Hallucinations in schizophrenia, sensory impairment, and brain disease: a unifying model

Based on recent insight into the thalamocortical system and its role in perception and conscious experience, a unified pathophysiological framework for hallucinations in neurological and psychiatric conditions is proposed, which integrates previously unrelated neurobiological and psychological findings. Gamma-frequency rhythms of discharge activity from thalamic and cortical neurons are facilitated by cholinergic arousal and resonate in networks of thalamocortical circuits, thereby transiently forming assemblies of coherent gamma oscillations under constraints of afferent sensory input and prefrontal attentional mechanisms. If perception is based on synchronisation of intrinsic gamma activity in the thalamocortical system, then sensory input to specific thalamic nuclei may merely play a constraining role. Hallucinations can be regarded as underconstrained perceptions that arise when the impact of sensory input on activation of thalamocortical circuits and synchronisation of thalamocortical gamma activity is reduced. In conditions that are accompanied by hallucinations, factors such as cortical hyperexcitability, cortical attentional mechanisms, hyperarousal, increased noise in specific thalamic nuclei, and random sensory input to specific thalamic nuclei may, to a varying degree, contribute to underconstrained activation of thalamocortical circuits. The reticular thalamic nucleus plays an important role in suppressing random activity of relay cells in specific thalamic nuclei, and its dysfunction may be implicated in the biological vulnerability to hallucinations in schizophrenia. Combined with general activation during cholinergic arousal, this leads to excessive disinhibition in specific thalamic nuclei, which may allow cortical attentional mechanisms to recruit thalamic relay cells into resonant assemblies of gamma oscillations, regardless of their actual sensory input, thereby producing an underconstrained perceptual experience.     

Perceptual functions in prosopagnosia

Some patients with prosopagnosia may have an apperceptive basis to their recognition defect. Perceptual abnormalities have been reported in single cases or small series, but the causal link of such deficits to prosopagnosia is unclear. Our goal was to identify candidate perceptual processes that might contribute to prosopagnosia, by subjecting several prosopagnosic patients to a battery of functions that may be necessary for accurate facial perception. We tested seven prosopagnosic patients. Three had unilateral right occipitotemporal lesions, two had bilateral posterior occipitotemporal lesions, and one had right anterior-to-occipital temporal damage along with a small left temporal lesion. These lesions all included the fusiform face area, in contrast to one patient with bilateral anterior temporal lesions. Most patients had impaired performance on face-matching tests and difficulty with subcategory judgments for non-face objects. The most consistent deficits in patients with lesions involving the fusiform face area were impaired perception of spatial relations in dot patterns and reduced contrast sensitivity in the 4 to 8 cycles deg(-1) range. Patients with bilateral lesions were impaired in saturation discrimination. Luminance discrimination was normal in all but two patients, and spatial resolution was uniformly spared. Curvature and line-orientation discrimination were impaired in only one patient, who also had the most difficulty with more basic-level object recognition. We conclude that deficits in luminance, spatial resolution, curvature, line orientation, and contrast at low spatial frequencies are unlikely to contribute to apperceptive prosopagnosia. More relevant may be contrast sensitivity at higher spatial frequencies and the analysis of object spatial structure. Deficits in these functions may impair perception of subtle variations in object shape, and may be one mechanism by which the recognition defect in prosopagnosia can extend to other classes of object subcategorization.     

Eye movements during intentional car following

Does intentional car following capture visual attention to the extent that driving may be impaired? We tested fifteen participants on a rudimentary driving simulator. Participants were either instructed to follow a vehicle ahead through a simulated version of London, or were given verbal instructions on where to turn during the route. The presence or absence of pedestrians, and the simulated time of the drive (day or night) were varied across the trials. Eye movements were recorded along with behavioural measures including give-way violations, give-way accidents, and kerb impacts. The results revealed that intentional car following reduced the spread of search and increased fixation durations, with a dramatic increase in the time spent processing the vehicle ahead (controlled for exposure). The effects were most pronounced during nighttime drives. During the car-following trials participants were also less aware of pedestrians, produced more give-way violations, and were involved in more give-way accidents. The results draw attention to the problems encountered during car following, and we relate this to the cognitive demands placed on drivers, especially police drivers who often engage in intentional car following and pursuits.     

Early computation of contour curvature and part structure: evidence from holes

We used holes to study unilateral border ownership and in particular the information carried by the sign of the curvature along the contour (ie the difference between convex and concave regions). When people perceive a hole, its shape has a reversed curvature polarity (ie a changed sign of curvature) compared to the same region perceived as an object. Bertamini (2001 Perception 30 1295-1310), and Bertamini and Croucher (2003 Cognition 87 33-54) suggested and found evidence to support the hypothesis that, because convex regions are perceived as parts, positional information is more readily available for convex regions. Therefore a change is predicted when a given region is perceived as either a hole or a figure. We confirm that finding in this study, using holes defined by binocular disparity. We conclude that a change from figure to hole always reverses the encoding of curvature polarity. In turn, polarity obligatorily affects perceived part structure and the processing of position.     

Bistability and biasing effects in the perception of ambiguous point-light walkers

The perceptually bistable character of point-light walkers has been examined in three experiments. A point-light figure without explicit depth cues constitutes a perfectly ambiguous stimulus: from all viewpoints, multiple interpretations are possible concerning the depth orientation of the figure. In the first experiment, it is shown that non-lateral views of the walker are indeed interpreted in two orientations, either as facing towards the viewer or as facing away from the viewer, but that the interpretation in which the walker is oriented towards the viewer is reported more frequently. In the second experiment the point-light figure was walking backwards, making the global orientation of the point-light figure opposite to the direction of global motion. The interpretation in which the walker was facing the viewer was again reported more frequently. The robustness of these findings was examined in the final experiment, in which the effects of disambiguating the stimulus by introducing a local depth cue (occlusion) or a more global depth cue (applying perspective projection) were explored.     

Distance estimation in a dynamic simulated environment: a visual field dependence problem?

Egocentric distance estimation implies that the subject perceives his own location in the environment. In a simulated environment, subjects have to transpose perceptively their own observation point in order to assume their virtual body position. As only visual information specifies this position, their ability to perceive the camera viewpoint should be linked to the field dependence independence factor (FDI). Field-independent subjects underestimated the mid-distance between their own virtual body position and a far-located target; their bisection was located between the projective (2-D) and the simulated (3-D) mid-distance which indicates their difficulty in considering the in-depth simulated spatial references. High correlations between the vertical estimation in the rod-and-frame test and the egocentric-distance-estimation task (with shifted camera) suggest a similar perceptive process in both tasks. This result is confirmed by the better performances of the field-dependent subjects in this condition. In conclusion, we discuss the relationship between FDI and the sense of presence in virtual environments.     

Differential effects of cast shadows on perception and action

In two experiments we investigated the effects of cast shadows on different real-life tasks. In experiment 1, participants were required to make a speeded verbal identification of the target object (perceptual task), whereas in experiment 2 participants were required to reach for and grasp the target object (motor task). In both experiments real three-dimensional (3-D) objects were presented, one at a time, either with their own natural cast shadow (congruent condition) or with the cast shadow of a different object (incongruent condition). Shadows were cast either to the left or to the right of the object. We asked whether the features of the shadow (ie whether it is congruent or incongruent with the object, and whether it is cast to the left or to the right of the object) could influence perception and action differently. Results showed that cast shadows did not influence identification of real 3-D objects (experiment 1), but they affected movement kinematics, producing distractor-like interference, particularly on movement trajectory (experiment 2). These findings suggest a task-dependent influence of cast shadows on human performance. In the case of object-oriented actions, cast shadows may represent further affordances of the object, and as such compete for the control of the action.     

Removing eyebrows impairs recognition of famous faces, or doesn't, depending on how the eyebrows are removed

Matching photos of famous people's faces with their names was as fast with photos that had eyebrows masked by adhesive plaster as with unretouched original photos. But matching was slower with photos that had eyebrows erased and replaced with adjacent skin texture and colour than with original photos. The conjecture that erasure impairs recognition because it alters a face's configuration was examined by repeating the experiment with the photos shown upside down, the rationale being that because configural information is difficult to encode from inverted faces the erasure effect should diminish. With inverted faces, matching was no different for original, eyebrows-masked, and eyebrows-erased photos. Eyebrows appear to be less important for face recognition as informative parts and features than as sources of information about a face's configuration.     

Location and shape in inhibition of return

Inhibition of return (IOR) is characterized by a delay in responding to a target preceded by an irrelevant stimulus (cue) at the same location, or, in other words, by a response delay when the same location is stimulated twice. It is not clear, however, if the same phenomenon is present when there is a repetition of a simple nonspatial attribute of the stimulus. The present study examines the repetition of shape. Six experiments were conducted. In Experiments 1, 2, 3, and 6 the stimuli were geometrical shapes, whereas in Experiments 4 and 5 letters were used. In Experiment 1 all the stimuli were presented at fixation, whereas in Experiments 2 to 6 they were presented in the periphery. In Experiments 2 and 3, location and shape of the cue and the target were independently manipulated in order to test independently inhibitory effects (IOR?) attributable to location repetition and shape repetition. Results showed an inhibitory effect for location and a much smaller inhibitory effect for shape. The latter was restricted to the cued location. Experiments 4 and 5 tested the possibility that inhibition caused by shape repetition is due to repetition blindness. Experiment 6 contrasted the presence or absence of a central neutral attractor in the delay interval between presentation of the cue and the target. Taken together the results seem to favor IOR as the basis for the inhibition caused by shape repetition.