Eugene R. Wist

Identification of the visual motion area (area V5) in the human brain by dipole source analysis.

The retinal periphery of nine healthy subjects was stimulated with computer-generated random-dot kinematograms. These stimuli provided almost isolated visual motion information and minimal position cues. Pattern-reversal stimuli at the same location in the visual field were used for control. Stimulus-related electrical brain activity was recorded from 29 scalp electrodes. Total mean and individual data were analyzed with a spatiotemporal multiple dipole model. The scalp potentials showed a different spatial distribution for motion and pattern stimulation in the time range of 160–200 ms. In this epoch, the predominant motion-related source activity was localized in the region of the contralateral occipital-temporal-parietal border. A significant ipsilateral source activity was not found. The predominant source activity related to the pattern stimulus occurred in the same epoch. The corresponding equivalent dipole was localized more medially and deeper in the brain. The orientation of these major dipole activities was markedly different. These dipoles appeared to represent activity of distinct extrastriate areas, in contrast to earlier activity which was modelled by more posterior dipoles in the occipital lobe. The latter dipoles were at comparable contralateral locations and had similar peak activities around 100 ms, suggesting an origin in the striate cortex.

Development of dynamic vision based on motion contrast

Abstract The development of dynamic vision was investigated in 400 healthy subjects (200 females and 200 males) aged between 4 and 24 years. The test consisted of a computer-generated random-dot kinematogram in which a Landolt ring was briefly presented as a form-from-motion stimulus. Motion contrast between the ring and background was varied in terms of the percentage of dots moving coherently within the ring in four levels (100%, 50%, 30%, and 20%). The subject’s task was to indicate the position of a gap in the ring (left, right, top, bottom). Results show a clear increase in performance with age for all motion contrast levels, with the greatest changes for the lowest levels. Adult performance was reached at the age of 15 years. Luminance-based static acuity measured with the Landolt test was poorly correlated with acuity for its form-from-motion analogue.

The Scintillating Grid Illusion

Disk-shaped luminance increments were added to the intersections of a Hermann grid consisting of medium grey bars on a black background. Illusory spots, darker than the background, were perceived as flashing within the white disks with each flick of the eye. This striking phenomenon may be referred to as the scintillating grid illusion. We determined the conditions necessary for cancelling the Hermann grid illusion, as well as the luminance requirements and the size ratio between disks and bars that elicits the scintillation effect. The fact that scanning eye movements are necessary to produce the scintillation effect sets it apart from the Hermann grid illusion.

Electrophysiological evidence for visual-vestibular interaction in man

The aim of the experiments reported here was to confirm electrophysiologically the results of psychophysical experiments, which demonstrated that thresholds for object-motion detection are significantly raised during both concurrent active or passive sinusoidal head oscillations and during visually induced self-motion perception (circularvection, CV). This intersensory inhibition could now be demonstrated electrophysiologically by recording visual motion evoked potentials both during concurrent sinusoidal head oscillations and during visually induced apparent self-motion of the objectively stationary subject. Recordings of visual contrast reversal evoked potentials failed to reveal such an interaction. Perceptual phenomena with multisensory stimulation are well described in the literature. Berthoz et al. demonstrated the dominant influence of the visual channel on vestibular thresholds such that the detection of a suprathreshold vestibular stimulation was clearly impaired by a simultaneously moving visual pattern inducing linearvection and vice versa. Comparable results are reported for circularvection. Evidence for inhibitory interaction between object-motion and simultaneous self-motion perception also exists. Electrophysiological data on intersensory interaction in humans have only been reported between electrical stimulation of a limb and its concurrent movement by means of scalp-recorded somatosensory-evoked potentials (SSEPs) (e.g. refs. 3, 5). Electrophysiological evidence for the interaction of visual object-motion and vestibular self-motion perception in humans has never been reported in the literature thus far, though Hood and Kayan demonstrated that retinal image motion makes a contribution to the vestibularly evoked bioelectric response.

Motion Aftereffects with Random-Dot Chequerboard Kinematograms: Relation between Psychophysical and VEP Measures

A random-dot chequerboard kinematogram was used to investigate the effect of motion adaptation both on evoked potentials and on motion aftereffects (MAEs). The experimental paradigm used allowed simultaneous measurement of both variables. Each adaptation period was followed by a series of 5 short test stimuli to which evoked potentials were recorded. Motion aftereffects were observed in the intervals between test stimuli. An inverse relationship between mean N2–P1 amplitude and mean reported MAEs was found as a function of adaptation durations of 1.4, 5.6, and 17.5 s. When the shortest and longest adaptation durations were compared, this relationship held for thirteen of fourteen subjects tested when adaptation-motion and test-motion directions corresponded and for twelve of fourteen subjects when they were opposed. The possibility that the effect of motion adaptation on N2–P1 amplitude was due to local luminance-contrast adaptation is discussed and shown to be unlikely. The suitability of this paradigm for the combined psychophysical and electrophysiological assessment of disturbances in motion perception is discussed.

Motion evoked brain potentials parallel the consistency of coherent motion perception in humans.

The perception of global coherent motion perception in complex motion patterns containing different direction vectors was investigated. Random dot kinematograms (RDK), plaids and fragmented plaid pattern were presented in which direction vectors of the moving elements were varied. In order to elicit coherent motion perception, all elements were displaced in the same direction (delta0 degrees). In a second condition, fifty percent of the elements were moved diagonally downwards to the left, with the remaining elements moving orthogonally (delta 90 degrees). Simultaneously with psychophysical judgements on the perceived motion direction, visual evoked potentials (VEPs) were recorded at occipital electrode positions. Onset of a global coherent motion was associated with a VEP negativity occurring at about 200 ms. The amplitude of this component was clearly reduced when local ambiguous signals could not be integrated to produce the perception of global coherent motion.

A computer-assisted test for the electrophysiological and psychophysical measurement of dynamic visual function based on motion contrast

A new test is described that allows for electrophysiological and psychophysical measurement of visual function based on motion contrast. In a computer-generated random-dot display, completely camouflaged Landolt rings become visible only when dots within the target area are moved briefly while those of the background remain stationary. Thus, detection of contours and the location of the gap in the ring rely on motion contrast (form-from-motion) instead of luminance contrast. A standard version of this test has been used to assess visual performance in relation to age, in screening professional groups (truck drivers) and in clinical groups (glaucoma patients). Aside from this standard version, the computer program easily allows for various modifications. These include the option of a synchronizing trigger signal to allow for recording of time-locked motion-onset visual-evoked responses, the reversal of target and background motion, and the displacement of random-dot targets across stationary backgrounds. In all instances, task difficulty is manipulated by changing the percentage of moving dots within the target (or background). The present test offers a short, convenient method to probe dynamic visual functions relying on surprathreshold motion-contrast stimuli and complements other routine tests of form, contrast, depth, and color vision.

The scintillating grid illusion during smooth pursuit, stimulus motion, and brief exposure in humans

The Scintillating Grid Illusion occurs when small white disks are superimposed onto the intersections of a grey-on-black Hermann grid. As a result illusory dark spots are seen at numerous crossings, flashing with each flick of the eye and changing their location and distribution with each saccade. The illusion is absent with steady fixation. The present study shows that saccadic eye movements are not necessary to produce the illusion. Rather, the illusion was also found to occur (i) during smooth pursuit movements when the grid was stationary, (ii) during smooth displacement of the grid with the gaze kept steady, and (iii) during brief exposures of the stationary grid. It is concluded that, while transient stimulation is essential for generating the illusion, reduction in effective luminance contrast resulting from brief exposure and high stimulus speed are responsible for reductions in its strength.

MARDER--multi-axes rotation device for experimental research. A new concept for investigations of the vestibular, oculomotor, and visual systems of humans in three-dimensional space.

A hydraulically driven, digitally servo-controlled multi-axes rotary chair is described. This device generates motion profiles with the subjects head in the center of rotation mainly in order to adequately stimulate the semicircular canals which are sensitive sensors for angular accelerations. This newly developed apparatus allows for motion stimuli which are below the vestibular threshold up to accelerations of 12 rad/s2 (688 degrees/s2) and is thus suitable for a variety of experiments in the field of vestibular, oculomotor, and intersensory research in 3-dimensional space.

Perception of direction of visual motion. I. Influence of angular body acceleration and tilt

We investigated, psychophysically, the influence of body rotation on visual motion direction thresholds for both upright sitting and tilted observers. Four angular accelerations (0, 20, 40 and 60 degrees/s2) were combined with 3 concurrent backward-tilt positions (0, 45 and 90 degrees). This led to combined stimulation of the semicircular canals and otoliths. Vestibular stimulation was combined with a visual motion stimulus. Random-dot kinematograms in which varying percentages of pixels coherently moving to the left were presented upon a background of otherwise randomly moving pixels (random walk). The smallest percentage of coherently moving pixels leading to a clear perception of motion direction represented as the perceptual threshold. Angular accelerations about the longitudinal body axis significantly increased motion-direction thresholds. Concurrent backward tilt did not influence thresholds. These results differ from those of studies in which translational linear acceleration was employed. Our results support the view that it is necessary to distinguish between linear acceleration caused by gravitational forces and that caused by additional linear accelerations about the x-, y-, and z-axes.