Processing of semantic relations in normal aging and Alzheimer's disease.

The decline in semantic memory observed in Alzheimer's disease is presumed to result from progressive loss of the attributes underlying category representation. Here, we explored the possibility that semantic deterioration would affect attributes differently, depending on the type of semantic relationship connecting the subject and the object of the attribution. We compared the performance of 50 patients with probable Alzheimer's disease (APs) to that of 30 elderly controls in two semantic tasks: a verbal sentence verification task and a visual test of analogical relations, both including several types of semantic relations. On the sentence verification task, the performance of APs was comparable to that of elderly controls when statements were true, but deteriorated significantly when statements were false. This result was interpreted as a failure of controlled processes to successfully search semantic space when statements were incongruent or false. In addition, all participants found some semantic relations more difficult to process than others, with relative difficulty being consistent across tasks. Taxonomic semantic relations were the most difficult, while part/whole relations were the easiest, but also the ones to deteriorate most rapidly. In contrast, functional attributes were comparatively preserved as the disease progressed. These results emphasize the role of attention and semantic context in jointly determining access to relevant attributes and categories. Furthermore, they suggest that semantic memory impairments in Alzheimer's are affected by the type of processing and semantic relationship required by the task.

Electrophysiological analysis of cortical mechanisms of selective attention to high and low spatial frequencies.

OBJECTIVES: This study investigated whether short-latency (<100ms) event-related potential (ERP) components were modulated during attention to spatial frequency (SF) cues. METHODS: Sinusoidally modulated checkerboard stimuli having high (5 cycles per degree (cpd)) or low (0.8cpd) SF content were presented in random order at intervals of 400-650ms. Subjects attended to either the high or low SF stimuli, with the task of detecting targets of slightly higher or lower SF, respectively, than the above standards. ERPs were recorded from 42 scalp sites during task performance and spatio-temporal analyses were carried out on sensory-evoked and attention-related components. RESULTS: Attended high SF stimuli elicited an early negative difference potential (ND120) starting at about 100ms, whereas attended low SF stimuli elicited a positivity (PD130) in the same latency range. The neural sources of both effects were estimated with dipole modeling to lie in dorsal, extrastriate occipital areas. Earlier evoked components evoked at 60-100ms that were modeled with striate and extrastriate cortical sources were not affected by attention to SF. Starting at 150ms, attended stimuli of both SFs elicited a broad selection negativity (SN) that was localized to ventral extrastriate visual cortex. The SN was larger over the left/right cerebral hemisphere for attended stimuli of high/low SF. CONCLUSIONS: These results support the view that attention to SF does not involve a mechanism of amplitude modulation of early-evoked components prior to 100ms. Attention to high and low SF information involves qualitatively different and hemispherically specialized neural processing operations.

Putting spatial attention on the map: timing and localization of stimulus selection processes in striate and extrastriate visual areas.

This study investigated the cortical mechanisms of visual-spatial attention in a task where subjects discriminated patterned targets in one visual field at a time. Functional magnetic imaging (fMRI) was used to localize attention-related changes in neural activity within specific retinotopic visual areas, while recordings of event-related brain potentials (ERPs) traced the time course of these changes. The earliest ERP components enhanced by attention occurred in the time range 70-130 ms post-stimulus onset, and their neural generators were estimated to lie in the dorsal and ventral extrastriate visual cortex. The anatomical areas activated by attention corresponded closely to those showing increased neural activity during passive visual stimulation. Enhanced neural activity was also observed in the primary visual cortex (area V1) with fMRI, but ERP recordings indicated that the initial sensory response at 50-90 ms that was localized to V1 was not modulated by attention. Modeling of ERP sources over an extended time range showed that attended stimuli elicited a long-latency (160-260 ms) negativity that was attributed to the dipolar source in area V1. This finding is in line with hypotheses that V1 activity may be modulated by delayed, reentrant feedback from higher visual areas.

The role of spatial selective attention in working memory for locations: evidence from event-related potentials.

We investigated the hypothesis that the covert focusing of spatial attention mediates the on-line maintenance of location information in spatial working memory. During the delay period of a spatial working-memory task, behaviorally irrelevant probe stimuli were flashed at both memorized and nonmemorized locations. Multichannel recordings of event-related potentials (ERPs) were used to assess visual processing of the probes at the different locations. Consistent with the hypothesis of attention-based rehearsal, early ERP components were enlarged in response to probes that appeared at memorized locations. These visual modulations were similar in latency and topography to those observed after explicit manipulations of spatial selective attention in a parallel experimental condition that employed an identical stimulus display.

Involvement of striate and extrastriate visual cortical areas in spatial attention.

We investigated the cortical mechanisms of visual-spatial attention while subjects discriminated patterned targets within distractor arrays. Functional magnetic resonance imaging (fMRI) was used to map the boundaries of retinotopic visual areas and to localize attention-related changes in neural activity within several of those areas, including primary visual (striate) cortex. Event-related potentials (ERPs) and modeling of their neural sources, however, indicated that the initial sensory input to striate cortex at 50-55 milliseconds after the stimulus was not modulated by attention. The earliest facilitation of attended signals was observed in extrastriate visual areas, at 70-75 milliseconds. We hypothesize that the striate cortex modulation found with fMRI may represent a delayed, re-entrant feedback from higher visual areas or a sustained biasing of striate cortical neurons during attention. ERP recordings provide critical temporal information for analyzing the functional neuroanatomy of visual attention.

Spatio-temporal dynamics of attention to color: evidence from human electrophysiology.

This study characterized patterns of brain electrical activity associated with selective attention to the color of a stimulus. Multichannel recordings of event-related potentials (ERPs) were obtained while subjects viewed randomized sequences of checkerboards consisting of isoluminant red or blue checks superimposed on a grey background. Stimuli were presented foveally at a rapid rate, and subjects were required to attend to the red or blue checks in separate blocks of trials and to press a button each time they detected a dimmer target stimulus of the attended color. An early negative ERP component with an onset latency of 50 ms was sensitive to stimulus color but was unaffected by the attentional manipulation. ERPs elicited by attended and unattended stimuli began to diverge after approximately 100 ms following stimulus onset. Inverse dipole modelling of the attended-minus-unattended difference waveform indicated that an initial positive deflection with an onset latency of 100 ms had a source in lateral occipital cortex, while a subsequent negative deflection with an onset at 160 ms had a source in inferior occipito-temporal cortex. Longer-latency attention-sensitive components were localized to premotor frontal areas (onset at 190 ms) and to more anterior regions of the fusiform gyrus (onset at 240 ms). These source localizations correspond closely with cortical areas that were identified in previous neuroimaging studies as being involved in color-selective processing. The present ERP data thus provide information about the time course of stimulus selection processes in cortical areas that subserve attention to color.

Event-related brain potentials in the study of visual selective attention.

Event-related brain potentials (ERPs) provide high-resolution measures of the time course of neuronal activity patterns associated with perceptual and cognitive processes. New techniques for ERP source analysis and comparisons with data from blood-flow neuroimaging studies enable improved localization of cortical activity during visual selective attention. ERP modulations during spatial attention point toward a mechanism of gain control over information flow in extrastriate visual cortical pathways, starting about 80 ms after stimulus onset. Paying attention to nonspatial features such as color, motion, or shape is manifested by qualitatively different ERP patterns in multiple cortical areas that begin with latencies of 100-150 ms. The processing of nonspatial features seems to be contingent upon the prior selection of location, consistent with early selection theories of attention and with the hypothesis that spatial attention is "special."

Selective attention to the color and direction of moving stimuli: electrophysiological correlates of hierarchical feature selection.

Event-related brain potentials (ERPs) were recorded from subjects who attended to pairs of adjacent colored squares that were flashed sequentially to produce a perception of movement. The task was to attend selectively to stimuli in one visual field and to detect slower moving targets that contained the critical value of the attended feature, be it color or movement direction. Attention to location was reflected by a modulation of the early P1 and N1 components of the ERP, whereas selection of the relevant stimulus feature was associated with later selection negativity components. ERP indices of feature selection were elicited only by stimuli at the attended location and had distinctive scalp distributions for features mediated by "ventral" (color) and "dorsal" (motion) cortical areas. ERP indices of target selection were also contingent on the prior selection of location but initially did not depend on the selection of the relevant feature. These ERP data reveal the timing of sequential, parallel, and contingent stages of visual processing and support early-selection theories of attention that stipulate attentional control over the initial processing of stimulus features.

Shifting attention in visual space: the effects of peripheral cueing on brain cortical potentials.

The effects of attentional shifts following peripheral cues were studied in humans using event-related potentials (ERPs) and reaction times. Subjects released a key following the presentation of a target preceded by a predictive cue in the same (valid) or the opposite (invalid) visual field, or a bilateral, non-predictive cue. The stimulus onset asynchrony (SOA) separating cue and target was either 200 or 600 ms. Subjects were faster and more accurate when responding to validly cued targets. Attentional modulation of the ERP was manifested as an enhancement of P1-N1 amplitude at posterior electrode sites following a validly cued target. Furthermore, the latencies of P1, N1 and P3 were significantly shorter in valid trials than in invalid trials. These results only reached significance with the longer SOA, since ERP refractoriness distorted the response evoked by the target when the SOA was only 200 ms. The findings are discussed in the context of previous behavioral and ERP cueing studies.

Event-related potentials, spatial orienting, and reading disabilities.

Event-related potentials (ERPs) were used to investigate the effects of visual-spatial orienting on selective neural processing in boys with learning disabilities. Twenty-seven 8-12 year old boys were classified into four groups depending on whether or not they had a reading disability or attention deficit disorder. Event-related potentials were recorded over the left and right occipital, central, and frontal cortical regions. The behavioral task required the subjects 1) to rapidly switch their attention from the center to the periphery of the visual field (spatial component), and 2) to selectively respond to a target versus a nontarget flash when the target was presented in the relevant visual field (nonspatial component). The amplitude of two ERP components was enhanced in response to relevant as compared to irrelevant stimuli. The enhancement of an early negative occipital-central component, which peaked 180-200 ms following targets (N1), indicated that selective neural processing associated with spatial attention could be switched in 600 ms. This enhancement of N1 was greater in boys with than without a reading disability, which implies that reading disability is associated with enhanced spatial attention. The enhancement of a later positive component, which peaked 300-340 ms following targets (P3), suggested that nonspatial target selection was reduced in boys with a reading disability, particularly over the left occipital hemisphere. Target relevance and reading disability also influenced trial-to-trial variability in the ERP waveform. The effects of reading disability on event-related potentials did not vary as a function of attention deficit disorder, indicating that these two disorders are distinct.

Separate brain potential characteristics in children with reading disability and attention deficit disorder: color and letter relevance effects.

This experiment was on event-related potential (ERP) indicants of the selective neural processing of black vs. white and letter vs. nonletter stimuli in boys (8-12 years of age) with and without a reading disability (RD) and/or attentional deficit disorder (ADD). Selective neural processing was measured by the increase or difference in ERP amplitude in response to stimuli that were relevant as compared to irrelevant to the color or letter attention task. The 52 children that participated in the study constituted four groups: 25 normal reading children (17 without ADD and 8 with ADD), and 27 RD children (11 without ADD and 16 with ADD). ERPs were recorded over the left and right occipital, central, and frontal regions. Selective neural processing due to stimulus relevance was reduced in boys with RD as compared to normal readers. This reduced selectivity was indicated by a predominantly symmetrical reduction in the magnitude of a positive difference potential over the central regions, between 300 and 360 msec, and then by a left greater than right hemisphere reduction in the magnitude of a positive difference potential over the occipital regions, at about 400 msec. Task relevance increased the within-subject and condition variability of this occipital positive component and this effect was greater for boys without than with RD, particularly over the left hemisphere. Selective neural processing due to stimulus relevance was greater in boys with ADD as compared to those without ADD. This was indicated by an increase in the magnitude of a positive difference potential between 320 and 400 msec over the central and frontal regions and a slow, late, negative difference potential between 600 and 800 msec over the central and occipital regions. These ADD effects tended to be greater over the right than left hemisphere. The unique polarity, scalp distribution, and time course of the effects of RD as compared to ADD on ERPs to relevant stimuli clearly indicated these two disorders, in part, involve different underlying brain deficits.