Marjan Persuh

Unconscious priming requires early visual cortex at specific temporal phases of processing

Although examples of unconscious shape priming have been well documented, whether such priming requires early visual cortex (V1/V2) has not been established. In the current study, we used TMS of V1/V2 at varying temporal intervals to suppress the visibility of preceding shape primes while the interval between primes and targets was kept constant. Our results show that, although conscious perception requires V1/V2, unconscious priming can occur without V1/V2 at an intermediate temporal interval but not at early (5–25 msec) or later (65–125 msec) stages of processing. Because the later time window of unconscious priming suppression has been proposed to interfere with feedback processing, our results further suggest that feedback processing is also essential for unconscious priming and may not be a sufficient condition for conscious vision.

Context-dependent brightness priming occurs without visual awareness

Our visual systems account for stimulus context in brightness perception, but whether such adjustments occur for stimuli that we are unaware of has not been established. We therefore assessed whether stimulus context influences brightness processing by measuring unconscious priming with metacontrast masking. When a middle-gray disk was presented on a darker (or brighter) background, such that it could be consciously perceived as brighter (or darker) via simultaneous brightness contrast (SBC), reaction times were significantly faster to a bright (or dark) annulus than to a dark (or bright) annulus. We further show that context-dependent brightness priming does not correlate with visibility using an objective measure of awareness (Experiment 1) and that context-dependent, but not context-independent brightness priming, occurs equally strongly for stimuli below or above the subjective threshold for awareness (Experiment 2). These results suggest that SBC occurs at early levels of visual input and is not influenced by conscious perception.

Saliency affects feedforward more than feedback processing in early visual cortex.

Early visual cortex activity is influenced by both bottom-up and top-down factors. To investigate the influences of bottom-up (saliency) and top-down (task) factors on different stages of visual processing, we used transcranial magnetic stimulation (TMS) of areas V1/V2 to induce visual suppression at varying temporal intervals. Subjects were asked to detect and discriminate the color or the orientation of briefly-presented small lines that varied on color saliency based on color contrast with the surround. Regardless of task, color saliency modulated the magnitude of TMS-induced visual suppression, especially at earlier temporal processing intervals that reflect the feedforward stage of visual processing in V1/V2. In a second experiment we found that our color saliency effects were also influenced by an inherent advantage of the color red relative to other hues and that color discrimination difficulty did not affect visual suppression. These results support the notion that early visual processing is stimulus driven and that feedforward and feedback processing encode different types of information about visual scenes. They further suggest that certain hues can be prioritized over others within our visual systems by being more robustly represented during early temporal processing intervals.

Perceptual overloading reveals illusory contour perception without awareness of the inducers

Unconscious perception is frequently examined by restricting visual input (e.g., using short stimulus durations followed by masking) to prevent that information from entering visual awareness. Failures to demonstrate perception without awareness may thus be a consequence of this restricted input rather than of limitations in unconscious perception. Here, we demonstrate a novel method that circumvents these significant drawbacks inherent in other methods. Using this new perceptual overloading technique (POT), in which stimuli are repeatedly presented in alternation with a stream of variable masks, we demonstrate illusory contour perception and modal completion even when subjects are completely unaware of the inducing elements. In addition to demonstrating a powerful new method to study consciousness by effectively gating robust visual input from visual awareness, we show that more complex contextual effects, previously considered to be a privilege only of conscious vision, can occur without awareness.

The Fata Morgana of Unconscious Perception

The human nervous system contains numerous visual pathways, suggesting that visual information is represented differently for different purposes. For example, an influential view holds that two separate but interacting processing streams result in vision for action and vision for perception (Milner and Goodale, 1995). Although much evidence in support of this view comes from patient studies (Milner et al., 1991), studies with healthy participants suggest that processing of visual information for perception and action (e.g., eye movements) can be dissociated (Spering et al., 2011). Most studies of perception use stimuli that are consciously perceived; however, the possibility of unconscious perception continues to intrigue the research community. In this paper I critically examine a large body of evidence for unconscious perception in healthy human participants obtained through a variety of methods that render stimuli invisible (Kim and Blake, 2005; Breitmeyer, 2015). Most researchers in the field have not been convinced by few authors skeptical about unconscious perception (Dulany, 1997; Holender and Duscherer, 2004; Peters et al., 2017). Here I question evidence for unconscious perception based on some general principles of studying mechanisms of biological pathways. I do not address studies on patients with disorders of awareness such as blindsight, neglect, and visual agnosias. It is difficult to establish the absence of phenomenology from patients’ reports because of response bias. Additionally, there is always a possibility that brain damage affects patients’ reports of conscious perception. Furthermore, some recent studies cast doubt on the absence of phenomenology in these patients. For example, a recent study that used a graded measure of awareness, suggests that stimuli in blindsight patients might in fact be weakly consciously perceived (Mazzi et al., 2016). Unconscious perception has been purportedly demonstrated in a number of studies (for excellent reviews see Kouider and Dehaene, 2007; Faivre et al., 2014; Sterzer et al., 2014; Yang et al., 2014). These studies have been plagued with methodological problems that still linger and repeatedly cast doubt on the validity of the phenomenon itself. Here, I suggest that we should focus on a different question. Sidestepping for the moment issues with measuring awareness, what can we say about unconscious perception?