Ocular following responses of monkeys to the competing motions of two sinusoidal gratings

Short-latency ocular following responses to motion stimuli are strongly affected by temporal modulations of the visual content during the initial fixation period

Neuronal and psychophysical responses to a visual stimulus are known to depend on the preceding history of visual stimulation, but the effect of stimulation history on reflexive eye movements has received less attention. Here, we quantify these effects using short-latency ocular following responses (OFRs), a valuable tool for studying early motion processing. We recorded, in human subjects, the horizontal OFRs induced by drifting vertical 1D pink noise. The stimulus was preceded by 600 to 1000 ms of maintained fixation (on a visible cross), and we explored the effect of different stimuli (“fixation patterns”) presented during the fixation period. We found that any temporal modulation present during the fixation period reduced the magnitude of the subsequent OFRs. Even changes in the overall luminance during the fixation period induced significant suppression. The magnitude of the effect was a function of both spatial and temporal structure of the fixation pattern. Suppression that was selective for both relative orientation and relative spatial frequency accounted for a considerable fraction of total suppression. Finally, changes in stimulus temporal structure alone (i.e. “flicker” versus “transparent motion”) led to changes in the spatial frequency tuning of suppression. In the time domain, the suppression developed quickly: 100 ms of temporal modulation in the fixation pattern produced up to 80% of maximal suppression. Recovery from suppression was instead more gradual, taking up to several seconds. By presenting transparent motion during the fixation period, with opposite motion signals having different spatial frequency content, we also discovered a direction-selective component of suppression, which depended on both the frequency and the direction of the moving stimulus.

Macaque monkeys show reversed ocular following responses to two-frame-motion stimulus presented with inter-stimulus intervals

When two-frame apparent motion stimuli are presented with an appropriate inter-stimulus interval (ISI), motion is perceived in the direction opposite to the actual image shift. Herein, we measured a simple eye movement, ocular following responses (OFRs), in macaque monkeys to examine the ISI reversal effect on oculomotor. Two-frame movies with an ISI induced reversed OFRs. Without ISI, the OFRs to the two-frame movie were induced in the direction of the stimulus shift. However, with ISIs ≥10 ms, OFRs in the direction opposite to the phase shift were observed. This directional reversal persisted for ISIs up to 160 ms; for longer ISIs virtually no ocular response was observed. Furthermore, longer exposure to the initial image (Motion onset delay: MOD) reduced OFRs. We show that these dependences on ISIs/MODs can be explained by the motion energy model. Furthermore, we examined the dependence on ISI reversal using various spatial frequencies. To account for our findings, the optimal frequency of the temporal filters of the energy model must decrease between 0.5 and 1 cycles/°, suggesting that there are at least two channels with different temporal characteristics. These results are consistent with those from humans, suggesting that the temporal filters embedded in human and macaque visual systems are similar. Thus, the macaque monkey is a good animal model for the early visual processing of humans to understand the neural substrates underlying the visual motion detectors that elicit OFRs.

Short-latency ocular-following responses: Weighted nonlinear summation predicts the outcome of a competition between two sine wave gratings moving in opposite directions

We recorded horizontal ocular-following responses to pairs of superimposed vertical sine wave gratings moving in opposite directions in human subjects. This configuration elicits a nonlinear interaction: when the relative contrast of the gratings is changed, the response transitions abruptly between the responses elicited by either grating alone. We explore this interaction in pairs of gratings that differ in spatial and temporal frequency and show that all cases can be described as a weighted sum of the responses to each grating presented alone, where the weights are a nonlinear function of stimulus contrast: a nonlinear weighed summation model. The weights depended on the spatial and temporal frequency of the component grating. In many cases the dominant component was not the one that produced the strongest response when presented alone, implying that the neuronal circuits assigning weights precede the stages at which motor responses to visual motion are generated. When the stimulus area was reduced, the relationship between spatial frequency and weight shifted to higher frequencies. This finding may reflect a contribution from surround suppression. The nonlinear interaction is strongest when the two components have similar spatial frequencies, suggesting that the nonlinearity may reflect interactions within single spatial frequency channels. This framework can be extended to stimuli composed of more than two components: our model was able to predict the responses to stimuli composed of three gratings. That this relatively simple model successfully captures the ocular-following responses over a wide range of spatial/temporal frequency and contrast parameters suggests that these interactions reflect a simple mechanism.

Contribution of color signals to ocular following responses

Ocular following responses (OFRs) are elicited at ultra-short latencies (< 60 ms) by sudden movements of the visual scene. In this study, we investigated the roles of color signals in OFRs in monkeys. To make physiologically isoluminant sinusoidal color gratings, we estimated the physiologically isoluminant points using OFRs and found that the physiologically isoluminant points were nearly independent of the spatiotemporal frequency of the gratings. We recorded OFRs induced by the motion of physiologically isoluminant color gratings and found that OFRs elicited by the motion of color gratings had different spatiotemporal frequency tuning from those elicited by the motion of luminance gratings. Additionally, OFRs to isoluminant color gratings had smaller peak responses, suggesting that color signals weakly contribute to OFRs compared with luminance signals. OFRs to the motion of stimuli composed of luminance and color signals were also examined. We found that color signals largely contributed to OFRs under low luminance signals regardless of whether color signals moved in the same or opposite direction to luminance signals. These results provide evidence of the multichannel visual computations underlying motor responses. We conclude that, in everyday situations, color information contributes cooperatively with luminance information to the generation of ocular tracking behaviors.

Difference in perceptual and oculomotor responses revealed by apparent motion stimuli presented with an interstimulus interval

To understand the mechanisms underlying visual motion analyses for perceptual and oculomotor responses and their similarities/differences, we analyzed eye movement responses to two-frame animations of dual-grating 3f5f stimuli while subjects performed direction discrimination tasks. The 3f5f stimulus was composed of two sinusoids with a spatial frequency ratio of 3:5 (3f and 5f), creating a pattern with fundamental frequency f. When this stimulus was shifted by 1/4 of the wavelength, the two components shifted 1/4 of their wavelengths and had opposite directions: the 5f forward and the 3f backward. By presenting the 3f5f stimulus with various interstimulus intervals (ISIs), two visual-motion-analysis mechanisms, low-level energy-based and high-level feature-based, could be effectively distinguished. This is because response direction depends on the relative contrast between the components when the energy-based mechanism operates, but not when the feature-based mechanism works. We found that when the 3f5f stimuli were presented with shorter ISIs (<100 ms), and 3f component had higher contrast, both perceptual and ocular responses were in the direction of the pattern shift, whereas the responses were reversed when the 5f had higher contrast, suggesting operation of the energy-based mechanism. On the other hand, the ocular responses were almost negligible with longer ISIs (>100 ms), whereas perceived directions were biased toward the direction of pattern shift. These results suggest that the energy-based mechanism is dominant in oculomotor responses throughout ISIs; however, there is a transition from energy-based to feature-tracking mechanisms when we perceive visual motion.

Postmicrosaccadic enhancement of slow eye movements

Active sensation poses unique challenges to sensory systems because moving the sensor necessarily alters the input sensory stream. Sensory input quality is additionally compromised if the sensor moves rapidly, as during rapid eye movements, making the period immediately after the movement critical for recovering reliable sensation. Here, we studied this immediate postmovement interval for the case of microsaccades during fixation, which rapidly jitter the "sensor" exactly when it is being voluntarily stabilized to maintain clear vision. We characterized retinal-image slip in monkeys immediately after microsaccades by analyzing postmovement ocular drifts. We observed enhanced ocular drifts by up to ~28% relative to premicrosaccade levels, and for up to ~50 ms after movement end. Moreover, we used a technique to trigger full-field image motion contingent on real-time microsaccade detection, and we used the initial ocular following response to this motion as a proxy for changes in early visual motion processing caused by microsaccades. When the full-field image motion started during microsaccades, ocular following was strongly suppressed, consistent with detrimental retinal effects of the movements. However, when the motion started after microsaccades, there was up to ~73% increase in ocular following speed, suggesting an enhanced motion sensitivity. These results suggest that the interface between even the smallest possible saccades and "fixation" includes a period of faster than usual image slip, as well as an enhanced responsiveness to image motion, and that both of these phenomena need to be considered when interpreting the pervasive neural and perceptual modulations frequently observed around the time of microsaccades.

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.

Principal Fourier component of motion stimulus dominates the initial optokinetic response in mice

Optokinetic responses (OKRs) are reflexive eye movements elicited by a moving visual pattern, and have been recognized in a variety of species. Several brainstem and cortical structures are known to be implicated in the generation of OKRs in primates, while the OKRs of afoveate mammals have been posited to be dominated by subcortical structures. To understand the subcortical mechanism underlying OKRs, the initial OKRs to horizontal quarter-wavelength steps applied to vertical grating patterns were studied in adult C57BL/6J mice under the monocular viewing conditions. The initial OKRs to sinusoidal gratings showed directional asymmetry with temporal-to-nasal predominance, a common characteristic of afoveate mammals that uses the subcortical structures to elicit OKRs. We then examined whether the OKRs of afoveate mammals are driven by the same visual features of the moving images as those in primates. The OKRs in mice were elicited by using the missing fundamental (mf) stimuli and its variants that had been used to understand the mechanism(s) underlying the cortical control of eye movements in primates. We obtained the results indicating that the OKRs of mice are driven by the principal Fourier component of moving visual image as in primates despite the differences in neural circuitries.

Sensory versus motor loci for integration of multiple motion signals in smooth pursuit eye movements and human motion perception

We have investigated how visual motion signals are integrated for smooth pursuit eye movements by measuring the initiation of pursuit in monkeys for pairs of moving stimuli of the same or differing luminance. The initiation of pursuit for pairs of stimuli of the same luminance could be accounted for as a vector average of the responses to the two stimuli singly. When stimuli comprised two superimposed patches of moving dot textures, the brighter stimulus suppressed the inputs from the dimmer stimulus, so that the initiation of pursuit became winner-take-all when the luminance ratio of the two stimuli was 8 or greater. The dominance of the brighter stimulus could be not attributed to either the latency difference or the ratio of the eye accelerations for the bright and dim stimuli presented singly. When stimuli comprised either spot targets or two patches of dots moving across separate locations in the visual field, the brighter stimulus had a much weaker suppressive influence; the initiation of pursuit could be accounted for by nearly equal vector averaging of the responses to the two stimuli singly. The suppressive effects of the brighter stimulus also appeared in human perceptual judgments, but again only for superimposed stimuli. We conclude that one locus of the interaction of two moving visual stimuli is shared by perception and action and resides in local inhibitory connections in the visual cortex. A second locus resides deeper in sensory-motor processing and may be more closely related to action selection than to stimulus selection.

How Motion Signals Are Integrated Across Frequencies: Study on Motion Perception and Ocular Following Responses Using Multiple-Slit Stimuli

Visual motion signals, which are initially extracted in parallel at multiple spatial frequencies, are subsequently integrated into a unified motion percept. Cross-frequency integration plays a crucial role when directional information conflicts across frequencies due to such factors as occlusion. We investigated the human observers' open-loop oculomotor tracking responses (ocular following responses, or OFRs) and the perceived motion direction in an idealized situation of occlusion—multiple-slits viewing (MSV)—in which a moving pattern is visible only through an array of slits. We also tested a more challenging viewing condition, contrast-alternating MSV (CA-MSV), in which the contrast polarity of the moving pattern alternates when it passes the slits. We found that changes in the distribution of the spectral content of the slit stimuli, introduced by variations of both the interval between the slits and the frame rate of the image stream, modulated the OFR and the reported motion direction in a rather complex manner. We show that those complex modulations could be explained by the weighted sum of the motion signal (motion contrast) of each spatiotemporal frequency. The estimated distribution of frequency weights (tuning maps) indicate that the cross-frequency integration of supra-threshold motion signals gives strong weight to low spatial frequency components (<0.25 cpd) for both OFR and motion perception. However, the tuning map estimated with the MSV stimuli were significantly different from those estimated with the CA-MSV (and from those measured in a more direct manner using grating stimuli), suggesting that inter-frequency interactions (e.g., interaction producing speed-dependent tuning) was involved.

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.

The effects of prolonged viewing of motion on short-latency ocular following responses

The adaptive effects of prolonged viewing of conditioning motion on ocular following responses (OFRs) elicited by brief test motion of a random-dot pattern were studied in humans. We found that the OFRs were significantly reduced when the directions of the conditioning and test motions were the same. The effect of conditioning motion was still observed when the speeds of the conditioning and test motions did not match. The effect was larger when the conditioning duration was longer, and decayed over time with increased temporal separation between the conditioning and test periods. These results are consistent with the characteristics of motion adaptation on the initial smooth pursuit responses. We also obtained data suggesting that the persistence of the effect depends on visual stimulation in the time between the conditioning and test periods, and that the presence of a stationary visual stimulus facilitates recovery from the motion adaptation.

The initial disparity vergence elicited with single and dual grating stimuli in monkeys: evidence for disparity energy sensing and nonlinear interactions

We recorded the initial vertical vergence eye movements elicited in monkeys at short latency ( approximately 70 ms) when the two eyes see one-dimensional (1D) horizontal grating patterns that are identical except for a phase difference (disparity) of one-quarter wavelength. With gratings composed of single sine waves, responses were always compensatory, showing Gaussian dependence on log spatial frequency (on average: peak = 0.75 cycles/deg; SD = 0.74; r(2) = 0.980) and monotonic dependence on log contrast with a gradual saturation well described by the Naka-Rushton equation (on average: n = 0.89; C(50) = 4.1%; r(2) = 0.978). With gratings composed of two sine waves whose spatial frequencies were in the ratio 3:5 and whose disparities were of opposite sign (the 3f5f stimulus), responses were determined by the disparities and contrasts of the two sine-wave components rather than the disparity of the features, consistent with early spatial filtering of the monocular inputs before their binocular combination and mediation by detectors sensitive to disparity energy. In addition, responses to the 3f5f stimulus showed a nonlinear dependence on the relative contrasts of the two sine waves. Thus on average, when the contrast of one sine wave was 2.3 times greater than that of the other, the one with the lower contrast was largely ineffective as though suppressed, and responses were determined almost entirely by the sine wave of higher contrast: Winner-Take-All. These findings are very similar to those published previously on the vertical vergence responses of humans, indicating that the monkey provides a good animal model for studying these disparity vergence responses.

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.