The temporal properties of first- and second-order vision

Temporal mechanisms in frontoparallel stereomotion revealed by individual differences analysis

Masking experiments, using vertical and horizontal sinusoidal depth corrugations, have suggested the existence of more than two spatial-frequency disparity mechanisms. This result was confirmed through an individual differences approach. Here, using factor analytic techniques, we want to investigate the existence of independent temporal mechanisms in frontoparallel stereoscopic (cyclopean) motion. To construct stereomotion, we used sinusoidal depth corrugations obtained with dynamic random-dot stereograms. Thus, no luminance motion was present monocularly. We measured disparity thresholds for drifting vertical (up-down) and horizontal (left-right) sinusoidal corrugations of 0.4 cyc/deg at 0.25, 0.5, 1, 2, 4, 6, and 8 Hz. In total, we tested 34 participants. Results showed a small orientation anisotropy with lower thresholds for horizontal corrugations. Disparity thresholds as a function of temporal frequency were almost constant from 0.25 up to 1 Hz, and then they increased monotonically. Principal component analysis uncovered two significant factors for vertical and two for horizontal corrugations. Varimax rotation showed that one factor loaded from 0.25 to 1-2 Hz and a second factor from 2 to 4 to 8 Hz. Direct Oblimin rotation indicated a moderate intercorrelation of both factors. Our results suggest the possible existence of two somewhat interdependent temporal mechanisms involved in frontoparallel stereomotion.

Selectivity for local orientation information in visual mirror symmetry perception

Mirror symmetry is a global percept formed from specific arrangements of matching local information. It has been shown that some features of this local information can interact with the global percept, interfering with symmetry perception. One such feature is orientation; it is well established that the orientation of the symmetry axis has a significant impact on symmetry perception, but the role of local orientation of individual elements is still equivocal. Some studies have argued for no role of local orientation in symmetry perception, while other studies have shown a detrimental effect of certain local orientation combinations. Using dynamic stimuli composed of oriented Gabor elements with increasing temporal delay (SOA) between the onset of the first and second element in a symmetric pair, we systematically map how orientation variation within and between symmetric pairs affected the temporal integration of symmetric patterns in five observers. This method allows consideration of both sensitivity to symmetry (threshold, or T₀) as well as the duration of visible persistence of each condition through the visual system (P). Our results show a clear role for local orientation in symmetry perception and highlight the importance of local orientation in symmetry perception. Our findings reinforce the need for more nuanced perceptual models incorporating local element orientation, which is currently unaccounted for.

The temporal integration windows for visual mirror symmetry

Symmetry perception in dot patterns is tolerant to temporal delays of up to 60 ms within and between element pairs. However, it is not known how factors effecting symmetry discrimination in static patterns might affect temporal integration in dynamic patterns. One such feature is luminance polarity. Using dynamic stimuli with increasing temporal delay (SOA) between the onset of the first and second element in a symmetric pair, we investigated how four different luminance-polarity conditions affected the temporal integration of symmetric patterns. All four luminance polarity conditions showed similar upper temporal limits of approximately 60 ms. However psychophysical performance over all delay durations showed significantly higher symmetry thresholds for unmatched-polarity patterns at short delays, but also significantly less sensitivity to increasing temporal delay relative to matched-polarity patterns. These varying temporal windows are consistent with the involvement of a fast, sensitive first-order mechanism for matched-polarity patterns, and a slower, more robust second-order mechanism for unmatched-polarity patterns. Temporal integration windows for unmatched-polarity patterns were not consistent with performance expected from attentional mechanisms alone, and instead supports the involvement of second-order mechanisms that combines information from ON and OFF channels.

Contrast-modulated stimuli produce more superimposition and predominate perception when competing with comparable luminance-modulated stimuli during interocular grouping

Interocular grouping (IOG) is a binocular visual function that can arise during multi-stable perception. IOG perception was initiated using split-grating stimuli constructed from luminance (L), luminance-modulated noise (LM) and contrast-modulated noise (CM). In Experiment 1, three different visibility levels were used for L and LM (or first-order) stimuli, and compared to fixed-visibility CM (or second-order) stimuli. Eight binocularly normal participants indicated whether they perceived full horizontal or vertical gratings, superimposition, or other (piecemeal and eye-of-origin) percepts. CM stimuli rarely generated full IOG, but predominantly generated superimposition. In Experiment 2, Levelt's modified laws were tested for IOG in nine participants. Split-gratings presented to each eye contained different visibility LM gratings, or LM and CM gratings. The results for the LM-vs-LM conditions mostly followed the predictions of Levelt's modified laws, whereas the results for the LM-vs-CM conditions did not. Counterintuitively, when high-visibility LM and low-visibility CM split-gratings were used, high-visibility LM components did not predominate IOG perception. Our findings suggest that higher proportions of superimposition during CM-vs-CM viewing are due to binocular combination, rather than mutual inhibition. It implies that IOG percepts are more likely to be mediated at an earlier monocular, rather than a binocular stage. Our previously proposed conceptual framework for conventional binocular rivalry, which includes asymmetric feedback, visual saliency, or a combination of both (Skerswetat et al. Sci Rep 8:14432, 2018), might also account for IOG. We speculate that opponency neurons might mediate coherent percepts when dissimilar information separately enters the eyes.

Levelt's laws do not predict perception when luminance- and contrast-modulated stimuli compete during binocular rivalry

Incompatible patterns viewed by each of the two eyes can provoke binocular rivalry, a competition of perception. Levelt's first law predicts that a highly visible stimulus will predominate over a less visible stimulus during binocular rivalry. In a behavioural study, we made a counterintuitive observation: high visibility patterns do not always predominate over low visibility patterns. Our results show that none of Levelt's binocular rivalry laws hold when luminance-modulated (LM) patterns compete with contrast-modulated (CM) patterns. We discuss visual saliency, asymmetric feedback, and a combination of both as potential mechanisms to explain the CM versus LM findings. Competing orthogonal LM stimuli do follow Levelt's laws, whereas only the first two laws hold for competing CM stimuli. The current results provide strong psychophysical evidence for the existence of separate processing stages for LM and CM stimuli.

Apparent shift in long-range motion trajectory by local pattern orientation

The present study shows that the apparent direction of a moving pattern is systematically affected by its orientation. We found that the perceived direction of motion of a single Gabor grating changing position in discrete steps interleaved by blank inter-stimulus interval (ISI) is biased toward the orientation of the grating. This orientation-induced motion shift peaks for grating orientations ~±15 deg away from the physical motion trajectory and was profound for relatively short distances. Orientation adaptation revealed that the directional shift is determined by the apparent –not the physical –orientation of the grating, and a subsequent experiment demonstrated that directional shift is also influenced by the orientation of the contrast-defined stimulus envelope. Results provide further evidence that the apparent trajectory of a motion stimulus is determined by interactions between motion and pattern information at relatively high levels of visual processing.

More superimposition for contrast-modulated than luminance-modulated stimuli during binocular rivalry

Luminance-modulated noise (LM) and contrast-modulated noise (CM) gratings were presented with interocularly correlated, uncorrelated and anti-correlated binary noise to investigate their contributions to mixed percepts, specifically piecemeal and superimposition, during binocular rivalry. Stimuli were sine-wave gratings of 2 c/deg presented within 2 deg circular apertures. The LM stimulus contrast was 0.1 and the CM stimulus modulation depth was 1.0, equating to approximately 5 and 7 times detection threshold, respectively. Twelve 45 s trials, per noise configuration, were carried out. Fifteen participants with normal vision indicated via button presses whether an exclusive, piecemeal or superimposed percept was seen. For all noise conditions LM stimuli generated more exclusive visibility, and lower proportions of superimposition. CM stimuli led to greater proportions and longer periods of superimposition. For both stimulus types, correlated interocular noise generated more superimposition than did anti- or uncorrelated interocular noise. No significant effect of stimulus type (LM vs CM) or noise configuration (correlated, uncorrelated, anti-correlated) on piecemeal perception was found. Exclusive visibility was greater in proportion, and perceptual changes more numerous, during binocular rivalry for CM stimuli when interocular noise was not correlated. This suggests that mutual inhibition, initiated by non-correlated noise CM gratings, occurs between neurons processing luminance noise (first-order component), as well as those processing gratings (second-order component). Therefore, first- and second-order components can contribute to overall binocular rivalry responses. We suggest the addition of a new well to the current energy landscape model for binocular rivalry that takes superimposition into account.

Binocular functional architecture for detection of contrast-modulated gratings

Combination of signals from the two eyes is the gateway to stereo vision. To gain insight into binocular signal processing, we studied binocular summation for luminance-modulated gratings (L or LM) and contrast-modulated gratings (CM). We measured 2AFC detection thresholds for a signal grating (0.75c/deg, 216ms) shown to one eye, both eyes, or both eyes out-of-phase. For LM and CM, the carrier noise was in both eyes, even when the signal was monocular. Mean binocular thresholds for luminance gratings (L) were 5.4dB better than monocular thresholds - close to perfect linear summation (6dB). For LM and CM the binocular advantage was again 5-6dB, even when the carrier noise was uncorrelated, anti-correlated, or at orthogonal orientations in the two eyes. Binocular combination for CM probably arises from summation of envelope responses, and not from summation of these conflicting carrier patterns. Antiphase signals produced no binocular advantage, but thresholds were about 1-3dB higher than monocular ones. This is not consistent with simple linear summation, which should give complete cancellation and unmeasurably high thresholds. We propose a three-channel model in which noisy monocular responses to the envelope are binocularly combined in a contrast-weighted sum, but also remain separately available to perception via a max operator. Vision selects the largest of the three responses. With in-phase gratings the binocular channel dominates, but antiphase gratings cancel in the binocular channel and the monocular channels mediate detection. The small antiphase disadvantage might be explained by a subtle influence of background responses on binocular and monocular detection.

Interactions between Orientation and Contrast Modulations Suggest Limited Cross-Cue Linkage

Recent studies of texture segmentation and second-order vision have proposed very similar models for the detection of orientation modulation and contrast modulation (OM and CM). From the similarity of the models it is tempting to assume that the two cues might be processed by a single generalised texture mechanism; however, recent results (Kingdom et al, 2003 Visual Neuroscience2 65–76) have suggested that these cues are detected independently, or at least in a mechanism that is able to maintain an apparent independence between the cues. We tested new combinations of OM and CM and found that CM at 0.4 cycle deg−1 facilitates the detection of OM at 0.2 cycle deg−1 when the peaks of contrast align with the extremes of orientation. There is also some evidence of weak facilitation of CM by OM under the same conditions. Further, this facilitation can be predicted by filter-rectify-filter channels optimised for the detection of each cue, adding weight to the argument that texture cues are processed in a single generalised mechanism that nonetheless achieves cue independence or near-independence in many circumstances. We also found that the amount of suprathreshold masking produced by an orientation cue depends on the overall percept formed by that cue.

First- and Second-Order Stimuli Reaction Time Measures Are Highly Sensitive to Mild Traumatic Brain Injuries

Mild traumatic brain injury (mTBI) has subtle effects on several brain functions that can be difficult to assess and follow up. We investigated the impact of mTBI on the perception of sine-wave gratings defined by first- and second-order characteristics. Fifteen adults diagnosed with mTBI were assessed at 15 days, 3 months, and 12 months postinjury. Fifteen matched controls followed the same testing schedule. Reaction times (RTs) for flicker detection and motion direction discrimination were measured. Stimulus contrast of first- and second-order patterns was equated to control for visibility, and correct-response RT means, standard deviations (SDs), medians, and interquartile ranges (IQRs) were calculated. The level of symptoms was also evaluated to compare it to RT data. In general in mTBI, RTs were longer, and SDs as well as IQRs larger, than those of controls. In addition, mTBI participants' RTs to first-order stimuli were shorter than those to second-order stimuli, and SDs as well as IQRs larger for first- than for second-order stimuli in the motion condition. All these observations were made over the three sessions. The level of symptoms observed in mTBI was higher than that of control participants, and this difference did also persist up to 1 year after the brain injury, despite an improvement. The combination of RT measures with particular stimulus properties is a highly sensitive method for measuring mTBI-induced visuomotor anomalies and provides a fine probe of the underlying mechanisms when the brain is exposed to mild trauma.

Double dissociation between first- and second-order processing

To study the difference of sensitivity to luminance- (LM) and contrast-modulated (CM) stimuli, we compared LM and CM detection thresholds in LM- and CM-noise conditions. The results showed a double dissociation (no or little inter-attribute interaction) between the processing of these stimuli, which implies that both stimuli must be processed, at least at some point, by separate mechanisms and that both stimuli are not merged after a rectification process. A second experiment showed that the internal equivalent noise limiting the CM sensitivity was greater than the one limiting the carrier sensitivity, which suggests that the internal noise occurring before the rectification process is not limiting the CM sensitivity. These results support the hypothesis that a suboptimal rectification process partially explains the difference of LM and CM sensitivity.

Oscillatory motion induces change blindness

Change blindness is the relative inability of normally sighted observers to detect large changes in scenes when the low-level signals associated with those changes are either masked or of extremely low magnitude. Change detection can be inhibited by saccadic eye movements, artificial saccades or blinks, and 'mud splashes'. We now show that change detection is also inhibited by whole image motion in the form of sinusoidal oscillations. The degree of disruption depends upon the frequency of oscillation, which at 3 Hz is equivalent to that produced by artificial blinks. Image motion causes the retinal image to be blurred and this is known to affect object recognition. However, our results are inconsistent with good change detection followed by a delay due to poor recognition of the changing object. Oscillatory motion can induce eye movements that potentially mask or inhibit the low-level signals related to changes in the scene, but we show that eye movements promote rather than inhibit change detection when the image is moving.

Why is second-order vision less efficient than first-order vision?

Research has shown that the sensitivity to second-order modulations of carrier contrast is lower than that to first-order luminance modulations stimuli. We sought to compare the efficiency of processing first- and second-order information. Employing a phase-discrimination paradigm we found that when humans were given sufficient a priori information of signal parameters they detected both luminance and contrast modulations of 0.6 and 2c/deg by a phase-sensitive algorithm. The overall detection efficiency for second-order patterns, however, was lower that that for first-order stimuli. To study the factors which limit the efficiency of first- and second-order vision, we measured detection performance for luminance and contrast modulations of 0.6 and 2c/deg embedded in Gaussian noise. The results showed that the detection of second-order patterns had lower sampling efficiency and higher additive internal noise as compared to the detection of first-order stimuli. Classification images for detecting contrast modulations of 2c/deg resembled the side-band component of the contrast modulations which suggests that human observers may detect contrast modulations of a sinusoidal carrier using first-order luminance channels. The lower sensitivity of the mechanism detecting second-order patterns might be due to higher levels of additive internal noise and lower sampling efficiency than those of the mechanism analysing first-order patterns.

Bistable Glass-pattern motion reveals two different processes

Examining the two motion processes is an elusive task due to the difficulty of finding a proper stimulus paradigm. A rotational Glass pattern created with a random-dot array by superimposing its rotated version on the top of it can provide such a paradigm. If we displace only its rotated part in the vertical or horizontal direction, a bistable motion occurs; local dot motion in the same direction and Glass-pattern motion in the orthogonal direction. From two experiments, we found local dot motion is predominant in short spatiotemporal range and global pattern motion in long spatiotemporal range. Since the stimulus allows us to maintain all of its properties identical except for the changes in spatiotemporal parameters, this result shows more robustly that the energy-based first-order motion favors short spatiotemporal ranges while the pattern-based second-order motion favors long spatiotemporal ranges.

The influence of spatial and temporal noise on the detection of first-order and second-order orientation and motion direction

Thresholds for identifying the direction of second-order motion (contrast-modulated dynamic noise) are consistently higher than those for identifying spatial orientation, unlike first-order gratings for which the two thresholds are typically the same. Two explanations of this phenomenon have been proposed: either first-order and second-order patterns are encoded by separate mechanisms with different properties, or dynamic noise selectively impairs ("masks") sensitivity to second-order motion direction but not orientation. The former predicts the two thresholds should remain distinct for second-order patterns, irrespective of the temporal structure (static vs. dynamic) of the noise carrier. The latter predicts direction thresholds should be higher than orientation thresholds, for both second-order and first-order motion patterns, when dynamic (but not static) noise is present. To resolve this issue we measured direction and orientation thresholds for first-order (luminance) and second-order (contrast or polarity) modulations of static or dynamic noise. Results were decisive: The two thresholds were invariably the same for first-order stimuli but markedly different (direction thresholds approximately 50% higher) for second-order stimuli, regardless of the temporal properties (static or dynamic) and the overall contrast of the noise, or the drift temporal frequency of the envelope. This suggests that first-order and second-order motion are encoded separately and that the mechanisms encoding second-order stimuli cannot determine direction at the absolute threshold for spatial form.

The effects of amplitude-spectrum statistics on foveal and peripheral discrimination of changes in natural images, and a multi-resolution model

Psychophysical thresholds were measured for discriminating small changes in spatial features of naturalistic scenes (morph sequences), for foveal and peripheral vision, and under M-scaling. Sensitivity was greatest for scenes with near natural Fourier amplitude slope, perhaps implying that human vision is optimised for natural scene statistics. A low-level model calculated differences in local contrast between pairs of images within a few spatial frequency channels with bandwidth like neurons in V1. The model was "customised" to each observer's contrast sensitivity function for sinusoidal gratings, and it could replicate the "U-shaped" relationships between discrimination threshold and spectral slope, and many differences between picture sets and observers. A single-channel model and an ideal-observer analysis both failed to capture the U-shape.

Sensitivity to spatial and temporal modulations of first-order and second-order motion

This study characterises the spatiotemporal "window of visibility" for first-order motion (luminance-modulated noise) and three varieties of second-order motion (contrast-modulated, polarity-modulated and spatial length-modulated noise). Direction-identification thresholds (minimum modulation depth producing 79.4% correct) were measured for each motion pattern (acuity permitting) over a five octave range of spatial and temporal frequencies (0.5-16 c/deg and 0.5-16 Hz respectively). Thresholds were converted into modulation sensitivity (1/threshold). For first-order motion patterns, sensitivity functions were generally bandpass. However, for second-order motion patterns, functions were predominantly lowpass in nature. In particular, the functions corresponding to contrast-modulated and polarity-modulated noise were virtually identical in terms of shape and sensitivity. However, sensitivity to modulations of spatial length was extremely poor and more lowpass, suggesting that additional strategies, perhaps a feature-based system, may be required for encoding motion of images of this type.

Human cortical responses to contrast modulations of visual noise

We studied visual evoked potentials (VEPs) elicited by second-order contrast modulations of binary dynamic noise and first-order luminance modulations. Using a 3-point Laplacian operator centred on Oz, we found that contrast modulations of both low and higher spatial frequencies elicited a negative component whose latency was about 200 ms. The latency of this component was significantly longer than that of the early Laplacian components to first-order luminance modulations. These findings could be due to slower first-stage linear filters and additional processing stages of the second-order pathway. The topographical analysis of scalp recorded VEPs to central and half-field stimulation has suggested that the responses to second-order patterns are likely to be generated by neuronal structures within the primary visual cortex which may have inputs from extrastriate neurons via feedback connections.

Poor encoding of position by contrast-defined motion

Second-order (contrast-defined) motion stimuli lead to poor performance on a number of tasks, including discriminating form from motion and visual search. To investigate this deficiency, we tested the ability of human observers to monitor multiple regions for motion, to code the relative positions of shapes defined by motion, and to simultaneously encode motion direction and location. Performance with shapes from contrast-defined motion was compared with that obtained from luminance-defined (first-order) stimuli. When the position of coherent motion was uncertain, direction-discrimination thresholds were elevated similarly for both luminance-defined and contrast-defined motion, compared to when the stimulus location was known. The motion of both luminance- and contrast-defined structure can be monitored in multiple visual field locations. Only under conditions that greatly advantaged contrast-defined motion, were observers able to discriminate the positional offset of shapes defined by either type of motion. When shapes from contrast-defined and luminance-defined motion were presented under comparable conditions, the positional accuracy of contrast-defined motion was found to be poorer than its luminance-defined counterpart. These results may explain some, but possibly not all, of the deficits found previously with second-order motion.

Contrast conservation in human vision

Visual experience, which is defined by brief saccadic sampling of complex scenes at high contrast, has typically been studied with static gratings at threshold contrast. To investigate how suprathreshold visual processing is related to threshold vision, we tested the temporal integration of contrast in the presence of large, sudden changes in the stimuli such occur during saccades under natural conditions. We observed completely different effects under threshold and suprathreshold viewing conditions. The threshold contrast of successively presented gratings that were either perpendicularly oriented or of inverted phase showed probability summation, implying no detectable interaction between independent visual detectors. However, at suprathreshold levels we found complete algebraic summation of contrast for stimuli longer than 53 ms. The same results were obtained during sudden changes between random noise patterns and between natural scenes. These results cannot be explained by traditional contrast gain-control mechanisms or the effect of contrast constancy. Rather, at suprathreshold levels, the visual system seems to conserve the contrast information from recently viewed images, perhaps for the efficient assessment of the contrast of the visual scene while the eye saccades from place to place.

How is complex second-order motion processed?

Converging psychophysical and electrophysiological evidence suggests that first-order (luminance-defined) complex motion types i.e., radial and rotational motion, are processed by specialized extrastriate motion mechanisms. We ask whether radial and rotational second-order (texture-defined) motion patterns are processed in a similar manner. The motion sensitivity to translating, radiating and rotating motion patterns of both first-order (luminance-modulated noise) and second-order (contrast-modulated noise) were measured for patterns presented at four different exposure durations (106, 240, 500 and 750 ms). No significant difference in motion sensitivity was found across motion type for the first-order motion class across exposure duration (i.e., from 240 to 750 ms) whereas direction-identification thresholds for radiating and rotating second-order motion were significantly greater than that of the second-order translational stimuli. Furthermore, thresholds to all second-order motion stimuli increased at a significantly faster rate with decreasing exposure duration compared to those of first-order motion. Interestingly, simple and complex second-order thresholds increased at similar rates. Taken together, the results suggest that complex second-order motion is not analyzed in a sequential manner. Rather, it seems that the same 'hard-wired' mechanisms responsible for complex first-order motion processing also mediate complex second-order motion, but not before the pre-processing (i.e., rectification) of local second-order motion signals.

Temporal properties of the visual responses to luminance and contrast modulated noise

Vision is sensitive to first-order luminance modulations and second-order modulations of carrier contrast. Our knowledge of the temporal properties of second-order vision is insufficient and contradictory. Using temporal summation and reaction time paradigms, we found that the type of visual noise (static or dynamic) determines the temporal properties of the responses to luminance and contrast modulations. In the presence of static noise, the temporal responses to both types of modulation of low and higher spatial frequencies were transient. When dynamic noise was used, the temporal responses to luminance and contrast modulations of higher spatial frequencies were sustained. At low spatial frequency, however, luminance modulations elicited transient responses, while contrast modulated dynamic noise produced sustained responses. The reaction times to near-threshold contrast modulations of low spatial frequency were slower than those to first-order patterns and they did not significantly differ at modulations of higher spatial frequency. The results suggest that the temporal characteristics of first-stage linear filters which feed the second-order pathway may determine the temporal responses to contrast modulated noise.

Failure of direction identification for briefly presented second-order motion stimuli: evidence for weak direction selectivity of the mechanisms encoding motion

We sought to investigate why the direction of second-order motion, unlike first-order motion, cannot be identified when the stimulus exposure duration is brief (<200 ms). In a series of experiments observers identified both the orientation (vertical or horizontal) and the direction (left, right, down or up) of a drifting sinusoidal modulation (0.93 c/ degrees ) in either the luminance (first order) or the contrast (second order) of a two-dimensional noise carrier. All motion stimuli were equated for visibility, and the duration was varied using the method of constant stimuli. Performance was measured for second-order motion over a range of drift temporal frequencies (0.63-5.04 Hz) and for first-order motion stimuli composed of two, opposite drifting modulations in luminance of unequal modulation depth. Orientation-identification performance was nearly 100% correct for both first-order and second-order motion stimuli, even at the briefest stimulus duration tested (26.49 ms). Direction identification for first-order motion was also typically good with brief presentations, but was poor for second-order motion when the exposure duration was < approximately 200 ms. Importantly increasing either the drift temporal frequency of second-order motion or the bidirectional nature of the first-order motion patterns produced comparable levels of performance for the two varieties of motion (i.e. the minimum duration required for reliable direction identification could be equated). As orientation-identification performance for the first-order and second-order motion stimuli was comparably good and minimally affected by duration, the marked differences on the direction-identification task must be specific to mechanisms that encode drift direction, rather than spatial structure. We propose that second-order motion detectors are much less selective for stimulus direction than first-order motion sensors, and thus are more susceptible to the deleterious effects of limiting stimulus duration (which introduces spurious motion in the opposite direction, particularly at low drift rates). Alternative explanations based on the delayed propagation of second-order motion signals or the temporal characteristics of the underlying motion mechanisms are not supported by our findings.

Processing of second-order stimuli in the visual cortex

Naturally occurring visual stimuli are rich in examples of objects delineated from their backgrounds simply by differences in luminance, so-called first-order stimuli, as well as those defined by differences of contrast or texture, referred to as second-order stimuli. Here we provide a brief overview of visual cortical processing of second-order stimuli, as well as some comparative background on first-order processing, concentrating on single-unit neurophysiology, but also discussing relationships to human psychophysics and to neuroimaging. The selectivity of visual cortical neurons to orientation, spatial frequency, and direction of movement of first-order, luminance-defined stimuli is conventionally understood in terms of simple linear filter models, albeit with some minor nonlinearities such as thresholding and gain control. However, these kinds of models fail entirely to account for responses of neurons to second-order stimuli such as contrast envelopes, illusory contours, or texture borders. Second-order stimuli constructed from sinusoidal components have been used to analyze the neurophysiological mechanisms of such responses; these experiments demonstrate that the same neuron can exhibit three distinct kinds of tuning to spatial frequency, and also to orientation. These results can be understood in terms of a type of nonlinear 'filter-->rectify-->filter' model, which has been widely used in human psychophysics. Finally, several general issues will be discussed, including potential artifacts in experiments with second-order stimuli, and strategies for avoiding or controlling for them; caveats about definitions of first- vs. second-order mechanisms and stimuli; the concept of form-cue invariance; and the functional significance of second-order processing.

Three-systems theory of human visual motion perception: review and update

Lu and Sperling [Vision Res. 35, 2697 (1995)] proposed that human visual motion perception is served by three separate motion systems: a first-order system that responds to moving luminance patterns, a second-order system that responds to moving modulations of feature types-stimuli in which the expected luminance is the same everywhere but an area of higher contrast or of flicker moves, and a third-order system that computes the motion of marked locations in a "salience map," that is, a neural representation of visual space in which the locations of important visual features ("figure") are marked and "ground" is unmarked. Subsequently, there have been some strongly confirmatory reports: different gain-control mechanisms for first- and second-order motion, selective impairment of first- versus second- and/or third-order motion by different brain injuries, and the classification of new third-order motions, e.g., isoluminant chromatic motion. Various procedures have successfully discriminated between second- and third-order motion (when first-order motion is excluded): dual tasks, second-order reversed phi, motion competition, and selective adaptation. Meanwhile, eight apparent contradictions to the three-systems theory have been proposed. A review and reanalysis here of the new evidence, pro and con, resolves the challenges and yields a more clearly defined and significantly strengthened theory.