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

Neural mechanisms underlying conscious and unconscious vision. Evidence from event-related potentials and transcranial magnetic stimulation

Vision affords us with the ability to consciously see, and use this information in our behavior. While research has produced a detailed account of the function of the visual system, the neural processes that underlie conscious vision are still debated. One of the aims of the present thesis was to examine the time-course of the neuroelectrical processes that correlate with conscious vision. The second aim was to study the neural basis of unconscious vision, that is, situations where a stimulus that is not consciously perceived nevertheless influences behavior. According to current prevalent models of conscious vision, the activation of visual cortical areas is not, as such, sufficient for consciousness to emerge, although it might be sufficient for unconscious vision. Conscious vision is assumed to require reciprocal communication between cortical areas, but views differ substantially on the extent of this recurrent communication. Visual consciousness has been proposed to emerge from recurrent neural interactions within the visual system, while other models claim that more widespread cortical activation is needed for consciousness. Studies I-III compared models of conscious vision by studying event-related potentials (ERP). ERPs represent the brain’s average electrical response to stimulation. The results support the model that associates conscious vision with activity localized in the ventral visual cortex. The timing of this activity corresponds to an intermediate stage in visual processing. Earlier stages of visual processing may influence what becomes conscious, although these processes do not directly enable visual consciousness. Late processing stages, when more widespread cortical areas are activated, reflect the access to and manipulation of contents of consciousness. Studies IV and V concentrated on unconscious vision. By using transcranial magnetic stimulation (TMS) we show that when early visual cortical processing is disturbed so that subjects fail to consciously perceive visual stimuli, they may nevertheless guess (above chance-level) the location where the visual stimuli were presented. However, the results also suggest that in a similar situation, early visual cortex is necessary for both conscious and unconscious perception of chromatic information (i.e. color). Chromatic information that remains unconscious may influence behavioral responses when activity in visual cortex is not disturbed by TMS. Our results support the view that early stimulus-driven (feedforward) activation may be sufficient for unconscious processing. In conclusion, the results of this thesis support the view that conscious vision is enabled by a series of processing stages. The processes that most closely correlate with conscious vision take place in the ventral visual cortex ~200 ms after stimulus presentation, although preceding time-periods and contributions from other cortical areas such as the parietal cortex are also indispensable. Unconscious vision relies on intact early visual activation, although the location of visual stimulus may be unconsciously resolved even when activity in the early visual cortex is interfered with.Siirretty Doriast

The role of primary visual cortex in unconscious visual processing

Lesion in primary visual cortex (V1) causes blindness in the visual field that is processed by the damaged area. Some patients with V1 lesion can unconsciously process the stimuli presented in their blind visual field. This kind of processing can be observed, for example, using a paradigm in which a patient is required to make a guess about the visual stimulus that he or she reports not seeing. If the accuracy of the responses is better than chance, it can be interpreted as unconscious processing, which is a measurable effect on behavior without conscious perception. This phenomenon of unconscious processing without V1 is commonly referred to as blindsight. The findings of unconscious processing without V1 in blindsight patients cannot be straightforwardly generalized to neurologically healthy people. Neural plasticity changes these patients’ brains; therefore, blindsight could be explained by these neural changes. Transcranial magnetic stimulation (TMS) is a method used to study the function of a cortical area in neurologically healthy people. With TMS, it is possible to interfere with the normal functioning of the brain. TMS of V1 can suppress the conscious perception of a visual stimulus. In this thesis, I studied whether the unconscious processing of visual stimuli is possible in neurologically healthy participants when the conscious perception of the stimulus is suppressed by TMS of V1. I found that both conscious and unconscious processing of visual information depend on V1 in neurologically healthy participants. Unconscious processing was observed only with the simplest task, in which participants responded to stimulus appearance. In this task, unconscious processing was observed only when the early activation of V1 was intact. The unconscious processing of chromaticity and motion was not observed when the stimulus was suppressed by TMS of V1. Therefore, I conclude that unconscious visual processing depends on V1 in neurologically healthy participants.Aivovaurio primaarilla näköaivokuorella (V1) aiheuttaa sokeuden siihen osaan näkökenttää, jonka prosessoinnista vaurioitunut aivokuoren osa vastaa. Jotkut tällaisista primaarin näköaivokuoren vauriosta johtuvasta sokeudesta kärsivistä potilaista pystyvät kuitenkin tiedostamattomasti prosessoimaan visuaalisia ärsykkeitä, jotka on esitetty sokealle näkökentälle. Tällaista prosessointia voidaan mitata esimerkiksi pyytämällä koehenkilö esittämään paras arvaus visuaalisen ärsykkeestä, jota hän ei raportoi nähneensä. Mikäli vastaustarkkuus on arvaustodennäköisyyttä parempi, voidaan todeta, että visuaalinen ärsyke vaikutti henkilön käyttäytymiseen tiedostamattomalla tasolla. Tästä ilmiöstä käytetään yleisesti nimeä sokeanäkö. Neurologisilla potilailla tehdyistä havainnoista ei kuitenkaan voida suoraan tehdä päätelmiä, jotka yleistyisivät koskemaan kaikkia ihmisiä. Potilaiden aivoissa tapahtuu plastisiteetin vuoksi muutoksia, jotka voivat selittää näitä säästyneitä visuaalisia kykyjä. Transkraniaalisella magneettistimulaatiolla (TMS) voidaan tutkia neurologisesti terveiden koehenkilöiden aivojen osien funktioita, sillä TMS häiritsee hetkellisesti aivojen normaalia viestinvälitystä. Primaarille näköaivokuorelle kohdistetut TMS-pulssit häiritsevät tietoista näkemistä. Tässä tutkielmassa selvitin, onko tiedostamaton prosessointi mahdollista, kun ärsykkeen tietoista havaintoa on häiritty primaarin näköaivokuoren stimulaatiolla. Tutkimuksissamme havaitsimme, että sekä tietoinen että tiedostamaton näköinformaation prosessointi vaativat primaarin näköaivokuoren toimintaa neurologisesti terveillä. Tiedostamatonta näköinformaation prosessointia havaittiin ainoastaan yksinkertaisimmassa tehtävässä, jossa koehenkilö reagoi ärsykkeen ilmestymiseen. Silloinkin ainoastaan, kun aikaista prosessointia primaarilla näköaivokuorella ei häiritty. Liikkeen ja värin prosessointia mittaavissa kokeissa tiedostamatonta prosessointia V1-stimulaation aikana ei havaittu. Tiedostamaton näköinformaation prosessointi riippuu siis primaarin näköaivokuoren toiminnasta neurologisesti terveillä koehenkilöillä

Visual Processing in Rapid-Chase Systems: Image Processing, Attention, and Awareness

Visual stimuli can be classified so rapidly that their analysis may be based on a single sweep of feedforward processing through the visuomotor system. Behavioral criteria for feedforward processing can be evaluated in response priming tasks where speeded pointing or keypress responses are performed toward target stimuli which are preceded by prime stimuli. We apply this method to several classes of complex stimuli. (1) When participants classify natural images into animals or non-animals, the time course of their pointing responses indicates that prime and target signals remain strictly sequential throughout all processing stages, meeting stringent behavioral criteria for feedforward processing (rapid-chase criteria). (2) Such priming effects are boosted by selective visual attention for positions, shapes, and colors, in a way consistent with bottom-up enhancement of visuomotor processing, even when primes cannot be consciously identified. (3) Speeded processing of phobic images is observed in participants specifically fearful of spiders or snakes, suggesting enhancement of feedforward processing by long-term perceptual learning. (4) When the perceived brightness of primes in complex displays is altered by means of illumination or transparency illusions, priming effects in speeded keypress responses can systematically contradict subjective brightness judgments, such that one prime appears brighter than the other but activates motor responses as if it was darker. We propose that response priming captures the output of the first feedforward pass of visual signals through the visuomotor system, and that this output lacks some characteristic features of more elaborate, recurrent processing. This way, visuomotor measures may become dissociated from several aspects of conscious vision. We argue that “fast” visuomotor measures predominantly driven by feedforward processing should supplement “slow” psychophysical measures predominantly based on visual awareness

Change blindness: eradication of gestalt strategies

Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

Top-down contingent feature-specific orienting with and without awareness of the visual input

In the present article, the role of endogenous feature-specific orienting for conscious and unconscious vision is reviewed. We start with an overview of orienting. We proceed with a review of masking research, and the definition of the criteria of experimental protocols that demonstrate endogenous and exogenous orienting, respectively. Against this background of criteria, we assess studies of unconscious orienting and come to the conclusion that so far studies of unconscious orienting demonstrated endogenous feature-specific orienting. The review closes with a discussion of the role of unconscious orienting in action control

Unconscious priming of "freely" chosen voluntary actions: Behavioral and electrophysiological evidence

In the course of development organisms learn to associate their actions with the effects these actions have in the environment. Recent studies have shown that perceiving or anticipating action-effects automatically activates actions, which formerly have been experienced to cause these effects (Elsner & Hommel, 2001). Using subliminal priming paradigms and electrophysiological measures I investigated whether subliminally (i.e., not consciously perceivable) presented action-effects can automatically activate associated actions and if so, whether this response priming by action-effects can bias free-choice actions. Secondly I investigated whether action-effects with different emotional valences influence response selection differently. To address the first question three experiments were performed. Each experiment consisted of two experimental phases. The first phase, the acquisition-phase, was a learning phase were simple key-press actions were associated with simple visual stimuli (i.e., action-effects; diamond or square) that were contingent on the actions. Immediately after the acquisition-phase the test-phase followed, in which participants performed free-choice actions after the presentation of a Go-signal. In Experiments 2 and 3 a NoGo-signal indicating that responses had to be withheld could appear with the same likelihood as the Go-signal. Unknown to the participants, one of the former action-effects (diamond or square) was presented subliminally prior to each Go- and NoGo-signal to investigate the influence of unconscious action-effects on response selection. Taken together, the results of the test-phases provided strong evidence that even subliminally presented (i.e., unconscious) action-effects can automatically activate associated responses. The response priming by action-effects became evident in the lateralized readiness potential (LRP), an electrophysiological indicator of specific response activation processes. Under certain circumstanced this automatic response activation can bias free-choice actions although participants experienced the actions as freely chosen. In the test-phase of the first experiment more acquisition-phase-consistent than –inconsistent responses were chosen. If, for instance, a left key-press had been associated with a square during the acquisition-phase, the left key was chosen significantly more often after the subliminal presentation of a square in the test-phase. At least three factors seemed to influence which responses were chosen and executed: The strength of the priming effect, the complexity of the task (i.e., pure Go-blocks or intermixed Go/NoGo-blocks), and the elapsed time between the prime stimulus and the Go-signal. To address the second question simple key-press actions were linked to action-effects with different emotional valences (positive vs. negative pictures accompanied by high or low tones) during the acquisition-phase. In the subsequent test-phase, the effects-tones that had been associated with negative or positive pictures were presented and followed by a Go-signal, after which participants had to freely choose to press one of the two response keys. Results indicated that the anticipation of the emotional valence of an action-effect influenced free-choice action. Whereas the effect-tones induced a clear response bias (i.e., more acquisition-consistent than –inconsistent key-choices) if they had been associated with a positive emotional valence, this response bias was not reliable for action-effects associated with negative emotional features. In summary, the present results provide further proof for ideomotor theories of action control (James, 1890; Elsner & Hommel, 2001) which state that actions are automatically activated by anticipating their consequences

Overt orienting of spatial attention and corticospinal excitability during action observation are unrelated

Observing moving body parts can automatically activate topographically corresponding motor representations in the primary motor cortex (M1), the so-called direct matching. Novel neurophysiological findings from social contexts are nonetheless proving that this process is not automatic as previously thought. The motor system can flexibly shift from imitative to incongruent motor preparation, when requested by a social gesture. In the present study we aim to bring an increase in the literature by assessing whether and how diverting overt spatial attention might affect motor preparation in contexts requiring interactive responses from the onlooker. Experiment 1 shows that overt attention-although anchored to an observed biological movement-can be captured by a target object as soon as a social request for it becomes evident. Experiment 2 reveals that the appearance of a short-lasting red dot in the contralateral space can divert attention from the target, but not from the biological movement. Nevertheless, transcranial magnetic stimulation (TMS) over M1 combined with electromyography (EMG) recordings (Experiment 3) indicates that attentional interference reduces corticospinal excitability related to the observed movement, but not motor preparation for a complementary action on the target. This work provides evidence that social motor preparation is impermeable to attentional interference and that a double dissociation is present between overt orienting of spatial attention and neurophysiological markers of action observation

Induction of Kanizsa Contours Requires Awareness of the Inducing Context

It remains unknown to what extent the human visual system interprets information about complex scenes without conscious analysis. Here we used visual masking techniques to assess whether illusory contours (Kanizsa shapes) are perceived when the inducing context creating this illusion does not reach awareness. In the first experiment we tested perception directly by having participants discriminate the orientation of an illusory contour. In the second experiment, we exploited the fact that the presence of an illusory contour enhances performance on a spatial localization task. Moreover, in the latter experiment we also used a different masking method to rule out the effect of stimulus duration. Our results suggest that participants do not perceive illusory contours when they are unaware of the inducing context. This is consistent with theories of a multistage, recurrent process of perceptual integration. Our findings thus challenge some reports, including those from neurophysiological experiments in anaesthetized animals. Furthermore, we discuss the importance to test the presence of the phenomenal percept directly with appropriate methods

Demonstrating perception without visual awareness: Double dissociations between priming and masking

A double dissociation impressively demonstrates that visual perception and visual awareness can be independent of each other and do not have to rely on the same source of information (T. Schmidt & Vorberg, 2006). Traditionally, an indirect measure of stimulus processing and a direct measure of visual awareness are compared (dissociation paradigm or classic dissociation paradigm, Erdelyi, 1986; formally described by Reingold & Merikle, 1988; Merikle & Reingold, 1990; Reingold, 2004). If both measures exhibit opposite time courses, a double dissociation is demonstrated. One tool that is well suited to measure stimulus processing as fast visuomotor response activation is the response priming method (Klotz & Neumann, 1999; Klotz & Wolff, 1995; see also F. Schmidt et al., 2011; Vorberg et al., 2003). Typically, observers perform speeded responses to a target stimulus preceded by a prime stimulus, which can trigger the same motor response by sharing consistent features (e.g., shape) or different responses due to inconsistent features. While consistent features cause speeded motor responses, inconsistent trials can induce response conflicts and result in slowed responses. These response time differences describe the response priming effect (Klotz & Neumann, 1999; Klotz & Wolff, 1995; see also F. Schmidt et al., 2011; Vorberg et al., 2003). The theoretical background of this method forms the Rapid-Chase Theory (T. Schmidt et al., 2006, 2011; see also T. Schmidt, 2014), which assumes that priming is based on neuronal feedforward processing within the visuomotor system. Lamme and Roelfsema (2000; see also Lamme, 2010) claim that this feedforward processing does not generate visual awareness because neuronal feedback and recurrent processes are needed. Fascinatingly, while prime visibility can be manipulated by visual masking techniques (Breitmeyer & Öğmen, 2006), priming effects can still increase over time. Masking effects are used as a direct measure of prime awareness. Based on their time course, type-A and type-B masking functions are distinguished (Breitmeyer & Öğmen, 2006; see also Albrecht & Mattler, 2010, 2012, 2016). Type-A masking is most commonly shown with a typically increasing function over time. In contrast, type-B masking functions are rarely observed, which demonstrate a decreasing or u-shaped time course. This masking type is usually only found under metacontrast backward masking (Breitmeyer & Öğmen, 2006; see also Albrecht & Mattler, 2010, 2012, 2016). While priming effects are expected to increase over time by Rapid-Chase Theory (T. Schmidt et al., 2006, 2011; see also T. Schmidt, 2014), the masking effect can show an opposite trend with a decreasing or u-shaped type-B masking curve, forming a double dissociation. In empirical practice, double dissociations are a rarity, while historically simple dissociations have been the favored data pattern to demonstrate perception without awareness, despite suffering from statistical measurement problems (T. Schmidt & Vorberg, 2006). Motivated by this shortcoming, I aim to demonstrate that a double dissociation is the most powerful and convincing data pattern, which provides evidence that visual perception does not necessarily generate visual awareness, since both processes are based on different neuronal mechanisms. I investigated which experimental conditions allow for a double dissociation between priming and prime awareness. The first set of experiments demonstrated that a double-dissociated pattern between priming and masking can be induced artificially, and that the technique of induced dissociations is of general utility. The second set of experiments used two awareness measures (objective vs. subjective) and a response priming task in various combinations, resulting in different task settings (single-, dual-, triple tasks). The experiments revealed that some task types constitute an unfavorable experimental environment that can prevent a double dissociation from occurring naturally, especially when a pure feedforward processing of the stimuli seems to be disturbed. The present work provides further important findings. First, stimulus perception and stimulus awareness show a general dissociability in most of the participants, supporting the idea that different neuronal processes are responsible for this kind of data pattern. Second, any direct awareness measure (no matter whether objective or subjective) is highly observer-dependent, requiring the individual analysis at the level of single participants. Third, a deep analysis of priming effects at the micro level (e.g., checking for fast errors) can provide further insights regarding information processing of different visual stimuli (e.g., shape vs. color) and under changing experimental conditions (e.g. single- vs. triple tasks)