Human short-latency ocular vergence responses produced by interocular velocity differences.

We studied human short-latency vergence eye movements to a novel stimulus that produces interocular velocity differences without a changing disparity signal. Sinusoidal luminance gratings moved in opposite directions (left vs. right; up vs. down) in the two eyes. The grating seen by each eye underwent »-wavelength shifts with each image update. This arrangement eliminated changing disparity cues, since the phase difference between the eyes alternated between 0° and 180°. We nevertheless observed robust short-latency vergence responses (VRs), whose sign was consistent with the interocular velocity differences (IOVDs), indicating that the IOVD cue in isolation can evoke short-latency VRs. The IOVD cue was effective only when the images seen by the two eyes overlapped in space. We observed equally robust VRs for opposite horizontal motions (left in one eye, right in the other) and opposite vertical motions (up in one eye, down in the other). Whereas the former are naturally generated by objects moving in depth, the latter are not part of our normal experience. To our knowledge, this is the first demonstration of a behavioral consequence of vertical IOVD. This may reflect the fact that some neurons in area MT are sensitive to these motion signals (Czuba, Huk, Cormack, & Kohn, 2014). VRs were the strongest for spatial frequencies in the range of 0.35-1 c/°, much higher than the optimal spatial frequencies for evoking ocular-following responses observed during frontoparallel motion. This suggests that the two motion signals are detected by different neuronal populations. We also produced IOVD using moving uncorrelated one-dimensional white-noise stimuli. In this case the most effective stimuli have low speed, as predicted if the drive originates in neurons tuned to high spatial frequencies (Sheliga, Quaia, FitzGibbon, & Cumming, 2016).

Anisotropy in spatial summation properties of human Ocular-Following Response (OFR).

Using sinusoidal gratings we show that an increase in stimulus size confined to the dimension orthogonal to the axis of motion leads to stronger Ocular Following Responses (OFRs) up to a certain optimal size. An increase beyond this optimum produces smaller responses, indicating suppressive interactions. In sharp contrast, when the stimulus growth occurs parallel to the axis of motion OFR magnitudes increase monotonically both for horizontal and vertical directions of motion. Similar results are obtained with 1D white noise patterns. However, the OFR spatial anisotropy is minimal with 2D white noise patterns, revealing a pivotal role of orientation-selective (i.e., cortical) mechanisms in mediating this phenomenon. The lack of anisotropy for 2D patterns suggests that directional signals alone are not sufficient to elicit this suppression. The OFR spatial anisotropy is potentiated if a stationary grating is presented for 600-1000ms before its motion commences, further emphasizing the importance of static orientation signals. These results suggest that the strength of cortical spatial interactions is asymmetric-i.e., larger in the direction of the ends than the flanks of an orientation-selective receptive field-which corroborates the existing neurophysiological evidence.

Retinal visual processing constrains human ocular following response.

Ocular following responses (OFRs) are the initial tracking eye movements elicited at ultra-short latency by sudden motion of a textured pattern. We wished to evaluate quantitatively the impact that subcortical stages of visual processing might have on the OFRs. In three experiments we recorded the OFRs of human subjects to brief horizontal motion of 1D vertical sine-wave gratings restricted to an elongated horizontal aperture. Gratings were composed of a variable number of abutting horizontal strips where alternate strips were in counterphase. In one of the experiments we also utilized gratings occupying a variable number of horizontal strips separated vertically by mean-luminance gaps. We modeled retinal center/surround receptive fields as a difference of two 2-D Gaussian functions. When the characteristics of such local filters were selected in accord with the known properties of primate retinal ganglion cells, a single-layer model was capable to quantitatively account for the observed changes in the OFR amplitude for stimuli composed of counterphase strips of different heights (Experiment 1), for a wide range of stimulus contrasts (Experiment 2) and spatial frequencies (Experiment 3). A similar model using oriented filters that resemble cortical simple cells was also able to account for these data. Since similar filters can be constructed from the linear summation of retinal filters, and these filters alone can explain the data, we conclude that retinal processing determines the response to these stimuli. Thus, with appropriately chosen stimuli, OFRs can be used to study visual spatial integration processes as early as in the retina.

Spatial summation properties of the human ocular following response (OFR): dependence upon the spatial frequency of the stimulus.

Ocular following responses (OFRs) are the initial tracking eye movements that can be elicited at ultra-short latency by sudden motion of a textured pattern. The OFR magnitude depends upon stimulus size, and also upon the spatial frequency (SF) of sine-wave gratings. Here we investigate the interaction of size and SF. We recorded initial OFRs in human subjects when 1D vertical sine-wave gratings were subject to horizontal motion. Gratings were restricted to elongated horizontal apertures-"strips"-aligned with the axis of motion. In Experiment 1 the SF and the height of a single strip was manipulated. The magnitude of the OFR increased with strip height up to some optimum value, while strip heights greater than this optimum produced smaller responses. This effect was strongly dependent on SF: the optimum strip height was smaller for higher SFs. In order to explore the underlying mechanism, Experiment 2 measured OFRs to stimuli composed of two thin horizontal strips-one in the upper visual field, the other in the lower visual field-whose vertical separation varied 32-fold. Stimuli of different sizes can be reconstructed from the sum of such horizontal strips. We found that the OFRs in Experiment 1 were smaller than the sum of the responses to the component stimuli, but greater than the average of those responses. We defined an averaging coefficient that described whether a given response was closer to the sum or to the average. For any one SF, the averaging coefficients were similar over a wide range of stimulus sizes, while they varied considerably (7-fold) for stimuli of different SFs.

Clinical guidelines for the management of craniofacial fibrous dysplasia.

Fibrous dysplasia (FD) is a non-malignant condition caused by post-zygotic, activating mutations of the GNAS gene that results in inhibition of the differentiation and proliferation of bone-forming stromal cells and leads to the replacement of normal bone and marrow by fibrous tissue and woven bone. The phenotype is variable and may be isolated to a single skeletal site or multiple sites and sometimes is associated with extraskeletal manifestations in the skin and/or endocrine organs (McCune-Albright syndrome). The clinical behavior and progression of FD may also vary, thereby making the management of this condition difficult with few established clinical guidelines. This paper provides a clinically-focused comprehensive description of craniofacial FD, its natural progression, the components of the diagnostic evaluation and the multi-disciplinary management, and considerations for future research.

Knowledge and utilization of palliative radiotherapy by pediatric oncologists.

BACKGROUND AND PURPOSE: Palliative radiotherapy (PRT) is a routine part of oncology care in adult patients, but it is used much less frequently among children with incurable cancer. We surveyed Canadian pediatric oncologists to learn about their knowledge and use of PRT and to identify potential barriers to referral. METHODS: A 13-item questionnaire assessing PRT knowledge and utilization was sent to 80 Canadian pediatric oncologists. RESULTS: The survey completion rate was 80%, with most respondents being providers of palliative care for children and making referrals for PRT. Although 62% had received training in radiation oncology, only 28% had received formal palliative care training. Respondents with palliative care training were found to be significantly more knowledgeable about PRT and were more likely to refer children for PRT (p < 0.01). Only 59% of respondents thought that they had adequate knowledge about the indications for PRT. A positive correlation was found between knowledge about the indications for PRT and referral for treatment (p < 0.01). Among survey respondents, 51% believed that PRT was underutilized, and the perceived barriers to PRT referral included patient or family reluctance, distance to the cancer centre, belief that PRT has little impact on quality of life, and concerns about toxicity. CONCLUSIONS: Palliative radiotherapy is considered to be underutilized among children. This situation appears to be related, in part, to inadequate knowledge and training among pediatric oncologists, suggesting that more emphasis needs to be placed on pediatric palliative care education.

The initial torsional Ocular Following Response (tOFR) in humans: a response to the total motion energy in the stimulus?

We recorded the initial torsional Ocular Following Responses (tOFRs) elicited at short latency by visual images that occupied the frontal plane and rotated about the lines of sight. Using 1-D radial gratings, the local spatio-temporal characteristics of these tOFRs closely resembled those we previously reported for the hOFRs to horizontal motion with 1-D vertical gratings. When the 1-D radial grating was subdivided into a number of concentric annuli, each with the same radial thickness, tOFRs were less than predicted from the sum of the responses to the individual annuli: spatial normalization. However, the normalization was much weaker than that which we previously reported for the hOFRs. Further, when the number, thickness and contrast of these concentric annuli were varied systematically, the latency and magnitude of the tOFRs were well described by single monotonic functions when plotted against the product of the total area of the annuli and the square of their Michelson contrast ("A*C(2)"), consistent with the hypothesis that the onset and magnitude of the initial tOFR are determined by the total motion energy in the stimulus. When our previously published hOFR data were plotted against A*C(2), a single monotonic function sufficed to describe the latency but not the magnitude.

Human ocular following: evidence that responses to large-field stimuli are limited by local and global inhibitory influences.

Large-field visual motion elicits tracking eye movements at ultra-short latency, often termed ocular following responses (OFRs). We recorded the initial OFRs of three human subjects when vertical sine-wave gratings were subject to horizontal motion in the form of successive 1/4-wavelength steps. The gratings could occupy the full screen (45 degrees wide, 30 degrees high) or a number of horizontal strips, each 1 degrees high and extending the full width of the display. These strips were always equally spaced vertically. In a first experiment, the gratings always had a contrast of 32%. Increasing the number of strips could reduce the response latency by up to 20 ms, so the magnitude of the initial OFRs was estimated from the change in eye position over the initial open-loop period measured with respect to response onset. A single (centred) strip (covering 3.3% of the screen) always elicited robust OFRs, and three strips (10% coverage) were sufficient to elicit the maximum OFR. Increasing the number of strips to 15 (50% coverage) had little impact, i.e., responses had asymptoted, and further increasing the coverage to 100% (full screen image) actually decreased the OFR so that it was now less than that elicited with only one strip. In a second experiment, the contrast of the gratings could be fixed at one of the four levels ranging from 8% to 64%, and the OFR showed essentially the same pattern of dependence on screen coverage except that the lower the contrast, the lower the level at which the response asymptoted. This indicated that the asymptote was not due simply to some upper limit on the magnitude of the eye movement or the underlying motion signals. We postulate that this asymptote is the result of normalization due to global divisive inhibition, which has often been described in visual-motion-selective neurons in the cortex. We further suggest that the decrease in the OFR when the image filled the screen was due to the increased continuity of the gratings which we postulate would favour the local inhibitory surround mechanisms over the central excitatory ones. This study indicates that robust OFRs can be elicited by much smaller motion stimuli than is commonly supposed and that introducing spatial discontinuities can increase the efficacy of the motion stimuli even while decreasing the area stimulated.

Spatial summation properties of the human ocular following response (OFR): evidence for nonlinearities due to local and global inhibitory interactions.

Ocular following responses (OFRs) are the initial tracking eye movements that can be elicited at ultra-short latency by sudden motion of a textured pattern. A recent study used motion stimuli consisting of two large coextensive sine-wave gratings with the same orientation but different spatial frequency and moving in (1/4)-wavelength steps in the same or opposite directions: when the two gratings differed in contrast by more than about an octave then the one with the higher contrast completely dominated the OFR and the one with lower contrast lost its influence as though suppressed [Sheliga, B. M., Kodaka, Y., FitzGibbon, E. J., & Miles, F. A. (2006). Human ocular following initiated by competing image motions: Evidence for a winner-take-all mechanism. Vision Research, 46, 2041-2060]. This winner-take-all (WTA) outcome was attributed to nonlinear interactions in the form of mutual inhibition between the mechanisms sensing the competing motions. In the present study, we recorded the initial horizontal OFRs to the horizontal motion of two vertical sine-wave gratings that differed in spatial frequency and were each confined to horizontal strips that extended the full width of our display (45 degrees ) but were only 1-2 degrees high. The two gratings could be coextensive or separated by a vertical gap of up to 8 degrees , and each underwent motion consisting of successive (1/4)-wavelength steps. Initial OFRs showed strong dependence on the relative contrasts of the competing gratings and when these were coextensive this dependence was always highly nonlinear (WTA), regardless of whether the two gratings moved in the same or opposite direction. When the two gratings moved in opposite directions the nonlinear interactions were purely local: with a vertical gap of 1 degrees or more between the gratings OFRs approximated the linear sum of the responses to each grating alone. On the other hand, when the two gratings moved in the same direction the nonlinear interactions were more global: even with a gap of 8 degrees -the largest separation tried-OFRs were still substantially less than predicted by the linear sum. When the motions were in the same direction, we postulate two nonlinear interactions: local mutual inhibition (resulting in WTA) and global divisive inhibition (resulting in normalization). Motion stimuli whose responses were totally suppressed by coextensive opponent motion of higher contrast were rendered invisible to normalization, suggesting that the local interactions responsible for the WTA behavior here occur at an earlier stage of neural processing than the global interactions responsible for normalization.

The vergence eye movements induced by radial optic flow: some fundamental properties of the underlying local-motion detectors.

Radial optic flow applied to large random dot patterns is known to elicit horizontal vergence eye movements at short latency, expansion causing convergence and contraction causing divergence: the Radial Flow Vergence Response (RFVR). We elicited RFVRs in human subjects by applying radial motion to concentric circular patterns whose radial luminance modulation was that of a square wave lacking the fundamental: the missing fundamental (mf) stimulus. The radial motion consisted of successive 1/4-wavelength steps, so that the overall pattern and the 4n+1 harmonics (where n=integer) underwent radial expansion (or contraction), whereas the 4n-1 harmonics--including the strongest Fourier component (the 3rd harmonic)--underwent the opposite radial motion. Radial motion commenced only after the subject had fixated the center of the pattern. The initial RFVRs were always in the direction of the 3rd harmonic, e.g., expansion of the mf pattern causing divergence. Thus, the earliest RFVRs were strongly dependent on the motion of the major Fourier component, consistent with early spatio-temporal filtering prior to motion detection, as in the well-known energy model of motion analysis. If the radial mf stimulus was reduced to just two competing harmonics--the 3rd and 5th--the initial RFVRs showed a nonlinear dependence on their relative contrasts: when the two harmonics differed in contrast by more than about an octave then the one with the higher contrast completely dominated the RFVRs and the one with lower contrast lost its influence: winner-take-all. We suggest that these nonlinear interactions result from mutual inhibition between the mechanisms sensing the motion of the different competing harmonics. If single radial-flow steps were used, a brief inter-stimulus interval resulted in reversed RFVRs, consistent with the idea that the motion detectors mediating these responses receive a visual input whose temporal impulse response function is strongly biphasic. Lastly, all of these characteristics of the RFVR, which we attribute to the early cortical processing of visual motion, are known to be shared by the Ocular Following Response (OFR)--a conjugate tracking (version) response elicited at short-latency by linear motion-and even the quantitative details are generally very similar. Thus, although the RFVR and OFR respond to very different patterns of global motion-radial vs. linear-they have very similar local spatiotemporal properties as though mediated by the same low-level, local-motion detectors, which we suggest are in the striate cortex.

Human vergence eye movements initiated by competing disparities: evidence for a winner-take-all mechanism.

Vergence eye movements were elicited in human subjects at short latencies (approximately 70 ms) by applying binocular disparities briefly (200 ms) to large grating patterns (46 degrees wide, 35 degrees high). The positions of both eyes were recorded with the electromagnetic search coil technique. Using a dichoptic viewing arrangement (Wheatstone stereoscope), each eye viewed two overlapping 1-D sine waves that had the same orientation but different spatial frequencies. These two sine waves each had a binocular disparity that was 1/4 of its wavelength and the effect of varying their relative contrasts was examined (15 contrast ratios ranging from 0.125 to 8). The first experiment used horizontal gratings and recorded the vertical vergence responses when the two sine waves had spatial frequencies in the ratio 3:5 and vertical disparities of opposite sign. Initial vergence responses showed a highly nonlinear dependence on the contrast ratio. On average, when the contrast of one sine wave exceeded that of the other by a factor of >2.2, the sine wave with the higher contrast dominated responses and the sine wave with the lower contrast had almost no influence: winner-take-all. A second experiment, which used vertical gratings and recorded the horizontal vergence responses when the two sine waves had spatial frequencies in the ratio 3:5 and horizontal disparities of opposite sign, also uncovered nonlinear interactions but these were much more variable from one subject to another and, on average, one sine wave did not achieve complete dominance until its contrast exceeded that of the other by a factor of >4.5. When these two experiments were repeated with grating patterns in which the two sine waves had spatial frequencies in the ratio 3:7 and disparities of the same sign, similar nonlinear interactions were apparent. We attribute the nonlinear dependence on relative contrast to mutual inhibition between the neural elements processing the disparities of the two sine waves. We further suggest that this interaction will help to maintain binocular alignment on the objects in the plane of regard because the retinal images of those objects will tend to be better focused-and hence tend to have higher contrasts-than the images of objects in other depth planes.

Short-latency disparity vergence eye movements: a response to disparity energy.

Vergence eye movements were elicited in human subjects by applying disparities to square-wave gratings lacking the fundamental ("missing fundamental", mf). Using a dichoptic arrangement, subjects viewed gratings that were identical at the two eyes except for a phase difference of 1/4 wavelength so that, based on the nearest-neighbor matches, the features and the 4n+1 harmonics (5th, 9th, etc.) all had binocular disparities of one sign, whereas the 4n-1 harmonics (3rd, 7th, etc.) all had disparities of the opposite sign. Further, the amplitude of the ith harmonic was proportional to 1/i. Using the electromagnetic search coil technique to record the positions of both eyes indicated that the earliest vergence eye movements elicited by these disparity stimuli had ultra-short latencies (minimum, <65 ms) and were always in the direction of the most prominent harmonic, the 3rd, but their magnitudes fell short of those elicited when the same disparities were applied to pure sinusoids whose spatial frequency and contrast matched those of the 3rd harmonic. This shortfall was evident in both the horizontal vergence responses recorded with vertical grating stimuli and the vertical vergence responses recorded with horizontal grating stimuli. When the next most prominent harmonic, the 5th, was removed from the mf stimulus (creating the "mf-5" stimulus) the vertical vergence responses showed almost no shortfall-indicating that it had been almost entirely due to that 5th harmonic-but the horizontal vergence responses still showed a small shortfall, at least with higher contrast stimuli. This small shortfall might represent a very minor contribution from higher harmonics and/or distortion products and/or a feature-based mechanism. We conclude that the earliest disparity vergence responses-especially vertical-were strongly dependent on the major Fourier components of the binocular images, consistent with early spatial filtering of the monocular visual inputs prior to their binocular combination as in the disparity-energy model of complex cells in striate cortex [Ohzawa, I., DeAngelis, G. C., & Freeman, R. D. (1990). Stereoscopic depth discrimination in the visual cortex: neurons ideally suited as disparity detectors. Science, 249, 1037-1041].

Human ocular following initiated by competing image motions: evidence for a winner-take-all mechanism.

The initial ocular following responses (OFRs) elicited by 1/4-wavelength steps applied to the missing fundamental (mf) stimulus are in the backward direction and largely determined by the principal Fourier component, the 3rd harmonic [Sheliga, B. M., Chen, K. J., FitzGibbon, E. J., & Miles, F. A. (2005). Initial ocular following in humans: A response to first-order motion energy. Vision Research, 45, 3307-3321]. When the contrast of the 3rd harmonic was selectively reduced below that of the next most prominent harmonic-the 5th, which moves in the opposite (forward) direction-then the OFR reversed direction and the 3rd harmonic effectively lost all of its influence as the OFR was now largely determined by the 5th harmonic. Restricting the stimulus to just two sine waves (of equal efficacy when of equal contrast and presented singly) with the spatial frequencies of the 3rd and 5th harmonics of the mf stimulus indicated that the critical factor was the ratio of their two contrasts: when of similar contrast both were effective (vector sum/averaging), but when the contrast of one was <1/2 that of the other then the one with the lower contrast became ineffective (winner-take-all). This nonlinear dependence on the contrast ratio was attributed to mutual inhibition and was well described by a weighted-average model with just two free parameters. Further experiments with broadband and dual-grating stimuli indicated that nonlinear interactions occur not only in the neural processing of stimuli moving in opposite directions but also of stimuli that share the same direction and differ only in their spatial frequency and speed. Clearly, broad-band and dual-grating stimuli can uncover significant nonlinearities in visual information processing that are not evident with single sine-wave stimuli.

The initial ocular following responses elicited by apparent-motion stimuli: reversal by inter-stimulus intervals.

Transient apparent-motion stimuli, consisting of single 1/4-wavelength steps applied to square-wave gratings lacking the fundamental ("missing fundamental stimulus") and to sinusoidal gratings, were used to elicit ocular following responses (OFRs) in humans. As previously reported [Sheliga, B. M., Chen, K. J., FitzGibbon, E. J., & Miles, F. A. (2005). Initial ocular following in humans: a response to first-order motion energy. Vision Research, in press], the earliest OFRs were strongly dependent on the motion of the major Fourier component, consistent with early spatio-temporal filtering prior to motion detection, as in the well-known energy model of motion analysis. Introducing inter-stimulus intervals (ISIs) of 10-200 ms, during which the screen was gray with the same mean luminance, reversed the initial direction of the OFR, the peak reversed responses (with ISIs of 20-40 ms) being substantially greater than the non-reversed responses (with an ISI of 0 ms). When the mean luminance was reduced to scotopic levels, reversals now occurred only with ISIs > or=60 ms and the peak reversed responses (with ISIs of 60-100 ms) were substantially smaller than the non-reversed responses (with an ISI of 0 ms). These findings are consistent with the idea that initial OFRs are mediated by first-order motion-energy-sensing mechanisms that receive a visual input whose temporal impulse response function is strongly biphasic in photopic conditions and almost monophasic in scotopic conditions.

Awareness and use of the Rapid Palliative Radiotherapy Program by family physicians in Eastern Ontario: a survey.

The Ottawa Rapid Palliative Radiotherapy Program (RPRP) was established in 1999 with the goal of facilitating access by family physicians to radiotherapy services for patients with advanced symptomatic cancer. Two years later, an audit revealed that of the 148 patients treated by the program, only 19 had been referred by family physicians.We therefore assessed awareness of the RPRP and perceptions of the effectiveness of palliative radiotherapy on the part of family physicians by surveying a random sample of family physicians in Eastern Ontario.Response rate was 50%. Only 18% of family physicians were aware of the RPRP, although 56% had previously referred patients for palliative radiotherapy. Among responders, 80% regularly provided palliative care, and these physicians were much more likely to be aware of and to refer patients for palliative radiotherapy.Our survey confirms the key role that family physicians play in providing care to patients with advanced cancer. However, significant deficits in family physician awareness of palliative radiotherapy programs and in knowledge of the effectiveness of palliative radiotherapy should be addressed to improve patient care.

Initial ocular following in humans: a response to first-order motion energy.

Visual motion is sensed by low-level (energy-based) and high-level (feature-based) mechanisms. Ocular following responses (OFR) were elicited in humans by applying horizontal motion to vertical square-wave gratings lacking the fundamental ("missing fundamental stimulus"). Motion consisted of successive 1/4-wavelength steps, so the features and 4n+1 harmonics (where n=integer) shifted forwards, whereas the 4n-1 harmonics--including the strongest Fourier component (the 3rd harmonic)--shifted backwards (spatial aliasing). Initial OFR, recorded with the electromagnetic search coil technique, were always in the direction of the 3rd harmonic, e.g., leftward steps resulted in rightward OFR. Thus, the earliest OFR were strongly dependent on the motion of the major Fourier component, consistent with early spatio-temporal filtering prior to motion detection, as in the well-known energy model of motion analysis.

Short-latency disparity vergence in humans: evidence for early spatial filtering.

Our study was concerned with the disparity detectors underlying the initial disparity vergence responses (DVRs) that are elicited at ultrashort latencies by binocular disparities applied to large images. DVRs were elicited in humans by applying horizontal disparity to vertical square-wave gratings lacking the fundamental (termed here, the "missing fundamental"). In the frequency domain, a pure square wave is composed of odd harmonics--first, third, fifth, seventh, etc.--such that the third, fifth, seventh, etc., have amplitudes that are one-third, one-fifth, one-seventh, etc., that of the first, and the missing fundamental lacks the first harmonic. The patterns seen by the two eyes have a phase difference of one-quarter wavelength, so the disparity of the features and 4n + 1 harmonics (where n = integer) has one sign (crossed or uncrossed), whereas the 4n - 1 harmonics--including the strongest Fourier component (the third harmonic)--has the opposite sign (uncrossed or crossed): spatial aliasing. The earliest DVRs, recorded with the search-coil technique, had minimum latencies of 70 to 80 ms and were generally in the direction of the third harmonic, that is, uncrossed disparities resulted in convergent eye movements. In other experiments on the DVRs, one eye saw a missing fundamental and the other saw a pure sine wave with the contrast and wavelength of the third harmonic but differing in phase by one-quarter wavelength. This resulted in short-latency vergence in accordance with matching of the third harmonic. These data all indicate the importance of the Fourier components, consistent with early spatial filtering prior to binocular matching.

Initial ocular following in humans depends critically on the fourier components of the motion stimulus.

Visual motion is sensed by low-level (energy-based) and high-level (feature-based) mechanisms. Our interest is in the motion detectors underlying the initial ocular following responses (OFR) that are elicited at ultrashort latencies by sudden motions of large images. OFR were elicited in humans by applying horizontal motion to vertical square-wave gratings lacking the fundamental. In the frequency domain, a pure square wave is composed of the odd harmonics--first, third, fifth, seventh, etc.--such that the third, fifth, seventh, etc., have amplitudes that are one-third, one-fifth, one-seventh, etc., that of the first, and the missing fundamental stimulus lacks the first harmonic. Motion consisted of successive quarter-wavelength steps, so the features and 4n+1 harmonics (where n = integer) shifted forward, whereas the 4n-1 harmonics--including the strongest Fourier component (the third harmonic)--shifted backward (spatial aliasing). Thus, the net Fourier energy and the non-Fourier features moved in opposite directions. Initial OFR, recorded with the search coil technique, had minimum latencies of 60 to 70 ms and were always in the direction of the third harmonic, for example, leftward steps resulted in rightward OFR. Thus, the earliest OFR were strongly dependent on the motion of the major Fourier component, consistent with mediation by oriented spatiotemporal visual filters as in the well-known energy model of motion detection. Introducing interstimulus intervals of 10 to 100 ms (during which the screen was uniform gray) reversed the initial direction of tracking, consistent with extensive neurophysiological and psychophysical data suggesting that the visual input to the motion detectors has a biphasic temporal impulse response.

Short-latency disparity-vergence eye movements in humans: sensitivity to simulated orthogonal tropias.

Small disparity stimuli applied to large random-dot patterns elicit machine-like vergence eye movements at short latency. We have examined the sensitivity of these eye movements to simulated orthogonal tropias in three normal subjects by recording (1) the effects of vertical disparities on the initial horizontal vergence responses elicited by 2 degrees crossed and uncrossed (horizontal) disparity stimuli, and (2) the effects of horizontal disparities on the initial vertical vergence responses elicited by 1.2 degrees left-hyper and 0.8 degrees right-hyper (vertical) disparity stimuli. Initial vergence responses were strongest when the orthogonal disparity was close to zero, and decreased to zero as the orthogonal disparity increased to 3 degrees -5 degrees, i.e., there was only a limited tolerance for orthogonal disparity. Tuning curves describing the dependence of the initial change in the vergence angle on the orthogonal disparity were well fit by a Gaussian function. An additional subject, who had an esotropia of approximately 10 degrees in our experimental setup, showed almost no horizontal vergence responses but did show vertical vergence responses to vertical disparity stimuli at short latency (albeit slightly longer than normal) despite the fact that her esotropia resulted in uncrossed disparities that would have totally disabled the vertical vergence mechanism of a normal subject, cf., anomalous retinal correspondence.

Onset of oscillopsia after visual maturation in patients with congenital nystagmus.

PURPOSE: To describe the clinical and oculographic characteristics of a cohort of five patients with congenital nystagmus (CN) and late-onset oscillopsia caused by a coincidental decline in other visual and/or ocular motor functions. DESIGN: Retrospective, observational, case series. PARTICIPANTS: Five visually mature patients with CN and recent-onset oscillopsia were evaluated clinically and with motility recordings. INTERVENTION: Eye movement analysis was performed off-line by computer analysis of digitized data. Nystagmus was analyzed for null-zone characteristics, waveforms, frequency, amplitudes, and slow-phase drift velocity during foveation. Surgical and medical treatment of associated ocular conditions in four of five patients. MAIN OUTCOME MEASURES: Presence of symptomatic oscillopsia and average time during foveation periods of slow-phase drift velocity less than 10 degrees /second. RESULTS: One of the five patients had associated rod-cone dystrophy, and another had recurrence of childhood head posturing with return of an eccentric null zone. The remaining three patients had decompensated strabismus associated with their oscillopsia. All five patients complained of oscillopsia in primary position that was relieved in the four who received treatment. Treatment included prismatic correction in one patient and surgery in three. Recordings in primary position after treatment showed increased duration during foveation periods of slow-phase drift velocity less than 10 degrees /second and an overall decreased intensity (amplitude/frequency) of the nystagmus. CONCLUSIONS: Symptomatic oscillopsia in patients with CN is unusual. This visually disturbing symptom can be precipitated by new or changing associated visual sensory conditions (e.g., decompensating strabismus, retinal degeneration). If the associated conditions can be treated, then accompanying oscillopsia may be relieved.