The polyketide synthase StlA is involved in inducing aggregation in Polysphondylium violaceum.

In the social amoeba Dictyostelium discoideum, the polyketide MPBD (4-methyl-5-pentylbenzene-1,3-diol) regulates the gene expressions of cAMP signaling to make cells aggregation-competent and also induces spore maturation. The polyketide synthase StlA is responsible for MPBD biosynthesis in D. discoideum and appears to be conserved throughout the major groups of the social amoeba (Dictyostelia). In this study, we analyzed the function of StlA in Polysphondylium violaceum by identifying the gene sequence and creating the knockout mutants. We found that Pv-stlA- mutants had defects only in cell aggregation but not in spore maturation, indicating that the function of StlA in inducing spore maturation is species-specific. We also found that MPBD could rescue the aggregation defect in Pv-stlA- mutants whereas the mutants normally exhibited chemotaxis to their chemoattractant, glorin. Our data suggest that StlA is involved in inducing aggregation in P. violaceum by acting on signaling pathways other than chemotaxis in P. violaceum.

Front-Presented Looming Sound Selectively Alters the Perceived Size of a Visual Looming Object.

In spite of accumulating evidence for the spatial rule governing cross-modal interaction according to the spatial consistency of stimuli, it is still unclear whether 3D spatial consistency (i.e., front/rear of the body) of stimuli also regulates audiovisual interaction. We investigated how sounds with increasing/decreasing intensity (looming/receding sound) presented from the front and rear space of the body impact the size perception of a dynamic visual object. Participants performed a size-matching task (Experiments 1 and 2) and a size adjustment task (Experiment 3) of visual stimuli with increasing/decreasing diameter, while being exposed to a front- or rear-presented sound with increasing/decreasing intensity. Throughout these experiments, we demonstrated that only the front-presented looming sound caused overestimation of the spatially consistent looming visual stimulus in size, but not of the spatially inconsistent and the receding visual stimulus. The receding sound had no significant effect on vision. Our results revealed that looming sound alters dynamic visual size perception depending on the consistency in the approaching quality and the front-rear spatial location of audiovisual stimuli, suggesting that the human brain differently processes audiovisual inputs based on their 3D spatial consistency. This selective interaction between looming signals should contribute to faster detection of approaching threats. Our findings extend the spatial rule governing audiovisual interaction into 3D space.

Processing of self-initiated sound motion in the human brain.

Interacting with objects in our environment usually leads to audible noise. Brain responses to such self-initiated sounds have been shown to be attenuated, in particular the so-called N1 component measured with electroencephalography (EEG). This attenuation has been proposed to be the effect of an internal forward model that allows for cancellation of the sensory consequences of a motor command. In the current study we asked whether the attenuation due to self-initiation of a sound also affects a later event-related potential - the so-called motion-onset response - that arises in response to moving sounds. To this end, volunteers were instructed to move their index fingers either left or rightward which resulted in virtual movement of a sound either to the left or to the right. In Experiment 1, sound motion was induced with in-ear head-phones by shifting interaural time and intensity differences and thus shifting the intracranial sound image. We compared the motion-onset responses under two conditions: a) congruent, and b) incongruent. In the congruent condition, the sound image moved in the direction of the finger movement, while in the incongruent condition sound motion was in the opposite direction of the finger movement. Clear motion-onset responses with a negative cN1 component peaking at about 160 ms and a positive cP2 component peaking at about 230 ms after motion-onset were obtained for both the congruent and incongruent conditions. However, the motion-onset responses did not significantly differ between congruent and incongruent conditions in amplitude or latency. In Experiment 2, in which sounds were presented with loudspeakers, we observed attenuation for self-induced versus externally triggered sound motion-onset, but again, there was no difference between congruent and incongruent conditions. In sum, these two experiments suggest that the motion-onset response measured by EEG can be attenuated for self-generated sounds. However, our result did not indicate that this attenuation depended on congruency of action and sound motion direction.

Size-Distance Scaling With Absolute and Relative Auditory Distance Information.

In the dynamic 3D space, it is critical for survival to perceive size of an object and rescale it with distance from an observer. Humans can perceive distance via not only vision but also audition, which plays an important role in the localization of objects, especially in visually ambiguous environments. However, whether and how auditory distance information contributes to visual size perception is not well understood. To address this issue, we investigated the efficiency of size-distance scaling by using auditory distance information that was conveyed by binaurally recorded auditory stimuli. We examined the effects of absolute distance information of a single sound sequence (Experiment 1) and relative distance information between two sound sequences (Experiment 2) on visual size estimation performances in darkened and well-lit environments. We demonstrated that humans could perform size-distance disambiguation by using auditory distance information even in darkness. Curiously, relative distance information was more efficient in size-distance scaling than absolute distance information, suggesting a high reliance on relative auditory distance information in our visual spatial experiences. The results highlight a benefit of audiovisual interaction for size-distance processing and calibration of external events under visually degraded situations.

The effect of illusionary perception on mismatch negativity (MMN): An electroencephalography study.

Mismatch negativity (MMN) is a unique brain response elicited by any discernible change of features in a tone sequence. Although the occurrence of MMN is dependent upon the difference of a stimulus parameter, such as frequency or intensity, recent studies have suggested that MMN occurs as a result of a comparison between an internal representation created by perception and an incoming tone. The present study aimed to investigate MMN occurs based upon the physical properties of stimuli or as a result of the perception of the scale illusion. A scale illusion occurs during presentation of ascending and descending musical scales between C4 and C5. The tones of these scales are presented to the right and left ear alternately using a dichotic listening paradigm. Although the ascending/descending sequences are alternated between ears after each tone, we perceive the illusion of progressively ascending/descending tones as being separated by ear. The experiment was designed as an oddball task using the illusionary sequence and three different types of tone sequences as control conditions. Brain response to these sequences and infrequently presented deviants was measured using electroencephalography (EEG). All of the control sequences showed MMN in response to the deviant. However, the illusionary sequence did not result in a significant MMN. These results suggest that in the case of scale illusion, the occurrence of MMN is based upon the representation of tones created by perception, but not upon the physical properties of a tone sequence.