Differentiating spatial and object-based effects on attention: an event-related brain potential study with peripheral cueing.

Do spatial attention and object attention modulate visual processing in similar ways? Previously we have found a dissociation between these two forms of attention on ERP measures of sensory processing under conditions of peripheral cueing, with spatial attention effects associated with changes over anterior scalp regions and object attention effects associated with changes over posterior regions (He, X., Fan, S., Zhou, K., Chen, L., 2004. Cue validity and object-based attention. J. Cogn. Neurosci. 16, 1085-1097). However, under conditions of central cueing recent data suggest that spatial and object attention have similar effects over posterior cortical areas (e.g., Martínez, A., Teder-Sälejärvi, W., Hillyard, S.A., 2007. Spatial attention facilitates selection of illusory objects: evidence from event-related brain potentials. Brain Res. 1139, 143-152). In the present study we present further evidence for dissociation between spatial and object-based attention under conditions in which spatial attention effects were enhanced by increasing the cue validity and the task load. The data replicated our previous results, with the effects of spatial attention found in an enhanced anterior N1, while the effects of object-based attention emerged in an enhanced posterior N1. Analyses of attention effect maps and current source density maps confirmed the distinct scalp distributions. These results support the proposal that, under peripheral cueing, spatial attention and object attention are associated with activity respectively in anterior and posterior brain structures, and further suggest a distinction between how attention modulates processing under conditions of central cueing and peripheral cueing.

Both frequency and interaural delay affect event-related potential responses to binaural gap.

Human listeners are extraordinarily sensitive to a transient break in interaural correlation (called binaural gap). In this study, a binaural gap embedded in interaurally correlated noise markers elicited marked scalp event-related potentials (ERPs). ERPs to the binaural gap in narrowband noise with the center frequency of 1600 Hz were significantly weaker than those for narrowband noise with the center frequency of 400 or 800 Hz. Introducing the interaural time difference (ITD) of 4 ms weakened the ERPs for either 400-Hz or 800-Hz noise. Introducing the ITD of 2 ms, however, only weakened the ERPs for 800-Hz but not 400-Hz noise. Thus central representations of a transient break in interaural correlation for narrowband noises are affected by both frequency and ITD.

An electrophysiological signature for proactive interference resolution in working memory.

We used event-related potentials (ERPs) to study the temporal dynamics of proactive interference in working memory. Participants performed a Sternberg item-recognition task to determine whether a probe was in a target memory set. Familiar negative probes were found to be more difficult to reject than less familiar ones. A fronto-central N2 component peaking around 300 ms post-probe-onset differentiated among target probes, familiar and less familiar non-target probes. The study identifies N2 as the ERP signature for proactive interference resolution. It also indicates that the resolution process occurs in the same time window as target/non-target discrimination and provides the first piece of electrophysiological evidence supporting a recent interference resolution model based on localization data [Jonides, J., Nee, D.E., 2006. Brain mechanisms of proactive interference in working memory. Neuroscience 139, 181-193].

Neurophysiological correlates of memory illusion in both encoding and retrieval phases.

False recognition of a critical lure at retrieval in the Deese-Roediger-McDermott (DRM) paradigm depends on different processing of its corresponding associates in the encoding phase. The current study recorded ERPs in both the encoding and retrieval phases to investigate the neural correlates of differential processing of true and false memories, and the roles of encoding and retrieval in eliciting memory illusion. The ERPs recorded at the study phase were characterized by a smaller N170 component and a larger amplitude late positive component (LPC) for associates that elicited later memory illusion than those that did not elicit later memory illusion. These ERP results suggest that increased active semantic associative processing or a gist representation was established for those items that elicited later memory illusion. This interpretation was supported by the serial-position analysis of the ERPs at encoding. Three ERP components were identified at retrieval. The equal early ERP old/new effects for true and false recognition reflected similar semantic priming. The parietal ERP old/new effect was greater for true than for false recognition, reflecting the recollection processes. A late slow negativity ERP distributed at the parietal and right frontal electrode sites differentiated between true and false recognition. The ERP results confirmed that both encoding and retrieval processes are involved in eliciting false memory. The parietal and frontal distributions of LPC at encoding and the late negativity at retrieval may imply a common neural mechanism in monitoring memory encoding and retrieval.

Neural substrates of visual perceptual learning of simple and complex stimuli.

OBJECTIVE: The two experiments described here used event-related potentials (ERPs) to investigate whether perceptual learning of different complexities of stimuli involves different levels of visual cortical processing in human adults. METHODS: Reaction times and ERPs were recorded during 3 consecutive training sessions in which subjects discriminated between simple stimuli made of line segments or complex stimuli made of compound shapes. RESULTS: Reaction times in both experiments were shortened across training sessions. For simple stimuli, training resulted in a decreased N1 (125-155ms) and an increased P2 (180-240ms) over the occipital area. For complex stimuli, however, training resulted in a decreased N1 (125-155ms) and N2 (290-340ms) and an increased P3 (350-550ms) over the central/parietal areas. CONCLUSIONS: These findings suggest that perceptual learning modifies the response at different levels of visual cortical processing related to the complexity of the stimulus. SIGNIFICANCE: The neuronal mechanisms involved in perceptual learning may depend on the nature (e.g. the complexity) of the stimuli used in the discrimination task.

Cue validity and object-based attention.

In a previous study, Egly, Driver, and Rafal (1994) observed both space- and object-based components of visual selective attention. However, the mechanisms underlying these two components and the relationship between them are not well understood. In the present research, with a similar paradigm, these issues were addressed by manipulating cue validity. Behavioral results indicated the presence of both space- and object-based components under high cue validity, similar to the results of Egly et al.'s study. In addition, under low cue validity, the space-based component was absent, whereas the object-based component was maintained. Further event-related potential results demonstrated an object-based effect at a sensory level over the posterior areas of brain, and a space-based effect over the anterior region. The present data suggest that the space- and object-based components reflect mainly voluntary and reflexive mechanisms, respectively.

An electrophysiological investigation of memory encoding, depth of processing, and word frequency in humans.

Memory encoding can be studied by monitoring brain activity correlated with subsequent remembering. To understand brain potentials associated with encoding, we compared multiple factors known to affect encoding. Depth of processing was manipulated by requiring subjects to detect animal names (deep encoding) or boldface (shallow encoding) in a series of Chinese words. Recognition was more accurate with deep than shallow encoding, and for low- compared to high-frequency words. Potentials were generally more positive for subsequently recognized versus forgotten words; for deep compared to shallow processing; and, for remembered words only, for low- than for high-frequency words. Latency and topographic differences between these potentials suggested that several factors influence the effectiveness of encoding and can be distinguished using these methods, even with Chinese logographic symbols.

Event-related potentials reveal involuntary processing of orientation changes in the visual modality.

The present study investigated whether there is a visual counterpart of the auditory mismatch negativity. Event-related potentials were recorded while subjects performed a spatial frequency discrimination task. "Match" and "nonmatch" stimuli were specifically categorized according to whether the second stimulus had the same orientation as the first stimulus in each trial. Nonmatch stimuli elicited larger occipital P84 and occipital and temporal N192 components than match stimuli, indicating the existence of involuntary processing in the visual modality. Moreover, the amplitude of the change-related N 192 was larger at the short stimulus onset asynchrony (SOA) than at the long SOA, suggesting that the visual modality involves a mismatch process similar to that of the auditory modality. The underlying neural representation (i.e., the visual memory trace) seems to develop easier and decay faster than its auditory counterpart.

Specificity and generalization of visual perceptual learning in humans: an event-related potential study.

To investigate the neural correlates of specificity and generalization of visual perceptual learning, we recorded event-related potentials from human adults when they were trained with a simple visual discrimination task. While reaction times decreased significantly across training sessions, event-related potentials showed larger P2 amplitudes ( approximately 210 ms) over the left occipital/parietal areas and smaller N1 amplitudes ( approximately 140 ms) at the left parietal site with more practice. Similar to reaction times, the training effect on the P2 amplitudes was specific to stimulus orientation. However, the N1 effect was generalized over differently oriented stimuli. These results indicated the complexity of the neural substance underlying perceptual learning, relative to behavioral level.

Spatiotemporal activation of the two visual pathways in form discrimination and spatial location: a brain mapping study.

To address the question of the relationship between the two visual pathways, a ventral stream for object and form vision and a dorsal stream for spatial and motion vision, we measured the spatiotemporal activation patterns in the two pathways responding to an integrated visuospatial task to which form discrimination and spatial location were assigned simultaneously. The two cognitive components of form discrimination and spatial location were interwoven in the task; however, the fMRI data demonstrated that such a task still activated both ventral GTi/GF (the inferior temporal gyrus/the fusiform gyrus) and dorsal Ga/PCu (the angular gyrus/Precuneus), which are supposed to mediate form discrimination and spatial location, respectively. In addition, the source waveforms of the fMRI foci based on the source analysis of the fMRI-seeded dipole modeling and the moving dipole modeling indicated that in responding to the task combining simultaneously form perception and spatial location, the activity in Ga/PCu begins earlier than that in GTi/GF, but it peaks later and lasts longer.

An event-related potential study on visual perceptual learning under short-term and long-term training conditions.

The present study investigated learning-induced changes of event-related potentials in a visual discrimination task, focusing on making a comparison between short-term and long-term learning perceptual processes. Event-related potentials were recorded from two groups of human adults. One group (short-term training group) was given 1.5 h of training in a single day, and another group (long-term training group) was given the same training procedure (1.5 hour of training) on each of 3 consecutive days. The results demonstrated that for short-term training group, along with the reduction of reaction times, the amplitudes of N1 and N2 negativities over the central/parietal areas decreased during the training. For long-term training group, however, after long-term training, the N2 effect disappeared and the N1 effect occurred over the posterior areas. In addition, the amplitudes of N2 for long-term training group were less than those for short-term training group. Our results suggest that neural activity depends not only on perceptual mechanisms and on the parameters of the physical stimuli but also on the extent of the observer's previous learning.