Interaction between cortico-tectal and retino-tectal inputs as revealed by analysis of field potentials of the cat's superior colliculus

Response Recovery Cycles in the Visual Cortex and Superior Colliculus Following Conditioning ‘ON” and ‘OFF” Stimulation in the Rabbit

The responsiveness of the visual cortex (VC) and superior colliculus (SC) was simultaneously compared following conditioning “ON” or “OFF” stimulation, in the rabbit.

Average evoked responses were recorded simultaneously from the visual cortex and superior colliculus. “ON” or “OFF” steps constituted the conditioning stimuli whereas the test stimulus consisted of optic nerve stimulation. All evoked responses exhibited a reversal of their polarity when the electrode was moved in the dorsoventral direction (Negative-Positive in the SC, Positive-Negative in the VC). This assured the somato-dentritic origin of the potentials. The results showed that responsiveness in both structures was significantly higher following an “OFF” stimulus than after an “ON” step. Collicular responsiveness was higher than in the VC when the same conditioning stimulus was applied. The spatial distribution of the source of “OFF” responses was circumscribed to the ventral part of the superficial layer of the superior colliculus. These results suggest specific properties associated with the brightening and dimming systems.

The corticotectal projection of the rat in vitro: development, anatomy and physiological characteristics

In this study, the formation of the corticotectal projection of the rat in organotypic slice culture was investigated, using both anatomical and physiological approaches. The establishment of fibre connections from visual cortex to superior colliculus explants was monitored after 3, 6, 14, 20 and 30 days in vitro by cortical injections of Dil. As in cortical cultures without cocultured colliculus, fibres anterogradely labelled by this procedure spread radially from the injection site into the surroundings of the explant, without displaying any directional preference. Especially, layer V pyramidal cells could be seen to extend processes not only to the collicular target, but also in the opposite direction, suggesting that no axonal guidance was exerted by the projection target. The total number of fibres projecting in the direction of the colliculus was not higher than of those projecting in the opposite direction. However, fibres projecting into the colliculus were significantly longer. This was also the case when the colliculus was placed next to the pial side of the cortical explant, indicating that outgrowth direction was not related to this observation. We therefore assume a chemotrophic rather than a chemotactic influence of the projection target on cortical axons, which is based on direct contact of axons to the target tissue. It cannot be excluded, however, that the failure to detect chemotactic guidance was caused by the lack of diffusion gradients in our culture system. Innervation of the collicular slice exclusively originated from layer V pyramidal cells, irrespective of the position of the collicular target. Fibre courses suggested that discrimination of the projection target was achieved upon encounter with the collicular surface by direct membrane contact. Inside the collicular tissue, fibre arborizations occurred preferredly in up to three layers perpendicular to the surface. Even after the smallest tracer injections, termination fields were diffusely distributed over the collicular slice. Also, the spatial distribution of retrogradely stained projection neurons did not differ statistically from an equal distribution. Thus, a high degree of convergence and divergence was observed anatomically in the corticotectal projection formed in vitro, corresponding to the immature state in vivo. The functionality of the corticotectal projection was assessed by intracellular recordings from collicular neurons. Electrophysiological properties, such as membrane potential (-68 +/- 11 mV), membrane resistance (35.4 +/- 27.7 M omega) and the time constant (3.0 +/- 2.1 ms) were comparable to reference values, confirming the viability of our culture preparation. The functionality of corticotectal transmission was revealed by intracellularly recorded responses of collicular cells to extracellular cortical stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Laminar distribution and the response of superior colliculus neurons to stimulation of the periodontal ligament in the rabbit

Unit discharges of superior colliculus neurons fired by electrical stimulation of the periodontal ligament and/or mechanical stimulation of the mandibular incisor tooth were extracellularly recorded, and their laminar distribution and response properties were examined. The neurons were widely distributed in the deep layer of the anterior two-thirds of the superior colliculus. Most of them were located in the lateral site of the profound layer, a few in the lateral site of the intermediate layer. Their locations correspond to the lower nasal quadrant of the visual field. The mean first-spike latency of the intermediate layer neurons (7.2 ms) elicited by electrical stimulation of the periodontal ligament was shorter than that of the profound-layer neurons (13.8 ms). About one-third of the neurons were suppressed by conditioning electrical stimulation of the optic chiasma. These results suggested that in addition to visual, auditory, and body surface tactile sensations, intraoral somesthetic sensations also participate in the functioning of the superior colliculus which appears to play a role in orienting the animal toward visual, somesthetic, and auditory stimuli.

Innervation by W-type retinal ganglion cells of superior colliculus neurons projecting to pulvinar nuclei in cats

Neurons of the cat superior colliculus (SC) sending their axons to the pulvinar nuclei were identified electrophysiologically as the ones responding antidromically to electrical stimulation of the pulvinar nuclei. They were located in the superficial layers of SC and found to be innervated by axons of W-type ganglion cells of the retina.

Influence of the visual cortex upon collicular evoked responses in the rabbit

In rabbit, the depression of the visual cortex by KCl showed an enhancement of the collicular evoked potentials. This enhancement revealed that the visual cortex had exerted a tonic type influence upon the superior colliculus.

Simultaneous recordings of visual cortex and superior colliculus field potentials in the rabbit

The field potentials recorded simultaneously at various depth of the rabbit's visual cortex and superior colliculus were analysed following light ON and light OFF. The collicular ON and OFF potentials exhibited three slow components superimposed by fast rhythmic oscillations. Only the first slow component reversed its polarity with penetration from surface negative to positive in depth. The cortical ON and OFF responses similarly contained three slow waves which all reversed their polarity with electrode penetration: from surface positive to negative in deeper layers. The most striking difference between ON and OFF cortical responses is the absence of fast rhythmic oscillations in the cortical ON response.