Robert Sekuler

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.

Direction-specific improvement in motion discrimination

With training, an observers ability to discriminate similar directions of motion gradually improves. A series of studies reveals that this improvement, (1) is restricted to the trained direction and other, similar directions, (2) persists for at least several months, (4) shows appreciable, but not complete, transfer between the two eyes, and (5) is largely restricted to the stimulated region of the field. Moreover, the improvement in direction discrimination does not produce a concomitant change in detection thresholds. In all likelihood, most of the improvement in direction discrimination represents a change in visual function, rather than changes in nonsensory processes.

Size-Detecting Mechanisms in Human Vision

Inspecting a pattern of alternating dark and light bars makes it difficult to see a similar pattern presented afterward. This phenomenon can be used to isolate mechanisms responsive to bars of a given width. Our results suggest that the human visual system contains several different classes of size detectors, each maximally sensitive to visual targets with sizes in a particular range.

Visual localization: age and practice

Older people seem to be highly susceptible to the distracting effects of irrelevant or interfering visual stimuli. We studied this susceptibility using visual displays in which observers had to localize the position of a face. When a face appeared in isolation, observers of all ages did equally well; when distracting stimuli surrounded the face, older observers alone performed poorly. Brief periods of practice produce substantial and long-lasting improvement in performance.

Aftereffect of Seen Motion with a Stabilized Retinal Image

Prolonged inspection of uniformly moving contours affects differentially the luminance threshold for the detection of test contours as a function of the direction of motion of the test contours. This finding supports a new explanation of the well-known aftereffect.

Motion processing in peripheral vision: Reaction time and perceived velocity

Reaction times to motion onset were measured as a function of eccentricity of presentation. These were compared with measurements of perceived speed at various eccentricities. For slowly moving targets, both dependent measures changed substantially with eccentricity: RT increased and perceived speed declined. For more rapidly moving targets, both dependent measures were unchanged by eccentricity. These results may be related to the difference between retinotopic distribution of neural mechanisms responsive to low rates of temporal modulation and the retinotopic distribution of neural mechanisms responsive to higher rates of temporal modulation.

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.

The effects of aging on motion detection and direction identification.

Random dot cinematograms were used to probe motion perception in human observers ranging from 23 to 81 years of age. Stimuli were either broadband directional Noise, which produces no experience of global motion flow, or a narrower band directional Signal, which tended to produce experiences of coherent, global direction flow. On each trial, subjects rated their certainty that a Signal had been presented, and used a computer mouse to indicate the direction of perceived global flow. At all ages, sensitivity to motion and accuracy of perceived direction improved significantly as stimulus duration increased from 75 to 470 ms. However, older subjects (>70 years of age) were significantly less sensitive to motion, and were significantly less accurate at identifying the direction of movement. A control experiment, which found that older subjects accurately perceived and remembered the orientation of a line, ruled out the possibility that the observed deficits in motion perception were due to an inability on the part of older subjects to manipulate the computer mouse. Those control results also showed that both younger and older observers maintained robust visual representations over durations ranging from .24 to 6.0s. The motion detection and identification results obtained from subjects less than 70 years of age were well fit by a simple multichannel model of motion, although different levels of additive internal noise were needed to fit detection data and direction-identification data, suggesting that motion direction and identification are constrained by different mechanisms. To fit the data from the oldest subjects, however, the values of model parameters had to be significantly altered, either by increasing the level of additive internal noise substantially, or by a smaller increase in noise coupled with an increase in the bandwidth of the models directionally selective channels. These results are qualitatively consistent with recent neurophysiological studies showing weaker directional selectivity and higher spontaneous noise in visual neurons of senescent monkeys and cats.

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.

Models of stimulus uncertainty in motion perception.

A model is proposed to account for the loss in visibility of moving targets that occurs when an observer is uncertain about the targets direction of motion. The models key features are an array of directionally selective visual mechanisms and a rule governing the mechanisms from which an observer will derive sensory data. In response to uncertainty about two possible directions of motion, the observer is assumed to use a mechanism whose peak sensitivity is to a direction midway between the two possible directions. Seven experiments, using both reaction time and forced-choice data, demonstrate the predictive advantages of this midway model over competing single-band and multiple-band models. Additionally, the experiments reveal several new properties of human motion perception: (a) Direction and velocity information have orthogonal representations in the visual system; (b) although motion sensitivity does not vary with direction, the precision with which small changes in direction can be recognized does, reflecting differential breadth of tuning for directionally selective mechanisms sensitive to various directions; and (c) motion-analyzing mechanisms are broadly tuned for direction as well as speed. Human psychophysics provides extensive evidence for the existence of visual mechanisms tuned to different directions of motion. Converging on this point are data from a variety of paradigms including selective adaptation (Sekuler & Ganz, 1963), subthreshold summation (Levinson & Sekuler, 1975), and aftereffects (Keck, Palella, & Pantle, 1976). In addition, we recently used a masking procedure to define the direction sensitivity profiles of these mechanisms (Ball & Sekuler, 1979). In general, these studies tell us how much information is potentially available in motion-sensit ive visual elements under ideal conditions. But they tell us little about the use to which that information can be put under nonideal conditions outside the laboratory.