Hulusi Kafaligonul

Meta- and paracontrast reveal differences between contour- and brightness-processing mechanisms.

We investigated meta- and paracontrast masking using tasks requiring observers to judge the surface brightness or else the contours of target stimuli. The contour task revealed strongest metacontrast at SOAs shorter than those obtained for the brightness task. Paracontrast revealed related temporal differences between the tasks. Additionally, the paracontrast results support the existence not only of prolonged inhibitory effects but also of facilitatory effects. The combined results comport with the existence of cortical mechanisms for: (i) fast contour processing, (ii) slow surface-brightness processing, (iii) prolonged inhibition, and (iv) facilitation.

Feedforward and feedback processes in vision.

Hierarchical processing is key to understanding vision. The visual system consists of hierarchically organized distinct anatomical areas functionally specialized for processing different aspects of a visual object (Felleman and Van Essen, 1991). These visual areas are interconnected through ascending feedforward projections, descending feedback projections, and projections from neural structures at the same hierarchical level (Lamme et al., 1998). Even though accumulating evidence suggests that these three projections play fundamentally different roles in perception, their distinct functional roles in visual processing are still subject to debate (Lamme and Roelfsema, 2000). The focus of this Research Topic was the roles of feedforward and feedback projections in vision. In fact, our motivation to edit this Research Topic was threefold: (i) to provide current views on the functional roles of feedforward and feedback projections for the perception of specific visual features, (ii) to invite recent views on how these functional roles contribute to the distinct modes of visual processing, (iii) to provide recent methodological views to identify distinct functional roles of feedforward and feedback projections and corresponding neural signatures. As summarized below, these aims are largely achieved thanks to fourteen contributions to this issue.

Metacontrast masking and stimulus contrast polarity

A recent report [Becker, M. W., & Anstis S. (2004). Metacontrast masking is specific to luminance polarity. Vision Research, 44, 2537-2543] of a failure to obtain metacontrast with target and mask stimuli of opposite contrast polarity is reexamined in an experiment that systematically varies not only stimulus contrast polarity but also target size and target-mask onset asynchrony (SOA). The results show that (a) although, as previously shown [Breitmeyer, B. G. (1978a). Metacontrast with black and white stimuli: Evidence of inhibition of on and off sustained activity by either on or off transient activity. Vision Research, 18, 1443-1448], metacontrast is weaker with stimuli of opposite contrast polarity, (b) substantial metacontrast can be obtained with targets and masks of opposite contrast polarity, especially (c) when the target is small. We conclude that Becker and Anstiss failure to obtain metacontrast with stimuli of opposite contrast polarity is due to their use of a single, relatively large, SOA value.

Audiovisual associations alter the perception of low-level visual motion

Motion perception is a pervasive nature of vision and is affected by both immediate pattern of sensory inputs and prior experiences acquired through associations. Recently, several studies reported that an association can be established quickly between directions of visual motion and static sounds of distinct frequencies. After the association is formed, sounds are able to change the perceived direction of visual motion. To determine whether such rapidly acquired audiovisual associations and their subsequent influences on visual motion perception are dependent on the involvement of higher-order attentive tracking mechanisms, we designed psychophysical experiments using regular and reverse-phi random dot motions isolating low-level pre-attentive motion processing. Our results show that an association between the directions of low-level visual motion and static sounds can be formed and this audiovisual association alters the subsequent perception of low-level visual motion. These findings support the view that audiovisual associations are not restricted to high-level attention based motion system and early-level visual motion processing has some potential role.

Effects of contrast polarity in paracontrast masking

The visibility of a target stimulus can be suppressed (inhibition) or increased (facilitation) during paracontrast masking. Three processes have been proposed to be involved in paracontrast masking: brief inhibition, facilitation, and prolonged inhibition (Breitmeyer et al., 2006). Brief inhibition is observed when the mask precedes the target at short stimulus onset asynchronies (SOAs) ranging from -10 to -30 msec, whereas prolonged inhibition is effective up to very large SOAs of -450 msec. Facilitation, enhancement in target visibility, can be observed at SOA values between -20 and -110 msec. We further investigated these processes by changing target-mask spatial separation and the contrast polarity of the mask. Our results show that (1) facilitation weakens when spatial separation between the target and mask is increased or when they have opposite contrast polarity, and (2) brief inhibition turns into facilitation for the opposite-polarity mask, whereas prolonged inhibition does not change significantly. These results suggest a fast inhibition mechanism realized in the contrast-specific center-surround antagonism of classical receptive fields for brief inhibition and a slower, higher level cortical processing that is indifferent to contrast polarity for prolonged inhibition.

Rapid Motion Adaptation Reveals the Temporal Dynamics of Spatiotemporal Correlation between ON and OFF Pathways

At the early stages of visual processing, information is processed by two major thalamic pathways encoding brightness increments (ON) and decrements (OFF). Accumulating evidence suggests that these pathways interact and merge as early as in primary visual cortex. Using regular and reverse-phi motion in a rapid adaptation paradigm, we investigated the temporal dynamics of within and across pathway mechanisms for motion processing. When the adaptation duration was short (188 ms), reverse-phi and regular motion led to similar adaptation effects, suggesting that the information from the two pathways are combined efficiently at early-stages of motion processing. However, as the adaption duration was increased to 752 ms, reverse-phi and regular motion showed distinct adaptation effects depending on the test pattern used, either engaging spatiotemporal correlation between the same or opposite contrast polarities. Overall, these findings indicate that spatiotemporal correlation within and across ON-OFF pathways for motion processing can be selectively adapted, and support those models that integrate within and across pathway mechanisms for motion processing.

The Involvement of Centralized and Distributed Processes in Sub‐second Time Interval Adaptation: An ERP Investigation of Apparent Motion

Accumulating evidence suggests that the timing of brief stationary sounds affects visual motion perception. Recent studies have shown that auditory time interval can alter apparent motion perception not only through concurrent stimulation but also through brief adaptation. The adaptation after‐effects for auditory time intervals was found to be similar to those for visual time intervals, suggesting the involvement of a central timing mechanism. To understand the nature of cortical processes underlying such after‐effects, we adapted observers to different time intervals using either brief sounds or visual flashes and examined the evoked activity to the subsequently presented visual apparent motion. Both auditory and visual time interval adaptation led to significant changes in the ERPs elicited by the apparent motion. However, the changes induced by each modality were in the opposite direction. Also, they mainly occurred in different time windows and clustered over distinct scalp sites. The effects of auditory time interval adaptation were centred over parietal and parieto‐central electrodes while the visual adaptation effects were mostly over occipital and parieto‐occipital regions. Moreover, the changes were much more salient when sounds were used during the adaptation phase. Taken together, our findings within the context of visual motion point to auditory dominance in the temporal domain and highlight the distinct nature of the sensory processes involved in auditory and visual time interval adaptation.

Temporal ventriloquism along the path of apparent motion: speed perception under different spatial grouping principles

The coordination of intramodal perceptual grouping and crossmodal interactions plays a critical role in constructing coherent multisensory percepts. However, the basic principles underlying such coordinating mechanisms still remain unclear. By taking advantage of an illusion called temporal ventriloquism and its influences on perceived speed, we investigated how audiovisual interactions in time are modulated by the spatial grouping principles of vision. In our experiments, we manipulated the spatial grouping principles of proximity, uniform connectedness, and similarity/common fate in apparent motion displays. Observers compared the speed of apparent motions across different sound timing conditions. Our results revealed that the effects of sound timing (i.e., temporal ventriloquism effects) on perceived speed also existed in visual displays containing more than one object and were modulated by different spatial grouping principles. In particular, uniform connectedness was found to modulate these audiovisual interactions in time. The effect of sound timing on perceived speed was smaller when horizontal connecting bars were introduced along the path of apparent motion. When the objects in each apparent motion frame were not connected or connected with vertical bars, the sound timing was more influential compared to the horizontal bar conditions. Overall, our findings here suggest that the effects of sound timing on perceived speed exist in different spatial configurations and can be modulated by certain intramodal spatial grouping principles such as uniform connectedness.