Manipulating the Fourier spectra of stimuli comprising a two-frame kinematogram to study early visual motion-detecting mechanisms: Perception versus short latency ocular-following responses.

Two-frame kinematograms have been extensively used to study motion perception in human vision. Measurements of the direction-discrimination performance limits (Dmax) have been the primary subject of such studies, whereas surprisingly little research has asked how the variability in the spatial frequency content of individual frames affects motion processing. Here, we used two-frame one-dimensional vertical pink noise kinematograms, in which images in both frames were bandpass filtered, with the central spatial frequency of the filter manipulated independently for each image. To avoid spatial aliasing, there was no actual leftward-rightward shift of the image: instead, the phases of all Fourier components of the second image were shifted by ±» wavelength with respect to those of the first. We recorded ocular-following responses (OFRs) and perceptual direction discrimination in human subjects. OFRs were in the direction of the Fourier components' shift and showed a smooth decline in amplitude, well fit by Gaussian functions, as the difference between the central spatial frequencies of the first and second images increased. In sharp contrast, 100% correct perceptual direction-discrimination performance was observed when the difference between the central spatial frequencies of the first and second images was small, deteriorating rapidly to chance when increased further. Perceptual dependencies moved closer to the OFR ones when subjects were allowed to grade the strength of perceived motion. Response asymmetries common for perceptual judgments and the OFRs suggest that they rely on the same early visual processing mechanisms. The OFR data were quantitatively well described by a model which combined two factors: (1) an excitatory drive determined by a power law sum of stimulus Fourier components' contributions, scaled by (2) a contrast normalization mechanism. Thus, in addition to traditional studies relying on perceptual reports, the OFRs represent a valuable behavioral tool for studying early motion processing on a fine scale.

Weighted summation and contrast normalization account for short-latency disparity vergence responses to white noise stimuli in humans.

Natural images are typically broadband, whereas detectors in early visual processing are selective for narrow ranges of spatial frequency. White noise patterns are widely used in laboratory settings to investigate how responses are derived from Fourier components in the image. Here, we report disparity vergence responses (DVRs) to white noise stimuli in human subjects and compare these with responses to white noise patterns filtered with bandpass filters and notch filters and to sinusoidal gratings. Although the contribution of these short-latency eye movements to the overall vergence response to a given stimulus is generally small, they have proven to be a valuable tool for the study of the early mechanisms that process disparity stimuli in human subjects. Removing lower spatial frequency (SF) components reduced DVR amplitude, whereas removing higher SF components led to an increase in DVR amplitude. For larger disparities, the transition occurred at lower SFs. All of these effects were quantitatively well described by a model that combined two factors: (a) an excitatory drive determined by a weighted sum of stimulus Fourier components, which was scaled by (b) a contrast normalization mechanism.

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.

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.

Ocular-following responses to white noise stimuli in humans reveal a novel nonlinearity that results from temporal sampling.

White noise stimuli are frequently used to study the visual processing of broadband images in the laboratory. A common goal is to describe how responses are derived from Fourier components in the image. We investigated this issue by recording the ocular-following responses (OFRs) to white noise stimuli in human subjects. For a given speed we compared OFRs to unfiltered white noise with those to noise filtered with band-pass filters and notch filters. Removing components with low spatial frequency (SF) reduced OFR magnitudes, and the SF associated with the greatest reduction matched the SF that produced the maximal response when presented alone. This reduction declined rapidly with SF, compatible with a winner-take-all operation. Removing higher SF components increased OFR magnitudes. For higher speeds this effect became larger and propagated toward lower SFs. All of these effects were quantitatively well described by a model that combined two factors: (a) an excitatory drive that reflected the OFRs to individual Fourier components and (b) a suppression by higher SF channels where the temporal sampling of the display led to flicker. This nonlinear interaction has an important practical implication: Even with high refresh rates (150 Hz), the temporal sampling introduced by visual displays has a significant impact on visual processing. For instance, we show that this distorts speed tuning curves, shifting the peak to lower speeds. Careful attention to spectral content, in the light of this nonlinearity, is necessary to minimize the resulting artifact when using white noise patterns undergoing apparent motion.

Temporal evolution of pattern disparity processing in humans.

Stereo matching, i.e., the matching by the visual system of corresponding parts of the images seen by the two eyes, is inherently a 2D problem. To gain insights into how this operation is carried out by the visual system, we measured, in human subjects, the reflexive vergence eye movements elicited by the sudden presentation of stereo plaids. We found compelling evidence that the 2D pattern disparity is computed by combining disparities first extracted within orientation selective channels. This neural computation takes 10-15 ms, and is carried out even when subjects perceive not a single plaid but rather two gratings in different depth planes (transparency). However, we found that 1D disparities are not always effectively combined: when spatial frequency and contrast of the gratings are sufficiently different pattern disparity is not computed, a result that cannot be simply attributed to the transparency of such stimuli. Based on our results, we propose that a narrow-band implementation of the IOC (Intersection of Constraints) rule (Fennema and Thompson, 1979; Adelson and Movshon, 1982), preceded by cross-orientation suppression, underlies the extraction of pattern disparity.

Ocular following in humans: spatial properties.

Ocular following responses (OFRs) are tracking eye movements elicited at ultrashort latency by the sudden movement of a textured pattern. Here we report the results of our study of their dependency on the spatial arrangement of the motion stimulus. Unlike previous studies that looked at the effect of stimulus size, we investigated the impact of stimulus location and how two distinct stimuli, presented together, collectively determine the OFR. We used as stimuli vertical gratings that moved in the horizontal direction and that were confined to either one or two 0.58° high strips, spanning the width of the screen. We found that the response to individual strips varied as a function of the location and spatial frequency (SF) of the stimulus. The response decreased as the stimulus eccentricity increased, but this relationship was more accentuated at high than at low spatial frequencies. We also found that when pairs of stimuli were presented, nearby stimuli interacted strongly, so that the response to the pair was barely larger than the response to a single strip in the pair. This suppressive effect faded away as the separation between the strips increased. The variation of the suppressive interaction with strip separation, paired with the dependency on eccentricity of the responses to single strips, caused the peak response for strip pairs to be achieved at a specific separation, which varied as a function of SF.

Evidence from vergence eye movements that disparities defined by luminance and contrast are sensed by independent mechanisms.

We recorded the initial disparity vergence responses (DVRs) elicited by 1-D sinusoidal gratings differing in phase at the two eyes by 1/4 wavelength and defined by luminance modulation (LM) or contrast modulation (CM) of dynamic binary noise. Both LM and CM stimuli elicited DVRs, but those to CM had longer latency (on average by ∼20 ms). DVRs showed sigmoidal dependence on depth of modulation, with higher thresholds for CM than for LM. With both LM and CM stimuli, fixing the modulation at one eye well above threshold rendered the DVR hypersensitive to low-level modulation at the other eye (dichoptic facilitation). Disparities defined by LM at one eye and CM at the other generated weak DVRs in the "wrong" direction, consistent with mediation entirely by distortion products associated with the CM stimulus. These (reversed) DVRs could be nulled by adding LM to the CM stimulus (in phase), and the greater the depth of the CM, the greater the added LM required for nulling, exactly as predicted by a simple compressive non-linearity. We conclude that disparities defined by LM and by CM are sensed by independent cortical mechanisms, at least for the purposes of generating short-latency vergence eye movements to disparity steps.

Contrast sensitivity, first-order motion and Initial ocular following in demyelinating optic neuropathy.

The ocular following response (OFR) is a measure of motion vision elicited at ultra-short latencies by sudden movement of a large visual stimulus. We compared the OFR to vertical sinusoidal gratings (spatial frequency 0.153 cycles/ degrees or 0.458 cycles/ degrees) of each eye in a subject with evidence of left optic nerve demyelination due to multiple sclerosis (MS). The subject showed substantial differences in vision measured with stationary low-contrast Sloan letters (20/63 OD and 20/200 OS at 2.5% contrast) and the Lanthony Desaturated 15-hue color test (Color Confusion Index 1.11 OD and 2.14 OS). Compared with controls, all of the subject's OFR to increasing contrast showed a higher threshold. The OFR of each of the subject's eyes were similar for the 0.153 cycles/ degrees stimulus, and psychophysical measurements of his ability to detect these moving gratings were also similar for each eye. However, with the 0.458 cycles/ degrees stimulus, the subject's OFR was asymmetric and the affected eye showed decreased responses (smaller slope constant as estimated by the Naka-Rushton equation). These results suggest that, in this case, optic neuritis caused a selective deficit that affected parvocellular pathways mediating higher spatial frequencies, lower-contrast, and color vision, but spared the field-holding mechanism underlying the OFR to lower spatial frequencies. The OFR may provide a useful method to study motion vision in individuals with disorders affecting anterior visual pathways.

Perception can influence the vergence responses associated with open-loop gaze shifts in 3D.

We sought to determine if perceived depth can elicit vergence eye movements independent of binocular disparity. A flat surface in the frontal plane appears slanted about a vertical axis when the image in one eye is vertically compressed relative to the image in the other eye: the induced size effect (Ogle, 1938). We show that vergence eye movements accompany horizontal gaze shifts across such surfaces, consistent with the direction of the perceived slant, despite the absence of a horizontal disparity gradient. All images were extinguished during the gaze shifts so that eye movements were executed open-loop. We also used vertical compression of one eye's image to null the perceived slant resulting from prior horizontal compression of that image, and show that this reduces the vergence accompanying horizontal gaze shifts across the surface, even though the horizontal disparity is unchanged. When this last experiment was repeated using vertical expansions in place of the vertical compressions, the perceived slant was increased and so too was the vergence accompanying horizontal gaze shifts, although the horizontal disparity again remained unchanged. We estimate that the perceived depth accounted, on average, for 15-41% of the vergence in our experiments depending on the conditions.