Blood vessels casting a shadow.
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Biomedical subjects
W R Levick
Publications and source records attributed to W R Levick.
Dendritic computation of direction selectivity by retinal ganglion cells.
Direction-selective ganglion cells (DSGCs) in the retina respond strongly when stimulated by image motion in a preferred direction but are only weakly excited by image motion in the opposite null direction. Such coding represents an early manifestation of complex information processing in the visual system, but the cellular locus and the synaptic mechanisms have yet to be elucidated. We recorded the synaptic activity of DSGCs using strategies to observe the asymmetric inhibitory inputs that underlie the generation of direction selectivity. The critical nonlinear interactions between the excitatory and inhibitory inputs took place postsynaptically within the dendrites of the DSGCs.
Action Potentials↗
Spatial-temporal response characteristics of the ON-OFF direction selective ganglion cells in the rabbit retina.
We studied the response properties of the ON-OFF direction selective (DS) ganglion cells in the rabbit retina. The activities of the ganglion cells were recorded extracellularly with tungsten-in-glass electrodes. Drifting sine-wave gratings with various spatial and temporal frequencies were used to stimulate the ON-OFF DS cells. Individual ON-OFF DS cell yielded strongest response to a particular spatial frequency irrespective of temporal frequency. The optimal spatial wavelength increased with increasing eccentricity and its half-width approximately matched the size of the receptive field. The optimal spatial frequency implies that the directional inhibition is effective over a range of about a receptive field diameter. The characteristics of the ON-OFF DS cells suggest that they might function as local motion detectors.
Animals↗
Changes in the receptive fields of cat retinal ganglion cells affected by pressure on the optic nerve.
In the cat optic nerve a lesion was induced by brief application of pressure. It selectively blocked impulse conduction in large diameter fibres of the retinal ganglion cells. Electrophysiological examination of single optic axons several weeks later demonstrated a gross alteration of the visual properties of the affected BT/Y ganglion cells. It is suggested that the alteration of receptive field properties may reflect the cellular and dendritic response to distant focal injury of the axon.
Animals↗
Lesion topography and new histological features in feline taurine deficiency retinopathy.
Previous studies of feline taurine deficiency retinopathy have established that the retinopathy begins as a focal lesion at the area centralis (feline central retinal degeneration) and progresses to form a horizontal band of retinal degeneration which, in some cases, eventually involves the whole retina. Several theories have been proposed to account for the strikingly unusual geographic distribution of lesions in this disease, including a preferential cone photoreceptor effect, and damage by exposure to light. In this paper evidence is provided that the regional lesions in advanced cases of taurine deficiency retinopathy extend circumferentially to form a peripheral annular band-like arrangement around the retinal extremity. The circumferential annulus is confluent with both temporal and nasal extremes of the horizontal band-shaped lesion. The topographic arrangement of the advanced lesion thus approximates the shape of the Greek letter theta, theta. A previously unreported sclerad displacement of photoreceptor cells into the subretinal space was commonly observed histologically. Radially sectioned material also revealed pathological changes in photoreceptors over a much more widespread region than the focal lesions observable ophthalmoscopically and in whole-mounts of the retina. The issue is therefore to account for the advanced photoreceptor cell loss in the focal lesions. A preferential effect on cone photoreceptors is insufficient to explain the lesion distribution, and unilateral tarsorrhaphy experiments exclude light damage as a major factor. Some other factor, or combination of factors, is most likely involved in determining the unique geographic distribution of lesions in feline taurine deficiency retinopathy.
Animals↗
Hypokalaemic episodic polymyopathy in cats fed a vegetarian diet.
A previously undocumented hypokalaemic condition with a cyclical nature, comprising acute bouts of polymyopathy followed by spontaneous recoveries, is described in the cat. Cats being fed a high protein vegetarian diet developed recurrent episodes of polymyopathy, characterised by ventroflexion of the head and neck, stiff forelimb gait, lateral head-resting and generalised muscle weakness. Plasma potassium concentrations (mean +/- standard deviation) were reduced from 3.28 +/- 0.33 mmol/l at the beginning of the experiment to 2.45 +/- 0.24 mmol/l during bouts of myopathy. This hypokalaemia was associated with increased creatine kinase activities indicative of muscle damage, and decreased urinary potassium concentrations, and was caused by insufficient dietary potassium. Cats that received the same diet supplemented with potassium did not develop hypokalaemic polymyopathy. Spontaneous recoveries of affected cats were not associated consistently with increases in plasma potassium concentrations. Plasma taurine concentrations decreased and glutamic acid increased markedly in all cats fed the experimental diet. There was no evidence of thiamin deficiency associated with the high glutamic acid intake. Veterinarians should be aware that hypokalaemic cats, and in particular those on potassium-deficient diets, may show cyclical disease with episodes of polymyopathy recurring after periods of spontaneous clinical recovery. This condition in cats may be a useful animal model for familial hypokalaemic periodic paralysis in humans.
Amino Acids↗
Nonlinear measurement and classification of receptive fields in cat retinal ganglion cells.
Originally, modeling of ganglion-cell responses in cat was based mainly on linear analysis. This is satisfactory for those cells in which spatial summation of excitation is approximately linear (X-cells) but it fails for Y-cells, where summation has strong nonlinear components. Others have shown the utility of using sinusoidal analysis to study harmonic and intermodulation nonlinearities in the temporal frequency domain. We have used Wiener-kernel analysis to obtain directly both temporal and spatial impulse responses and their nonlinear interactions. From these, we were able to predict accurately the responses that a counterphase modulated grating elicited in both X-cells and Y-cells. In addition, we show that the first-order responses can measure the two-dimensional spatial features of the receptive field with high resolution. Thus, nonlinear analysis of responses to white-noise stimuli may be sufficient to both classify and measure the receptive fields of many different types of ganglion cells.
Animals↗
Retinotopic organization in the dorsal lateral geniculate nucleus of the tammar wallaby (Macropus eugenii).
Electrophysiological recordings were made from 187 single cells in the tammar wallaby (Macropus eugenii) dorsal lateral geniculate nucleus (LGNd). The results show that it is topographically organized such that the superior visual field is represented dorsally, the inferior field is represented ventrally, the nasal visual field is represented caudally, and the temporal visual field is represented rostrally. The visual field of one eye ranges from -30 degrees nasal to +179 degrees temporal in azimuth and +73 degrees superior to -49 degrees inferior in elevation. Ganglion cells that had receptive field positions between -9 degrees and +179 degrees projected to the contralateral LGNd while the ganglion cells that projected to the ipsilateral LGNd had visual fields from 0 to +30 degrees. The binocular visual field extends 60 degrees in azimuth. This representation in the LGNd is expanded relative to the monocular representation. There is also an increased representation of the horizon in the temporal field corresponding to the visual streak of retinal ganglion cells. The binocular visual field is located where contralateral and ipsilateral laminae are shown to interdigitate by proline autoradiography. There are nine eye-specific laminae in the LGNd. Four receive afferents from the contralateral eye and five receive afferents from the ipsilateral eye. The lines of isoelevation are perpendicular to the coronal plane of section while the lines of isoazimuth are nearly parallel to the coronal plane. The lines of projection representing one visual direction are inferred to be perpendicular to the tangent of curvature of the laminae as in the LGNd of other mammals. The majority of cells (85%) recorded had on- or off-centre responses. On- and off-centre responses were not apparently segregated in the LGNd but segregation may not have been revealed by the single-unit recording technique.
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Orientation bias of brisk-transient y-cells of the cat retina for drifting and alternating gratings.
Brisk-transient ganglion cells of the cat's retina were examined for orientation bias using two different stimuli: drifting gratings and alternating gratings, both of fixed contrast (50%) and fixed temporal frequency (2 Hz). Some cells were strongly biassed for both stimuli, some were not biassed for either while still others were strongly biassed for only one or other stimulus. The preferred orientations for the two types of grating tended to be the same, on average, but substantial differences were not uncommon. A systematic preference for radially-oriented gratings was evident when the stimulus was drifting but there was an additional preference for tangentially-oriented gratings when the stimulus was alternating. Orientation bias for drifting gratings often extended over a broad range of spatial frequencies and was maximum near the resolution limit. For alternating gratings, bias was evident only at the highest spatial frequencies. Results indicate that the arrangement of receptive field components responsible for linear and nonlinear kinds of behaviour may sometimes possess different axes of symmetry.
Animals↗
Spatial frequency characteristics of brisk and sluggish ganglion cells of the cat's retina.
Receptive fields of cat retinal ganglion cells were stimulated by a drifting sinusoidal luminance pattern of fixed (50%) contrast and the amplitude of the fundamental frequency component of response was determined as a function of spatial frequency. Frequency response functions for most cells were unimodal and skewed towards zero frequency when plotted on linear scales. At a fixed retinal location, cells of different classes had different frequency response functions. Heterogeneity within some of the classes could be largely removed by normalizing the axes, thus, revealing a common shape of function for the class. At a fixed retinal location, the maximum response obtained at each spatial frequency was always obtained from a cell of the brisk, rather than sluggish, classes. Spatial frequency resolution was highest for brisk-sustained cells and usually lowest for brisk transient cells.
Animals↗
Bimodal receptive fields of cat retinal ganglion cells.
Receptive fields of cat retinal ganglion cells were stimulated by a drifting, sinusoidal luminance pattern of fixed contrast. The amplitudes and phases of the harmonic components in the response were determined as a function of spatial frequency. For most cells, the graphs of response vs spatial frequency (when plotted on linear scales) were unimodal and skewed towards zero frequency for all stimulus orientations. However, some cells had bimodal frequency response functions when the stimulus was in the non-preferred orientation. These unusual cells also exhibited a sudden phase-reversal of pi radians which occurred at the frequency of the changeover between the two modes. Calculations based on the experimental data predicted two distinctly separate regions of high sensitivity within the receptive centres of such cells. A narrow bar stimulus was used to confirm that the receptive fields had, in effect, double centres.
Action Potentials↗
Performance of cat retinal ganglion cells at low light levels.
Responses of brisk-sustained cat retinal ganglion cells were examined using receiver operating characteristic (ROC) analysis. Stimuli were brief luminance changes superimposed upon a weak steady pedestal ranging from 27 to 47,000 quanta (507 nm) per second at the cornea. Overall quantum efficiencies of cells ranged up to approximately 13% and were compatible with previous estimates at absolute threshold. The main work was done on on-center cells, but a small sample of off-center units behaved similarly. Experimental ROC curves verified a set of qualitative predictions based on a theoretical treatment of performance, assuming that response variability resulted solely from quantum fluctuations. However, quantitative predictions were not fulfilled. The discrepancy could be resolved by postulating a source of added internal variance, R, the value of which could then be deduced from the experimental measurements. A ganglion cell model limited by a fixed amount of added variance from physiological sources and having access to a fixed fraction of incident quanta can account quantitatively for (a) slopes of ROC curves, (b) variation of detectability with magnitude of both increments and decrements, and (c) performance over a range of pedestal intensities. Estimates of the proportion of incident quanta used ranged up to 29% under some conditions, a figure approximately matching estimates of the fraction of corneal quanta that isomerize rhodopsin in the cat.
Animals↗
Astigmatic visual and deprivation in cat: behavioral, optical and retinophysiological consequences.
Kittens were deprived of clear vision in one eye only by +7D or -7D cylindrical lenses worn from eye opening until 7.5 months of age. Behavioral measurements subsequently demonstrated diminished acuity through the affected eye relative to the opposite control eye. However, single unit recordings at 3.5 years of age from retinal ganglion cells located within 1-11 degrees of the centre of the area centralis showed neither loss of grating resolution nor any abnormal orientation bias. Furthermore, the astigmatic rearing had minimal effect on the development of the cat's natural optics to the emmetropic state. The experiments indicate that the amblyopic defect induced by the cylindrical lenses occurs in the brain rather than the eye.
Age Factors↗
Analysis of orientation bias in cat retina.
1. Responses of cat retinal ganglion cells to a drifting sinusoidal grating stimulus were measured as a function of the grating orientation and spatial frequency.2. The response at fixed frequency and contrast varied with orientation in the manner of a cosine function. A new measure was introduced to quantify this orientation bias in the response domain on an absolute scale of 0-100%. Under experimental conditions designed to maximize the effect, the mean bias for 250 cells was 16% and the range was 0-46%. In 70% of cells there was significant bias.3. Orientation bias varied with spatial frequency and was maximal near the high-frequency limit. The majority of biassed cells preferred the same orientation at high and low frequencies but in some cells a reversal occurred: the orientation which gave maximum response at high frequencies gave minimum response at low frequencies. The greatest variation of cut-off frequency with orientation was (2/3) octave.4. Orientation bias was due to neural, not optical, factors. Nevertheless, the phenomenon could often be imitated by deliberately introduced optical astigmatism of up to 4 dioptres for brisk-sustained units and over 10 dioptres for brisk-transient units.5. The grating orientation preferred by cells varied systematically with position in the visual field. The central tendency was for the grating which yielded maximum response to lie parallel to the line joining the cell to the area centralis. This generalization failed for units within 2 degrees of the centre of the area centralis.6. Analysis of orientation bias indicates a functional asymmetry of receptive fields such that the centre mechanism, and sometimes also the surround mechanism, is elongated along the line joining cell to area centralis.
Animals↗
Rabbit retinal ganglion cells. Receptive field classification and axonal conduction properties.
The classification of concentrically organized receptive fields of rabbit retinal ganglion cells was extended along similar lines to that in the cat by distinguishing brisk and sluggish classes and then sustained and transient types of each. Quantitative measures of responsiveness to stationary and to moving stimuli revealed characteristic features which distinguished these classes. Brisk-transient and brisk-sustained classes are not as distinct from each other as in the cat: centre size distributions overlapped almost completely and there was also substantial overlap of axonal conduction properties whether expressed in terms of latency or conduction velocity between two central stimulus sites. Representatives of every class of rabbit ganglion cells sent axons to the superior colliculus.
Action Potentials↗
Neurophysiological tracing of a projection from temporal retina to contralateral visual cortex of the cat.
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Animals↗
Orientation bias of cat retinal ganglion cells.
Kuffler described the receptive fields of cat retinal ganglion cells as having a concentric arrangement. This has usually been taken to mean that they are approximately circular in form (see, for example, ref. 2). Hammond tested the circularity of the centre component of receptive fields by plotting a contour of isosensitivity to a small flashed spot. He concluded that centres were often ellipitical (average ratio of major to minor axis 1.23) and that more than 50% of the recorded cells had the major axis oriented within /+- 20 degrees of the horizontal. Such data are important for discussions of the neurophysiological basis of the 'oblique effect' (reduced visibility for periodic grating patterns when oriented away from the vertical or horizontal) observed in psychophysical experiments on humans because subcortical units are often assumed to be orientationally unbiased. Orientation selectivity is a prominent attribute of visual cortical neurones so analysis has usually emphasized the distribution of orientation selectivity at that level. The results presented here redirect attention to the retinal level since they reveal a previously unsuspected systematic relation between orientation bias of ganglion cells and their location relative to the area centralis.
Action Potentials↗
Retinal ganglion cells and optic decussation of white cats.
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