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A B Bonds

Publications and source records attributed to A B Bonds.

At least 37 records · Page 2Linked to original sources
PubMed1991

Visual evoked potential responses of the anesthetized cat to contrast modulation of grating patterns.

Contrast modulation affords independent control of static contrast (C) and changes in contrast (delta C). We found that in anesthetized, paralyzed cats, the visual evoked potential (VEP) was dependent only on magnitude of delta C at each pattern transition, and was independent of the starting or ending contrast level. Increasing modulation frequency to above 2 Hz reduced the VEP monotonically, implying that the time constant for differentiation by the VEP is of the order of 250 msec. The essentially perfect a.c. coupling suppresses standing contrast completely, permitting the full dynamic range of the VEP response system to be used for detection of contrast increments (which results in a decreasing Weber fraction). The difference between our results and those of behavioral studies using contrast modulation can be explained by eye movements present in the behavioral studies which refresh the retinal image of the static contrast in a way uncorrelated to temporal modulation of the stimulus, thus introducing a masking effect.

Anesthesia, General↗
PubMed1991

Inhibitory refinement of spatial frequency selectivity in single cells of the cat striate cortex.

Single cells in the cat striate cortex are more selective for the spatial frequency of sinewave grating stimuli than are cells of the retina or lateral geniculate nucleus. We have explored the possibility that this enhancement of selectivity results from spatial-frequency-selective inhibition. Stimulation with two superimposed gratings, one to excite the cell and one to prove for inhibition, revealed spatial frequency-dependent response suppression in 74% of the total population studied. Suppression was slightly more prevalent in simple cells (80%) than in complex cells (68%). In 93% of the cases where suppression was found, its tuning was complementary to excitatory spatial frequency tuning, and the strongest suppression was usually found where the excitatory tuning function approached zero imp./sec. Characteristics of the phenomenon were independent of cortical layers. We conclude that organized inhibitory mechanisms serve to refine the spatial frequency bandpass of striate cortical cells. This provides evidence for another degree of nonlinearity in the organization of cortical receptive fields and supports the hypothesis that a fundamental function of the visual cortex is image dissection in the domain of spatial frequency.

Adaptation, Physiological↗
PubMed1991

Temporal dynamics of contrast gain in single cells of the cat striate cortex.

The response amplitude of cat striate cortical cells is usually reduced after exposure to high-contrast stimuli. The temporal characteristics and contrast sensitivity of this phenomenon were explored by stimulating cortical cells with drifting gratings in which contrast sequentially incremented and decremented in stepwise fashion over time. All responses showed a clear hysteresis, in which contrast gain dropped on average 0.36 log unit and then returned to baseline values within 60 s. Noticeable gain adjustments were seen in as little as 3 s and with peak contrasts as low as 3%. Contrast adaptation was absent in responses from LGN cells. Adaptation was found to depend on temporal frequency of stimulation, with greater and more rapid adaptation at higher temporal frequencies. Two different tests showed that the mechanism controlling response reduction was influenced primarily by stimulus contrast rather than response amplitude. These results support the existence of a rapid and sensitive cortically based system that normalizes the output of cortical cells as a function of local mean contrast. Control of the adaptation appears to arise at least in part across a population of cells, which is consistent with the idea that the gain control serves to limit the information converging from many cells onto subsequent processing areas.

Adaptation, Ocular↗
PubMed1989

Role of inhibition in the specification of orientation selectivity of cells in the cat striate cortex.

Mechanisms supporting orientation selectivity of cat striate cortical cells were studied by stimulation with two superimposed sine-wave gratings of different orientations. One grating (base) generated a discharge of known amplitude which could be modified by the second grating (mask). Masks presented at nonoptimal orientations usually reduced the base-generated response, but the degree of reduction varied widely between cells. Cells with narrow orientation tuning tended to be more susceptible to mask presence than broadly tuned cells; similarly, simple cells generally showed more response reduction than did complex cells. The base and mask stimuli were drifted at different temporal frequencies which, in simple cells, permitted the identification of individual response components from each stimulus. This revealed that the reduction of the base response by the mask usually did not vary regularly with mask orientation, although response facilitation from the mask was orientation selective. In some sharply tuned simple cells, response reduction had clear local maxima near the limits of the cell's orientation-tuning function. Response reduction resulted from a nearly pure rightward shift of the response versus log contrast function. The lowest mask contrast yielding reduction was within 0.1-0.3 log unit of the lowest contrast effective for excitation. The temporal-frequency bandpass of the response-reduction mechanism resembled that of most cortical cells. The spatial-frequency bandpass was much broader than is typical for single cortical cells, spanning essentially the entire visual range of the cat. These findings are compatible with a model in which weak intrinsic orientation-selective excitation is enhanced in two stages: (1) control of threshold by nonorientation-selective inhibition that is continuously dependent on stimulus contrast; and (2) in the more narrowly tuned cells, orientation-selective inhibition that has local maxima serving to increase the slope of the orientation-tuning function.

Animals↗
PubMed1987

Visual resolution and sensitivity in a nocturnal primate (galago) measured with visual evoked potentials.

Visual resolution and contrast sensitivity were examined in anesthetized, paralyzed galagos using visual evoked potentials (VEPs) resulting from stimulation with phase-reversed sinewave gratings. Spatial frequency vs contrast response functions were band-pass with peak sensitivity at 0.2-0.4 c/deg and a high frequency cut-off between 1.6 and 3 c/deg. Peak contrast sensitivities (estimated from extrapolation of contrast response functions) varied across animals from 10 to 170. Variation of the stimulus modulation rate showed that best responses occurred at 1 Hz with an upper limit of 6-16 Hz. As in other primates, an oblique effect was seen in 6 of 8 animals. The contrast sensitivity function (CSF) determined from cortical VEPs agrees well with the CSFs of cells in the lateral geniculate nucleus, but peak sensitivity and spatial frequency are slightly lower than found for the behavioral CSF. Overall visual performance resembled closely that of another nocturnal species, the cat.

Animals↗
PubMed1986

Anticholinesterase agents affect contrast gain of the cat cortical visual evoked potential.

The effects of the anticholinesterase agents physostigmine, pyridostigmine, diisopropylfluorophosphate and soman on the cortical visual evoked potential (VEP) were examined using phase reversed sine wave gratings of varying contrasts. All 4 agents decreased the slope (gain) of the response vs log contrast function while the intercept with the noise level (threshold, an indicator of contrast sensitivity) was unaffected. These results suggest that anticholinesterases modify the VEP by changing a contrast gain control mechanism.

Acetylcholine↗
PubMed1986

Contrast adaptation in cat visual cortex is not mediated by GABA.

The possible involvement of gamma-aminobutyric acid (GABA) in contrast adaptation in single cells in area 17 of the cat was investigated. Iontophoretic application of N-methyl bicuculline increased cell responses, but had no effect on the magnitude of adaptation. These results suggest that contrast adaptation is the result of inhibition through a parallel pathway, but that GABA does not mediate this process.

Adaptation, Physiological↗
PubMed1984

Spatial adaptation of the cortical visual evoked potential of the cat.

Adaptation that is spatially specific for the adapting pattern has been seen psychophysically in humans. This is indirect evidence for independent analyzers (putatively single units) that are specific for orientation and spatial frequency in the human visual system, but it is unclear how global adaptation characteristics may be related to single unit performance. Spatially specific adaptation was sought in the cat visual evoked potential (VEP), with a view towards relating this phenomenon with what we know of cat single units. Adaptation to sine-wave gratings results in a temporary loss of cat VEP amplitude, with induction and recovery similar to that seen in human psychophysical experiments. The amplitude loss was specific for both the spatial frequency and orientation of the adapting pattern. The bandwidth of adaptation was not unlike the average selectivity of a population of cat single units.

Adaptation, Ocular↗
PubMed1982

An "oblique effect" in the visual evoked potential of the cat.

An oblique effect was observed in the amplitude of the VEP recorded from area 17 of the cat. The ratio of the responses to oblique gratings compared with responses to horizontal and vertical gratings averaged 0.77. Orientation dependence was strongest at low spatial frequencies, unlike the effect found in primates.

Animals↗
PubMed1981

Spatial consequences of bleaching adaptation in cat retinal ganglion cells.

1. Experiments were conducted to study the effects of localized bleaching on the centre responses of rod-driven cat retinal ganglion cells. 2. Stimulation as far as 2 degrees from the bleaching site yielded responses which were reduced nearly as much as those generated at the bleaching site. Bleaching in the receptive field middle reduced responsiveness at a site 1 degrees peripheral more than bleaching at that peripheral site itself. 3. The effectiveness of a bleach in reducing centre responsiveness is related to the sensitivity of the region in which the bleach is applied. 4. Response reduction after a 0.2 degree bleach followed the same temporal pattern for concentric test spots of from 0.2 to 1.8 degrees in diameter, implying a substantially uniform spread of adaptation within these bounds. 5. A linear trade-off between fraction of rhodopsin and area bleached over a range of 8:1 yields the same pattern of response reduction, implying that the non-linear nature of bleaching adaptation is a property of the adaptation pool rather than independent photoreceptors.

Adaptation, Ocular↗
PubMed1981

Induced movement of receptor alignment toward a new pupillary aperture.

Bonds and MacLeod (1978) reported on an individual whose receptors, as measured by the peak of the Stiles-Crawford function, were oriented in the direction of a traumatically displaced pupil. In an attempt to demonstrate that this orientational response is caused by an active alignment process and not simply a result of passive forces due to the traumatic ocular injury, we attempted to influence receptor alignment in the same eye by artificially creating a centered pupil. This was accomplished by placing on the subject's dilated eye a soft contact lens containing a centered 2mm artificial pupil. Receptor alignment (inferred from the entrance pupil location of the peak of the Stiles-Crawford function) significantly changed (0.8 mm) in the direction of the "new pupil" over a time course of 5 days. This alignment persisted for as long as the pupil was worn (1 month). With removal of the artificial pupil, the receptor alignment returned to its original eccentric orientation in 5 days. The results indicate that an active phototropic mechanism significantly influences receptor alignment toward the pupillary aperture.

Contact Lenses, Hydrophilic↗
PubMed1979

Cortical plasticity in monocularly deprived immobilized kittens depends on eye movement.

A marked reduction of binocular cells in striate cortex is found if 4-week-old kittens are visually stimulated monocularly while anesthetized and held in a stereotaxic apparatus. If the kittens are paralyzed and artificially respirated, changes are not found unless an eye is moved mechanically. It appears that eye movement and visual stimulation are necessary conditions for deactivation of binocular connections, but neither is sufficient to induce such changes alone.

Animals↗
PubMed1979

Recovery of cat retinal ganglion cell sensitivity following pigment bleaching.

1. The threshold illuminance for small spot stimulation of on-centre cat retinal ganglion cells was plotted vs. time after exposure to adapting light sufficiently strong to bleach significant amounts of rhodopsin. 2. When the entire receptive field of an X- or Y-type ganglion cell is bleached by at most 40%, recovery of the cell's rod-system proceeds in two phases: an early relatively fast one during which the response appears transient, and a late, slower one during which responses become more sustained. Log threshold during the later phase is well fit by an exponential in time (tau = 11.5-38 min). 3. After bleaches of 90% of the underlying pigment, threshold is cone-determined for as long as 40 min. Rod threshold continues to decrease for at least 85 min after the bleach. 4. The rate of recovery is slower after strong than after weak bleaches; 10 and 90% bleaches yield time constants for the later phase of 11.5 and 38 min, respectively. This contrasts with an approximate time constant of 11 min for rhodopsin regeneration following any bleach. 5. The relationship between the initial elevation of log rod threshold extrapolated from the fitted exponential curves and the initial amount of pigment bleached is monotonic, but nonlinear. 6. After a bleaching exposure, the maintained discharge is initially very regular. The firing rate first rises, then falls to the pre-bleach level, with more extended time courses of change in firing rate after stronger exposures. The discharge rate is restored before threshold has recovered fully. 7. The change in the response vs. log stimulus relationship after bleaching is described as a shift of the curve to the right, paired with a decrease in slope of the linear segment of the curve.

Action Potentials↗
PubMed1978

A displaced Stiles-Crawford effect associated with an eccentric pupil.

Receptors tend to be oriented toward a point near the center of the pupil in the normal eye. We report psychophysical studies of receptor orientation and directional sensitivity in a subject whose right pupil is displaced nearly 3 mm nasally as a result of injury sustained 25 years ago. The Stiles-Crawford effect was measured for foveal cones and for parafoveal cones and rods. Greatest sensitivity was found in all cases at a point close to the center of the natural pupil, indicating that the receptors in this eye are trained toward the abnormally situated pupil. At large angles of incidence, foveal cones exhibited a clear asymptote of sensitivity 0.83 log units below the sensitivity for axially incident light. Parafoveal cones were more directionally sensitive, with a suggestion of an asymptote for oblique incidence about 1.2 log units below the sensitivity for axial incidence. Rods showed a sensitivity pattern decentered like that of the cones, with a greatest observed sensitivity loss of 0.28 log units. Best acuity for cones was observed for entrance pupils close to the optical axis of the eye, remote from the pupillary region for best sensitivity.

Child, Preschool↗
PubMed1974

The bleaching and regeneration of rhodopsin in the cat.

1. The processes of bleaching and regeneration were monitored by retinal densitometry in living cats.2. Neither bleaching nor regeneration of rhodopsin can be described by the simple kinetic equation (Alpern, 1971) found valid for man.3. After a strong 1 min bleach, the retina contains more unbleached rhodopsin than expected on the basis of the initial bleaching rate.4. During the first 9 min after a 1 min bleach, cats regenerate rhodopsin only slowly; density changes during this period are dominated by formation and decay of metarhodopsin III. Subsequently, rhodopsin regeneration accelerates to a rate of 50%/11 min.5. No such delay precedes recovery from a prolonged (20 min) bleach.

Animals↗
PubMed1974

Optical quality of the living cat eye.

1. The optical quality of the living cat eye was measured under conditions similar to those of cat retinal ganglion cell experiments by recording the aerial image of a nearly monochromatic thin line of light.2. Experiments were performed to assess the nature of the fundal reflexion of the cat eye, which was found to behave essentially as a diffuser.3. The optical Modulation Transfer Function (MTF) was calculated from the measured aerial linespread using Fourier mathematics; the MTF of a ;typical' cat eye was averaged from data collected from ten eyes.4. The state of focus of the optical system, the pupil size and the angle of the light incident on the eye were all varied to determine their effect on image quality.5. By using an image rotator, the aerial linespread was measured for several orientations of the line; these measurements yielded an approximation of the two-dimensional pointspread completely characterizing the optical system.6. Evidence is reviewed to show that the optical resolution of the cat, albeit some 3-5 times worse than that of human, appears to be better than the neural resolution of its retina and its visual system as a whole.

Animals↗