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M A Georgeson

Publications and source records attributed to M A Georgeson.

At least 37 records · Page 2Linked to original sources
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↗
PubMed1987

Temporal properties of spatial contrast vision.

The temporal characteristics of spatial contrast vision at and above threshold were assessed psychophysically using sinusoidal gratings and a contrast-matching method. Temporal frequency response curves became flatter as contrast level increased. An impulse response model was fitted to these flicker data, and used to make predictions about temporal integration and two-pulse summation. The predictions fitted the experimental data well, except under conditions where the Broca-Sulzer effect occurred. The latter was not predicted by the biphasic impulse response that otherwise worked well. Temporal filtering became moderately transient and operated over a shorter time-scale as contrast increased, and above threshold temporal properties varied little with spatial frequency. The data and modelling supported the idea that subjects exploited probability summation at threshold while using a peak detection criterion above threshold. Systematic visual field asymmetries in contrast perception are also described. Single- and multiple-channel models of temporal processing are discussed.

Flicker Fusion↗
PubMed1987

Facilitation and masking of briefly presented gratings: time-course and contrast dependence.

We measured two-alternative forced-choice contrast thresholds for briefly presented sinusoidal gratings in the presence of superimposed masking gratings of various contrasts, and at a range of onset asynchronies. Facilitation (lower thresholds) occurred when the mask was simultaneous, in-phase, and near-threshold, but was abolished at asynchronies of 50 msec or more and by presenting the test grating as a brief contrast reversal instead of a pulse. We argue that facilitation requires temporal summation of responses within the same neural channels, but our results do not distinguish between transducer and uncertainty models. Masking (threshold elevation) occurred over a broader range of asynchronies, and was not abolished by test contrast reversal. Masking and facilitation probably depend on different processes with different time-courses. The occurrence of masking at asynchronies outside the range of temporal summation suggests that a static, compressive transducer does not, in general, account for masking. Brief masking and prolonged contrast adaptation are very similar in magnitude, and as a function of contrast and relative spatial frequency. Masking and adaptation may have a common origin, but differ in speed of recovery.

Adaptation, Ocular↗
PubMed1986

Sustained and transient temporal integration functions depend on spatial frequency, not grating area.

Contrast sensitivity for sinusoidal gratings was measured by a two-alternative forced-choice method and plotted as a function of duration. Gratings of large or small area and low or high spatial frequency (0.75 or 8.25 c/deg) were used. The temporal integration curve was much flatter in the low spatial frequency conditions than in the high frequency conditions, but its shape did not depend on the area of the grating. The results support the established view that the visual system exhibits transient temporal properties at low spatial frequencies and sustained properties at higher frequencies, but disconfirm the recent proposal that temporal integration depends on the area of the grating (expressed in square-cycles) rather than its spatial frequency.

Fovea Centralis↗
PubMed1985

Afterimages of sinusoidal, square-wave and compound gratings.

Negative afterimages were observed after steady fixation of sinusoidal gratings at low spatial frequencies, and quantified using contrast-matching and cancellation methods. Afterimage contrast increased as a power function of "exposure", defined as the product of inspection contrast and inspection duration. A single function, linear on a log-log plot, describes the afterimages of gratings at different spatial frequencies, contrasts and durations. The matching method yielded a lower slope (about 0.42) than the cancellation method (0.62), probably because contrast adaptation attenuates perceived afterimage contrast in the first method, but not the second. Square-wave gratings, and those containing two or three harmonic components gave much weaker afterimages (assessed by contrast-matching) than sine-waves did. Contrast adaptation may again be responsible. Hemi-field asymmetries in contrast perception were noted. The contrast of real gratings and afterimages was nearly additive, especially at lower exposures, but the results cannot distinguish between linear filter and gain control models of "local adaptation".

Adaptation, Ocular↗
PubMed1985

Apparent spatial frequency and contrast of gratings: separate effects of contrast and duration.

Variations in apparent spatial frequency of sinusoidal gratings were assessed using a matching method. Perceived spatial frequency increased as contrast decreased and as duration of presentation decreased (from 320 to 20 msec). The effect of duration was not produced via changes in apparent contrast (a) because the changes in perceived spatial frequency were much the same when subjective contrasts were matched as when physical contrasts were matched and (b) because reducing duration to 20 msec produced very little change in perceived contrast, despite a factor of four reduction in threshold contrast sensitivity. The effects of duration and contrast on apparent spatial frequency are therefore dissociable, but both might exert their effects via changes in the balance of centre-surround antagonism within receptive fields; the "selectivity shift" theory of these size perception phenomena is discussed.

Form Perception↗
PubMed1985

On seeing temporal gaps between gratings: a criterion problem for measurement of visible persistence.

Previous estimates of visible persistence based on subjects' ability to see a "blank" between brief flashes of a grating are remarkably long when compared with studies of temporal integration. We show firstly that subjects can detect gaps as short as 5 msec at low spatial frequencies, rising to about 20 msec at high spatial frequencies, and secondly that artefactual luminance transients such as might be found in tachistoscopes do not greatly increase the detection thresholds. Much longer estimates were obtained through the use of a more conservative criterion, which was subjectively ill-defined and very variable across subjects. Studies which use this method may not measure visible persistence very effectively.

Female↗
PubMed1985

The effect of spatial adaptation on perceived contrast.

Perceived contrast was assessed by contrast-matching between adjacent sinusoidal gratings of the same spatial frequency (3 cycles/degree), before and after adaptation to gratings of various contrasts. On logarithmic axes, the effect of adaptation was large for test contrasts below the adapting contrast, but absent for test contrasts higher than the adapting contrast. This result rules out a simple gain-reduction hypothesis, in which adaptation would attenuate all test contrasts by the same proportion. Instead, results for all combinations of adapting and test contrast levels (including threshold elevation) conformed fairly closely to a simple subtractive rule: any test contrast presented after adaptation behaved as if one-third of the adapting contrast were subtracted from it. Though not exact, this may be a useful descriptive rule. Deviations from the simple rule may be explained by increased variance in the visual response at high adapting contrasts, combined with nonlinearity at low test contrasts. A subtractive effect at the psychophysical level does not necessarily conflict with evidence for contrast gain reduction at the single-cell level.

Adaptation, Ocular↗
PubMed1984

Stability of phase recognition in complex spatial waveforms.

Observers viewed 200 msec presentations of gratings containing first (0.5 c/deg) and third (1.5 c/deg) harmonic components. The phase of the third harmonic and the absolute position of the grating on the screen varied randomly from trial to trial. Classification of the phase relation (0, 90, 180 or 270 deg was 99% perfect. When a 2 sec period of inspection of the grating or its fundamental preceded the test presentation, strong shifts in perceived waveform were observed that depended on the test grating's position relative to the inspection grating, and resembled the effects seen during continuous viewing. No phase-specific effects were obtained. The pattern of results was exactly that predicted by negative afterimage. Phase recognition at low contrast, and at a high frequency, was also good. Triangular-wave gratings were misperceived (the "square-wave illusion") only when real or simulated afterimages were present. We conclude that recognition of phase relations in a complex waveform is stable when the predictable variation due to afterimages is eliminated.

Afterimage↗
PubMed1984

Eye movements, afterimages and monocular rivalry.

The eye-movement/afterimage theory of "monocular rivalry" (MR) between gratings was tested and strongly supported. In three experiments perceptual dominance of vertical or horizontal components of the pattern and fluctuations in perceived contrast of a single grating were shown to depend on the nature of the preceding shift in fixation position in the manner predicted by the theory. In a fourth experiment the angular selectivity of these fluctuations was eliminated, as predicted, when appropriate eye movements were made. Fixation-contingent fluctuations became equally strong for 15 degrees and 90 degrees angles. Taken together with data on afterimages, the results appear to resolve most of the problems recently raised against the theory.

Afterimage↗
PubMed1984

Spatial selectivity of contrast adaptation: models and data.

Contrast threshold elevation was measured in human observers at a single spatial frequency (4 or 8 c/deg) after adaptation to gratings at different contrasts and spatial frequencies. When plotted against adapting contrast, the threshold elevation functions for different adapting frequencies were neither straight nor parallel, which poses difficulties for the "equivalent contrast transformation". The different functions appear to form a single family, differing by a scaling factor that depends on spatial frequency but is independent of contrast. This is similar to expressing the aftereffect as a proportion of its peak value. A multiple channel, "fatigue" model of adaptation is shown to be consistent with the results, provided the channel shape is appropriate and channels overlap considerably in the frequency domain. Channel bandwidth is estimated at 1.4 octaves, similar to that of cortical cells.

Adaptation, Ocular↗
PubMed1980

Spatial frequency analysis in early visual processing.

The existence of multiple channels, or multiple receptive field sizes, in the visual system does not commit us to any particular theory of spatial encoding in vision. However, distortions of apparent spatial frequency and width in a wide variety of conditions favour the idea that each channel carries a width- or frequency-related code or 'label' rather than a 'local sign' or positional label. When distortions of spatial frequency occur without prior adaptation (e.g. at low contrast or low luminance) they are associated with lowered sensitivity, and may be due to a mismatch between the perceptual labels and the actual tuning of the channels. A low-level representation of retinal space could be constructed from the spatial information encoded by the channels, rather than being projected intact from the retina.

Form Perception↗