Biomedical subjects

A Toet

Publications and source records attributed to A Toet.

17 recordsLinked to original sources

PubMed1994

Factors limiting large-scale localisation.

The mechanisms mediating relative spatial localisation in the visual system are still unclear. There is a growing amount of evidence that this capability is not merely limited by the processing of the front-end visual system. Models of localisation should, therefore, include higher-level processing stages. A careful study of the sources of error in localisation tasks may further our understanding of the nature of these processes. A study is reported in which the possible role of higher-order processing in relative spatial localisation is explicitly addressed. For this purpose the error sources of threshold performance were investigated for two similar relative-spatial-localisation tasks: two-dot separation discrimination and two-dot orientation discrimination. Fovea-centered stimuli with large dot separations were used. The front-end processing for these stimuli is probably identical in both tasks. Hence, differential effects of the variation of the experimental parameters on threshold performance for both tasks may reveal the characteristics of the higher-level processing involved. The effects of dot separation, stimulus orientation, and experimental procedure (single-stimulus binary forced choice versus two-alternative forced choice) on threshold performance for both tasks are reported. The results show that thresholds for both tasks increase proportionally with dot separation. However, separation-discrimination thresholds are always significantly higher than orientation-discrimination thresholds. Thresholds for separation discrimination are independent of stimulus orientation. In contrast, orientation-discrimination thresholds show an oblique effect: thresholds are consistently lower for horizontal stimuli. Both tasks also show a different dependency of threshold behaviour on the experimental procedure. For a horizontal stimulus orientation, separation discrimination is better with an explicit (physical) reference standard, whereas orientation discrimination is better with an implicit referent. These differential effects cannot be explained by any of the known characteristics of the front-end visual system. They suggest that large-scale spatial-localisation performance is probably limited at a processing level at which spatial relations are explicitly represented.

Adult↗

PubMed1994

The effect of similarity and duration on spatial interaction in peripheral vision.

Spatial interactions are extensive in the peripheral visual field, extending up to about half the retinal eccentricity of the target (Toet and Levi, Vision Res. 32, 1349-1357, 1992). In the present study it is shown that the degree and extent of peripheral spatial interaction depends in large measure on the similarity between test and flanking stimuli. The stimulus consisted of a test T surrounded by four distracting flanking Ts, each randomly oriented. The task was to determine the orientation of the test T. The test and flanking Ts differed in contrast polarity, shape, depth, color, eye of origin, or contrast. When the target and flanks differed in contrast polarity, depth, or shape, performance improved markedly for all observers. A color difference enhanced the performance of most but not all observers. Eye-of-origin had no effect, that is, spatial interaction was identical when the target and flanks were presented to the same eye, or to opposite eyes. The role of stimulus duration in spatial interaction was examined in two additional experiments. In the first, the stimulus viewing duration was increased in order to allow the observer time to serially search for the test T. In the second experiment, a postmask was presented at the location of the test T. The results of these experiments showed that the influence of similarity was independent of stimulus duration and the postmask, and suggest that serial search does not play an important role in the spatial interaction effects reported here. The extent of spatial interaction is correlated with the ability to do parallel search.

Color Perception↗

PubMed1992

Visual processing of optic acceleration.

We present data on the human sensitivity to optic acceleration, i.e. temporal modulations of the speed and direction of moving objects. Modulation thresholds are measured as a function of modulation frequency and speed for different periodical velocity vector modulation functions using a localized target. Evidence is presented that human detection of velocity vector modulations is not directly based on the acceleration signal (the temporal derivative of the velocity vector modulation). Instead, modulation detection is accurately described by a two-stage model: a low-pass temporal filter transformation of the true velocity vector modulation followed by a variance detection stage. A functional description of the first stage is a second order low-pass temporal filter having a characteristic time constant of 40 msec. In effect, the temporal low-pass filter is an integration of the velocity vector modulation within a temporal window of 100-140 msec. A non-trivial link of this low-pass filter stage to the temporal characteristics of standard motion detection mechanisms will be discussed. Velocity vector modulations are detected in the second-stage, whenever the variance of the filtered velocity vector exceeds a certain threshold variance in either the speed or direction dimension. The threshold standard deviations for this variance detection stage are estimated to be 17% for speed modulations and 9% for motion direction modulations.

Fixation, Ocular↗

PubMed1992

The two-dimensional shape of spatial interaction zones in the parafovea.

The spatial analysis of a target may be strongly degraded by the simultaneous presentation of nearby pattern elements. The present study investigated the shape and extent of the region of interaction as a function of retinal location. The stimuli consisted of 3 collinear [symbol: see text] s which were randomly oriented up ([symbol: see text]) or down ([symbol: see text]). The task was to discriminate the orientation of the middle [symbol: see text]. The retinal locations studied were at 0, 2.5, 5 and 10 degrees, on the lower vertical meridian and on the nasal halves of both the horizontal and the 45 degrees diagonal visual field meridians. The extent of the interaction region was defined as the separation between the midpoint of two adjacent [symbol: see text] s that resulted in 75% correct discrimination. The shape of the interaction region was determined by using several orientations (horizontal, vertical, left diagonal and right diagonal) for the virtual line joining the 3. [symbol: see text] s. Our results show that the size of the interaction regions varies linearly with eccentricity as does the size of a just resolved individual [symbol: see text]. However, the size of the interaction region varies much more rapidly than does the resolution threshold for an individual [symbol: see text]. The spatial interaction zones appear to be elongated radially, so that they have an elliptical shape. The size of the major axis is about 2-3 times the size of the minor axis. The major axis is along the meridian through the central visual field (i.e. it is oriented radially) while the minor axis is oriented tangentially (i.e. isoeccentrically).

Adult↗

PubMed1991

Spatiotemporal representation of moving luminance edges in human vision.

The edges of straight bars in a square-wave luminance grating appear undulating to an observer when the retinal image of this pattern is in motion. The amplitude of the perceived undulations increases linearly with retinal image speed with an average slope of 30 +/- 4 ms. The period of the motion-induced bulges is 2.5 +/- 0.5 degree and shows no consistent variation with the retinal image velocity of the pattern. The close quantitative agreement between the spatiotemporal extent of this effect and recent estimates of the spatiotemporal parameters of human motion-sensitive mechanisms suggests the existence of motion-sensitive cells in the central nervous system that have a fixed time constant but change the shape and size of their retinal support with retinal image velocity.

Humans↗

PubMed1989

Differential spatial displacement discrimination with interfering stimuli.

Differential spatial displacement discrimination thresholds were determined for a configuration of three blobs with Gaussian spatial and temporal contrast envelopes. This task is similar to the well known three-dot alignment hyperacuity task. Thresholds determined in the presence of interfering stimuli were identical to thresholds determined without these flanking stimuli. The thresholds scale linearly with stimulus size over at least two decades. We conclude that (i) the mechanisms that compute differential spatial displacement for the three-blob alignment task are not disturbed by the presence of neighbouring stimuli, even when these enter the region over which the computations are performed and (ii) at all levels of resolution similar mechanisms are used to compute differential spatial displacement.

Adult↗

PubMed1989

Visibility of blobs with a Gaussian luminance profile.

Contrast detection thresholds for circular symmetric stimuli with a Gaussian luminance profile and Gaussian temporal modulation were measured as a function of their spatial scale parameter (varied over two and a half decades) for four different presentation times. The stimulus was presented on a very large uniform background field. For sufficiently large stimuli the threshold is constant over a size variation of at least one and a half decades. When the stimulus is presented on a restricted background field, a loss of sensitivity is found at long presentation times when the stimulus size exceeds a certain fraction of the background diameter. The measurements show (i) that the visual system has a very large bandwidth and (ii) that the presence of edges or luminance transitions in the visual field can diminish the sensitivity to a stimulus considerably, even when they are rather distant. The results can be explained in a natural way with a multi-layer receptive field model.

Adult↗

PubMed1988

Local spatial scale for three-dot alignment acuity.

Three-dot alignment discrimination thresholds were determined for blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. Thresholds were determined as a function of the blur parameter of the stimuli. This was done for a range of eccentricities in the visual field (from 45 degrees nasal to 65 degrees temporal). The thresholds were corrected for variations of the stimulus extent with the blur parameter. The results were used to estimate the local spatial scale for three-dot alignment acuity. This was done by a method recently introduced by Watson (1987). It was found that the local spatial scale for three-dot alignment acuity is approximately linearly proportional to eccentricity.

Adult↗

PubMed1988

The construction of a simultaneous functional order in nervous systems. IV. The influence of physical constraints on the resulting functional order.

The signal activity in a neural net will be constrained both by its physical structure and by environmental constraints. By monitoring its signal activity a neural system can build up a simultaneous functional order that encodes these constraints. We have previously (Part I) presented two models that construct a simultaneous functional order in a collection of neural elements using either signal-covariances or signal-coincides. In this paper we present the results of simulation experiments that were performed to study the influence of the physical constraints of a neural system on the simultaneous functional order produced by both models. In the simulation experiments we used a one-dimensional detector array. We delineate the physical constraints such an array has to satisfy in order to induce a functional order relation that allows an isomorphism with a geometrical order. We show that for an appropriate choice of the system parameters both models can produce a simultaneous functional order with sufficient internal coherence to allow isomorphisms with a triangulation. In this case the dimensionality and the coherence of the detector array are objectively available to the system itself.

Environment↗

PubMed1988

The visual assessment of the spatial location of a bright bar.

Vernier acuity, three-line interval bisection and line-width discrimination experiments were performed for a target bar stimulus with an asymmetrical orthoaxial contrast profile. This was done in an attempt to identify the nature of the spatial primitives that are involved in the visual coding of spatial location. We conclude that both the zero-crossings and the centroid or extremum of the zero-bounded region of the neural activity distribution that is elicited by the presentation of the target bar, are in principle available to perception. It probably depends on the spatial characteristics of the applied stimulus pattern and the adopted strategy which features are actually used in different localization tasks.

Adult↗

PubMed1988

Effects of blur and eccentricity on differential spatial displacement discrimination.

Differential spatial displacement discrimination thresholds were determined for stimuli consisting of blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. The tasks were similar to the two-point discrimination acuity task and the three-dot alignment hyperacuity task. Thresholds were determined as a function of eccentricity along the horizontal meridian of the visual field (from 45 degrees nasal to 65 degrees temporal). The spatial spread or blur parameter of the blobs was adopted as a scale parameter. The results show that the performance of the visual system in differential spatial displacement discrimination tasks becomes progressively more homogeneous for a progressive increase in the blur parameter of the stimuli. Scaling (i) the three-blob alignment results with estimates of the cortical magnification factor and (ii) the two-blob separation discrimination results with their corresponding neural blur parameter shows an impressive isotropy and blur scale-invariance for the mechanisms mediating differential spatial displacement discrimination across the visual field. These results are interpreted in terms of a scaled sampling lattice model of the visual system, in combination with an automatic scale-selection mechanism.

Adult↗

PubMed1988

Differential spatial displacement discrimination thresholds for Gabor patches.

Differential spatial displacement discrimination thresholds were determined for a configuration of three identical Gaussian modulated patches of sinusoidal grating presented with equal orientation and at threshold luminance contrast. The patches were arranged one above the other at equal spatial intervals. The orientation of the bars of the sinusoidal grid constituting the patches was orthogonal to the axis joining the centres of the outer two patches. Thresholds were determined for displacements of the middle patch both orthogonal to and along the axis joining the outer two patches. Thresholds for both tasks were obtained as a function of both the spatial frequency of the sinusoidal grating and the spatial scale parameter of the Gaussian envelope of the patches. We found that the differential spatial displacement discrimination thresholds for both tasks are a constant fraction of the spatial scale parameter of the Gaussian envelope of the sinusoidal grating patches and are independent of the spatial frequency of the modulated grating. We conclude that the human visual system is capable of assigning a single location tag to an entire Gaussian modulated patch of sinusoidal grating. The accuracy with which the relative position of such a Gabor patch can be determined in a constant fraction of its spatial extent (spread of the Gaussian spatial envelope).

Adult↗

PubMed1987

The construction of a simultaneous functional order in nervous systems. I. Relevance of signal covariances and signal coincidences in the construction of a functional order.

We have developed two algorithms that construct a simultaneous functional order in a collection of neural elements using purely functional relations. The input of the first algorithm is a matrix describing the total of covariances of signals carried by the members of the neural collection. The second algorithm proceeds from a matrix describing a primitive inclusion relation among the members of the neural collection that can be determined from coincidences in their signal activity. From this information both algorithms compute a partial functional order in the collection of neural elements. Such an order has an objective existence for the system itself and not only for an external observer. By either merging individual neurons or recruiting previously unspecified ones the partial order is locally transformed into a lattice order. Thus, the simultaneous functional order in a nervous net may become isomorphic with a geometrical order if the system has enough internal coherence. Simulation experiments were done, both for the neuron-merging and the neuron-recruitment routines, to study the number of individuals in the resulting lattice order as a function of the number of individuals in the underlying partially ordered set.

Algorithms↗

PubMed1987

The construction of a simultaneous functional order in nervous systems. II. Computing geometrical structures.

The functional order of a collection of nervous elements is available to the system itself, as opposed to the anatomical geometrical order which exists only for external observers. It has been shown before (Part I) that covariances or coincidences in the signal activity of a neural net can be used in the construction of a simultaneous functional order in which a modality is represented as a concatenation of districts with a lattice structure. In this paper we will show how the resulting functional order in a nervous net can be related to the geometry of the underlying detector array. In particular, we will present an algorithm to construct an abstract geometrical complex from this functional order. The algebraic structure of this complex reflects the topological and geometrical structure of the underlying detector array. We will show how the activated subcomplexes of a complex can be related to segments of the detector array that are activated by the projection of a stimulus pattern. The homology of an abstract complex (and therefore of all of its subcomplexes) can be obtained from simple combinatorial operations on its coincidence scheme. Thus, both the geometry of a detector array and the topology of projections of stimulus patterns may have an objective existence for the neural system itself.

Animals↗

PubMed1987

The construction of a simultaneous functional order in nervous systems. III. The influence of environmental constraints on the resulting functional order.

In a previous paper (Part I) we introduced a model that constructs a simultaneous functional order in a set of neuronal elements by monitoring the coincidences in their signal activities (the so-called coincidence-model). The simultaneous signal activity in a neural net will be constrained both by its physical restrictions and by environmental constraints. In this paper we present the results of simulation experiments that were performed to study the influence of environmental constraints on the resulting functional order in a set of neural elements corresponding to a one-dimensional detector array. We show that the coincidence-model produces a functional order that encodes the physical constraints of the environment. Moreover, we demonstrate that the signal activity in the neural net (the "perceptions") can be related to events in the outer world. We provide some examples to demonstrate that our model may prove useful to gain insight into certain developmental disorders.

Algorithms↗

PubMed1987

Scale invariant features of differential spatial displacement discrimination.

For a configuration of three blobs, with Gaussian spatial contrast profiles, at threshold luminance contrast, differential spatial displacement discrimination thresholds were determined. The blobs were arranged one above the other, at equal spatial intervals. Thresholds were determined for displacements of the middle blob both orthogonal to and along the axis joining the outer two blobs. Thresholds for both tasks were obtained as a function of both the resolution and the separation of the blobs. The thresholds for both tasks increased linearly with decreasing resolution (increasing blur), for a constant ratio of the resolution parameter and the separation of the outer two blobs. At all levels of resolution there are two blob-separation regimes, in which different strategies are used to compute differential spatial displacements. Independent of the level of resolution, transition between those regions occurs when the separation of the outer two blobs is a constant multiple (approx. 25) of their blur parameter. We interpret these results as further evidence for a scale-invariant mechanism for differential spatial displacement computation.

Adult↗

PubMed1987

Two-point resolution near detection threshold.

Resolution thresholds were determined for a configuration of two blobs with overlapping Gaussian spatial and temporal envelopes presented near detection-threshold luminance contrast. The resolution thresholds are a constant fraction of the blur parameter of the stimuli over a range of at least two decades. There is no indication for a segmented nature of the resolution discrimination function.

Adult↗