Attentional effects in the visual pathways: a whole-brain PET study

Attentional effects in the visual pathways were investigated by contrasting the distribution of regional cerebral blood flow (rCBF) measured by H₂ ¹⁵O positron emission tomography (PET) during performance of a shape-matching task with the distribution of rCBF during a less demanding color-matching task. The two tasks were performed using the same stimuli: pairs of colored random shapes shown at a fixed rate (2 s per pair). In the shape-matching task, the subjects determined whether the two stimuli were the same in shape regardless of differences in size or color. In the color-matching task, the subjects determined whether the two stimuli were the same in color regardless of differences in size or shape. Mean reaction time for shape-matching exceeded mean reaction time for color-matching by nearly 200 ms. The corresponding shape-color comparison showed extensive bilateral increases in rCBF in visual areas in the occipital and parietal lobes, including the primary visual cortex. Subcortical activations were found in cerebellum (particularly the vermis) and in the thalamus with the focus in a region comprising the lateral geniculate nucleus, the pulvinar, and adjacent parts of the reticular nucleus. Frontal activations were found in a region that seems implicated in visual short-term memory (posterior parts of the superior sulcus and the middle gyrus). The reverse, color-shape comparison showed bilateral increases in rCBF in the anterior cingulate gyri, superior frontal gyri, and superior and middle temporal gyri. The attentional effects found by the shape-color comparison in the thalamus and the primary visual cortex may have been generated by feedback signals preserving visual representations of selected stimuli in short-term memory. Electronic Publication     

Working memory processes show different degrees of lateralization: evidence from event-related potentials

This study aimed to identify different processes in working memory, using event-related potentials (ERPs) and response times. Abstract polygons were presented for memorization and subsequent recall in a delayed matching-to-sample paradigm. Two polygons were presented bilaterally for memorization and a cue indicated whether one (and if so, which one of the two) or both polygons had to be memorized. A subsequent test figure was presented unilaterally to the left or right visual field and had to be compared with the memorized figure(s). ERP results suggested that memorization takes place in a visual buffer in contralateral posterior brain areas, whereas identification of the test stimulus as a target appears to be mainly a left hemispheric process. Increased response times were found for nontarget test stimuli as compared to targets, and for target test stimuli that were presented contralaterally with respect to the location of the memorized stimulus. In addition, response times were slower when two figures were memorized than when only one was memorized.     

Electrophysiological localization of brain regions involved in perceptual memory

Event-related potentials (ERP) were recorded during perceptual discrimination and short-term memory, varying the interstimulus interval (1–10 s) in delayed spatial frequency discrimination. Accuracy of discrimination remained unimpaired across this time interval, but choice reaction times increased. A brain source localization (BESA) model showed that the activity of the parietal and right temporal sources increased with long retention intervals in a sequential activation pattern where a long-latency component of the parietal source specific to the memory condition was observed, the latency of which matched a memory-related increase in choice reaction times in the cognitive task. It is suggested that the temporal sources are involved in encoding and storage of visual information, and the parietal source is involved in memory retrieval. Received: 9 April 1998 / Accepted: 11 September 1998     

A mismatch process in brief delayed matching-to-sample task: an fMRI study

Our previous ERP studies have consistently demonstrated that a negativity N270 elicited by incongruent information in visual S1–S2 matching task represents a type of conflict process, which is distinct from that represented by the classic conflict task or other negative components in delayed matching-to-sample stimulus presentation patterns, since the inter-stimulus interval between S1 and S2 was presented as shorter as 500 ms. The N270 component of ERP was shown to reflect conflict processing during the simple working memory operations. In the present study, a functional MRI (fMRI) was used to investigate the visuospatial characteristics of brain activation associated with the task eliciting N270. The fMRI data showed an increased activation in the right anterior cingulate cortex (ACC, BA 24) and right dorsolateral prefrontal cortex (DLPFC, BA 46), and activation biased to the left occipitotemporal cortex (BA 37) in the incongruent condition. It is suggested that the greater activations of the right ACC coupling with right DLPFC to incongruent task reflect functional efficiency of the right cingulo-prefrontal network during the brief visual delayed period discrimination performance and mismatched information processing. This work was approved by the local ethics committee.     

Changes in brain activity patterns in aging: The novelty oddball

A decrease in the frontal lobes' efficiency is supposed to play a role in age-related changes in cognitive function. If frontal lobes are involved in the maintenance of working memory, the elderly may require increased frontal activity because of more rapid memory decay. This is consistent with the fact that the P3 component of the eventrelated potential (ERP) has a more frontal orientation with increasing age. However, frontally distributed P3s are also observed in young people when novel stimuli are unexpectedly presented in an oddball paradigm. Young and old subjects were run in an auditory novelty oddball in which ERPs were recorded from 30 scalp sites. The young adults' P3s showed either a posterior (targets) or more frontally oriented (novels) scalp focus. The elderly were less accurate in their memory for the novel stimuli, and their P3s showed anterior and posterior foci to both targets and novels. The young adults' target P3s changed over time from a frontal to a posterior focus, whereas the old adults' did not. These results are consistent with decreased ability of the elderly to maintain the templates needed for stimulus categorization.     

Neuronal activity of somatosensory cortex in a cross-modal (visuo-haptic) memory task

Studies have shown that in the monkey′s associative cerebral cortex, cells undergo sustained activation of discharge while the animal retains information for a subsequent action. Recent work has revealed the presence of such ″memory cells″ in the anterior parietal cortex (Brodmann′s areas 3a, 3b, 1, and 2) – the early stage of the cortical somatosensory system. Here we inferred that, in a cross-modal visuo-haptic short-term memory task, somatosensory cells would react to visual stimuli associated with tactile features. Single-unit discharge was recorded from the anterior parietal cortex – including areas of hand representation – of monkeys performing a visuo-haptic delayed matching-to-sample task. Units changed firing frequency during the presentation of a visual cue that the animal had to remember for making a correct tactile choice between two objects at the end of a delay (retention period). Some units showed sustained activation during the delay. In some of them that activation differed depending on the cue. These findings suggest that units in somatosensory cortex react to visual stimuli behaviorally associated with tactile information. Further, the results suggest that some of these neurons are involved in short-term active memory and may, therefore, be part of cross-modal memory networks. Received: 24 March 1997 / Accepted: 8 May 1997     

Memory for drawings in locations: spatial source memory and event-related potentials

Event-related potentials (ERPs) were recorded during recognition tasks for line drawings (items) or for both drawings and their spatial locations (sources). Recognized drawings elicited more positive ERPs than new drawings. Independent of accuracy in the spatial judgment, the old/new effect in the source recognition task was larger over the prefrontal scalp, and of longer temporal duration than in the item recognition task, suggesting that the source memory task engaged a qualitatively distinct memory process. More posterior scalp sites were sensitive to the accuracy of the source judgment, but this effect was delayed relative to the difference between studied and unstudied drawings, suggesting that source memory processes are completed after item recognition. Similarities and differences between spatial source memory and memory for conjunctions of other stimulus attributes are discussed, together with the role of prefrontal cortex in memory.     

Tracking the voluntary control of auditory spatial attention with event-related brain potentials

A lateralized event-related potential (ERP) component elicited by attention-directing cues (ADAN) has been linked to frontal-lobe control but is often absent when spatial attention is deployed in the auditory modality. Here, we tested the hypothesis that ERP activity associated with frontal-lobe control of auditory spatial attention is distributed bilaterally by comparing ERPs elicited by attention-directing cues and neutral cues in a unimodal auditory task. This revealed an initial ERP positivity over the anterior scalp and a later ERP negativity over the parietal scalp. Distributed source analysis indicated that the anterior positivity was generated primarily in bilateral prefrontal cortices, whereas the more posterior negativity was generated in parietal and temporal cortices. The anterior ERP positivity likely reflects frontal-lobe attentional control, whereas the subsequent ERP negativity likely reflects anticipatory biasing of activity in auditory cortex.     

An ERP study on the time course of phonological and semantic activation in Chinese word recognition

With the event-related potential (ERP) technique, we examined the time course of phonological and semantic activation in Chinese word recognition. Participants did a semantic judgment task and a homophone judgment task over the same set of word pairs. Each pair was of either high or low word frequency and the two words were either unrelated or related semantically or phonologically, i.e., being homophones. For high-frequency words, both the semantically related pairs in the homophone task and the homophonic pairs in the semantic task elicited ERP responses different from the unrelated control pairs in the N400 component but not any component earlier, suggesting a similar time course for semantic and phonological activation. For low-frequency words, the semantically related pairs in the homophone task were associated with a similar modulation of N400. However, compared to the unrelated controls, the homophonic pairs in the semantic task elicited a larger P200, a component implicated in phonological processing in the literature, and thus demonstrated a phonological activation earlier than semantic activation. The results showed that word frequency affects the time course of semantic and phonological activation suggesting that phonology is not invariably activated before semantics in Chinese word recognition.     

Neural activities of tactile cross-modal working memory in humans: an event-related potential study

In the present study, we examined the neural mechanisms underlying cross-modal working memory by analyzing scalp-recorded event-related potentials (ERPs) from normal human subjects performing tactile-tactile unimodal or tactile-auditory cross-modal delay tasks that consisted of stimulus-1 (S-1, tactile), interval (delay), and stimulus-2 (S-2, tactile or auditory). We hypothesized that there would be sequentially discrete task-correlated changes in ERPs representing neural processes of tactile working memory, and in addition, significant differences would be observed in ERPs between the unimodal task and the cross-modal task. In comparison to the ERP components in the unimodal task, two late positive ERP components (LPC-1 and LPC-2) evoked by the tactile S-1 in the delay of the cross-modal task were enhanced by expectation of the associated auditory S-2 presented at the end of the delay. Such enhancement might represent neural activities involved in cross-modal association between the tactile stimulus and the auditory stimulus. Later in the delay, a late negative component (LNC) was observed. The amplitude of LNC depended on information retained during the delay, and when the same information was retained, this amplitude was not influenced by modality or location of S-2 (auditory S-2 through headphones, or tactile S-2 on the left index finger). LNC might represent the neural activity involved in working memory. The above results suggest that the sequential ERP changes in the present study represent temporally distinguishable neural processes, such as the cross-modal association and cross-modal working memory.     

Working memory and preparation elicit different patterns of slow wave event-related brain potentials

Some event-related brain potential (ERP) studies of working memory have used delayed match-to-sample designs in which a stimulus (S1) is held in memory for comparison with a subsequent stimulus (S2). During the S1-S2 interval, ERP slow negativities varied with both the type and amount of material held in working memory. One interpretation is that these slow waves index working memory operations. An alternative explanation is that they only reflect general preparatory processing for the response to S2. To decide between these explanations, we used two visual processing tasks that required similar preparation for S2. In one task, visual memory rehearsal operations were required. During the S1-S2 interval, there were clear differences between the amplitudes, topographies, and the effect of information load on the slow waves in the two tasks, thus ruling out preparation only as an explanation.     

Large lateralized EDAN‐like brain potentials in a gaze‐shift detection task

Attentional cueing tasks using gaze direction as spatial cues have sometimes yielded an early directing attention negativity (EDAN) component in the ERP, presumably reflecting the initial orienting toward the cued location. However, other studies have failed to identify an EDAN component for gaze cues, yielding an inconsistent picture. In the present study, we re‐examined the EDAN to gaze cueing, using a continuous task where the specific direction of the gaze changes was task irrelevant. Face stimuli changed gaze direction several times during each trial between direct, left‐, and right‐averted positions. Participants counted the number of gaze shifts during the trial. Results showed an unusually large EDAN‐like ERP asymmetry at posterior scalp sites that was of similar amplitude for large and small gaze shifts into the periphery. Shifts from an averted position toward a direct gaze elicited a qualitatively similar but smaller effect than shifts into the periphery. Together, these findings shed new light on gaze‐elicited spatial attention as they indicate a reflexive attention orienting, following the direction of gaze motion, even when the gaze direction itself is irrelevant for the task.     

Event-related potentials index cognitive style differences during a serial-order recall task.

Working memory and attentional inhibition processes (jointly symbolized here as WM/I) have been proposed to explain cognitive style differences in Field Dependence-Independence (FDI). FI relative to FD subjects have been found to use more effectively WM/I to operate on task-relevant information. The purpose of this study was to determine whether cognitive style differences are revealed as differences in ERP activity in a novel WM/I task. A serial-order recall task served to manipulate memory load by varying the amount and kind of information to be elaborated and retained in WM in order of temporal appearance (S1, S2); recall demand of the serial-order judgment (S3) was also concurrently varied. FI subjects engaged in deeper WM processing during the high memory load conditions relative to FD subjects; and this was measured as a higher amplitude slow negative wave (SNW), over the centro-parietal scalp extending to the frontal scalp, during the retention interval. In contrast, P300 amplitude was larger for FD subjects in the high memory load conditions following S1, which corresponded with a reduced amplitude SNW. We suggest that inhibitory processes indexed by P300, which FD subjects must mobilize to change their usually global-perceptual (i.e. shallow) attentional strategy for processing task information, may have resulted in less mental-attentional (WM/I) resources available to them during the task's retention phase (Rosen and Engle, 1997). Thus, ERP methods can be used to investigate differences in cognitive style.     

Electrocortical Signs of Levels of Processing: Perceptual Analysis and Recognition Memory

The electrocortical manifestations of levels of perceptual processing and memory performance were investigated by recording event related potentials (ERPs) during a verbal comparison task and a subsequent test of recognition memory. Two words were judged, on each trial, to be the same or different according to an orthographic, phonemic, or semantic criterion. Orthographic processing of words led to poor performance on a test of recognition memory whereas phonemic and semantic processing led to increasingly better performance. “Same” judgments led to better memory performance for phonemically and semantically processed items. The ERP waveforms included a late positive component (LPC) and a slow wave, both of which were responsive to the comparison criterion and the type of judgment. Further, the LPC appeared to index recognition accuracy in the memory test. The ERP data are interpreted as reflecting associative activation in the human memory system.     

Multiple Sources of P3b Associated with Different Types of Information

This experiment investigated how the P3a, P3b, and Slow Wave components of the event-related brain potential (ERP) respond to manipulations of the nature, timing, and extent of information delivery. There were two experiments in which the total amount of task information was distributed between pairs of successive stimuli (S1 and S2) within each trail. The task was to predict the relation between S1 and S2. In Experiment 1, the S1 could resolve no, partial, or all uncertainty with respect to the prediction outcome (correct or incorrect). Each S1 delivered three types of information: 1) outcome information--which resolved the subjects' uncertainty about the correctness of their prediction; 2) procedural information--which resolved uncertainty about how much outcome information would be delivered by S1; and 3) memory information--the identity of S1, which had to be stored for subsequent comparison with S2. In Experiment 2, the activity of these components was contrasted in two conditions in which the S1 delivered either memory information alone or both memory and procedural information. P3a and Slow Wave were sensitive only to outcome information. P3b was sensitive to all three types of information, and its scalp topography varied as a function of the type of information. The topographic variations indicate that P3b is not a unitary phenomenon but rather is a composite of activity arising from multiple intracranial sources of bioelectric activity.     

Deviance detection in congruent audiovisual speech: Evidence for implicit integrated audiovisual memory representations

Detection of deviant speech syllables embedded in continuous noise was investigated in an oddball paradigm. Behavioral results showed improvement of detecting and identifying the syllables when congruent visual speech accompanied the utterances. A centrally maximal negative ERP difference wave peaking at approximately 290 ms post-stimulus was elicited by audiovisual but not by auditory- or visual-only task-irrelevant deviant syllables. Whereas the circumstances of the elicitation of this ERP response are similar to those of the mismatch negativity component (MMN and its visual counterpart, vMMN), its scalp distribution differs from that of both unimodal MMNs. Elicitation of an MMN-like ERP response (termed here as the audiovisual MMN: avMMN) suggests that detection of the audiovisual deviants involved integrated audiovisual memory representations. The pattern of behavioral and ERP results suggest that the formation of such cross-modal memory representation does not require voluntary operations and may even proceed for stimuli outside the focus of attention.     

Modality-independent involvement of the left BA 44 during lexical decision making

During a lexical decision task, lexical decision making can be distinguished from lexical retrieval. Lexical decision making is independent of stimulus modality and not reflected in the decision times, whereas the opposite holds for lexical retrieval. In neuroimaging studies investigating lexical decision tasks with either visual or auditory stimuli these two processes have so far been confused. Therefore, it is not clear whether the activation of Broca’s region, consisting of the left Brodmann’s area (BA) 44 and BA 45, reported in such studies really reflects lexical decision making. The present event-related fMRI study investigated the role of Broca’s region in lexical decision by analyzing brain activation that is independent of stimulus modality or decision times. Twenty-two healthy participants performed lexical decisions on visual and auditory real words and pseudo-words. The left BA 44 was conjointly activated during visual and auditory lexical decisions as compared to rest indicating the modality-independent involvement of BA 44 in lexical decision tasks in general. To identify brain activation related to lexical decision making rather than lexical retrieval the decision times were entered as covariates into the fMRI analysis. In this analysis, the left BA 44 was activated for visual and for auditory lexical decision making. These results indicate that the left BA 44 as a distinct sub-part of Broca’s region plays an important role in lexical decision making independently of stimulus modality and decision times.     

人脑内不同机制参与视觉图形冲突的处理

目的：研究脑内对具有冲突信息的立体图形及平面图形处理的机制。方法：刺激对由觉的平面图形及立体图形随机组合而成,分为四种状态,a,刺激对由两个完全相同的立体图形组成;b.刺激对由两个完全相同的平面图形组成;c．刺激对由两个构型不同的立体图形组成;d.刺激对由两个构型不同的平面图形组成,受试者的任务;辨别前后呈现的两个刺激（第一个刺激S1,第二个刺激S2）形状是否相同。结果：在c,d两种状态下,已记录到冲突负波N270,该负波不出现于a,b状态,同时,N270的最大振幅在c状态下出现在右侧后头部,而d状态下则出现在前头部,结论;脑对具有冲突特征的立体及平面图形具有不同处理机制。     

Effects of aging on event-related brain potentials and reaction times in an auditory oddball task

Auditory event-related potentials (ERPs) were recorded from 71 healthy individuals between 18 and 82 years of age during performance of a disjunctive reaction time task in an auditory oddball paradigm. The effects of aging on reaction times and on the latencies, amplitudes, and distributions of each of the main ERP components were examined. No significant slowing of the reaction times of the elderly subjects was observed in relation to the younger ones. The peak latencies of both the N1 and P2 components elicited by standard tones were slightly but significantly slowed with age. In the ERPs of target tones, the later, endogenous components (N2, P3, and SW) showed linear increases in latency as a function of age; the later the component, the longer the age-related delay. In general, aging was associated with less negativity (both N2 and SW) and more positivity (P3) over the anterior scalp, together with a smaller P3 and a more pronounced N2 over posterior scalp areas. Most of the effects observed in target ERPs were also evident in the difference waves derived from subtraction of the standard from the target ERPs, although the slope of the age-related latency increase of N2 was shallower and that of the P3 was steeper in the difference ERPs. These findings are discussed in relation to previous accounts of ERP changes with aging.     

Effects of emotion and semantic relatedness on recognition memory: Behavioral and electrophysiological evidence

Some aspects of our memory are enhanced by emotion, whereas others can be unaffected or even hindered. Previous studies reported impaired associative memory of emotional content, an effect termed associative “emotional interference”. The current study used EEG and an associative recognition paradigm to investigate the cognitive and neural mechanisms associated with this effect. In two experiments, participants studied negative and neutral stimulus-pairs that were either semantically related or unrelated. In Experiment 1 emotions were relevant to the encoding task (valence judgment) whereas in Experiment 2 emotions were irrelevant (familiarity judgment). In a subsequent associative recognition test, EEG was recorded while participants discriminated between intact, rearranged, and new pairs. An associative emotional interference effect was observed in both experiments, but was attenuated for semantically related pairs in Experiment 1, where valence was relevant to the task. Moreover, a modulation of an early associative memory ERP component (300–550 ms) occurred for negative pairs when valence was task-relevant (Experiment 1), but for semantically related pairs when valence was irrelevant (Experiment 2). A later ERP component (550–800 ms) showed a more general pattern, and was observed in all experimental conditions. These results suggest that both valence and semantic relations can act as an organizing principle that promotes associative binding. Their ability to contribute to successful retrieval depends on specific task demands.