Simulating self-motion

In general, vehicle motions far exceed the mechanical constraints of an earth-fixed simulator base. Inertial motions can, therefore, only be simulated in partial agreement with those of the actual vehicle. As a consequence, physical mismatches between inertial and environmental motion are inevitable. Here, the concept of a subjective reference frame is introduced, relative to which perceived self-motion is defined. This frame must be released from the earth-fixed frame to evoke simulated self-motion. In addition, self-motion and environmental motion need to be perceived reciprocal, in order to evoke a stationary perceived environment. Due to the only limited accuracy of human self-motion perception, however, perceived self-motion and perceived environmental motion need not to be exactly reciprocal. The extent to which self-motion and environmental motion may differ can be expressed by a just noticeable difference. This just noticeable difference denotes the threshold at which the environment is perceived to move. In this article, a self-motion perception model is outlined in which perceived environmental motion and perceived self-motion are separated. The perception model and the just noticeable differences can then be applied to determine the inertial stimulation that is needed to evoke perceived self-motion, in which the environment is perceived stationary throughout simulation.     

Object motion perception during ego-motion: Patients with a complete loss of vestibular function vs. normals

Object motion perception was assessed in avestibular patients and normal controls. Two experiments were conducted, in which subjects were required to assess the motion of a visual stimulus with respect to earth. In the first experiment, we measured the velocity at which a briefly presented (200 ms) grating was perceived as earth fixed, while the subject maintained fixation on a visual target fixed relative to the body, during whole-body yaw rotation (VOR suppression). In this experimental setup, the influence of the semicircular canal signals on object motion perception was evaluated. The avestibular patients judged the grating to be stationary with respect to earth, when it was moving at the same velocity as their body, whereas for normal controls, the grating was perceived as stationary when it moved at a velocity slower than their body motion, but greater than zero. The difference between the two subject groups was significant, and showed the strong contribution of the vestibular system to object motion perception. Similarly, a measurement of the velocity at which a grating was perceived as stationary was obtained during smooth pursuit eye movements. In this experiment the contribution of the efference copy of the oculomotor signal and proprioceptive signals to object motion perception were assessed. As with the first experiment, the normal controls displayed a more veridical sense of object motion perception than the patients, although the difference was only just significant. We suggest that the difference could be an adaptive change in the patients perception of motion, which allows them to reduce the effects of oscillopsia.     

Differential effects of ambivalent visual-vestibular-somatosensory stimulation on the perception of self-motion

The direction of perceived self-motion was determined as a function of combined visual-vestibular and vestibulo-somatosensory stimulations about the earth's vertical z-axis by means of a rotary chair and drum system. The predominant influence of concurrent antagonistic vestibular stimulation on circular vection (CV) even at small accelerations has been demonstrated by several studies in the past. The results of the present paper do not confirm the generally assumed influence of the vestibular system on exocentric motion perception, but instead produce evidence of the dominance of the visual channel even at high acceleration levels. Using a joystick to indicate perceived self-motion, we found the following: Constant CV (visual stimulus velocity: 40 degrees/s) could only be cancelled by vestibular stimulations at oppositely directed mean angular accelerations of 26.9 +/- 9.1 degrees/s2. Smaller accelerations led merely to an acceleration level-related decrease in the perceived velocity of CV. Despite a clear decision with respect to the direction of the perceived motion, subjects (Ss) reported dizziness or even strong motion sickness symptoms. Similar results were obtained with vestibulo-somatosensory interactions. The results are interpreted in terms of an intensive visual support in the processing of visual-vestibular signals, particularly at cortical level, assuming a cortical velocity generator (CVG).     

Nystagmus slow-phase velocity during vestibular,optokinetic, and combined stimulation in the monkey

Summary In Rhesus monkeys the slow-phase velocity of nystagmus was measured during optokinetic, vestibular, and combined stimulation. Accelerations and decelerations of 2.5–40°/s², and rotation at constant velocities of 70–160°/s were applied. During combined visual-vestibular stimulation, nystagmus slow-phase velocity is a function only of the instantaneous stimulus velocity: It has a gain near unity and is independent of the duration and value of the acceleration. The limited linear working range of the vestibular or optokinetic system is thus extended. During deceleration an inappropriate nystagmus response is elicited only when the previous constant velocity rotation was above the saturation velocity of optokinetic nystagmus (OKN). These results are related to single neuron activity recorded in the vestibular nuclei and the flocculus under identical stimulus conditions.     

Detection thresholds to stimulation of ventrobasal complex in cats

Cats were trained to indicate, by bar pressing for food rewards, their detection of stimulation of the ventrobasal (VB) complex delivered through implanted bipolar electrodes. By varying stimulus intensity it was possible to determine thresholds for detection. Scaling stimulus intensity relative to the appearance of a minimal evoked potential allowed comparisons between animals and also comparisons with results obtained by stimulation of peripheral nerve. Animals could detect VB stimulation, but only at stimulus intensities consistently stronger than those required for minimal appearance of an evoked response in ipsilateral primary somatosensory cortex. Results of VB activation differed from cutaneous nerve effects in that VB detection thresholds were markedly influenced by stimulus frequency. They were lowest at frequencies above 30 Hz and increased greatly at lower frequencies. Discomfort or pain did not seem to result even from relatively high stimulus intensities. The results compare well with observations obtained from stimulation of VB in humans. The appearance of an evoked cortical response is not necessarily correlated with behavior. Under appropriate conditions, behavior can be elicited predictably with minimal electrocortical activity; under other conditions detection may be absent even when large numbers of cortical neurons are activated. We suggest that regions of the cerebral cortex receiving thalamocortical projections from VB may not be essential in the detection process.     

Differential involvement of the cerebellum in biological and coherent motion perception

Perception of biological motion (BM) is a fundamental property of the human visual system. It is as yet unclear which role the cerebellum plays with respect to the perceptual analysis of BM represented as point-light displays. Imaging studies investigating BM perception revealed inconsistent results concerning cerebellar contribution. The present study aimed to explore the role of the cerebellum in the perception of BM by testing the performance of BM perception in patients suffering from circumscribed cerebellar lesions and comparing their performance with an age-matched control group. Perceptual performance was investigated in an experimental task testing the threshold to detect BM masked by scrambled motion and a control task testing the detection of motion direction of coherent motion masked by random noise. Results show clear evidence for a differential contribution of the cerebellum to the perceptual analysis of coherent motion compared with BM. Whereas the ability to detect BM masked by scrambled motion was unaffected in the patient group, their ability to discriminate the direction of coherent motion in random noise was substantially affected. We conclude that intact cerebellar function is not a prerequisite for a preserved ability to detect BM. Because the dorsal motion pathway as well as the ventral form pathway contribute to the visual perception of BM, the question of whether cerebellar dysfunction affecting the dorsal pathway is compensated for by the unaffected ventral pathway or whether perceptual analysis of BM is performed completely without cerebellar contribution remains to be determined.     

The timing of feature-based attentional effects during object perception

Allocating attention to basic features such as colour enhances perception of the respective features throughout the visual field. We have previously shown that feature-based attention also plays a role for more complex features required for object perception. To investigate at which level object perception is modulated by feature-based attention we recorded high-density event-related potentials (ERPs). Participants detected contour-defined objects or motion, and were informed to expect each feature dimension. Participants perceived contour-defined objects and motion better when they expected the congruent feature. This is consistent with modulation of the P1 when attending to lower-level features. For contours, modulation occurred at 290 ms, first at frontal electrodes and then at posterior sites, associated with sources in ventral visual areas accompanied by greater signal strength. This pattern of results is consistent with what has been observed in response to illusory contours. Our data provide novel insights into the contribution of feature-based attention to object perception that are associated with higher tier brain areas.     

Direction-specific motion blindness induced by focal stimulation of human extrastriate cortex

Motion blindness (MB) or akinetopsia is the selective disturbance of visual motion perception while other features of the visual scene such as colour and shape are normally perceived. Chronic and transient forms of MB are characterized by a global deficit of direction discrimination (pandirectional), which is generally assumed to result from damage to, or interference with, the motion complex MT+/V5. However, the most characteristic feature of primate MT-neurons is not their motion specificity, but their preference for one direction of motion (direction specificity). Here, we report that focal electrical stimulation in the human posterior temporal lobe selectively impaired the perception of motion in one direction while the perception of motion in other directions was completely normal (unidirectional MB). In addition, the direction of MB was found to depend on the brain area stimulated. It is argued that direction specificity for visual motion is not only represented at the single neuron level, but also in much larger cortical units.     

Audiovisual synchrony perception for music, speech, and object actions

We investigated the perception of synchrony for complex audiovisual events. In Experiment 1, a series of music (guitar and piano), speech (sentences), and object action video clips were presented at a range of stimulus onset asynchronies (SOAs) using the method of constant stimuli. Participants made unspeeded temporal order judgments (TOJs) regarding which stream (auditory or visual) appeared to have been presented first. Temporal discrimination accuracy was significantly better for the object actions than for the speech video clips, and both were significantly better than for the music video clips. In order to investigate whether or not these differences in TOJ performance were driven by differences in stimulus familiarity, we conducted a second experiment using brief speech (syllables), music (guitar), and object action video clips of fixed duration together with temporally reversed (i.e., less familiar) versions of the same stimuli. The results showed no main effect of stimulus type on temporal discrimination accuracy. Interestingly, however, reversing the video clips resulted in a significant decrement in temporal discrimination accuracy as compared to the normally presented for the music and object actions clips, but not for the speech stimuli. Overall, our results suggest that cross-modal temporal discrimination performance is better for audiovisual stimuli of lower complexity as compared to stimuli having continuously varying properties (e.g., syllables versus words and/or sentences).     

A neural architecture for visual motion perception: Group and element apparent motion

A neural network model of motion segmentation by visual cortex is described. The model's properties are illustrated by simulating on the computer data concerning group and element apparent motion, including the tendency for group motion to occur at longer ISIs and under conditions of short visual persistence. These phenomena challenge recent vision models because the switch between group and element motion is determined by changing the timing of image displays whose elements flash on and off but do not otherwise move through time. The model clarifies the dependence of short-range and long-range motion on a spatial scale. Its design specifies how sustained response cells and transient response cells cooperate and compete in successive processing stages to generate motion signals that are sensitive to direction-of-motion, yet insensitive to direction-of-contrast. Properties of beta motion, phi motion, gamma motion, and Ternus motion are explained. A number of prior motion models are clarified, transformed, and unified, including the Reichardt model, Marr-Ullman model, Burt-Sperling model, Nakayama-Loomis model, and NADEL model. Apparent motion and real motion generate testably different model properties. The model clarifies how preprocessing of motion signals by a motion OC Filter is joined to long-range cooperative motion mechanisms in a motion CC Loop to control phenomena such as induced motion, motion capture, and motion after effects. The total model system is a motion Boundary Contour System (BCS) that is computed in parallel with the static BCS of Grossberg and Mingolla before both systems cooperate to generate a boundary representation for 3-D visual form perception.     

Repetitive transcranial magnetic stimulation of human area MT/V5 disrupts perception and storage of the motion aftereffect

Following adaptation to a moving stimulus, the introduction of a stationary pattern creates the illusion of motion. This phenomenon, known as the motion aftereffect (MAE), can be delayed by placing a blank storage interval between the adapting and test stimuli. Human motion selective area MT/V5 has been proposed as the likely neural origin of MAEs. To examine the role of MT/V5 in perceiving and storing MAEs, we applied repetitive transcranial magnetic stimulation (rTMS) to this area during a 10 s storage interval and while subjects perceived illusory motion. Our results show that rTMS disrupts perception of the MAE when it is delivered in the early parts of the storage period and when it is applied during the perceptual MAE itself. Stimulation of control regions corresponding to V1 or Cz did not affect the MAE. In addition, magnetic stimulation of dorsolateral prefrontal and posterior parietal cortices did not disrupt MAE perception. These data provide experimental support for the notion that MT/V5 subserves perception and storage of the motion aftereffect.     

Perceived state of self during motion can differentially modulate numerical magnitude allocation

Although a direct relationship between numerical allocation and spatial attention has been proposed, recent research suggests that these processes are not directly coupled. In keeping with this, spatial attention shifts induced either via visual or vestibular motion can modulate numerical allocation in some circumstances but not in others. In addition to shifting spatial attention, visual or vestibular motion paradigms also (i) elicit compensatory eye movements which themselves can influence numerical processing and (ii) alter the perceptual state of ‘self’, inducing changes in bodily self‐consciousness impacting upon cognitive mechanisms. Thus, the precise mechanism by which motion modulates numerical allocation remains unknown. We sought to investigate the influence that different perceptual experiences of motion have upon numerical magnitude allocation while controlling for both eye movements and task‐related effects. We first used optokinetic visual motion stimulation (OKS) to elicit the perceptual experience of either ‘visual world’ or ‘self’‐motion during which eye movements were identical. In a second experiment, we used a vestibular protocol examining the effects of perceived and subliminal angular rotations in darkness, which also provoked identical eye movements. We observed that during the perceptual experience of ‘visual world’ motion, rightward OKS‐biased judgments towards smaller numbers, whereas leftward OKS‐biased judgments towards larger numbers. During the perceptual experience of ‘self‐motion’, judgments were biased towards larger numbers irrespective of the OKS direction. Contrastingly, vestibular motion perception was found not to modulate numerical magnitude allocation, nor was there any differential modulation when comparing ‘perceived’ vs. ‘subliminal’ rotations. We provide a novel demonstration that numerical magnitude allocation can be differentially modulated by the perceptual state of self during visual but not vestibular mediated motion.     

Distinct cerebellar regions for body motion discrimination

Visual processing of human movements is critical for adaptive social behavior. Cerebellar activations have been observed during biological motion discrimination in prior neuroimaging studies, and cerebellar lesions may be detrimental for this task. However, whether the cerebellum plays a causal role in biological motion discrimination has never been tested. Here, we addressed this issue in three different experiments by interfering with the posterior cerebellar lobe using transcranial magnetic stimulation (TMS) during a biological discrimination task. In Experiments 1 and 2, we found that TMS delivered at onset of the visual stimuli over the vermis (vermal lobule VI), but not over the left cerebellar hemisphere (left lobule VI/Crus I), interfered with participants’ ability to distinguish biological from scrambled motion compared to stimulation of a control site (vertex). Interestingly, when stimulation was delivered at a later time point (300 ms after stimulus onset), participants performed worse when TMS was delivered over the left cerebellar hemisphere compared to the vermis and the vertex (Experiment 3). Our data show that the posterior cerebellum is causally involved in biological motion discrimination and suggest that different sectors of the posterior cerebellar lobe may contribute to the task at different time points.     

Visual object and face processing in mild-to-moderate Alzheimer's disease: from segmentation to imagination

Little is known about the fate of higher level visual perception and visual mental imagery in the early stages of Alzheimer's disease (AD). In this study, we assessed these abilities in a group of mild-to-moderate AD patients using tasks selected to satisfy two main criteria. First, they have been shown to be sensitive to impairments of perception and imagery caused by other neurological conditions. Second, they test specific stages of visual perception and cognition in a reasonably selective manner. These stages were (in their normal order of occurrence during perception): the segmentation of different local points of the visual field into regions belonging to distinct objects; the representation of the shapes of these segmented regions in the image; the construction of more abstract shape representations that possess constancy over changes in size, location, orientation or illumination (assessed separately for faces and objects); the use of these perceived shape representations to access stored shape representations; and the access of lexical semantic representations from these high-level visual representations. Additional tasks tested the top-down activation of earlier visual representations from the semantic level in visual mental imagery. Our findings indicate small, but in most cases reliable, impairments in visual perception, which are independent of degree of cognitive decline. Deficits in basic shape processing influenced performance on some higher level visual tasks, but did not contribute to poor performance on face processing, or to the profound deficit on object naming. The latter of these is related to semantic-lexical impairment.     

Neural correlates of object size and object location during grasping actions

The visuo‐motor channel hypothesis (Jeannerod, 1981) postulates that grasping movements consist of a grip and a transport component differing in their reliance on intrinsic vs. extrinsic object properties (e.g. size vs. location, respectively). While recent neuroimaging studies have revealed separate brain areas implicated in grip and transport components within the parietal lobe, less is known about the neural processing of extrinsic and intrinsic properties of objects for grasping actions. We used functional magnetic resonance imaging adaptation to examine the cortical areas involved in processing object size, object location or both. Participants grasped (using the dominant right hand) or passively viewed sequential pairs of objects that could differ in size, location or both. We hypothesized that if intrinsic and extrinsic object properties are processed separately, as suggested by the visuo‐motor channel hypothesis, we would observe adaptation to object size in areas that code the grip and adaptation to location in areas that code the transport component. On the other hand, if intrinsic and extrinsic object properties are not processed separately, brain areas involved in grasping may show adaptation to both object size and location. We found adaptation to object size for grasping movements in the left anterior intraparietal sulcus (aIPS), in agreement with the idea that object size is processed separately from location. In addition, the left superior parietal occipital sulcus (SPOC), primary somatosensory and motor area (S1/M1), precuneus, dorsal premotor cortex (PMd), and supplementary motor area (SMA) showed non‐additive adaptation to both object size and location. We propose different roles for the aIPS as compared with the SPOC, S1/M1, precuneus, PMd and SMA. In particular, while the aIPS codes intrinsic object properties, which are relevant for hand preshaping and force scaling, area SPOC, S1/M1, precuneus, PMd and SMA code intrinsic as well as extrinsic object properties, both of which are relevant for digit positioning during grasping.     

The visual perception of human and animal motion in point-light displays

Abstract

Mounting neurophysiological evidence indicates that the visual analysis of human movement differs from the visual analysis of other categories of complex movement. If different patterns of neural activity underlie visual percepts of human and nonhuman movement, then psychophysical measures should elucidate different patterns of visual sensitivity to human movement and similarly complex, but nonhuman movement. To test this prediction, two psychophysical studies compared visual sensitivity to human and animal motions. Using a simultaneous masking paradigm, observers performed a coherent motion detection task with point-light displays of human and horse gait, presented upright and inverted. While task performance indicated the use of configural processing during the detection of both human and horse motion, observers demonstrated greater visual sensitivity to coherent human motion than coherent horse motion. Recent experience influenced orientation dependence for both types of motion. Together with previous neurophysiological findings, these psychophysical results suggest that the visual perception of human movement is both distinct from and shares commonalities with the visual perception of similarly complex, nonhuman movement.     

Current perception thresholds in toe-to-digit transplantation and digit-to-digit replantation

Recovery of digital nerve function in toe-to-digit transplantation and digit-to-digit replantation was evaluated by transcutaneous constant current sine wave stimulation at 5-, 250-, and 2000-Hz frequencies to determine the current perception thresholds (CPT). For toe transplantation and digit replantation, the mean interval between injury and surgery was 9 months and 7 h, respectively, while the mean interval between surgery and CPT study was 52 months and 20 months, respectively. Control CPTs evoked by three frequency stimuli were obtained from contralateral corresponding normal finger and normal toe. Normal finger had significantly lower 250- and 2000-Hz CPTs than normal toe, but the 5-Hz CPT was not different between them. Replanted digit achieved nearly complete recovery of these three frequency CPTs when compared to normal finger. In toe transplantation, 2000-Hz CPT was comparable to normal finger, while 5- and 250-Hz CPTs were comparable to normal toe. The present findings suggest that the transplanted toe was intermediate between normal finger and normal toe, but more like normal toe than normal finger with regard to detection thresholds of the current-evoked sensation. © 1996 John Wiley & Sons, Inc.     

Walking performance of vestibular-defective patients before and after unilateral vestibular neurotomy

The present study investigated goal-directed linear locomotion in nine Menière’s patients before and after (1 week, 1 and 3 months) a curative unilateral vestibular neurotomy (UVN). Experiments were done using a 3D motion analysis system in subjects walking eyes open (EO) and eyes closed (EC) towards a real or memorized target, respectively. Locomotor pattern (velocity, step length, step frequency and walk ratio) and walking trajectory deviations were evaluated for normal and fast speeds of locomotion and compared to those recorded in 10 healthy subjects. Before UVN, patients showed no walking deviation but gait pattern changes characterized by slower walks compared to the controls, mainly due to step length and step frequency reductions for both visual conditions and locomotion speeds. In the acute stage after UVN, locomotor pattern impairments were significantly accentuated. On the other hand, patients showed strong walking deviations towards the lesioned side with EC. Opposite lateral deviation towards the intact side were observed with EO for normal speed only. Recovery from impaired locomotor pattern was achieved within 1 month for normal speed but remained uncompensated 3 months post-lesion for fast speed particularly in EC condition. Finally, the walking trajectory deviation towards the lesioned side in the dark was maintained up to 3 months after UVN. The results show that central processing of visual and vestibular cues contributes to an accurate locomotor pointing. They argue for an increased weight of visual reference frame on locomotor functions when vestibular function is unilaterally impaired.     

Lack of visual evoked potentials amplitude decrement during prolonged reversal and motion stimulation in migraineurs

ObjectiveWe evaluated response decrement during a short time repetitive low and high contrast reversal and low contrast motion stimulation in controls and migraineurs.MethodsA total of 39 migraine patients (out of which 19 were in the interictal period and without prophylactic treatment) and 36 healthy volunteers were examined using pattern-reversal (PR-VEP) and motion-onset (M-VEP) visual evoked potentials. Binocular stimulation lasted 2.5 min and the decrement assessment was blinded.ResultsEvidence of significant decrement was observed in healthy volunteers for high contrast PR-VEP amplitude of P100-N75 ratios between the fifth and first blocks (0.9; p = 0.001) with a linear decline (−0.7 μV/min, p = 0.001) and in the P100-N145 amplitude with linear decline (−0.5 μV/min, p = 0.004). Significant decrement was also observed for the ratio between the fifth and first block P1-N2 amplitudes in M-VEP (0.9, p = 0.006). No significant decrement was noted in the low contrast PR-VEP or among migraineurs.ConclusionsWe confirm differences in decrease of VEPs amplitude during short term examination between controls and migraineurs. We showed the decrement deficit also in the extrastriatal regions of the migraineurs’ visual cortex.SignificanceLow contrast and motion-onset stimuli in short time decrement assessment did not increase the test sensitivity.