Biomedical subjects

A M Derrington

Publications and source records attributed to A M Derrington.

At least 19 recordsLinked to original sources

PubMed1996

Rapid colour-specific detection of motion in human vision.

The human visual system is much better at analysing the motion of luminance (black and white) patterns than it is at analysing the motion of colour patterns, especially if the pattern is presented very briefly or moves rapidly. We report here that observers reliably distinguish the direction of motion of a colour pattern presented for only 17 milliseconds, provided that the contrast is several times the threshold value (the contrast needed to detect the presence of the pattern). A control experiment, in which a static luminance 'mask' is added to the moving colour pattern, proves that discrimination of the direction of motion of these brief stimuli is colour-specific. The mask drastically impairs discrimination of the direction of motion of a luminance pattern, but it has little effect on a colour pattern. We conclude that the human visual system contains colour-specific motion-detection mechanisms that are capable of analysing very brief signals.

Color Perception↗

PubMed1995

Refraction, aliasing, and the absence of motion reversals in peripheral vision.

Reversals in perceived direction of motion of a grating when its spatial frequency exceeds half that of the sampling mosaic provide a potential tool for estimating sampling frequency in peripheral retina. We used two-alternative forced-choice tasks to measure performance of three observers detecting or discriminating direction of motion of high contrast horizontal or vertical sinusoidal luminance gratings presented either 20 or 40 deg from the fovea along the horizontal meridian. A foveal target at a comfortable viewing distance aided fixation and accommodation. A Maxwellian view optometer with 3 mm artificial pupil was used to correct the refraction of the peripheral grating, which was presented in a circular patch, 1.8 deg in diameter, in a surround of similar colour and mean luminance (47.5 cd.m-2). The refractive correction at each eccentricity was measured by recording the aerial image of a point after a double pass through the eye. The highest frequency which can reliably be detected (7-14 c/deg at 20 deg, 5.5-7.5 c/deg at 40 deg) depends critically on refraction. Refraction differs by up to 5 D from the fovea to periphery, and by up to 6 D from horizontal to vertical. Direction discrimination performance shows no consistent reversals, and depends less on refraction. It falls to chance at frequencies as low as one-third of the highest that can be detected. Gratings which can be detected but whose direction of motion cannot be discriminated appear as irregular speckle patterns whose direction of motion varies from trial to trial.(ABSTRACT TRUNCATED AT 250 WORDS)

Discrimination, Psychological↗

PubMed1994

Implications of motion detection for early non-linearities.

Analysis of motion may be accomplished using the spatiotemporal variations produced when a spatially varying luminance waveform moves across linear receptive fields. Moving contrast-modulated patterns which consist of coarse-scale spatial variations in the contrast of fine-scale luminance patterns cannot be analysed in this way. The human visual system can analyse the motion of contrast-modulated patterns and this suggests it may contain mechanisms that use non-linear transformations. Non-linear transformation of contrast-modulated patterns would give rise to a component (a distortion product) that varies on the same spatial scale as the contrast variation; this can be analysed to extract motion. Is the non-linearity simply an inherent part of the transduction process or is it a characteristic of a mechanism specialized for the analysis of the motion of such patterns? Comparisons of the spatial and temporal limitations of motion discrimination using luminance and contrast-modulated patterns suggest that the mechanisms which analyse the two types of patterns are different, although recent physiological evidence suggests that they may have common elements.

Animals↗

PubMed1994

Motion of chromatic stimuli: first-order or second-order?

This paper measures the minimum velocity required to discriminate the direction of motion (the lower threshold of motion--LTM) for patterns which consisted of spatial variations in luminance, chromaticity or luminance contrast in an attempt to distinguish between the underlying directionally-selective mechanisms. The characteristics of these patterns can be defined as first-order/Fourier stimuli (luminance and chromatic gratings) or second-order/non-Fourier stimuli (contrast gratings or "beats"). Measurements for each pattern were made at durations ranging from 0.015 to 0.96 sec and at contrasts of 0.5 log units above detection threshold and 1.5 log units above the threshold for detecting the stationary pattern. Observers were able to discriminate the direction of motion in luminance gratings and high contrast chromatic gratings at all durations above 0.015 sec. The direction of motion of beats and low contrast chromatic gratings was indiscriminable until they had been presented for at least 0.12 sec. This was taken to indicate the existence of a fast-acting and a slow-acting system dealing with the first- and second-order patterns respectively. When defined on this basis, the chromatic stimulus acts as a first-order (luminance) pattern at high contrasts and a second-order (beat) pattern at low contrasts.

Color Perception↗

PubMed1994

The analysis of motion of two-dimensional patterns: do Fourier components provide the first stage?

Human observers were required to report the direction of motion of simple two-dimensional (2-D) "plaid" patterns made by adding together two sinusoidal gratings of identical contrast (0.5 or 1.5 log units above threshold), spatial frequency (1 or 5 c/deg) and orthogonal orientations (horizontal and vertical, or +/- 45 deg). The patterns were made to move either by moving both gratings at the same speed (pattern motion) or by moving one component with the other stationary (component motion). In one task (direction discrimination) the observer knew the axis of motion, and was required to discriminate the direction of motion along that axis in a temporal two-alternative forced-choice paradigm; in the other task (direction identification) the observer did not know the axis of motion and was required to identify the direction of motion and the axis of motion. In both tasks the discrimination of pattern motion was consistently better than the discrimination of component motion, contrary to the predictions of the "two-stage" model of motion analysis, in which it is assumed that the motion of a 2-D pattern is calculated from the 1-D motions of its Fourier components. The variation in direction discrimination of pattern motion with speed did not have the form predicted under the assumption that the direction of motion of the pattern could be discriminated using the motion of either of its two component gratings. Finally, an elaborated version of the Adelson and Movshon [(1982) Nature, 300, 523-525] two-stage model, in which noise affects the two stages fails to predict the performance in identifying the pattern motion of the plaid pattern, except for 1 c/deg low contrast plaids. These results suggest that when 2-D patterns contain moderately high contrasts or high spatial frequencies observers may use other attributes, instead of, or in addition to Fourier components, to analyse their 2-D motion.

Discrimination, Psychological↗

PubMed1994

Speed, spatial-frequency, and temporal-frequency comparisons in luminance and colour gratings.

The perceived speed, temporal frequency, and spatial frequency of matched colour and luminance gratings were compared in separate experiments. The large factor by which colour gratings are perceived to be slower moving than matched luminance gratings cannot be explained by systematic differences in the perceived spatial frequency or in the perceived temporal frequency of the two types of grating.

Color Perception↗

PubMed1993

Errors in direction-of-motion discrimination with dichoptically viewed stimuli.

At durations shorter than about 150 msec, a complex grating comprising a static 1-c/deg grating and a moving 3-c/deg grating is perceived as moving in the direction opposite that of the physical direction of motion. Here the phenomenon is further examined by measuring the perceived direction of motion of the fused images of a 1-c/deg grating presented to one eye and a moving 3-c/deg grating presented to the other. The strength of the illusion is almost unaffected by dichoptic presentation. This observation is consistent with the hypothesis that perceived motion is a consequence of the way the visual system integrates signals arising from different detectors tuned to the two component gratings.

Humans↗

PubMed1993

Discriminating the direction of second-order motion at short stimulus durations.

We measured the ability of human observers to discriminate the direction of motion of different spatial patterns presented for durations ranging from 0.021 to 0.67 sec. The patterns were: (1) a vertical grating (spatial frequency 0.93 c/deg at 5% contrast); (2) a "beat" pattern made by adding vertical gratings of 6.3 and 5.4 c/deg both at 5% contrast moving in opposite directions (this pattern appears as a horizontally moving, 0.93 c/deg "beat"; i.e. spatial variation in the contrast of a stationary vertical grating of 5.8 c/deg); and (3) a "plaid" pattern made by adding gratings of 5.9 c/deg orientated +/- 81 deg from vertical (this pattern can also be expressed as a horizontally moving 1.9 c/deg beat in a horizontal grating of 5.8 c/deg). The direction of motion of the grating and the plaid pattern were discriminable at all durations tested. The direction of motion of the beat could only be discriminated at durations above approx. 200 msec. We suggest that this is a consequence of the fact that the moving beat is only visible to second-order mechanisms, and that second-order mechanisms for the analysis of motion operate more slowly than first-order mechanisms.

Discrimination, Psychological↗

PubMed1992

Errorless performance in a two-frame apparent motion task using high contrast stimuli, a failure to replicate Cleary (1990)

Explore the source record for details and available documents.

Contrast Sensitivity↗

PubMed1992

Two-stage analysis of the motion of 2-dimensional patterns, what is the first stage?

Humans↗

PubMed1992

Analysis of the motion of 2-dimensional patterns: evidence for a second-order process.

The sum of two differently orientated moving sinusoidal gratings of similar spatial frequency, contrast, and velocity appears as a single coherent "plaid" pattern. The visual system is thought to analyse the motion of plaids in two stages, first analysing the motion of the (1-D) components, and then calculating a speed and direction which is consistent with those 1-D motions. We studied the apparent direction of motion of plaids made by adding two components that had the same spatial frequency and contrast, and were symmetrically oriented about the vertical axis. The gratings moved in jumps, and we studied the effect of varying the size of the jump, the angle between the component gratings, and the temporal interval between the jumps, on the perceived direction of motion. When the size of the jumps was increased to 3/8 of their spatial period, the perceived direction of motion of the plaid pattern reversed, although if one component were presented alone, its direction of movement did not reverse. Reversed motion of this type was consistently obtained if the angle between the components was greater than about 140 degrees, if the interval between jumps was at least 25 msec, and if the spatial frequency of the component gratings was less than about 4 c/deg. When the angle between the components was smaller, or the time between jumps was greater, most observers saw normal motion in the direction predicted by the two-stage hypothesis. When the spatial frequency was raised, observers saw no consistent motion.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗

PubMed1989

Detecting the displacements of spatial beats: no role for distortion products.

When two sinusoidal gratings of the same orientation and similar spatial frequency are summed, the resulting pattern has a periodic spatial variation or beat in contrast. Although the pattern contains no luminance modulation component at the beat frequency, it behaves in some respects as if it did: human observers for example are very good at detecting spatial displacements of the beat. We wished to test the possibility that a non-linearity in the visual system generates a component (a "distortion product") at the beat frequency, and that it is displacement of the distortion product that observers detect. Attempting to "null" the distortion product by adding to the beat pattern a sinusoidal component of the same spatial frequency as the distortion product but 180 deg out of phase with it does not impair performance in detecting motion of the beat; there is no nulling at any amplitude of the added component. Reducing the phase shift of the hypothetical distortion product by adding a static sinusoid to the moving beat pattern fails to produce the predicted fall in performance. These results suggest that distortion products do not contribute to our sensitivity to the displacement of beat patterns. Reversing the contrast of a beat pattern when it is displaced, slightly increases sensitivity to displacement, the same manipulation impairs performance with luminance patterns. This is consistent with the notion that the beat is detected as an unsigned local contrast signal.

Contrast Sensitivity↗

PubMed1989

Some observations on the masking effects of two-dimensional stimuli.

Gratings that differ in orientation by as much as 62.5 deg from that of a signal grating raise the signal's threshold by nearly a log unit. The spatial-frequency tuning of the masking effect reaches a maximum slightly below the spatial frequency of the maskers but far from that of any quadratic distortion product. Further, the location of the peak does not depend much on the relative orientation of the signal and maskers thus making it unlikely that the masking effect can be explained in any simple way by the presence of visual nonlinearities. This illustrates the difficulty of attempting to explain human performance in even relatively simple discrimination experiments with models based on mechanisms tuned for spatial frequency and orientation.

Form Perception↗

PubMed1989

Failure of motion discrimination at high contrasts: evidence for saturation.

The ability of human observers to discriminate the direction of motion of a briefly-presented, slowly moving, 1 c/deg sinusoidal grating varies non-monotonically with the contrast of the grating. At low contrasts, performance improves with increasing contrast, but it reaches a peak between 95% and 100% correct at a contrast of 0.02-0.05. With further increases in contrast performance declines, reaching chance levels at a contrast of about 0.4. Detection of the same stimulus improves with increasing contrast to 100% correct and stays there. This behaviour would be expected if the visual signal which determines direction-of-motion is given by the difference between the responses of paired direction-selective filters tuned to opposite directions of motion and if the responses of these paired filters saturate at modest contrasts.

Contrast Sensitivity↗

PubMed1988

Direction-of-motion discrimination with complex patterns: further observations.

Moving one component of a stimulus comprising two sinusoidal gratings of the same orientation sometimes results in mistaken judgments of the direction of motion. If the component with the higher spatial frequency moves and the stimulus is presented briefly, observers report motion in the direction opposite that which actually occurs. The illusory, or backward, motion appears whether the movement producing it occurs smoothly or as a discrete jump at the midpoint of the stimulus presentation. At durations at which motion appears reversed, smooth and discrete motion are indistinguishable. Measurement of the speed of the illusory motion by a cancellation technique permits comparison with results from classical induced-motion paradigms; the classical effect, obtained with spatially separated components, is smaller but in the same direction as the errors in perceived direction of motion that we measure. We suggest that the errors in judging the direction of motion may result from interactions among motion detectors tuned to the different spatial-frequency components of the stimulus.

Discrimination, Psychological↗

PubMed1987

Detecting the displacements of spatial beats: a monocular capability.

Sensitivity to the sudden displacement (phase shift) of a single monocularly presented sinusoidal grating is increased when a static grating of similar spatial frequency is presented to the same eye. If the static grating is presented to the other eye instead sensitivity is, at best, halved. This demonstration implies that monocular and binocular visual pathways differ in their sensitivity to spatial variations of contrast. In addition it provides another example in which the monocular visual pathways are more sensitive to spatial displacements than the binocular pathways.

Depth Perception↗

PubMed1987

Errors in direction-of-motion discrimination with complex stimuli.

The direction of apparent motion in a complex pattern comprising a static 1-cycle/degree (c/deg) grating and a moving 3-c/deg grating changes with stimulus duration. At durations longer than about 150 msec, motion is seen almost veridically; the motion of the 3-c/deg grating, which is seen correctly, merely induces in the 1-c/deg grating a weak apparent motion in the opposite direction. At shorter durations, however, the only motion seen is in the opposite direction from that which, in fact, occurs. The reversed apparent motion is both compelling and consistent; it is reported both by naive and by experienced observers, and, although it only occurs for certain ranges of spatial frequency, contrast and duration, the ranges are substantial. The reversal appears to be almost independent of the temporal frequency and the spatial phase of the stimulus; it occurs both for discrete and for continuous motion. It seems likely that the apparent motion with short duration stimuli reveals properties of local visual movement detection previously unknown and difficult to account for within the framework of current models of motion perception.

Discrimination, Psychological↗

PubMed1987

Distortion products in geniculate X-cells: a physiological basis for masking by spatially modulated gratings?

The responses of X-cells in cat lateral geniculate nucleus, to complex grating patterns moving across the receptive field, were recorded with microelectrodes. The patterns were multi-component gratings, composed by adding a low spatial-frequency sinusoidal "signal" to a high spatial frequency "mask" which was either unmodulated, contrast-modulated (AM) or quasi-frequency modulated (QFM) at the signal frequency. The response to AM and QFM gratings has a component at the same frequency as the response to the "signal". This low frequency component has the properties of a distortion product generated by a quadratic non-linearity in the LGN. These properties may account for the psychophysical masking which occurs between modulated high-spatial-frequency gratings and gratings of the modulation frequency [Henning, Hertz and Broadbent (1975) Vision Res. 15, 887-897].

Action Potentials↗