Nonspecific influences on neuronal firing in the central visual pathway

Human flash-VEP and quantitative EEG are independently affected by acute scopolamine

Scopolamine in acute intramuscular doses of 0.25-0.75 mg reduced the P2-N3 flash-VEP amplitude and, in the quantitative EEG, the 8.5-12.0 Hz power and total power in 8 healthy young male volunteers. The effects on flash-VEP and EEG total power were dose dependent and were evident 30 min and 90 min respectively after drug administration, regardless of dose. The reduction in 8.5-12.0 Hz power was limited to the 0.50 and 0.75 mg doses. No systematic effects on the pattern-VEP were observed. Possible interferences with flash- or pattern-VEP amplitude of the scopolamine-induced EEG changes were identified and removed by regression analysis and computation of VEP residuals from the regression function. The P2-N3 flash-VEP residuals proved EEG independent and showed relationships with dose and time after drug administration that were superimposable on those of the original data, with comparable significance levels at the drug/placebo and pre/postdrug statistical comparisons. The results indicate that VEP estimates of drug effects which are independent from EEG changes can be identified in human studies and allow some inference on the cholinergic specificity of the systems affecting late flash-VEP components. The statistical approach used in this study is suitable for application in VEP studies when effects of interacting factors are to be expected.

Increasing PGO spike density by auditory stimulation increases the duration and decreases the latency of rapid eye movement (REM) sleep

The purpose of this study was to determine whether ponto-geniculo-occipital (PGO) spikes play a role in triggering or maintaining sleep. During the recording of the sleep cycle of cats, the appearance of the first PGO spike automatically triggered an auditory stimulus through a speaker placed in the cat's recording cage. The effect of this procedure was compared to similar period when no such stimulus was given. The results showed that the auditory stimulus increased PGO spike density during REM sleep. It also produced a spectacular increase in the duration of REM, while decreasing the latency of its appearance from the first PGO spike. It is suggested that the auditory stimulus reinforces the 'PGO system', which in turn may function as a pace-setter for priming and maintaining REM sleep.

Auditory evoked phosphenes in optic nerve disease

Five patients with optic neuropathy, four vascular and one demyelinating, are described who each complained of an unusual symptom. Bright flashes of light (phosphenes) occurred in the affected eyes and were evoked by sudden unexpected sounds. Movement of the eye alone did not reproduce the symptom. In all patients the phenomenon was sufficiently prominent to interfere with sleep and was the main complaint of one patient. An anticonvulsant (phenytoin) greatly reduced the frequency and intensity of the phosphene in one patient.

Relationship of geniculate and occipital PGO waves and the effects of nonvisual sensory activity

The nature of occipital PGO waves (REM) was addressed by comparing the occipital PGO waves measured at the marginal gyrus with the geniculate PGO waves and the occipital PGO waves "directly" transmitted to the occipital cortex. The directly transmitted PGO waves lack the first short positive-going spike that precedes the negative phase of the occipital PGO waves. The "directly" transmitted PGO wave was recorded from cats with either deafferented or excised LG nucleus. The "directly" transmitted PGO wave resembled pontine negative-going PGO wave more regarding amplitude and wave shape changes than the occipital PGO waves of intact cats. The ascending negativity of pontine PGO waves is indirectly transmitted through phasic increase of the local excitatory processes, more in the LG than in the occipital cortex. The PGO waves seem to be affected by endogenous and exogenous nonvisual processes, more in the LG than the occipital cells alone. The occipital PGO waves seem to result from the vectorial sum of geniculate and "directly" transmitted occipital PGO waves.

Studies of visual function and its decay in mice with hereditary retinal degeneration

Functional implications of mouse hereditary retinal degeneration have been studied at the level of the superior colliculus and visual cortex in the C57BL/6J-le rd strain. On autoradiography at a light-microscopic level, following eye injection with radioactive compounds, central visual structures appeared normal. A slight reduction in ipsilateral retinal projection was probably related to reduced retinal pigmentation associated with the light ear (le) mutation. In recordings from visual cortex and tectum in rd mice older than five months the cells discharged with highly rhythmic maintained activity. This ongoing activity depended on retinal input, since temporary asphyxia of the eye stopped it immediately. The frequency of the rhythm was influenced by the anesthesia. In these older mice no visual receptive fields could be mapped, but in a few tectal recordings it was possible to suppress the maintained activity by diffuse, very intense illumination. As in normal mice, no auditory or somatosensory responses were observed in the visual cortex or upper tectal layers. In recordings from tectum before the age of three weeks retinotopic topography and receptive fields were normal. By day 24 no receptive fields could be recorded from parts of the tectum representing the central 90--100 degrees of the visual field, whereas within a peripheral ring responses were still roughly normal under photopic conditions. Over the following four months these peripheral responses faded away slowly. Incremental thresholds, especially in the scotopic range, were elevated, rising slowly to unmeasurable values. Similarly during dark adaptation the thresholds fell to values several log units above those reached in normal mice; these values of dark adapted thresholds in rd mice rose with age. This is consistent with morphological changes known to occur in the retina as a consequence, of the rd mutation the rods degenerating before the cones.

Reticular suppression of flash-evoked IPSPs in visual cortex neurons

In a majority of visual cortex (VC) neurons recorded intracellularly in chronically implanted rabbits, light flashes evoked responses consisting of protracted IPSPs (duration 160-200 msec) followed in some cases by a rebound. Inhibition of spontaneous activity through high frequency reticular stimulation (MRF) or through presentations of non-visual arousing stimuli, was not associated with strong hyperpolarization of the cell membrane. Reticular stimulation evoking both activation and inhibition of spontaneous neuronal activity elicited attenuation or complete elimination elimination (disinhibition) of flash-evoked protracted IPSPs and of the postinhibitory rebounds. Rhythmic neuronal discharges elicited by stimulation of the visual pathway was also reduced during reticular activation, due to attenuation of the IPSP-rebound sequence.