Biomedical subjects

I P Howard

Publications and source records attributed to I P Howard.

At least 19 recordsLinked to original sources

PubMed1992

Visual pursuit over textured backgrounds in different depth planes.

Several investigators have reported that voluntary pursuit of a moving object is less smooth when it moves over a textured background compared with when it moves over a dark background. Furthermore, when a person fixates a stationary target on a moving background, microdrifts of the eyes occur in the direction of motion of the background. These two facts suggest that OKN is not completely switched off during voluntary pursuit or fixation. Howard and Simpson (1989) found that optokinetic nystagmus (OKN) is only weakly evoked by moving stimuli which are out of the plane of convergence. This led to the prediction that voluntary pursuit of a visual target should be disrupted more by a stationary background in the same depth plane than by a background which is nearer or further away than the pursuit target. Pursuit disruption was measured by the frequency and total amplitude of saccadic intrusions. For horizontal pursuit, disruption was reduced, compared with the coplanar condition, when the textured background was further away than the target but not when it was nearer. For vertical pursuit, disruption was reduced in both the far and near conditions compared with the coplanar condition. In both cases disruption was least when the background was dark.

Adult↗

PubMed1992

Accuracy of aimed arm movements in changed gravity.

We studied the accuracy of aimed arm movements in normal gravity, and during the hypergravity (hyper-G) and microgravity (micro-G) episodes of KC-135 parabolic flights. Subjects pointed at mirror-viewed targets without sight of their arm, and final pointing position was measured by a digitizing pad. Compared with the normal gravity (normal-G) baseline, subjects pointed consistently higher in hyper-G, and still higher in micro-G. Results were not different if subjects viewed targets only during normal-G and pointed at their memorized position under changed gravity (changed-G); this suggests that the "elevator illusion" played a minor role in our study. The observed impairments were attributed to degraded proprioceptive feedback and/or inappropriate motor programs in changed-G. Pointing accuracy improved movement-to-movement but not parabola-to-parabola, indicating that prolonged exposure is needed for sustained adaptation.

Aerospace Medicine↗

PubMed1991

Human cyclovergence as a function of stimulus frequency and amplitude.

By the use of scleral search coils a continuous record of human cyclovergence was obtained while two identical 80 degrees textured patterns, presented dichoptically, oscillated in the frontal plane in counterphase through 1, 3 and 6 degrees of cyclorotation at frequencies between 0.05 and 2 Hz. The amplitude and gain of the response decreased exponentially with increasing stimulus frequency. As stimulus amplitude increased, response amplitude also increased but gain was highest for low-amplitude cyclorotations. For an amplitude of 1 degrees and a frequency of 0.05 Hz the gain reached 0.87 for two subjects. The phase lag increased from a few degrees at a frequency of 0.05 Hz to over 100 degrees at a frequency of 2 Hz. These results suggest that cyclovergence is designed to correct for small, slow drifts in the stereoscopic alignment of the images in the two eyes. Although the disparity in the textured display was not interpreted as slant, it provided a strong stimulus for cyclovergence. The cyclovergence caused a transfer of cyclodisparity into a superimposed vertical line, which was then perceived as slanting in depth.

Adult↗

PubMed1991

Optokinetic torsion: dynamics and relation to circularvection.

Continuous records of optokinetic torsion to sinusoidal inputs were obtained using the electromagnetic scleral search-coil technique. We measured the gain and phase lag of optokinetic torsion in response to a spherical visual display rotating steadily at various angular velocities and sinusoidally at frequencies from 0.2 to 2.0 Hz and at amplitudes from 10 to 80 deg. Gain (peak slow-phase eye velocity over stimulus angular velocity) of up to 0.12 were obtained with stimulus frequencies of 0.2 Hz and declined to an average value of about 0.02 at a frequency of 2.0 Hz. Phase lag was virtually zero at a frequency of 0.2 Hz and increased to over 80 deg at 2.0 Hz. The records from the sinusoidal stimuli show very few quick phases. With increasing stimulus amplitudes, the amplitude of the response increased but its gain declined. We found no evidence of torsional after-nystagmus nor any relation between the torsional response and reports of vection or sensation of body tilt induced by the rotating display. Torsional optokinetic nystagmus is most suited to compensate for low-amplitude, low-frequency stimulus rotation and normally supplements torsion induced by head tilt.

Adult↗

PubMed1991

Induced rotary motion and ocular torsion.

When a large patterned annulus rotates around a stationary sectored disc the latter appears to rotate in the opposite direction. Such induced rotary motion was examined with central discs subtending 5, 20 and 40 deg at the eye, with the surround filling the remainder of the visual field. The annular surround or the central disc could be oscillated sinusoidally around the fixation point through 20 deg at 0.2 Hz. In each case, subjects estimated the angles through which the moving and stationary parts of the display appeared to rotate on one half-cycle. Subjects also estimated the angle of rotation of an oscillating display that filled the visual field. Induced rotation of the centre was around 100% of the inducing amplitude for all disc sizes, but there was no induced motion of the surround when the centre rotated. Ocular torsion was measured under the same conditions, using the scleral search-coil technique. The amplitude of ocular torsion was a function of the size of the stationary or rotating field. Thus, variations in stimulus conditions affected induced rotary motion and ocular torsion in different ways. The implications of the results for theories of induced motion in terms of underregistered eye movements are discussed.

Eye Movements↗

PubMed1991

Induced motion: isolation and dissociation of egocentric and vection-entrained components.

Induced motion (IM) is illusory motion of a stationary test target opposite to the direction of the real motion of the inducing stimulus. We define egocentric IM as an apparent motion of the test target relative to the observer, and vection-entrained IM as an apparent motion of a stationary object along with an apparent motion of the self (vection) induced by the same stimulus. These two forms of IM are often confounded, and tests for distinguishing between them have not been devised. We have devised such tests. Our test for egocentric IM relies on evidence that this form of IM is due mainly to a misregistration of eye movements when optokinetic nystagmus (OKN) is inhibited, and on evidence that OKN is evoked only by stimuli in the plane of convergence. Our test for vection-entrained IM relies on evidence that vection is evoked only by the more distant of two superimposed inducing stimuli. Thus we found egocentric IM to be induced without vection or vection-entrained IM when subjects converged on a foreground moving display with a stationary display in the background, and vection-entrained IM to be induced without egocentric IM when subjects converged on a stationary-foreground display with a moving display in the background. The two types of IM were evoked in opposite directions at the same time when subjects converged on a foreground moving display while a background display moved in the opposite direction. The two forms of IM showed no signs of interaction, and we conclude that they rely on independent motion mechanisms that operate within distinct frames of reference. A control experiment suggested that the depth adjacency effect in IM is determined by the depth adjacency of the inducing stimulus to convergence, not just to the test target.

Acceleration↗

PubMed1991

Visually-induced sickness in normal and bilaterally labyrinthine-defective subjects.

A group of nine normal subjects (with no overt vestibular dysfunction) and a group of 6 bilaterally labyrinthine-defective subjects were exposed to a visual field rotating about an Earth-horizontal axis (orthogonal to the gravity axis). The visual stimulus was provided by a 3-m diameter sphere with random dots rotating at 30, 45, and 60 degrees per second (degree/s) about the stationary subject's roll, pitch and yaw axes. The subject's head was positioned at the center of the sphere such that it experienced apparent motion in all three axes. Results indicated that in the normal group, symptoms of motion sickness were reported in 21 of 27 test-trials. When labyrinthine-defective subjects were exposed to the roll and pitch stimulus, no sickness symptoms were reported or observed. These results strongly suggest that the vestibular system is necessary for sickness induced by moving visual fields.

Adult↗

PubMed1990

Visually-induced tilt during parabolic flights.

A helmet-mounted visual display system was used to study visually induced sensations of self-motion (vection) about the roll, pitch and yaw axes under normal gravity condition (1g) and during the microgravity and hypergravity phases of parabolic flights aboard the NASA KC-135 aircraft. Under each gravity condition, the following parameters were investigated: (1) the subject's perceived body vertical with eyes closed and with eyes open gazing at a stationary random dot display; (2) the magnitude of sensations of body tilt with respect to the subjective vertical, while the subject viewed displays rotating about the roll, pitch and yaw axes; (3) the magnitude of vection; (4) latency of vection. All eleven subjects perceived a definite "up and down" orientation throughout the course of the flight. During the microgravity phase, the average magnitudes of perceived body tilt and self-motion increased significantly, and there was no significant difference in vection latency. These results show that there is a rapid onset of increased dependence on visual inputs for perception of self-orientation and self-motion in weightlessness, and a decreased dependence on otolithic and somatosensory graviceptive information. Anti-motion sickness drugs appear not to affect the parameters measured.

Aerospace Medicine↗

PubMed1990

Shape from shading in different frames of reference.

It has often been reported that, in the absence of information about the direction of illumination, people interpret surface convexities and concavities in accordance with the assumption that illumination comes from above. However, 'above' could mean with reference to gravity, the head or the retina. Yonas et al reported that four-year-old infants use the head more than gravity as the frame of reference in interpreting surface relief but that seven-year-olds make about equal use of the two frames of reference. The potency of these two frames of reference when acting separately and when pitted against each other was measured on adult subjects. For all subjects the 'assumption' about the direction of illumination was predominantly with respect to the head. The gravitational frame was used only when the headcentric frame was irrelevant, and then not consistently.

Adult↗

PubMed1989

Human optokinetic nystagmus is linked to the stereoscopic system.

It was previously proposed that a linkage between the optokinetic system and the stereoscopic system in higher mammals serves to allow these animals to selectively stabilize those parts of the visual scene which lie in the plane of convergence as the animals move forward in a three-dimensional world (Howard and Ohmi, 1984). A new procedure is now described by which OKN gain can be measured as a function of the binocular disparity of the stimulus. With vergence locked on a vertical line, the gain of the slow phase of vertical optokinetic nystagmus (OKN) was recorded in four human subjects as the binocular disparity (stereo depth) of the moving display was changed from -3 degrees to +3 degrees. The gain of OKN was found to be inversely proportional to binocular disparity. Evidence for cells in the visual cortex, MT and MST that are sensitive both to visual motion and binocular disparity is reviewed. It is argued that the activity of cells responsive to direction of motion and zero disparity selectively augments OKN and that this enables humans to stabilize the images of parts of the scene in the plane of regard while ignoring competing motion signals arising from other distances.

Functional Laterality↗

PubMed1989

Suppression of OKN and VOR by afterimages and imaginary objects.

Optokinetic nystagmus (OKN) is suppressed if attention is directed to a centrally placed afterimage superimposed on a moving display. Imagining a stationary object has little or no effect. An afterimage does not provide the retinal slip and misfoveation error signals provided by a stationary object and we have shown that an effective error signal does not arise from occlusion or masking of the display by the afterimage. Although a lack of relative motion between afterimage and moving display could indicate when OKN gain is one, there is no unique relative motion signal associated with a gain of zero. Subjects could partially inhibit the vestibulo-ocular reflex (VOR) in the dark when they imagined a head-fixed object. They could suppress the response more effectively by attending to an afterimage, but the suppression was still only partial. When OKN and VOR were evoked simultaneously, pursuit movements of the eyes could not be suppressed until the vestibular inputs had subsided. We conclude that signals associated with OKN, are fully available to the mechanism that assesses the headcentric motion of objects but that signals associated with VOR are only partially available to that mechanism.

Adult↗

PubMed1989

Up-down asymmetry in human vertical optokinetic nystagmus and afternystagmus: contributions of the central and peripheral retinae.

The vertical optokinetic nystagmus (OKN) of 10 normal subjects and the optokinetic afternystagmus (OKAN) of 3 subjects were measured with the magnetic search coil technique. In order to assess the relative contributions of various retinal areas to the up-down asymmetry in OKN the central and peripheral visual fields were selectively stimulated in four OKN conditions. In the full-field OKN condition the stimulus was a 61 degrees x 64 degrees display of moving random-dots. Overall, full-field OKN gains elicited by upward motion were significantly higher than those elicited by downward motion at stimulus velocities between 30 and 70 degrees/s. In the periphery-only OKN condition a 3 degrees or 6 degrees-wide vertical band occluded the center of the full-field display. Nine of the 10 subjects displayed OKN in this condition. For 6 subjects, the addition of the 6 degrees band to the full field resulted in an increase in the up-down asymmetry at stimulus velocities above 30 degrees/s. For the other three subjects there was a decline in the gains of both upward and downward OKN when the 3 degrees or 6 degrees band was present; the result was directionally symmetric OKN gains. In the central-strip OKN condition only a 6 degrees-wide central vertical strip of moving dots was visible. The gains of central-strip OKN were not significantly different from the full-field responses. A servo controlled centrally-located 10 degrees x 6 degrees moving display was used in the center-only OKN condition. In this condition both upward and downward gains were attenuated and there was no up-down asymmetry. OKAN was measured following a 50-s exposure to either the full-field or center-only OKN display. The stimulus velocity was 30 degrees/s. After viewing the full-field display the 3 subjects displayed OKAN with slow phases upward following upward OKN but there was no downward OKAN following downward OKN. In contrast, there was no consistent directional asymmetry following exposure to the center-only display. The disappearance of the upward preponderance in OKN and OKAN with occlusion of the peripheral retina suggests that the directional asymmetry in vertical OKN exists in the slow OKN system.

Adult↗

PubMed1989

Human horizontal optokinetic nystagmus elicited by the upper versus the lower visual fields.

A 30-deg-high horizontally rotating random-dot display was presented to the central field, and with its more central edge at vertical eccentricities of 0, 2.5, 5, and 10 deg above or below the horizon. Stimulus velocities of 25-100 deg/s and two directions of motion were presented. The mean gain of the slow phases of optokinetic nystagmus (OKN) for five subjects was significantly higher when the stimulus was presented to the lower visual field than when the stimulus was presented to the upper field. This difference was most pronounced when the display was displaced 5 deg from the fovea and moving below 100 deg/s. Our results are consistent with existing psychophysical and physiological evidence for the superiority of the upper retina. In addition, four of the five observors showed significant directional asymmetries.

Adult↗

PubMed1989

Circular vection as a function of the relative sizes, distances, and positions of two competing visual displays.

In studies where it is reported that illusory self-rotation (circular vection) is induced more by peripheral displays than by central displays, eccentricity may have been confounded with perceived relative distance and area. Experiments are reported in which the direction and magnitude of vection induced by a central display in the presence of a surround display were measured. The displays varied in relative distance and area and were presented in isolation, with one moving and one stationary display, or with both moving in opposite directions. A more distant display had more influence over vection than a near display. A central display induced vection if seen in isolation or through a 'window' in a stationary surrounding display. Motion of a more distant central display weakened vection induced by a nearer surrounding display moving the other way. When the two displays had the same area their effects almost cancelled. A moving central display nearer than a textured stationary surround produced vection in the same direction as the moving stimulus. This phenomenon is termed 'contrast-motion vecton' because it is probably due to illusory motion of the surround induced by motion of the centre. Unequivocal statements about the dominance of an eccentric display over a central display cannot be made without considering the relative distances and sizes of the displays and the motion contrast between them.

Adult↗

PubMed1989

Human optokinetic nystagmus: competition between stationary and moving displays.

We reported earlier that occlusion of the central retina and stationary edges have highly interactive effects on the gain of optokinetic nystagmus (OKN; Murasugi, Howard, & Ohmi, 1986). In this study, we explored this effect in more detail. A central occluding band of variable height, flanked by vertical bars, was superimposed onto an array of dots moving at 30 degrees per second. The height of the occluding band required to abolish OKN increased with the separation of the vertical bars. For bars 3.5 degrees apart, OKN was abolished in most subjects when a band only 6' high ran between them. For bars 75 degrees apart, a band at least 20 degrees in height was required to abolish the response. The effects of the stationary figure depended to some extent on the subject's attention, but only at intermediate values of bar separation. Both low- and high-level mechanisms are proposed to account for the results.

Adult↗

PubMed1988

Effect of stationary objects on illusory forward self-motion induced by a looming display.

It has previously been shown that when a moving and a stationary display are superimposed, illusory self-rotation (circular vection) is induced only when the moving display appears as the background. Three experiments are reported on the extent to which illusory forward self-motion (forward vection) induced by a looming display is inhibited by a superimposed stationary display as a function of the size and location of the stationary display and of the depth between the stationary and looming displays. Results showed that forward vection was controlled by the display that was perceived as the background, and background stationary displays suppressed forward vection by about the same amount whatever their size and eccentricity. Also, the perception of foreground-background properties of competing displays determined which controlled forward vection, and this control was not tied to specific depth cues. The inhibitory effect of a stationary background on forward vection was, however, weaker than that found with circular vection. This difference makes sense because, for forward body motion, the image of a distant scene is virtually stationary whereas, when the body rotates, it is not.

Attention↗

PubMed1987

Human optokinetic nystagmus in response to moving binocularly disparate stimuli.

Physiological and behavioral evidence shows that the directionally preponderant subcortical control of optokinetic nystagmus (OKN) in lower mammals is supplemented in higher mammals by bidirectional cortical control. It is hypothesized that this cortical control allows higher mammals to cope with the parallactic movement of the scene produced by linear motion of the body. In particular, it is hypothesized that a coupling between OKN and stereopsis allows higher mammals to stabilize the images of objects within the plane of fixation while ignoring motion signals from objects at other distances. According to this hypothesis the gain of the slow phase of OKN should be highest for binocularly fused moving stimuli and attenuated for binocularly disparate displays. The results of Experiment 1 confirmed this prediction although the effects of accommodation were not ruled out completely. In Experiment 2 a display moving in one direction was presented across the central retina at the same time as one moving in the opposite direction was presented in the upper and lower periphery. It was found that subjects do not show OKN in the direction of the peripheral display unless it is binocularly fused and the central display is disparate. In Experiment 3 a stationary display of dots was superimposed on a moving display. It was found that OKN is not inhibited by the stationary display when it has a horizontal disparity and the moving display is fused. Experiment 4 found that horizontal OKN is disrupted by the sudden introduction of a vertical disparity in the stimulus. Since accommodative state was kept constant in the last three experiments, the data show that binocular disparities can help a person to stabilize selectively the image of one moving display while ignoring conflicting motion signals from another display.

Accommodation, Ocular↗

PubMed1987

Circular vection as a function of foreground-background relationships.

It has previously been reported that illusory self-rotation (circular vection) is most effectively induced by the more distant of two moving displays. Experiments are reported in which the relative effectiveness of two superimposed displays in generating circular vection as a function of (i) the separation in depth between them, (ii) their perceived relative distances, and (iii) which display was in the plane of focus was investigated. Circular vection was governed by the motion of the display that was perceived to be the more distant, even when it was actually nearer. However, actual or perceived distance was found to be not the crucial factor in circular vection because even when the distance between the two displays was virtually zero, vection was controlled by the display perceived to be in the background. When the displays were well separated in depth, vection was not affected by whether the near or the far display was in the plane of focus, nor by which display was fixed or pursued by the eyes.

Depth Perception↗