Ágoston Török

Context-dependent spatially periodic activity in the human entorhinal cortex

Significance In the mammalian brain, neurons in the hippocampus represent unique places and neurons in the entorhinal cortex, so-called grid cells, provide an internal coordinate system of the environment. Dysfunction in this circuit causes memory problems and disorientation, which are early symptoms of Alzheimer’s disease. Much of what we have learned from this circuit is based on rodent studies. Here, we report that grid cells in the human brain represent space in a more environmentally adaptive manner than in rodents. Human grids show an increased scalability with the environment relative to rodents. The enhanced context dependency of spatial representations is probably a late development of evolution. The spatially periodic activity of grid cells in the entorhinal cortex (EC) of the rodent, primate, and human provides a coordinate system that, together with the hippocampus, informs an individual of its location relative to the environment and encodes the memory of that location. Among the most defining features of grid-cell activity are the 60° rotational symmetry of grids and preservation of grid scale across environments. Grid cells, however, do display a limited degree of adaptation to environments. It remains unclear if this level of environment invariance generalizes to human grid-cell analogs, where the relative contribution of visual input to the multimodal sensory input of the EC is significantly larger than in rodents. Patients diagnosed with nontractable epilepsy who were implanted with entorhinal cortical electrodes performing virtual navigation tasks to memorized locations enabled us to investigate associations between grid-like patterns and environment. Here, we report that the activity of human entorhinal cortical neurons exhibits adaptive scaling in grid period, grid orientation, and rotational symmetry in close association with changes in environment size, shape, and visual cues, suggesting scale invariance of the frequency, rather than the wavelength, of spatially periodic activity. Our results demonstrate that neurons in the human EC represent space with an enhanced flexibility relative to neurons in rodents because they are endowed with adaptive scalability and context dependency.

ERP correlates of prosody and syntax interaction in case of embedded sentences

Abstract Understanding spoken language depends on processing the delicate combination of grammatical structure, meaning and prosody of utterances. Previous studies have established that prosody influences the processing of sentences when the grammatical structure is ambiguous, however it is unclear how closely prosody and syntax are related when there is no ambiguity. In an event-related brain potential (ERP) study, we investigated the processing of embedded normal and pseudosentences in which all function and content words were replaced by meaningless words. Sentences could have either natural prosodic structure or incongruent prosodic structure, where the prosody deviated from the one expected based on the syntactic structure, but otherwise the sentences were unambiguous. The resulting ERP components (CPS) showed that the construction of prosodic structure was similar in normal and pseudosentences, thus suggesting that prosody has an abstract, recursive representation, independent of other linguistic information. Moreover, we found evidence that the incongruent prosody was not only detected (shown by the RAN), but it induced neural reintegration processes (shown by the P600) in spite of the syntactic structure of sentences being intact. These results suggest that the prosodic structure is a mandatory constituent of sentence structure building whenever it is present.

Up, Down, Near, Far: An Online Vestibular Contribution to Distance Judgement

Whether a visual stimulus seems near or far away depends partly on its vertical elevation. Contrasting theories suggest either that perception of distance could vary with elevation, because of memory of previous upwards efforts in climbing to overcome gravity, or because of fear of falling associated with the downwards direction. The vestibular system provides a fundamental signal for the downward direction of gravity, but the relation between this signal and depth perception remains unexplored. Here we report an experiment on vestibular contributions to depth perception, using Virtual Reality. We asked participants to judge the absolute distance of an object presented on a plane at different elevations during brief artificial vestibular inputs. Relative to distance estimates collected with the object at the level of horizon, participants tended to overestimate distances when the object was presented above the level of horizon and the head was tilted upward and underestimate them when the object was presented below the level of horizon. Interestingly, adding artificial vestibular inputs strengthened these distance biases, showing that online multisensory signals, and not only stored information, contribute to such distance illusions. Our results support the gravity theory of depth perception, and show that vestibular signals make an on-line contribution to the perception of effort, and thus of distance.

It sounds real when you see it. Realistic sound source simulation in multimodal virtual environments

Designing multimodal virtual environments promises revolutionary advances in interacting with computers in the near future. In this paper, we report the results of an experimental investigation on the possible use of surround-sound systems to support visualization, taking advantage of increased knowledge about how spatial perception and attention work in the human brain. We designed two auditory-visual cross-modal experiments, where noise bursts and light-blobs were presented synchronously, but with spatial offsets. We presented sounds in two ways: using free field sounds and using a stereo speaker set. Participants were asked to localize the direction of sound sources. In the first experiment visual stimuli were displaced vertically relative to the sounds, in the second experiment we used horizontal offsets. We found that, in both experiments, sounds were mislocalized in the direction of the visual stimuli in each condition (ventriloquism effect), but this effect was stronger when visual stimuli were displaced vertically, as compared to horizontally. Moreover we found that the ventriloquism effect is strongest for centrally presented sounds. The analyses revealed a variation between different sound presentation modes. We explain our results from the viewpoint of multimodal interface design. These findings draw attention to the importance of cognitive features of multimodal perception in the design of virtual environment setups and may help to open new ways to more realistic surround based multimodal virtual reality simulations.

Effect of stimulus intensity on response time distribution in multisensory integration

To increase the efficiency of multimodal user interfaces, one has to design them according to how multimodal features appear in the real world. Although spatial coincidence and matching intensity levels are important for perception, these factors received little attention in human–computer interaction studies. In our present study we aimed to map how spatial coincidence and different intensity levels influence response times. Sixteen participants performed a simple auditory localization task, where sounds were presented either alone or together with visual non-targets. We found that medium intensity visual stimuli facilitated responses to low intensity sounds. Analyses of response time distributions showed that intensity of target and non-target stimuli affected different parameters of the ex-Gaussian distribution. Our results suggest that multisensory integration and response facilitation may occur even if the non-target has low predictive power to the location of the target. Furthermore, we show that the parameters of the ex-Gaussian distribution can be related to distinct cognitive processes. The current results are potentially applicable in the design of an intelligent warning system that employs the user’s reaction time to adapt the warning signal for optimal results.

Temporal dynamics of object location processing in allocentric reference frame

The spatial location of objects is processed in egocentric and allocentric reference frames, the early temporal dynamics of which have remained relatively unexplored. Previous experiments focused on ERP components related only to egocentric navigation. Thus, we designed a virtual reality experiment to see whether allocentric reference frame-related ERP modulations can also be registered. Participants collected reward objects at the end of the west and east alleys of a cross maze, and their ERPs to the feedback objects were measured. Participants made turn choices from either the south or the north alley randomly in each trial. In this way, we were able to discern place and response coding of object location. Behavioral results indicated a strong preference for using the allocentric reference frame and a preference for choosing the rewarded place in the next trial, suggesting that participants developed probabilistic expectations between places and rewards. We also found that the amplitude of the P1 was sensitive to the allocentric place of the reward object, independent of its value. We did not find evidence for egocentric response learning. These results show that early ERPs are sensitive to the location of objects during navigation in an allocentric reference frame.

A novel virtual plus-maze for studying electrophysiological correlates of spatial reorientation

Quick reorientation is an essential part of successful navigation. Despite growing attention to this ability, little is known about how reorientation happens in humans. To this aim, we recorded EEG from 34 participants. Participants were navigating a simple virtual reality plus-maze where at the beginning of each trial they were randomly teleported to either the North or the South alley. Results show that the teleportation event caused a quick reorientation effect over occipito-parietal areas as early as 100 msec; meaning that despite the known stochastic nature of the teleportation, participants built up expectations for their place of arrival. This result has important consequences for the optimal design of Virtual reality locomotion.