Biomedical subjects

Jun Saiki

Publications and source records attributed to Jun Saiki.

11 recordsLinked to original sources

PubMed2006

A neural network implementation of a saliency map model.

The saliency map model proposed by Itti and Koch [Itti, L., & Koch, C. (2000). A saliency-based search mechanism for overt and covert shifts of visual attention. Vision Research, 40, 1489-1506] has been a popular model for explaining the guidance of visual attention using only bottom-up information. In this paper we expand Itti and Koch's model and propose how it could be implemented by neural networks with biologically realistic dynamics. In particular, we show that by incorporating synaptic depression into the model, network activity can be normalized and competition within the feature maps can be regulated in a biologically plausible manner. Furthermore, the dynamical nature of our model permits further analysis of the time course of saliency computation, and also allows the model to calculate saliency for dynamic visual scenes. In addition to explaining the high saliency of pop-out targets in visual search tasks, our model explains attentional grab by sudden-onset stimuli, which was not accounted for by previous models.

Attention↗

PubMed2006

Feature binding in visual working memory evaluated by type identification paradigm.

Memory for feature binding comprises a key ingredient in coherent object representations. Previous studies have been equivocal about human capacity for objects in the visual working memory. To evaluate memory for feature binding, a type identification paradigm was devised and used with a multiple-object permanence tracking task. Using objects defined by shape and color, observers identified types of changes in feature combinations across an occlusion event, and the effects of object motion and number of switches were investigated. With only one switch, task performance was impaired even under stationary conditions, suggesting highly limited capacity of binding memory. Second switch improved performance only in the stationary condition, suggesting that object motion strongly disrupts feature binding. Further analyses and experiments suggest that improvement by the second switch reflects transition of binding memory by selective attention.

Attention↗

PubMed2006

[Examination of eye-movement related factors of representational momentum].

The judged final position of a moving target that disappears is displaced forward (representational momentum: RM), a phenomenon called representational momentum (RM). Recently, Kerzel (2000) suggested that RM was elicited by SPEMs after a target's offset that moved the persisting image of the target in the direction of the motion after the target dissapeared. We examined RM for a target that was not pursued by eyes. In Experiment 1 the target and a small dot moved in the same or opposite direction. Participants were instructed to pursue the small dot and locate the final position of the target. In both conditions the target's motion on the retina was expected to be equal to each other. Although the Kerzel's hypothesis (2000) predicted negative RM for the target moving in the opposite direction of SPEMs, no consistent negative RM was observed. Results of Experiment 2 suggested that the individual strategy to use the small dot affected RM. Taken together, it was shown that Kerzel's lower-level model was not very successful in explaining RM, but instead data suggested that predictive mental extrapolation and a higher-order individual strategy were involved in the production of RM.

Eye Movements↗

PubMed2005

Maintaining coherence of dynamic objects requires coordination of neural systems extended from anterior frontal to posterior parietal brain cortices.

Object representation in visual working memory enables humans to perceive a consistent visual world and must satisfy two attributes: coherence and dynamic updating. The present study measured brain activity using functional magnetic resonance imaging (fMRI) during the multiple object permanence tracking (MOPT) task, which requires observers to process simultaneously both coherence maintenance and dynamic updating of objects. Whole brain analysis revealed anterior and ventral parts of frontal area and dorsal frontoparietal activation during both object-moving and object-stationary conditions. Subsequent region-of-interest analyses in the anterior/ventral frontal and the dorsal frontoparietal regions revealed that these two systems engage the two different cognitive processes involved in the MOPT task, with coherency maintenance processed in the anterior/ventral frontal areas and spatial processing in the dorsal frontoparietal network. These results suggest that cooperation between these two systems underpins object representations in visual working memory.

Frontal Lobe↗

PubMed2005

Visual search asymmetry with uncertain targets.

The underlying mechanism of search asymmetry is still unknown. Many computational models postulate top-down selection of target-defining features as a crucial factor. This feature selection account implies, and other theories implicitly assume, that predefined target identity is necessary for search asymmetry. The authors tested the validity of the feature selection account using a singleton search task without a predefined target. Participants conducted a target-defined and a singleton search task with a circle (O) and a circle with a vertical bar (Q). Search asymmetry was observed in both tasks with almost identical magnitude. The results were not due to trial-by-trial feature selection, because search asymmetry persisted even when the target was completely unpredictable. Asymmetry in the singleton search was also observed with more complex stimuli, Kanji characters. These results suggest that feature selection is not necessary for search asymmetry, and they impose important constraints on current visual search theories.

Choice Behavior↗

PubMed2005

Illusory motion and representational momentum.

When a moving target vanishes abruptly, participants judge its final position as being ahead of its actual final position, in the direction of motion (representational momentum; Freyd & Finke, 1984). In the present study, we presented illusory motion and examined whether or not forward displacement was affected by the perceived direction and speed of the target. Experiments 1A and 1B showed that an illusory direction of movement of a target was perceived, and Experiment 2 showed that an illusory speed of a moving target was observed. However, neither the direction nor the magnitude of forward displacement was affected by these illusions. Therefore, it was suggested that the mechanism underlying forward displacement (or some extrapolation processing) uses different motion signals than does the perceptual mechanism.

Humans↗

PubMed2005

[Effect of spatial information in the blindness to response-compatible stimuli].

Targets are identified or detected more accurately when they are presented before or during a response which does not share the same feature with the target, compared with when the response does share the same feature (Müsseler & Hommel, 1997a, 1997b, blindness to response-compatible stimuli). We investigated the effect of spatial information in the blindness effect. Experiment 1 showed that the blindness effect also occurred when the task was to detect the location of stimuli as well as to identify the orientation of an arrow. Experiment 2 showed that when the location of stimuli and response were compatible, the blind effect was reduced although the location was perfectly task-irrelevant. It was suggested that spatial information had a critical role in the blindness effect from action to perception, similar to the case of stimulus-response compatibility where the stimuli affected the response.

Humans↗

PubMed2003

Spatiotemporal characteristics of dynamic feature binding in visual working memory.

It has been proposed that visual working memory can hold a set of four to five coherent object representations. As a test of this proposal, I devised a paradigm called multiple object permanence tracking (MOPT) that measures memory for feature-location binding in dynamic situations. Observers were asked to detect any feature switch in the middle of a regular rotation of a pattern with multiple objects behind an occluder. The feature switch detection performance dramatically declined as the pattern rotation velocity increased. The behavioral evidence for the use of multiple color-shape conjunction was observed only when the objects were stationary. These results cast doubt on the view that the functional unit of visual working memory involves coherent object representation, where object features are tightly bound and dynamically updated.

Color Perception↗

PubMed2003

Feature binding in object-file representations of multiple moving items.

Maintenance of episodic representations by feature-location binding is important for visual cognition. It has been proposed that we can hold and update coherent episodic representations of up to four objects. This study investigated the dynamic maintenance of feature-location bindings with multiple objects. In a series of seven experiments, participants judged whether a sequence of rotating patterns of three or four colored disks contains any color switch between two disks. Color-switch detection is in general difficult, even when tracking of objects' motions is successful, suggesting that our ability for dynamic maintenance is limited. The performance improved when the interframe rotation angle became sufficiently small. Moreover, spatiotemporal predictability was necessary for this improvement, suggesting that the maintenance of multiple episodic representations is an interactive process between our prediction and sensory mechanisms.

Cognition↗

PubMed2003

[Perceptual judgments of novel contour shapes and hierarchical descriptions of geometrical properties].

Previous studies of pattern psychophysics have suggested that a form property such as the number of turns and a structural property such as symmetry were useful cues for perceptual judgments of simple forms. However, it is necessary for complete descriptions of more complex forms to use hierarchical indices reflecting global and local characteristics. In this study, we clarified what geometrical properties contributed to complexity and similarity judgments of novel shapes, and examined differences between the two judgments, using Fourier descriptors as a form property, and symmetropy as a structural property. Global and local unevenness were derived from the amplitude of Fourier descriptors, and the hierarchical representation was found in both judgment data. Whereas complexity judgment was based on local unevenness and global symmetry, similarity judgment was, it was suggested, mainly based on global unevenness and symmetry. Moreover, it became clear that geometrical properties important for complexity data were a subset of those for similarity data. These results suggested that more dimensions in geometrical properties were necessary for similarity judgment than complexity judgment.

Form Perception↗

PubMed2002

Multiple-object permanence tracking: limitation in maintenance and transformation of perceptual objects.

Research on change blindness and transsaccadic memory revealed that a limited amount of information is retained across visual disruptions in visual working memory. It has been proposed that visual working memory can hold four to five coherent object representations. To investigate their maintenance and transformation in dynamic situations, I devised an experimental paradigm called multiple-object permanence tracking (MOPT) that measures memory for multiple feature-location bindings in dynamic situations. Observers were asked to detect any color switch in the middle of a regular rotation of a pattern with multiple colored disks behind an occluder. The color-switch detection performance dramatically declined as the pattern rotation velocity increased, and this effect of object motion was independent of the number of targets. The MOPT task with various shapes and colors showed that color-shape conjunctions are not available in the MOPT task. These results suggest that even completely predictable motion severely reduces our capacity of object representations, from four to only one or two.

Algorithms↗