Visual evoked potentials in a patient with prosopagnosia

Visual evoked potentials (VEPs) were recorded from a 53-year-old man with prosopagnosia during presentation of slides of known and unknown faces and under two control conditions. ANOVA comparisons with a normal male group showed no differences in P100 amplitude, P300 amplitude or P300 latency. There were no significant evoked potential differences between the patient and controls specifically related to the face conditions.There was, however, a significant delay in the latency of P100 from both hemispheres during all types of stimuli. This prolonged latency was asymmetrical, showing a right sided emphasis with the control conditions: pattern reversal and slides of geometric designs. This finding, of a dissociation in the interhemispheric delay, provides physiological evidence of stimulus-specific organisation at an early, sensory level.The fact that the P100 component showed a marked delay, yet P300 fell within normal limits for amplitude and latency, suggests that this patient's problem lies at a perceptual level.     

Visual evoked potentials and event related potentials in congenitally deaf subjects

The purpose was to test parameters of visual evoked potentials (VEPs) and of event-related potentials (ERPs) in deaf subjects to verify visual and cognitive CNS functions in a handicapped group of the population. Three types of visual stimuli (with dominating parvocellular or magnocellular system activation or with cognitive tasks) were used in the study. Six deaf persons (4 women, 2 men, mean age 17 years) and 6 persons with normal hearing (sex- and age-matched) were included in this pilot study. In all types of stimulation, latencies and amplitudes of main VEPs and ERPs components were evaluated. No significant latency differences were found. However, significantly reduced amplitudes were found in the occipital area for responses to motion and cognitive stimuli which might be interpreted as a part of functional reorganization of the extrastriate and cognitive cortical areas of deaf subjects.     

Visual event-related potentials evoked by using a virtual reality display

This study aimed at investigating whether a virtual reality display (VRD) is an appropriate tool for evoking visual event-related potentials (VEPs). VEPs evoked by VRD stimuli were highly similar in form to VEPs evoked by using a computer monitor, both having two dominant peaks, labeled P100 and N200. Monitor and VRD N200 latencies and amplitudes were highly correlated. However, peak latencies were longer and the peaks were broader when stimuli were presented on the VRD. Besides, VRD P100 amplitude was smaller, and an N75 peak could be seen usually only on monitor VEPs.     

Visual evoked potentials elicited by a moving unidimensional noise pattern

(1) Motion onset and offset visual evoked potentials (VEPs) were recorded in normal human subjects using a unidimensional noise pattern moving at 1, 8 and 64°/s. The maximum N1-P1 amplitude of the motion onset response was obtained when using a fine noise pattern (maximum energy at 5.2 cpd) moving at 8°/s. (2) At a velocity of 8°/s, the motion onset response (fine pattern, 0.70 contrast) showed a morphology similar to the pattern disappearance response. Both at a lower (1°/s) and a higher velocity (64°/s) the N1-P1 amplitude of the motion onset complex was significantly reduced. The latency of the motion onset response (8°/s) and the pattern disappearance complex were significantly different. (3) The effect of lowering the spatial content of the noise pattern on the amplitude of the motion onset response was different for the 3 velocities tested: the largest effect was at the lower velocity of 1°/s; there was no similar effect on the pattern disappearance response. (4) With decreasing contrast, the N1-P1 amplitude of the motion onset response at 8°/s decreased, but this reduction in amplitude was much less than that of the disappearance response. The contrast dependency of the motion onset complex was identical for binocular and monocular recordings. (5) Increasing the motion duration or the duration of the interstimulus interval did not alter the general morphology of the motion response.     

Visual evoked potentials to structured stimuli in children with cataracts

EPs elicited by diffuse light flashes and check patterns, were studied in children and juveniles with congenital and traumatic cataract, during different periods and various durations of deprivation, as well as in a control group of children and juveniles with normal vision. Specific age characteristics were found in the EPs to check patterns in seven years-old children. In children with congenital cataracts significant changes in the shape and amplitude-temporal EP parameters were recorded. In children with traumatic cataracts, presumably developed after the end of the sensitive period, the changes in EPs were much less pronounced. The obtained data are discussed from the point of view of the duration of the sensitive period of the formation of the visual function in humans and of informative value of different EP components.     

Short-latency somatosensory evoked potentials in brain-dead patients

Ten adult brain-dead patients were evaluated for the presence of clearly defined median nerve short-latency somatosensory evoked potentials (SSEPs). All met clinical criteria recommended by the President's Commission report (1981), had positive apnea tests, and had electrocerebral silent EEGs. P13-P14 and N20 were absent in all scalp-scalp channels, although 3 patients showed P13-P14 in scalp-non-cephalic channels. Of 6 patients showing N13, 3 lacked P13-P14. Our data suggest a characteristic destruction of N20 and rostral P13-P14 generators, with variable rostral-caudal loss of lower generators, SSEPs can provide valuable information about brain-stem activity in the evaluation of suspected brain-dead patients.     

Visual cortical evoked potentials in emotionally unstable persons threatened with electrocutaneous stimulation

In preliminarily selected (by a number of parameters) highly emotional subjects, evoked potentials were recorded from parietal, central, frontal and temporal cortical areas, in the first part of the experiment -- in response to indifferent light flashes (which served as a signal to achieve a precise movement), and in the second part -- in response to the same flashes, but with preliminary information of the subject on a possible electrocutaneous stimulation (in case of an error in task performing). It is found that with the threat of the electro-cutaneous stimulation the major changes are observed in the frontal, temporal and central areas. An increase in the latencies of the IV, V, VI components and P300 wave was recorded. At the same time the amplitude of the V component and that of the P300 wave decreased. The IV component was enhanced. In extreme situation the activity directed to avoid the electrostimulation was disturbed (errors of movement performance increased by 3.7 kg). The cortical mechanisms of the organization of the emotional behaviour in extreme situations are discussed.     

Visual evoked potentials in the vicinity of the optic tract during stereotactic pallidotomy

We recorded visual evoked responses in eight patients with Parkinson's disease, using a depth electrode either at or below the stereotactic target in the ventral part of the globus pallidus internus (GPi), which is located immediately dorsal to the optic tract. Simultaneously, scalp visual evoked potentials (VEPs) were also recorded from a mid-occipital electrode with a mid-frontal reference electrode. A black-and-white checkerboard pattern was phase reversed at 1 Hz; check size was 50 min of arc. Pallidal VEPs to full field stimulation showed an initial positive deflection, with a latency of about 50 ms (P50), followed by a negativity with a mean latency of 80 ms (N80). The mean onset latency of P50 was about 30 ms. P50 and N80 were limited to the ventralmost of the GPi and the ansa lenticularis. Left half field stimulation evoked responses in the right ansa lenticularis region while right half field stimulation did not, and vice versa. These potentials thus seemed to originate posterior to the optic chiasm. The scalp VEPs showed typical triphasic wave forms consisting of N75, P100 and N145. The location of the recording electrode in the ansa lenticularis region did not modify the scalp VEP. These results suggest that P50 and N80 are near-field potentials reflecting the compound action potentials from the optic tract. Therefore, N75 of the scalp VEPs may represent an initial response of the striate cortex but not of the lateral geniculate nucleus.     

Abnormalities of short-latency somatosensory evoked potentials in parkinsonian patients

Twenty-two patients (16 affected by parkinsonian syndromes, 6 by other neurological diseases) and 12 age-matched controls were examined. Short-latency somatosensory evoked potentials were recorded from 30 scalp electrodes in the 45–52 msec following separate left and right median nerve stimulation at the wrist. Bit-colour maps were generated on a 4096 pixel matrix via quadratic interpolation. Peak latencies and amplitudes of the parietal, central and frontal components were evaluated. Moreover, the amplitude ratios between parietal and frontal components on the same hemiscalp and between peaks on homologous right and left scalp districts were taken into account. The unique significant difference between parkinsonians and controls was represented by a depressed frontal N₃₀ wave. This peak was absent in 3 and reduced in 7 out of 16 parkinsonians, with an overall abnormality rate of 47% of the examined arms. Average maps pooling data of parkinsonians and controls confirmed the presence of reduced evoked activity for the whole duration of wave N₃₀ on those mid- and parasgittal frontal districts where this peak is maximally represented in normals. A similar abnormality was found in 1 of the 6 non-parkinsonian neurological patients suffering from a meningioma of the falx compressing the left supplementary motor area. Possible pathophysiology of such wave N₃₀ abnormalities in parkinsonians is discussed.     

Visual evoked potentials in response to dichoptic presentation of sinusoidal grating and noise background

Dichoptic stimulation was used in comparison of visual evoked potentials (VEPs) with those obtained with monocular stimulation (recordings made from the occipital area). 16 subjects viewed sinusoidal gratings with the right eye while a visual noise was added via a mirror for the left eye. In presence of the noise, amplitude of the early VEP components' N1, P1b, and the late component P2 decreased, P1a is not changed in presence of the noise, and the late negative wave N2 increased for all spatial frequencies. The effect of noise on the amplitude of VEPs obtained for monocular and dichoptic stimulation was similar. The data suggest that external noise is filtered by the V1 cortical neurons--matched filters for the gratings.     

Visual acuity of the midland banded water snake estimated from evoked telencephalic potentials

The visual acuity of seven midland banded water snakes was measured by recording evoked responses from telencephalon to temporally modulated square wave grating patterns. Using conventional electrophysiological techniques and signal averaging, high contrast square wave gratings of different spatial frequencies were presented. Acuity was estimated by extrapolating relative response amplitude/log₁₀ spatial frequency functions which yielded an average acuity of 4.25 cycles/degree. Refractive state was also estimated by recording evoked potentials to intermediate spatial frequencies with different lenses in front of the eye. Polynomial fits indicated that under the experimental conditions the snakes were around 6.4 diopters hyperopic suggesting a corrected acuity of 4.89 cycles/degree. Reduction of grating luminance resulted in a reduction in evoked potential acuity measurements. These results indicate that the spatial resolution of midland banded water snakes is the equal of cat; about 20/120 in human clinical terms.     

Differential changes of laser evoked potentials,late auditory evoked potentials and P300 under morphine in chronic pain patients

The present study investigates the differential behavior of laser evoked brain potentials (LEPs), late auditory evoked potentials (AEP) and the endogenous P300 in response to morphine treatment, examined in 6 chronic pain patients. The main result was that in parallel with marked clinical pain relief, amplitudes of the long latency LEP positivity (P400) were significantly reduced under morphine. One patient suffering from extremely painful osteoporosis for 20 years exhibited a large middle latency component (N170) which was prominently attenuated by morphine. In contrast to LEP amplitude reductions, auditory N1 and P2 potentials appeared either unchanged or even enlarged during morphine treatment. Also P300 amplitude was slightly increased under morphine. Reaction time and mood scales also failed to indicate any sedation. Obviously, LEPs reflected specifically the analgesic morphine effect in this study, while stability or enhancement of AEPs and P300 during morphine treatment indicated lack of sedation or even improved perception and concentration due to the removal of persistent pain as a disruptive perceptual-cognitive stressor.     

Sex and ear differences of brain-stem acoustic evoked potentials in a sample of normal full-term newborns. Normative study

The present paper reports a normative study of the BAEPs recorded from 80 normal full-term newborns by using a rarefaction click at 70, 60, 40, 20 dB HL.Positive, negative peaks and the relative amplitudes were measured. Means, standard deviations and 95% and 99% confidence limits were calculated for each component, for positive and negative IPLs and amplitude ratios. A normality test distribution showed that each component had normal distribution except for wave PIII at 70 dB and at 60 dB HL beacause of its index of kurtosis and for IPLs PII-PIII and NII-NIII at 70 and 60 dB HL. MLRA was performed and significant statistical differences were found for sex, ears and intensities. Females had the latencies of waves PIII, PIV, PV, NII and NIV shorter than males. The BAEPs obtained from the left ear had shorter latencies for positive and negative peaks. Latencies, amplitude and morphology of the BAEPs vary with variations of the intensity. The IPLs PII-PV, PIII-PV and PIV-PV were not affected by changes of intensity.