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Biomedical subjects

A M Derrington

Publications and source records attributed to A M Derrington.

At least 19 recordsLinked to original sources
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↗
PubMed1992

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

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 find that the direction of motion of a plaid (components 1.6 c/deg orientated +60 degrees and -60 degrees) can be discriminated at velocities so low that the direction of motion of its components is not discriminable. This finding is not consistent with the "two-stage" hypothesis in the form that it is usually expressed. We suggest that mechanisms sensitive to the motion of local elements in the pattern, such as edges, could also contribute to the first stage of the analysis of plaid motion.

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↗
PubMed1986

Detection of spatial beats: non-linearity or contrast increment detection?

We measured sensitivity of human observers to the 1 c/deg beat between sinusoidal gratings of 9 and 10 c/deg, at different contrasts of the 2 components. Raising the contrast of one component increases the contrast required in the other component to detect the beat. This is consistent with the hypothesis that the beat is detected because of the local increments in contrast which it produces, but not with the hypothesis that it is detected when a difference-frequency distortion product generated by non-linear transduction of luminance reaches threshold.

Discrimination, Psychological↗
PubMed1985

Separate detectors for simple and complex grating patterns?

Grating having two sinusoidal components show a periodic variation in contrast which is visible as a "beat" pattern. The spatial frequency of the beat is the difference between the frequencies of the two components. Thresholds for a number of detection and discrimination tasks were measured using beat patterns of 1 c/deg (with components of 9 and 10 c/deg), and gratings of 1 and 10 c/deg. Temporal modulation at 6 Hz lowered detection thresholds for 1 c/deg gratings, but not for beats or 10 c/deg gratings. The effect of contrast on the range of temporal frequencies over which direction of movement can be discriminated differs for the three types of pattern: beats resemble neither low nor high spatial frequency gratings. Low and (for 2 of 3 observers) high spatial frequency gratings, but not beat patterns, are susceptible to a movement after effect induced by a low spatial-frequency grating. Beat patterns induce little or no movement after effect. We conclude that beat patterns are not detected by the same mechanisms that detect simple gratings.

Figural Aftereffect↗
PubMed1985

The low level motion system has both chromatic and luminance inputs.

Adaptation to moving isoluminant gratings induces a motion after-effect (MAE). Isoluminant gratings are less effective at inducing and at nulling MAEs than are luminance gratings. These results are consistent with a low-level motion detection system which operates on signals from mechanisms which show both spatial and chromatic opponency.

Adaptation, Ocular↗
PubMed1985

Detecting the displacement of periodic patterns.

Observers were asked to detect the direction of displacement of a 30 c/deg grating. They were virtually unable to perform this task when the component was presented alone but when either a 28 or a 32 c/deg component (neither of which moved) was added to the 30 c/deg component observers were extremely sensitive to displacements of the 30 c/deg component. These results suggest that the detection of displacements cannot take place within narrow-band spatial channels, but relies on a mechanism which compares the output of channels in different spatial positions.

Form Perception↗
PubMed1984

Development of spatial frequency selectivity in striate cortex of vision-deprived cats.

Single unit activity was recorded in the striate cortex of vision-deprived cats aged between 3 and 8 weeks. Contrast sensitivity or response measurements made using moving sinusoidal gratings were used to construct spatial frequency tuning curves. At 3 weeks sensitivity, selectivity (assessed both as the narrowness of the tuning curve bandwidth and as the proportion of selective cells), and optimal spatial frequency, are all better than in 2 week old normally reared cats, and comparable with those of 3 week old normally reared cats. After 3 weeks of age no further improvements take place, although distributions of sensitivity, best spatial frequency and bandwidth overlap with those of normal cortical cells, and selectivity is clearly better than in adult LGN cells. These results are consistent with the idea that the spatial properties of cortical cells are at least partly predetermined, but that many cells require visual experience to develop normally.

Age Factors↗
PubMed1984

Spatial frequency selectivity of remote pattern masking.

Sudden movement of a high contrast grating in peripheral retina selectively reduces the sensitivity of human observers to low spatial frequency sinusoidal gratings, presented at the fovea. The suppression is similar to that observed using spots as targets. It is suggested that both these effects result from masking by the physiological periphery effect.

Form Perception↗