An introduction to object recognition.

In this report we present a general introduction to object recognition. We begin with brief discussions of the terminology used in the object recognition literature and the psychophysical tasks that are used to investigate object recognition. We then discuss models of shape representation. We dispense with the idea that shape representations are like the 3-D models used in computer aided design and explore instead models of shape representation that are based on future descriptions. As these descriptions encode only the features that are visible from a particular viewpoint, they are generally viewpoint-specific. We discuss various means of achieving viewpoint-invariant recognition using such descriptions, including reliance on diagnostic features visible from a wide range of viewpoints, storage of multiple descriptions for each object, and the use of transformation mechanisms. Finally, we discuss how differences in viewpoint dependence that are often observed for within-category and between-category recognition tasks could be due to differences in the types of features that are naturally available to distinguish among different objects in these tasks.

The contribution of qualitative and quantitative shape features to object recognition across changes of view.

Two experiments investigated the influence of qualitative and quantitative shape features on recognition of novel, four-component objects. Quantitatively different objects had different connection angles between the components. Qualitatively different objects had different connection angles and differently shaped components in some of the four positions. Old-new recognition declined less with changes of view for qualitatively different objects (Experiment 1). However, recognition of these objects was made to decline sharply with changes of view if subjects were biased to attend to the connection angles rather than the component shapes (Experiment 2), suggesting that the influence of different features depends on visual experience with those features. These results favor a feature-based model of shape representation that utilizes multiple feature types and that can rely on different features depending on particulars of the objects and the task.

The relationship of vertical and horizontal velocity gradients in the perception of shape, rotation, and rigidity.

Perceived shape, rotation, and rigidity were investigated in displays with linear velocity gradients in the vertical and horizontal directions. Different temporal relationships between these gradients simulated perspective projections of frontally oriented or rotated translating dihedral angles, orthographic projections of rotating dihedral angles, or nonrigid motion. Displays with nonzero horizontal gradients were judged to represent greater angle magnitudes (less relative depth) than displays with 0 horizontal gradients. The temporal relationship between the vertical and horizontal gradients did not influence judged shape but did affect rotation and rigidity judgments; rigid rotations were judged to rotate most, and nonrigid displays were rated as least rigid. These results indicate that the visual system integrates information from more than 1 velocity field. Possible integration methods based on first-order optic flow analysis are discussed.

Mild traumatic brain injury does not produce post-traumatic stress disorder.

It has been widely assumed that patients who sustain mild traumatic brain injury (MTBI) or post-concussive syndrome develop post-traumatic stress disorder (PTSD) in response to their cognitive difficulties, diminished coping skills, or other losses. This study examined 70 patients who had previously been diagnosed as having either PTSD or MTBI. Each patient was asked to provide a highly detailed chronological history of the events which preceded, followed, and occurred during the traumatic event, to indicate whether they were rendered unconscious or had amnesia for the event, and to describe the various symptoms they developed. All (100.0%) of the PTSD patients were able to provide a highly detailed and emotionally charged recollection of the events which occurred within 15 minutes of the traumatic event in comparison to none (0.0%) of the MTBI patients. None of the MTBI patients reported symptoms such as intrusive recollections of the traumatic event, nightmares, hypervigilance, phobic or startle reactions, or became upset when they were asked to describe the traumatic event or were exposed to stimuli associated with it. These data suggest that PTSD and MTBI are two mutually exclusive disorders, and that it is highly unlikely that MTBI patients develop PTSD symptoms. Furthermore, these findings suggest that clinicians should exercise considerable caution in ruling out PTSD prior to making the diagnosis of MTBI.

Recovery of function following severe traumatic brain injury: a retrospective 10-year follow-up.

It has been widely assumed that most of the recovery following severe traumatic brain injury (TBI) occurs within the first 6 months, and that virtually all of the recovery occurs within the first 1-2 years post-injury. In an effort to evaluate the long-term recovery of patients who had sustained severe TBI, we interviewed the relatives and significant others of 20 patients who had sustained TBI at least 5 years earlier, using a modified version of the Portland Adaptability Inventory. Retrospective ratings were collected to evaluate the patients' psychosocial, cognitive, physical, and emotional status prior to their injury, and at 1, 2, 5, and an average of 10.3 years post-injury. The results indicated that TBI patients exhibit significant improvements in their social, cognitive, physical, and emotional functioning after 2 years post-injury regardless of the severity of their initial brain trauma. These data suggest that patients who sustain severe TBI continue to make gradual improvements in their functioning for at least 10 years post-injury. Our findings contradict the widely held assumption that the recovery process ends after 1 or 2 years post-injury.

Detection of one versus two objects in structure from motion.

The ability of subjects to detect whether a structure-from-motion display depicts one or two rigid objects was examined in the presence or the absence of noise points. Each object was composed of a set of points chosen randomly within the volume of a sphere. The objects rotated rigidly about different axes passing through the center of the sphere. For displays without noise points, detection increased with larger angles between the rotation axes and with more points in each object. For displays in which noise points were present, detection was above chance but, in general, worse than that for displays without noise points. The implications of these results for image segmentation in complex motion patterns is discussed.

Recovering three-dimensional shape from perspective translations and orthographic rotations.

Perceived orientation in depth and 3D shape was investigated for perspective projections of translations and orthographic projections of rotations of 3D dihedral angles. The principal findings were that (a) perceived orientation in depth depends on the sign of the velocity gradient, even in the case of orthographic projections; (b) the relationship between perceived orientation and the sign of the velocity gradient is greater for shallower gradients in orthographic projections of rotations, consistent with previous findings for perspective translations; (c) the magnitudes of simulated dihedral angles were underestimated (relative depth overestimated) for orthographic projections of rotations but were overestimated for perspective projections of translations; and (d) the judged magnitude of the dihedral angle depends on the velocity ratio and on image compression; it cannot be predicted from the velocity ratio or the velocity gradient alone.

Inferring structure from motion in two-view and multiview displays.

Five experiments were conducted to examine constraints used to interpret structure-from-motion displays. Theoretically, two orthographic views of four or more points in rigid motion yield a one-parameter family of rigid three-dimensional (3-D) interpretations. Additional views yield a unique rigid interpretation. Subjects viewed two-view and thirty-view displays of five-point objects in apparent motion. The subjects selected the best 3-D interpretation from a set of 89 compatible alternatives (experiments 1-3) or judged depth directly (experiment 4). In both cases the judged depth increased when relative image motion increased, even when the increased motion was due to increased simulation rotation. Subjects also judged rotation to be greater when either simulated depth or simulated rotation increased (experiment 4). The results are consistent with a heuristic analysis in which perceived depth is determined by relative motion.

Evidence that the substantia nigra is a component of the endogenous pain suppression system in the rat.

The present study sought to determine whether opiate receptors in the substantia nigra may mediate antinociception produced by systemic morphine. Bilateral intranigral microinjection of naloxone-HCl (0.3-10 micrograms) suppressed the antinociceptive effects of systemically administered morphine sulfate (5 mg/kg, s.c.) on the tail-flick and hot-plate tests in a dose-related manner. Injection of naloxone (3 micrograms) into the ventral tegmental area did not alter antinociception produced by systemic morphine (5 mg/kg, s.c.). These findings support the argument that the substantia nigra is an essential, and previously unrecognized, component of the endogenous pain suppression system.