Biomedical subjects

Joseph S Lappin

Publications and source records attributed to Joseph S Lappin.

10 recordsLinked to original sources

PubMed2006

Weakened center-surround interactions in visual motion processing in schizophrenia.

Schizophrenia is often accompanied by a range of visual perception deficits, with many involving impairments in motion perception. The presence of perceptual abnormalities may impair neural processes that depend on normal visual analysis, which in turn may affect overall functioning in dynamic visual environments. Here, we examine the integrity of suppressive center-surround mechanisms in motion perception of schizophrenic patients. Center-surround suppression has been implicated in a range of visual functions, including figure-ground segregation and pursuit eye movements, visual functions that are impaired in schizophrenia. In control subjects, evidence of center-surround suppression is found in a reduced ability to perceive motion of a high-contrast stimulus as its size increases. This counterintuitive finding is likely a perceptual correlate of center-surround mechanisms in cortical area MT. We now show that schizophrenic patients exhibit abnormally weak center-surround suppression in motion, an abnormality that is most pronounced in patients with severe negative symptoms. Interestingly, patients with the weakest surround suppression outperformed control subjects in motion discriminations of large high-contrast stimuli. This enhanced motion perception of large high-contrast stimuli is consistent with an MT abnormality in schizophrenia and has a potential to disrupt smooth pursuit eye movements and other visual functions that depend on unimpaired center-surround interactions in motion.

Adult↗

PubMed2006

Fine temporal properties of center-surround interactions in motion revealed by reverse correlation.

Center-surround interactions are a key property of visual motion mechanisms. Using a temporal reverse correlation method with human observers, we investigated perceptual interactions between a brief center motion (approximately 20 ms) and a surround that moved up-down with a new direction chosen randomly every 5 ms. The aim was to reveal interactions between center and surround motions and their dependency on relative direction, contrast, and timing. Hypothesizing that surround computation involves different neural circuitry than the center response, we manipulated surround contrast to affect the relative timing of center and surround signals. The reverse correlation analysis yielded temporal profiles of surround influence indicating, in 5 ms steps, the time course of the effect of the surround on the discriminability of center motion. The resulting temporal profiles varied systematically with contrast: as surround contrast decreased, both the latency and duration of its influence increased. This finding, consistent with longer and variable neural response latencies at low contrast, psychophysically reveals fine-scale temporal interactions between center and surround signals. Additionally, the strength of surround influence was correlated with psychophysical thresholds for discriminating center motion. The directionality of this relationship, however, depended only on center contrast. When center motion was high contrast, poor direction discrimination was associated with an increased probability of same-direction surround motions. Low-contrast center motion, however, was more discriminable when surrounded by motion in the same direction, regardless of surround contrast. This suggests that the previously reported adaptive nature of center-surround interactions in motion is driven primarily by the visibility of the center motion signals.

Contrast Sensitivity↗

PubMed2006

Environmental context influences visually perceived distance.

What properties determine visually perceived space? We discovered that the perceived relative distances of familiar objects in natural settings depended in unexpected ways onthe surrounding visual field. Observers bisected egocentric distances in a lobby, in a hallway, and on an open lawn. Three key findings were the following: (1) Perceived midpoints were too far from the observer, which is the opposite of the common foreshortening effect. (2) This antiforeshortening constant error depended on the environmental setting--greatest in the lobby and hall but nonsignificant on the lawn. (3) Context also affected distance discrimination; variability was greater in the hall than in the lobby or on the lawn. A second experiment replicated these findings, using a method of constant stimuli. Evidently, both the accuracy and the precision of perceived distance depend on subtle properties of the surrounding environment.

Adult↗

PubMed2005

Optimal size for perceiving motion decreases with contrast.

Visual patterns have widely varying contrasts and elicit local signals of varying reliability, ranging from noisy to relatively noise-free. One way to deal efficiently with the variable visual input is to employ flexible neural mechanisms that adapt to changing conditions. We investigated whether the spatial properties of motion mechanisms change with stimulus contrast and found that the optimal size for perceiving motion decreases with increasing contrast. These data were well-described by a model in which spatial summation increases with decreasing contrast.

Computer Graphics↗

PubMed2004

The visual perception of length along intrinsically curved surfaces.

The ability of observers to perceive three-dimensional (3-D) distances or lengths along intrinsically curved surfaces was investigated in three experiments. Three physically curved surfaces were used: convex and/or concave hemispheres (Experiments 1 and 3) and a hyperbolic paraboloid (Experiment 2). The first two experiments employed a visual length-matching task, but in the final experiment the observers estimated the surface lengths motorically by varying the separation between their two index fingers. In general, the observers' judgments of surface length in both tasks (perceptual vs. motoric matching) were very precise but were not necessarily accurate. Large individual differences (overestimation, underestimation, etc.) in the perception of length occurred. There were also significant effects of viewing distance, type of surface, and orientation of the spatial intervals on the observers' judgments of surface length. The individual differences and failures of perceptual constancy that were obtained indicate that there is no single relationship between physical and perceived distances on 3-D surfaces that is consistent across observers.

Depth Perception↗

PubMed2003

Perceptual consequences of centre-surround antagonism in visual motion processing.

Centre-surround receptive field organization is a ubiquitous property in mammalian visual systems, presumably tailored for extracting image features that are differentially distributed over space. In visual motion, this is evident as antagonistic interactions between centre and surround regions of the receptive fields of many direction-selective neurons in visual cortex. In a series of psychophysical experiments we make the counterintuitive observation that increasing the size of a high-contrast moving pattern renders its direction of motion more difficult to perceive and reduces its effectiveness as an adaptation stimulus. We propose that this is a perceptual correlate of centre-surround antagonism, possibly within a population of neurons in the middle temporal visual area. The spatial antagonism of motion signals observed at high contrast gives way to spatial summation as contrast decreases. Evidently, integration of motion signals over space depends crucially on the visibility of those signals, thereby allowing the visual system to register motion information efficiently and adaptively.

Contrast Sensitivity↗

PubMed2003

Exocentric pointing to opposite targets.

We use an exocentric pointing task to study exocentric visual directions to targets that are opposite to a pointer relative to the observer. (The apparent distance between the target and the pointer always exceeded 90 degrees of visual angle.) All pointing takes place in the horizontal plane at eye height. Observers could not see both target and pointer at a single glance. They had to look back and forth between them, using combinations of eye movements, head turns, twists at the waist and turning on the feet. In the limit of diametrically opposite targets we find that the observers pick either one of two distinct orientations of the pointer as equally "visually correct". Which one results depends on the stance assumed by the observer. The difference between the two equally acceptable pointings is between 5 degrees and 10 degrees. Such a result is predicted from earlier measurements in the context of a model that describes the geometry of the horizon as a Riemannian space with varying intrinsic curvature. The present results thus fit--perhaps surprisingly--very well in such a picture.

Eye Movements↗

PubMed2002

Visual coherence of moving and stationary image changes.

Detection thresholds were compared for moving and stationary oscillations with equivalent contrast changes. Motion was more detectable than stationary oscillation, and the difference increased with size of the feature (a Gaussian blob). Phase discriminations between a center and two flanking features were much better for motion than for stationary oscillation. Motion phase discriminations were similar to motion detection and were robust over increases in spatial separation and temporal frequency, but not so for stationary oscillations. Separate visual motion signals were positively correlated, but visual signals for stationary oscillation were negatively correlated. Evidently, motion produces visually coherent changes in image structure, but stationary contrast oscillation does not.

Contrast Sensitivity↗

PubMed2002

What constitutes an efficient reference frame for vision?

Vision requires a reference frame. To what extent does this reference frame depend on the structure of the visual input, rather than just on retinal landmarks? This question is particularly relevant to the perception of dynamic scenes, when keeping track of external motion relative to the retina is difficult. We tested human subjects' ability to discriminate the motion and temporal coherence of changing elements that were embedded in global patterns and whose perceptual organization was manipulated in a way that caused only minor changes to the retinal image. Coherence discriminations were always better when local elements were perceived to be organized as a global moving form than when they were perceived to be unorganized, individually moving entities. Our results indicate that perceived form influences the neural representation of its component features, and from this, we propose a new method for studying perceptual organization.

Discrimination, Psychological↗

PubMed2002

Large-scale visual frontoparallels under full-cue conditions.

We determined the curvature of apparent frontoparallels in a natural scene (a large lawn in broad daylight). Data on frontoparallels in these conditions are very sparse and reveal idiosyncratic curvatures of frontoparallels and irregular variation with distance. We used a method of bisection of linear segments indicated through pairs of stakes at angular separations (from the vantage point) of up to 120 deg. Distances of 2 m and 10 m (in the forward direction) were used. The bisection was carried out by the observer through maneuvering a radio-controlled vehicle carrying a third stake. Four observers participated in the experiment; they had no problems with the task and yielded mutually consistent results. We found that the frontoparallels are significantly curved and are concave towards the observer. Surprisingly, the sign of the curvature is opposite to that found when the frontoparallels are defined through an exocentric pointing task. Available theory (Luneburg's) does not predict this, but the theory is hardly applicable to the case of vision in natural scenes. This interesting discrepancy has not been reported before.

Cues↗