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Biomedical subjects

T Caelli

Publications and source records attributed to T Caelli.

At least 19 recordsLinked to original sources
PubMed1989

Perceptual spatial frequency--orientation surface: psychophysics and line element theory.

We have investigated the discriminability of gratings which simultaneously vary in spatial frequency and orientation. Thirteen and nine reference gratings were used with two observers, and bivariate discrimination probability surfaces were determined around each grating. These data were then fitted to a general bivariate Gaussian function. The results clearly demonstrate local separability in this log frequency and orientation discrimination domain. Our results also show that the factor contributing most to the non-Euclidean nature of such frequency/orientation discrimination is orientation anisotropia, although we also find some evidence for smaller changes in the associated Riemannian line-element at different frequency ranges. These results cast doubt upon claims for a "pseudo line-element" for frequency discrimination based upon the nonlinear outputs of a fixed set of detectors.

Discrimination, Psychological↗
PubMed1988

On the discrimination of compound Gabor signals and textures.

We compared the discrimination of compound Gabor signals when presented pairwise and in isolation to that of textures consisting of an array of such signals. Textures composed of mirror-image compound Gabor signals were found indistinguishable even when the individual micropatterns were easily discriminated. No such blindness exists for textures composed of non-mirror-image compound Gabor signals. These results are analogous to earlier findings concerning the foveal and extrafoveal rapid discrimination of compound gratings [Rentschler and Treutwein, Nature 313, 308-310 (1985)]. This suggests that both texture perception and extrafoveal vision are, under the conditions of the given experiments, characterized by the absence of neural interactions that otherwise allow the analysis of form. Our results do also imply that the texton concept of Julesz can be extended to grey-level Gabor textures, irrespectively of whether the space domain (geometric features) or the spatial frequency domain (local spectral characteristics) representations of the micropatterns are referred to.

Discrimination, Psychological↗
PubMed1987

Visual pattern recognition in humans. I. Evidence for adaptive filtering.

We have investigated how observers learn to classify compound Gabor signals as a function of their differentiating frequency components. Performance appears to be consistent with decision processes based upon the least squares minimum distance classifier (LSMDC) operating over a cartesian feature space consisting of the real (even) and imaginary (odd) components of the signals. The LSMDC model assumes observers form prototype signals, or adaptive filters, for each signal class in the learning phase, and classify as a function of their degree of match to each prototype. The underlying matching process can be modelled in terms of cross-correlation between prototype images and the input sample.

Form Perception↗
PubMed1987

Is pattern masking predicted by the cross-correlation between signal and mask?

We sought to determine whether the cross-correlation between signals and masks consisting of Gaussian modulated grating patterns is consistent with observed psychophysical masking effects. Our experimental results support this relationship, and suggest further specifications for the determinants of pattern masking effects.

Form Perception↗
PubMed1987

On the detection of signals in non-white noise.

We have studied the detection, by human observers, of suprathreshold bandlimited signals embedded at various locations in non-white, Gaussian filtered noise. Detection models based upon the direct cross-correlation between the signal and the noise image (matched filtering) cannot account for the results of our experiments. Our findings point instead at a detection process occurring at the level of signal decomposition, and jointly determined by: (a) the differential outputs of discrete, bandlimited spatial analyzers selectively responsive to different components of the signal; and (b) variable detection rules adaptively related to such outputs and to the type of signal information available to the observer.

Humans↗
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↗