Biomedical subjects

T Ledgeway

Publications and source records attributed to T Ledgeway.

8 recordsLinked to original sources

PubMed1997

Changes in perceived speed following adaptation to first-order and second-order motion.

To investigate whether or not adaptation to second-order motion can cause changes in perceived speed, measurements of perceived speed were obtained for two varieties of motion: (i) contrast-modulated two-dimensional static noise (second-order motion); and (ii) luminance-modulated noise (first-order motion). The test stimulus (either first-order or second-order) was presented to one side of a central fixation spot and a comparison stimulus (always first-order) was simultaneously presented on the opposite side. The observer's task was to indicate which of the two motion stimuli appeared to drift faster. The perceived speed of the test stimulus was measured with and without prior adaptation to motion on one side of the fixation spot only (that of the test stimulus). The modulation depth of the adaptation stimulus was always half that of the test stimulus and all test patterns were equated for visibility. The pattern of results for second-order motion was similar to that for first-order motion. Typically, adaptation reduced perceived speed, particularly when the adaptation speed was faster than the test speed. However, when the adaptation speed was low relative to the test speed, increases in perceived speed were found. Cross-over adaptation effects between first-order and second-order motion were also observed. Robust velocity aftereffects were found for second-order motion when the noise was dynamic or was high-pass filtered, suggesting that first-order (luminance) artifacts were not responsible for the velocity aftereffects observed. We conclude that the perceived speeds of first-order and second-order motion appear to be encoded in human vision using similar computational principles (but not necessarily utilizing the same mechanism), since the same pattern of results was found for the two varieties of motion.

Adaptation, Physiological↗

PubMed1997

Separate detection of moving luminance and contrast modulations: fact or artifact?

We have investigated first-order artifacts in second-order motion perception. Subjects were required to identify the orientation and direction of a drifting sinusoidal contrast modulation. When the carrier consisted of static two-dimensional noise, performance often reflected the use of first-order artifacts that arise from stochastic local biases in the noise, rather than the detection of the contrast modulation per se. This stimulus, which has been used widely for studying second-order motion, therefore appears to be inappropriate for that purpose. In contrast, global distortion products arising from luminance non-linearities do not appear to provide usable artifacts. Two manipulations were employed to eliminate local first-order artifacts: the use of dynamic noise and the use of high-pass filtered static noise. These two manipulations gave similar results, which were quite different from those obtained with broadband static noise. We argue that performance with both of these image types reflects the activity of a true second-order motion mechanism. A characteristic property of this mechanism is that it cannot specify direction at the threshold for detecting orientation. Direction thresholds are around 50% higher than orientation thresholds when first-order artifacts are eliminated.

Contrast Sensitivity↗

PubMed1996

How similar must the Fourier spectra of the frames of a random-dot kinematogram be to support motion perception?

Direction-discrimination performance was measured for two-frame random-dot kinematograms in which one or both frames were spatial frequency filtered with a one octave band-pass filter and the centre frequency of this filter was varied in the range 0.75-9 c/deg independently for each frame. When both frames were filtered so that they contained common (overlapping) spatial frequencies direction discrimination was extremely good but it deteriorated rapidly as the degree of spectral overlap between the two frames decreased. These results are consistent with previous findings that suggest that the mechanisms that mediate the initial stages of motion detection are narrowly tuned for spatial frequency and cannot combine information conveyed at disparate frequencies in order to compute an unambiguous estimate of the direction of local motion. However, when only one of the frames was band-pass filtered and the other was unfiltered (broadband), the correct direction of stimulus motion could be discriminated reliably for a broad range of filter centre frequencies. Performance was best when the centre frequency of the filtered frame was at medium spatial frequencies and tended to deteriorate as the centre frequency approached either extreme of the spatial frequency range examined. This basic pattern of results may be attributed to the visual system's differential sensitivity to the Fourier components present in the unfiltered frame.

Discrimination, Psychological↗

PubMed1995

The perceived speed of second-order motion and its dependence on stimulus contrast.

Speed matches were obtained, using a spatial two-alternative forced-choice task, between a second-order motion stimulus and a first-order motion stimulus. The second-order motion stimulus was composed of contrast-modulated noise [produced by multiplying two-dimensional (2-d), static noise by a drifting, one-dimensional (1-d) sinusoid]. The first-order motion stimulus was composed of luminance-modulated noise (produced by summing, rather than multiplying, 2-d noise and a drifting sine grafting). In Expt 1, the relationship between the perceived speed of first- and second-order motion was examined. The motion stimuli had the same spatial frequency (1 or 3 c/deg) and were equated for visibility by presenting them at the same multiple of direction-identification threshold. Over a range of physical speeds, the perceived speeds of the first-order and second-order motion stimuli were identical when their physical speeds were the same. In Expt 2, the effect of varying stimulus "contrast" (contrast modulation depth) on the perceived speed of second-order motion was examined. The contrast of the first-order motion stimulus was fixed and speed matches were obtained for second-order motion stimuli at several contrast modulation depths. The motion stimuli had the same spatial (1 or 4 c/deg) and temporal (5 or 20 Hz) frequencies. It was found that the perceived speed of second-order motion was approximately linearly related to log modulation depth. In agreement with previous studies we also confirmed that the perceived speed of first-order motion is similarly dependent on stimulus contrast (luminance modulation depth).(ABSTRACT TRUNCATED AT 250 WORDS)

Contrast Sensitivity↗

PubMed1994

Adaptation to second-order motion results in a motion aftereffect for directionally-ambiguous test stimuli.

The magnitude of the motion aftereffect (MAE) obtained following adaptation to first- or second-order motion was measured in two experiments using a nulling method. The second-order motion adaptation stimulus was composed of contrast-modulated noise produced by multiplying two-dimensional random noise by a drifting, 1 c/deg, vertical sine grating. The first-order motion adaptation stimulus was composed of luminance-modulated noise produced by adding, rather than multiplying, the sine grating and noise field. The test stimuli were directionally-ambiguous first- or second-order motion patterns composed of either two oppositely drifting sine gratings added to static noise or its contrast-modulated equivalent. The amplitudes of the two drifting components were manipulated such that as one increased in amplitude the other decreased in amplitude by the same degree. This technique was employed to estimate the null point at which the test no longer appeared to drift in the direction opposite the adaptation direction. In the first experiment all stimuli were equated for visibility by presenting them at the same multiple of threshold and all possible combinations of first- and second-order motion adaptation and test stimuli were examined. The results were similar for all conditions: following adaptation the amplitude of the test component drifting in the same direction as adaptation needed to be approximately twice that of the oppositely drifting component in order to null the perception of unidirectional motion of the test. In a second experiment, the effects of manipulating the amplitude (visibility) of the first- and second-order motion adaptation stimuli on MAE magnitude were investigated. This revealed an approximately linear relationship between MAE magnitude and the amplitudes of the adaptation stimuli. The results demonstrate that, contrary to the findings of several previous studies, adaptation to second-order motion does produce a substantial movement aftereffect. Cross-adaptation between first- and second-order motion stimuli also occurs under appropriate conditions and produces aftereffects that are comparable in magnitude when the stimuli are equated for visibility.

Adaptation, Ocular↗

PubMed1994

Evidence for separate motion-detecting mechanisms for first- and second-order motion in human vision.

Current theories of second-order motion perception postulate that such motion is detected by either a high-level mechanism which computes the temporal correspondences between "features" extracted from the image, or low-level motion mechanisms which operate on a nonlinear, neural transformation of the luminance profile of the image. Theories which favour the latter strategy either suggest that first- and second-order motion are detected by a common mechanism or else that distinct mechanisms exist for the two types of motion, both operating on similar principles. The aim of this study was to differentiate between these possibilities. Observers were required to judge the direction of multiframe motion sequences in which the frames alternated between sinusoidal variations in luminance (first order) and similar variations in contrast (second order). On each frame the modulation signal was displaced by some fraction of its spatial period. The motion sequences were designed such that integration of both types of frame (first and second order) would lead to unambiguous motion in a particular direction whilst separate analysis of first- or second-order frames alone would yield ambiguous motion. The results show clearly that observers were unable to integrate the first- and second-order frames of such motion sequences. However, when observers were presented with motion sequences in which the frames alternated between two, different types of second-order image (variations in the contrast or size of the elements constituting a random noise field) perceived direction was always consistent with integration of both image types. This is taken as support for models that suggest that first- and second-order motion are processed by distinct mechanisms in the visual system and that each mechanism is only sensitive to one type of motion. It is suggested that several varieties of second-order motion stimuli may be regarded as equivalent to contrast-modulated images when considered in terms of the effects of local spatiotemporal filtering operations carried out by the human visual system. In this respect, our results are consistent with the "texture grabber" concept of Werkhoven, Sperling and Chubb [(1993) Vision Research, 33, 463-485].

Contrast Sensitivity↗

PubMed1994

The duration of the motion aftereffect following adaptation to first-order and second-order motion.

The magnitude of the motion aftereffect (MAE) obtained following adaptation to first-order or to second-order motion was measured by estimating its duration. The second-order adaptation stimulus was composed of contrast-modulated noise produced by multiplying two-dimensional (2-D) noise by a drifting 1 cycle deg-1 sine grating. The first-order adaptation stimulus was composed of luminance-modulated noise produced by summing, rather than multiplying, the noise and the sine grating. The test stimuli were directionally ambiguous motion patterns composed of either two oppositely drifting sine gratings added to noise or the contrast-modulated equivalent. The adaptation and test stimuli were equated for visibility by presenting them at the same multiple of direction-identification threshold. All possible combinations of first-order and second-order adaptation and test stimuli were examined in order to compare the magnitudes of the MAEs obtained following same adaptation and cross adaptation. After adaptation the test stimuli always appeared to drift coherently in the direction opposite to that of adaptation and the magnitudes of this MAE were very similar for all conditions examined. Statistical analyses of the results showed that there was no significant difference between the durations of the MAEs obtained in the same-adaptation and cross-adaptation conditions. The cross-adaptation effects suggest that either first-order or second-order motion are detected by a common low-level mechanism, or that separate parallel motion-detecting mechanisms exist, for the two types of motion, that interact at some later stage of processing.

Humans↗

PubMed1993

Transparent motion from feature- and luminance-based processes.

The apparent motion of a discretely displaced complex waveform with a periodic contrast modulation or "beat" of frequency f and sinusoidal components of frequencies 3f and 4f was examined at various interstimulus intervals (ISIs). At short ISIs perception of motion of both the beat and an aliased component of the waveform results in transparent motion. At longer ISIs motion is perceived only in the direction of the features of the waveform. The transparent motion observed at short ISIs indicates that, under certain conditions, "short-range" motion sensors do not constrain "long-range" feature processing and both may be active simultaneously.

Adaptation, Ocular↗