Masking Disrupts Reentrant Processing in Human Visual Cortex

Abstract

In masking, a stimulus is rendered invisible through the presentation of a second stimulus shortly after the first. Over the years, authors have typically explained masking by postulating some early disruption process. In these feedforward-type explanations, the mask somehow “catches up” with the target stimulus, disrupting its processing either through lateral or interchannel inhibition. However, studies from recent years indicate that visual perception—and most notably visual awareness itself—may depend strongly on cortico-cortical feedback connections from higher to lower visual areas. This has led some researchers to propose that masking derives its effectiveness from selectively interrupting these reentrant processes. In this experiment, we used electroencephalogram measurements to determine what happens in the human visual cortex during detection of a texture-defined square under nonmasked (seen) and masked (unseen) conditions. Electro-encephalogram derivatives that are typically associated with reentrant processing turn out to be absent in the masked condition. Moreover, extrastriate visual areas are still activated early on by both seen and unseen stimuli, as shown by scalp surface Laplacian current source-density maps. This conclusively shows that feedforward processing is preserved, even when subject performance is at chance as determined by objective measures. From these results, we conclude that masking derives its effectiveness, at least partly, from disrupting reentrant processing, thereby interfering with the neural mechanisms of figure-ground segmentation and visual awareness itself.

Act Quickly, Decide Later: Long-latency Visual Processing Underlies Perceptual Decisions but Not Reflexive Behavior

“…The size of the individual squares of the checkerboard was 2.5°by 2.5°visual arc. Although this results in relatively long presentation times for the texture stimuli compared to earlier work, we cannot expect that the longer presentation times will have a large effect on the processing of textures (compare e.g., Fahrenfort, Scholte, & Lamme, 2007, who have used very brief presentation times, and Scholte et al, 2008, using long presentation times, yielding very similar results).…”

“…In fact, they even report that some participants in their study could reliably detect targets in the absence of visual awareness, resembling a blindsight-like condition. We propose that the poor overall quality of visual information may have triggered the perceptual decision network to start accumulating evidence as soon as sensory information became available, instead of waiting for a more precise representation that does not become available as a result of masking (Fahrenfort et al, 2007;Lamme, Zipser, & Spekreijse, 2002).…”

Dissociable Brain Mechanisms Underlying the Conscious and Unconscious Control of Behavior

“…Each time information reaches a successive stage in this hierarchy, this higher level area also starts to sent information back to lower level areas through feedback connections. Single-cell recordings in monkeys (Super, Spekreijse, & Lamme, 2001) and TMS (Pascual-Leone & Walsh, 2001), fMRI (Haynes, Driver, & Rees, 2005), and EEG (Fahrenfort, Scholte, & Lamme, 2007) experiments in humans have revealed that the feedforward sweep probably remains unconscious, whereas recurrent interactions trigger awareness of a stimulus (for reviews, see Dehaene et al, 2006;Lamme, 2006). Interestingly, masking probably disrupts feedback activations but leaves feedforward activations relatively intact (Del Cul, Baillet, & Dehaene, 2007;Fahrenfort et al, 2007;Lamme, Zipser, & Spekreijse, 2002).…”

Processing of natural images is feedforward: A simple behavioral test

“…Simulation studies of rapid stimulus classification in artificial neuronal networks indeed have suggested that most of the stimulusrelevant information could be extracted from the temporal distribution of the very first spikes in the feedforward wavefront (Serre, Oliva, & Poggio, 2007;VanRullen, Delorme, & Thorpe, 2001;VanRullen, Gautrais, Delorme, & Thorpe, 1998). The issue of feedforward versus recurrent processing is theoretically interesting because many authors have assumed that feedforward processing alone is insufficient to generate visual awareness and that a stimulus must be processed recurrently to become consciously accessible (DiLollo, Enns, & Rensink, 2000;Fahrenfort, Scholte, & Lamme, 2007;Lamme, 2002;Lamme, Rodriguez-Rodriguez, & Spekreijse, 1999;Lamme & Roelfsema, 2000;Lamme, Zipser, & Spekreijse, 2002;Pascual-Leone & Walsh, 2001;Ro, Breitmeyer, Burton, Singhal, & Lane, 2003;Roelfsema, Tolboom, & Khayat, 2007;Tong, 2003Tong, ). et al, 2006Schmidt & Seydell, 2008;Vath & Schmidt, 2007)-a variant of the response priming paradigm (Dehaene et al, 1998;Eimer & Schlaghecken, 1998;Neumann & Klotz, 1994;Schmidt, 2002;Verleger, Jaśkowski, Aydemir, van der Lubbe, & Groen, 2004;Vorberg, Mattler, Heinecke, Schmidt, & Schwarzbach, 2003)-and has also been confirmed in the time course of lateralized readiness potentials (Vath & Schmidt, 2007).…”