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Biomedical subjects

L R Harris

Publications and source records attributed to L R Harris.

47 records · Page 3Linked to original sources
PubMed1980

The superior colliculus and movements of the head and eyes in cats.

1. The superior colliculus has been studied in alert cats which were restrained and whose head and eye movements were monitored.2. Microstimulation within the rostral part of the colliculus, which represents the central 25 deg of the visual field, evokes saccadic eye movements that carry the area centralis to that region of visual space previously occupied by the receptive fields of the cells that were stimulated (;foveation'). These saccades are not generally accompanied by a movement of the head.3. At more caudal locations the visual receptive fields of collicular neurones lie at a greater eccentricity relative to the area centralis than the maximum possible deviation of the eyes from the central position in normal circumstances. At these sites electrical stimulation produces a combined movement of the head and eyes whose co-ordination is identical to that of natural gaze changes in response to novel stimuli. Prolonged stimulation results in the addition of further co-ordinated eye-head movements.4. The addition of a movement of the head does not increase the area of visual space that may be foveated in a single gaze change. Movements of the head are compensated by the vestibulo-ocular reflex. The visual receptive fields of cells at more caudal locations cannot be foveated by a single gaze change.5. A third class of response to electrical stimulation is also occasionally found in the caudal part of the colliculus. The head movement often begins before an accompanying eye movement and continues smoothly for the entire stimulation duration or until limited by the range of mobility.6. Electrical microstimulation was never found to produce so-called ;goal-directed eye movement, in which the eyes move, in a single saccade, to a fixed orbital position regardless of their starting position.7. Ninety-nine cells were recorded from the superior colliculus and classified into four types based on their responses, or lack of responses, during or preceding eye and head movements. Type 1 cells did not show changes in activity prior to gaze changes Type 2 cells were inhibited prior to and during eye movements. Cells discharging before normal saccadic eye movements (type 3) were found only in the rostral part of the colliculus. Cells discharging before head movements (type 4) were found only in the caudal part.8. These results are discussed with respect to the production of gaze changes in the cat.

Action Potentials↗
PubMed1980

Coal liquefaction: recent findings in occupational safety and health.

Some coal liquefaction materials are potentially hazardous because of similarities to materials in other related coal processes that have been associated with a high cancer risk. Limited survey data obtained by NIOSH at two coal liquefaction pilot plants have shown that workers were exposed to low concentrations of certain polynuclear aromatic hydrocarbons (PNA's) and aromatic amines, some of which are suspected carcinogens. The degree of risk incurred by such exposures cannot be determined because toxicologic data allowing for the evaluation of effects at low exposure levels are unavailable. These industrial hygiene studies are discussed as well as recent health and process aspects of this technology.

Air Pollutants↗
PubMed1977

The effects of remote retinal stimulation on the responses of cat retinal ganglion cells.

1. Action potentials were recorded from optic nerve fibres of lightly anaesthetized cats while parts of the retina remote from the receptive field were stimulated by a shifting grating. 2. Vigorous responses can be obtained under these conditions, confirming McIlwain (1966), Krüger & Fischer (1973), and others. 3. These 'shift responses' are not caused by fluctuations of stray light because (a) they cannot be reduced by deliberately increasing or decreasing the light falling on the receptive field synchronously with the shifting grating; (b) a steady adapting light applied to the receptive field does not raise the threshold for the responses, whereas adapting light on the peripheral retina does, and (c) the threshold for the responses is elevated more following bleaching adaptation of the periphery than following bleaching adaptation of the centre. 4. Shift responses are strong, of short latency, and brief in duration in brisk-transient (Y-type) neurones. With few exceptions they are weak but long-lasting in brisk-sustained (X-type) neurones. 5. Shift responses are unlike responses from the main receptive field in having a distinct threshold; the magnitude of the response to weak gratings is not simply proportional to contrast, as is the case with weak stimuli applied to the receptive field. 6. It is thought that the excitatory pathway may involve amacrine cells, and that this mechanism may be concerned with the detection of the shifts of the image that occur with saccadic eye movements.

Action Potentials↗
PubMedDate not supplied

The relative role of visual and non-visual cues in determining the perceived direction of "up": experiments in parabolic flight.

In order to measure the perceived direction of "up", subjects judged the three-dimensional shape of disks shaded to be compatible with illumination from particular directions. By finding which shaded disk appeared most convex, we were able to infer the perceived direction of illumination. This provides an indirect measure of the subject's perception of the direction of "up". The different cues contributing to this percept were separated by varying the orientation of the subject and the orientation of the visual background relative to gravity. We also measured the effect of decreasing or increasing gravity by making these shape judgements throughout all the phases of parabolic flight (0 g, 2 g and 1 g during level flight). The perceived up direction was modeled by a simple vector sum of "up" defined by vision, the body and gravity. In this model, the weighting of the visual cue became negligible under microgravity and hypergravity conditions.

Cues↗
PubMedDate not supplied

Gravity and perceptual stability during translational head movement on earth and in microgravity.

We measured the amount of visual movement judged consistent with translational head movement under normal and microgravity conditions. Subjects wore a virtual reality helmet in which the ratio of the movement of the world to the movement of the head (visual gain) was variable. Using the method of adjustment under normal gravity 10 subjects adjusted the visual gain until the visual world appeared stable during head movements that were either parallel or orthogonal to gravity. Using the method of constant stimuli under normal gravity, seven subjects moved their heads and judged whether the virtual world appeared to move "with" or "against" their movement for several visual gains. One subject repeated the constant stimuli judgements in microgravity during parabolic flight. The accuracy of judgements appeared unaffected by the direction or absence of gravity. Only the variability appeared affected by the absence of gravity. These results are discussed in relation to discomfort during head movements in microgravity.

Gravitation↗
PubMedDate not supplied

Brain stem and cortical contributions to the generation of horizontal optokinetic eye movements in humans.

We evaluated the subcortical pathways' contribution to human adults' horizontal OKN by using a method similar to that used previously with cats (Harris & Smith, 1990; Smith & Harris, 1991). Five normal adults viewed plaids composed of two drifting sinusoidal gratings arranged such that their individual directions of drift were 60 deg or more from the direction of coherent motion of the overall pattern. Physiological evidence indicates that under monocular viewing, nasalward coherent motion gives advantage to any crossed subcortical contribution while temporalward coherent motion minimizes it. We recorded horizontal eye movement by infrared reflection and asked subjects to report the perceived direction of motion. During both binocular and monocular viewing, the direction of the slow phase of OKN fell closer to the direction of coherent movement than to that of the oriented components. Monocular viewing produced no nasal-temporal asymmetries in the influence of coherent motion on the direction of OKN. This suggests that in humans the influence of coherent motion is mediated primarily by cortical mechanisms and, unlike in cats, with little or no involvement of subcortical mechanisms in the generation of horizontal OKN.

Adult↗
PubMedDate not supplied

Sensitivity to full-field visual movement compatible with head rotation: variations with eye-in-head position.

Variations in velocity detection thresholds for full-field visual rotation about various axes are compatible with a simple channel-based system for coding the axis and velocity of the rotation (Harris & Lott, 1995). The present paper looks at the frame of reference for this system. The head-centered, craniotopic reference system and the retinal-based, retinotopic reference systems were separated by using eccentric eye positions. We measured the threshold for detecting full-field visual rotation about a selection of axes in the sagittal plane with the eyes held either 22 1/2 degs up, straight ahead or 22 1/2 degs down in the head. The characteristic features of the variation in detection thresholds did not stay stable in craniotopic coordinates but moved with the eyes and were constant in retinotopic coordinates. This suggests that the coding of head rotation by the visual system is in retinotopic coordinates.

Adult↗
PubMedDate not supplied

Sensitivity to full-field visual movement compatible with head rotation: variations among axes of rotation.

Movement detection thresholds for full-field visual motion about various axes were measured in three subjects using a two-alternative forced-choice staircase method. Thresholds for 1-s exposures to rotation about different rotation axes varied significantly over the range 0.139 +/- 0.05 deg/s to 0.463 +/- 0.166 deg/s. The highest thresholds were found in response to rotation about axes closely aligned to the line of sight. Variations among the thresholds for different axes could not be explained by different movement patterns in the fovea or variations in motion sensitivity with eccentricity. The variations can be well simulated by a three-channel model for coding the axis and velocity of full-field visual motion. A three-channel visual coding system would be well suited for extracting information about self-rotation from a complex pattern of retinal image motion containing components due to both rotation and translation. A three-channel visual motion system would also be readily compatible with vestibular information concerning self-rotation arising from the semicircular canals.

Adult↗