elongated relative to the trailing edge. We furthermore showed that the apparent-position shifts due to motion can be negated by displacing the centroid in the opposite direction to the motion.

Contrast Sensitivity↗

PubMed2006

The role of luminance contrast in the detection of global structure in static and dynamic, same- and opposite-polarity, Glass patterns.

Perception of global structure conveyed in static Glass patterns is difficult, though not impossible, when the constituent dipoles are formed by partnering opposite polarity dots. We investigate whether the addition of motion signals to opposite-polarity Glass patterns can act to restore the perception of global structure. The stimuli were concentric Glass patterns consisting of 200 dipoles concentrically orientated, or oriented at random orientations, placed on a grey background. For each dipole, one luminance-increment dot (Weber contrast of 1) was paired with another dot set to a contrast ranging between luminance increment and luminance decrement (i.e., a Weber contrast range of approximately -1 to 1). Dipoles were either stationary (Experiment 1), or randomly re-positioned at 17Hz (Experiment 2), on each frame transition. A two-interval forced-choice paradigm, in conjunction with an adaptive staircase, was used to obtain Glass-pattern detection thresholds. The task required observers to identify the interval that contained concentric Glass structure; the other interval contained randomly orientated dipoles. Generally, lower global form thresholds were observed for dynamic and same-polarity Glass patterns than for static and opposite-polarity Glass patterns. In particular, for dynamic presentations improvement in sensitivity was more evident for opposite-polarity than for same-polarity Glass patterns. These findings suggest that motion plays an important role in the detection of global structure in dynamic Glass patterns.

Contrast Sensitivity↗

PubMed2006

Apparent position in depth of stationary moving three-dimensional objects.

Motion signals contained within a stationary object projected on the fronto-parallel plane shift the object's apparent spatial position in the direction of the motion [see De Valois, R. L., & De Valois, K. K. (1991). Vernier acuity with stationary moving Gabors. Vision Research, 31(9), 1619-1626]. We report an analogous apparent position shift of three-dimensional objects that contain local elements that move in depth. Our stimulus was a transparent three-dimensional cylinder defined by 150 limited-lifetime dots, oriented such that it was end on and its tangent plane was circular. Dots moved in depth by changes in their binocular disparities. In the first experiment, observers judged the positions of the near and far ends of the cylinder, by moving marker lines in depth, for different dot speeds. The results showed that when dots moved towards the observer, the perceived location of the two ends of the cylinder appeared closer in depth. When dots moved away from the observer, the opposite effect was produced. Additionally, the amount of apparent position shift produced was dependent on dot speed, with faster speeds producing larger positional offsets. However, we found in the second experiment that when the cylinder contained randomly moving dots, or when the cylinder contained equal amounts of dots moving towards and away from the observer, positional shifts were very much reduced, or abolished. Our findings suggest that motion signals can induce a misperception of position in depth that is similar manner to that produced by motion within an object in the two-dimensional image plane.

Cues↗

PubMed2005

Global speed averaging is tuned for binocular disparity.

The extent to which local speeds at different depths are averaged to determine global speed was determined using a version of the Global Dot Motion (GDM) stimulus. Judgments of the apparent speed of fast moving dots (4.05-10.53 deg/s) in the presence of slow moving dots (4.05 deg/s) were measured using GDM stimuli which simulated radially expanding motion, and which confined slow and fast speed dots exclusively to alternating wedge shaped sectors. The presence of slow moving dots in the stimulus reduced the apparent speed of high-speed dots in a manner consistent with speed averaging. However with increases in depth, produced by a difference in binocular disparity between dots in alternating sectors, speed averaging became less effective, and the relationship between speed and disparity resembled a tuning function. We discuss our results in light of research that clarifies the functional properties of global motion mechanisms in the primate cortex.

Animals↗

PubMed2005

Glass-pattern detection is tuned for stereo-depth.

We investigated the role of disparity information in the detection of global form. Glass patterns, which allow insight into processing at both local and global stages of form analysis, were used as stimuli. We determined how detection of concentric Glass patterns is affected by a disparity difference introduced between partner dots forming local dipoles (Experiment 1), and how detection is affected by the addition of randomly oriented dot-pairs (noise dots) at crossed and uncrossed disparities (Experiment 2). The first experiment showed that detection thresholds increased when partner dots were separated in depth at disparities greater than approximately 17 min arc; the second experiment showed that noise dots disrupted the detection of form if they were presented at disparities of between approximately +/-20 min arc from the Glass pattern's presentation depth plane. Our findings suggest that disparity information plays a role in the recovery of the image structure and, importantly, local and global form mechanisms were found to be selective for a small range of stereo-depths. We discuss the findings of our study in the light of current evidence indicating that a common neural substrate is responsible for the analysis of form and binocular disparity.

Attention↗

PubMedDate not supplied

Mechanism independence for texture-modulation detection is consistent with a filter-rectify-filter mechanism.

The ability of the visual system to detect stimuli that vary along dimensions other than luminance or color--"second-order" stimuli--has been of considerable interest in recent years. An important unresolved issue is whether different types of second-order stimuli are detected by a single, all purpose, mechanism, or by mechanisms that are specific to stimulus type. Using a conventional psychophysical paradigm, we show that for a class of second-order stimuli--textures sinusoidally modulated in orientation (OM), spatial frequency (FM), and contrast (CM)--the human visual system employs mechanisms that are selective to stimulus type. Whereas the addition of a subthreshold mask to a test pattern of the same stimulus type was found to facilitate the detection of the test, no facilitation was observed when mask and test were of different types, suggesting mechanism independence for the different types of stimulus. This finding raises the important question of whether mechanism independence is compatible with the well-known filter-rectify-filter (FRF) model of second-order stimulus detection, since FRF mechanisms, in principle, do not discriminate between stimulus types. We show that for all mask/test combinations except those with CM masks, the FRF mechanism giving the largest response to the test modulation is largely unaffected by subthreshold levels of a different stimulus-type mask. For this reason, we cannot rule out the possibility that FRF mechanisms mediate the detection of our stimuli. For combinations involving CM masks, however, we propose that a process of contrast normalization renders the test stimulus insensitive to the mask stimulus.

Contrast Sensitivity↗