Biomedical subjects

B G Cleland

Publications and source records attributed to B G Cleland.

At least 19 recordsLinked to original sources

PubMed1996

Tailoring of variability in the lateral geniculate nucleus of the cat.

Variability is usually considered an unwanted component in a sensory signal, yet the visual system does not seem to filter out the noise. On the contrary, noise is 'tailored' to scale with the signal size. We show that this tailoring occurs in the lateral geniculate nucleus, preferentially in X-cells, which are the cells most likely to transmit pattern information. Tailoring the variability to the signal size may be the visual system's way of providing the right amount of variability for a signal of any magnitude at all times during the computation.

Animals↗

PubMed1996

A conceptual basis for pacing waveforms.

A model is developed that allows evaluation of the pacing efficacy of different stimulus waveforms. It treats the heart as having a first order time constant and enables ready visualization of the time course of the effective voltage within the heart. For pacemakers, where the stimulus pulse is produced by the discharge of a capacitor, the voltage within the heart rapidly rises to a peak and then more slowly decays to zero. The time interval at which the peak occurs defines the optimal duration, i.e., the shortest duration with minimal pacing voltage. Characteristics are developed that show the changes in optimal duration and pacing threshold for changes in the pacemaker's output capacitor and for differences in lead impedance and time constant of the heart.

Animals↗

PubMed1996

A conceptual basis for defibrillation waveforms.

A model is developed for defibrillation that treats the heart as a first order time constant. Such a model allows ready evaluation of different monophasic waveforms. For implantable devices where the voltage is provided by the discharge of a capacitor, it can be seen that the effective voltage within the heart rises rapidly to a peak and then decays to zero. The time interval at which this peak occurs is defined as the optimal duration, and there is no advantage in extending the pulse beyond this point. Characteristics are presented that show how the time course of this voltage within the heart changes with different device capacitors and load impedances. The effect of different heart time constant are also examined. For biphasic waveforms, a contour plot of threshold voltage is presented with phase 1 and phase 2 durations on the two axes. It is seen that there is a region of reliable low threshold defibrillation from 3.5/1.5 ms to 9/6 ms.

Animals↗

PubMed1992

The rod-cone shift and its effect on ganglion cells in the cat's retina.

We examined how several characteristics of cat retinal ganglion cells--receptive field size, spatial resolution, and centre-surround antagonism--change with background illumination. Spectral sensitivity was also measured to see how these changes depend on the rod-cone shift. The radius of the centre mechanism changed very little across the mesopic range. The absence of a change can be attributed to the connections rods make with cones, and to the small spatial spread of rods which connect to a cone. The highest spatial frequency to which a cell could respond dropped sharply with falling background illumination. This loss of spatial resolution is due partly to increasing receptive field size, and partly to loss of contrast gain. Centre-surround antagonism approached zero as background illumination fell. The loss of antagonism could have been due to either a change in the subtractive relationship between centre and surround, or due to a loss of surround strength relative to centre strength; the latter was shown to be the case.

Adaptation, Ocular↗

PubMed1992

Variability of responses of cat retinal ganglion cells.

Previous studies of the variability of firing of retinal ganglion cells have led to apparently contradictory conclusions. To a first approximation, the variance of rate of maintained discharges of ganglion cells in cat is independent of the mean firing rate. On the other hand, the variability of responses to abrupt changes in lighting of ganglion cells in goldfish increases with increasing firing rate. To examine whether the difference is due to differences between species, we examined the variability of responses of cat ganglion cells, and find it similar to that of goldfish ganglion cells. The variance of rate of ganglion cells is neither independent of mean rate, as might be expected from maintained discharges, nor directly proportional to the mean rate, as it is for cat cortical cells. Rather, there is a nonlinear relationship between variance of rate and mean rate.

Animals↗

PubMed1992

Effects of selective pressure block of Y-type optic nerve fibers on the receptive-field properties of neurons in the striate cortex of the cat.

In an aseptic operation under surgical anesthesia, one optic nerve of a cat was exposed and subjected to pressure by means of a special cuff. The conduction of impulses through the pressurized region was monitored by means of electrodes which remained in the animal after the operation. The pressure was adjusted to selectively eliminate conduction in the largest fibers (Y-type) but not in the medium-size fibers (X-type). The conduction block is probably due to a demyelination and remains complete for about 3 weeks. Within 2 weeks after the pressure-block operation, recordings were made from single neurons in the striate cortex (area 17, area V1) of the cat anesthetized with N2O/O2 mixture supplemented by continuous intravenous infusion of barbiturate. Neurons were activated visually via the normal eye and via the eye with the pressure-blocked optic nerve ("Y-blocked eye"). Several properties of the receptive fields of single neurons in area 17 such as S (simple) or C (complex) type of receptive-field organization, size of discharge fields, orientation tuning, direction-selectivity indices, and end-zone inhibition appear to be unaffected by removal of the Y-type input. On the other hand, the peak discharge rates to stimuli presented via the Y-blocked eye were significantly lower than those to stimuli presented via the normal eye. As a result, the eye-dominance histogram was shifted markedly towards the normal eye implying that there is a significant excitatory Y-type input to area 17. In a substantial proportion of area 17 neurons, this input converges onto the cells which receive also non-Y-type inputs. In one respect, velocity sensitivity, removal of the Y input had a weak but significant effect. In particular, C (but not S) cells when activated via the normal eye responded optimally at slightly higher stimulus velocities than when activated via the Y-blocked eye. These results suggest that the Y input makes a distinct contribution to velocity sensitivity in area 17 but only in C-type neurons. Overall, our results lead us to the conclusion that the Y-type input to the striate cortex of the cat makes a significant contribution to the strength of the excitatory response of many neurons in this area. However, the contributions of Y-type input to the mechanism(s) underlying many of the receptive-field properties of neurons in this area are not distinguishable from those of the non-Y-type visual inputs.

Animals↗

PubMed1992

Effects of selective pressure block of Y-type optic nerve fibers on the receptive-field properties of neurons in area 18 of the visual cortex of the cat.

Recordings were made from single neurons in area 18 of anesthetized cats (N2O/O2 mixture supplemented by continuous intravenous infusion of barbiturate) in which one optic nerve had been pressure blocked to selectively block conduction in the largest (Y-type) fibers. Cortical neurons were stimulated visually via the normal eye or via the eye with the pressure-blocked optic nerve ("Y-blocked eye"). Several properties of the receptive fields such as their spatial organization (S or C cells), orientation tuning, and the presence and strength of end-zone inhibition appear to be unaffected by removal of the Y input. By contrast, the removal of the Y input resulted in a small but significant reduction in the size of the discharge field and in the direction-selectivity index. In three respects, peak response discharge rate, eye dominance, and velocity sensitivity, removal of the Y input had strong and highly significant effects. Thus, the mean peak discharge frequency of responses evoked by the stimulation of binocular neurons via the Y-blocked eye was significantly lower than that of responses evoked by the stimulation via the normal eye. Accordingly, the eye-dominance histogram was shifted markedly towards the normal eye (more so than in the homologous experiment conducted on area 17-Burke et al., 1992). Finally, the mean preferred velocity of responses of cells activated via the normal eye was in the vicinity of 145 deg/s, whereas for cells activated via the Y-blocked eye the value was about 35 deg/s. Overall, the results of the present study imply that (1) apart from Y-type excitatory input there are significant excitatory non-Y-inputs to area 18; these inputs at least partially consist of indirect X-type input relayed via area 17; (2) in neurons of area 18 that receive both Y-type and non-Y-type excitatory inputs, the Y-type input has a major influence on strength of the response and velocity sensitivity and a lesser influence on the direction selectivity and size of the discharge fields; and (3) area 18 contains mechanisms determining such receptive-field properties as S- or C-type organization, orientation tuning, and direction selectivity which can be accessed either by the Y input or by non-Y input.

Afferent Pathways↗

PubMed1988

Visual adaptation is highly localized in the cat's retina.

1. The aim of this study was to determine how the spatial pattern of steady light in a visual stimulus affects the state of adaptation of the retina. 2. Impulse rate was recorded from single X and Y ganglion cells in the cat's retina. The luminance of a narrow bar of light centred over the receptive field was modulated sinusoidally in time about a steady background, and a cell's contrast gain was measured as the ratio of impulse rate modulation to bar contrast. 3. The contrast gain of a cell was set by the background, a fixed luminance level about which luminance varied in the form of a grating; grating luminance varied sinusoidally with distance but did not vary in time. When the spatial frequency of the grating was low, contrast gain was increased by a grating with a trough centred over the receptive field, and decreased by a peak-centred grating. 4. As the spatial frequency of the grating increased, its effect on contrast gain disappeared. For cells around 10 deg from the central area, this change occurred at spatial frequencies close to 1 cycle deg-1. 5. For each cell the effect on contrast gain of the background's spatial frequency was compared with the spatial frequency response to a time-varying grating. It was found that the summation area for adapting light in both X and Y cells is very close in size to an X cell centre mechanism, and that the summation area for adapting light in Y cells is therefore considerably smaller than a Y cell centre. 6. From this and other evidence it was shown that sub-areas of the Y cell centre mechanism can be independently adapted. 7. A background grating with a trough centred over the receptive field raised contrast gain more at mid-range spatial frequencies than at low frequencies, producing a hump in the contrast gain versus frequency curve. A peak-centred grating reduced contrast gain more at mid-range frequencies than at low, producing a dip. 8. The dip in the contrast gain versus frequency curve for a peak-centred grating was always greater than the hump for a trough-centred grating. 9. These humps and dips were interpreted in terms of a model containing two antagonistic pathways. One pathway had a smaller summation area for adapting light than the other.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Ocular↗

PubMed1985

Synaptic delay in the lateral geniculate nucleus of the cat.

Unitary, presynaptic spike potentials were observed in single cell recordings from the dorsal lateral geniculate nucleus of the cat. In 11 cells, spontaneous S potentials (extracellularly recorded excitatory postsynaptic potentials) were preceded at a fixed interval by a small wave (the 'T' potential). In another 14 cells, a T potential, although not detected in single traces, was revealed by averaging 20-100 samples synchronized to the peak of the S potential. Provided the field response was not too large a T potential could also be detected in the response to a stimulus to the optic nerve. The T potential would appear to be the spike potential of the afferent optic axon which is excitatory to the geniculate cell because it precedes the S potential at a very exact interval and also follows the corresponding retinal ganglion cell spike at a very exact interval and because the interval between T potential and S potential is reversibly decreased by cooling with a temperature coefficient characteristic of synapses. T potentials ranged in amplitude from 8 to 134 microV and were all predominantly positive-going suggesting a failure of the nerve impulse to invade fully the terminals of the optic nerve. The time from the positive peak of the T potential to the start of the S potentials was taken as a good measure of the synaptic delay. The T-S interval averaged 0.29 ms (+/- 0.045 ms S.D.).

Action Potentials↗

PubMed1985

Is the retina sensitive to the effects of prolonged blur?

Two preparations were used to study the developmental effects of prolonged blurring of retinal images on the acuities of retinal ganglion cells. Five kittens were raised from three weeks to six months of age with daily administration of atropine to one eye. Another two kittens were raised from three weeks to 16 weeks with a contact lens of high refractive power fitted to one eye. Behavioural estimates of the visual acuity were made for two animals from each group. Animals of both groups demonstrated an amblyopia in the experimental eye: visual acuity varied from 1.8 to 2.5 cycles per degree compared with 6.0 to 7.5 cycles per degree when using the normal eye. The spatial resolving properties were measured for retinal ganglion cells within the amblyopic eyes of two lens-reared cats and three atropinized cats. Brisk-sustained (X) cells were recorded from along the naso-temporal division. The acuities of ganglion cells from the lens-reared cats were indistinguishable from those from normal cats at comparable eccentricities. However, for the cats raised with atropine administration, sub-normal acuities were determined for retinal ganglion cells from all regions that were studied in the experimental eye. We conclude that blur of retinal images produced by external means has no effect on the resolving power of retinal ganglion cells. The lowered ganglion cell acuities encountered with the atropinized cats must be attributable to a secondary effect of the atropine administration. Organic changes in the retinal blood vessel pattern support this contention.

Animals↗

PubMed1985

Convergent strabismic amblyopia in cats.

Experiments were carried out to determine the effects of different types of experimental strabismus on the acuities of retinal ganglion cells. Six kittens were raised from twenty-one days of age with an esotropia surgically induced by myectomy of the lateral rectus muscle and a large portion of the superior oblique muscle. The results are compared with those, previously reported, from five other cats also made esotropic, but by tenotomy of the lateral rectus. All animals tested behaviourally were amblyopic in the strabismic eye. For square wave gratings, the visual acuities were 1.0 to 2.5 cyc/deg through the strabismic eye compared with 6.0 to 7.5 cyc/deg through the non-deviating eye. The cut-off spatial frequencies were determined for 132 brisk sustained cells from five of the myectomized strabismic cats. There was a loss of approximately 20% in cut-off spatial frequency when compared with both normal and tenotomized cats. A correlate of the physiologically observed difference between the tenotomized cats and the myectomized cats was also found in the morphology of cells in the lateral geniculate nucleus. The tenotomized cats showed no evidence of cell shrinkage in laminae receiving a projection from the amblyopic eye whereas in the myectomized cats large differences were observed in cell cross-sectional areas between laminae receiving input from the amblyopic eye and those receiving input from the non-deviating eye. Together, these findings indicate that the presence of a neural deficit in the retina of strabismic cats is associated with the actual removal of extra-ocular muscle and probably has little to do with the optical quality of images arriving at the retina.

Accommodation, Ocular↗

PubMed1985

The cat as a model for visual deprivation.

Both behavioural and neurophysiological changes can be observed in cats that have experienced interference with their normal visual environment. This visual deprivation may result from alterations to the path of light forming the normal image on the retina, and includes changes that cause the image to fall on an inappropriate part of the retina so that normal binocular interactions are affected. While some neurophysiological changes can be observed at the level of the lateral geniculate nucleus they become more prominent as information reaches the visual cortex, where cells commonly receive neural excitation from both eyes and require the information to come from corresponding parts of the two retinas and that the stimulus should have appropriate orientation and direction of movement. Many of the observations of deprivation in animals have clear parallels in the human environment.

Amblyopia↗

PubMed1985

A comparison of visual responses of cat lateral geniculate nucleus neurones with those of ganglion cells afferent to them.

We compared visual responses of cat lateral geniculate nucleus (l.g.n.) neurones with those of retinal ganglion cells providing their afferent inputs. Quantitative studies were made on twenty such pairs; eight X on-centre, seven Y on-centre, two X off-centre and three Y off-centre pairs. Receptive field centre locations of cell pairs with correlated activities were very closely superimposed, having a mean centre displacement of 1.6 minutes of arc for X cells and 11 minutes of arc for Y cells. With flashed spots and annuli, responses of l.g.n. cells were almost always smaller than those of their retinal afferents, with peaks and troughs in ganglion cell responses being faithfully followed in the geniculate neurones. This is consistent with almost all impulses from the l.g.n. cell being triggered by the afferent feeding its centre. With spots of different sizes and contrasts, modulation of responses by l.g.n. inhibition was obvious, but effects were complex. With moving bright-bar stimuli, although response histograms were clearly reshaped to some extent in the l.g.n., peak firing rates under different stimulus conditions were often merely attenuated by a constant factor for most l.g.n. cells in comparison with their retinal inputs. For velocity tuning curves, a few cell pairs showed selective attenuation at high speeds, while others showed it at low speeds. All the latter group appeared to have more than one major excitatory afferent. These changes in velocity tuning occurred across the X/Y classification, so that differences in velocity preference of the X and Y systems is more blurred in the l.g.n. than in the retina.

Action Potentials↗

PubMed1983

Sensitivity to stationary flashing spots of the brisk classes of ganglion cells in the cat retina.

The responses of brisk-sustained (X) and brisk-transient (Y) cat retinal ganglion cells to small stationary spots flashing on and off in their receptive fields were recorded. Response magnitudes were determined as the amplitude of the fundamental frequency component. At 0.5 and 8 Hz the sensitivity of brisk-sustained cells is independent of the size of their receptive field centre, with the mean sensitivity at 0.5 Hz being 16% lower than at 8 Hz. At 8 Hz the sensitivity of brisk-transient cells is inversely proportional to the centre diameter of their receptive fields. The relationship is quite different from that found in brisk-sustained cells. The ratio of the sensitivity at 0.5 Hz to the sensitivity at 8 Hz can be used as a measure of the 'sustainedness' of a ganglion cell. Using this measure brisk-sustained cells show no change in their sustained nature as a function of eccentricity. In contrast, brisk-transient cells appear to become increasingly transient with eccentricity. Within the area centralis the sensitivities of brisk-transient cells at 0.5 and 8 Hz are very similar to the sensitivities of brisk-sustained cells, and hence they have a very obvious sustained component to a light flashed on and off at low frequency. These brisk-transient units meet all other criteria of their class and so there can be no question as to their classification.

Action Potentials↗

PubMed1983

Response to the length of moving visual stimuli of the brisk classes of ganglion cells in the cat retina.

Response histograms were collected for brisk-sustained and brisk-transient ganglion cells in the cat retina as narrow bars were moved backwards and forwards across their receptive fields. When a bar of fixed length was moved across the centre of the receptive field with contrast proportional to velocity, a constant response was obtained as long as the centre of the receptive field was crossed within the summation time. However, if the length of the bar was such that it extended beyond the centre, then there was a small but steady increase in surround antagonism for an increase in velocity. The same response was produced by a brief whole-field flash as by an extended bar moving across the receptive field at high velocity if both stimulus conditions delivered the same energy uniformly across the receptive field. With brisk-sustained cells it was observed, for small bar lengths, that bar length and contrast could be exchanged to give a constant response, even when there was considerable non-linearity in the over-all stimulus-response relationship. Thus conditions that resulted in constant stimulus flux produced a constant response. This property was seen at both high and low velocities for the majority of brisk-sustained units. The stimulus-response relationship had a greater range of linearity at high velocities than at low velocities. From similar experiments with brisk-transient cells it was observed that bar length and contrast could only be exchanged to give a constant response at high velocities. At low velocities there was considerable non-linearity: there appeared to be saturation of the response from local regions and it was necessary to extend the bar outside such a region to obtain an increase in response. At lower velocities, despite the changes seen in length-response curves under different conditions of contrast and velocity, the degree of surround antagonism remained constant for a given cell. Further, both brisk-sustained and brisk-transient cells showed the same degree of surround antagonism.

Action Potentials↗

PubMed1983

Response to the velocity of moving visual stimuli of the brisk classes of ganglion cells in the cat retina.

Extracellular recording of the responses of cat retinal ganglion cells to narrow moving bars revealed systematic response variations with changes in stimulus velocity. These response variations were studied by collecting peri-stimulus histograms from brisk-sustained (X) and brisk-transient (Y) ganglion cells as narrow elongated bars were moved backwards and forwards across their receptive fields. Velocity-response curves were produced from plots of the amplitude of the main peak of the histograms as a function of velocity. The shape of these curves was found to be reasonably constant for both classes of ganglion cells. For a given cell, the peak of the velocity-response curve shifted to both a higher response level and a higher velocity as the stimulus contrast was increased. Within both classes of cells there was a systematic shift in the velocity-response curve as a function of the size of the receptive field centre. For brisk-sustained cells this was seen as an increase in both the response and velocity at the peak for larger centre sizes, while for brisk-transient cells it was an increase in velocity at the peak with negligible change in response. When the velocity required to produce a small criterion response was determined, there were distinct differences between the two classes of cells. When plotted on a double-logarithmic scale as a function of centre size the brisk-sustained cells had a slope of 2.00 while brisk-transient cells had a slope of 1.20. Within the area centralis brisk-transient cells responded more readily at high velocities than brisk-sustained cells. This was not the case in the peripheral retina, where both cell classes responded about equally at high velocities.

Action Potentials↗

PubMed1983

Response of neurons in the cat's lateral geniculate nucleus to moving bars of different length.

It is well recognized that in the visual cortex of the cat, some of the cells (hypercomplex) are sharply tuned for the length of a bar moving backwards and forwards across their receptive fields. Other cells (simple) exhibit no such tuning but appear to respond proportionately over a range of bar lengths. The tuning seen in hypercomplex cells is already observable to a lesser degree in retinal ganglion cells. Our experiments were carried out to determine the extent of this tuning in the lateral geniculate nucleus, which relays information from the retina to the cortex. Results show that geniculate cells have tuning properties intermediate between those of ganglion cells and hypercomplex cells. By adding together a linear array of geniculate cells, it is possible to model the characteristics of a simple cell and to demonstrate that while an elongated bar gives a minimal response in hypercomplex cells, it should have little effect on the response of simple cells.

Animals↗

PubMed1982

Normality of spatial resolution of retinal ganglion cells in cats with strabismic amblyopia.

1. A convergent or divergent strabismus was induced surgically in eight kittens and a cyclotropia of about 90 deg in two additional kittens. 2. Behavioural measurements were made of the visual acuity of each eye for square-wave gratings. All eight animals that were so tested displayed a reduction of acuity in one eye relative to the other of 1.3-2.5 octaves. 3. The activity of retinal ganglion cells was recorded within the amblyopic eye of six cats, three with a convergent strabismus, two with a divergent strabismus and one with a cyclotropia. Measurements were made of the spatial resolution with 215 on-centre cells for horizontal and vertical gratings. 4. In contrast to other reports, we found the spatial resolution of ganglion cells in the amblyopic eye of the strabismic animals to be comparable to those of normal cats at all retinal eccentricities. In particular there was no evidence for a loss of resolution in the vicinity of the area centralis. 5. Measurement of the cross-sectional area of cells in the lateral geniculate nucleus (l.g.n.) revealed no evidence of cell shrinkage in laminae receiving a projection from the amblyopic eye. 6. Together, these findings lead to the conclusion that the neural deficit responsible for the strabismic amblyopia in these animals did not lie in the retina but rather at more central levels of the visual pathway.

Action Potentials↗