The minimum temporal thresholds for motion detection of grating patterns

Inferring visual space from ultra-fine extra-retinal knowledge of gaze position

It has long been debated how humans resolve fine details and perceive a stable visual world despite the incessant fixational motion of their eyes. Current theories assume these processes to rely solely on the visual input to the retina, without contributions from motor and/or proprioceptive sources. Here we show that contrary to this widespread assumption, the visual system has access to high-resolution extra-retinal knowledge of fixational eye motion and uses it to deduce spatial relations. Building on recent advances in gaze-contingent display control, we created a spatial discrimination task in which the stimulus configuration was entirely determined by oculomotor activity. Our results show that humans correctly infer geometrical relations in the absence of spatial information on the retina and accurately combine high-resolution extraretinal monitoring of gaze displacement with retinal signals. These findings reveal a sensory-motor strategy for encoding space, in which fine oculomotor knowledge is used to interpret the fixational input to the retina.

The effect of simultaneous flickering light stimulation on global form and motion perception thresholds

The question regarding the exact function of the primary visual cortex (V1) in vision has been around ever since the description of residual vision after damage to this cortical area by Riddoch in 1917. In 2002, Schoenfeld and colleagues proposed that V1 can be saturated by flashes of light, by which the function of V1-bypassing visual pathways can be "unmasked". The Schoenfeld group found that light flashes applied on stimulus onset led to the elevation of brightness increment detection thresholds, but left motion detection thresholds unaltered. Although the proposed method (i.e. the use of light flashes to induce refractoriness in V1) could be a simple, cheap and elegant way of exploring V1 functions, no study has followed up on this. Therefore it is not known if it works at all with other types of stimuli. For that reason, we decided to revisit the idea in a modified form. Global form and motion perception thresholds were assessed with static Glass pattern stimuli and random dot kinematograms, with and without 12Hz flickering light stimulation. Global motion thresholds were almost unaltered by flickering stimulation, while a significant threshold elevation was caused in the global form perception task. The strongest conclusion allowed by our data is that simultaneous flickering photostimulation elevates global form perception thresholds but not global motion perception thresholds. This is in some way related to the refractoriness generated in an unsatisfactorily defined part of V1. We suggest that this does not necessarily reflect the activity of V1-bypassing pathways, and propose that the application of light flashes is a method that deserves more attention in the exploration of the V1-dependent and independent elements of visual consciousness in human subjects.

A motion illusion reveals mechanisms of perceptual stabilization

Visual illusions are valuable tools for the scientific examination of the mechanisms underlying perception. In the peripheral drift illusion special drift patterns appear to move although they are static. During fixation small involuntary eye movements generate retinal image slips which need to be suppressed for stable perception. Here we show that the peripheral drift illusion reveals the mechanisms of perceptual stabilization associated with these micromovements. In a series of experiments we found that illusory motion was only observed in the peripheral visual field. The strength of illusory motion varied with the degree of micromovements. However, drift patterns presented in the central (but not the peripheral) visual field modulated the strength of illusory peripheral motion. Moreover, although central drift patterns were not perceived as moving, they elicited illusory motion of neutral peripheral patterns. Central drift patterns modulated illusory peripheral motion even when micromovements remained constant. Interestingly, perceptual stabilization was only affected by static drift patterns, but not by real motion signals. Our findings suggest that perceptual instabilities caused by fixational eye movements are corrected by a mechanism that relies on visual rather than extraretinal (proprioceptive or motor) signals, and that drift patterns systematically bias this compensatory mechanism. These mechanisms may be revealed by utilizing static visual patterns that give rise to the peripheral drift illusion, but remain undetected with other patterns. Accordingly, the peripheral drift illusion is of unique value for examining processes of perceptual stabilization.

Visual motion detection in hierarchical spatial frames of reference

Neurophysiological and neuroimaging work has uncovered modulatory influence of long-range lateral connections from outside of the classical receptive field on neuronal and behavioral responses to localized targets. We report two psychophysical experiments investigating visual detection of real and apparent motion in central vision with and without remote and immediate stationary references. At a particular temporal frequency (0.1-12.8 Hz), participants adjusted the amplitude of either triangle-wave (real) or square-wave (stroboscopic/apparent) oscillatory motion of a vertical bar along a straight, horizontal trajectory for the first impression of the target's stationarity/nonstationarity (the displacement threshold). In the relative motion conditions, a stationary reference bar was positioned 23' apart from the target; in the absolute motion conditions, the bar was absent. The thresholds were measured with a dimly-lit uniform background (13 x 13 degrees ) and either in the darkness (experiment 1) or moving-background conditions (experiment 2). For both real and apparent motion, varying the observation conditions yields three sensitivity levels: irrespective of the background, the lowest thresholds occur in the presence of an immediate reference, followed by the moderately increased thresholds obtained with a dimly-lit background alone. The equally high thresholds occur in the darkness and moving-background conditions without any visible stationary references. The results suggest that the spatial frames of reference for visual motion detection are hierarchically nested, yet independent. The findings provide support for the view that absolute motion perception should be considered relative, extending neurophysiological evidence for the existence of long-range lateral connections across the visual field.

The Spatial and Temporal Conditions for Perceiving Velocity as Constant

Perceived velocities during a brief period of exposure (<1.2 s) were measured to examine how much time is necessary to perceive velocity as constant. Moving sinusoidal gratings were used as stimuli at relatively low velocities. At the beginning of each stimulus presentation, a moving pattern was perceived as stationary until a critical time had passed. After that, perceived velocity was positively correlated with moving distance, irrespective of physical velocity below a critical moving distance. Beyond the critical moving distance, velocity was perceived as constant. A simple model is presented to explain these results.