André W Keizer

Leiden University, Leiden, South Holland, Netherlands

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Publications (10)33.48 Total impact

  • André W Keizer, Bernhard Hommel, Victor A F Lamme
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    ABSTRACT: When visual information enters the brain, it is relayed to different specialized regions, processing features such as shape, color, or motion. And yet, in our conscious experience of a colored, moving shape, all the different features seem to be integrated into one unified percept. Therefore, it has been hypothesized that consciousness and feature binding share an intimate relationship. To study this relationship, we used a paradigm in which the behavioral effects of feature binding can be measured. Using masks, we investigated whether spontaneous binding between the orientation and location of a Gabor patch takes place when the Gabor patch is processed consciously or unconsciously. The results of our study suggest that orientation and location of a visually presented object are automatically integrated, even when subjects are unaware of that object. We conclude that binding and consciousness share a less intimate relationship than previously hypothesized, since consciousness is not a necessary condition for binding to occur.
    Psychonomic Bulletin & Review 08/2014; · 2.99 Impact Factor
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    ABSTRACT: Recent research showed a correlation between cognitive decline and a decrease of EEG gamma activity. In the present double-blind randomized control study, we investigated whether gamma and beta neurofeedback protocols, that have been shown to modulate performance on cognitive control and memory in young adults, also leads to increased brain activity and cognitive performance in elderly. Twenty older adults either performed eight 30-minute gamma neurofeedback session or beta neurofeedback session within a period of 21 days. Cognitive performance was determined before and after the training through an IQ and memory task and we added a subjective well-being questionnaire. Both neurofeedback training protocols resulted in a significant increase of the brain activity within each training session, suggesting that the aging brain is still trainable. However, we found no effects on cognitive performance or transfer of the feedback beyond the trainings. We discuss several possible reasons for the lack of training on rest measurements and cognition and ways to improve the feedback protocols for future studies.
    Biological psychology 06/2013; · 4.36 Impact Factor
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    Bernhard Hommel, André W Keizer
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    ABSTRACT: Humans represent perceptual events in a distributed, feature-specific fashion, which calls for some sort of feature integration. It has been suggested that processing an event leads to the creation of a temporary binding of the corresponding feature codes - an object file. Here we show that object files do not only comprise of perceptual feature codes but also include codes that reflect evaluations of the perceptual event.
    Frontiers in Psychology 01/2012; 3:581. · 2.80 Impact Factor
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    ABSTRACT: Ideomotor theory claims that actions are cognitively represented and accessed via representations of the sensory effects they evoke. Previous studies provide support for this claim by showing that the presentation of action effects primes activation in corresponding motor structures. However, whether people actually use action-effect representations to control their motor behavior is not yet clear. In our fMRI study, we had participants prepare for manual or facial actions on a trial-by-trial basis, and hypothesized that preparation would be mediated by the cortical areas that code for the perceptual effects of these actions. Preparing for manual action induced higher activation of hand-related areas of motor cortex (demonstrating actual preparation) and of the extrastriate body area, which is known to mediate the perception of body parts. In contrast, preparing for facial action induced higher activation of face-related motor areas and of the fusiform face area, known to mediate face perception. These observations provide further support for the ideomotor theory and suggest that visual imagery might play a role in voluntary action control.
    Journal of Cognitive Neuroscience 01/2011; 23(1):214-20. · 4.49 Impact Factor
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    ABSTRACT: The present fMRI study tested the assumption that a single pairing of a stimulus and a logically unrelated response is sufficient for binding the corresponding stimulus and response codes into an event representation (event file) that is automatically retrieved upon processing of at least one of its components. In particular, we investigated whether repeating a face or a house stimulus and/or a left or a right manual response induces the automatic retrieval of the response or stimulus that it previously accompanied. ROI analyses of fusiform face area, parahippocampal place area, and right and left motor cortex revealed that repeating one component of a previously encountered stimulus-response episode leads to the suppression of cortical areas processing the other components, suggesting that these components were indeed automatically retrieved and conflicted with ongoing processing. The particular pattern obtained is consistent with predictions from diffusion models of decision making, which suggest a crucial role of local competition in response selection.
    Journal of Cognitive Neuroscience 04/2010; 23(4):896-904. · 4.49 Impact Factor
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    ABSTRACT: Neural synchronization in the gamma band has been associated with feature binding and intelligence. Using neurofeedback, we aimed at changing the power of the gamma band and investigated whether these changes would influence behavioral measures of feature binding and intelligence. The results show that people are indeed able to alter the power in the gamma band if provided with neurofeedback. Moreover, the increase of gamma band power was related to a decrease of binding costs and an increase in intelligence, suggesting that the control of feature binding and intelligence share a common underlying mechanism.
    International journal of psychophysiology: official journal of the International Organization of Psychophysiology 11/2009; 75(1):25-32. · 3.05 Impact Factor
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    André W Keizer, Roland S Verment, Bernhard Hommel
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    ABSTRACT: Neural synchronization has been proposed to be the underlying mechanism for exchanging and integrating anatomically distributed information and has been associated with a myriad of cognitive domains, including visual feature binding, top-down control, and long-term memory. Moreover, it seems that separate frequency bands have different functions in these cognitive processes. Here we studied whether neurofeedback training designed either to increase local gamma band activity (GBA+; 36-44 Hz), or local beta band activity (BBA+; 12-20 Hz), would have an impact on performance of behavioral tasks measuring short-term and long-term episodic binding. Our results show that GBA-enhancing neurofeedback training increased occipital GBA within sessions, and occipital and frontal GBA across sessions. Both groups showed an increase of GBA coherence between frontal and occipital areas, but the BBA+ group increased BBA coherence between these areas as well. Neurofeedback training had profound effects on behavior. First, we replicated earlier findings that enhancing GBA led to greater flexibility in handling (selectively retrieving) episodic bindings, which points to a role of GBA in top-down control of memory retrieval. Moreover, the long-term memory task revealed a double dissociation: GBA-targeted training improved recollection, whereas BBA-targeted training improved familiarity memory. We conclude that GBA is important for controlling and organizing memory traces of relational information in both short-term binding and long-term memory, while frontal-occipital coherence in the beta band may facilitate familiarity processes.
    NeuroImage 11/2009; 49(4):3404-13. · 6.25 Impact Factor
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    ABSTRACT: The visual cortex of the human brain contains specialized modules for processing different visual features of an object. Confronted with multiple objects, the system needs to attribute the correct features to each object (often referred to as 'the binding problem'). The brain is assumed to integrate the features of perceived objects into object files - pointers to the neural representations of these features, which outlive the event they represent in order to maintain stable percepts of objects over time. It has been hypothesized that a new encounter with one of the previously bound features will reactivate the other features in the associated object file according to a kind of pattern-completion process. Fourteen healthy volunteers participated in an fMRI experiment and performed a task designed to measure the aftereffects of binding visual features (houses, faces, motion direction). On each trial, participants viewed a particular combination of features (S1) before carrying out a speeded choice response to a second combination of features (S2). Repetition and alternation of all three features was varied orthogonally. The behavioral results showed the standard partial repetition costs: a reaction time increase when one feature was repeated and the other feature alternated between S1 and S2, as compared to complete repetitions or alternations of these features. Importantly, the fMRI results provided evidence that repeating motion direction reactivated the object that previously moved in the same direction. More specifically, perceiving a face moving in the same direction as a just-perceived house increased activation in the parahippocampal place area (PPA). A similar reactivation effect was not observed for faces in the fusiform face area (FFA). Individual differences in the size of the reactivation effects in the PPA and FFA showed a positive correlation with the corresponding partial repetition costs. Our study provides the first neural evidence that features are bound together on a single presentation and that reviewing one feature automatically reactivates the features that previously accompanied it.
    Behavioral and Brain Functions 11/2008; 4:50. · 2.79 Impact Factor
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    ABSTRACT: Perceiving an event requires the integration of its features across numerous brain maps and modules. Visual object perception is thought to be mediated by a ventral processing stream running from occipital to inferotemporal cortex, whereas most spatial processing and action control is attributed to the dorsal stream connecting occipital, parietal, and frontal cortex. Here we show that integration operates not only on ventral features and objects, such as faces and houses, but also across ventral and dorsal pathways, binding faces and houses to motion and manual action. Furthermore, these bindings seem to persist over time, as they influenced performance on future task-relevant visual stimuli. This is reflected by longer reaction times for repeating one, but alternating other features in a sequence, compared to complete repetition or alternation of features. Our findings are inconsistent with the notion that the dorsal stream is operating exclusively online and has no access to memory.
    Acta Psychologica 02/2008; 127(1):177-85. · 2.26 Impact Factor
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    André Willem Keizer
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    ABSTRACT: One of the most striking features of the brain is that it is modular; it consists of often highly specialized areas. This modular organization requires efficient communication in order to integrate the information that is represented in distinct brain areas. In my thesis, I studied the neural basis of this communication. With various brain imaging techniques (fMRI, EEG), I found support for the idea that fast communication is omnipresent in the brain and that cognitive control processes can influence this communication. Using a method (neurofeedback) with which subjects were trained to enhance a particular type of brain activity (gamma band activity, or ‘GBA’), I demonstrated that enhanced GBA resulted in an increase of performance on tasks measuring short-term and long-term memory of associated information and fluid intelligence. These findings suggest that GBA underlies the control of associated information, presumably originating from frontal brain regions.