Barry Mapperson

The effect of spatial frequency and contrast on visual persistence

The visual persistence of sinusoidal gratings of varying spatial frequency and contrast was measured. It was found that the persistence of low-contrast gratings was longer than that of high-contrast stimuli for all spatial frequencies investigated. At higher contrast levels of 1 and 4 cycles deg−1 gratings, a tendency for persistence to be independent of contrast was observed. For 12 cycles deg−1 gratings, however, persistence continued to decrease with increasing contrast. These results are compared with recently published data on other temporal responses, and are discussed in terms of the different properties of sustained and transient channels.

Dissociation of visual and spatial processing in working memory

Although models of working memory originally included visuo-spatial memory as an undifferentiated component, recent work indicates that spatial location is separable from other visual characteristics. The evidence which is derived from interference, however, typically requires the division of conscious attention between primary and interference tasks. In the present experiments the differential effects of an unattended flickering surround upon serial recall of a central stimulus presentation were investigated. Colour changes in the surround created more interference than achromatic flicker when the task was to report either the colours or the patterns presented in the target. When the locations of the targets were required, however, the situation was reversed and the achromatic flicker created more interference than did the colour flicker. It is suggested that this double dissociation of identification and location is best understood in terms of differential involvement of the magnocellular and parvocellular pathways.

Orientation and spatial-frequency-specific surround effects on binocular rivalry

The presence of an annular grating surrounding rivaling gratings was found to significantly reduce the dominance of a component that was similar in both spatial frequency and orientation. This effect, which transfers interocularly, would seem to implicate cortical or higher mechanisms in binocular rivalry suppression.

Spatial frequency and orientation specificity of simultaneous and successive inhibition on monocular rivalry

For several forms of perceptual instability, the relative dominance of one percept is reduced by the prior inspection of a similar percept in isolation.’ This effect. reported for figure-ground reversals. binocular rivalry and a reversible perspective figure, is attributed to “satiation” (Carlson, 1953: Hochberg. 1950: Kakizaki, 195Oa, b). If some form of neural inhibition is involved. the amount of change would be expected to vary with the extent to which common populations of neurones process the inspection and the test stimuli. Fluctuations in dominance (“monocular rivalry”-Breese, 1899) are perceived when viewing crossed gratings. Electrophysiological and psychophysical studies suggest that in this case substantial effects would be produced only by an inspection stimulus similar both in orientation and in spatial frequency to one component of the test stimulus (Blakemore and Campbell, 1969; Campbell and MatTei, 1970). This wouId apply whether the test stimulus was presented to the same or to the contralateral eye of subjects who exhibited normal stereopsis (Ware and Mitchell, 1974). With a number of psychophysical techniques, effects similar to those produced after prolonged inspection of a grating are found when the inspection stimulus is presented simultaneo~ly as an annulus surrounding the test stimulus (e.g. Klein, Stromeyer and Ganz, 1974; Tolhurst and Thompson, 1975). It would therefore be further predicted that an orientation and spatial-frequency specific surround effect on relative dominance could be demonstrated. The following experiments were designed to test these hypotheses.

Stimulus variables affecting induced rotation

Dunker (1929/1938) reported that a stationary pattern surrounded by a rotating annulas appears to rotate in the opposite direction. Our experiments show that this rotary induced motion is affected by varying the velocity, pattern, and contrast of the inducing stimulus, is unaffected by the test-stimulus pattern, and is affected by test-stimulus contrast only at low contrast levels.

Problems for an after-image explanation of monocular rivalry

Two optically superimposed gratings observed with free eye movements are perceptually unstable under certain conditions, with continual fadings of one or the other component occurring. In a recent paper, Georgeson and Phillips (1980) argue that this “monocular rivalry” between gratings of different orientations may be simply due to negative after-images which reinforce or attenuate components of the pattern as random eye movements occur. Unfortunately this seemingly plausible explanation would appear to be invalidated by a number of reported findings, some of which are detailed below. Georgeson and Phillips (1980) argue that their approach would explain why the disappearances are partial rather than complete, citing Wade (1975) in this context (complete disappearance would require not only eye movements very precisely positioned but after-images as strong as the inducing stimulus). While partial disappearance of one component is the usually adopted criterion, the fact that complete disappearances of one pattern do occur has been noted by Breese (1899), Campbell and Howell (1972), Atkinson et al. (1973) and was in fact directly measured in Experiment 4 of Wade (1975).* Since the frequency with which after-images are seen is drastically reduced when contrast is lowered (Corwin et al., 1976) an after-image explanation would predict corresponding reductions in the rate of monocular rivalry as a function of contrast. Atkinson et ul. (1973) however, report substantial rivalry rates at contrast levels where Corwin et al. (1976) were unable to obtain after-images. They further show a slight tendency for rivalry rates to increase with decreasing contrast over that portion of the contrast

Presence and absence of color selectivity in the motion aftereffect

It is controversial whether the magnitude of the motion aftereffect is greater when both inspection and test stimuli are the same color rather than different colors (color selectivity). The present experiments show that the extent of color selectivity in the classical motion aftereffect depends upon (1) the duration of the interval between inspection and test, and (2) the nature of the stimulation during this interval. These findings are consistent with previous reports of two phases in the motion aftereffect and are interpreted in terms of the known properties of sustained and transient cells in the human visual system.

Colour selectivity in the tilt after-effect: Comments upon Wade and Wenderoth

Abstract Three experiments were performed to isolate some factors influencing colour-selectivity in the tilt aftereffect. Colour-selectivity was found with an interstimulus interval of 20 msec with both a random staircase and PEST method of measurement. Colour-selectivity was reduced with a zero interstimulus interval and eliminated when the inspection and test fields were the same size. It is concluded that while colour-selectivity in the tilt aftereffect is a robust effect, its presence may be modified by a number of methodological and procedural factors.

The Dependence of Monocular Rivalry on Spatial Frequency: Some Interaction Variables:

The effect of spatial frequency upon the rate of monocular rivalry has been investigated on several occasions, with conflicting results. With increases in spatial frequency the rivalry rate has been variously reported as declining monotonically, increasing slightly and briefly before declining sharply, or showing a sharp increase before a sharp decrease. Experiments are reported which show that all three functions may be obtained depending on whether the number of cycles of the gratings is held constant across spatial frequency, or the field size is held constant (and, if the latter, what field size is used). The point at which any high-frequency decline occurs is also shown to increase with increasing luminance. All functions obtained correspond to visual sensitivity functions obtained under comparable conditions.

Sources of Individual Differences in IT

Abstract Interest in inspection time (IT), uniformly thought to index ‘speed-of-processing’, has been maintained because of its established empirical correlation with general mental ability and performance IQ measures. The IT procedure generally consists of a simple visual discrimination task displayed at various critical exposure durations and immediately followed by a suitable mask. Processing is assumed to terminate at mask onset, which seems inappropriate in light of the target/mask composite necessarily available with integration theories of backward masking. The present paper reports five experiments involving attended and unattended secondary stimuli and additional factor analyses of previous studies involving IT. From these it is proposed that IT is better thought of as indexing the power an individual can bring to bear within a specific cognitive domain rather than ‘speed-of-processing’ per se . The consequence is that the observed IT-IQ correlation is merely the inevitable outcome of measuring the same domain with two different tasks; rather than due to some elemental factor such as mental speed underlying both tasks. Ways in which this model could be tested are discussed.