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

AbstractRadial 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 ¼–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

Keep looking ahead? Re-direction of visual fixation does not always occur during an unpredictable obstacle avoidance task

Visual information about the environment, especially fixation of key objects such as obstacles, is critical for safe locomotion. However, in unpredictable situations where an obstacle suddenly appears it is not known whether central vision of the obstacle and/or landing area is required or if peripheral vision is sufficient. We examined whether there is a re-direction of visual fixation from an object fixated ahead to a suddenly appearing obstacle during treadmill walking. Furthermore, we investigated the temporal relationship between the onset of muscle activity to avoid the obstacle and saccadic eye and head movements to shift fixation. Eight females (mean SD; age = 24.8 2.3 years) participated in this experiment. There were two visual conditions: a central vision condition where participants fixated on two obstacles attached to a bridge on the treadmill and a peripheral vision condition where participants fixated an object two steps ahead. There were two obstacle release conditions: only an obstacle in front of the left foot was released or an obstacle in front of either foot could be released. Only trials when the obstacle was released in front of the left foot were analyzed such that the difference in the two obstacle conditions was whether there was a choice of which foot to step over the obstacle. Obstacles were released randomly in one of three phases during the step cycle corresponding to available response times between 219 and 462 ms. We monitored eye and head movements along with muscle activity and spatial foot parameters. Performance on the task was not different between vision conditions. The results indicated that saccades are rarely made (< 18% of trials) and, when present, are initiated ∼ 350 ms after muscle activity for limb elevation, often accompanied by a downward head movement, and always directed to the landing area. Therefore, peripheral vision of a suddenly appearing obstacle in the travel path is sufficient for successful obstacle avoidance during locomotion: visual fixation is generally not re-directed to either the obstacle or landing area

Perception, Action, and Roelofs Effect: A Mere Illusion of Dissociation

A prominent and influential hypothesis of vision suggests the existence of two separate visual systems within the brain, one creating our perception of the world and another guiding our actions within it. The induced Roelofs effect has been described as providing strong evidence for this perception/action dissociation: When a small visual target is surrounded by a large frame positioned so that the frame's center is offset from the observer's midline, the perceived location of the target is shifted in the direction opposite the frame's offset. In spite of this perceptual mislocalization, however, the observer can accurately guide movements to the target location. Thus, perception is prone to the illusion while actions seem immune. Here we demonstrate that the Roelofs illusion is caused by a frame-induced transient distortion of the observer's apparent midline. We further demonstrate that actions guided to targets within this same distorted egocentric reference frame are fully expected to be accurate, since the errors of target localization will exactly cancel the errors of motor guidance. These findings provide a mechanistic explanation for the various perceptual and motor effects of the induced Roelofs illusion without requiring the existence of separate neural systems for perception and action. Given this, the behavioral dissociation that accompanies the Roelofs effect cannot be considered evidence of a dissociation of perception and action. This indicates a general need to re-evaluate the broad class of evidence purported to support this hypothesized dissociation

Spatial working memory and Inhibition of Return

Recently we showed that maintaining a location in spatial working memory affects saccadic eye movement trajectories, in that the eyes deviate away from the remembered location (Theeuwes, Olivers, &amp; Chizk, 2005). Such saccade deviations are assumed to be the result of inhibitory processes within the oculomotor system. The present study investigated whether this inhibition is related to the phenomenon of inhibition of return (IOR), the relatively slow selection of previously attended locations as compared with new locations. The results show that the size of IOR to a location was not affected by whether or not the location was kept in working memory, but the size of the saccade trajectory deviation was affected. We conclude that inhibiting working memory–related eye movement activity is not the same as inhibiting a previously attended location in space. Working memory is a system that allows for the temporary storage of information until a task is completed (see, e.g., Baddeley, 1986). Awh and colleagues (Awh &amp; Jonides, 2001; Awh, Jonides, &amp; Reuter-Lorenz, 1998) provided evidence for a strong link between working memory and attention. For example, they showed that when a locatio

Reading Text Increases Binocular Disparity in Dyslexic Children

Children with developmental dyslexia show reading impairment compared to their peers, despite being matched on IQ, socio-economic background, and educational opportunities. The neurological and cognitive basis of dyslexia remains a highly debated topic. Proponents of the magnocellular theory, which postulates abnormalities in the M-stream of the visual pathway cause developmental dyslexia, claim that children with dyslexia have deficient binocular coordination, and this is the underlying cause of developmental dyslexia. We measured binocular coordination during reading and a non-linguistic scanning task in three participant groups: adults, typically developing children, and children with dyslexia. A significant increase in fixation disparity was observed for dyslexic children solely when reading. Our study casts serious doubts on the claims of the magnocellular theory. The exclusivity of increased fixation disparity in dyslexics during reading might be a result of the allocation of inadequate attentional and/or cognitive resources to the reading process, or suboptimal linguistic processing per se

Setting things straight: a comparison of measures of saccade trajectory deviation

In eye movements, saccade trajectory deviation has often been used as a physiological operationalization of visual attention, distraction, or the visual system’s prioritization of different sources of information. However, there are many ways to measure saccade trajectories and to quantify their deviation. This may lead to noncomparable results and poses the problem of choosing a method that will maximize statistical power. Using data from existing studies and from our own experiments, we used principal components analysis to carry out a systematic quantification of the relationships among eight different measures of saccade trajectory deviation and their power to detect the effects of experimental manipulations, as measured by standardized effect size. We concluded that (1) the saccade deviation measure is a good default measure of saccade trajectory deviation, because it is somewhat correlated with all other measures and shows relatively high effect sizes for two well-known experimental effects; (2) more generally, measures made relative to the position of the saccade target are more powerful; and (3) measures of deviation based on the early part of the saccade are made more stable when they are based on data from an eyetracker with a high sampling rate. Our recommendations may be of use to future eye movement researchers seeking to optimize the designs of their studies

Relative contributions to vergence eye movements of two binocular cues for motion-in-depth

When we track an object moving in depth, our eyes rotate in opposite directions. This type of "disjunctive" eye movement is called horizontal vergence. The sensory control signals for vergence arise from multiple visual cues, two of which, changing binocular disparity (CD) and inter-ocular velocity differences (IOVD), are specifically binocular. While it is well known that the CD cue triggers horizontal vergence eye movements, the role of the IOVD cue has only recently been explored. To better understand the relative contribution of CD and IOVD cues in driving horizontal vergence, we recorded vergence eye movements from ten observers in response to four types of stimuli that isolated or combined the two cues to motion-in-depth, using stimulus conditions and CD/IOVD stimuli typical of behavioural motion-in-depth experiments. An analysis of the slopes of the vergence traces and the consistency of the directions of vergence and stimulus movements showed that under our conditions IOVD cues provided very little input to vergence mechanisms. The eye movements that did occur coinciding with the presentation of IOVD stimuli were likely not a response to stimulus motion, but a phoria initiated by the absence of a disparity signal

Perceptual judgment and saccadic behavior in a spatial distortion with briefly presented stimuli.

When observers are asked to localize the peripheral position of a small probe with respect to the mid-position of a spatially extended comparison stimulus, they tend to judge the probe as being more peripheral than the mid-position of the comparison stimulus. This relative mislocalization seems to emerge from differences in absolute localization, that is the comparison stimulus is localized more towards the fovea than the probe. The present study compared saccadic behaviour and relative localization judgements in three experiments and determined the quantitative relationship between both measures. The results showed corresponding effects in localization errors and saccadic behaviour. Moreover, it was possible to estimate the amount of the relative mislocalization by means of the saccadic amplitude