Coronal slices, containing part of the medial hyperstriatum ventrale (MHV), were cut from the left forebrains of domestic chicks and maintained in vitro. Records were made of the field responses evoked in the MHV by local electrical stimuli provided at 0.1 Hz. Two 1 min periods of stimulation at 5 Hz, separated by 10 min, were used in attempts to induce a persistent increase in the size of the postsynaptic response to test stimulation at 0.1 Hz. This procedure produced a potentiation which usually lasted longer than 2 h. The probability of inducing this persistent potentiation of the response (PPR) is not distributed evenly over the whole anteroposterior length of the MHV but is higher in slices that also contain the septo-mesencephalic tract ventrally. These are the slices that contain the intermediate part of the medial hyperstriatum ventrale (IMHV); an area that is essential for early behavioural learning. At this level PPR is not confined to the IMHV. It can also be produced in the lateral neostriatum in response to similar local stimulation at 5 Hz. No PPR was observed in either the caudal ectostriatum, or the paleostriatum.
A single coronal slice, containing the intermediate part of the medial hyperstriatum ventrale (IMHV) was cut from the left forebrain of a series of domestic chicks and maintained in vitro. Records were made of field responses evoked in the IMHV by local electrical stimuli. Two 1-min periods of 5 Hz stimulation, separated by 10 min, were used in attempts to induce persistent potentiation of the responses (PPR) to test stimulation at 0.1 Hz. In dark-hatched chicks the probability of producing PPR is much higher in slices from chicks aged 2-5 days post-hatch than in those from either younger or older birds. As an independent measure of plasticity in dark-hatched chicks, the probability of eliciting unit responses to repeated stimulation of remote sites in the slice at 3.3 Hz was analysed. This probability was greater in slices from chicks aged 2-5 days than in those from either younger or older birds. In light-hatched chicks the probability of inducing PPR is significantly higher during the first day post-hatch, than in dark-hatched chicks of this age. The probability of producing PPR in slices from light-hatched chicks aged 2-3 days is less than that in slices from either younger or older birds. It is clear that both the age and past experience of the domestic chick affect the neurophysiological properties of slices of brain, tested in vitro.
In a previous paper it has been shown that interval distributions derived from the activity of single cortical neurones can be described by log-normal curves. A cell’s temporal pattern of discharge can therefore be defined by the values of two parameters – a modal interval, and a geometric standard deviation (g. s. d.). It has also been shown that the values of both parameters change when an animal falls asleep. The modal interval becomes shorter, and the g. s. d. usually becomes larger. This paper deals with the effects of changes in arousal of animals which are awake; and, in particular, with the effects of the transition from relaxation to alarm. Single unit recordings have been made from neurones in the post-lateral and supra-sylvian gyri of unrestrained cats. In order to eliminate the direct effects of eye-movements, the experiments were carried out in complete darkness, and the animal was observed through an infrared telescope. Alarm was produced by the hiss of compressed air. An animal was said to be alarmed when he stood up abruptly and turned towards the source of the noise. Alarm produced a marked fall in the discharge frequency of those cells in post-lateral cortex which initially showed a low ( < 2 action potentials per second) rate of spontaneous activity. The discharge rate of the remaining neurones (whether in suprasylvian or post-lateral cortex) was unaffected by the sudden increase in arousal. But the temporal pattern of discharge of every cell was altered. The modal interval became longer when the animal was alarmed, and the g. s. d. usually became smaller. Such changes could have been predicted from a knowledge of the neural concomitants of the transition from sleep to wakefulness. These results suggest that the activity of all cortical neurones is affected by the level of arousal of the animal, and that this modulation takes the form of a continuum of possible modal intervals, and possible g.s.ds.
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