Nam-Gyoon Kim

Learning to Perceive Structure from Motion and Neural Plasticity in Patients with Alzheimer's Disease.

Recent research has demonstrated that Alzheimers disease (AD) affects the visual sensory pathways, producing a variety of visual deficits, including the capacity to perceive structure-from-motion (SFM). Because the sensory areas of the adult brain are known to retain a large degree of plasticity, the present study was conducted to explore whether the degradation of a visual function impaired by AD can be reversed or slowed through perceptual learning. Whereas many studies directed at learning in AD examined learning capacities involving the implicit memory system, a largely preserved system in AD, the present study focused on perceptual learning involving visual deficits impaired by AD. Patients with mild or moderately severe AD (N=8 in each group) were presented with computer displays depicting SFM. Participants completed three sessions a day on three consecutive days with each session comprised of 48 trials. Displays showed eight different geometric solids rendered in three densities of a random dot texture. Participants identified the displayed object by pointing to a corresponding wooden object. Results showed impaired capacity for motion perception and SFM perception in both AD groups. However, performance of patients with mild AD improved over the nine sessions, whereas that of patients with moderate AD remained unchanged. These results suggest that the cortical circuits for SFM are still plastic in the mild AD stage.

Dynamic Occlusion and Optical Flow From Corrugated Surfaces

Dynamic occlusion (i.e., accretion and deletion of optical texture at the occluding edge) can occur under many different environmental conditions, for example, objects hidden behind other objects when viewed by a moving observer, objects moving in front of other objects, or an observer approaching a brink. Because each of these conditions may require the actor to respond differently, the actor may need to be able to differentiate these situations reliably. This study was directed at the optical pattern induced by dynamic occlusion that occurs when one locomotes over a rolling terrain (i.e., a corrugated surface). Two experiments were conducted for this purpose. Participants viewed displays simulating their translation along a corrugated surface in which surface corrugation, texture type, and texture density varied as part of experimental control. Results demonstrated that the visual system reliably extracts the global flow pattern for accurate perception of heading direction despite the presence of optical disturbances in optical flow. However, performance nearly failed in the unstructured texture displays wherein optical disturbances were less salient. Still, the results provide strong evidence that human observers are sufficiently sensitive to dynamic occlusion to extract information about heading direction.

The Effect of Retinal Eccentricity on Perceiving Collision Impacts

Three experiments examined the role of central and peripheral vision in collision detection and control of braking. Displays simulated observer movement over a ground plane toward obstacles lying in the observers path (Experiments 1 & 3) or toward a vertical 2-D plane (Experiment 2). Optical expansion was depicted under the constraint that remains constant throughout the approach. Displays employed in Experiments 1 and 2 were masked centrally (peripheral vision) or peripherally (central vision) with mask size ranging from 10° to 30° in diameter in steps of 5°. Participants responded to the optical pattern engendered by with more consistency in the peripheral vision condition than in the central vision condition. Experiment 3 further examined the peripheral advantage by masking the displayed peripheral region from 20% to 80%. Largely unaffected performance across masked areas of the periphery confirmed that the efficacy of the peripheral retina does not lie in more extensive flow vectors but in its inherent sensitivity to optical consequences engendered by . These results were compared with psychophysical findings in self-motion perception and clinical findings in patients with partial vision loss.

Loss of Sensitivity to Dynamic Occlusion in Patients with Alzheimer's Disease

The current study examined whether diminished sensitivity to dynamic occlusion in Alzheimers disease (AD) contributes to reduced capacity to recover 3D shape from motion. Young controls, age-matched elderly controls, and AD patients participated in the study. Participants watched computer simulations of an object, depicted as either transparent or opaque, rotating about the vertical axis against a background rendered in random dot texture. Six geometric solids were graphically simulated, each rendered in three texture densities, against three different levels of background texture densities. Participants identified the displayed object by pointing to the matching wooden object. Young controls were most accurate (79%), followed by elderly controls (61%) and AD patients (43%). Both control groups identified opaque objects better than transparent objects, but AD patients identified both objects equally poorly. These results demonstrate that dynamic occlusion (i.e., accretion and deletion of optical texture at the occluding edge) facilitates recovery of 3D shape from motion but such capacity is severely impaired in AD. The current results suggest the need for more research into dynamic occlusion, not only as source of information to recover 3D shape from motion, but also as a visual deficit in AD.

Schizophrenia: An Impairment in the Capacity to Perceive Affordances

Phenomenological psychopathologists conceptualize schizophrenia as a self-disorder involving profound distortions of selfhood. For James Gibson, “to perceive the world is to coperceive oneself.” If the sense of self is disturbed in individuals with schizophrenia, this could also lead to disturbances in these individuals’ ability to perceive affordances, environmental properties taken with reference to the perceiver’s action capabilities (e.g., a rigid surface affording ‘walk-on-able,’ chairs ‘sit-on-able,’ and so on). To test this hypothesis, three experiments investigated schizophrenia patients’ affordance perception. Participants were presented with a photo of a common object on the computer and then asked to judge its secondary affordance (a non-designed function) in a two-choice reaction time task in Experiment 1 and in a yes/no task in Experiment 2. Schizophrenia participants performed less accurately and more slowly than controls. To rule out visual impairment as a contributing factor, in Experiment 3, participants identified physical properties (color, shape, material composition) of the objects. Schizophrenia participants were as accurate as controls and responded faster than in the previous experiments. Results suggest that the capacity to perceive affordances is likely impaired in people with schizophrenia, although the capacity to detect the object’s physical properties is kept intact. Inability to perceive affordances, those functionally significant properties of the surrounding environment, may help explain why schizophrenia patients may appear as somewhat detached from the world.

A Binocular Information Source for Size Perception

For too long, size perception research has been guided by the size distance invariance hypothesis. Although research to validate this hypothesis has been largely inconclusive, the hypothesis has endured, perhaps in part because alternative information sources for size perception were lacking. Here, I propose a binocular information source for size perception. The model, drawing on the binocular geometry of viewing a physical extent, is expressed solely in terms of four angular measures and interocular distance, with the explicit exclusion of egocentric distance information. Thus, the proposed information source, if utilized by the binocular system, should be able to augment the few existing sources of information for size perception (e.g., familiar size, texture gradient, and horizon ratio).

How Facial Expressions of Emotion Affect Distance Perception.

Facial expressions of emotion are thought to convey expressers’ behavioral intentions, thus priming observers’ approach and avoidance tendencies appropriately. The present study examined whether detecting expressions of behavioral intent influences perceivers’ estimation of the expresser’s distance from them. Eighteen undergraduates (nine male and nine female) participated in the study. Six facial expressions were chosen on the basis of degree of threat—anger, hate (threatening expressions), shame, surprise (neutral expressions), pleasure, and joy (safe expressions). Each facial expression was presented on a tablet PC held by an assistant covered by a black drape who stood 1, 2, or 3 m away from participants. Participants performed a visual matching task to report the perceived distance. Results showed that facial expression influenced distance estimation, with faces exhibiting threatening or safe expressions judged closer than those showing neutral expressions. Females’ judgments were more likely to be influenced; but these influences largely disappeared beyond the 2 m distance. These results suggest that facial expressions of emotion (particularly threatening or safe emotions) influence others’ (especially females’) distance estimations but only within close proximity.

Perceiving Collision Impacts in Alzheimer’s Disease: The Effect of Retinal Eccentricity on Optic Flow Deficits

The present study explored whether the optic flow deficit in Alzheimer’s disease (AD) reported in the literature transfers to different types of optic flow, in particular, one that specifies collision impacts with upcoming surfaces, with a special focus on the effect of retinal eccentricity. Displays simulated observer movement over a ground plane toward obstacles lying in the observer’s path. Optical expansion was modulated by varying τ˙. The visual field was masked either centrally (peripheral vision) or peripherally (central vision) using masks ranging from 10° to 30° in diameter in steps of 10°. Participants were asked to indicate whether their approach would result in “collision” or “no collision” with the obstacles. Results showed that AD patients’ sensitivity to τ˙ was severely compromised, not only for central vision but also for peripheral vision, compared to age- and education-matched elderly controls. The results demonstrated that AD patients’ optic flow deficit is not limited to radial optic flow but includes also the optical pattern engendered by τ˙. Further deterioration in the capacity to extract τ˙ to determine potential collisions in conjunction with the inability to extract heading information from radial optic flow would exacerbate AD patients’ difficulties in navigation and visuospatial orientation.

Oculomotor Effects in the Size-Distance Paradox and the Moon Illusion

Drawing on 2 concepts—the resting position of the eyes and a binocular geometry for perceived size, the moon illusion is explained as the consequence of different oculomotor adjustments caused by change in the direction of gaze contingent upon the viewing conditions of the moon. Hence, each particular moon will be viewed with a different vergence state which, in turn, yields a different amount of binocular disparity. The vergence state will determine the perceived size of an object whereas disparity will determine its perceived distance. It is further contended that the perceived size of the moon is based on a new binocular information source for size perception enabling the size of an object to be perceived even in the absence of egocentric distance information. Discussion focuses on the paradoxical aspect of the moon illusion and how the size-distance invariance hypothesis may have contributed to its effect.

Independence of Size and Distance in Binocular Vision

For too long, the size distance invariance hypothesis (SDIH) has been the prevalent explanation for size perception. Despite inconclusive evidence, the SDIH has endured, primarily due to lack of suitable information sources for size perception. Because it was derived using the geometry of monocular viewing, another issue is whether the SDIH can encompass binocular vision. A possible alternative to SDIH now exists. The binocular source of size information proposed by Kim (2017) provides metric information about an object’s size. Comprised of four angular measures and the interpupillary distance (IPD), with the explicit exclusion of egocentric distance information, Kim’s binocular variable demands independence of perceived size and perceived distance, whereas the SDIH assumes interdependence of the two percepts. The validity of Kim’s proposed information source was tested in three experiments in which participants viewed a virtual object stereoscopically then judged its size and distance. In Experiments 1 and 2, participants’ size judgments were more accurate and less biased than their distance judgments, a finding further reinforced by the results of partial correlation analyses, demonstrating that perceived (stereoscopic) size and distance are independent, rather than interdependent as the SDIH assumes. Experiment 3 manipulated participants’ IPDs, one component of Kim’s proposed variable. Size and distance judgments were overestimated under a diminished IPD, but underestimated under an enlarged IPD, a result consistent with predictions based on participants’ utilization of the proposed information source. Results provide unequivocal evidence against the SDIH as an account of size perception and corroborate the utility of Kim’s proposed variable as a viable alternative for the binocular visual system.