Temporal filtering enhances direction discrimination in random-dot patterns

Encoding of rapid time-varying information is impaired in poor readers

A characteristic set of eye movements and fixations are made during reading, so the position of words on the retinae is constantly being updated. Effective decoding of print requires this temporal stream of visual information to be segmented or parsed into its constituent units (e.g., letters or words). Poor readers' difficulties with word recognition could arise at the point of segmenting time-varying visual information, but the mechanisms underlying this process are little understood. Here, we used random-dot displays to explore the effects of reading ability on temporal segmentation. Thirty-eight adult readers viewed test stimuli that were temporally segmented by constraining either local motions or analogous form cues to oscillate back and fourth at each of a range of rates. Participants had to discriminate these segmented patterns from comparison stimuli containing the same motion and form cues but these were temporally intermingled. Results showed that the motion and form tasks could not be performed reliably when segment duration was shorter than a temporal resolution (acuity) limit. The acuity limits for both tasks were significantly and negatively correlated with reading scores. Importantly, the minimum segment duration needed to detect the temporally segmented stimuli was longer in relatively poor readers than relatively good readers. This demonstrates that adult poor readers have difficulty segmenting temporally changing visual input particularly at short segment durations. These results are consistent with evidence suggesting that precise encoding of rapid time-varying information is impaired in developmental dyslexia.

Influence of correspondence noise and spatial scaling on the upper limit for spatial displacement in fully-coherent random-dot kinematogram stimuli

Correspondence noise is a major factor limiting direction discrimination performance in random-dot kinematograms [1]. In the current study we investigated the influence of correspondence noise on Dmax, which is the upper limit for the spatial displacement of the dots for which coherent motion is still perceived. Human direction discrimination performance was measured, using 2-frame kinematograms having leftward/rightward motion, over a 200-fold range of dot-densities and a four-fold range of dot displacements. From this data Dmax was estimated for the different dot densities tested. A model was proposed to evaluate the correspondence noise in the stimulus. This model summed the outputs of a set of elementary Reichardt-type local detectors that had receptive fields tiling the stimulus and were tuned to the two directions of motion in the stimulus. A key assumption of the model was that the local detectors would have the radius of their catchment areas scaled with the displacement that they were tuned to detect; the scaling factor k linking the radius to the displacement was the only free parameter in the model and a single value of k was used to fit all of the psychophysical data collected. This minimal, correspondence-noise based model was able to account for 91% of the variability in the human performance across all of the conditions tested. The results highlight the importance of correspondence noise in constraining the largest displacement that can be detected.

Attentional selection of moving objects by a serial process

How does the efficiency of attentional selection depend on the number of attended objects in a display? We measured the channel capacity (CC) of human observers during the attentional tracking of moving targets. The relation between CC and target number was used to estimate target-sampling rate. The sampling rate was halved when the number of targets was doubled, indicating that tracking was accomplished by a mechanism whose processing rate did not vary with target number. Systematically varying the dynamic parameters of the display provided inconclusive evidence for the idea that the time interval between successive samples of the same target increased with target number. These results are consistent with the hypothesis that the selection of multiple moving objects involves a limited capacity processor.

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.

Discrimination of spatial displacements by patients with retinitis pigmentosa

We compared maximum displacement thresholds (Dmax) with minimum displacement thresholds (Dmin) in patients with retinitis pigmentosa (RP) in order to characterize the nature of their visual disability, as well as to assess possible models of foveal vision loss. Thresholds for discriminating the direction of the spatial displacement of random dot patterns were measured in a group of 20 patients with typical RP or Usher syndrome whose visual acuities were 20/40 or better and who had minimal or no clinical evidence of changes in the ocular media. Findings were compared with those from an age-similar group of 15 visually normal subjects. Displacement thresholds were measured using a two-frame random dot cinematogram and a four-alternative forced-choice procedure. Measurements were made at each of three dot contrasts and three dot sizes. For the patients with RP, reducing either the dot contrast or dot size increased Dmin and decreased Dmax such that the range of discriminable displacements became considerably restricted, even at modest reductions in dot contrast or size. This restriction in the displacement thresholds of the patients with RP was correlated significantly with their visual acuity. By comparison, the control subjects showed little change in either Dmin or Dmax under these conditions. These results indicate that patients with RP who have only relatively minor reductions in their visual acuity can have severely compromised motion perception. The pattern of findings suggests that an abnormal contrast response of the foveal cone system is a major determinant of the impaired displacement thresholds of these patients with RP.

Motion detection in interleaved random dot patterns: evidence for a rectifying nonlinearity preceding motion analysis

Three experiments examined direction discrimination in temporally interleaved random dot patterns. The stimulus consisted of two or more uncorrelated random patterns presented in a repeating temporal sequence, so that each pattern appeared only once every n frames, separated by uncorrelated patterns. Each pattern shifted either leftward or rightward at each re-appearance (all patterns shifted in the same direction in any one presentation). Subjects could specify shift direction correctly even when eight different patterns were interleaved, provided that the duration of each frame was brief. An explanation based on responses in first-order motion energy detectors tuned to low spatiotemporal frequencies (effectively summating the interleaved patterns over time) was tested using a stimulus in which each pattern inverted in contrast mid-way through each frame. Contrary to predictions based on temporal summation, performance with contrast-inverting patterns was only slightly lower than with non-inverting patterns. An alternative explanation was examined, based on responses in motion detectors that full-wave rectify image contrast before extracting motion energy. Computed responses from such detectors successfully predicted psychophysical performance with interleaved random patterns. Implications for models of motion analysis are discussed.