The effect of acute trichloroethylene exposure on electroretinogram components

The effects of acute exposure to trichloroethylene (TCE) on the electroretinogram (ERG) and oscillatory potentials (OPs) were studied on adult New Zealand rabbits anesthetized with urethane. The ERG and OPs were recorded under photopic conditions with a speculum electrode, with pupils fully dilated and the cornea anesthetized. Emulsified (Intralipid) TCE was given intravenously in three consecutive injections of equal volume for a total dose varying between 24 and 85 mg/kg. Averaged ERG and OPs (50 sweeps) were taken up to 250 minutes after the last TCE injection. Our results indicated that, irrespective of the dose used, there was a significant (p less than 0.001) reduction (average: 50%) in the amplitude of the b-wave of the ERG following TCE injection. In contrast, the OPs showed a differential effect. At low doses, OP4 showed a significant (p less than 0.001) reduction resulting in some cases to its complete extinction, while at higher dose there is a marked increase in amplitude. Our results would suggest that the OPs are a better retinal potential to assess the level of TCE toxicity than is the b-wave of the ERG.

The effect of 2-amino-4-phosphonobutyric acid on the oscillatory potentials of the electroretinogram

Previous reports have shown that an intravitreal injection of 2-amino-4-phosphonobutyric acid, a glutamate analogue that selectively blocks the photoreceptors' input to the on-bipolar cells, produces a rapid decrease in the amplitude of the electroretinographic b-wave. To our knowledge, the effect of this glutamate analogue has not been examined on the oscillatory potentials. We therefore conducted such a study. A needle electrode was inserted into the anterior chamber of the eyes of seven anesthetized and paralyzed rabbits to record simultaneously the electroretinogram and the oscillatory potentials. These responses were evoked by flashes of white light delivered in mesopic conditions. As expected, an injection of 2-amino-4-phosphonobutyric acid, produced a rapid decrease in the amplitude of the b-wave, while the a-wave was relatively spared. The amplitude of the oscillatory potentials also decreased rapidly. At maximal effect, the electroretinogram essentially consisted of a normal a-wave followed by small oscillations and no evidence of a b-wave. The 100-1000 Hz recording confirmed that the oscillations seen on the postinjection electroretinograms were remnants of the original oscillatory potentials.

The effect of iodoacetic acid on the electroretinogram and oscillatory potentials in rabbits

It has been shown that a single injection of iodoacetic acid selectively (but temporarily) abolishes the b-wave of the electroretinogram. We examined whether such use of this chemical further substantiate our claim that the b-wave of the electroretinogram is a composite potential resulting from the summation (or integration) of faster retinal potentials, usually referred to as the oscillatory potentials. Full-field electroretinograms were recorded from adult New Zealand rabbits before and after a single, bolus injection of 15 mg/kg of buffered iodoacetic acid. Both the 1-1000 Hz electroretinogram and the 100-1000 Hz oscillatory potentials were recorded simultaneously. The oscillatory potentials considered in this study were those normally seen on the rising phase of the b-wave. Following the intravenous injection of iodoacetic acid, there was a progressive decrease in the amplitude and peak time of the b-wave. This observation also was reflected in the oscillatory potential recordings, in which the long-latency oscillatory potentials (3 and 4) progressively disappeared while oscillatory potential 2 remained. We believe that these findings further support our contention that the oscillatory potentials are major components of the b-wave.

Oscillatory potentials as predictors to amplitude and peak time of the photopic b-wave of the human electroretinogram

The oscillatory potentials are viewed by many as small oscillations of a high-frequency domain that ride on the b-wave of the electroretinogram. A study of electroretinograms and oscillatory potentials performed in 25 normal subjects was undertaken to substantiate my claim that oscillatory potentials are fast retinal potentials that are integrated to form the b-wave. The prominence of the OPs on the ascending limb of the b-wave was found to be only weakly correlated (r = -0.37) to the amplitude of the oscillatory potentials (measured in the 100-1000 Hz recordings). There was, however, a high correlation (r = 0.78) between the prominence of the oscillatory potentials and their frequency domain as determined by the peak-to-peak timing. Furthermore, the peak-to-peak timing of the oscillatory potentials was highly correlated with the b-wave peak time (r = 0.86) as well as with the 'a-wave trough to b-wave peak' time (r = 0.90), while the amplitude of the oscillatory potentials was correlated to the amplitude of the b-wave (r = 0.78). Interestingly, when combining the amplitude of the oscillatory potentials with the time interval between oscillatory potentials 2 and 3 and 3 and 4, a higher correlation (r = 0.88) was found with the b-wave amplitude. The latter finding would support my claim that the b-wave represents an integration (amplitude as a function of time) of the oscillatory potentials.

A new speculum electrode for electroretinography

A Storz model E4110 infant speculum was modified to record electroretinograms (ERGs) from anesthetized rabbits. The electrode combines the advantage of a corneal contact lens electrode (with blepharostat) without its major disadvantage, namely: corneal abrasion. Addition of i.v. tubings glued to both arms of the speculum allows for constant wetting of the eyeball with 0.9% NaCl. The latter not only prevents the eye (and cornea) from drying, but also favors the recording of reproducible ERGs with a noise level comparable to ERGs recorded with a corneal contact lens electrode.

Maturation of the photopic b-wave and oscillatory potentials of the electroretinogram in the neonatal rabbit

We examined the ontogenesis of the b-wave of the rabbit electroretinogram (ERG) in light adaptation and looked at the contribution of the oscillatory potentials (OPs) to the waveform at each stage of development. Recordings were obtained weekly from 2 to 5 weeks of age in six rabbits and every other day during the second week of life in six others. Comparative analysis of the ERGs showed a changing peak time and amplitude of the photopic b-wave with increasing age, consistent with the development of new components corresponding to the OPs. The results illustrate the importance of the OPs in determining both the amplitude and peak time of the photopic b-wave in the maturing rabbit.

The oscillatory potentials in response to stimuli of photopic intensities delivered in dark-adaptation: an explanation for the conditioning flash effect

Previous studies reported that the oscillatory potentials (OPs) evoked to the first flash of a series were always smaller than those produced by the later flashes. This conditioning flash effect (CFE) was suggested to arise from rod inhibition of cone-mediated OPs. We investigated this CFE with the use of two stimulus intensities: 10 cd sec m-2 and 1 cd sec m-2. While the highest intensity did yield the previously reported CFE, the dimmest intensity did not. Our results further indicated that with the brightest stimulus, there is a significant increase in the interpeak interval of the OPs, while dimmest stimuli failed to reveal a similar marked increase. We also noted a significant correlation between the frequency domain of the OPs (as estimated with the interpeak interval) and the amplitude of the OPs (individual or collective: SOPs). Our results would also suggest that the observed CFE could result from a cone inhibition of rod-mediated OPs.

Temporal relationship between ERG components and geniculate unit activity in rabbit

The purpose of this study was to examine if there is an ERG component (alpha-wave, b-wave, oscillatory potentials) which can be used to account for the intraretinal processing time. To address this issue, ERGs and LGN unit responses were recorded simultaneously from anesthetized and paralyzed rabbits. A gradual decrease in the intensity of the light stimulus yields a progressive increase in the latency of the LGN unit response. A similar, highly correlated (r = 0.91 +/- 013), latency shift was also noted for one oscillatory potential (OP2). In comparison, correlation coefficients of r = 0.63 +/- 0.27 and r = 0.70 +/- 0.29 were obtained for the alpha- and b-wave respectively. Furthermore, in 77.6% of the cells examined, OP2 preceded LGN unit activity while the b-wave preceded LGN activity in only 17.8% of the cases. Our results suggest that, of all the retinal potentials considered, the peak time of OP2 better reflects the primary visual processes and the intraretinal time taken for visual processing.

The diagnostic use of the second oscillatory potential in clinical electroretinography

Of all the electroretinogram (ERG) components (a-wave, b-wave, and oscillatory potentials) only one oscillatory potential, OP2, was found to be significantly correlated with the absolute intensity of the flash stimulus (i.e., the intensity of the stimulus irrespective of the state of retinal adaptation). Our finding was further confirmed in single cell recordings of lateral geniculate unit activity in rabbits in which peak time of OP2 was found to correlate better with the geniculate activity. For these reasons we have identified OP2 as the "intensity coding" oscillatory potential of the ERG. In order to investigate if this new feature could have some clinical significance, we examined photopic ERGs recorded from patients affected with various retinopathies. In most instances the peak time of OP2 paralleled that of the b-wave, that is, in the ERG with delayed b-wave the peak time of OP2 was also delayed, while in ERGs with normal b-wave peak time the peak time of OP2 was also normal. However, in some conditions (especially in cone-rod diseases) a delayed OP2 was found in ERGs with normal b-wave peak times.

The electroretinogram in Stargardt's disease and fundus flavimaculatus

A retrospective study was performed comparing the ERG results of 15 patients with Stargardt's disease and fundus flavimaculatus. Patients with fundus flavimaculatus had "fish-tail" lesions with or without macular changes, while the Stargardt's group had macular atrophy without fish-tail flecks. The mean visual acuity was 20/200 for the Stargardt's patients compared with a mean of 20/80 for the fundus flavimaculatus patients. The Stargardt's photopic and scotopic amplitudes were respectively 33% and 34% of normal, while the fundus flavimaculatus values were less impaired at 58% and 64% of normal.

The effect of diphenylhydantoin on the electroretinogram

Acute administration of diphenylhydantoin (DPH) in rabbits produces a significant increase in the amplitude of the a-wave. A marked increase in the amplitude of the b-wave is also noted but the time course is slower than that for the a-wave. While in controls the oscillatory potential (OP) recordings essentially consist of three major types, recordings taken after DPH injection consist of one major OP (OP2), which appears to be a result of the fusion of the original OP2 with another OP produced by the DPH injection. A similar blend of OPs was also seen in ERGs recorded from three human subjects on DPH therapy.

Optic nerve blockade influences the retinal responses to flash in rabbits

The presence of retinopetal fibers in mammals has been debated many times in the past two decades. Do rabbits have a retinopetal system? This question is addressed with the present investigations. In anesthetized and paralysed rabbits the b-wave and the oscillatory potentials (OP) are recorded at the cornea. The optic nerve is isolated retrobulbarly and is gently hooked to a curved injecting capillary. Through the latter lidocaine hydrochloride is pressure injected. This drug interrupts the neuronal flow travelling along the nerve. A steel electrode is positioned in the optic chiasm allowing us to monitor the evoked field potentials from the tested and untested eyes. The optic nerve blockade produces the following observations: (1) the amplitude of the b-wave is not significantly altered; and (2) the amplitudes of the long latency OP are significantly increased. The retinal capacity to respond to a second flash after the application of an initial light pulse was evaluated by varying the interval between the two flashes. After optic nerve blockade the recovery of the retinal responsiveness is considerably slower. Fourier analysis indicated that the highest power increases occurred around 200 Hz. It is difficult to escape the suggestion that rabbits possess retinopetal fibers.

Abnormal dark-adapted electroretinogram in Best's vitelliform macular degeneration

It is generally well accepted that in Best's vitelliform macular degeneration (BVMD) the electroretinogram (ERG) is normal whereas the electro-oculogram (EOG) is markedly abnormal. We describe a patient in whom BVMD was suspected on the basis of the clinical findings, EOG and family history (one of her daughters had the typical vitelliform lesion). However, her dark-adapted ERG was markedly abnormal. Similar anomalies were found in the dark-adapted ERG of the daughter. While the temporal features of the various ERG waves were well preserved, a substantial decrease in the amplitude of specific segments of the ERG signal was observed. A similar decrease in the amplitude of the oscillatory potentials was also found. We believe that this unusual combination of BVMD and abnormal dark-adapted ERG may be due to the reported reduced penetrance and variable expressivity of the BVMD gene(s).

Maturation of the electroretinogram of the neonatal rabbit

The development of the electroretinogram in the rabbit has been said to proceed with first the a-wave, then the b-wave and last the oscillatory potentials. The aim of our study was to reexamine this claim with special attention to the oscillatory potentials. Albino rabbits from the same litter were studied at weekly intervals for five weeks from the first week of life. A Grass photostimulator was used in light and dark adaptation and 50 amplified responses were averaged. Both 1-1000 Hz (electroretinogram) and 100-1000 Hz (oscillatory potential) band-widths were recorded simultaneously. The a-wave was the earliest signal to appear, at the second week of life. A rapid growth of the b-wave and oscillatory potentials was then noted between the second and third weeks, followed by a slower change. They evolved at the same rate, each with an increase in amplitude and decrease in peak time. The change in form of the b-wave was consistent with the sequential formation of each potential as the rabbit matured. The finding of simultaneous development of the b-wave and oscillatory potentials in the aging neonatal rabbit is contrary to previous reports.

Analysis of the photopic electroretinogram recorded before and after dark adaptation

To evaluate the effect of a long period of dark adaptation on the light-adapted electroretinogram (ERG), photopic ERGs were recorded before (light-adapted) and immediately after (light-readapted) 15 minutes of dark adaptation. The amplitude of the b-wave in the light-readapted ERG was 48% +/- 4% of that in the light-adapted ERG, and the peak time of the b-wave in the light-readapted ERG was delayed by an average of 3.49 +/- 1.09 ms. Segmental analysis of the b-wave showed that the last segment of the ascending limb of the b-wave (from the second oscillatory potential [OP] to the peak) was attenuated the most (61% +/- 8%). In fact, the reduction in the amplitude of the last segment alone accounted for more than 60% +/- 13% of the overall reduction observed for the entire b-wave. This last segment appears to correspond to OP4 in the 100- to 1000-Hz recordings. The amplitude of OP4 in the light-readapted ERG was 56.5% +/- 10% of that in the light-adapted ERG, a similar reduction to that observed for the last segment of the 1- to 1000-Hz b-wave. The results strongly support the concept that the b-wave of the photopic ERG may actually represent a composite of potentials whose individual contributions are best visualized when the ERG is recorded with the 100- to 1000-Hz bandwidth.

Modulations of collicular visual responses by acoustic stimuli in rabbits

This study analyzes the influences of an acoustic stimulus upon neuronal light responses of superficial layers of the superior colliculus in anesthetized and paralyzed rabbits. The results have revealed that even if visually-responsive cells fail to be excited by sound, the latter is still capable of modifying light-evoked discharge. The influence may be "short-term" (the discharge rate recovers within 500 ms) or it may be "long-term" (the firing rate remains modified for several seconds). This audio-visual interaction depends upon several factors: the time of occurrence of both stimuli, the physical aspects of the visual target, the relative positions of the speaker and the visual receptive field, and finally, the sensitivity of the unit to movement direction. Data indicates that cells of the most dorsal (hence visual) layers of the superior colliculus are influenced by sound. It is concluded that the colliculus may use the sound as an additional cue to orientate the animal. Also, collicular cells could "memorize" for several seconds various features present in the environment.

Components of the electroretinogram: a reappraisal

During light adaptation a progressive increase in the intensity of a flash stimulus yields ERGs in which the a-wave shows a gradual reduction in peak time, while the b-wave appears to do the opposite. However, close inspection of the different ERG waveforms indicates that the wave identified as the b-wave for a threshold stimulus actually decreases in peak time with progressively stronger stimulus. Furthermore, what is identified as the b-wave for a threshold stimulus delivered in light adaptation will become a part of the a-wave complex if the same stimulus is used in dark adaptation. Data presented here indicate that the identification of the a- and b-waves of the ERG must be constantly revised according to the intensity of the stimulus and/or the level of retinal adaptation. These findings seriously challenge the validity of the a-b-wave nomenclature currently used in electroretinography. An alternative nomenclature, based on the first derivative (dv/dt) of the ERG wave, is proposed.

Influence of a local inactivation in the superior colliculus on lateral geniculate responses in rabbits

The influence of the superficial layers of the superior colliculus (SC) on responses of the lateral geniculate nucleus was studied in anesthetized and paralyzed rabbits. Fifty to one hundred nanoliters of inactivating drugs were injected through a micropipette positioned in the SC in register with the geniculate cells. The latter were tested with discrete moving and stationary targets presented in random sequences. Results revealed that the SC exerted a dual facilitative (n = 19) and inhibitory (n = 25) influence. The latter segment of the response pattern was modified while the initial portion remained unchanged. In some cases differential effects were observed, that is, the responses to one particular stimulus were more affected than responses to other modalities. However, in the majority of cells it was the on-off responses which were most affected. These findings point toward complex influences of the SC upon geniculate responses in rabbits. This complexity is also supported by findings that in cats the colliculo-geniculate synapses are of various types.

A composite nature for the photopic b-wave of the human electroretinogram as evidenced by the use of the 60 Hz notch filter

The impact of the 60 Hz notch (rejection) filter was examined in typical photopic electroretinograms recorded with two different bandwidths: 1-1000 Hz and 100-1000 Hz. The peak times of the major ERG components (a- and b-waves) are not modified by the line filter. The amplitude of the b-wave decreases by 30%, and the morphology of the ERG wave changes slightly. Segmental analysis of the ascending limb of the b-wave reveals that the sole reduction in the amplitude of its first step (from the a-wave trough to the peak of the first oscillatory potential) accounts for more than 70% of the overall reduction measured for the b-wave. Similar results are also found in ERGs recorded with the 100-1000 Hz bandwidth. The data suggests that the b-wave of the ERG should be considered as a composite of potentials rather than as a single potential.

Modifications of light responses in the superior colliculus following cortical cryoblockade

Responses to light stimuli of cells in the superior colliculus (SC) were studied in anesthetized and paralyzed rabbits after reversible inactivation of corticocollicular impulses by cooling of the visual cortex. The recording of flash-evoked responses and EEG allowed us to monitor the time course of the inactivation and its recovery. The cell's responses were measured prior to, during, and after cortical blockade. In about half of the collicular cells, disruption of cortical functions resulted in a sharp decline of collicular responsivity. All types of response were affected equally. Hence, the cortex has a global tonic excitatory impact.

Oscillations on the electroretinogram: a synthetic approach

A synthetic approach was used to examine the contribution that the oscillatory potentials (OPs) bring to the genesis of the electroretinogram. The data suggest that the OPs may be more important contributors than has generally been recognized.

Impact of the recording bandwidth on the electroretinogram

Previous investigators have suggested using a recording bandwidth of 100 to 1000 Hz to selectively amplify the oscillatory potentials (OPs) of the electroretinogram (ERG). The data reported in this article do not support this recommendation. Comparisons between the usual broadband (1- to 1000-Hz) ERG and that recorded with a narrower bandwidth (100 to 1000 Hz) indicate that the two ERGs have the same temporal features. The only differences noted are in amplitude and configuration, and these filter-induced modifications are predictable. The 100- to 1000-Hz ERG thus appears more like a low-voltage version of the 1- to 1000-Hz one than a recording with selective amplification of some hidden high-frequency subcomponents. A novel interpretation of ERG-wave synthesis that could account for these findings is suggested.

Local excitability in the superior colliculus influences evoked responses of lateral geniculate cells in rabbits

In anesthetized, immobilized rabbits recordings were made simultaneously from cells in the Lateral Geniculate Nucleus (CGL) and Superior Colliculus (CS), in order to study how the CS influences the CGL. The experimental protocol consisted of three steps. In the initial step (first control) the light stimulus was triggered electronically. In the second step (Test), the same stimulus was triggered by a spontaneous spike arising from a collicular cell. Thus the stimulus presentation was time-locked to collicular endogenous activity. The third step was the same as the first and constituted a second control. The frequencies of stimulus application were gated to be approximately the same. The results indicated that the CS exerts two separate effects on CGL units. In 37 pairs (26%), conditioning the stimulus presentation to collicular firing produced a significant enhancement of geniculate responses. In 24 pairs (17%), the geniculate responses declined. In 82 pairs (57%), no significant influence was noted. The colliculo-geniculate influence is transient. The effects peaked between 100 to 200 msec after the collicular spike and returned to their control levels within 300 msec. Collicular cells producing a decline were encountered mostly in the ventral part of the stratum griseum superficiale, and the stratum opticum, whereas collicular cells that were related to an increased geniculate response were more frequently found dorsally. Increments were more pronounced if the distance (D) between receptive fields was short (0 degrees less than D less than 40 degrees) or if the collicular and geniculate fields were far apart (120 degrees less than D less than 180 degrees). The decrement effect was attenuated as the distance separating the two receptive fields. This study suggests that the superior colliculus is capable of generating an internal signal powerful enough to modulate at the geniculate nucleus the visual message conveyed toward the visual cortex. A possible role of the CS in the initiation of the corollary discharge is briefly discussed.

The photopic electroretinogram in congenital stationary night blindness with myopia

Previous studies have reported that subjects affected with congenital stationary night blindness and myopia demonstrated some photopic (cone) abnormalities in their electroretinogram (ERG). By comparing the photopic ERG elicited with a threshold and a suprathreshold stimulus it was found that, at threshold, no significant differences were noted both in the peak time and in the amplitude of ERGs evoked from CSNB and normal subjects. However, a more powerful stimulus (16 times the threshold) yields a significant difference in the ERGs recorded from the two groups. ERGs recorded from CSNB patients are decreased in amplitude with a b-wave peak time that remains normal. First derivative analysis of the ERG wave along with a selective recording of the oscillatory components of the ERG suggest that the only visible anomaly in the suprathreshold photopic ERG of CSNB patients is an absence of the two oscillations normally seen on the ascending portion of the b-wave. Data obtained on normal subjects are also reported that try to explain the functional significance of these two oscillatory potentials.

Evidence of a collicular input to the dorsal lateral geniculate nucleus in rabbits -- electrophysiology

In anesthetized and paralyzed rabbits, unit responses of lateral geniculate nucleus (LGN) cells to focal electrical stimulation of the superior colliculus were studied. Geniculate responses to collicular stimulation (SCS) were compared with responses to optic nerve shock (ONS). A weak correlation coefficient suggested that collicular stimulation did not fire geniculate cells through collateral activation. Further differentiation between collicular and retinofugal inputs to LGN was made possible by repetitive stimulation. Geniculate cells which responded to collicular stimulation were relay cells as they were antidromically invaded from the visual cortex. This ruled out recordings from the ventral geniculate, since this area does not project to the visual cortex. A direct colliculo-geniculate pathway was revealed by antidromic activation of collicular cells by stimulation of the dorsal LGN. Finally, triggering flashes by collicular firing resulted in a marked modification of the geniculate test response. The results suggest that the superior colliculus sends fibers to the LGN and is capable of modulating the retino-cortical neuronal message at the level of the LGN.

Influence of the visual cortex upon receptive field organization of lateral geniculate cells in rabbits

In anesthetized rabbits, the receptive fields of lateral geniculate cells were mapped prior to and following the interruption of the corticogeniculate feed-back. Visual cortex (V.C.) was depressed by a focal application of 3 M KCl. The responsiveness of the V.C. was verified by monitoring the visually evoked potentials. In off- and on-center cells, the surround excitatory responses were remarkably reduced and even fully abolished in most units. In contrast, the center excitation remained unmodified. These effects were reversible. In some on-center units the center response had also decreased, and was replaced by an evoked inhibitory response. Relay cells and interneurons which yielded on and off responses over the entire area of the receptive field exhibited a loss of only one of the evoked discharges. It is concluded that the V.C. exerts mostly a specific desinhibitory action upon the geniculate network. This action affects either the center or the surround responses differentially. The results are compared with those obtained from cats.

Latency distribution from orthodromic stimulation at the optic nerve, in the lateral geniculate nucleus and superior colliculus of rabbits

Latency distribution of responses to optic nerve stimulation has been compared between the Lateral Geniculate Nucleus (L.G.N.) and Superior Colliculus (S.C.), in rabbits. Histograms revealed that in both structures the latency distributions are very similar. Results suggest that all groups of retinofugal fibers project to both sites, although longer latencies are more frequently met at the collicular level. In addition evidence is presented that in rabbit's optic nerve there are at least three populations of axons with different conduction velocities.

Lateral geniculate cell responses to electrical stimulation of the retina

Electrical stimulation of the retina evokes at the optic tract level rhythmic bursts of activity whose temporal structure is predictable from the polarity of the stimulation and the receptive field type. The reaction of lateral geniculate units to this input was studied in fast and slow relay cells as well as in interneurons. The results revealed that fast relaty cells presented a response whose temporal structure remained essentially unmodified in comparison to that observed at the optic tract level: that both anodal and cathodal polarities produced rythmic pattern of excitation the latency of which depended upon receptive field type and polarity applied. In slow relay cells and interneurons responses with equal latencies could be evoked for both polarities. Following cortical depression with 3 M KCl the latency of first bursts was unaffected in relay cells, while about one third of interneurons showed a temporal pattern which was similar to that recorded at the optic tract level after the treatment. This suggests that both ON and OFF retinal networks converge upon one geniculate slow P cell and interneuron, whereas fast relay cells are mostly driven by one of the two systems. Furthermore this convergence may be achieved through visual cortex in some units.

Corticofugal influence on evoked activity of lateral geniculate neurons in the rabbit

Corticofugal influence upon lateral geniculate units was evaluated by inactivating the visual cortex with a topical application of 3M KCL in rabbits. The results revealed a decrease of spontaneous firing in slow P cells following cortical depression suggesting a tonic corticofugal influence. The most consistently observed alteration was an enhancement of the ON response with a decrease of the response to the opposite stimulus (OFF); and vice versa: the short latency excitation being augmented. It is suggested that the visual cortex exerts a powerful influence on intra-geniculate inhibitory processes.