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Henk Spekreijse

Publications and source records attributed to Henk Spekreijse.

At least 19 recordsLinked to original sources
PubMed2006

The influence of inattention on the neural correlates of scene segmentation.

Numerous experiments show that people are unable to report about unattended information. It is also clear that there is extensive processing in the absence of attention. Here, we study, by using an 'inattentional blindness' paradigm while measuring BOLD responses or MEG to texture displays, to what level of scene segmentation visual information is processed when subjects are not capable of reporting that segregating textures were present. We presented non-segregating and occasionally segregating textures in two different conditions: 1. a condition where subjects were not informed about the presence of the segregating textures while at the same time engaged in a foveal task, resulting in prolonged inattentional blindness and 2. a condition with similar task demands, in which, however, the subjects perceived the segregating textures. BOLD responses from early visual areas (V1, V2, V3, V4) and MEG responses up to 240 ms showed a significant difference between segregating and not segregating textures in both conditions and did not differ from each other, indicating that scene segmentation processes proceed normally during inattention. A difference between the two conditions, and hence an influence of attention, was signaled in area V3a and more parietal MEG sensors around 400 ms.

Adult↗
PubMed2006

Attention lights up new object representations before the old ones fade away.

We investigated how attention shifts from one object to another by recording neuronal activity in the primary visual cortex. Monkeys performed a contour-grouping task in which they had to select a target curve and ignore a distractor curve. Some trials required a shift of attention, because the target and distractor curves were switched during the course of the trial. We monitored the dynamics of this attention shift in area V1, in which neuronal responses evoked by the target curve are stronger than those evoked by the distractor. The reallocation of attention was associated with a rapid and strong enhancement of responses to the newly attended curve, followed, after approximately 60 ms, by a weaker suppression of responses to the curve from which attention was removed. We conclude that attention can be rapidly allocated to a new object before it disengages from the previously attended one.

Animals↗
PubMed2006

Elasticity, viscosity, and deformation of orbital fat.

PURPOSE: For development of a finite element analysis model of orbital mechanics, it was necessary to determine the material properties of orbital fat and its degree of deformation in eye rotation. METHODS: Elasticity and viscosity of orbital fat of eight orbits of four calves and two orbits of one rhesus monkey were measured with a parallel-plate rheometer. The degree of deformation of orbital fat was studied in two human subjects by magnetic resonance imaging (MRI) through the optic nerve in seven (first subject) or fourteen positions of gaze from left to right. Bifurcations of veins in the fat were used as markers for displacement of the fat. RESULTS: The elastic shear modulus (G') of calf orbital fat was between 250 Pa and 500 Pa, and of monkey orbital fat it was between 500 Pa and 900 Pa. The viscous shear modulus (G'') of calf orbital fat was between 80 Pa and 150 Pa, and for monkey orbital fat it was between 300 Pa and 500 Pa. In the MRI scans, it was found that markers in the fat, 1 to 5 mm posterior to the sclera, rotated with the eye for 36% to 53% of eye rotation; the remainder was accounted for by sliding of the eye within the Tenon capsule and within the orbital fat. CONCLUSIONS: Elastic and viscous shear moduli of orbital fat are low. Little energy is dissipated in the fat. The required deformation of the fat during eye rotation is limited because the eye slides, to some extent, within the Tenon capsule.

Adipose Tissue↗
PubMed2005

Synchrony dynamics in monkey V1 predict success in visual detection.

Behavioral measures such as expectancy and attention have been associated with the strength of synchronous neural activity. On this basis, it is hypothesized that synchronous activity affects our ability to detect and recognize visual objects. To investigate the role of synchronous activity in visual perception, we studied the magnitude and precision of correlated activity, before and after stimulus presentation within the visual cortex (V1), in relation to a monkey's performance in a figure-ground discrimination task. We show that during the period of stimulus presentation a transition in synchronized activity occurs that is characterized by a reduction of the correlation peak height and width. Before stimulus onset, broad peak correlations are observed that change towards thin peak correlations after stimulus onset, due to a specific decrease of low-frequency components. The magnitude of the transition in correlated activity is larger, i.e. a stronger desynchronization occurs, when the animal perceives the stimulus correctly than when the animal fails to detect the stimulus. These results therefore show that a transition in synchronous firing is important for the detection of sensory stimuli. We hypothesize that the transition in synchrony reflects a change from loose and global neuronal interactions towards a finer temporal and spatial scale of neuronal interactions, and that such a change in neuronal interactions is required for figure-ground discrimination.

Animals↗
PubMed2005

Neural responses in cat visual cortex reflect state changes in correlated activity.

Cortical state is characterized by ongoing rhythmic neural activity. Changes in rhythmic activity and thus in cortical state are shown to occur spontaneously in the anesthetized cat. We were interested in whether these state changes have an affect on the cortical processing of sensory stimuli. This was investigated by recording spontaneous and stimulus-evoked local field potentials and multi-unit neuronal activity (MUA) from trans-cortical electrode arrays in the visual cortex of the anesthetized cat. Changes in cortical state were identified by calculating the cross-correlation strength and cross-coherency, between MUA channels at different layers and on separate electrode arrays. Spontaneous changes in rhythmic activity were associated with changes in the strength of stimulus-evoked multiple unit responses of cortical neurons. The highest multi-unit responses were found in periods when low-frequency rhythms of the electroencephalogram increase in magnitude and high-frequency rhythms decrease. Such changes in evoked responses were maximal at layer IV, the input layer of the visual cortex. Our findings suggest that stimulus response magnitude depends on rhythmic state and reflects changes in functional connectivity within the visual cortex.

Action Potentials↗
PubMed2004

Relationship between change detection and pre-change [corrected] activity in visual area V1.

Humans are poor at detecting changes to visual scenes occurring during brief disruptions. It is unclear whether this 'change blindness' results from failure to process the relevant item before the change, or failure to compare/recall the item after the change. We recorded pre-change multi-unit activity in area V1 of monkeys performing a change detection task. The animals were rewarded for making a saccade to the changing figure. Figure-ground related activity was observed, even when no correct saccade was made. However, for the changing figure, pre-change activity was stronger in correct trials than in incorrect trials. We conclude that small differences in pre-change figure-ground segregation have predictive value in whether the change will be successfully detected.

Animals↗
PubMed2004

Synchrony and covariation of firing rates in the primary visual cortex during contour grouping.

The visual system imposes structure onto incoming information, by grouping image elements of a single object together, and by segregating them from elements that belong to other objects and the background. One influential theory holds that the code for grouping and segmentation is carried by the synchrony of neuronal discharges on a millisecond time scale. We tested this theory by recording neuronal activity in the primary visual cortex (area V1) of monkeys engaged in a contour-grouping task. We found that synchrony was unrelated to contour grouping. The firing rates of V1 neurons are also correlated across trials. We demonstrate that this rate covariation is mainly determined by fluctuations in visual attention. Moreover, we show that rate covariation depends on perceptual grouping, as it is strongest between neurons that respond to features of the same object.

Animals↗
PubMed2004

Correlates of transsaccadic integration in the primary visual cortex of the monkey.

We make several eye movements per second when we explore a visual scene. Each eye movement sweeps the scene's projection across the retina and changes its representation in retinotopic areas of the visual cortex, but we nevertheless perceive a stable world. Here we investigate the neuronal correlates of visual stability in the primary visual cortex. Monkeys were trained to make two saccades along a single curve and to ignore another, distracting curve. Attention enhanced neuronal responses to the entire relevant curve before the first saccade. This response enhancement was rapidly reestablished after the saccade, although the image was shifted across the primary visual cortex. We argue that this fast postsaccadic restoration of the attentional response enhancement contributes to the stability of vision across eye movements, and reduces the impact of saccades on visual cognition.

Animals↗
PubMed2004

Correspondence of presaccadic activity in the monkey primary visual cortex with saccadic eye movements.

We continuously scan the visual world via rapid or saccadic eye movements. Such eye movements are guided by visual information, and thus the oculomotor structures that determine when and where to look need visual information to control the eye movements. To know whether visual areas contain activity that may contribute to the control of eye movements, we recorded neural responses in the visual cortex of monkeys engaged in a delayed figure-ground detection task and analyzed the activity during the period of oculomotor preparation. We show that approximately 100 ms before the onset of visually and memory-guided saccades neural activity in V1 becomes stronger where the strongest presaccadic responses are found at the location of the saccade target. In addition, in memory-guided saccades the strength of presaccadic activity shows a correlation with the onset of the saccade. These findings indicate that the primary visual cortex contains saccade-related responses and participates in visually guided oculomotor behavior.

Animals↗
PubMed2004

Visual information transfer across eye movements in the monkey.

During normal viewing, the eyes move from one location to another in order to sample the visual environment. Information acquired before the eye movement facilitates post-saccadic processing. This "preview effect" indicates that some information is maintained in transsaccadic memory and combined with information acquired at the next fixation. However, the nature of transsaccadic memory remains a subject of debate. Here, we investigate preview effects in monkeys that carry out a contour-grouping (curve-tracing) task, by manipulating the consistency between pre- and post-saccadic information. The results show that consistent information causes a preview benefit, whereas inconsistent information causes a preview cost. These preview effects are relatively independent of the pre-saccadic viewing duration, and they occur even when the stimulus is exposed for only approximately 10 ms. The results further demonstrate that an entire relevant curve is stored in transsaccadic memory, instead of just the items at the saccade goal. They suggest that preview effects are caused by a mechanism that stores attended sensory information to make it available at the next fixation. The results are discussed within a theoretical framework that establishes an intimate relationship between attention, short-term memory and transsaccadic memory.

Animals↗
PubMed2004

The role of figure-ground segregation in change blindness.

Partial report methods have shown that a large-capacity representation exists for a few hundred milliseconds after a picture has disappeared. However, change blindness studies indicate that very limited information remains available when a changed version of the image is presented subsequently. What happens to the large-capacity representation? New input after the first image may interfere, but this is likely to depend on the characteristics of the new input. In our first experiment, we show that a display containing homogeneous image elements between changing images does not render the large-capacity representation unavailable. Interference occurs when these new elements define objects. On that basis we introduce a new method to produce change blindness: The second experiment shows that change blindness can be induced by redefining figure and background, without an interval between the displays. The local features (line segments) that defined figures and background were swapped, while the contours of the figures remained where they were. Normally, changes are easily detected when there is no interval. However, our paradigm results in massive change blindness. We propose that in a change blindness experiment, there is a large-capacity representation of the original image when it is followed by a homogeneous interval display, but that change blindness occurs whenever the changed image forces resegregation of figures from the background.

Cues↗
PubMed2003

Set size effects in the macaque striate cortex.

Attentive processing is often described as a competition for resources among stimuli by mutual suppression. This is supported by findings that activity in extrastriate cortex is suppressed when several stimuli are presented simultaneously, compared to a single stimulus. In this study, we randomly varied the number of simultaneously presented figures (set size) in an attention-demanding change detection task, while we recorded multiunit activity in striate cortex (V1) in monkeys. After figure-background segregation, activity was suppressed as set size increased. This effect was stronger and started earlier among cells stimulated by the background than those stimulated by the figures themselves. As a consequence, contextual modulation, a correlate of figure-background segregation, increased with set size, approximately 100 msec after its initial generation. The results indicate that suppression of responses under increasing attentional demands differentially affects figure and background responses in area V1.

Animals↗
PubMed2003

Figure-ground activity in primary visual cortex (V1) of the monkey matches the speed of behavioral response.

To look at an object its position in the visual scene has to be localized and subsequently appropriate oculo-motor behavior needs to be initiated. This kind of behavior is largely controlled by the cortical executive system, such as the frontal eye field. In this report, we analyzed neural activity in the visual cortex in relation to oculo-motor behavior. We show that in a figure-ground detection task, the strength of late modulated activity in the primary visual cortex correlates with the saccade latency. We propose that this may indicate that the variability of reaction times in the detection of a visual stimulus is reflected in low-level visual areas as well as in high-level areas.

Animals↗
PubMed2003

Internal state of monkey primary visual cortex (V1) predicts figure-ground perception.

When stimulus information enters the visual cortex, it is rapidly processed for identification. However, sometimes the processing of the stimulus is inadequate and the subject fails to notice the stimulus. Human psychophysical studies show that this occurs during states of inattention or absent-mindedness. At a neurophysiological level, it remains unclear what these states are. To study the role of cortical state in perception, we analyzed neural activity in the monkey primary visual cortex before the appearance of a stimulus. We show that, before the appearance of a reported stimulus, neural activity was stronger and more correlated than for a not-reported stimulus. This indicates that the strength of neural activity and the functional connectivity between neurons in the primary visual cortex participate in the perceptual processing of stimulus information. Thus, to detect a stimulus, the visual cortex needs to be in an appropriate state.

Animals↗
PubMed2003

Subtask sequencing in the primary visual cortex.

Complex visual tasks can usually be decomposed into a number of simpler subtasks. Whether such subtasks are solved serially or in parallel is subject to considerable debate. Here we investigate how subtasks are coordinated in time by recording from the primary visual cortex of macaque monkeys. The animals were trained to perform both a simple and a composite task. In the simple task, they had to mentally trace a target curve while ignoring a distractor curve. Neuronal responses in the primary visual cortex to the target curve were enhanced relative to responses to the distractor curve 130 ms after stimulus appearance. In the composite task, the monkeys searched for a colored marker and traced a curve that was attached to this marker. In an initial phase of the trials, neuronal responses reflected visual search, and the response enhancement due to curve tracing now occurred after 230 ms, 100 ms later than in the simple task. We conclude that subtasks of the composite task are carried out in a structured and sequential manner that can be monitored in the primary visual cortex.

Animals↗
PubMed2003

Large capacity storage of integrated objects before change blindness.

Normal people have a strikingly low ability to detect changes in a visual scene. This has been taken as evidence that the brain represents only a few objects at a time, namely those currently in the focus of attention. In the present study, subjects were asked to detect changes in the orientation of rectangular figures in a textured display across a 1600 ms gray interval. In the first experiment, change detection improved when the location of a possible change was cued during the interval. The cue remained effective during the entire interval, but after the interval, it was ineffective, suggesting that an initially large representation was overwritten by the post-change display. To control for an effect of light intensity during the interval on the decay of the representation, we compared performance with a gray or a white interval screen in a second experiment. We found no difference between these conditions. In the third experiment, attention was occasionally misdirected during the interval by first cueing the wrong figure, before cueing the correct figure. This did not compromise performance compared to a single cue, indicating that when an item is attentionally selected, the representation of yet unchosen items remains available. In the fourth experiment, the cue was shown to be effective when changes in figure size and orientation were randomly mixed. At the time the cue appeared, subjects could not know whether size or orientation would change, therefore these results suggest that the representation contains features in their 'bound' state. Together, these findings indicate that change blindness involves overwriting of a large capacity representation by the post-change display.

Attention↗
PubMed2002

Masking interrupts figure-ground signals in V1.

In a backward masking paradigm, a target stimulus is rapidly (<100 msec) followed by a second stimulus. This typically results in a dramatic decrease in the visibility of the target stimulus. It has been shown that masking reduces responses in V1. It is not known, however, which process in V1 is affected by the mask. In the past, we have shown that in V1, modulations of neural activity that are specifically related to figure-ground segregation can be recorded. Here, we recorded from awake macaque monkeys, engaged in a task where they had to detect figures from background in a pattern backward masking paradigm. We show that the V1 figure-ground signals are selectively and fully suppressed at target-mask intervals that psychophysically result in the target being invisible. Initial response transients, signalling the features that make up the scene, are not affected. As figure-ground modulations depend on feedback from extrastriate areas, these results suggest that masking selectively interrupts the recurrent interactions between V1 and higher visual areas.

Animals↗