Invisibility and interpretation

Invisibility is often thought to occur because of the low-level limitations of the visual system. For example, it is often assumed that backward masking renders a target invisible because the visual system is simply too slow to resolve the target and the mask separately. Here, we propose an alternative explanation in which invisibility is a goal rather than a limitation and occurs naturally when making sense out of the plethora of incoming information. For example, we present evidence that (in)visibility of an element can strongly depend on how it groups with other elements. Changing grouping changes visibility. In addition, we will show that features often just appear to be invisible but are in fact visible in a way the experimenter is not aware of

Effects of a red background on magnocellular functioning in average and specifically disabled readers

AbstractTwo experiments were conducted using metacontrast masking to examine responses in the magno system of adults, average reading adolescents and adolescents with specific reading disability. In Experiment 1 the effects of a red background field on the metacontrast functions of adult subjects were investigated. Results showed that a red, compared to a photometrically matched white background field, significantly attenuated metacontrast magnitude, supporting the interpretation of metacontrast as due to magno system suppression of parvo system responses. The finding of a red background effect was replicated in Experiment 2 with the two adolescent groups. The metacontrast functions of the adolescent groups also differed significantly, with those with specific reading disability exhibiting weaker metacontrast than the average readers. This result is consistent with a deficit in the magno system of individuals with specific reading disability and indicates the continuation of the deficit beyond childhood

Combining simultaneous with temporal masking

Simultaneous and temporal masking are two frequently used techniques in psychology and vision science. Although there are many studies and theories related to each masking technique, there are no systematic investigations of their mutual relationship, even though both techniques are often applied together. Here, the authors show that temporal masking can both undo and enhance the deteriorating effects of simultaneous masking depending on the stimulus onset asynchrony between the simultaneous and temporal masks. For the task and stimuli used in this study, temporal masking was largely unaffected by the properties of the simultaneous mask. In contrast, simultaneous masking seems to depend strongly on spatial grouping and was strongly affected by the properties of the temporal mask. These findings help to identify the nature of both temporal and simultaneous masking and promote understanding of the role of spatial and temporal grouping in visual perception

Spatial grouping determines temporal integration

To make sense out of a continuously changing visual world, people need to integrate features across space and time. Despite more than a century of research, the mechanisms of features integration are still a matter of debate. To examine how temporal and spatial integration interact, the authors measured the amount of temporal fusion (a measure of temporal integration) for different spatial layouts. They found that spatial grouping by proximity and similarity can completely block temporal integration. Computer simulations with a simple neural network capture these findings very well, suggesting that the proposed spatial grouping operations may occur already at an early stage of visual information processing

Saccades influence the visibility of targets in rapid stimulus sequences: the roles of mislocalization, retinal distance and remapping

Briefly presented targets around the time of a saccade are mislocalized towards the saccadic landing point. This has been taken as evidence for a remapping mechanism that accompanies each eye movement, helping maintain visual stability across large retinal shifts. Previous studies have shown that spatial mislocalization is greatly diminished when trains of brief stimuli are presented at a high frequency rate, which might help to explain why mislocalization is rarely perceived in everyday viewing. Studies in the laboratory have shown that mislocalization can reduce metacontrast masking by causing target stimuli in a masking sequence to be perceived as shifted in space towards the saccadic target and thus more easily discriminated. We investigated the influence of saccades on target discrimination when target and masks were presented in a rapid serial visual presentation (RSVP), as well as with forward masking and with backward masking. In a series of experiments, we found that performance was influenced by the retinal displacement caused by the saccade itself but that an additional component of un-masking occurred even when the retinal location of target and mask was matched. These results speak in favor of a remapping mechanism that begins before the eyes start moving and continues well beyond saccadic termination

Grouping based feature attribution in metacontrast masking

The visibility of a target can be strongly suppressed by metacontrast masking. Still, some features of the target can be perceived within the mask. Usually, these rare cases of feature mis-localizations are assumed to reflect errors of the visual system. To the contrary, I will show that feature "mis-localizations" in metacontrast masking follow rules of motion grouping and, hence, should be viewed as part of a systematic feature attribution process

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

Inter-Individual Differences as Instrument to Investigate the Mechanisms in Metacontrast Masking

In der Metakontrastmaskierung wird die Sichtbarkeit des ersten Stimulus (Target) durch das Auftreten eines zweiten Stimulus (Maske) reduziert. Zwei Maskierungsfunktionen (MF) treten hauptsächlich auf: Typ A, wenn die Sichtbarkeit mit ansteigender SOA zumimmt, und Typ-B, wenn die Sichtbarkeit in kurzer und langer SOA hoch ist und auf ein Minimum in mittlerer SOA abfällt. In fünf Studien wurde systematisch untersucht welchen Einfluss experimentelle Parameter auf das Auftreten der MF haben. Je länger die Maske im Verhältnis zum Target präsentiert wird, desto weiter verschiebt sich das Minimum der MF hin zu kürzerer SOA und desto mehr ähnelt sie einer Typ-A-MF (monoton ansteigend). Die Maskierung ist in kleinen Stimuli stärker als in großen Stimuli, sowohl im Zentrum als auch in der Peripherie. Bei beiden Stimulusgrößen findet bei der kürzesten SOA die stärkste Maskierung statt. MF unterscheiden sich nicht, wenn sich die Vorhersagbarkeit der Präsentationsorte der Stimuli unterscheidet. Scheinbewegungen werden in langen SOAs eher wahrgenommen als in kurzen SOAs. Darüber hinaus wurden inter-individuelle Unterschiede gefunden, die Einblicke in die Mechanismen erlauben, die in der Metakontrastmaskierung beteiligt sind. Einige Versuchspersonen zeigen eine Typ-A-MF und berichten Scheinbewegungen in der Abfolge der Stimuli wahrzunehmen, hauptsächlich in langen SOAs. Andere zeigen eine Typ-B-MF und berichten negative Nachbilder in Form des Tagets im Inneren der Maske wahrzunehmen, hauptsächlich in kurzen SOAs. Typ-A- und Typ-B-Versuchspersonen unterscheiden sich in ihrer Top-Down-Verarbeitung der Stimuli, nicht aber in ihrer Bottom-Up-Verarbeitung. Zwei Prozesse stellten sich heraus, die in der Verarbeitung der Metakontraststimuli beteiligt sein könnten. Nach der Integrations-Segregations-Theorie steht die getrennte (segregierte) Wahrnehmung zweier Stimuli, die Scheinbewegungen ermöglicht, mit dem Segregationsprozess in Zusammenhang. Dagegen steht die gleichzeitige (integrierte) Wahrnehmung zweier nacheinander folgender Stimuli mit dem Integrationsprozess in Zusammenhang. Da Prozess 1 stark mit einer Typ-A-MF verknüpft ist, einhergehend mit der Wahrnehmung von Scheinbewegungen, könnte dieser Prozess dem Segregationsprozess entsprechen. Da Prozess 2 stark mit einer Typ-B-MF verknüpft ist, einhergehend mit der Wahrnehmung negativer Nachbilder, könnte dieser Prozess dem Integrationsprozess entsprechen. Es wird angenommen, dass diese beiden Prozesse an der bewussten Wahrnehmung des Targets in der Metakontrastmaskierung beteiligt sind