Biomedical subjects
Publications and source records attributed to R J Watt.
Data are presented which indicate that a length limit of 20 min arc applies to the highly accurate discrimination of curvature difference. Within this constraint and the previously reported orientational restriction (Watt and Andrews, 1982, Vision Res. 22, 449-460), curvature may determine the level of performance.
A wide range of pattern acuity measurements are presented, and detailed comparisons are made between different acuities for different stimuli and tasks. The most striking finding is that only a small number of cues and mechanisms are involved, and there are thus considerable inhomogeneities in spatial perception, even under focused attention and foveal viewing. A theory for pattern acuity is elaborated, bringing the pattern acuity phenomena into a more general scheme for the visual perception of spatial pattern. Four distinct mechanisms are required to account for all the data. Two of these analyse the shape of small segments of contour using high grade position and slope information; the other two relate the positions of these segments also using position and slope information, but of lower grade.
Relative location acuities similar to vernier acuity were measured for a variety of different luminance profiles and contrasts. The results are interpreted in terms of the extraction of zero-bounded centroids in the responses of a limited range of bandpass spatial filters, with centre frequencies between 3.5 and 30 c/deg. The process by which the outputs of the filters are combined is examined, and MIRAGE, a specific proposal, tested and found to be supported. Analysis of the results obtained at lower contrasts suggests that the major source of noise lies not in the amplification of the filters, but is due to the location of the centroids being distorted by spurious noise zero-crossings. The pattern of noise intrusion is readily explained in terms of MIRAGE.
If an edge having a particular blur profile is exchanged for an edge at the same location but having a different blur profile, apparent movement between the two edges may be seen. The spatial extent of this movement was measured by a cancellation technique. Implications for the theory of edge location are discussed, including the possibility that spatial filtering is preceded by a nonlinear transduction stage.
The roles of the various cues in the traditional vernier target are examined. We conclude that there are at least two mechanisms by which vernier acuities of the order of 5 sec arc may be obtained. The two cues are the overall slope of the target, and the relative positional differences. By using vernier targets that are degraded in two different ways, we can demonstrate each mechanism.
A vernier target moving discontinuously between discrete spatial stations is seen as having a spatial offset if its component bars are presented at spatially aligning stations with a temporal delay (the interpolation effect). In agreement with previous work, we find that the threshold for detecting this virtual spatial offset is the same as that for detecting an explicit spatial offset, provided that certain spatial and temporal constraints are met. Interpolation is a completely efficient process provided that the spatial interval between stations in the movement trajectory is less than 3-4 min arc, but thereafter it shows a gradual decline in efficiency. This paper presents a model of the decline in efficiency which arises, we suggest, not because of a failure of sampling to represent the original stimulus, but because of a progressive mismatch between the sampled signal and the bandwidth of the spatial filters involved in interpolation. Our model is compared with the constant velocity model of Fahle & Poggio [Proc. R. Soc. Lond. B 213, 451-477 (1981)].
Vernier acuity demonstrates a fine sense of relative visual location, which can be exploited to examine the question of mechanisms for the extraction of position information. By determining the appearance and location of composite stimuli consisting of a number of unresolved parallel bars of different luminances, we have demonstrated that vernier acuities of the order of 5 sec arc are due to mechanisms with an inflexible manner of encoding local intensity changes in the retinal image. The results are compared and contrasted with current theoretical approaches to the problems of early visual coding, and are best described by a model which encodes only the occurrence and location of zero-crossings in the second derivative of the retinal light distribution.
Threshold vernier acuity was measured under different conditions of target movement and exposure duration. In the case of a simple two-line vernier target, image motion up to about 3 deg/sec had little effect upon threshold for a briefly exposed (150 msec) target, which is relatively poor even for a stationary stimulus, but produced a decrement in acuity for a continuously exposed stimulus. This finding was repeated in a second experiment, which used a centroid cue to vernier offset, and which compared the effects of horizontal and vertical target orientation. It is suggested that image motion and reduced exposure duration restrict the proportion of the light spread function that can be usefully sampled by the neural networks responsible for hyperacuity.
Experiments are reported that measure the precision with which observers can (a) discern the degree of blur of an edge, (b) distinguish between two different types of blur, and relate the locations of two such edges by judging their (c) misalignment or (d) separation. Three blurring functions were used, and detailed examination of the retinal stimulus in each case leads to a simple hypothesis for the visual coding of edge blur. This is interpreted in terms of a general and primitive code for the transduction of local and widespread spatial luminance changes, in which stationary points in an approximation to the second derivative are detected and localized. The phenomena of illusory brightness changes in Mach bands and the Chevreul illusion find a ready explanation in this theory.
A modification to the standard vernier target that has a detrimental effect on acuity is described. The addition of an extra bar alongside one of the test bars and directly underneath the other increases thresholds by an amount that is a monotonic function of its luminance. This allows for the hypothesis that the location of a bright bar is a function of some widespread description of the light distribution arising from such a bar on the retina, rather than some local feature of such a distribution. In particular, the data are not consistent with any simple notion of boundary extraction and support the conjecture that position of a bar is assigned to the mean or centroid of its light distribution.
Explore the source record for details and available documents.
A moving target with identical trajectories in the two eyes appeared shifted in depth if flickered with a phase difference between the eyes. The direction of the depth shift was the same as if the phase-lagging eye had been seeing the target with a spatial advance in the direction of target motion. Maximum acuity for this effect was in the region of 7 deg of phase angle, which may be expressed as a spatial difference between corresponding luminance points of about 13 sec arc. Acuity was approximately constant over a wide range of modulation periods from 50 to 150 msec when expressed in terms of phase angle, but fell off rapidly at higher and at lower frequencies. The results cannot be explained simply by either conventional disparity theory or by spatio-temporal interpolation, but suggest that inter-ocular correspondence is influenced by dynamic luminance information.
It is pointed out that "spot size" of high resolution oscilloscope displays typically refers to the width of the spot at its half peak amplitude point, not to the width of the whole light distribution. The modulation transfer function of oscilloscope displays can have a more pronounced effect upon stimulus contrast than is generally realized, particularly when short viewing distances are used. We present measures of the spot profiles of a typical display, and use these to derive the MTF of the instrument. A simple correction is described to derive the actual contrast of a sine wave grating, given the manufacturer's specification of spot size.
Spatial and temporal vernier offset thresholds were determined for a target moving in discrete spatial steps between stations. In the spatial case, the two bars comprising the target had a relative spatial offset at each station but were synchronous. In the temporal case the two bars appeared at aligning stations with a temporal asynchrony. Temporal thresholds were converted into spatial units by calculating the distance the target would have travelled during the temporal delay. When compared in this way, temporal and spatial thresholds were very similar if the duration of the motion sequence was 300 msec or greater, but at briefer durations temporal thresholds became progressively higher. The possibility that eye pursuit might account for these data was ruled out by recording eye movements and rejecting trials on which tracking occurred. Analysis of eye records revealed the surprising fact that some tracking can occur even when the duration of the motion sequence is as little as 150 msec and random in direction.
Curvature discrimination thresholds were measured for a wide range of stimulus curvatures and sizes. Results are compared with an ideal processor of curvature to provide relative efficiencies. The results lead to three major findings. (i) Curved lines may be processed with the same precision as straight lines for decisions of shape, demonstrating a new class of hyperacuity. (ii) High relative efficiencies are obtained for all curved lines with an orientation range of less than 40 deg. (iii) This performance is not however consistent with common processing for straight and curved lines. The results are discussed, and two or possibly three separate processes are suggested as the basis for detailed shape perception.
Explore the source record for details and available documents.
Explore the source record for details and available documents.