Time-till-breakdown and scalp electrical potential maps of long-range apparent motion

Apparent Motion Induces Activity Suppression in Early Visual Cortex and Impairs Visual Detection

Apparent motion (AM) is induced when two stationary visual stimuli are presented in alternating sequence. Intriguingly, AM leads to an impaired detectability of stimuli along the AM path (i.e., AM-induced masking). It has been hypothesized that AM triggers an internal representation of a moving object in early visual cortex, which competes with stimulus-evoked representations of visual stimuli on the motion path in early visual cortex of 25 human adults (16 female). We tested this hypothesis by measuring BOLD responses in early visual cortex during the process of AM-induced masking, using fMRI and population receptive field methods. Surprisingly, and counter to our hypothesis, we showed that AM suppressed, rather than increased, BOLD responses along early visual (V1 and V2) representations of the AM path, including regions that were not directly activated by the AM inducer stimuli. This activity suppression of the visual response predicted the subsequent reduction in detectability of the target that appeared in the middle of the AM path. Our data thereby provide direct empirical evidence for suppressive neural mechanisms underlying AM and suggest that illusory motion can render us blind to objects on the motion path by suppressing neural activity at the earliest cortical stages of visual perception.SIGNIFICANCE STATEMENT When two spatially distinct visual objects are presented in alternating sequence, apparent motion (AM) occurs and impairs detectability of stimuli along its path. The underlying mechanism is thought to be that increased activation in human early visual cortex evoked by AM interferes with the representation of the stimulus. Strikingly, however, we show that AM suppresses neural activity along the motion path, and the strength of activity suppression predicts the subsequent behavioral performance decrement in terms of detecting a stimulus along the AM path. Our findings provide empirical evidence for a suppressive, rather than faciliatory, mechanism underlying AM.

Veridical stimulus localization is linked to human area V5/MT+ activity

How the brain represents visual space is an unsolved mystery. Spatial localization becomes particularly challenging when visual information processing is briefly disrupted, as in the case of saccadic eye movements, blinks, or visual masks. As we have recently reported, a compression of visual space, illustrated by displacements of shortly flashed stimuli, can be observed in the temporal vicinity of masking stimuli during ocular fixation (Zimmermann et al., 2013). We here aimed at investigating the neural mechanisms underlying these displacements using functional magnetic resonance imaging. On the behavioral level, we detected significant stimulus displacement when visual masks were simultaneously presented. At the neural level, we observed decreased human motion complex V5/MT+ activation associated with these displacements: When comparing trials with a perceived stimulus shift in space to trials of veridical perception of stimulus localization, human V5/MT+ was significantly less activated although no differences in perceived motion can account for this. Data suggest an important role of human V5/MT+ in the process of spatial localization of briefly presented objects and thus extend current concepts of the functions of human V5/MT+.

BOLD responses in the superior colliculus and lateral geniculate nucleus of the rat viewing an apparent motion stimulus

In rats, the superior colliculus (SC) is a main destination for retinal ganglion cells and is an important subcortical structure for vision. Electrophysiology studies have observed that many SC neurons are highly sensitive to moving objects, but complementary non-invasive functional imaging studies with larger fields of view have been rarely conducted. In this study, BOLD fMRI is used to measure the SC and nearby lateral geniculate nucleus' (LGN) hemodynamic responses, in normal adult Sprague Dawley (SD) rats, during a dynamic visual stimulus similar to those used in long-range apparent motion studies. The stimulation paradigm consists of four light spots arranged in a linear array and turned on and off sequentially at different rates to create five effective speeds of motion (7, 14, 41, 82, and 164°/s across the visual field). Stationary periods (same light spot always on) are interleaved between the moving periods. The speed response function (SRF), the hemodynamic response amplitude at each speed tested, is measured. Significant responses are observed in the SC and LGN at all speeds. In the SC, the SRF increases monotonically from 7 to 82°/s. The minimum response amplitude occurs at 164°/s. The results suggest that the SC is sensitive to slow moving visual stimuli but the hemodynamic response is reduced at higher speeds. In the LGN, the SRF exhibits a similar trend to that of the SC, but response amplitude during 7°/s stimulation is comparable to that during 164°/s stimulation. These findings are in good agreement with previous electrophysiology studies conducted on albino rats like the SD strain. This work represents the first fMRI study of stimulus speed dependence in the SC and is also the first fMRI study of motion responsiveness in the rat.

Primary visual cortex activity along the apparent-motion trace reflects illusory perception

The illusion of apparent motion can be induced when visual stimuli are successively presented at different locations. It has been shown in previous studies that motion-sensitive regions in extrastriate cortex are relevant for the processing of apparent motion, but it is unclear whether primary visual cortex (V1) is also involved in the representation of the illusory motion path. We investigated, in human subjects, apparent-motion-related activity in patches of V1 representing locations along the path of illusory stimulus motion using functional magnetic resonance imaging. Here we show that apparent motion caused a blood-oxygenation-level-dependent response along the V1 representations of the apparent-motion path, including regions that were not directly activated by the apparent-motion-inducing stimuli. This response was unaltered when participants had to perform an attention-demanding task that diverted their attention away from the stimulus. With a bistable motion quartet, we confirmed that the activity was related to the conscious perception of movement. Our data suggest that V1 is part of the network that represents the illusory path of apparent motion. The activation in V1 can be explained either by lateral interactions within V1 or by feedback mechanisms from higher visual areas, especially the motion-sensitive human MT/V5 complex.

Using fMRI in humans, the authors reveal a clear role for V1 cortex in forming an illusory perception of motion when stationary stimuli are successively flashed in different locations.

Alternative perceptual states 'apparent motion' and 'perceived simultaneity' lead to differences of induced EEG rhythms

An investigation of the cortical response (EEG) to periodically presented stimuli producing an ambiguity between long-range apparent motion and flicker is reported. ERPs to stimulus onsets differed slightly between the two percepts, in accordance with the results of Manning et al. (1988), Selmes et al. (1997). Induced rhythms exhibited a strong increase in induced beta and gamma powers at electrode positions T7 and T8 during the perception of apparent motion in two out of 10 participants. In addition, a small overall increase in alpha power at 12-13 Hz and a decrease in delta power below 3.5 Hz during perceived motion were found. The results indicate that a variety of different neural rhythms are involved in the perception of long-range apparent motion.