The effect of experimental 'scotomata' on the ipsilateral and contralateral responses to pattern-reversal in one half-field.

The averaged cortical responses to a reversing checkerboard pattern presented monocularly in either left or right visual half-fields have been recorded from the occipital scalp using a transverse chain of widely spaced electrodes referred to a common mid-frontal electrode. The half-field responses showed a consistent asymmetry, the dominant feature of which was a positive wave (P100) that was widespread on the ipsilateral scalp and maximally recorded from the midline and ipsilateral electrodes. This formed part of the triphasic negative-positive-negative complex, the other two components being an N75 and an N145. On the contralateral scalp it was generally possible to record a triphasic complex of opposite polarity, but this was usually of smaller amplitude and its components (P75, N105, P135) showed greater variation in latency and morphology than the ipsilateral components. With progressive occlusion of the pattern stimulus from the central regions of the visual half-field, the ipsilateral positive wave (P100) was increasingly attenuated, while components of the contralateral complex were relatively unaffected, or, in some cases, enhanced. By contrast, reducing the radius of the stimulated area had relatively little effect on the ipsilateral P100, while the contralaterally recorded response was attenuated. These differential effects on the half-field response components are discussed in relation to the anatomy of the central and paracentral cortical representation of the visual field. The implications for the interpretation of evoked potential recordings in patients with field defects are considered.     

Appropriate stimulation in visual evoked potential to evaluate visual perception state of athletes★

BACKGROUND: Several studies have demonstrated that visual evoked potentials can be influenced by sport events. To the best of our knowledge, there are no specific parameters for the most appropriate stimulation for evaluating the functional state of athletes. OBJECTIVE: To investigate the best stimulation in visual evoked potential to apply to functional evaluation of athletes. DESIGN, TIME AND SETTING: Ninety-five, healthy students from the Shandong Normal University took part in an observational, contrast study. PARTICIPANTS: All active participants were male. Sixty-five students majored in physical education, and had participated in exercise for the duration of (4.26 ± 3.08) years. An additional 30 students majored in other subjects. METHODS: The neural electricity tester, NDI-200, was adapted to examine and record visual evoked potential with varying probes using bipolar electrodes attached to the head of all the participants in a dark room. The visual evoked potential values were analyzed transversally. A chessboard pattern reversal method was applied with the following parameters: 2 Hz stimulation frequency, brightness of 90 cdp, 80% contrast, 1–100 Hz bandpass filters, and 10 μV sensitivity; 100 responses were averaged. MAIN OUTCOME MEASURES: latency, peak latencies, and inter-peak latencies were measured in N75, P100, N145 with varying probe stimulations. RESULTS: (1) Comparisons between the little check, middle check, and big check stimulation, demonstrated that the common tendencies in visual evoked potential indexes of the two groups of N75 latency were successively shorter and N145 were longer. P100-N145 peak latency was decreased and each inter-peak latency was longer. (2) Changes of N75, P100, and N145 with different check stimulations in the physical education student group: after compared with the middle check stimulation, N75 latency was significantly longer (P < 0.01), and N75-P100 inter-peak latency (P < 0.05) and N75-N145 inter-peak latency were both shorter (P < 0.05). N75-P100 inter-peak latency was shorter (P < 0.01) in the little check stimulation. When compared with the big check stimulation, N75 latency was significantly longer (P < 0.01) and N145 was shorter (P < 0.01). Compared with the big check stimulation, N145 latency was significantly shorter (P < 0.05). (3) Changes of N75, P100, and N145 with different check stimulations in the normal students: when compared with the big check stimulation, N75 latency was significantly longer (P < 0.05) and N145 latency was shorter (P < 0.05). Each inter-peak latency was shorter (P < 0.05) in the little check stimulation. When comparing the middle check stimulation, N75-N145 inter-peak latency was shorter (P < 0.05). CONCLUSION: Large visual evoked potential differences were observed between students majoring in physical education and other subjects when medium probe stimulation was applied. These results suggest that the use of medium probe stimulation (25 mm×25 mm) should be adopted when evaluating the functional state of athletes. Key Words: visual evoked potential; functional evaluation; probe stimulation     

Spatial dissociation of early and late colour evoked components

Evoked potentials to the primary colours red, green, yellow and blue were recorded and compared with those evoked by white. The unpatterned 10 degrees X13 degrees stimuli were generated with the aid of a colour monitor. Activity was depicted with 5 electrodes, the central electrode 5 cm above the inion and two on each side 5 and 10 cm apart from the central electrode. With equally bright colour stimuli a previously described early negative colour-dominated component N87 was localized in all subjects at the central occipital electrode while the following positivity P100 was clearly more lateralized to the peripheral electrodes. With half-field stimulation N87 showed a similar--paradoxical--lateralization to the ipsilateral electrodes as has been demonstrated for pattern reversal. The existence and localization of N87 could be confirmed also for blue colours, its amplitude independent of the blue luminance, its latency decreasing for definite additional brightness increments and decrements. The N87 to blue was of the same latency as the N87 components to other colours. N87 is mainly generated foveally and parafoveally, its amplitude saturates with stimuli larger than 6-8 degrees in diameter.     

Effect of central scotomata on pattern reversal visual evoked potentials in patients with maculopathy and healthy subjects

The effect of central scotomata on pattern reversal visual evoked potential (PVEP) was investigated in patients with maculopathy and healthy subjects. PVEP was evoked monocularly by both full-field and half-field stimulations. Since the latency of 'the major positivity at Oz' (P100-Oz) is used as the most reliable parameter in the clinical application of PVEP, special attention was focused on its changes, comparing with 'ipsilateral major positivity of half-field PVEP' (P100-IHF). Although the incidence of modification was lower in the patients, central scotomata modified PVEPs of the healthy subjects and of the affected eye of the patients in a similar manner: full-field PVEP showed prolonged latency and reduced amplitude of P100-Oz. Half-field PVEP disclosed prolonged P100-Oz latency with intact P100-IHF latency. Only difference was that amplitude reduction of both P100-Oz and P100-IHF of half-field PVEP was observed only in the healthy subjects. The prolonged P100-Oz latency of half-field PVEP was accompanied, both in the healthy subjects and in the patients, by a contralateral negative-positive complex (N105-P135) which was augmented and extended to Oz. The prolonged P100-Oz latency, thus, was due to the pronounced P135. These observations suggested that an attenuation of the afferent impulses from the central retina may cause a prolongation of the P100-Oz latency in both healthy subjects and patients, but this is not a reflection of the truly prolonged P100-IHF latency. It was concluded that, in the clinical application of PVEP, recordings of half-field PVEP from the lateral electrodes seem to be essential to distinguish true prolongation of the P100-Oz latency.     

Topographic aspects of visual evoked potentials recorded after flash-pattern stimulation in normal subjects

Visual evoked responses have been recorded on 14 leads placed on the scalp. They were localised on the central, parietal, temporal and occipital regions. The common reference was linked-ears. The stimulus consisted of a flash pattern. The size of the pattern and of each square was respectively of 20 degrees and 30 min. The stimulation of the total visual field had evoked two families of curves. One of these types of curves was posterior (occipital, temporal and parietal) and consisted in a positive, negative, positive wave (100, 136, 200 msec on average) and the second type was localised on the central leads and much more rhythmic than the posterior response. The initial complex (negative, positive, negative, positive: 70, 92, 120, 178 msec) was followed by a slow after-discharge. The half-field stimulation evoked a P100 component on the contralateral posterior leads, but this P100 was less prominent on the ipsilateral posterior leads. The modifications were particularly evident at the level of the temporal posterior electrodes. This P100 component was the only wave to be modified by half-field visual stimulation. The posterior N136 and P200 wave and all the rhythmic central response were exactly the same with half or total visual field stimulation. The results have obviously shown that the different waves of the visual evoked responses are not coming from the same sources. The interpretation of multichannel evoked recordings was therefore very difficult.     

The influence of pattern size on amplitude, latency and wave form of retinal and cortical potentials elicited by checkerboard pattern reversal and stimulus onset-offset.

Transient pattern electroretinograms (PERGs) and visual evoked potentials (VEPs) were recorded with checkerboard pattern reversal and equiluminance stimulus onset-offset, elicited by a high quality moving mirror stimulator. Different sized checkerboard patterns (0.35-4.2 c/deg) were used as stimulus patterns. The wave forms of the equiluminance stimulus onset responses were similar to ERGs evoked with luminance decrease and the stimulus offset PERGs were like ERGs elicited by luminance increase. The PERG c wave and the VEP showed spatial frequency tuning with pattern reversal and stimulus offset. Spatial frequency tuning was not detectable with PERG a and b waves. Pattern reversal and stimulus onset evoked PERGs had no major spectral components above 40 Hz; stimulus offset evoked PERGs contained components up to 55.3 Hz. Retino-cortical time--measured as a latency difference of the PERG b wave to VEP P100--was identical with pattern reversal and stimulus onset and about 12 msec longer with stimulus offset. Our results suggest that the 3 stimulation modes, reversal, onset and offset induce different types of processing at the retinal and cortical levels. PERG a and b waves to our high luminance/contrast stimuli contain no pattern specific information and the c waves are the sum of luminance and pattern specific responses.     

Visual event-related potentials index focused attention within bilateral stimulus arrays. I. Evidence for early selection.

Event-related potentials (ERPs) were recorded from the scalp while subjects attended to sequences of bilaterally symmetrical arrays of 4 letters (2 in each visual half-field) that were flashed briefly at intervals of 280-520 msec. These sequences also included randomized presentations of unilateral 'probe' stimuli consisting of irrelevant bars (experiment 1) or potentially relevant letter pairs (experiment 2). The task was to pay attention to the letter pairs in either the left or the right half-field on a given run and to press a button when the two letters matched one another (targets). The ERPs to the bilateral arrays included an early positive wave (P1, peaking at 135 msec) that was enhanced over posterior scalp sites contralateral to the attended visual field. Both types of probe stimulus also elicited a larger early positivity in the P1 latency range (100-200 msec) when delivered to the attended half-field, followed in some cases by a more prolonged positive deflection. Notable for its absence was any sign of an enlarged posterior N1 component (160-200 msec), which was prominent in the ERP to attended-field stimuli in previous studies using randomized sequences of unilateral stimuli. Attended-field targets elicited large N2 and P3 (P300) components, which were greatly reduced or absent when targets occurred in the unattended field. The observed ERP effects were interpreted in terms of early sensory selection during visual spatial attention.     

Dissociation of visual evoked responses to hemi-field or full-field flash-checkerboard stimulation

Unexpected visual evoked responses (VERs) were recorded in 5 subjects with tumoral, ischemic or hemorrhagic lesions of the retrochiasmatic visual pathways. The flash pattern stimulation was always binocular and involved full-field and half-field stimuli. In these 5 cases, the total field VER was asymmetrical with anomalies on the affected occipital region. However half-field VERs P100 contralateral to the stimulus were noted both on the normal and on the affected occipital region. One can ask if this is not an electrophysiological equivalent of the clinical relative hemianopsia.     

Pattern reversal evoked potentials: age, sex and hemispheric asymmetry

Binocular pattern reversal evoked potentials (PREPs) and EEG power were recorded bilaterally from occipital scalp of 50 boys and 50 girls aged 5-14. PREP latency was unaffected by either age or gender. Amplitudes, however, were consistently larger for girls than boys, with differences diminishing by adolescence. Significant electrophysiological asymmetries were also found; larger PREP amplitudes were measured at the right occipital electrode than the left for the two waves investigated. These asymmetries were apparent in both girls and boys across all ages, although they tended to diminish with maturation. EEG alpha asymmetry did not correlate with PREP amplitude asymmetry. PREPs elicited by half-field stimulation of 8 girls aged 9-10 years determined that 'paradoxical lateralization' of the half-field response was the same for both left and right half-fields, thus failing to account for the asymmetry of the full-field binocular response.     

Flash-pattern visual evoked potentials and limited lesions of the occipital lobe

Four patients with restricted lobe lesions and one patient with large occipital lesions were selected for VEP studies to full-field and half-field flash pattern stimuli. The abnormalities of the responses were more important with full-field stimuli. In three patients with inner side occipital lobe damage, the P100 component disappeared ipsilaterally to the lesion, but N140 remained normal on bilateral occipital leads. In one patient with outer side occipital lobe damage, P100 was normal on bilateral occipital leads, but N140 disappeared ipsilaterally to the lesion. In one patient with large occipital lesion, P100 and N140 both disappeared ipsilaterally to the lesion. It appears that the P100 component is related to the inner and that N140 is related to the outer area of the occipital lobe.     

Pattern evoked potentials in human albinism: Evidence of two different topographical asymmetries reflecting abnormal retino-cortical projections

In fifteen subjects with oculo-cutaneous or ocular albinism pattern-reversal visual evoked potentials (VEP) were recorded to monocular whole-field (16° radius) and right and left vertical half-field (16° radius) stimulation from an array of occipital electrodes.In all 15 subjects the VEP was of low amplitude and the monocular response to whole-field stimulation showed abnormal asymmetry in scalp topography. This asymmetry was similar to that produced by stimulation of the temporal half-field in the same eye. Two distinct types of VEP asymmetry were readily identified in different subjects. In 5 the major positivity of the response ($\text{P100}$) was distributed in the channels ipsilateral to the stimulated eye or temporal half-field, being similar in distribution to the $\text{P100}$ from the temporal hemifield of normal individuals. In 9 other subjects the converse occurred; the $\text{P100}$ was distributed contralaterally. The remaining subject was unique in that the responses from each eye had a distribution that resembled one of the two main groups. The nasal half-field responses also differentiated the two groups. On stimulation of the nasal field in the first group the $\text{P100}$ was usually absent or attenuated and when present it had the same topography as the temporal half-field $\text{P100}$. In contrast the nasal half-field $\text{P100}$ from the second group was always present and, although almost invariably smaller than the temporal half-field $\text{P100}$, it had an identical distribution.There was no correlation between these two patterns of VEP asymmetry and clinical or known genetic features.The findings confirm that in human albinos each hemisphere receives a predominantly monocular input from the contralateral eye; i.e. in addition to the temporal half-field, approximately 20° of the nasal half-field is projected to the hemisphere contralateral to each eye and not, as in normal subjects, to the ipsilateral hemisphere. Furthermore, the finding of two distinct and in some respects opposite types of VEP topographical asymmetry raises the possibility of two variants of the geniculo-cortical projection patterns existing in human albinos similar to those described in the Siamese cat.     

Hemispheric differences for visual evoked potentials from checkerboard stimuli

Visual evoked potentials (VEPs) were elicited in right-handed male subjects with an alternating checkerboard pattern stimulus presented to either the left or right visual half-field. The sizes of the individual checks in different conditions were 0.25, 0.5, 2.0, or 4.0 cycles/degree of visual angle. The amplitude of the P100 VEP decreased while peak latency increased as check size decreased across both visual field conditions. Left hemisphere responses demonstrated significantly larger P100 amplitudes compared to the right hemisphere responses, although the interaction between hemisphere and stimulus size was not significant. No hemispheric effects of P100 latency were observed. The results suggest that the left hemisphere is engaged more than the right hemisphere for the sensory analysis of visual stimuli composed of straight edges over a wide range of spatial frequencies.     

White noise effects on pattern-reversal visual evoked potentials.

Visual evoked potentials (VEPs) were recorded in 9 subjects from occipital and temporal leads. The stimulus was a checkerboard phase-reversed at the frequency of 1 Hz, binocularly viewed by the subject. VEPs were recorded during white noise stimulation (9 different levels of intensity) and without noise stimulation. P100 latency was not affected by the white noise stimulation, whereas the N75-P100 amplitude turned out to be affected by the simultaneous auditory stimulation with different patterns in relation to the site of the recording. The results are discussed in terms of general activation aroused by the white noise on visual information processing.     

The attentional effects of peripheral cueing as revealed by two event-related potential studies.

OBJECTIVE: The mechanism of visual spatial attention elicited by peripheral cueing was investigated in two studies. METHOD: Event-related potentials (ERPs) were recorded when the subjects were performing a spatial frequency discrimination task and a location discrimination task. Stimuli were randomly flashed in the left or right visual field. Prior to each stimulus a peripheral cue was presented with a validity of 75%. RESULTS: The subjects responded faster to valid trials than to invalid trials. The earliest visual ERP component, C1, was not modulated by the cue validity, suggesting that visual spatial attention elicited by peripheral cueing does not involve striate cortex. Valid trials elicited larger contralateral P1 but a smaller contralateral N1 than invalid trials. The early onsets of these attentional effects show that spatial attention affects stimulus processing at early sensory/perceptual stages. The latencies of contralateral P1 and contralateral N1 were shorter for invalid trials, however. The ipsilateral N1 was enhanced by valid trials in the spatial frequency discrimination task but was not in the location discrimination task, whereas the contralateral N1 was larger for invalid trials than for valid trials in both tasks. CONCLUSION: The results indicate that involuntary allocation of attention involves different mechanisms from voluntary allocation of attention.     

Bimodal evoked potentials during long-term therapy with conventional or slow release preparations of carbamazepine and valproic acid in children and adolescents with epilepsy

The measurement of evoked potentials (EPs) may be particularly useful in clinical neuropharmacology for investigation of drug effects of afferent nerve conduction within CNS. The study aims at estimating the long term effects of conventional or slow release formulation (CR) of carbamazepine (CBZ) and valproid acid (VPA) on visual (VPA) and brainstem auditory (BAEP) evoked potentials. Investigation covered 125 patients 8 to 18 years old to whom both formulations of CBZ or VPA were administered in monotherapy. Everyone received a drug dosage which gave an adequate therapeutic plasma concentration and satisfactory seizure control. Antiepileptic drugs (AEDs) plasma levels were measured by means of fluorescence polarization immunoassay method aided of TDx Analyzer (Abbott, Diagnostic). EPs were registered by means of Multiliner (Toennies, Germany). A pattern of reversal stimulation for VEP was used. The latencies of N75, P100, N145 as well as interpeak amplitudes of N75/P100, P100/N145 were evaluated. The following BAEP parameters were considered: morphology of the potential, absolute latencies of waves I, III, V and I-III, III-V, I-V. EP were always performed in the same conditions and with the same equipment for the epileptic and control groups. The obtained values were compared with age-matched control group. The following BAEP abnormalities were observed: prolonged absolute latencies of waves I, III, V as well as prolonged IPLs I-III. The BAEP V/I amplitude ratio and morphology of the waves were normal in all patients. The VEPs abnormalities manifested as prolongation of P100 or N145 latencies and reduction of amplitudes N75/P100, P100/N145. Results of these electrophysiological studies with CBZ and VPA demonstrate that EP are sensitive, noninvasive reflectors of AEDs effects within the CNS.     

Flexibility in spatial and non-spatial feature grouping: an event-related potentials study

Early perceptual grouping was studied using 256-channels event-related potentials in a choice response task. The task involved the detection of a triangle configuration of Gabor patches among patches of different spatial frequencies. The influence of two task-irrelevant factors was compared. One was the spatial proximity relation between the target patches and the other their relative orientation, a non-spatial relationship. Non-spatial effects were predominant in early peaks N64 and P100 in the occipital areas, and were reduced in size for later peaks. Spatial effects started from N180 in the occipital areas and continued in P250 and P430 in the central areas, increasing in size with time. These findings constitute a case of reversal of the usual order of spatial and non-spatial feature processing, illustrating that the flexibility in the early visual system may be greater than previously assumed.     

Chlordimeform produces contrast-dependent changes in visual evoked potentials of hooded rats

Previous experiments found that acute exposure to the insecticide/acaricide, chlordimeform (CDM), produced large increases in the amplitude of pattern reversal evoked potentials (PREPs) without changing the amplitude of flash evoked potentials (FEPs) in the same rats (W. K. Boyes and R. S. Dyer, Exp. Neurol. 86: 434-447, 1984). Current work investigated the influence of physical characteristics of the evoking stimuli on the action of CDM. Adult male Long-Evans rats with epidural visual cortex electrodes were used. In experiment 1, PREPs were elicited with alternating gratings having equal contrast (99%) and a square wave spatial luminance profile at several spatial frequencies. Rats treated 1 h previously with 40 mg/kg CDM had increased PREP amplitudes at 0.1, 0.2, and 0.4 cycles per degree (cpd), but not at 0.8 cpd. No changes were found after 5 mg/kg CDM. In experiment 2, PREPs were elicited with gratings oriented at 0 degrees (horizontal), 45 degrees, 90 degrees, or 135 degrees. Treatment with 40 mg/kg CDM increased PREP amplitudes and latencies regardless of orientation. In experiment 3, FEPs elicited with strobe flashes spanning four log units of intensity showed a small but significant CDM dose X intensity interaction on P2N2 peak-to-peak amplitude. In experiment 4, PREPs were elicited with alternating gratings having a sinusoidal spatial luminance profile, spatial frequency of 0.2 or 0.8 cpd, and contrast ranging from noise levels to 65%. Rats treated with 40 mg/kg CDM showed increased peak-to-peak amplitudes only at 0.2 cpd and only at contrast values above 10%. The failure of CDM to alter PREPs at 0.8 cpd was attributed to low contrast sensitivity at that spatial frequency. The results demonstrated that the action of CDM on visual evoked potentials was dependent on the amount of contrast in the stimulus pattern, and suggested that CDM alters the encoding of visual contrast.     

Contrast and stereoscopic visual stimuli yield lateralized scalp potential fields associated with different neural generators.

The use of dynamic random-dot stereograms (RDS) allows to investigate evoked potential components generated exclusively by cortical structures. We analyzed the scalp distribution of stereoscopically evoked or contrast evoked potential field by recording electrical brain activity in 20 channels simultaneously from an electrode array covering the occipital scalp areas. Evoked brain activity was obtained from 13 healthy adults with dynamic RDS stimuli presented as a stereoscopic checkerboard pattern in the center, or in the right or left visual half-field. Such stereoscopically evoked scalp potential distributions were compared to those elicited by a conventional 2-dimensional checkerboard reversal stimulus of the same mean luminance and retinal extent. We found that the latencies of the major evoked components were similar for contrast and stereoscopic stimuli, while significant differences were observed when we compared the strength of the evoked potential fields or the topographical pattern elicited by lateralized stereoscopic and contrast stimuli. The functional relation of evoked electrical brain activity to the retinal stimulus location was significantly different for stereoscopic and contrast stimuli. We present evidence that stereoscopic perception relies on the activation of cortical structures in the human visual system that are different from those activated by comparable contrast stimuli, supporting the conclusions derived from our earlier electrophysiological experiments on stereoscopic vision. These data on the physiological correlates of processing of stereoscopic information in humans are in line with the results obtained with single neuron recordings from the cat and monkey visual cortex.     

Further investigation of visual evoked potentials elicited by lateralized stimuli: effects of stimulus eccentricity and reference site

Visual evoked potentials (VEPs) to small lateralized flashes were recorded from the parietal midline, homotopic lateral central and occipital electrodes, and from left and right mastoid processes, all referred to a balanced non-cephalic reference. Two stimulus eccentricities, 4 degrees and 10 degrees, were employed, and a comparison made between the non-cephalic and linked mastoid references. P120 (measured at lateral occipital sites only) peaked earlier and was of smaller amplitude at the electrode contralateral to the visual field of stimulus exposure. N160 peaked earlier at central than occipital sites, was larger from electrodes over the contralateral hemisphere, and at the occipital sites only, peaked earlier in the electrode contralateral to the visual field of stimulus exposure. These effects were virtually unaffected by the eccentricity manipulation and it is concluded that light scatter across the visual midline is unlikely to be responsible for the observed pattern of ipsilateral-contralateral VEP asymmetries. The mastoids were found to detect significant stimulus-locked activity in the same latency range as the occipital N160 component. However, comparison of the non-cephalic and linked mastoid references revealed only non-specific effects, and no sign of any change in the pattern of ipsilateral-contralateral VEP asymmetries, or the magnitude of the associated latency differences. It is concluded that these effects may be of value in the study of callosal transfer.     

Luminance and spatial attention effects on early visual processing

Event-related potentials (ERPs) were recorded from healthy subjects in response to unilaterally flashed high and low luminance bar stimuli presented randomly to left and right field locations. Their task was to covertly and selectively attend to either the left or right stimulus locations (separate blocks) in order to detect infrequent shorter target bars of either luminance. Independent of attention, higher stimulus luminance resulted in higher ERP amplitudes for the posterior N95 (80–110 ms), occipital P1 (110–140 ms), and parietal N1 (130–180 ms). Brighter stimuli also resulted in shorter peak latency for the occipital N1 component (135–220 ms); this effect was not observed for the N1 components over parietal, central or frontal regions. Significant attention-related amplitude modulations were obtained for the occipital P1, occipital, parietal and central N1, the occipital and parietal P2, and the parietal N2 components; these components were larger to stimuli at the attended location. In contrast to the relatively short latencies of both spatial attention and luminance effects, the first interaction between luminance and spatial attention effects was observed for the P3 component to the target stimuli (350–750 ms). This suggests that interactions of spatial attention and stimulus luminance previously reported for reaction time measures may not reflect the earliest stages of sensory/perceptual processing. Differences in the way in which luminance and attention affected the occipital P1, occipital N1 and parietal N1 components suggest dissociations among these ERPs in the mechanisms of visual and attentional processing they reflect. Nonetheless, scalp current density mappings of the attention effects throughout the latency ranges of the P1 and N1 components show the most prominent attention-related activity to be in lateral occipital scalp areas. Such a pattern is consistent with the spatially selective filtering of information into the ventral stream of visual processing which is reponsible for complex feature analysis and object identification.