Ryohei Nakayama

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.

Discretized Theta-Rhythm Perception Revealed by Moving Stimuli

Despite the subjective continuity of perception over time, increasing evidence suggests that the human nervous system samples sensory information periodically, a finding strongly exemplified by discretized perception in the alpha-rhythm frequency band. More recently, studies have revealed a theta-band cyclic process that manifests itself as periodical fluctuations in behavioral performance. Here, we used a simple stimulus to demonstrate that the theta-cyclic system can produce a vivid experience of slow discrete visual sampling: a Gabor texture pattern appears as a series of flickering snapshots if its spatial window moves continuously over a carrier grating that remains still or drifts continuously in the opposite direction. While the perceptual magnitude of this illusory saltation varied with the speed difference between grating and window components in head-centered coordinates, the perceived rhythm of saltation remained nearly constant (3–8 Hz) over a wide range of stimulus parameters. Results provide further evidence that the slow cyclic neural processes play a critical role not only in attentional task performance but also in conscious perception.

Sensitivity to Acceleration in the Human Early Visual System

It is widely believed that the human visual system is insensitive to acceleration in moving stimuli. This notion is supported by evidence that detection sensitivity for velocity modulation in moving stimuli is a lowpass function of the velocity modulations temporal frequency. However, the lowpass function might be a mixture of detection by attention-based tracking and low-level mechanisms sensitive to acceleration. To revisit the issue of acceleration perception in relation to attentive tracking, we measured detection sensitivities for velocity modulations at various temporal frequencies (0.25–8 Hz) by using drifting gratings within long or short spatial windows that make the tracking of grating easier or more difficult respectively. Results showed that modulation sensitivity is lowpass for gratings with long windows but bandpass for gratings with short windows (peak at ~1 Hz). Moreover, we found that lowpass sensitivity becomes bandpass when we removed observer attention by a concurrent letter identification task. An additional visual-search experiment showed that a target dot moving with a velocity modulation at relatively high temporal frequencies (~2–4 Hz) was most easily detected among dots moving at various constant velocities. These results support the notion that high sensitivity to sluggish velocity modulation is a product of attentively tracking of moving stimuli and that the visual system is directly sensitive to accelerations and/or decelerations at the preattentive level.

The Roles of Non-retinotopic Motions in Visual Search

In visual search, a moving target among stationary distracters is detected more rapidly and more efficiently than a static target among moving distracters. Here we examined how this search asymmetry depends on motion signals from three distinct coordinate systems—retinal, relative, and spatiotopic (head/body-centered). Our search display consisted of a target element, distracters elements, and a fixation point tracked by observers. Each element was composed of a spatial carrier grating windowed by a Gaussian envelope, and the motions of carriers, windows, and fixation were manipulated independently and used in various combinations to decouple the respective effects of motion coordinate systems on visual search asymmetry. We found that retinal motion hardly contributes to reaction times and search slopes but that relative and spatiotopic motions contribute to them substantially. Results highlight the important roles of non-retinotopic motions for guiding observer attention in visual search.

Directional asymmetry in contrast sensitivity during smooth pursuit eye movement depends on spatial frequency

It is known that smooth pursuit eye movement (SPEM) modulates visual contrast sensitivity in an asymmetric manner. For instance, Schütz et al. (2007) reported that contrast sensitivity to a 1 c/deg grating is higher when it drifts in the same direction with the SPEM than in the opposite direction. They interpret this asymmetry in terms of the effect of spatial attention that moves together with, or even leads, the eye. The present study shows that this asymmetry is reversed at high spatial frequency bands. We measured contrast sensitivity for a spatially localized grating (Gabor patch) of 11 c/deg. The spatial window of the grating (s.d. of H0.8 x V0.4 deg) moved in the same velocity with SPEM (1.5 deg/sec), and thereby it was static on the retinal coordinate. The grating within spatial window drifted on the retina either in the same direction or in the opposite direction of SPEM. We found that for a range of retinal temporal frequencies tested (1.4-22.5 Hz), the observers contrast sensitivity was substantially higher (0.1-0.25 log unit) when the grating drifted in the opposite direction of SPEM than in the same direction. This asymmetry was also observed for the cut-off temporal frequencies for a grating with a contrast of 0.5. For gratings of a low spatial frequency (0.5 c/deg), the cut off frequency was little, or not, higher for gratings in the same same direction than in the opposite direction of SPEM, consistent with the previous study. These results indicate that during smooth pursuit, the visual system becomes more sensitive to high-spatial frequency signals that moves slower than the eye than those that moves faster than the eye. The results cannot be well accounted for neither by spatial attention nor by sensitivity modulations of parvocellular and magnocellular channels. Meeting abstract presented at VSS 2015.