PubMed Health⌕ Search

Biomedical subjects

P Perona

Publications and source records attributed to P Perona.

12 recordsLinked to original sources
PubMed1999

Image recognition: visual grouping, recognition, and learning.

Vision extracts useful information from images. Reconstructing the three-dimensional structure of our environment and recognizing the objects that populate it are among the most important functions of our visual system. Computer vision researchers study the computational principles of vision and aim at designing algorithms that reproduce these functions. Vision is difficult: the same scene may give rise to very different images depending on illumination and viewpoint. Typically, an astronomical number of hypotheses exist that in principle have to be analyzed to infer a correct scene description. Moreover, image information might be extracted at different levels of spatial and logical resolution dependent on the image processing task. Knowledge of the world allows the visual system to limit the amount of ambiguity and to greatly simplify visual computations. We discuss how simple properties of the world are captured by the Gestalt rules of grouping, how the visual system may learn and organize models of objects for recognition, and how one may control the complexity of the description that the visual system computes.

Humans↗
PubMed1998

Where is the sun?

Explore the source record for details and available documents.

Functional Laterality↗
PubMed1997

Shading and stereo in early perception of shape and reflectance.

Recent experiments involving shaded 2-D stimuli have shown that early-vision mechanisms are capable of interpreting 3-D shape from shading. In particular, target discrimination tasks suggest that a target 'pops out' when background distractors, but not the target, can be interpreted as convex and lit from above or top-left. Since the problem of extracting 3-D shape from shading is intrinsically ill-defined, early vision may need to make these twin assumptions of convexity and top-left lighting in order to constrain the problem. Would these assumptions be recognized as unnecessary and consequently discarded when 3-D shape could be unambiguously defined by some other cue, like stereo disparity? A 2AFC stimulus onset asynchrony paradigm with masking was used in target discrimination experiments. The performance of five naive subjects on tasks where only shading cues were present was compared with that on tasks involving shading as well as stereo cues that define shape unambiguously. The results show that although stereo disparity information is incorporated by early-vision 3-D mechanisms, it is not used to overturn the default assumptions of lighting and shape. Stereo information is interpreted within the framework of top-left lighting, and a consistent preference for convexity is seen over concavity.

Contrast Sensitivity↗
PubMed1996

Early computation of shape and reflectance in the visual system.

A compelling sense of three-dimensional shape may be conveyed by the photograph of an object. Cues such as contour, shading, perspective and occlusion, to name a few, contribute to this percept. Psychophysical experiments suggest that certain aspects of three-dimensional shape are computed rapidly and in parallel by the visual system. Here we report that reflectance is also computed rapidly; moreover, it is the apparent reflectance, rather than brightness or perceptual three-dimensional shape, that is the primary basis for discrimination during the early stages of visual processing.

Humans↗
PubMed1996

Preattentive perception of elementary three-dimensional shapes.

Experiments in which a single target pattern is discriminated from multiple background distractors show that certain shaded, two-dimensional (2-D) stimuli consistent with a top-lit, polyhedral interpretation can be processed fast (< 80 msec) and in parallel. Unshaded line drawings of the same shapes, however, are processed serially. Strong pop-out asymmetries and control experiments involving shaded patterns that do not have familiar 3-D interpretations suggest that such fast, parallel processing is dependent upon perception 3-D shape. Furthermore, this process can be influenced by contextual scene information, in a manner that is dependent upon whether the additional cues contribute to the perception of a consistent 3-D scene.

Adolescent↗
PubMed1991

Loading of the contralateral foot in peripheral vascular insufficiency below-knee amputees.

Seven ambulatory peripheral vascular insufficiency below-the-knee amputees underwent gait analysis to determine the local pressures on seven prominent plantar regions in their remaining limb. Patients occupied significantly less stance phase time on their remaining limbs. Peak, mean, and total pressures measured at each of the seven prominent areas was decreased as compared with laboratory age and sex matched controls. The results of this study suggest that the risk of contralateral limb amputation surgery is related to systemic peripheral vascular insufficiency or local factors, and not increased loading of the remaining contralateral "limb-at-risk."

Aged↗
PubMed1990

Preattentive texture discrimination with early vision mechanisms.

We present a model of human preattentive texture perception. This model consists of three stages: (1) convolution of the image with a bank of even-symmetric linear filters followed by half-wave rectification to give a set of responses modeling outputs of V1 simple cells, (2) inhibition, localized in space, within and among the neural-response profiles that results in the suppression of weak responses when there are strong responses at the same or nearby locations, and (3) texture-boundary detection by using wide odd-symmetric mechanisms. Our model can predict the salience of texture boundaries in any arbitrary gray-scale image. A computer implementation of this model has been tested on many of the classic stimuli from psychophysical literature. Quantitative predictions of the degree of discriminability of different texture pairs match well with experimental measurements of discriminability in human observers.

Algorithms↗
PubMed1979

[Breast pneumocystography. 96 tentative cases].

Successful execution of mammary pneumocystography in 87-96 attempts is reported. A brief description of the method and a statistical assessment of the incidence and features of the cysts observed are followed by the expression of a very favourably judgment on the soundness of the technique, and the presentation of several of the more meaningful cases.

Adult↗
PubMed1979

[Facial telethermography in the diagnosis of carotid insufficiency in the extracranial sector].

The usefulness of facial telethermography in the diagnosis of circulatory insufficiency owing to occlusion of the extracranial vessels is discussed. The method has proved of great assistance in the screening of patients with vascular diseases, and in the choice of a subsequent contrastographic examination. It gave a significant number of positive findings in cases of insufficiency of the internal carotid artery. The number of false positives and false negatives was virtually negligible.

Arterial Occlusive Diseases↗
PubMedDate not supplied

[The diagnostic and therapeutic validity of pneumoradiography of breast cysts in the light of personal experience with 236 cases].

Personal experience of 236 breast cyst aspirations is reported. In 224 cases, insufflation was also successful and pneumocystomammography was carried out, associated always with cytological examination of the aspirated fluid. In 2 cases, the examination made possible a diagnosis of cyst degeneration (0.8%). The number of recurrences was minimal (2.8%). It is concluded that the technique is diagnostically and therapeutically useful.

Adult↗