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Biomedical subjects

L R Harris

Publications and source records attributed to L R Harris.

At least 37 records · Page 2Linked to original sources
PubMed1993

The effect of gravity on the resting position of the cat's eye.

We measured rotation (horizontal, vertical and torisonal) and translation (horizontal and vertical) of the paralysed cat's eye in response to 45 degrees steps of orientation presented in a pseudorandom order around the roll and pitch axes (with respect to the horizontal canals). During changes of position of the animal in the roll plane, the eyes rotated towards the lowest part of the orbit (left with left ear down; top when the cat was upside down, etc.) by an average of 0.55 degree. Changing orientation in the pitch plane evoked vertical rotations of +/- 1.42 degrees (upwards eye movement during forwards head pitch) and torsional rotations of +/- 1.3 degrees. All these rotations taken together suggest that the centre of mass is in front of, below and temporal to the centre of rotation. The eyes translated temporally (thus separating by 0.72 mm) during forward pitching and there was a small vertical displacement (0.23 mm) when the animal was upside down. These findings are discussed with respect to a possible role of the extraocular proprioception system.

Animals↗
PubMed1993

Horizontal saccades to dichoptically presented targets of differing disparities.

Horizontal saccades were elicited to targets of various disparities displayed dichoptically. When the left eye and right eye targets were in the same hemifield, the resulting saccade demonstrated spatial averaging (42%, where 50% represents perfect averaging) between the left and right eye target positions. When the left eye and right eye targets were in opposite hemifields, the saccade was directed to one of the stimuli and was only minimally influenced by the presence of the other. This pattern is similar to that obtained when saccades are made to double targets, both of which are visible to both eyes. These data are discussed in terms of an ecological role for the global effect.

Adult↗
PubMed1992

Motion defined exclusively by second-order characteristics does not evoke optokinetic nystagmus.

We showed high-contrast, second-order motion stimuli to subjects whilst recording their horizontal eye movements. These stimuli were very poor at evoking optokinetic nystagmus. Smooth-pursuit eye movements and fixation were reduced by a masking band +/- 2.5 deg above and below an imaginary fixation point. First-order stimuli evoked vigorous optokinetic nystagmus (OKN) under identical conditions and also when matched for apparent contrast. These findings are discussed in terms of the site of detection of second-order motion.

Adult↗
PubMed1991

Use of plaid patterns to distinguish the corticofugal and direct retinal inputs to the brainstem optokinetic nystagmus generator.

We have recorded the direction of optokinetic nystagmus (OKN) elicited by moving plaid patterns in order to dissociate the pathways that mediate horizontal OKN. The plaids used comprised two drifting sinusoidal gratings arranged such that their individual directions of drift were very different from the direction of coherent motion of the overall pattern. The direction of OKN with binocular viewing was close to the mean of the component directions, suggesting a dominant influence of cortical visual neurons that respond to oriented one-dimensional components of the image. But the direction of OKN was consistently shifted slightly towards the direction of motion of the overall pattern, suggesting a secondary influence responsive to pattern direction. OKN recordings obtained during monocular viewing suggest that this secondary influence reflects the direct retinal pathway to the brainstem structures mediating OKN.

Animals↗
PubMed1989

Auditory and visual neurons in the cat's superior colliculus selective for the direction of apparent motion stimuli.

In the cat, cells of the superior colliculus (SC) and the tectofugal pathways of the visual system are highly selective for the direction of a moving visual stimulus. Deep layer units of SC in addition respond to auditory and somatosensory stimuli, but the proportion of such non-visual cells is usually found to be much lower than that of visual cells. We recorded the responses of 174 cells in the SC to sequentially presented, localized visual and/or auditory stimuli that produced the sensation of apparent motion to human observers. Controls using single LED flashes or tone pips or clicks at very long intervals that did not produce apparent motion were also used. We found both visual and auditory units that responded vigorously to the apparent motion stimuli and showed pronounced directional selectivity. However, in the auditory domain such units were rare and thus did not increase the proportion of auditory responses in SC substantially. Varying the interstimulus interval (ISI) of these stimuli, both visual and auditory, indicated that the mechanism of direction selectivity in these cells was suppression of the response in the 'non-preferred' direction rather than facilitation in the 'preferred' direction. With long ISI's of 200 ms or more, every single stimulus gave a discrete response peak of constant amplitude. For ISI's of 50 ms or less the discrete peaks merged to a continuous response. Maximal firing rate in the preferred direction remained the same as for longer ISI's, but was decreased for movement in the non-preferred directions. Very short ISI's (10 ms) produced little response in any direction.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1988

The contribution of the horizontal semicircular canals to the response to off-vertical-axis rotation in the cat.

The response to off-vertical-axis rotation (OVAR) was measured in cats under circumstances in which the signals from the horizontal semicircular canals and otoliths were opposed. Opposition was achieved by sudden acceleration or deceleration during constant velocity OVAR. The degree of opposition was expressed as a canal/otolith ratio where a ratio of unity indicated agreement. For a canal/otolith ratio of 1, the OVAR gain (eye velocity/stimulus velocity) was 0.73 (+/- 0.13). The steady-state OVAR response was, however, reduced if the canals and otoliths were opposed. The reduction depended on the degree of opposition with a fall-off of 0.15 gain/(unit of canal/otolith ratio). These findings are discussed with respect to the central velocity store and the mechanism underlying the generation of the OVAR response.

Acceleration↗
PubMed1987

The eye movements evoked by a rotating linear acceleration vector in the cat depend on a central velocity storage mechanism.

The otoliths are stimulated in a particular pattern by any head movement that is not about an earth-vertical axis and evoke compensatory eye movements in the cat. Such eye movements are not produced if the otolith stimulation is accompanied by vertical canal stimulation. Vertical canal stimulation inactivates the velocity store (a central neural representation of head velocity) as seen by the attenuation of optokinetic after-nystagmus. These observations provide further evidence for the involvement of the central velocity store in the generation of otolith-evoked nystagmus.

Acceleration↗
PubMed1987

Vestibular and optokinetic eye movements evoked in the cat by rotation about a tilted axis.

Horizontal and vertical eye movements were recorded from cats in response to either off-vertical axis rotation (OVAR) at a range of velocities (5-72 deg/s) and a range of tilts (0-60 deg) or horizontal (with respect to the cat) optokinetic stimulation (10-80 deg/s), also around a range of tilted axes (0-60 deg). The responses to stopping either of these stimuli were also measured: post-rotatory nystagmus (PRN) following actual rotation, and optokinetic after nystagmus (OKAN) following optokinetic stimulation. The response found during OVAR was a nystagmus with a bias slow-phase velocity that was sinusoidally modulated. The bias was dependent on the tilt and reached 50% of its maximum velocity (maximum was 73 +/- 23% of the table velocity) at a tilt of 16 deg. The phase of modulation in horizontal eye velocity bore no consistent relation to the angular rotation. The amplitude of this modulation was roughly correlated with the bias with a slope of 0.13 (deg/s) modulation/(deg/s) bias velocity. There was also a low-velocity vertical bias with the slow-phases upwardly directed. The vertical bias was also modulated and the amplitude depended on the bias velocity (0.27 (deg/s) modulation/(deg/s) bias velocity). When separated from the canal dependent response, the build up of the OVAR response had a time constant of 5.0 +/- 0.8 s. Following OVAR there was no decline in the time constant of PRN which remained at the value measured during earth-vertical axis rotation (EVAR) (6.3 +/- 2 s). The peak amplitude of PRN was reduced, dependent on the tilt, reaching only 20% of its EVAR value for a tilt of 20 deg. When a measurable PRN was found, it was accompanied by a slowly-emerging vertical component (time constant 5.4 +/- 2 s) the effect of which was to vector the PRN accurately onto the earth horizontal. OKN measured about a tilted axis showed no differences in magnitude or direction from EVAR OKN even for tilts as large as 60 deg. OKAN following optokinetic stimulation around a tilted axis appeared normal in the horizontal plane (with respect to the animal) but was accompanied by a slowly emerging (time constant 4.1 +/- 2 s) vertical component, the effect of which was to vector the overall OKAN response onto the earth horizontal for tilts less than 20 deg. These results are compared with data from monkey and man and discussed in terms of the involvement of the velocity storage mechanism.

Animals↗
PubMed1983

Auditory compensation of the effects of visual deprivation in the cat's superior colliculus.

Neurones in the superior colliculus of normal and visually deprived cats were analyzed for their responses to visual, auditory and somatosensory stimuli. The percentage of auditory-responsive cells throughout all layers had increased from 11% to 42% after binocular deprivation. Some auditory responses were found even in superficial layers. The number of somatosensory responses, though not systematically tested, was also higher in the visually deprived animals. Visually responsive units did not significantly decrease in number, thus resulting in an increased proportion of multisensory neurones. The vigour of auditory responses had increased after visual deprivation, while the vigour of visual responses had decreased significantly. In addition to the auditory effects of visual deprivation found, our study confirms previous findings on the visual effects of visual deprivation in the superior colliculus. Since only qualitative changes of visual responses, but no suppression of visual by non-visual activity was found, the neuronal mechanisms responsible for these changes may be different from competition as present in the visual cortex.

Acoustic Stimulation↗
PubMed1981

Temporal and spatial response characteristics of the cat superior colliculus.

We have examined the responses of 72 cells of the cat superior colliculus to drifting gratings of sinusoidal luminance profile as a function of spatial frequency velocity and contrast. Of 72 cells, 66 responded to gratings either by change in mean firing rate only (58/72) or in a temporally modulated pattern in addition to the change in mean firing rate (8/72). The remaining 6 showed no change in discharge rate in response to any of the gratings tested. Many cells (24/72) were inhibited or excited by particular combinations of spatial and temporal frequencies. Some (8/72) demonstrated selective inhibition or excitation to a particular temporal frequency independent of spatial frequency and velocity and could therefore be said to be tuned specifically to temporal frequency. No cells were tuned only to a constant spatial frequency or a constant velocity. (24/72) cells displayed maximum inhibition or excitation only at a particular combination of spatial and temporal frequencies. Some cells (8/72) demonstrated a temporal modulation synchronous with the drifting grating in addition to an elevated mean discharge rate. The change in discharge rates evoked by gratings are generally less than those evoked by presentation of moving small slits or spots of light. Collicular cells often demonstrate a center-surround organization in their response to gratings. The center and surround often differ in their spatial frequency and velocity preferences. Compared to cortical and retinal ganglion cells, individual collicular cells are extremely non-linear. On a cell population basis, however, a linear Fourier analysis on grating response predicts the collicular cells' preference for movement of small objects.

Animals↗
PubMed1981

The eye movements of the dark-reared cat.

Cats reared in total darkness to adulthood have abnormal eye movements. A spontaneous nystagmus is found in the dark before any visual experience. The eye movements evoked by vestibular or optokinetic stimulation are less effective at compensation than for a normal cat. The vestibulo-ocular reflex (VOR) has a low gain (around 0.3) and a frequency dependent phase relation. The efficiency of optokinetic nystagmus (OKN) is poorer than for a normal cat, except for downwards stimulus movement which is followed better than normal. OKN is poorest in response to a stimulus viewed monocularly moving in the nasal to temporal direction. Neither VOR nor OKN of a dark-reared cat recover in efficiency within 5 months of the animal being brought into the light. A normal cat put into the dark for 135 days shows none of these abnormalities except an occasional spontaneous nystagmus.

Acclimatization↗
PubMed1981

Modification of the balance and gain of the vestibulo-ocular reflex in the cat.

The characteristics of the vestibulo-ocular reflex (VOR) of a normal cat can be modified in response to visual demands. Two aspects of the VOR are modifiable independently by a normal cat: the gain and the balance. An imbalance results in a spontaneous nystagmus and an asymmetric VOR. Neither the gain nor the balance of a dark-reared cat's VOR is susceptible to visual modification. A cat whose crossed visual pathways are severed at the level of the optic chiasm is able to modify the gain of the VOR but not its balance. Both dark-reared and split-chiasm cats have only very short-lasting optokinetic after-nystagmus.

Acclimatization↗
PubMed1980

Integration of visual and auditory space in the mammalian superior colliculus.

Recordings of eye movements and single-neurone microelectrode recordings of the superior colliculus in cats show that, for each saccadic movement, their eyes start near to the centre of the orbit so that the coordinates of visual and auditory space are aligned, and complex neural compensation of auditory or visual inputs to the superior colliculus is unnecessary.

Animals↗
PubMed1980

Abolition of optokinetic nystagmus in the cat.

Combining a behavioral and a surgical manipulation, namely complete visual deprivation with surgical section of the optic chiasm, results in the abolition of optokinetic nystagmus in the cat. This basic optomotor reflex remains relatively unaffected by either of these manipulations performed singly.

Animals↗