Visual mechanisms of spatial disorientation in Alzheimer's disease.

Impaired optic flow perception may contribute to the visuospatial disorientation of Alzheimer's disease (AD). We find that 36% of AD patients have elevated perceptual thresholds for left/right outward radial optic flow discrimination. This impairment is related to independent visual motion processing deficits affecting the perception of left/right motion-defined boundaries and in/out radial motion. Elevated optic flow thresholds in AD are correlated with greater difficulty in the Road Map test of visuospatial function (r = -0.5) and in on-the-road driving tests (r = -0.83). When local motion cues are removed from optic flow, subjects must rely on the global pattern of motion. This reveals global pattern perceptual deficits that affect most AD patients (85%) and some normal elderly subjects (21%). This deficit might combine with impaired local motion processing to undermine the alternative perceptual strategies for visuospatial orientation. The greater prevalence of global pattern deficits suggests that it might precede local motion processing impairments, possibly relating to the sequence of early hippocampal and later posterior cortical damage that is typical of AD.

Dimension reduction and source identification for multispecies groundwater contamination.

Assessment of chemical contamination at large industrial complexes with long and sometimes unknown histories of operation represents a challenging environmental problem. The spatial and temporal complexity of the contaminant may be due to changes in production processes, differences in the chemical transport, and the physical heterogeneity of the soil and aquifer materials. Traditional mapping techniques are of limited value for sites where dozens of chemicals with diverse transport characteristics may be scattered over large spatial areas without documentation of disposal histories. In this context, a site with a long and largely undocumented disposal history of shallow groundwater contamination is examined using principal component analysis (PCA). The dominant chemical groups and chemical "modes" at the site were identified. PCA results indicate that five primary and three transition chemical groups can be identified in the space of the first three eigenvectors of the correlation matrix, which account for 61% of the total variance of the data. These groups represent a significant reduction in the dimension of the original data (116 chemicals). It is shown that each group represents a class of chemicals with similar chemo-dynamic properties and/or environmental response. Finally, the groups are mapped back onto the site map to infer delineation of contaminant source areas for each class of compounds. The approach serves as a preliminary step in subsurface characterization, and a data reduction strategy for source identification, subsurface modeling and remediation planning.

MST responses to pursuit across optic flow with motion parallax.

Self-movement creates the patterned visual motion of optic flow with a focus of expansion (FOE) that indicates heading direction. During pursuit eye movements, depth cues create a retinal flow field that contains multiple FOEs, potentially complicating heading perception. Paradoxically, human heading perception during pursuit is improved by depth cues. We have studied medial superior temporal (MST) neurons to see whether their heading selectivity is also improved under these conditions. The responses of 134 MST neurons were recorded during the presentation of optic flow stimuli containing one or three speed-defined depth planes. During pursuit, multiple depth-plane stimuli evoked larger responses (71% of neurons) and stronger heading selectivity (70% of neurons). Responses to the three speed-defined depth-planes presented separately showed that most neurons (54%) preferred one of the planes. Responses to multiple depth-plane stimuli were larger than the averaged responses to the three component planes, suggesting enhancing interactions between depth-planes. Thus speed preferences create selective responses to one of many depth-planes in the retinal flow field. The presence of multiple depth-planes enhances those responses. These properties might improve heading perception during pursuit and contribute to relative depth perception.

Optic flow illusion and single neuron behaviour reconciled by a population model.

Radial patterns of optic flow contain a centre of expansion that indicates the observer's direction of self-movement. When the radial pattern is viewed with transparently overlapping unidirectional motion, the centre of expansion appears to shift in the direction of the unidirectional motion [Duffy, C.J. & Wurtz, R.H. (1993) Vision Res., 33, 1481-1490]. Neurons in the medial superior temporal (MST) area of monkey cerebral cortex are thought to mediate optic flow analysis, but they do not shift their responses to parallel the illusion created by transparent overlap. The population-based model of optic flow analysis proposed by Lappe and Rauschecker replicates the illusory shift observed in perceptual studies [Lappe, M. & Rauschecker, J.P. (1995) Vision Res., 35, 1619-1631]. We analysed the behaviour of constituent neurons in the model, to gain insight into neuronal mechanisms underlying the illusion. Single model neurons did not show the illusory shift but rather graded variations of their response specificity. The shift required the aggregate response of the population. We compared the model's predictions about the behaviour of single neurons with the responses recorded from area MST. The predicted distribution of overlap effects agreed with that observed in area MST. The success of the population-based model in predicting the illusion and the neuronal behaviour suggests that area MST uses the graded responses of single neurons to create a population response that supports optic flow perception.

Visual loss and getting lost in Alzheimer's disease.

BACKGROUND: AD causes patients to get lost in familiar surroundings, in part because of visuospatial disorientation from parieto-occipital involvement. Parieto-occipital cortex analyzes the radial patterns of visual motion that create optic flow and guide movements through the environment by showing one's direction of self-movement. OBJECTIVE: To determine whether AD patients are impaired in perceiving the visual patterns of optic flow, suggesting a perceptual mechanism of visuospatial disorientation. METHODS: We studied the ability of young normal subjects, elderly normal subjects, and AD patients to see and interpret visual patterns, including the radial motion of optic flow. Each person sat in front of a panoramic computer display and gave push-button responses to indicate their perception of the projected visual stimuli. Spatial navigation was tested by asking questions about a recently traversed path. RESULTS: Half of the AD subjects showed impaired optic flow perception that was associated with poor performance on the spatial navigation test, even though their perception of simple moving patterns was relatively preserved. Some AD subjects also showed a separate impairment in interpreting optic flow, so that they could not use those stimuli to judge their direction of self-movement. CONCLUSIONS: AD greatly impairs the ability to see the radial patterns of optic flow. This may interfere with the use of visual information to guide self-movement and maintain spatial orientation.

MST neuronal responses to heading direction during pursuit eye movements.

As you move through the environment, you see a radial pattern of visual motion with a focus of expansion (FOE) that indicates your heading direction. When self-movement is combined with smooth pursuit eye movements, the turning of the eye distorts the retinal image of the FOE but somehow you still can perceive heading. We studied neurons in the medial superior temporal area (MST) of monkey visual cortex, recording responses to FOE stimuli presented during fixation and smooth pursuit eye movements. Almost all neurons showed significant changes in their FOE selective responses during pursuit eye movements. However, the vector average of all the neuronal responses indicated the direction of the FOE during both fixation and pursuit. Furthermore, the amplitude of the net vector increased with increasing FOE eccentricity. We conclude that neuronal population encoding in MST might contribute to pursuit-tolerant heading perception.

MST neurons respond to optic flow and translational movement.

We recorded the responses of 189 medial superior temporal area (MST) neurons by using optic flow, real translational movement, and combined stimuli in which matching directions of optic flow and real translational movement were presented together. One-half of the neurons (48%) showed strong responses to optic flow simulating self-movement in the horizontal plane, and 24% showed strong responses to translational movement. Combining optic flow stimuli with matching directions of translational movement caused substantial changes in both the amplitude of the best responses (44% of neurons) and the strength of direction selectivity (71% of neurons), with little effect on which stimulus direction was preferred. However, combining optic flow and translational movement such that opposite directions were presented together changed the preferred direction in 45% of the neurons with substantial changes in the strength of direction selectivity. These studies suggest that MST neurons combine visual and vestibular signals to enhance self-movement detection and disambiguate optic flow that results from either self-movement or the movement of large objects near the observer.

Multiple temporal components of optic flow responses in MST neurons.

Neurons in monkey medial superior temporal cortex (MST) respond to optic flow stimuli with early phasic, tonic, and after-phasic response components. In these experiments we characterized each response component to compare its potential contributions to visual motion processing. The early responses begin 60-100 ms after stimulus onset and last between 100 and 250 ms, the tonic responses begin 100-300 ms after stimulus onset and last for as long as the evoking stimulus persists, and the after-responses begin about 60 ms after the stimulus goes off and last for 100-350 ms. A neuron's tonic responses were evoked by specific optic flow stimuli: over two-thirds of the 264 neurons showed tonic responses evoked by two to five stimuli, whereas only 15% responded to either all or none of the stimuli. The tonic responses continued with stimulus presentations as long as 15 s, with their directional preferences being maintained throughout stimulation. However, the tonic response to a given stimulus was seen to change in amplitude when it was presented in random sequence with different sets of other stimuli. Thus, the tonic responses might convey substantial information about optic flow patterns, which continue with prolonged stimulation, but can be modified by the visual context created by other visual motion stimuli. Only about one-third of the 264 neurons had early responses that were selective for specific stimuli. In neurons yielding at least one early response, that neuron was most often activated by all the visual motion stimuli. After-responses occurred in only half the neurons, but they were more often specifically related to particular optic flow stimuli, regardless of whether those stimuli had evoked tonic excitatory or tonic inhibitory responses. The presence of early and after-responses complicates the interpretation of activity evoked when one stimulus immediately follows another. However, under those conditions, early responses and after-responses might contribute to signaling changes in the ongoing pattern of optic flow. We conclude that several components of MST responses should be recognized and that they potentially play different roles in the cortical analysis of optic flow. Tonic responses show the greatest specificity for particular optic flow stimuli, and possess characteristics which make them suitable neuronal participants in self-movement perception.

Medial superior temporal area neurons respond to speed patterns in optic flow.

The speed of visual motion in optic flow fields can provide important cues about self-movement. We have studied the speed sensitivities of 131 neurons in the dorsal region of the medial superior temporal area (MSTd) that responded to either radial or circular optic flow stimuli. The responses of more than two-thirds of these neurons were strongly modulated by changes in the mean speed of motion in optic flow stimuli, with response profiles resembling simple filter characteristics. When we removed the normal gradient of speeds in optic flow (slower speeds in the center, faster speeds in the periphery), approximately two-thirds of the neurons showed changes in their responses. When the speed gradient was altered rather than eliminated, almost nine in 10 neurons preferred either a normal speed gradient or an inverted one (slower speeds near the periphery) over stimuli with no speed gradient. These speed gradient preferences do not come simply from different speed preferences in the central and peripheral segments of the stimulus area. Rather, these speed gradient preferences seemed to reflect interactions between simultaneously presented speeds within an optic flow stimulus. The sensitivity of MSTd neurons to patterns of speed, as well as patterns of direction, strengthens the view that these neurons are well suited to the analysis of optic flow. Sensitivity to speed gradients in optic flow might contribute to neuronal mechanisms for spatial orientation during self-movement and for representing the three-dimensional structure of the visual environment.

Planar directional contributions to optic flow responses in MST neurons.

Many neurons in the dorsal region of the medial superior temporal area (MSTd) of monkey cerebral cortex respond to optic flow stimuli in which the center of motion is shifted off the center of the visual field. Each shifted-center-of-motion stimulus presents both different directions of planar motion throughout the visual field and a unique pattern of global motion across the visual field. We investigated the contribution of planar motion to the responses of these neurons in two experiments. In the first, we compared the responses of 243 neurons to planar motion and to shifted-center-of-motion stimuli created by vector summation of planar motion and radial or circular motion. We found that many neurons preferred the same directions of motion in the combined stimuli as in the planar stimuli, but other neurons did not. When we divided our sample into one group with stronger directionality to both planar and vector combination stimuli and one group with weaker directionality, we found that the neurons with the stronger directionality were those that showed the greatest similarity in the preferred direction of motion for both the planar and combined stimuli. In a second set of experiments, we overlapped planar motion and radial or circular motion to create transparent stimuli with the same motion components as the vector combination stimuli, but without the shifted centers of motion. We found that the neurons that responded most strongly to the planar motion when it was combined with radial or circular motion also responded best when the planar motion was overlapped by a transparent motion stimulus. We conclude that the responses of those neurons with stronger directional responses to both the motion of planar and vector combination stimuli are most readily understood as responding to the total planar motion in the stimulus, a planar motion mechanism. Other neurons that had weaker directional responses showed no such similarity in the preferred directions of planar motion in the vector combination and the transparent overlap stimuli and fit best with a mechanism dependent on the global motion pattern. We also found that neurons having significant responses to both radial and circular motion also responded to the spiral stimuli that result from a vector combination of radial and circular motion. The preferred planar-spiral vector combination stimulus was frequently the one containing that neurons' preferred direction of planar motion, which makes them similar to other MSTd neurons.

Response of monkey MST neurons to optic flow stimuli with shifted centers of motion.

Neurons in the dorsal region of the medial superior temporal area (MSTd) have previously been shown to respond to the expanding radial motion that occurs as an observer moves through the environment. In previous experiments, MSTd neurons were tested with radial and circular motion centered in the visual field. However, different directions of observer motion, relative to the direction of gaze, are accompanied by visual motion centered at different locations in the visual field. The present experiments investigated whether neurons that respond to radial and circular motion might respond differently when the center of motion was shifted to different regions of the visual field. About 90% of the 245 neurons studied responded differently when the center of motion was shifted away from the center of the field. The centers of motion preferred by each neuron were limited to one area of the visual field. All parts of the visual field were represented in the sample, with greater numbers of neurons preferring centers of motion closer to the center of the field. We hypothesize that each of the MSTd neurons has a center of motion field with a gradient of preferred centers of motion, and that there is an orderly arrangement of MSTd neurons with each region of the visual field being represented by a set of neurons. This arrangement creates the potential for graded responses from individual neurons for different directions of heading as an observer moves through the environment.

An illusory transformation of optic flow fields.

We compared a human observer's ability to locate the focus of expansion (FOE) of a radial optic flow field when this flow field was either combined with, or overlapped by, planar motion. With combined stimuli, in which the FOE was displaced in the direction opposite to the planar motion, subjects accurately located the displaced FOE. With overlapping (transparent) stimuli and the FOE remaining in the center of the display, we found an illusory transformation of the radial pattern: the focus of expansion appeared to be shifted in the direction of the planar motion. The speed of both the planar and radial patterns of motion influenced the illusion. Presence or absence of visual fixation had little effect. We suggest that this illusion might provide a clue as to the way the brain processes planar and radial motion which might in turn be relevant to the interaction of the planar and radial motion components of optic flow fields.

Optical frequency shifter technique based on stimulated Brillouin scattering in birefringent optical fiber.

An optical technique for producing a heterodyne carrier frequency suitable for electronic signalprocessing schemes in sensing applications is described. The technique exploits stimulated Brillouin scattering (SBS) generated in birefringent optical fiber. Systems based on single-fiber and dual-fiber topologies are reported that yield 10.6 ± 8.0- and 665 ± 10.0-MHz carrier frequencies, respectively. Frequency instabilities arose from nonlinear dynamical effects inherent to the SBS process. The implications of the results for future signal-processing and sensing schemes based on SBS are then discussed.

Neuronal correlates of optic flow stimulation.

Neurons in a region of monkey extrastriate cortex, MSTd, respond to the components of optic flow stimulation. Some of these neurons (single-component neurons) are selective for a single type of motion such as inward- or outward-radial motion. Other neurons respond to multiple types of rotation, for example, rightward planar, clockwise circular, and inward radial. Rather than forming discrete groups, we think these neurons represent a continuum covering the range from single-component sensitivity to multiple-component sensitivity. By combining the optic flow stimuli, we have also been able to recognize that such combinations alter the response of cells in the continuum to varying degrees. At this point, while our evidence is consistent with the hypothesis that cells in area MSTd contribute to the processing of optic flow stimuli, we do not know whether these neurons do in fact serve this function. As in all single-cell recording experiments, even those in awake animals performing tasks closer to real-world tasks than we have succeeded in emulating here, the activity of the cell in relationship to the visual stimulation is simply a correlate of the optic flow stimulation and may or may not contribute to the processing of optic flow stimulation upon which behavior depends. Further information on a number of characteristics of these cells might clarify their role. Information on such factors as whether heading in the environment is conveyed by individual neurons, or whether this property is more likely to be conveyed over a population of neurons, and the role of changes in the point of fixation of the eyes are critical points. Generation of behavior on the basis of the optic flow stimulation and determination that this behavior is modified by selective lesion of MSTd would also strengthen the argument that visual motion processing in this area is related to analyzing optic flow information.

Sensitivity of MST neurons to optic flow stimuli. I. A continuum of response selectivity to large-field stimuli.

1. Neurons in the dorsomedial region of the medial superior temporal area (MSTd) have large receptive fields that include the fovea, are directionally selective for moving visual stimuli, prefer the motion of large fields to small spots, and respond to rotating and expanding patterns of motion as well as frontal parallel planar motion. These characteristics suggested that these neurons might contribute to the analysis of the large-field optic flow stimulation generated as an observer moves through the visual environment. 2. We tested the response of MSTd neurons in two awake monkeys by systematically presenting a set of translational and rotational stimuli to each neuron. These 100 X 100 degrees stimuli were the motion components from which all optic flow fields are derived. 3. In 220 single neurons we found 23% that responded primarily to one component of motion (planar, circular, or radial), 34% that responded to two components (planocircular or planoradial, but never circuloradial), and 29% that responded to all three components. 4. The number of stimulus components to which a neuron responded was unrelated to the size or eccentricity of its receptive field. 5. Triple-, double-, and single-component neurons varied widely in the strength of their responses to the preferred components. Grouping these neurons together revealed that they did not form discrete classes but rather a continuum of response selectivity. 6. This continuum was apparent in other response characteristics. Direction selectivity was weakest in triple-component neurons, strongest in single-component neurons. Significant inhibitory responses were less frequent in triple-component neurons than in single-component neurons. 7. There was some indication that the neurons of similar component classes occupied adjacent regions within MSTd, but all combinations of component and direction selectivity were occasionally found in immediate juxtaposition. 8. Experiments on a subset of neurons showed that the speed of motion, the dot density, and the number of different speed planes in the display had little influence on these responses. 9. We conclude that the selective responses of many MSTd neurons to the rotational and translational components of optic flow make these neurons reasonable candidates for contributing to the analysis of optic flow fields.