Fergus W. Campbell

Saccadic omission: why we do not see a grey-out during a saccadic eye movement.

Abstract We investigated why we have no perception of a smeared image resulting from the reduction in contrast (grey-out) occurring at the retina during saccadic eye movements. By turning on light in the experimental room only during the eye movement, we were able to show that this grey-out was perceived as a smeared image of the visual scene. However, when the experimental room was illuminated before and/or after the saccade as well as during the saccade, perception of the grey-out was obliterated. During a period of fixation, perception of a blank image comparable in duration to an eye movement could also be eliminated by a preceding or following clear image. We conclude that lack of perception during saccadic eye movements made in normal contoured environments results primarily from the visual “masking” effect of a clear image before and/or after the eye movement acting on the grey-out during the eye movement. This “saccadic omission” is entirely a visual phenomenon and is far more powerful than the usually studied elevation of visual threshold for detection of a flash, “saccadic suppression.”

On the nature of the neural abnormality in human amblyopia; neural aberrations and neural sensitivity loss

In this investigation contrast threshold measurements are compared with supra-threshold perception for a group of human amblyopes. The results indicate that human amblyopia involves, in some cases, not only loss of sensitivity but spatial distortion. Thus a new group of amblyopes can now be identified in which only distortion occurs. These results have important physiological implications for both the normal and abnormal visual systems. Neurophysiologists investigating visual loss from deprivation should assess whether similar distortions occur in animals. This question may hold the answer to whether the present animal models are relevant to the human condition.

The Magic Number 4 ± 0: A New Look at Visual Numerosity Judgements

Visual numerosity judgements were made for tachistoscopically presented linear arrays of dots or lines. The interelement interval (which could be specified in spatial frequency terms) was constant for a given array but varied across conditions. A clear limit in the accuracy of numerosity judgements was found to be set at 4 for regularly spaced elements with spatial frequencies below approximately 10 cycles/deg (element and interelement interval of 0·05 deg). This limit in terms of accuracy is accompanied by a fast and almost constant response time for arrays of 4 or less, compared to response times for arrays of more than 4 elements. The limit in accuracy falls to 2 elements rather than 4 for spacing narrower than 0·05 deg although with such spacing the elements are still easily resolved. The limit of 4 is found if the stimulus is a bright afterimage, lasting for approximately 60 s. This result suggests that the limit is independent of the time allowed for a single fixation and is a perceptual limit rather than a limit in some memory buffer. ‘Numerosity’ units are proposed to account for the results.

A Demonstration of the Visual Importance and Flexibility of Spatial-Frequency Amplitude and Phase

To establish how little information the human visual system requires for recognition, common objects were digitally manipulated in the Fourier domain. The results demonstrate that it is not only possible, but also quite efficient, for a (biological) visual system to exist with very few phase relationships among the component spatial frequencies of the (retinal) image. A visual example is then presented which illustrates how certain phase relationships can hinder, or completely eliminate, the recognition of visual scenes.

Differences in the neural basis of human amblyopias: the effect of mean luminance.

Abstract The effect of reducing background luminance on contrast thresholds for different forms of amblyopia was investigated. As expected from previous acuity studies strabismic and organic amblyopia exhibit dramatically different behaviour; for a particular spatial frequency the degree of strabismic amblyopia reduces with reducing luminance whereas organic amblyopia does not. Unexpectedly, there is a similar difference in the behaviour of strabismic and anisometropic amblyopia. This difference suggests that these two forms of “functional” amblyopia do not have identical neural bases. An hypothesis concerning the extent of the abnormality within the visual field is advanced to explain these luminance dependent results.

Preliminary results of a physiologically based treatment of amblyopia.

An apparatus has been developed in which high-contrast square-wave gratings are rotated in front of the amblyopic eye while the child is performing some task requiring visual concentration. In the course of three 7-minute treatments 73% of patients treated achieved 6/12 or better; 75% of these patients had previously undertaken conventional or minimal occlusion.

Displacement detection in human vision

The displacement threshold is defined to be the smallest instantaneous target displacement that can be detected. Properties of the displacement threshold for a small, luminous spot were measured psychophysically. In a structureless field, the displacement threshold was near 1.5′, subject to individual variation. The effects of pattern were studied by measuring displacement thresholds at the centers of a set of annuli ranging from 2.85′–728′ dia. Displacement thresholds were reduced by the presence of the annuli and were as low as 0.3′. This threshold reduction could not be fully attributed to processes of relative spatial localization because displacement thresholds were lower than spatial localization (bullseye) thresholds for annulus diameters greater than 20′. The displacement threshold is virtually independent of orientation and pupil size. It increased about 75% with a three log unit decrease in photopic target luminance. Displacement detection appears to depend upon the motion sense rather than the position sense. It may be limited by fixation accuracy.

The discriminability of spatial phase relationships in amblyopia

Previous work has shown that anomalies in the contrast sensitivity functions of amblyopes are often insufficient to explain the degree of visual deficit in more complex tasks. Our stimuli were compound gratings composed of a fundamental and its third harmonic, added in either square-wave or triangle-wave phase. At medium to high spatial frequencies we find that many amblyopes, unlike normal observers, are unable to distinguish between such gratings which have identical power spectra, but different phase spectra. In this frequency range they can, however, easily distinguish between a compound grating and its fundamental component alone. It seems that in amblyopia visual processing occurs over a more truncated frequency range than is implied by detection experiments. Various explanations of this observation are considered.

Detection and discrimination of simple and complex patterns at low spatial frequencies

Abstract These experiments examined the extent to which low spatial frequencies are processed independently. The assessment was carried out with respect to both detection and discrimination performance. For simple sinusoidal gratings, pairs of stimuli could be discriminated when their contrasts reached threshold, if the ratio of their spatial frequencies was 3:1 or larger, suggesting idependent processing in separate channels. For smaller frequency ratios, slightly more contrast was required for discrimination than for detection, suggesting that stimuli were not processed by entirely separate channels. The detection and discrimination thresholds of complex grating stimuli fell within the ranges which would be expected if probability summation effects and summation of different closely spaced harmonic frequencies within single channels are considered, supporting the hypothesis of independent processing of low-spatial frequency information. The single exception to this involved discrimination of a square wave and a square wave with its fundamental component removed. In this case, discrimination required considerably more contrast than detection, even when factors of probability summation and within-channel summation of harmonics are considered.

The Dependence of the Visual Numerosity Limit on Orientation, Colour, and Grouping in the Stimulus

Visual numerosity judgements were made for tachistoscopically presented arrays of dots. The arrangement within the arrays was either linear or such that dots could be easily perceptually subdivided into two groups. Subdivision was either in terms of an orientation difference, a colour difference, or a spacing difference in the centre of the array. For a large difference in orientation between the two ‘arms’ of the array (90°), or a large central space (three times the interdot interval) up to 8 dots were accurately perceived. This numerosity limit was twice that found for equivalent linear arrays, with no grouping. Although in terms of accuracy it seems that in these conditions the two groups within each array can be counted independently, there is no evidence for independent processing in terms of response times. From the results of a subsidiary experiment it seems likely that the slow response times in the subgrouping conditions are due to the necessity of processes other than counting (such as judgements of symmetry). For arrays where subgrouping was in terms of a colour difference, or an orientation difference of between approximately 45° and 90°, or a small central space of twice the interdot interval, there was an improvement in accuracy compared to equivalent linear arrays, but no evidence of independent processing, up to a limit of 4, in each of the subgroups. From these preliminary results, tentative proposals concerning ‘numerosity’ units and their properties are made.