Search PubMed⌕ Search

Biomedical subjects

T Caelli

Publications and source records attributed to T Caelli.

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

Some task and signal dependent rules for spatial vision.

In this paper we consider the types of computational processes which may be involved in solving a variety of perceptual problems from the detection of signals in the presence of others, to texture discrimination, and some aspects of pattern recognition. These processes centre around the involvement of correlational computations, the transduction of their input/output values, and the apparent involvement of selective filtering mechanisms. Our results suggest that even if fixed detectors (in tuning characteristics) are involved in low-level vision, the human observer apparently employs much more adaptive (variable tuning characteristic) filters and nonlinear mechanisms in more complex spatial tasks.

Humans↗
PubMed1987

Localization of signals in images.

We have observed the ability of observers to locate signals embedded in white noise and other nonwhite and nonstationary images. Performance was found to be consistent with that predicted by a cross correlator that takes into account the variability of the background image (normalization) and that assumes a degree of uncertainty as to the precise location of the signal.

Form Perception↗
PubMed1987

The concept of spatial frequency channels cannot explain some visual masking effects.

Visual masking is assumed to be predictable by the degree to which mask and signal spatial frequency amplitudes are similar. In this masking experiment we have clearly shown this not to be the case by studying the effects of factorial combinations of signal-mask amplitude and phase spectra. Results show that the amplitude spectra characteristics do not predict masking, and a better predictor of these results appears to be the correlations between the mask and signal's luminance profiles. These results, therefore, show that the spatial similarity between two images, as may be processed by "spatial frequency channels" cannot be determined by the similarities between these channel outputs as defined by the (output) modulated amplitudes.

Humans↗
PubMed1986

On the extraction and alignment of image edges.

In this paper the types of image filtering, segmentation and correlational processes which are consistent with the ability of human observers to align edges in arbitrary two-dimensional images are considered. Edge-only versions of a number of images were computed by pre-filtering, segmenting and using laplacians to extract luminance boundaries. The matching of such edge-only images with the originals seems well predicted by a (derivative) matched filtering (cross-correlation) process showing some bias toward edges contained within the middle frequency range of the images used.

Form Perception↗
PubMed1986

Cross-correlation model for pattern acuity.

We have investigated whether the perceptual alignment of two images can be predicted from their cross-correlation characteristics. Results from two experiments demonstrate this to be the case with a variety of images varying from Gaussian-modulated sinusoid gratings to two-dimensional textures and face images.

Data Display↗
PubMed1986

On the discrimination of micropatterns and textures.

The Perceptron-type of approach of texture perception argues for decomposition of the patterns by detectors or filters which can interact to result in classification of spatial information. We consider a case of texture discrimination where textures composed of discriminable grey-level micropatterns may or may not be distinguished. Two types of models involving linear pattern decomposition are shown to be insufficient for explaining such results. This suggests that observers encode the signals by means of signal dependent or adaptive feature extractors. Such a concept of texture perception is related to connectionist ideas of information processing as pursued by Julesz in his texton theory.

Animals↗
PubMed1985

Amblyopic processing of positional information. Part II: Sensitivity to phase distortion.

Amblyopic sensitivities to spatial distortions of compound gratings and checker-board textures were studied. Iso-energy target signals were used by employing phase modulations which leave the stimulus power spectra unaltered. The non-amblyopic eyes of the 6 amplyopes tested showed supranormal sensitivities at 1 s exposure duration for both types of patterns. At 125 ms the amblyopic eyes were found virtually blind to the texture distortions while performance improved with prolonging the presentation time to 1 s. Comparable results were obtained with compound gratings. However, the correlation between amblyopic acuities and texture discrimination was higher than that between acuities and grating performance. This is probably due to the fact that both (misaligned) vernier targets and the texture are more 'natural' stimuli in the sense that they contain a broad range of spatial frequency components at many different orientations. Differences in behaviour between strabismic and anisometropic amblyopes did not occur. This suggests that the results depend upon defects of foveal visual function.

Amblyopia↗
PubMed1985

On the detection of Gabor signals and discrimination of Gabor textures.

We have explored the detection and discrimination of Gabor signals in two different paradigms in order to determine the signals' relative importance for spatial vision. In the first experiment we determined the "detection efficiency" of five such signals presented in isolation, and our results support the conclusion that, for optimal detection, the Gaussian profile should only span (to 1/e decay values) approximately two cycles. In three subsequent experiments we have observed the discriminability of textures composed of such signals over a range of spatial frequencies, Gaussian windows and orientation differences. Our results are consistent with the detection data only insofar as the discrimination thresholds vary in a predictable way with the Gaussian window parameters. Finally, we have determined the degree to which texture discrimination, in terms of differences in both orientation and spatial frequency, can be predicted from the independent manipulations of both codes within a given Gaussian window size.

Discrimination, Psychological↗
PubMed1985

What is perceived when two images are combined?

When two images are combined three perceptual outcomes are possible. One, they can be perceived as they are--separate and independent images. Two, they can become perceptually fused into a new image. Three, one image may dominate, or mask, the other. These possibilities are demonstrated with a variety of images and it is proposed that it is their spatial correlation rather than their spatial frequency similarities or differences which is critical in determining the particular outcome.

Form Perception↗
PubMed1985

Three processing characteristics of visual texture segmentation.

Three mechanisms are outlined which are sufficient to determine texture segmentation or discrimination. They are: (1) convolution of detector profiles with the input image; (2) impletion, where the perceptual 'filling in' of the input surface occurs via a nonlinear filtering operation on each detector's output (3) grouping, where areas are segregated according to their differences in detector responses after impletion occurs. These mechanisms are compared with those proposed to occur in human visual texture discrimination.

Form Perception↗
PubMed1984

On the number of intensity levels discriminated in textures.

The number of intensity levels detected in briefly displayed textures was studied as a function of two related spatial parameters: granularity and spatial frequency. A texture-discrimination paradigm was used to determine the discriminability of an original texture and of a version of it reduced (further "quantized') in intensity levels over a range of texture granularities and frequency bandwidths. Results show specific sensitivities to intensity level reductions as a function of the spatial-frequency composition of the textures, with particular loss of sensitivity in the higher spatial-frequency regions.

Discrimination, Psychological↗
PubMed1984

The waggon-wheel effect.

The waggon-wheel effect was studied by use of three wheels with different numbers of spokes (4, 8, 16) and a wide range of strobe temporal frequencies. The results obtained are discussed in terms of a model in which: (i) nearest-neighbour relationships predict the direction and speed of movement, (ii) persistence and masking occur over ranges consistent with values reported in the literature, (iii) apparent motion is generated between currently illuminated spokes and persisting images of spokes, (iv) duration of spoke illumination (sweep) is a determining factor and, (v) a top-down process finds the best fit (a sort of 'simple structure') for the complex spatiotemporal display.

Humans↗
PubMed1983

On the efficient two-dimensional energy coding characteristics of spatial vision.

Recent evidence indicates that the vertebrate visual system possesses units whose receptive fields may be represented as narrow-band orientation and frequency specific filters. It has been indirectly demonstrated that equivalent psychophysical units act in such a way as to not discriminate the individual frequency components within each frequency band. Here we directly demonstrate this to be the case with digital images and so show an efficient energy code for spatial vision.

Discrimination, Psychological↗
PubMed1983

Discrimination thresholds in the two-dimensional spatial frequency domain.

In two experiments we have determined the discriminability between two sinusoidal gratings as a function of orientation and spatial frequency differences. Twelve orientation (15 degrees steps) and four spatial frequencies (2, 4, 8, 12 c/deg) were considered and corresponding discrimination thresholds were determined. Results indicated that: (a) spatial frequency discrimination thresholds did not significantly vary over all frequency and orientation positions, and averaged at +/- 1/8 octave limits. (b) Orientation thresholds only slightly increased with spatial frequency (from +/- 5 degrees at 2 c/deg to +/- 6 degrees at 12 c/deg). (c) A large and consistent anisotropy occurred with orientation thresholds showing most sensitivity in the horizontal and vertical directions, with largest thresholds at the oblique angles. No oblique effect was observed with spatial frequency thresholds. These results confirm past observations, providing an extensive set of threshold measurements in the two-dimensional spatial domain.

Discrimination, Psychological↗