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M A Georgeson

Publications and source records attributed to M A Georgeson.

13 recordsLinked to original sources
PubMed1992

Human vision combines oriented filters to compute edges.

The experiments examined the perceived spatial structure of plaid patterns, composed of two or three sinusoidal gratings of the same spatial frequency, superimposed at different orientations. Perceived structure corresponded well with the pattern of zero crossings in the output of a circular spatial filter applied to the image. This lends some support to Marr & Hildreth's (Proc. R. Soc. Lond. B 207, 187 (1980)) theory of edge detection as a model for human vision, but with a very different implementation. The perceived structure of two-component plaids was distorted by prior exposure to a masking or adapting grating, in a way that was perceptually equivalent to reducing the contrast of one of the plaid components. This was confirmed by finding that the plaid distortion could be nulled by increasing the contrast of the masked or adapted component. A corresponding reduction of perceived contrast for single gratings was observed after adaptation and in some masking conditions. I propose the outlines of a model for edge finding in human vision. The plaid components are processed through cortical, orientation-selective filters that are subject to attenuation by forward masking and adaptation. The outputs of these oriented filters are then linearly summed to emulate circular filtering, and zero crossings (zcs) in the combined output are used to determine edge locations. Masking or adapting to a grating attenuates some oriented filters more than others, and although this changes only the effective contrast of the components, it results in a geometric distortion at the zc level after different filters have been combined. The orientation of zcs may not correspond at all with the orientation of Fourier components, but they are correctly predicted by this two-stage model. The oriented filters are not 'orientation detectors', but are precursors to a more subtle stage that locates and represents spatial features.

Adaptation, Ocular↗
PubMed1992

No evidence for dichoptic motion sensing: a reply to Carney and Shadlen.

Georgeson and Shackleton (1989, Vision Research, 29, 1511-1523) confirmed the existence of dichoptic apparent motion (AM), but argued that its basis was the spatio-temporal correspondence of visible features ("feature tracking"), not early motion sensors. Direction selectivity in motion sensors was probably purely monocular. The key evidence came from AM of missing-fundamental (MF) gratings. Monocular MF motion was always reversed, implying motion sensors responding to the third harmonic (3f) component. Dichoptic MF motion was in the correct direction at the higher contrasts and lower drift rates, and overall was highly correlated with judgements of pattern structure, suggesting feature-tracking. Carney and Shadlen (1992, Vision Research, 32, 187-191) criticized some of our methodology and the theoretical interpretation. Their central argument was that dichoptic AM for sine-waves did reflect dichoptic motion sensing, but was less reliable at higher contrasts. Hence forward motion of the dichoptic MF pattern should occur only where sine-wave (3f) motion-sensing declines. We discuss their critique, and find little support for it. We also present additional data on dichoptic AM for 3f and 5f gratings, showing that Carney and Shadlen's predictions were not upheld. Feature tracking remains the most plausible account of dichoptic AM.

Contrast Sensitivity↗
PubMed1991

The time course of adaptation to spatial contrast.

We explored the buildup and decay of threshold elevation during and after adaptation to sinewave gratings in a series of experiments investigating the effects of adapting time, adapting contrast, spatial frequency and retinal eccentricity. Contrast thresholds for vertical sinewave gratings truncated in space by a one-dimensional Gaussian envelope were measured before and after adaptation to a full-field suprathreshold grating of the same spatial frequency and orientation. Thresholds were measured intermittently after adaptation in a "seen/not-seen" single presentation procedure until these thresholds returned to baseline values. The first test grating was presented 300 msec after the offset of the adapting stimulus, and thereafter at regular intervals. At different times after adaptation, contrast thresholds were estimated by off-line analysis of the data using the QUEST algorithm. Adapting time was either 1, 10, 100 or 1000 sec and adapting contrast was either 9, 19, 29 or 39 dB (re. 1%). The test gratings were presented centered either at the fixation point or at 5 and 10 deg eccentricity along the horizontal meridian. The results suggest that up to the saturation level the buildup and the decay of adaptation to contrast is well described by a power function of time. The slope of the best fitting line on log-log axes is fairly constant for the adaptation times tested. As reported earlier, thresholds increased with adapting contrast and these contrast-dependent differences were evident 300 msec after the termination of adaptation. Adaptation at 10 deg eccentricity yielded slightly higher threshold elevations than for central vision. Based on these results, a description is given of the dynamic response of the underlying neural mechanisms.

Adaptation, Ocular↗
PubMed1991

Contrast overconstancy.

In foveal vision there is a tendency toward contrast constancy above threshold. Contrast sensitivity varies greatly across different spatial and temporal frequencies and at different luminances, but perception of contrast above threshold varies much less (in logarithmic terms). In parafoveal vision (1-2 degrees eccentricity) the contrast of high-spatial-frequency gratings appeared higher than a 3-cycle per degree (c/deg) comparison grating, even though their threshold sensitivity was lower. I call this overconstancy. For two experienced observers, the effect was greatest at high spatial frequencies (12-18 c/deg) and at contrasts close to threshold. A simple model of contrast coding was able to fit all the data accurately: R' = [(C-T)/(Cnorm-T)]m, where R' is the final response, C is contrast, T is threshold contrast, and Cnorm and m are parameters. Cnorm represents the contrast at which different response functions converge to a common value (R' = 1), while m is the slope of the upper branch of a given function on logarithmic axes. This model, incorporating threshold subtraction, nonlinear compression, and response normalization, was derived from research on contrast matching, magnitude estimation, and discrimination. On this model overconstancy results when C less than Cnorm, and the contrast response for the test condition is more compressive (lower m) than in the comparison condition. In the parafovea m decreased linearly with increasing spatial frequency for both subjects, but Cnorm was approximately constant at 40-50%.

Contrast Sensitivity↗
PubMed1990

The temporal range of motion sensing and motion perception.

Apparent motion (AM) was studied using the missing-fundamental square-wave grating, displaced discretely over time. At inter-stimulus intervals (ISIs) greater than about 40 msec, AM was seen in the direction of displacement of the visible features of the pattern, while at shorter ISIs AM was seen in the reversed direction, following the displacement of the third harmonic spatial frequency component. This confirms (a) that the "long-range", feature-based process can bridge much greater time-gaps than the "short-range" motion sensors and (b) that the missing-fundamental pattern is a particularly useful tool for teasing the two processes apart.

Humans↗
PubMed1989

Monocular motion sensing, binocular motion perception.

The two-process account of motion perception and its binocular organization were addressed in experiments on apparent movement (AM) with three types of grating: sinusoidal; random bar width; and square-wave with missing fundamental (MF). Monocular MF gratings sampled four times per cycle of drift always appeared to move backwards. Here AM was unrelated to the spatial appearance of the pattern, and followed the motion of the dominant spatial frequency component (the third harmonic). We take this reversed AM to be characteristic of "short-range" motion sensors. It did not occur dichoptically, implying that the direction-selective mechanism of motion sensors is purely monocular. AM was seen with dichoptic presentation for all three types of grating. Performance improved with the length of the stimulus sequence, as predicted by probability summation. This result reconciles previous positive and negative findings on dichoptic AM. The perceived direction of dichoptic AM was consistent with polarity-selective matching of features over time (the "long-range process"). The most telling effect supporting feature-matching in dichoptic motion was that dichoptic MF motion reversed direction with a change in the visible features of the pattern (induced by changes in contrast and pulse duration); monocular apparent motion did not. Two routes from spatial frequency channels to the perception of object motion are discussed.

Contrast Sensitivity↗
PubMed1988

Spatial phase dependence and the role of motion detection in monocular and dichoptic forward masking.

Contrast thresholds for briefly flashed gratings were measured by the QUEST procedure, under conditions of forward masking by gratings of the same spatial frequency (usually 1 c/deg). Low-contrast masks reduced threshold at short onset asynchronies (0 to 50 msec), while higher contrasts raised threshold over a broader temporal range (0 to 100-140 msec). Both effects depended on the spatial phase relation, but in different ways. Threshold reduction at 0- +/- 90 deg phases probably arises from spatio-temporal filtering by direction-selective mechanisms. This conclusion was supported by computer simulation of a motion detector model. The direction-selective stage of motion analysis may be entirely monocular, since facilitation at 90 deg was abolished by dichoptic presentation. Threshold elevation was phase-dependent at short SOA's (20-50 msec), with a minimum at +/- 90 deg, but was not phase-dependent at longer SOA's (70-140 msec). In-phase masking (0 deg) was about equally strong monocularly and dichoptically, but dichoptic threshold elevation showed no phase-dependence at any SOA. Threshold elevation at longer SOA's, and with dichoptic presentation, may reflect a purely suppressive binocular masking effect, unselective for spatial phase, and its basis may be the same as contrast adaptation. At short SOA's, monocular and binocular masking data apparently reflect a mixture of this phase-independent suppression and phase-selective facilitation.

Contrast Sensitivity↗
PubMed1988

Comparison of contrast responses across spatial mechanisms.

We have analysed several recent studies of suprathreshold contrast perception in terms of the normalization of CTFs originally suggested by Georgeson and Sullivan (1975). When suprathreshold responses for different spatial frequencies are compared it is necessary to consider the effects of normalization across CTFs of different spatial mechanisms.

Amblyopia↗
PubMed1978

Apparent foveofugal drift of counterphase gratings.

Counterphase gratings, and several other stimuli which consist of equal components of motion in both directions, appeared to drift foveofugally, rather than foveopetally, when presented to the retinal periphery. This 'foveofugal drift effect' was demonstrated by descriptive and nulling techniques and its magnitude was shown to vary across subjects. The effect was fairly brief under continuous fixation. Several lines of evidence suggested that eye movements were not responsible for the effect. The phenomenon implies a directional asymmetry in the human visual system which may be related to our consistent exposure to expanding patterns of visual flow.

Differential Threshold↗
PubMed1976

Psychohysical hallucinations of orientation and spatial frequency.

After inspection of vertical sinusoidal gratings at least three distinct types of subjective or "hallucinated" patterns can be seen on a uniform test field. One type, here called horizontal streaming (H), is already well-known from the work of MacKay. A second type (V) looks like aroughly sinusoidal grating about 1-5 octaves above the adapting spitial frequency. Under optimal conditions a second vertical component appears at about 2 octaves below the adapting frequency. The third category of aftereffect consists of diagonal lines (D) at two orientations (about +/-40 degrees from vertical). The spatial-frequency band at these two orientations appears to be fairly broad, but roughly similar to the adapting frequency. The duration and strength of D increased, while V declined, at higher adapting spatial frequencies. D and V were increasing functions of adapting contrast, while H appeared abruptly only after the highest adapting contrast. H, D, and V are thus all functionally distinct. A schematic model of cortical organization is proposed to account for these phenomena. Pattern channels selective for a given orientation are grouped together with movement channels selective for the orthogonal direction. Antagonism between channels within such "modules" accounts for the streaming effect (H). Inhibition between modules tuned to different orientations and spatial frequencies accounts for the D and V effects: after adaptation of channels in one module, neighbouring module(s) are released from inhibition to produce a spurious response which is seen as a grating-like object in the adapted part of the visual field. During flickering adaptation a "halluncinated" lattice can be seen superimposed on the adapting grating. It apparently consists of Fourier components more remote from the adapting pattern than D and V are. This disinhibitory effect is strong confirmation of the inhibitory model. The regular and highly organized matrix of channels implied by these experiments may constitute a cortical hypercolumn conducting a coarse, piecewise Fourier transformation of the retinal image.

Figural Aftereffect↗
PubMed1975

Contrast constancy: deblurring in human vision by spatial frequency channels.

The perception of contrast was measured in humans by a technique of subjective contrast-matching, and was compared with contrast sensitivity as defined by threshold measures. 2. Contrast-matching between different spatial frequencies was performed correctly (especially at frequencies above 5 c/deg) despite the attenuation by optical and neural factors which cause large differences in contrast thresholds. 3. Contrast-matching between single lines of different widths was also veridical, and was not limited by the spatial integration (Ricco's Law) present at threshold. Adaptation to gratings altered the appearance of lines, and this could be best understood in Fourier terms. 4. The generality of these results was shown by matching the contrast of pictures which had been filtered so that each contained a one octave band of spatial frequencies. 5. Within the limits imposed by threshold and resolution, contrast-matching was largely independent of luminance and position on the retina. 6. Six out of eleven astigmatic observers showed considerable suprathreshold compensation for their orientation-specific neural deficit in contrast sensitivity. 7. These results define a new property of vision: contrast constancy. It is argued that spatial frequency channels in the visual cortex are organized to compensate for earlier attenuation. This achieves a dramatic 'deblurring' of the image, and optimizes the clarity of vision.

Astigmatism↗