Biomedical subjects

J C Boulton

Publications and source records attributed to J C Boulton.

12 recordsLinked to original sources

PubMed1994

Rapid segmentation of one-dimensional noise textures across borders.

We investigated the segmentation of texture pairs that were samples of one-dimensional binary visual noise. The stimulus consisted of an array of 5 x 8 squares separated by two-dimensional noise borders of varying width. The squares were filled with vertical black or white stripes of random width. The task was to detect the presence of a target square which differed from the squares above and below in one of three possible ways: the target pattern was either a contrast inverted copy or a horizontal translation of the pattern in the vertically adjacent squares, or else an independent realization of the noise. The binary noise in the textures was sequentially high-pass filtered to preclude the use of coarse-scale receptive fields and minimize the presence of sparse, extended "features". The target could be detected reliably within 100 msec even when the border width was larger than the maximal stripe width. The border width at threshold saturated for longer presentation times. Our results show that the microstructure of the patterns, i.e. information on the scale of the linewidth in the patterns, is not used directly, even though it contains most of the signal energy and is objectively the most reliable cue to the segmentation.

Contrast Sensitivity↗

PubMed1993

Dependence on stimulus onset asynchrony in apparent motion: evidence for two mechanisms.

The detection of the direction of motion was measured as a function of the spatial and temporal offset for a kinematogram stimulus presented in two-frame apparent motion. The stimulus was made up of Gabor function micro-patterns randomly distributed across the stimulus field. We show that for short stimulus onset asynchronies (SOA) performance can be predicted from the spatio-temporal Fourier power spectrum of the stimulus, whereas for long SOAs the pattern of performance is qualitatively different from such a prediction. The dependence of motion perception on SOA exhibits an abrupt change from one mode of behaviour to the other. These findings are suggestive of the operation of distinct mechanisms, one "quasi-linear" and one "nonlinear", which can be separated by temporal parameters.

Discrimination, Psychological↗

PubMed1993

Different parameters control motion perception above and below a critical density.

The maximum displacement for the detection of apparent motion (Dmax) is measured using stimuli made up of Gabor function micro-patterns randomly distributed across the stimulus field. Previous studies using high densities of micro-patterns have demonstrated Dmax to be dependent on the spatial frequency content of the stimulus and not the size of the stimulus elements. Here we report that Dmax increases suddenly when the number of micro-patterns in the visual field is reduced beyond some critical point. The number of micro-patterns at which the transition in Dmax occurs is found to be inversely proportional to the width of the micro-patterns along the axis of motion. Beyond this transition, for low density stimuli, Dmax is found to be dependent on both the number and size of micro-patterns in the stimulus field. These results are suggestive of the operation of different motion mechanisms under conditions of low vs high micro-pattern density.

Female↗

PubMed1992

Absence of smooth motion perception in color vision.

We have tested the behavioral evidence for a separation of the processing of color contrast from motion in the human visual system. Two different aspects of motion perception are examined; the identification of the direction of movement of a chromatic grating and the perception of smooth motion. The results show that color vision is at no great disadvantage in the identification of direction of movement, since this can be done at color contrasts quite close to detection threshold over a wide range of spatial and temporal frequencies. However, we find that subjects can identify direction without having the genuine perception of smooth motion. Smooth motion perception is revealed to be highly impaired since it is detected only at very high color contrasts and over a narrow range of spatial temporal conditions.

Color Perception↗

PubMed1992

Spatial frequency selective mechanisms underlying the motion aftereffect.

The motion aftereffect (MAE) was used to study the spatial frequency selectivity of suprathreshold motion perception. Observers were adapted to drifting sine-wave gratings confined to a retinal eccentricity of approx. 4 deg. The magnitude of the subsequent MAE was measured while viewing a stationary sine-wave grating test surface of one of a number of spatial frequencies. The largest MAE was found when the spatial frequency of the test stimulus was the same as that of the adapting stimulus. This phenomenon held for spatial frequencies between 0.5 and 4 c/deg, and was robust with changes in contrast of either adapting or test gratings. However, at an adapting spatial frequency of 0.25 c/deg, the peak MAE was observed at 0.5 c/deg. Control experiments indicated that this peak shift was not the result of the reduced number of cycles in the stimulus, nor the temporal frequency. There was no measurable MAE at spatial frequencies lower than 0.25 c/deg. These results suggest the existence of a "lowest adaptable channel" for the motion aftereffect.

Adaptation, Ocular↗

PubMed1992

Interactions between colour and luminance contrast in the perception of motion.

It has been demonstrated widely that at isoluminance moving chromatic stimuli are seen to be stationary or moving more slowly than their luminance counterparts. We have examined the effect on perceived velocity of adding luminance contrast to an isoluminant chromatic stimulus. We show that moving luminance contrast 'captures' colour so that a combined colour and luminance stimulus is seen moving as a unified percept. However, in the presence of colour contrast, significantly higher levels of luminance contrast are required to achieve a veridical velocity than for monochromatic stimuli with only luminance contrast. We show that this interactive effect between colour and luminance contrast cannot be fully explained by a threshold masking of luminance by colour contrast. The effect suggests that a breakdown in the veridical perception of velocity should be expected for colours with a wide range of associated luminance contrasts and not just for those at the point of isoluminance.

Color Perception↗

PubMed1991

Optimal spatial displacement for direction selectivity in cat visual cortex neurons.

Responses of single neurons in cat visual cortex were measured in response to sinewave grating stimuli. Firstly, a neuron's spatial frequency tuning was determined, and subsequent stimuli were set at the optimal spatial frequency for that neuron. Then a "jumping grating" stimulus was used: a sinewave grating subjected to a series of abrupt spatial displacements, while remaining stationary for a fixed exposure time between displacements. The amount of direction selectivity elicited by this stimulus was measured as a function of the amount of spatial displacement. Visual cortex neurons generally showed an optimal spatial displacement, corresponding to somewhat less than one quarter of a spatial period of the neuron's optimal spatial frequency (close to, but systematically less than, "quadrature phase"). In a majority of neurons tested, this optimal displacement was not affected by increasing the exposure time between displacements, indicating that the measurements were not a simple consequence of temporal frequency tuning. These results closely parallel recent human psychophysical data obtained from measurements of motion aftereffect or direction discrimination elicited by jumping grating stimuli.

Animals↗

PubMed1991

Motion detection is dependent on spatial frequency not size.

The maximum displacement for the detection of apparent motion is measured using stimuli made up of micro-patterns randomly distributed across the stimulus field. The micro-patterns were Gabor functions or half-wave rectified Gabor functions. Dmax is shown to be dependent of the spatial frequency content of the stimulus, and to be largely independent of the size of the stimulus elements or the number of them in the stimulus field (within limits). Evidence is shown for Dmax to be considered a measure of motion detection that reveals properties of low level underlying motion mechanisms.

Contrast Sensitivity↗

PubMed1991

Motion thresholds in infants to sinusoidal gratings.

Motion thresholds were determined at 9 degrees eccentricity in infants (mean = 14 weeks old). The stimuli used were computer-generated sinusoidal gratings presented through a 7.45 degrees aperture at a contrast ratio of .83. The range of velocities (.5, 1, 2, 4, and 6 degrees per s) was examined at only one spatial frequency (1 cycle per degree). At low velocities (less than 2 degrees per s), the infants showed no clear preference for the moving stimulus over the stationary stimulus. At faster velocities (2-6 degrees per s), the infants exhibited a clear preference for the moving stimulus. The results were interpreted as indicating that infants at 3 months of age are relatively insensitive to slow motions for low spatial frequency stimuli.

Age Factors↗

PubMed1990

The optimal displacement for the detection of motion.

The optimal spatial displacement for the detection of motion by the human visual system was investigated using spatially narrow band stimuli. Direction discrimination was used for abruptly displaced stimuli. An optimal spatial displacement was found for the detection of motion and this bore a characteristic relationship to the spatial wavelength of the stimuli in motion; it was equivalent to 1/6 of the spatial wavelength of the stimulus for low contrast stimuli and 1/5 of the spatial wavelength for higher contrast stimuli. This finding, which in turn suggests that the spatial subunits of motion detectors may be separated by less than 1/4 spatial wavelength, receives some support from other psychophysical and neurophysiological studies.

Contrast Sensitivity↗

PubMed1990

Luminance contrast and motion detection.

Direction discrimination was used to measure the minimum and maximum displacement for the detection of motion (Dmin and Dmax) for abruptly displaced sinewave gratings. This was measured for a range of contrast levels from 2 to 32 times the detection threshold for a range of spatially narrow band stimuli. Performance for Dmin (but not Dmax) was found to deteriorate with an increase in contrast, with the most sensitive values for Dmin obtained at contrast levels of 4-8 times detection threshold. This dependence on luminance contrast is thought to be due to the physiology of the visual system, rather than the physics of the stimulus.

Contrast Sensitivity↗

PubMed1987

Two mechanisms for the detection of slow motion.

The lowest velocity at which motion can be detected, the lower threshold of motion (LTM), has been investigated as a function of eccentricity. Results show that temporal properties of the LTM change with eccentricity, suggesting a dichotomy in the temporal properties of the neural mechanisms subserving this aspect of central and peripheral vision. Suprathreshold drifting sinusoidal gratings were used with a method of constant stimuli (a two-alternative forced-choice procedure) to obtain the LTM for different stimulus durations at different eccentricities. Results for central vision support a displacement-based model of movement detection similar to that of Reichardt [in Sensory Communication, W.A. Rosenblith, ed. (Wiley, New York, 1961), p. 303]. The LTM decreases as the stimulus duration increases, thus maintaining a constant minimum displacement. However, the results for the far periphery (35 deg of eccentricity) suggest a different mechanism, in which detection depends not on a constant displacement but on a constant velocity; as the stimulus duration increases, the LTM remains constant. These results for gratings can also be generalized to other stimuli, for example, random-dot patterns presented in apparent motion.

Humans↗