Dirk Kerzel

Eye movements and visible persistence explain the mislocalization of the final position of a moving target

When observers are asked to localize the final position of a moving target, the judged position is usually displaced from the actual position in the direction of motion. The short-term time course of the displacement was investigated to test theories that attribute the localization error to spatial and temporal properties of human perception or to representational momentum. It was found that briefly after target offset, the judged position is already displaced in the direction of motion. It is argued that the shift results from eye movements after target offset that move the targets persisting image in the direction of motion.

Comparing mislocalizations with moving stimuli: The Fröhlich effect, the flash-lag, and representational momentum

When observers are asked to localize the onset or the offset position of a moving target, they typically make localization errors in the direction of movement. Similarly, when observers judge a moving target that is presented in alignment with a flash, the target appears to lead the flash. These errors are known as the Fröhlich effect, representational momentum, and flash-lag effect, respectively. This study compared the size of the three mislocalization errors. In Experiment 1, a flash appeared either simultaneously with the onset, the mid-position, or the offset of the moving target. Observers then judged the position where the moving target was located when the flash appeared. Experiments 2 and 3 are exclusively concerned with localizing the onset and the offset of the moving target. When observers localized the position with respect to the point in time when the flash was presented, a clear mislocalization in the direction of movement was observed at the initial position and the mid-position. In contrast, a mislocalization opposite to movement direction occurred at the final position. When observers were asked to ignore the flash (or when no flash was presented at all), a reduced error (or no error) was observed at the initial position and only a minor error in the direction of the movement occurred at the final position. An integrative model is proposed, which suggests a common underlying mechanism, but emphasizes the specific processing components of the Fröhlich effect, flash-lag effect, and representational momentum.

Neuronal Processing Delays Are Compensated in the Sensorimotor Branch of the Visual System

Moving objects change their position until signals from the photoreceptors arrive in the visual cortex. Nonetheless, motor responses to moving objects are accurate and do not lag behind the real-world position. The questions are how and where neural delays are compensated for. It was suggested that compensation is achieved within the visual system by extrapolating the position of moving objects. A visual illusion supports this idea: when a briefly flashed object is presented in the same position as a moving object, it appears to lag behind. However, moving objects do not appear ahead of their final or reversal points. We investigated a situation where participants localized the final position of a moving stimulus. Visual perception and short-term memory of the final target position were accurate, but reaching movements were directed toward future positions of the target beyond the vanishing point. Our results show that neuronal latencies are not compensated for at early stages of visual processing, but at a late stage when retinotopic information is transformed into egocentric space used for motor responses. The sensorimotor system extrapolates the position of moving targets to allow for precise localization of moving targets despite neuronal latencies.

Improved visual sensitivity during smooth pursuit eye movements

When we view the world around us, we constantly move our eyes. This brings objects of interest into the fovea and keeps them there, but visual sensitivity has been shown to deteriorate while the eyes are moving. Here we show that human sensitivity for some visual stimuli is improved during smooth pursuit eye movements. Detection thresholds for briefly flashed, colored stimuli were 16% lower during pursuit than during fixation. Similarly, detection thresholds for luminance-defined stimuli of high spatial frequency were lowered. These findings suggest that the pursuit-induced sensitivity increase may have its neuronal origin in the parvocellular retino-thalamic system. This implies that the visual system not only uses feedback connections to improve processing for locations and objects being attended to, but that a whole processing subsystem can be boosted. During pursuit, facilitation of the parvocellular system may reduce motion blur for stationary objects and increase sensitivity to speed changes of the tracked object.

Amygdala Activation for Eye Contact Despite Complete Cortical Blindness

Cortical blindness refers to the loss of vision that occurs after destruction of the primary visual cortex. Although there is no sensory cortex and hence no conscious vision, some cortically blind patients show amygdala activation in response to facial or bodily expressions of emotion. Here we investigated whether direction of gaze could also be processed in the absence of any functional visual cortex. A well-known patient with bilateral destruction of his visual cortex and subsequent cortical blindness was investigated in an fMRI paradigm during which blocks of faces were presented either with their gaze directed toward or away from the viewer. Increased right amygdala activation was found in response to directed compared with averted gaze. Activity in this region was further found to be functionally connected to a larger network associated with face and gaze processing. The present study demonstrates that, in human subjects, the amygdala response to eye contact does not require an intact primary visual cortex.

Memory for the position of stationary objects: disentangling foveal bias and memory averaging

The perceived and remembered position of stationary target objects is subject to a large number of distortions. Objects are localized toward the fovea, and when an additional object (distractor) is presented, a tendency to average target and distractor position was observed. These distortions in visual short-term memory have been referred to as foveal bias and memory averaging, respectively. Because most studies on memory averaging did not monitor eye fixation, foveal bias and memory averaging may have been confounded. That is, observers may have fixated the distractor. To disentangle these factors, target and distractor were presented in the periphery, and fixation was monitored. Memory averaging was not observed. Rather a bias away from the distractor occurred when the distractor was briefly presented during the retention interval, or when it was visible throughout the trial. In contrast, a foveal bias was observed regardless of whether an additional object was present.

Temporal contrast sensitivity during smooth pursuit eye movements

During smooth pursuit eye movements, stimuli other than the pursuit target move across the retina, and this might affect their detectability. We measured detection thresholds for vertically oriented Gabor stimuli with different temporal frequencies (1, 4, 8, 12, 16, 20, and 24 Hz) of the sinusoids. Observers kept fixation on a small target spot that was either stationary or moved horizontally at a speed of 8 deg/s. The sinusoid of the Gabor stimuli moved either in the same or in the opposite direction as the pursuit target. Observers had to indicate whether the Gabor stimuli were displayed 4 degrees above or below the target spot. Results show that contrast sensitivity was mainly determined by retinal-image motion but was slightly reduced during smooth pursuit eye movements. Moreover, sensitivity for motion opposite to pursuit direction was reduced in comparison to motion in pursuit direction. The loss in sensitivity for peripheral targets during pursuit can be interpreted in terms of space-based attention to the pursuit target. The loss of sensitivity for motion opposite to pursuit direction can be interpreted as feature-based attention to the pursuit direction.

Effects of contrast on smooth pursuit eye movements

It is well known that moving stimuli can appear to move more slowly when contrast is reduced (P. Thompson, 1982). Here we address the question whether changes in stimulus contrast also affect smooth pursuit eye movements. Subjects were asked to smoothly track a moving Gabor patch. Targets varied in velocity (1, 8, and 15 deg/s), spatial frequency (0.1, 1, 4, and 8 c/deg), and contrast, ranging from just below individual thresholds to maximum contrast. Results show that smooth pursuit eye velocity gain rose significantly with increasing contrast. Below a contrast level of two to three times threshold, pursuit gain, acceleration, latency, and positional accuracy were severely impaired. Therefore, the smooth pursuit motor response shows the same kind of slowing at low contrast that was demonstrated in previous studies on perception.

Conflicts During Response Selection Affect Response Programming: Reactions Toward the Source of Stimulation

In the Simon effect, participants make a left or right keypress in response to a nonspatial attribute (e.g., color) that is presented on the left or right. Reaction times (RTs) increase when the response activated by the irrelevant stimulus location and the response retrieved by instruction are in conflict. The authors measured RTs and movement parameters (MPs) of pointing responses in a typical Simon task. Their results show that the trajectories veer toward the imperative stimulus. This bias decreased as RTs increased. The authors suggest that the time course of trajectory deviations reflects the resolution of the response conflict over time. Further, time pressure did not affect the size of the Simon effect in MPs or its time course, but strongly reduced the Simon effect in RTs. In contrast, response selection before the onset of a go signal on the left or right did not affect the Simon effect in RTs, but reduced the Simon effect in MPs and reversed the time course. The authors speculate about independent Simon effects associated with response selection and programming.

A Simon effect with stationary moving stimuli

To clarify whether motion information per se has a separable influence on action control, the authors investigated whether irrelevant direction of motion of stimuli whose overall position was constant over time would affect manual left-right responses (i.e., reveal a motion-based Simon effect). In Experiments 1 and 2, significant Simon effects were obtained for sine-wave gratings moving in a stationary Gaussian window. In Experiment 3, a direction-based Simon effect with random-dot patterns was replicated, except that the perceived direction of motion was based on the displacement of single elements. Experiments 4 and 5 studied motion-based Simon effects to point-light figures that walked in place--displays requiring high-level analysis of global shape and local motion. Motion-based Simon effects occurred when the displays could be interpreted as an upright human walker, showing that a high-level representation of motion direction mediated the effects. Thus, the present study establishes links between high-level motion perception and action.