Corticolimbic interactions associated with performance on a short-term memory task are modified by age.

Aging has been associated with a decline in memory abilities dependent on hippocampal processing. We investigated whether the functional interactions between the hippocampus and related cortical areas were modified by age. Young and old subjects' brain activity was measured using positron emission tomography (PET) while they performed a short-term memory task (delayed visual discrimination) in which they determined which of two successively presented sine-wave gratings had the highest spatial frequency. Behavioral performance was equal for the two groups. Partial least squares (PLS) analysis of PET images identified a hippocampal voxel whose activity was similarly correlated with performance across groups. Using this voxel as a seed, a second PLS analysis identified cortical regions functionally connected to the hippocampus. Quantification of the neural interactions with structural equation modeling suggested that a different hippocampal network supported performance in the elderly. Unlike the neural network engaged by the young, which included prefrontal cortex Brodmann's area (BA) 10, fusiform gyrus, and posterior cingulate gyrus, the network recruited by the old included more anterior areas, i.e., dorsolateral prefrontal cortex (BA 9/46), middle cingulate gyrus, and caudate nucleus. Recruitment of a distinct corticolimbic network for visual memory in the elderly suggests that age-related neurobiological deterioration not only results in focal changes but also in the modification of large-scale network operations.

Optic flow helps humans learn to navigate through synthetic environments.

Self-movement through an environment generates optic flow, a potential source of heading information. But it is not certain that optic flow is sufficient to support navigation, particularly navigation along complex, multi-legged paths. To address this question, we studied human participants who navigated synthetic environments with and without salient optic flow. Participants used a keyboard to control realistic simulation of self-movement through computer-rendered, synthetic environments. Because these environments comprised series of identically textured virtual corridors and intersections, participants had to build up some mental representation of the environment in order to perform. The impact of optic flow on learning was examined in two experiments. In experiment 1, participants learned to navigate multiple T-junction mazes with and without accompanying optic flow. Optic flow promoted faster learning, mainly by preventing disorientation and backtracking in the maze. In experiment 2, participants found their way around a virtual city-block environment, experiencing two different kinds of optic flow as they went. By varying the rate at which the display was updated, we created optic flow that was either fluid or choppy. Here, fluid optic flow (as compared with choppy optic flow) enabled participants to locate a remembered target position more accurately. When other cues are unavailable, optic flow can be a significant aid in wayfinding. Among other things, optic flow can facilitate path integration, which involves updating a mental representation of place by combining the trajectories of previously travelled paths [corrected].

Perceptual dimorphism in visual motion from stationary patterns.

Fraser and Wilcox [1979 Nature (London) 281 565-566] devised a series of complex stationary patterns that provoked episodes of compelling illusory motion, but only in about two-thirds of people tested. Using simplified versions of their stimuli, we have confirmed their claim of perceptual dimorphism. We show that the strength of the illusory motion depends upon stimulus duration, eccentricity, and contrast. The illusory motion does not require fluctuations in accommodation, as has been suggested for some other forms of illusory motion. Finally, we consider the relation of Fraser-type motion to other forms of illusory motion.

Recruitment of unique neural systems to support visual memory in normal aging.

The performance of many cognitive tasks changes in normal aging [1] [2] [3]. Recent behavioral work has identified some tasks that seem to be performed in an age-invariant manner [4]. To understand the brain mechanisms responsible for this, we combined psychophysical measurements of visual short-term memory with positron emission tomography (PET) in young and old individuals. Participants judged the differences between two visual stimuli, and the memory load was manipulated by interposing a delay between the two stimuli. Both age groups performed the task equally well, but the neural systems supporting performance differed between young and old individuals. Although there was some overlap in the brain regions supporting performance (for example, occipital, temporal and inferior prefrontal cortices, and caudate), the functional interconnections between these common regions were much weaker in old participants. This suggests that the regions were not operating effectively as a network in old individuals. Old participants recruited unique areas, however, including medial temporal and dorsolateral prefrontal cortices. These unique areas were strongly interactive and their activity was related to performance only in old participants. Therefore, these areas may have acted to compensate for reduced interactions between the other brain areas.

Human theta oscillations exhibit task dependence during virtual maze navigation.

Theta oscillations (electroencephalographic activity with a frequency of 4-8 Hz) have long been implicated in spatial navigation in rodents; however, the role of theta oscillators in human spatial navigation has not been explored. Here we describe subdural recordings from epileptic patients learning to navigate computer-generated mazes. Visual inspection of the raw intracranial signal revealed striking episodes of high-amplitude slow-wave oscillations at a number of areas of the cortex, including temporal cortex. Spectral analysis showed that these oscillations were in the theta band. These episodes of theta activity, which typically last several cycles, are dependent on task characteristics. Theta oscillations occur more frequently in more complex mazes; they are also more frequent during recall trials than during learning trials.

Collisions between moving visual targets: what controls alternative ways of seeing an ambiguous display?

When identical visual targets move directly toward and then past one another, they appear either to stream past one another or to bounce off each other. Bertenthal et al (1993 Perception 22 193-207) accounted for the relative strengths of these two percepts by invoking a directional bias, arising from cooperative interactions within a network of motion detectors. We tested this explanation by devising conditions that would enhance or diminish the strength of such a directional bias. In separate experiments we varied (i) the presence or absence of temporal transients (pausing, disappearance, occlusion); (ii) the distances travelled by the targets; and (iii) their acceleration or deceleration before and after collision. The tendency to see the objects stream past one another was not related to the strength of an hypothesized directional bias, suggesting that the perception of this ambiguous motion display was not mediated by directional recruitment. Instead, the results suggest that perceived direction reflects the operation of neural constraints that mirror the constraints operating upon moving objects in the three-dimensional natural world.

Blur and contrast as pictorial depth cues.

Studies have shown that blur can act as a pictorial cue to depth perception. But blurring a stimulus reduces its contrast, and studies have also shown that contrast can act as a pictorial cue to depth perception. To determine whether blur and contrast have separate influences on depth perception, each variable was independently manipulated in two experiments. Observers reported depth alternations in a simple reversible figure. Both contrast and blur were found to influence depth perception, but blur had its greatest effect at moderate contrasts. When blurred and sharp stimuli were equated on either Michelson or RMS contrast, blur continued to affect depth perception. Hence blur can act as a depth cue independently of contrast. It is speculated that blur is effective as a pictorial cue because of its usual association with other depth cues, particularly in pictures and photographs.

Motion perception: a modern view of Wertheimer's 1912 monograph.

Max Wertheimer's 1912 monograph on apparent motion is a seminal contribution to the study of visual motion, but its actual contents are not widely known. This article attempts to clarify what the monograph did and did not contribute, emphasizing links between Wertheimer's principal findings and the results of subsequent investigations of motion perception, including currently active lines of research. The topics discussed include Wertheimer's experimental tests of explanations for apparent motion; his work with motion phenomena that lie between succession and optimum motion; his studies of the influence of attention on motion; explorations of various forms of hysteresis and motion transparency; and Wertheimer's work with a motion-blind patient.

Detection of changes in speed and direction of motion: reaction time analysis.

Observers reacted to the change in the movement of a random-dot field whose initial velocity, V0, was constant for a random period and then switched abruptly to another value, V1. The two movements, both horizontally oriented, were either in the same direction (speed increments or decrements), or in the opposite direction but equal in speed (direction reversals). One of the two velocities, V0 or V1, could be zero (motion onset and offset, respectively). In the range of speeds used, 0-16 deg/sec (dps), the mean reaction time (MRT) for a given value of V0 depended on magnitude of V1-V0 only: MRT approximately r+c(V0)/magnitude of V1-V0 beta, where beta = 2/3, r is a velocity-independent component of MRT, and c(V0) is a parameter whose value is constant for low values of V0 (0-4 dps), and increases beginning with some value of V0 between 4 and 8 dps. These and other data reviewed in the paper are accounted for by a model in which the time-position function of a moving target is encoded by mass activation of a network of Reichardt-type encoders. Motion-onset detection (V0 = 0) is achieved by weighted temporal summation of the outputs of this network, the weights assigned to activated encoders being proportional to their squared spatial spans. By means of a "subtractive normalization," the visual system effectively reduces the detection of velocity changes (a change from V0 to V1) to the detection of motion onset (a change from 0 to V1-V0). Subtractive normalization operates by readjustment of weights: the weights of all encoders are amplified or attenuated depending on their spatial spans, temporal spans, and the initial velocity V0. Assignment of weights and weighted temporal summation are thought of as special-purpose computations performed on the dynamic array of activations in the motion-encoding network, without affecting the activations themselves.

Representational development of direction in motion perception: a fragile process.

Response to a change in direction is more rapid if the target moves in a predictable direction before the change than if the pre-change direction is not predictable. However, if the target trajectory is viewed for approximately half a second before the change in direction, the effect of directional predictability disappears. Visual information gathered prior to change in direction is used to construct an increasingly more accurate representation of target trajectory. To study this process, we inject various temporal transients into the trajectory prior to the change in direction. We find that extraction of directional information is interrupted if: (i) motion continues along a constant trajectory, but the target disappears briefly behind an implicit or real occluder, (ii) the target pauses briefly, but remains visible, or (iii) the target changes speed briefly, while continuing to move in the same direction. The theoretical implications for motion perception are discussed. These implications include a framework for understanding interactions between stimulus-derived information and a priori information.

Temporal and spatial integration in dynamic random-dot stimuli.

Random-dot cinematograms comprising many different, spatially intermingled local motion vectors can produce a percept of global coherent motion in a single direction. Thresholds for discriminating the direction of global motion were measured under various conditions. Discrimination thresholds increased with the width of the distribution of directions in the cinematogram. Thresholds decreased as the duration of area of the cinematogram increased. Temporal integration for global direction discrimination extends over about 465 msec (9.3 frames) while the spatial integration limit is at least as large as 63 deg2 (circular aperture diameter = 9 deg). The large spatial integration area is consistent with the physiology of higher visual areas such as MT and MST.

Vision, aging, and driving: the problems of older drivers.

Although there are well-recognized declines in visual functioning with age, their contribution to the problems of older persons on tasks in the natural environment, including driving, are largely unknown. Adults ranging in age from 22-92 years were surveyed in regard to their visual difficulties when driving and performing everyday tasks. The visual problems of drivers increased with age along five different visual dimensions: unexpected vehicles, vehicle speed, dim displays, windshield problems, and sign reading. Several of the age-related visual problems that were reported appear to be related to the types of automobile accidents more common among older drivers. The study also replicated the findings from an earlier investigation of non-driving tasks that showed visual declines with age on five dimensions: visual processing speed, light sensitivity, dynamic vision, near vision and visual search. These findings indicate promising areas of research regarding the effects of visual aging on tasks in the natural environment.

Using metamers to explore motion perception.

We examined conditions under which two quite different types of random-dot cinematograms were perceptually matched. In one stimulus type, directions of motion were defined by a uniform distribution; in the other, directions were drawn from a discrete set of just a few, widely separated directions. Cinematograms whose range of uniformly distributed directions lay between 180 and 270 deg could be matched by cinematograms containing just 6-10 discrete directions. The number of discrete directions required for a match was a nonmonotonic function of the range of directions present in the other cinematogram. The results are consistent with a line-element model in which the outputs of 12 direction-selective mechanisms, each with a half-amplitude half-bandwidth of 30 deg, are combined nonlinearly to produce the percept of motion.

Self-reported visual problems of older drivers.

We conducted a survey in order to gain insights into the reasons older persons decide to give up driving. Our survey focused on vision. We probed the relationship between visual deficiencies and driving status by asking older people about the problems they encountered while performing routine visual tasks. The results showed that older persons who had recently given up driving reported more visual problems than did their driving counterparts. These problems related to difficulties in dynamic vision, visual processing speed, visual search, light sensitivity, and near vision. The results suggested that older persons are sensitive to their own visual deficits and that this awareness influences decisions about driving.

Assimilation and contrast in motion perception: explorations in cooperativity.

Motions within one region of the field influence motion seen elsewhere. To explore this phenomenon we used cinematograms comprised of alternating strips within which dots (i) tended to move in one direction, or (ii) moved in random directions (dynamic noise). When alternating strips were narrow, motion in one direction induced a similar direction of illusory motion in the adjoining dynamic noise (assimilation); when alternating strips were wide, motion tended to induce an illusory opposed motion in the dynamic noise (contrast). Since this illusory motion exhibits hysteresis, it probably results from spatially distributed, cooperative processes. The shift from assimilation to contrast, as the cinematogram's strips increase in size, suggests that facilitatory and inhibitory influences of the network extend over different distances.

How the visual system detects changes in the direction of moving targets.

To determine how the visual system represents information about change in target direction, we studied the detection of such change under conditions of varying stimulus certainty. Target direction was either held constant over trials or was allowed to vary randomly. When target direction was constant the observer could be certain about that stimulus characteristic; randomizing the target direction rendered the observer uncertain. We measured response times (RTs) to changes in target direction following initial trajectories of varying time and distance. In different conditions, the observer was uncertain about either the direction of the initial trajectory, or the direction of change or both. With brief initial trajectories in random directions, uncertainty about initial direction elevated RTs by 50 ms or more. When the initial trajectories were at least 500 ms, this directional uncertainty ceased to affect RTs; then, only uncertainty about the direction of change affected RTs. We discuss the implications of these results for (i) schemes by which the visual system might code directional change; (ii) the visual integration time for directional information; and (iii) adaptational processes in motion perception.

Direction perception in complex dynamic displays: the integration of direction information.

We created random-dot cinematograms in which each dot's successive movements were independently drawn from a Gaussian distribution of directions of some characteristic bandwidth. Such a display, comprising many different, spatially intermingled local motion vectors, can produce a percept of global coherent motion in a single direction. Using pairs of cinematograms, direction discrimination of global motion was measured under various conditions of direction distribution bandwidth, exposure duration, and constancy of each dot's path. A line-element model gave an excellent account of the results: (i) over a considerable range, discrimination was unaffected by the cinematogram's direction distribution bandwidth; (ii) only for the briefest presentations did changes in duration have an effect; (iii) so long as the overall directional content of the cinematogram remained unchanged, the constancy or randomness of individual dots' paths did not affect discrimination. Finally, the line-element model continued to give a good account of the results when we made additional measurements with uniform rather than Gaussian distributions of directions.