Thalamocortical coupling and component properties of visually evoked afterdischarge

ON and OFF field potentials in the rat superior colliculus during development

The present investigation is aimed at characterizing the development of ON and OFF visually evoked responses in the rat superior colliculus from postnatal day 13 (P13) to postnatal day 25. Depth profiles of field potentials reveal that ON and OFF long latency biphasic field potentials are already present when collicular cells are first responsive to light (P13). There is an inversion in the polarity of these responses as the electrode penetrates the collicular layers, suggesting a synaptic organization similar to the one found in adult animals. At P15, OFF field potentials begin to exhibit oscillatory activity. Local cobalt injections within the superior colliculus abolishes these OFF oscillations, suggesting a postsynaptic origin. Fast Fourier transform (FFT) analysis of the OFF field potentials demonstrates that oscillatory activity increases in frequency during development. This increase is thought to reflect the myelination and stabilization of synaptic connections that occur during this period. To our knowledge, this is the first report of OFF oscillatory responses in the superior colliculus.

The effects of bilateral and unilateral frontal lesions on visual cortical hypersynchronous bursting and behavioral activity

Photically evoked after-discharge (PhAD) bursting was examined in albino rats following either bilateral or unilateral dorsal frontal lesions. The initial effect of either type of frontal lesion was to significantly suppress PhAD occurrence. However, after repeated exposure to the PhAD recording environment no difference could be seen between the lesioned and control animals. In addition, no difference in PhAD could be detected between the two visual cortices in any of the groups. Spontaneous alternation and open-field activity were also tested. Both frontally lesioned groups of animals were significantly more active than control animals in the initial stages of testing. These findings indicate that the frontal cortex can inhibit brain stem mechanisms which are responsible for behavioral arousal and the modulation of hypersynchronous bursting in the cortex. However, because no difference was seen between the two visual cortices in any of the groups, it is suggested that this mechanism is not a strictly unilateral mechanism.