Effects of voluntary attention on structured afterimages

Growing evidence for separate neural mechanisms for attention and consciousness

Our conscious experience of the world seems to go in lockstep with our attentional focus: We tend to see, hear, taste, and feel what we attend to, and vice versa. This tight coupling between attention and consciousness has given rise to the idea that these two phenomena are indivisible. In the late 1950s, the honoree of this special issue, Charles Eriksen, was among a small group of early pioneers that sought to investigate whether a transient increase in overall level of attention (alertness) in response to a noxious stimulus can be decoupled from conscious perception using experimental techniques. Recent years saw a similar debate regarding whether attention and consciousness are two dissociable processes. Initial evidence that attention and consciousness are two separate processes primarily rested on behavioral data. However, the past couple of years witnessed an explosion of studies aimed at testing this conjecture using neuroscientific techniques. Here we provide an overview of these and related empirical studies on the distinction between the neuronal correlates of attention and consciousness, and detail how advancements in theory and technology can bring about a more detailed understanding of the two. We argue that the most promising approach will combine ever-evolving neurophysiological and interventionist tools with quantitative, empirically testable theories of consciousness that are grounded in a mathematically formalized understanding of phenomenology.

The SSVEP tracks attention, not consciousness, during perceptual filling-in

Research on the neural basis of conscious perception has almost exclusively shown that becoming aware of a stimulus leads to increased neural responses. By designing a novel form of perceptual filling-in (PFI) overlaid with a dynamic texture display, we frequency-tagged multiple disappearing targets as well as their surroundings. We show that in a PFI paradigm, the disappearance of a stimulus and subjective invisibility is associated with increases in neural activity, as measured with steady-state visually evoked potentials (SSVEPs), in electroencephalography (EEG). We also find that this increase correlates with alpha-band activity, a well-established neural measure of attention. These findings cast doubt on the direct relationship previously reported between the strength of neural activity and conscious perception, at least when measured with current tools, such as the SSVEP. Instead, we conclude that SSVEP strength more closely measures changes in attention.

Concentrative (Sahaj Samadhi) meditation training and visual awareness: An fMRI study on color afterimages

All of us consciously experience the world around us through our sensory modalities. Empirical studies on the relationship between attention and awareness have shown that attention does influence perceptual experience or appearance in addition to better performance in perceptual tasks. The practice of meditation also changes perceptual experience in addition to better perceptual performance. For example, a study with Sahaj Samadhi meditators utilizing negative color afterimages had shown that concentrative meditation influences visual experience. However the brain regions that are modified by meditation practice leading to such changes in visual experience or awareness are still not known. Here using negative color afterimages in a functional MRI study, we investigated the brain mechanisms underlying the changes in visual awareness as a function of attentional enhancement achieved through long-term concentrative meditation practice. We found increased activity in right lateralized inferior occipital and inferior frontal cortex, which suggests the importance of attentional control in modulating visual awareness. The results of this study indicate that the link between attention and conscious experience is possibly changed by meditation practices.

Size Constancy is Preserved but Afterimages are Prolonged in Typical Individuals with Higher Degrees of Self-Reported Autistic Traits

Deficits in perceptual constancies from early infancy have been proposed to contribute to autism and exacerbate its symptoms (Hellendoorn et al., Frontiers in Psychology 6:1–16, 2015). Here, we examined size constancy in adults from the general population (N = 106) with different levels of self-reported autistic traits using an approach based on negative afterimages. The afterimage strength, as indexed by duration and vividness, was also quantified. In opposition to the Hellendoorn and colleagues’ model, we were unable to demonstrate any kind of relationship between abilities in size constancy and autistic traits. However, our results demonstrated that individuals with higher degrees of autistic traits experienced more persistent afterimages. We discuss possible retinal and post-retinal explanations for prolonged afterimages in people with higher levels of autistic traits.

Cortical mechanisms for afterimage formation: evidence from interocular grouping

Whether the retinal process alone or retinal and cortical processes jointly determine afterimage (AI) formation has long been debated. Based on the retinal rebound responses, recent work proposes that afterimage signals are exclusively generated in the retina, although later modified by cortical mechanisms. We tested this notion with the method of "indirect proof". Each eye was presented with a 2-by-2 checkerboard of horizontal and vertical grating patches. Each corresponding patch of the two checkerboards was perpendicular to each other, which produces binocular rivalry, and can generate percepts ranging from complete interocular grouping to either monocular pattern. The monocular percepts became more frequent with higher contrast. Due to adaptation, the visual system is less sensitive during the AIs than during the inductions with AI-similar contrast. If the retina is the only origin of AIs, comparable contrast appearance would require stronger retinal signals in the AIs than in the inductions, thus leading to more frequent monocular percepts in the AIs than in the inductions. Surprisingly, subjects saw the fully coherent stripes significantly more often in AIs. Our results thus contradict the retinal generation notion, and suggest that in addition to the retina, cortex is directly involved in the generation of AI signals.

Attention as a process of selection, perception as a process of representation, and phenomenal experience as the resulting process of perception being modulated by a dedicated consciousness mechanism

Equivalence of attention and consciousness is disputed and necessity of attentional effects for conscious experience has become questioned. However, the conceptual landscape and interpretations of empirical evidence as related to this issue have remained controversial. Here I present some conceptual distinctions and research strategies potentially useful for moving forward when tackling this issue. Specifically, it is argued that we should carefully differentiate between pre-conscious processes and the processes resulting in phenomenal experience, move the emphasis from studying the effects of attention on the modality-specific and feature-specific perception to studying attentional effects on panmodal universal attributes of whatever conscious experience may be the case, and acknowledge that there is a specialized mechanism for leading to conscious experience of the pre-consciously represented contents autonomous from the mechanisms of perception, attention, memory, and cognitive control.

Spatially localized motion aftereffect disappears faster from awareness when selectively attended to according to its direction

In searching for the target-afterimage patch among spatially separate alternatives of color-afterimages the target fades from awareness before its competitors (Bachmann, T., & Murd, C. (2010). Covert spatial attention in search for the location of a color-afterimage patch speeds up its decay from awareness: Introducing a method useful for the study of neural correlates of visual awareness. Vision Research 50, 1048-1053). In an analogous study presented here we show that a similar effect is obtained when a target spatial location specified according to the direction of motion aftereffect within it is searched by covert top-down attention. The adverse effect of selective attention on the duration of awareness of sensory qualiae known earlier to be present for color and periodic spatial contrast is extended also to sensory channels carrying motion information.

Consciousness and attention: on sufficiency and necessity

Recent research has slowly corroded a belief that selective attention and consciousness are so tightly entangled that they cannot be individually examined. In this review, we summarize psychophysical and neurophysiological evidence for a dissociation between top-down attention and consciousness. The evidence includes recent findings that show subjects can attend to perceptually invisible objects. More contentious is the finding that subjects can become conscious of an isolated object, or the gist of the scene in the near absence of top-down attention; we critically re-examine the possibility of "complete" absence of top-down attention. We also cover the recent flurry of studies that utilized independent manipulation of attention and consciousness. These studies have shown paradoxical effects of attention, including examples where top-down attention and consciousness have opposing effects, leading us to strengthen and revise our previous views. Neuroimaging studies with EEG, MEG, and fMRI are uncovering the distinct neuronal correlates of selective attention and consciousness in dissociative paradigms. These findings point to a functional dissociation: attention as analyzer and consciousness as synthesizer. Separating the effects of selective visual attention from those of visual consciousness is of paramount importance to untangle the neural substrates of consciousness from those for attention.

A dissociation of attention and awareness in phase-sensitive but not phase-insensitive visual channels

The elements most vivid in our conscious awareness are the ones to which we direct our attention. Scientific study confirms the impression of a close bond between selective attention and visual awareness, yet the nature of this association remains elusive. Using visual afterimages as an index, we investigate neural processing of stimuli as they enter awareness and as they become the object of attention. We find evidence of response enhancement accompanying both attention and awareness, both in the phase-sensitive neural channels characteristic of early processing stages and in the phase-insensitive channels typical of higher cortical areas. The effects of attention and awareness on phase-insensitive responses are positively correlated, but in the same experiments, we observe no correlation between the effects on phase-sensitive responses. This indicates independent signatures of attention and awareness in early visual areas yet a convergence of their effects at more advanced processing stages.

Opposing effects of attention and consciousness on afterimages

The brain's ability to handle sensory information is influenced by both selective attention and consciousness. There is no consensus on the exact relationship between these two processes and whether they are distinct. So far, no experiment has simultaneously manipulated both. We carried out a full factorial 2 x 2 study of the simultaneous influences of attention and consciousness (as assayed by visibility) on perception, correcting for possible concurrent changes in attention and consciousness. We investigated the duration of afterimages for all four combinations of high versus low attention and visible versus invisible. We show that selective attention and visual consciousness have opposite effects: paying attention to the grating decreases the duration of its afterimage, whereas consciously seeing the grating increases the afterimage duration. These findings provide clear evidence for distinctive influences of selective attention and consciousness on visual perception.

Types of attention matter for awareness: a study with color afterimages

It has been argued that attention and awareness might oppose each other given that attending to an adapting stimulus weakens its afterimage. We argue instead that the type of attention guided by spatial extent and perceptual levels is critical and might result in differences in awareness using afterimages. Participants performed a central task with small, large, local, or global letters and a blue square as an adapting stimulus in three experiments and indicated the onset and offset of the afterimage. We found that increases in the spatial spread of attention resulted in the decrease of afterimage duration. In terms of levels of processing, global processing produced larger afterimage durations with stimuli controlled for spatial extent. The results suggest that focused or distributed attention produce different effects on awareness, possibly through their differential interactions with polarity dependent and independent processes involved in the formation of color afterimages.

Focused and distributed attention

Recent studies on attention have emphasized distinctions between focused and distributed attention. Distributed attention has been shown to play a key role in obtaining statistical information or processing global aspects of a scene. In addition to differences in information processing, focused and distributed attention differ in terms of the way they interact with emotions. We review findings that indicate close relationship between focused attention and sad emotions as well as distributed attention and happy emotions. Given the potentially close relationship between attention and consciousness, these two types of attention may differ in terms of processes leading to awareness. We review different positions on the relationship between attention and consciousness and arguments for the existence of opposition between attention and awareness that have been made based on findings with color afterimages. We discuss our studies on attention and afterimages indicating the close linkage between different types of attention and awareness as indicated by differences in the strength of afterimages based on the type of attention deployed.

Interdependence of attention and consciousness

Research on attention has been closely linked with possible advances in the study of consciousness. Various theories and models have been proposed for attention in the past 50 years. Behavioural, computational, and neuroscientific approaches have been successful in improving our understanding of attentional processes. Given the current status of attention research, what can we say about the relationship between attention and consciousness? This paper discusses the possible relationships between attention and consciousness. Findings from cognitive science and neuroscience relevant to the elucidation of this relationship are discussed. Recent findings from phenomena that have a bearing on this relationship such as inattentional amnesia, change blindness, attentional blink, perceptual stabilization, and afterimages are described. The implications of the results of these phenomena for attention and awareness are also discussed. It is proposed that top-down attention is not a unitary phenomena and such a characterization may provide a way to interpret some of the results from these findings.

Troxler effect with dichoptic stimulus presentations: evidence for binocular inhibitory summation and interocular suppression

Whether the Troxler effect (TE) has to do with interocular suppression and/or summation was studied with dichoptically matched (binocular or dioptic) and unmatched (monocular) stimulus presentations. Perceptual disappearance was found to occur more slowly under the binocular condition (mean=14.2s) than the monocular condition (mean=8.4s), but much faster than predicted by probability summation of the experimentally obtained latencies and durations of the TE in the monocular conditions (>27 s), suggesting a binocular inhibitory summation, the opposite of the binocular summation found with detection and contrast matching tasks [(Blake, R., & Fox, R. (1973). The psychological inquiry into binocular summation. Perception and Psychophysics, 14, 161-185; Blake, R., Sloane, M., & Fox, R. (1981). Further developments in binocular summation. Perception &Psychophysics 30, 266-276.)]. In addition, Ss with poorer stereoacuity took longer to see the disappearance in the monocular condition, and showed a larger disparity between the TEs from the two monocular conditions, suggesting a contribution of interocular suppression to the TE.

Attentional effects on afterimages: Theory and data

We explore attentional effects on afterimages in the framework of the FACADE model of visual perception. We first show that the FACADE model can account for the experimental findings of Suzuki and Grabowecky [Suzuki, S., & Grabowecky, M. (2003). Attention during adaptation weakens negative afterimages. Journal of Experimental Psychology: Human Perception and Performance 29, 793-807] that afterimages are weaker when the inducing stimulus is attended. We then analyze the model's behavior with attentional influences on a two-stimulus afterimage studied by Francis and Rothmayer [Francis, G., & Rothmayer, M. (2003). Interactions of afterimages for orientation and color: Experimental data and model simulations. Perception & Psychophysics 65, 508-522]. The model predicts that attentional focus directed towards the first stimulus has little effect on afterimage strength. In contrast, the model predicts that attentional focus on the second stimulus should increase the strength of the afterimage compared to when attention is focused elsewhere. Moreover, the model predicts that the attentional effects on the second stimulus should vary with time after offset of the second inducing stimulus. All of the model predictions are validated in an experiment. The model and experimental results extend and clarify previous explanations of attentional effects and afterimages.

Attention and consciousness: two distinct brain processes

The close relationship between attention and consciousness has led many scholars to conflate these processes. This article summarizes psychophysical evidence, arguing that top-down attention and consciousness are distinct phenomena that need not occur together and that can be manipulated using distinct paradigms. Subjects can become conscious of an isolated object or the gist of a scene despite the near absence of top-down attention; conversely, subjects can attend to perceptually invisible objects. Furthermore, top-down attention and consciousness can have opposing effects. Such dissociations are easier to understand when the different functions of these two processes are considered. Untangling their tight relationship is necessary for the scientific elucidation of consciousness and its material substrate.

Continuous flash suppression reduces negative afterimages

Illusions that produce perceptual suppression despite constant retinal input are used to manipulate visual consciousness. Here we report on a powerful variant of existing techniques, continuous flash suppression. Distinct images flashed successively at approximately 10 Hz into one eye reliably suppress an image presented to the other eye. The duration of perceptual suppression is at least ten times greater than that produced by binocular rivalry. Using this tool we show that the strength of the negative afterimage of an adaptor was reduced by half when it was perceptually suppressed by input from the other eye. The more completely the adaptor was suppressed, the more strongly the afterimage intensity was reduced. Paradoxically, trial-to-trial visibility of the adaptor did not correlate with the degree of reduction. Our results imply that formation of afterimages involves neuronal structures that access input from both eyes but that do not correspond directly to the neuronal correlates of perceptual awareness.

Generalized flash suppression of salient visual targets

A pattern of light striking the retina of an alert observer is normally readily perceived. While a handful of conditions exist in which even salient visual stimuli can be rendered invisible, the mechanisms underlying such suppression remain poorly understood. Here, we describe experiments using a novel stimulation sequence that gives rise to the sudden and reliable subjective disappearance of a wide range of visual patterns. We found that a parafoveal target immediately vanished from perception following the abrupt onset of a surrounding texture. The probability of disappearance was influenced by the ocular configuration of the target and surround, as well as their spatial separation. In addition, suppression was critically dependent upon several hundred milliseconds of stimulus-specific adaptation. These findings demonstrate that the all-or-none disappearance of a salient visual target, which is reminiscent of a high-level selection process, is inextricably linked to topographic stimulus representations, presumably in the early visual cortex.