In order to test physiologically for cerebrotectal connections in a fish, averaged evoked potentials and unit responses were recorded from the optic tectum following electrical stimulation applied to the telencephalon in the siluroid teleost Ictalurus nebulosus. A single shock applied to the area dorsalis centralis (Dc) of the telencephalon, and only to this area, elicits a sequence of deflections in the ipsilateral optic tectum: an initial negative peak at about 8 ms, (= N8), a larger N25 and a slow P50-N95. The configurations, depth profiles, latencies and susceptibility to repetitive stimulation, together with the known tectal anatomy, suggest that the first wave is due to the afferent fibers from the telencephalon and that N25 is due to deep tectal neurons. Telencephalic input exerts a conditioning effect on the field potentials and unit responses evoked by direct optic nerve shock. Such a shock elicits, in the contralateral tectum, small negative, optic tract axon peaks followed by a large N6, believed to be postsynaptic, and a still later P12. As a first approximation it is argued that the telencephalic input and the retinal input are activating different sets of neuronal elements in the optic tectum, since the configuration and depth profile of the telencephalic and optic nerve shock-elicited potentials are different. A conditioning Dc stimulus has a long-lasting effect on the form of the optic nerve field potential, maximally when the pallial shock precedes the optic by about 90 ms. The effect, observed by subtracting the conditioned from the unconditioned tectal response to optic nerve shock, is a difference wave with N11 and P20. The unit activity from deep tectal laminae is either activated or accelerated following Dc stimulation, while superficially located neurons are not affected. In another group of tectal units, the optic nerve shock-induced response is depressed by a preceding pallial dorsalis centralis stimulus. The evidence is compatible with the assumption of direct projections from Dc to the deep layers of the tectum, but the timing could also permit indirect pathways. In any case, the influence is not simple or identical for different tectal cell classes.

The responses of cerebellar units following electrical stimulation applied to the area dorsalis centralis (Dc) of the telencephalon were studied in the siluroid teleost Ictalurus nebulosus. Two kinds of units are distinguished on physiological criteria, identified as Purkinje and eurydendroid cells (the efferent neurons of the cerebellum in teleosts equivalent to cells of the deep nuclei in other vertebrates). A high proportion of both kinds of units in the corpus cerebelli are sensitive to such stimulation. Each kind of unit shows several consistent response types. Purkinje cells fire simple spikes spontaneously at the rate of 8–50 spikes/s and respond to a single shock to Dc with an initial latency of 34–64 ms. The response can be one of the following types: (1) inhibition alone, with a duration of 0.3–1.5 s; (2) initial inhibition for 0.04–0.2 s, followed by postinhibitory rebound, or (3) initial excitation followed by inhibition which may or may not be followed by a late excitation. It is suggested that the initial excitation and initial inhibition reflect the activation of mossy fiber-granule cell-Purkinje cell circuitry and mossy fiber-granule cell-inhibitory interneuron-Purkinje cell circuitry, respectively. Indirect evidence suggests the involvement of climbing fibers, but their characteristic complex spikes are rarely seen. Changing the stimulation sites within Dc does not appear to change the response pattern but may alter the threshold intensities, latencies and amplitudes. Changing stimulation frequency has complex effects depending on the response type. Purkinje cells responding with initial excitation are located along the lateral edges and along the midline of the corpus cerebelli; units responding with initial inhibition are more often found in an intermediate zone. This suggests three sagittal bands on each side. The contralateral cerebellum has a relative excess of Purkinje cells with initially inhibitory response. Putative eurydendroid cells show either initial excitation or a pattern of inhibition, excitation, inhibition. The initial latency is longer than in Purkinje cells. The contralateral cerebellum has a relative excess of eurydendroid cells with initially excitatory responseThe high proportion of units that respond to Dc stimulation, their complex dynamics, diverse response types and compartmentalization point to the importance of the cerebrocerebellar influence in teleosts. Principal differences from mammals are the much longer latencies and less differentiation according to place stimulation in the cerebrum.

Extracellular spikes from Purkinje and eurydendroid cells as well as evoked field potentials were recorded from the cerebellum in a catfish following trains of 50–100 stimuli at 2–10 Hz delivered to the area dorsalis centralis (Dc) of the telencephalon. Forty out of 48 Purkinje cells tested showed some response, either an increase or a decrease in the level of ongoing spike discharge following the 10- to 25-second tetanic pulse trains. The altered level lasted for > 3 min. Four out of 16 eurydendroid cells showed an increase in posttrain ongoing discharge; none showed a decrease. In those Purkinje cells responding to single pulse stimulation with early excitation, that response was converted to early inhibition during the later cycles of a tetanic pulse train. When tested with posttrain stimulation, they resumed the pretrain early excitation about 100 s after the train; before that, the recovery stages are complex, neither a simple continuation of, nor a simple rebound from, the later cycles of the stimulus train. Bursting discharge of Purkinje cells is often induced or, if already present, enhanced and regularized, typically at approximately 3/s, by train stimulation of Dc. This pattern begins during the train but continues for tens of seconds after its end. The effects of a stimulus train on the mean frequency and on the pattern of ongoing discharge depend on the train parameters: intensity, duration and stimulus rate within the train; 4–5 Hz is most effective. Higher rates or repeated trains produce a more intense but shorter-lasting effect. Evoked field potentials change during a train, and posttrain test stimuli show a slow recovery, especially after longer pulse trains. The cerebrocerebellar influence in a teleost is nontrivial, widespread, differentiated and shows long-lasting posttetanic plasticity.