Tetsuko Kasai

Event-related brain potentials during selective attention to depth and form in global stereopsis

To elucidate the discriminating processes of surface depth and boundary form in global stereopsis, event-related brain potentials (ERPs) were investigated by the detection of combinations of the attributes in random sequences. The attentional relevance of depth or form was associated with selection negativities (SNs) over the lateral occipito-temporal regions initiating at about 200 ms post-stimulus. The initial SNs were elicited irrespective of relevance to the other feature; the following parts increased only for stimuli with crossed disparity having the relevant dimension of the other feature, suggesting the independent selection of stereoscopic depth and form followed by the perceptual integration into an object.

Attention-spreading based on hierarchical spatial representations for connected objects

Attention selects objects or groups as the most fundamental unit, and this may be achieved through a process in which attention automatically spreads throughout their entire region. Previously, we found that a lateralized potential relative to an attended hemifield at occipito-temporal electrode sites reflects attention-spreading in response to connected bilateral stimuli [Kasai, T., & Kondo, M. Electrophysiological correlates of attention-spreading in visual grouping. NeuroReport, 18, 93–98, 2007]. The present study examined the nature of object representations by manipulating the extent of grouping through connectedness, while controlling the symmetrical structure of bilateral stimuli. The electrophysiological results of two experiments consistently indicated that attention was guided twice in association with perceptual grouping in the early phase (N1, 150–200 msec poststimulus) and with the unity of an object in the later phase (N2pc, 310/330–390 msec). This suggests that there are two processes in object-based spatial selection, and these are discussed with regard to their cognitive mechanisms and object representations.

Time course of spatial and feature selective attention for partly-occluded objects

Attention selects objects/groups as the most fundamental units, and this may be achieved by an attention-spreading mechanism. Previous event-related potential (ERP) studies have found that attention-spreading is reflected by a decrease in the N1 spatial attention effect. The present study tested whether the electrophysiological attention effect is associated with the perception of object unity or amodal completion through the use of partly-occluded objects. ERPs were recorded in 14 participants who were required to pay attention to their left or right visual field and to press a button for a target shape in the attended field. Bilateral stimuli were presented rapidly, and were separated, connected, or connected behind an occluder. Behavioral performance in the connected and occluded conditions was worse than that in the separated condition, indicating that attention spread over perceptual object representations after amodal completion. Consistently, the late N1 spatial attention effect (180-220 ms post-stimulus) and the early phase (230-280 ms) of feature selection effects (target N2) at contralateral sites decreased, equally for the occluded and connected conditions, while the attention effect in the early N1 latency (140-180 ms) shifted most positively for the occluded condition. These results suggest that perceptual organization processes for object recognition transiently modulate spatial and feature selection processes in the visual cortex.

Individuals with Asperger's disorder exhibit difficulty in switching attention from a local level to a global level.

The purpose of the present study was to determine whether individuals with Asperger’s disorder exhibit difficulty in switching attention from a local level to a global level. Eleven participants with Asperger’s disorder and 11 age- and gender-matched healthy controls performed a level-repetition switching task using Navon-type hierarchical stimuli. In both groups, level-repetition was beneficial at both levels. Furthermore, individuals with Asperger’s disorder exhibited difficulty in switching attention from a local level to a global level compared to control individuals. These findings suggested that there is a problem with the inhibitory mechanism that influences the output of enhanced local visual processing in Asperger’s disorder.

Attention that covers letters is necessary for the left-lateralization of an early print-tuned ERP in Japanese hiragana

Extensive experience with reading develops expertise in acquiring information from print, and this is reflected in specific enhancement of the left-lateralized N170 component in event-related potentials. The N170 is generally considered to reflect visual/orthographic processing; while modulations of its left-lateralization related to phonological processes have also been indicated. However, in our previous study, N170-like response to Hiragana strings lacked left-lateralization when the stimuli were completely task-irrelevant in rapid-presentation sequences [Okumura et al. (2014). Early print-tuned ERP response with minimal involvement of linguistic processing in Japanese Hiragana strings. Neuroreport 25, 410-414]. This suggests that, despite the highly transparent character-to-syllable correspondence, the phonological mapping of Hiragana strings requires some kind of attention toward print. To verify this notion, the present study examined ERPs under the same experimental condition as in the previous study, except that the task required attention to a stimulus attribute (i.e., color). As a result, Hiragana words and nonwords elicited left-lateralized negative deflection in the occipito-temporal region during 130-170ms post-stimulus in comparison to symbol strings, but only when the print had a narrow intercharacter spacing. Moreover, we observed the enhancement of very early occipital ERP in response to words during 70-100ms. The present results suggest that visual attention plays a role in early print processing, which may contribute to our understanding of the mechanisms that underlie expert as well as impaired reading.

Early print-tuned ERP response with minimal involvement of linguistic processing in Japanese Hiragana strings.

The act of reading leads to the development of specific neural responses for print, the most frequently reported of which is the left occipitotemporal N170 component of event-related potentials. However, it remains unclear whether this electrophysiological response solely involves print-tuned neural activities. The present study examined an early print-tuned event-related potential response with minimal involvement of linguistic processing in a nonalphabetic language. Japanese Hiragana words, nonwords, and alphanumeric symbol strings were presented rapidly and the task was to detect the change in color of a fixation cross to restrict linguistic processing. As a result, Hiragana words and nonwords elicited a larger posterior N1 than alphanumeric symbol strings bilaterally, irrespective of intercharacter spacing. The fact that this N1 was enhanced specifically for rapidly presented Hiragana strings suggests the existence of print-tuned neural processes that are relatively independent of the influence of linguistic processing.

Early visual selection in near and far space: an event-related potential study.

Earlier behavioral studies have shown that near space (within reaching distance) is represented in the brain separately from far space (out of reaching distance), and the relationship between the different spatial coordinates and spatial attention is unclear. The typical event-related potentials of P1 and N1 in the near (viewing distance of 30 cm) and far (140 cm) conditions, with stimuli at a constant visual angle are examined in this study. An early P1 (100–130 ms poststimulus) attention effect at occipital parietal sites increased in response to stimuli at the left visual field only in near space. This suggests that near and far spatial representations are involved in early visual selection.

Print-specific N170 involves multiple subcomponents for Japanese Hiragana

Print-specific N170 in event-related potentials is generally considered to reflect relatively automatic processing for letter strings, which is crucial for fluent reading. However, our previous studies demonstrated that print-specific N170 for transparent Japanese Hiragana script consists of at least two subcomponents under rapid stimulus presentation: an attention-related left-lateralized N170 and a bilateral N170 associated with more automatic orthographic processes (Okumura, Kasai & Murohashi, 2014, 2015). The present study aimed to confirm the latter component by controlling presentation frequency of letters and nonlinguistic visual controls (i.e., symbols), but found a quite different pattern of results; an enhanced occipito-temporal positivity for words (80-120ms poststimulus) followed by the typical left-lateralized N170 and an enhanced parietal negativity for nonwords (150-200ms). These results should provide further insights into the interaction processes between attention and early stages of print processing.

Object-based spatial attention when objects have sufficient depth cues

Attention directed to a part of an object tends to obligatorily spread over all of the spatial regions that belong to the object, which may be critical for rapid object-recognition in cluttered visual scenes. Previous studies have generally used simple rectangles as objects and have shown that attention spreading is reflected by amplitude modulation in the posterior N1 component (150-200 ms poststimulus) of event-related potentials, while other interpretations (i.e., rectangular holes) may arise implicitly in early visual processing stages. By using modified Kanizsa-type stimuli that provided less ambiguity of depth ordering, the present study examined early event-related potential spatial-attention effects for connected and separated objects, both of which were perceived in front of (Experiment 1) and in back of (Experiment 2) the surroundings. Typical P1 (100-140 ms) and N1 (150-220 ms) attention effects of ERP in response to unilateral probes were observed in both experiments. Importantly, the P1 attention effect was decreased for connected objects compared to separated objects only in Experiment 1, and the typical object-based modulations of N1 were not observed in either experiment. These results suggest that spatial attention spreads over a figural object at earlier stages of processing than previously indicated, in three-dimensional visual scenes with multiple depth cues.