Observations on the response to photic stimulation in organic cerebral dysfunction

Effect of normal aging upon interhemispheric EEG coherence: analysis during rest and photic stimulation

The present study was conducted to examine and compare interhemispheric EEG coherence at rest and during photic stimulation (PS; 5, 10 and 15 Hz) in 30 younger subjects aged 22.1 +/- 2.2 (mean +/- S.D.) and 25 elderly subjects aged 56.8 +/- 4.9. The elderly subjects had significantly lower coherence in the resting EEG for the delta, theta, alpha-3, beta-1 and beta-2 frequency bands. In contrast, EEG analysis during PS showed that elderly subjects had significantly higher coherence in the frequency corresponding to PS at 15 Hz. In addition, when we examined the changes in coherence from the resting state to the stimulus condition (i.e., coherence reactivity), elderly subjects had significantly higher coherence reactivity to PS at 15 Hz. These findings suggest a low interhemispheric functional connectivity in elderly subjects under non stimulus conditions and a high connectivity during photic stimulus.

Aberrant functional organization in schizophrenia: analysis of EEG coherence during rest and photic stimulation in drug-naive patients

EEG coherence provides a measure of functional correlations between two EEG signals. The present study was conducted to examine intrahemispheric EEG coherence at rest and during photic stimulation (PS) in 18 drug-naive patients with paranoid schizophrenia and 30 control subjects. Compared with the controls, the schizophrenic patients had significantly higher intrahemispheric coherence of the resting EEG for the delta band, although no significant group differences were found for other frequency bands. EEG analysis during PS showed that the patients also had significantly higher EEG coherence over the left posterior regions. In this study, we also examined the changes in intrahemispheric coherence from rest to the stimulus condition (i.e., PS-related coherence reactivity); the patients were found to show significantly smaller changes, with significant group differences being also confined to the posterior regions in the left hemisphere. These findings provide evidence that schizophrenic patients have abnormal EEG coherence in both resting and stimulus conditions and suggest more diffuse, undifferentiated functional organization within hemispheres. In addition, diminished coherence reactivity suggests a failure of PS-related functional reorganization in schizophrenia.

Interhemispheric EEG coherence in never-medicated patients with paranoid schizophrenia: analysis at rest and during photic stimulation

We assessed functional relationships between hemispheres by calculating interhemispheric EEG coherence at rest and during photic stimulation in 18 never-medicated patients with paranoid schizophrenia and 30 control subjects. Although no significant group differences were found in the resting EEG, the schizophrenic patients had significantly higher coherence on EEGs recorded during photic stimulation, compared to the control subjects. In this study, we also examined the changes in interhemispheric coherence from rest to the stimulus condition (i.e., stimulation-related coherence reactivity); the patients were found to show significantly greater coherence reactivity to photic stimulation. These findings provide further evidence that schizophrenic patients have a higher degree of interhemispheric functional connectivity and thus have less lateralized cerebral organization than normal subjects. Our results also suggest that schizophrenic patients have excessive functional reorganization between hemispheres in association with photic stimulation.

Responses to photic stimulation in patients with occipital spikes

OBJECTIVE

To determine the effect of intermittent photic stimulation (IPS) and frequency of asymmetric driving responses in patients with occipital spikes.

METHODS

The amplitude of the driving response at 4 flash frequencies was measured from a referential montage in 60 patients with occipital spikes and in 60 normal EEG records from age-matched patients. Responses were classified as asymmetric if the amplitude at one occipital area was less than 50% of the amplitude at the other.

RESULTS

A measurable photic response occurred significantly less frequently in patients with occipital spikes (48%) compared to the control group (70%; Fisher's test P < 0.05). The driving response was asymmetric in 7/36 patients (37%) with unilateral spike foci versus none in the control group (Fisher's test, P < 0.001). The amplitude was suppressed ipsilateral to the focus in 5 patients, all of whom had an ipsilateral structural lesion or focal slowing. In two cases the amplitude was higher ipsilateral to the focus, neither having slowing or a structural lesion.

CONCLUSIONS

Patients with occipital spikes have an increased frequency of asymmetric driving response. An attenuated response ipsilateral to the focus seems to be related to an underlying lesion while the presence of an epileptiform focus in some cases with no slowing on EEG and normal imaging studies may lead to an accentuation of this response.

Topographic analysis of EEG photic driving in normal and schizophrenic subjects

In a 19-channel EEG photic driving paradigm the steady-state evoked response was studied in schizophrenic patients and normal controls. Seventeen drug-free patients and 15 normal controls were compared. Schizophrenic patients had lower EEG photic driving in the alpha range, particularly in the high alpha frequency band (p = 0.08). Topographic analysis revealed that the group differences were primarily located in the mid-frontal, central and parietal areas (p < 0.05). Temporal and lateral frontal lobe alpha remained the same in the two groups. Separate analysis of eye movement (EM) showed an opposite effect on the alpha-frequency photic driving as compared to that on the resting alpha EEG. EM decreased the resting alpha EEG and increased the photic driving. This finding suggested that the lower EEG photic driving in the schizophrenic patients could not be explained by the eye movement artifact. We hypothesize that the 10 Hz range photic driving that reflects the intrinsic EEG spindle generation in the thalamus may play an important role in the psychophysiology of schizophrenia.

Quantitation of photic driving in dementia with normal EEG

The EEG is frequently normal when early dementia is clinically suspected. The presence or absence of occipital driving with intermittent photic stimulation is not helpful in detecting cerebral pathology, but quantitation of photic driving was shown to distinguish between dementia and depression in geriatric patients. We quantified photic driving in twenty clinically demented patients without diffuse or focal EEG abnormality, and compared this in 20 age-matched normal controls. Intermittent stimulation at odd-numbered frequencies between 1 and 31 Hz was carried out. The product of frequency of stimulation and duration of driving response was determined, and the incidence of absent, subharmonic and supraharmonic photic driving was determined. The quantitative measure of frequency times following duration was significantly greater in the nondemented individuals than in demented patients with normal EEGs. More nondemented individuals demonstrated supraharmonic and subharmonic responses than did demented patients. The range of normal variability in persistence, duration, and symmetry of photic driving is great, and caution in determining abnormality on this basis is appropriate. Subtle but measurable changes in photic driving may occur early in dementing illnesses, however.

Intermittent photic stimulation. Clinical usefulness of non-convulsive responses

The clinical usefulness of non-convulsive responses induced by intermittent photic stimulation (IPS) was studied in 46 patients whose evaluation included both electroencephalography (EEG) and computerized cranial tomography (CT scan). Three patterns of photic responses were identified and related to background EEG activity and type of lesion seen on CT scan. An asymmetry in photic driving manifested only by a consistent amplitude difference greater than 50% was rarely associated with other EEG changes such as focal slowing or significant asymmetry in the alpha rhythm. In contrast, an asymmetry in development of the photic response correlated well with ipsilateral focal slowing and CT scan evidence of parenchymatous brain disease. Bilateral, symmetrical high amplitude single spikes evoked by individual light flashes were seen only in patients with diffuse encephalopathies. In no patient with structural brain disease as judged by CT scan was an abnormality in IPS the only EEG finding. There was also no apparent relationship between the anatomical site of a focal cerebral lesion and the type of response induced by IPS. Photic stimulation is of limited value in non-epileptic patients. Little clinical significance can be attached to nonconculsive responses in the absence of associated EEG changes which by themselves are generally more informative.