Controlling binocular rivalry.

Binocular rivalry is the alternating perception that occurs when the two eyes are presented with incompatible stimuli. We have developed a new method for controlling binocular rivalry and measuring its progress. One eye views a static grating while the fellow eye views a grating that smoothly and cyclically varies between two orientations, one the same as the static grating and the other orthogonal. Contrast sensitivity was tested monocularly a number of times during the stimulus cycle. When the eye viewing the static grating was tested, sensitivity varied between maximum and minimum values as the conditioning stimulus varied from binocularly compatible to incompatible. The interocular suppression thus demonstrated was limited to the eye viewing the static grating; variations in the fellow eye's sensitivity were due to interocular masking alone.

The depth and selectivity of suppression in binocular rivalry.

Binocular rivalry occurs when the two eyes are presented with incompatible stimuli and the perceived image alternates between the two stimuli. The aim of this study was to find out whether the periodic perceptual loss of a monocular stimulus during binocular rivalry is mirrored by a comparable loss of contrast sensitivity. We presented brief test stimuli to one eye while its conditioning stimulus was dominant or suppressed. The test stimuli were varied widely across four stimulus domains--namely, the relative stimulation of medium- and long-wavelength-sensitive cones, duration, spatial frequency, and grating orientation. The result in each case was the same. Suppression depended slightly or not at all on the type of test stimulus, and contrast sensitivity during suppression was around 64% of that during dominance. The effect of suppression on sensitivity is therefore very weak, relative to its effect on the perceived image. Furthermore, suppression was largely independent of the similarity between the conditioning and the test stimuli, indicating that our results are better explained by eye suppression than by stimulus suppression. A model is presented to account for the small, monocular sensitivity loss during suppression: It assumes that test detection precedes conditioning stimulus perception in the visual pathway.

Colour but not form patterns combine between the eyes.

PURPOSE: A visual pattern was broken into patches, some of which were shown to one eye and the remainder of which were shown to the other eye. Our aim was to find the conditions under which the subject could combine the information given to the two eyes separately to reconstruct the original pattern. METHODS: One green and four red patches were presented to one eye. Five patches were presented to the other eye at corresponding locations but with red substituted for green and green for red. RESULTS: When the chromatic contrasts were appropriately adjusted, subjects saw five patches of a single colour for most of the viewing time. Intermixed horizontal and vertical gratings did not separate into coherent percepts in the same way. CONCLUSIONS: This result indicates cooperation between the responses to colour patches presented to different eyes, a finding that may be of use in testing for normal binocular vision.

Visual sensitivity in the presence of a patterned background.

The sensitivity of the visual system depends on ambient illumination: Sensitivity is reduced in the presence of a bright, uniform background. We asked how sensitivity is adjusted when the background is spatially detailed and therefore contains both luminance peaks and troughs in the neighborhood of a foreground object. A test flash was superimposed on a static sinusoidal grating. As the grating's spatial frequency increased, sensitivity for flash detection declined, regardless of whether the flash was superimposed on a peak or a trough of the grating. We studied the mechanisms underlying this loss of sensitivity by delivering the test stimulus through one eye and the background through the other. The conclusion is that three mechanisms are involved. Luminance adaptation and a masking process adjust sensitivity at low- and mid-range spatial frequencies, respectively. The third mechanism, a contrast gain control, is localized (it occurs at spatial frequencies approaching the limit for resolution) and fast (complete in half a second), and it results from early processing in the visual pathway (it is absent during dichoptic viewing). This local adjustment of sensitivity may help to protect the clarity of even the smallest details in the visual scene.

Components of visual acuity loss in strabismus.

Strabismus, the misalignment of the visual axis of one eye relative to that of the other eye, reduces visual acuity in the affected eye. Several processes contributing to that loss are: amblyopia, which results in a chronic acuity loss whether or not the fellow eye is viewing; strabismic deviation, which shifts the image of an acuity target onto more peripheral, and therefore less acute, retina when the fellow eye fixates; interocular suppression and binocular masking, which reduce visibility in the strabismic eye due to neural influences from the other eye. We measured the losses due to these processes in nine small-angle strabismic subjects. Amblyopia reduced acuity by a median of 34% relative to its value in subjects with normal binocular vision, and strabismic deviation produced a loss of 44%. Suppression and masking together reduced acuity by 20%, and therefore had substantially less effect than the other factors.

Visual loss during interocular suppression in normal and strabismic subjects.

Visual acuity was measured in one eye during monocular vision, and while the fellow eye viewed stimuli not including the acuity target. The aim was to find how acuity in one eye is reduced by going from monocular to binocular viewing. In normal subjects, acuity was at its lowest during the suppressive phase of binocular rivalry, was reduced less when the fellow eye viewed a contoured nonrivalrous stimulus, and was not reduced at all when the stimulus to the fellow eye consisted of a uniformly lit field. In strabismic subjects, by contrast, acuity was markedly reduced in going from monocular to binocular viewing no matter what stimulus was viewed by the fellow eye. Pathological suppression is therefore largely independent of the inducing stimulus. It was also shown that acuity in the nonstrabismic eye of some of the strabismic subjects was improved by allowing the strabismic eye to view; these were the subjects with the greatest depths of amblyopia.

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.

Spatial characteristics of the contrast gain control in the cat's retina.

Impulse rate was recorded from X- and Y-type ganglion cells in the cat's retina. The stimuli were stationary gratings for which luminance varied sinusoidally with distance across the stimulus, and amplitude varied sinusoidally in time. Y cell fundamental and second harmonic responses recorded at medium to high spatial frequencies advanced in phase with increasing contrast, an effect attributable to the contrast gain control. As spatial frequency increased to the highest value capable of evoking a second harmonic response, the phase of this response became retarded, indicating that the contrast gain control was losing its effect. This result showed that the spatial resolution of the contrast gain control is very close to that of the Y cell's rectifying subunits. The observations strongly suggest that the contrast gain control has its effect early in retinal image processing.

Developing the art of successful clinical supervision in health care programs.

Supervision is a dynamic and ever-changing process. Excellent employees can be trained to be outstanding supervisors if the competencies and skills of the supervisory process are clarified. Supervision is a successful art that can be learned and enhanced if appropriate supervisory skills are identified, taught, and learned. Supervision is too important in developing the future health care work force to be left to chance.

A model for spatiotemporal frequency responses in the X cell pathway of the cat's retina.

A linear model is described for the cat eye's signal-processing pathway, from the visual stimulus at the cornea, to cones, to X-type ganglion cells. The model contains elements representing the eye's optics, phototransduction, gain control, spatiotemporal processing by cell layers, and pure delay. Centre-surround antagonism in the model arises through the presence of a centre element producing a small spatial spread of signals, and an antagonistic element producing a larger spread. Two arrangements were tried, feedforward and feedback, in which the antagonistic element's output was subtracted from the centre element's output, and input, respectively. The model was fitted to empirical spatial and temporal frequency responses collected by Frishman et al. (1987), and accounted qualitatively for these data in the feedback, but not the feedforward, arrangement. The model's centre pathway comprises a cascade of low-pass spatial filters, as does the surround pathway. As a consequence, the spatial frequency responses for these two pathways closely approximate Gaussian functions of spatial frequency, and the spatial frequency response of the complete model at low temporal frequency closely matches that of the difference of Gaussians model.

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)

Computer use in allied health programs.

This article presents a preliminary assessment of computer use in allied health programs. The findings of a survey among 60 allied health programs indicate that computer use in the classroom, in clinical education, and in simulation has increased. In preprofessional education, computers are used by less than 50% of the allied health programs. However, computers are used more in the professional phase for patient management, clinical simulations using branching and logic methods, physiological simulations, and therapeutic planning and management. Students in these programs are required to take three to six semester hours of computer literacy classes. In the classroom, computer-assisted instruction is used to provide remediation, reinforcement, enrichment, and test-taking drills in clinical and didactic learning. Students use microcomputers to gain application experience in health statistics, data bases, abstracting, and diagnosis-related group classifiers. Although progress has been made in computer use, greater efforts must be expended to ensure greater use of computer technology in the next decade in allied health disciplines. Recommendations for increased use of computer technology in allied health programs are provided.

Spatiotemporal frequency responses of cat retinal ganglion cells.

Spatiotemporal frequency responses were measured at different levels of light adaptation for cat X and Y retinal ganglion cells. Stationary sinusoidal luminance gratings whose contrast was modulated sinusoidally in time or drifting gratings were used as stimuli. Under photopic illumination, when the spatial frequency was held constant at or above its optimum value, an X cell's responsivity was essentially constant as the temporal frequency was changed from 1.5 to 30 Hz. At lower temporal frequencies, responsivity rolled off gradually, and at higher ones it rolled off rapidly. In contrast, when the spatial frequency was held constant at a low value, an X cell's responsivity increased continuously with temporal frequency from a very low value at 0.1 Hz to substantial values at temporal frequencies higher than 30 Hz, from which responsivity rolled off again. Thus, 0 cycles X deg-1 became the optimal spatial frequency above 30 Hz. For Y cells under photopic illumination, the spatiotemporal interaction was even more complex. When the spatial frequency was held constant at or above its optimal value, the temporal frequency range over which responsivity was constant was shorter than that of X cells. At lower spatial frequencies, this range was not appreciably different. As for X cells, 0 cycles X deg-1 was the optimal spatial frequency above 30 Hz. Temporal resolution (defined as the high temporal frequency at which responsivity had fallen to 10 impulses X s-1) for a uniform field was approximately 95 Hz for X cells and approximately 120 Hz for Y cells under photopic illumination. Temporal resolution was lower at lower adaptation levels. The results were interpreted in terms of a Gaussian center-surround model. For X cells, the surround and center strengths were nearly equal at low and moderate temporal frequencies, but the surround strength exceeded the center strength above 30 Hz. Thus, the response to a spatially uniform stimulus at high temporal frequencies was dominated by the surround. In addition, at temporal frequencies above 30 Hz, the center radius increased.

The receptive-field spatial structure of cat retinal Y cells.

1. Y-type ganglion cells in the cat's retina were stimulated with bars of light and grating patterns at photopic luminances. Stimuli were stationary, and luminance at each point was varied sinusoidally in time at 2 Hz. Impulse rates were recorded from single cells. 2. When the stimulus was a narrow bar of light, the impulse rate approached a sinusoidal function of time as contrast was reduced. The linear behaviour of each cell was therefore characterized by taking the limit of response parameters as contrast approached zero. 3. The ratio of surround strength to centre strength varied widely between cells but the two strengths were approximately equal on average. The difference between surround phase and centre phase averaged 168 deg. 4. As contrast increased, responses became rectified. Rectifier output was well described by a power law of stimulus amplitude, where the power was usually 1.4 or 1.5. 5. Response phase advanced with increasing contrast, and at high response amplitudes grew less than proportionally with contrast. These effects were assumed due to the contrast gain control described by Shapley & Victor (1978). 6. Gratings in which luminance varied sinusoidally with distance were used to determine Y cell spatial resolution. The second-harmonic amplitude of the response diminished rapidly with increasing spatial frequency: the radius of the best-fitting Gaussian mechanism was about 0.25 deg for a cell at 10 deg eccentricity. 7. This spatial resolution is close to the linear resolution of X cells as determined by Linsenmeier, Frishman, Jakiela & Enroth-Cugell (1982). 8. A receptive field model incorporating both linear and non-linear elements is described. The model consists of an array of subunit pathways, each of which has a centre-surround organization followed by a rectifier; a pool weights and sums subunit outputs, and signals are then passed through a contrast gain control. 9. The model accounts qualitatively for the over-all centre-surround organization of Y cell linear responses, the dependence of frequency-doubled responses on spatial frequency, and impulse rate as a function of time for a variety of bar and grating stimuli.

Cutaneous mechanoreceptors in macaque monkey: temporal discharge patterns evoked by vibration, and a receptor model.

1. Vibratory stimuli applied to the hand of a monkey evoke phase locked impulse trains in the three classes of low threshold mechanoreceptive afferents which innervate the area. The responses of each class of afferent (slowly adapting (SA), rapidly adapting (RA), and Pacinian (PC) vary in a systematic but complex way across the range of frequencies and intensities to which they are sensitive. The receptors are not accessible for electrophysiological recording. The aim in this study was to infer the mechanisms underlying their responses from detailed examination of the statistical properties of the impulse trains.2. A very simple receptor model with four degrees of freedom was chosen as a starting point. The independent variables consisted of the resting membrane time constant, tau, a variable membrane conductance, G(r), the fraction of each sinusoidal stimulus cycle producing depolarization, p(r), and the noise level, sigma, which was assigned to the impulse threshold. The aim was to use the deviations between observed data and predictions from the basic model to construct a more effective model. In fact, the deviations were minor and were mostly explained by periods of increased excitability in the wake of each action potential. Almost all of the differences between the responses of the three mechanoreceptive classes examined in this paper were accounted for by differences in time constants.3. The temporal structure of the responses from each mechanoreceptive class was examined at two levels of resolution, a coarse level where the resolution unit was a full cycle, and a fine level where the unit was 0.1 ms.4. The coarse structure of each response was represented by the presence or absence of an impulse on each stimulus cycle. In each mechanoreceptive class, the impulse sequences were random at low stimulus frequencies and regular at high frequencies. The transition frequencies were roughly 5 Hz for the slowly adapting afferents, 7 Hz for the rapidly adapting afferents, and 110 Hz for the Pacinian afferents. The model matched these data closely when the time constants were set at 80, 60 and 3.4 ms for SAs, RAs and PCs, respectively.5. The fine structure of the responses of each mechanoreceptive class exhibited impulse phase locking, phase advance with increasing intensity, and bimodal phase distributions at higher frequencies. Impulses contributing to the first mode of bimodal distributions always occurred in cycles following cycles in which no impulse occurred. Impulses contributing to the retarded mode always occurred in cycles following filled cycles. The mean phase differences between the two modes was called phase retardation. Phase retardation grew with stimulus frequency for both the receptors and the model; the time constants required to match the model against neural phase retardation curves were 123, 64 and 4.8 ms for SAs, RAs and PCs, respectively.

A model accounting for effects of vibratory amplitude on responses of cutaneous mechanoreceptors in macaque monkey.

1. A mechanoreceptor model, developed in the preceding paper (Freeman & Johnson, 1982), was used to study the effects of vibratory intensity and frequency on the responses of slowly adapting, rapidly adapting and Pacinian afferents in monkey hairless skin. As in the previous paper almost all of the response properties studied here were accounted for by the equivalent circuit model; changes in membrane time constant and amplitude sensitivity accounted for the differences between the three mechanoreceptive fibre types.2. The stimulus-response function of primary concern was the relationship between impulse rate and vibratory amplitude. This relationship had the same general form in each of the three fibre types. Amplitudes, I, less than I(0) produced no impulse on any stimulus cycles. Amplitudes greater than I(1) produced one impulse on every cycle. As I rose from I(0) to I(1) the impulse rate rose monotonically from 0 to 1 impulse/cycle. For each fibre type the form of this ramp depended on the stimulus frequency.3. At stimulus frequencies low in the frequency range of each fibre type the (I(0), I(1)) ramp tended to be steep and sigmoidal in shape. Two or more impulses occurred on some cycles and none on others.4. At intermediate frequencies the (I(0), I(1)) ramps became linear with at most one impulse on each cycle. A short plateau appeared at 0.5 impulses/cycle (i.e. there was a range of intensities yielding one impulse on alternate cycles). All of these response properties at low and intermediate frequencies were explained by the model.5. At higher frequencies the (I(0), I(1)) ramps became shallower and developed discontinuities in slope at impulse rates of 0.5 impulses/cycle. At stimulus frequencies greater than 20 Hz for SAs and RAs, the upper segment of the (I(0), I(1)) slope became steeper. For frequencies greater than 80 Hz, the upper segments of the Pacinian (I(0), I(1)) slopes were shallower than the lower segments. These effects suggested transient periods of hyperexcitability following each action potential, and reductions in sensitivity due to high impulse rates, respectively.6. The model's membrane time constant was adjusted to match the observed reduction in the (I(0), I(1)) slope with increasing stimulus frequency. The time constants required for least-squares fitting were 58, 29 and 4.2 msec for slowly adapting, rapidly adapting and Pacinian afferents, respectively; these values are of the same order as those obtained in the preceding paper.7. Receptor sensitivity varied across the frequency spectrum, slow adaptors being most sensitive at low frequencies, rapidly adapting units at mid-range, and Pacinians at the high frequencies. According to the model, the high frequency roll-off in a receptor's tuning curve is due to the current integrating properties of receptor membrane, and the low frequency roll-off is due to a high pass filter, presumably mechanical, situated in the tissues between the stimulus probe and receptor membrane.8. Impulse phase advances with increasing stimulus intensity in both receptor and model. The ability of the model to fit both the rate-intensity function and phase advance functions in individual receptors is demonstrated.