Role of vestibular and neck inputs for the perception of object motion in space

Summary The contribution of vestibular and neck inputs to the perception of visual object motion in space was studied in the absence of a visual background (in the dark) in normal human subjects (Ss). Measures of these contributions were obtained by means of a closed loop nulling procedure; Ss fixed their eyes on a luminous spot (object) and nulled its actual or apparent motion in space during head rotation in space (vestibular stimulus) and/ or trunk rotation relative to the head (neck stimulus) with the help of a joystick. Vestibular and neck contributions were expressed in terms of gain and phase with respect to the visuo-oculomotor/joystick feedback loop which was assumed to have almost ideal transfer characteristics. The stimuli were applied as sinusoidal rotations in the horizontal plane (f= 0.025–0.8 Hz; peak angular displacements, 1–16°). Results: (1) During vestibular stimulation, Ss perceived the object, when kept in fixed alignment with the moving body, as moving in space. However, they underestimated the object motion; the gain was only about 0.7 at 0.2–0.8 Hz and clearly decreased at lower stimulus frequencies, while the phase exhibited a small lead. (2) During pure neck stimulation (trunk rotating relative to the stationary head), the object, when stationary, appeared to move in space counter to the trunk excursion. This neck-contingent object motion illusion was small at 0.2–0.8 Hz, but increased considerably with decreasing frequency, while its phase developed a small lag. (3) Vestibular, neck, and visuo-oculomotor effects summed linearly during combined stimulations. (4) The erroneous vestibular and neck contributions to the object motion perception were complementary to each other, and the perception became about veridical (G1, 0°), when both inputs were combined during head rotation with the trunk stationary. The results are simulated by an extended version of a computer model that previously had been developed to describe vestibular and neck effects on human perception of head motion in space. In the model, the perception of object motion in space is derived from the superposition of three signals, representing object to head, (visuo-oculomotor; head coordinates), head on trunk (neck; trunk coordinates), and trunk in space (vestibular-neck interaction; space coordinates).Supported by Deutsche Forschungsgemeinschaft, SFB 325     

Haptic selective attention by foot and by hand

Nonvisual perceptions of a wielded object's spatial properties are based on the quantities expressing the object's mass distribution, quantities that are invariant during the wielding. The mechanoreceptors underlying the kind of haptic perception involved in wielding – referred to as effortful, kinesthetic, or dynamic touch – are those embedded in the muscles, tendons, and ligaments. The present experiment's focus was the selectivity of this muscle-based form of haptic perception. For an occluded rod grasped by the hand at some intermediate position along its length, participants can attend to and report selectively the rod's full length, its partial lengths (fore or aft of the hand), and the position of the grip. The present experiment evaluated whether participants could similarly attend selectively when wielding by foot. For a given rod attached to and wielded by foot or attached to (i.e. grasped) and wielded by hand, participants reported (by magnitude production) the rod's whole length or fractional length leftward of the point of attachment. On measures of mean perceived length, accuracy, and reliability, the degree of differentiation of partial from full extent achieved by means of the foot matched that achieved by means of the hand. Despite their neural, anatomical, and experiential differences, the lower and upper limbs seem to abide by the same principles of selective muscle-based perception and seem to express this perceptual function with equal facility.     

The selective processing of briefly presented affective pictures: an ERP analysis

Recent event-related potential (ERP) studies revealed the selective processing of affective pictures. The present study explored whether the same phenomenon can be observed when pictures are presented only briefly. Toward this end, pleasant, neutral, and unpleasant pictures from the International Affective Pictures Series were presented for 120 ms while event related potentials were measured by dense sensor arrays. As observed for longer picture presentations, brief affective pictures were selectively processed. Specifically, pleasant and unpleasant pictures were associated with an early endogenous negative shift over temporo-occipital sensors compared to neutral images. In addition, affective pictures elicited enlarged late positive potentials over centro-parietal sensor sites relative to neutral images. These data suggest that a quick glimpse of emotionally relevant stimuli appears sufficient to tune the brain for selective perceptual processing.     

Perceiving a stable world during active rotational and translational head movements

When a person moves through the world, the associated visual displacement of the environment in the opposite direction is not usually seen as external movement but rather as a changing view of a stable world. We measured the amount of visual motion that can be tolerated as compatible with the perception of moving within a stable world during active, sinusoidal, translational and rotational head movement. Head movements were monitored by means of a low-latency, mechanical head tracker and the information was used to update a helmet-mounted visual display. A variable gain was introduced between the head tracker and the display. Ten subjects adjusted this gain until the visual display appeared stable during sinusoidal yaw, pitch and roll head rotations and naso-occipital, inter-aural and dorso-ventral translations at 0.5 Hz. Each head movement was tested with movement either orthogonal to or parallel with gravity. A wide spread of gains was accepted as stable (0.8 to 1.4 for rotation and 1.1 to 1.8 for translation). The gain most likely to be perceived as stable was greater than that required by the geometry (1.2 for rotation; 1.4 for translation). For rotational motion, the mean gains were the same for all axes. For translation there was no effect of whether the movement was inter-aural (mean gain 1.6) or dorso-ventral (mean gain 1.5) and no effect of the relative orientation of the translation direction relative to gravity. However translation in the naso-occipital direction was associated with more closely veridical settings (mean gain 1.1) and narrower standard deviations than in other directions. These findings are discussed in terms of visual and non-visual contributions to the perception of an earth-stable environment during active head movement.     

Effects of the Ebbinghaus figure on grasping are not only due to misjudged size

It is not evident how the small effects of the flankers of the Ebbinghaus figure on peak grip aperture (PGA) should be interpreted. One interpretation is that the flankers influence the estimated size, which in turn influences the grasp. If this interpretation is correct, then only the size-dependent aspects of the grasping movement should depend on the spatial positions of the flankers. An alternative interpretation is that the effect on grip aperture is caused by a change in judgement of the required precision, in which case various aspects of the grasping movement could be influenced by the size and position of the flankers. We presented subjects with a display consisting of a central disk surrounded by four large or small flankers. The array of circular flankers could be rotated by 45°. There were two tasks: to reproduce the perceived size of the central disk, and to grasp the central disk. As in other studies, the reproduced size and the PGA were both influenced by the size of the flankers. The effect on reproduced size settings was independent of the flankers spatial position. Nevertheless, the flankers position did influence the final grip aperture and the grip orientation at PGA and at movement offset. Because the flankers changed more than only the PGA, we conclude that the effect of the flankers on prehension cannot only be because of misjudgement of the size of the central disk.     

Fusion of vestibular and podokinesthetic information during self-turning towards instructed targets

When observers step about their vertical axis ("active turning") without vision they dispose of essentially two sources of information that can tell them by how much they have turned: the vestibular cue which reflects head rotation in space and the "podokinesthetic" cue, a compound of leg proprioceptive afferents and efference copy signals which reflects the observer's motion relative to his support. We ask how these two cues are fused in the process leading to the perception of self-displacement during active turning. To this end we compared the performance of observers in three angular navigation tasks which differed with regard to the number and type of available motion cues: (1) Passive rotation, vestibular cue ( ves) only; observers are standing on a platform which is being rotated. (2) Treadmill stepping, podokinesthetic cue ( pod) only; observers step counter to the rotating platform so as to remain stable in space. (3) Active turning, ves and pod available; observers step around on the stationary platform. In all three tasks, angular velocity varied from trial to trial (15, 30, 60 degrees /s) but was constant during trials. Perception was probed by having the observers signal when they thought to have reached a previously instructed angular displacement, either in space or relative to the platform ("target"; range 60-1080 degrees ). Performance was quantified in terms of the targeting gain (displacement reached by the observer divided by target angle) and of the random error ( E(r)), which records an observer's deviation during single trials from his average performance. Confirming previous observations, E(r) was found to be significantly smaller during active turning than during passive turning, and we now complement these observations by showing that it is also significantly smaller than during treadmill stepping. This behaviour of E(r) is compatible with the idea that ves and pod be averaged during active turning. On the other hand, the observed characteristics of the targeting gain ( G(T)) support this idea only for the case of fast rotations (60 degrees /s); at lower velocities, the gain found during active turning was clearly not the average of the G(T) values recorded in the passive and the treadmill modes. We therefore also discuss alternative scenarios as to how ves and pod could interact, among these one based on the concept of a vestibular eigenmodel. A common denominator of these scenarios is that ves assumes the role of a prerequisite for an optimal use of pod during turning on a stationary support, without itself entering the calculation of displacement perception; this perception would be based exclusively on pod. Finally, it was a consistent observation that during passive rotations cognitive mechanisms fill in for the decaying vestibular signal in the context of the present navigation task, enabling observers to achieve large displacements surprisingly well although the duration of these movements exceeds by far the conventionally cited value of the central vestibular time constant (=20 s).     

Effects of selective NMDA and non-NMDA blockade in the nucleus accumbens on the plus-maze test

Effect of blocking N-methyl-D-aspartic acid (NMDA) and non-NMDA-glutamatergic receptors on performance in the plus-maze was studied in male rats bilaterally cannulated into the nucleus accumbens (Acc). Rats were divided into seven groups that received either 1 microl injections of saline, (+/-)2-amino-7-phosphonoheptanoic acid (AP-7, 0.2, 0.5, or 1 microg) or 2,3 dioxo-6-nitro-1,2,3,4,tetrahydrobenzo-(f)quinoxaline-7-sulphonamide disodium (NBQX, 0.2, 0.5, or 1 microg) 15 min before testing. Time spent in open arm, time per entry, end arrivals, open, closed, and total arm entries, relationship between open-, closed-, and total arm entries, rearing, face-, head-, and body grooming, and number of fecal boli were recorded. Time spent in the open arm increased under AP-7 (0.5 and 1 microg; P<.01) and NBQX (1 microg; P<.05) treatment, whereas time per entry was increased only with AP-7 (1 microg; P<.05). Open arm entries were increased by the intermediate doses of AP-7 (0.5 microg; P<.01) and NBQX (0.5 microg; P<.05); end arrivals were increased by the intermediate dose of AP-7 (0.5 microg/1 microl, P<.05). The frequency of rearing, grooming, and closed arm entries was not affected by the treatment. We conclude that NMDA and non-NMDA-glutamatergic blockade in the Acc lead to a behavioral disinhibition of cortical influences with the median doses, but that at higher doses the blockers have an anxiolytic-like effect.     

Human perception of horizontal trunk and head rotation in space during vestibular and neck stimulation

Summary The vestibular signal of head motion in space must be complemented by a neck signal of the trunk-to-head excursion in order to provide the individual with information on trunk motion in space. This consideration led us to study psychophysically the role of vestibular-neck interaction for human self-motion perception. Subjects (Ss) were presented with passive horizontal rotations of their trunk and/or head (sinusoidal rotations, f=0.025 –0.4 Hz) in the dark for vestibular and neck stimulation, as well as for combinations of both. Ss' perception was evaluated in terms of gain (veridical perception of stimulus magnitude, G=1), phase, and detection threshold. (1) Perception of trunk rotation in space. During vestibular stimulation (whole-body rotation) and neck stimulation (trunk rotation with the head kept stationary) the frequency-transfer characteristics underlying this perception were very similar. The gain fell short; it was only about 0.7 at 0.4 and 0.2 Hz stimulus frequency and was further attenuated with decreasing frequency. In contrast, the phase was close to that of actual trunk position. The gain attenuation was found to be a function of the peak angular velocity of the stimulus, a fact, which we related to a velocity threshold of the order of 1 deg/s. During the various vestibular-neck combinations used, Ss' perception was again erroneous, reflecting essentially the sum of its two non-ideal constituents. However, there was one noticeable exception; during the combination head rotation on stationary trunk, Ss veridically perceived their trunk as stationary (compatible with the notion that the sum yielded zero). (2) Perception of head rotation in space. During vestibular stimulation, Ss' estimates showed the same non-ideal gain-vs.-frequency characteristics as described above for the trunk. Neck stimulation induced an illusion as if the head had been rotated in space. This neck contribution was such that, when it was combined with its vestibular counterpart during head rotation on stationary trunk, the perception became almost veridical. On closer inspection, however, this neck contribution was found to reflect the sum of two components; one was the non-ideal neck signal contributing to the perception of trunk in space, the other was an almost ideal neck signal of head-on-trunk rotation. (3) The results could be described by a simple model. In this model, the erroneous vestibular signal head in space is primarily used to create an internal representation of trunk in space. To this end, it is combined with the closely matching neck signal of trunk to head. The perception of head rotation in space is achieved by summing this trunk in space signal with the almost ideal head on trunk signal, again of nuchal origin. These seeming complex interactions have two implications: (i) the head is referred to trunk coordinates, whereas the trunk is referred to space coordinates; (ii) there is at least one condition in the dark where orientation is correct (despite an erroneous vestibular signal), i.e., during head rotation on stationary trunk.Supported by Deutsche Forschungsgemeinschaft, SFB 325