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 septomesencephalic 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.

The responses to local stimulation have been recorded from neurons in the intermediate part of the medial hyperstriatum ventrale (IMHV) of the domestic chick, by using an in vitro slice preparation. When the slice is bathed in gassed Krebs’ solution, a single stimulus evokes a short-lasting diphasic response. The first phase is negative and lasts some 3 ms, whereas the second, positive phase is often of lower amplitude and usually persists for about 15 ms. The first phase is little altered by perfusion with either Ca 2+ -free Krebs’ solution or Krebs’ solution containing a high concentration of Mg 2+ . In contrast, the second phase is abolished by these procedures. The post-synaptic phase is positive when it is recorded anywhere between 0.1-1.25 mm from the stimulated point; however, in the immediate vicinity (0.0- 0.1 mm) of the stimulating electrodes, the post-synaptic response is strongly negative. A pair of stimuli has to be separated by at least 10 s to guarantee complete recovery of excitability of the post-synaptic response. The recovery curve for this response shows a refractory period of some 5 ms, a peak of excitability at an interval of about 20 ms, and then a sharp trough of relative inexcitability at about 200 ms. The post-synaptic response is considerably reduced in magnitude and duration by the addition of AP-5 to the perfusion fluid; the remaining post-synaptic response is completely abolished by kynurenic acid. The addition of bicuculline methiodide in concentrations of at least 1 x 10 -6 M increases both the magnitude and duration of the second, positive phase of the response to single stimuli. This extended positive response (which may last from 500-800 ms) is abolished by perfusion with bicuculline dissolved in Ca 2+ -free Krebs’ solution. For the entire duration of the extended post-synaptic positive response produced by bicuculline, the irregular discharge of single neurons can be recorded. Like the post-synaptic positive response in Krebs’ solution, the much larger response produced by bicuculline shows a very localized negativity beneath the stimulating electrodes and displays an almost identical timecourse for the recovery of excitability following a single stimulus. The bicuculline induced positive response is also considerably reduced by the presence of AP-5; the addition of kynurenic acid abolishes the remaining post-synaptic response completely. A post-synaptic response, similar to that produced under bicuculline, can be produced by the addition of a maximally effective dose of d-tubocurarine. This response has the same pharmacological and electrophysiological properties as that produced under bicuculline, only differing in that a maximal response evoked in the presence of curare, can be increased in amplitude and duration by the addition of bicuculline, whereas a maximal bicuculline response is unaffected by the addition of curare.

The responses to single electrical stimuli have been recorded from neurons in the brains of domestic chicks, by using an in vitro preparation consisting of a coronal slice taken from the forebrain. All slices were cut so that they contained the intermediate part of the medial hyperstriatum ventrale (IMHV). When such a slice is bathed in standard Krebs’ solution there is no evidence that the excitation produced by a single stimulus can be transmitted more than 1 mm either towards or away from the IMHV. The addition of bicuculline methiodide (more than 3 x 10 -6 M) to Krebs’ solution allows the excitation produced by a single stimulus to spread in all directions throughout the dorsal half of a coronal slice. At points remote (more than 1.5 mm) from the stimulated point, the magnitude of the spreading wave of excitation bears an all-or-nothing relation to the strength of stimulus used to excite it. This wave of excitation spreads from the excited point in all directions without attenuation at 0.9 + 0.017 (s. d.) m s -1 and consists of a prolonged burst of activity of the invaded neurons. The properties of coronal slices described above are also true of brain slices cut in a parasagittal plane. The spreading response to a single stimulus given in the presence of bicuculline, can be reduced in magnitude by the addition of AP-5 but it still spreads throughout the dorsal part of the slice at the same velocity. The response can be eliminated by the addition of kynurenic acid. The addition of curare to the bathing medium produces similar responses that spread in a similar fashion to those seen under bicuculline. These results suggest that the dorsal part of the forebrain of the domestic chick (in fact, the part derived from the embryological alar plate) contains a network of reciprocally connected local circuits. Transmission throughout the network is normally prevented by active inhibition.

Coronal sections were taken from the forebrains of domestic chicks, aged 1-20 days, and maintained in vitro. Extracellular recordings of neural activity were made from the intermediate part of the medial hyperstriatum ventrale (IMHV). Spontaneous activity was rarely recorded, but neuronal responses could be evoked by stimulation of various sites. Each recording point was surrounded by an arc of sites which, when stimulated, typically elicited a short-latency field potential. These 'local responses' could be recorded in slices from chicks of any age. Stimulation of more distant sites failed to evoke field potentials from the IMHV. Instead, trains of large, unit action potentials appeared on an undisturbed baseline. Such 'unit' responses could only be evoked by stimuli delivered at specific frequencies. They required facilitation, were of variable latency, and often finally decayed. The number of sites capable of evoking a 'unit' response from the IMHV fell dramatically in slices taken from chicks older than 4 days.

1. It has been shown that the spontaneous activity of many neurons in the cerebral cortex changes whenever the animal’s level of arousal alters (A. C. Webb, Proc. R. Soc. Lond. B 194, 225–237, 239–251, 1976). It is generally agreed that the temporal pattern of discharge is sensitive to fluctuations in level of arousal. There is less agreement concerning effects on discharge frequency. 2. We have re-examined our own records of the spontaneous discharge of single cells in the visual, parietal and auditory cerebral cortex of unanaesthetized and unrestrained cats. We have also searched the literature for all comparable observations of single neurons in various cortical sites, recorded in the same and in other species. Most of the available data seem consistent with the idea that mean frequency of discharge of all mammalian cortical cells is affected in the same way by the transition from waking to slow-wave, or motionless sleep. 3. If the mean frequency of a cortical neuron when the animal is awake be W/s, and that of the same neuron in the sleeping animal be S/s, then the data from various cortical sites in cat and monkey can be fitted by a straight line: S = 2.00 + 0.45 W, correlation coefficient = 0.64. 4. The changes in discharge frequency that occur when an alarmed animal relaxes can also be fitted by a straight line. If the mean frequency of a cortical neuron when a cat is peacefully awake be W/s and that of the same cell when the cat is alarmed be A/s, then the relation between W and A can be described by W = 0.92 + 0.84 A, correlation coefficient = 0.92. 5. Neither excitation nor inhibition alone can account for the changes in spontaneous activity that accompany shifts in level of arousal. We propose a simple neural model that can account for the observed changes. This model assumes that, as level of arousal falls, the output of some subcortical system increases, providing both excitation and inhibition in fixed proportions.

The purpose of the experiments was to find out whether neighbouring neurons in chronic preparations of neurally isolated cerebral cortex are more likely to fire synchronously than are similar neurons in the intact brain. Chronically implanted extracellular microelectrodes were used to obtain simultaneous records of the spontaneous discharges of neighbouring neurons in the suprasylvian gyrus of the unanesthetized, unrestrained cat. We have examined multi-unit records obtained from neurons in islands of neurally isolated cortex; these records have been compared with similar records from neurons in the same cortical region of the intact brains of control animals. In isolated cortex, neighbouring neurons showed a tendency to discharge in near synchrony. In contrast, there was a random temporal relation between the firing times of adjacent nerve cells of intact cortex, provided the cat was awake. These results, taken together with the relevant observations of other workers, may indicate the manner in which biologically important information is transmitted within the mammalian brain.

Slabs of cat parietal cortex with some 2 mm of underlying white matter were surgically isolated from the rest of the nervous system, without interference with the superficial blood supply. Wire micro-recording electrodes were inserted into the isolated cortex; bone, muscle and skin wounds were repaired and the animal allowed to recover from anaesthesia. The adequacy of surgical isolation was examined histologically 8--12 weeks after operation. Only one of the six preparations reported here showed surviving neural connections with the rest of the brain. Soon after operation, spontaneous bursts of neural activity appeared within the isolated area. These became more frequent until neural discharge was continuous but irregular. Our records were made from this time onwards. The interval distributions obtained from neurons within the isolated area did not differ significantly from log-normal curves. When the unrestrained animal fell asleep, there was no significant alteration in the model interval or geometric standard deviation of interval distributions recorded from cells in isolated cortex. The interval distributions of neurons in isolated cerebral cortex resembled those of neurons in the intact cortex of an alarmed animal. It is concluded that the reduction of modal interval that is shown by neurons in intact cortex when an animal falls asleep is probably due to the neural influence of infracortical structures.

Chronically implanted microelectrodes have been used to obtain extracellular records of trains of spontaneous action potentials from 30 neurones in the cerebral cortices of 13 unrestrained cats. Recorded neurones were in or near to primary visual cortex, primary auditory cortex, and in the supra-sylvian gyrus. Records were made with animals in several different behavioural states, which included sleep with rapid eye-movements and quiet sleep, peacefully awake, and alarmed. Interval distributions derived from trains of 200 action potentials recorded in less than 80 s did not differ significantly from curves in which the probability of any interval is normally distributed about a modal interval, when plotted on a logarithmic time-axis. Thus the complete interval distributions of neurones firing faster than 2.5/s can be described by two parameters – a modal interval, and a geometric standard deviation. This quantitative description of interval distributions proved equally applicable to neurones in all three cortical areas and was valid over the whole range of behavioural states examined. It does not usually hold when the discharge frequency of a neurone is lower than 2.5 action potentials per second. An acceptable fit for a log-normal curve can then only be obtained for intervals that are shorter than about ten times the modal interval. It is pointed out that mean frequency of discharge is a measure of neural activity which is a secondary parameter, since it is dependent upon both modal interval and geometric standard deviation. Our preliminary data show that the two parameters which define the best-fit log-normal curves can vary independently with the behavioural state of the animal in a way that suggests that they may be physiologically important.

This report describes the effects of stationary retinal patterns of light upon the behaviour of single neurons in the cat’s primary visual cerebral cortex. Records were obtained with extracellular microelectrodes from 20 neurons in the neurologically isolated forebrains of 11 cats. Black and white patterns were projected upon a translucent pearl-glass screen, and were focused upon the animal’s retinae through 10D spectacle lenses and optically neutral contact glasses. The stationary pattern was a straight light-dark edge. Experiments tested the effects of this stationary pattern upon both the frequency of spontaneous discharge and upon responses to a neighbouring mobile pattern. The mobile pattern consisted of a parallel thin white line passing through the neuron’s receptive field and provided with a rectangular oscillation of about 10´ amplitude and frequency of 2.5 Hz . Stationary patterns rarely produced any detectable modification of spontaneous discharge. Occasionally, however, such patterns produced a measurable change in the mean frequency of firing, which was greatest when the pattern crossed the edge of the receptive field. In contrast, stationary patterns almost invariably modified the responses of neurons to a neighbouring mobile pattern. This effect of the stationary pattern was as great when both patterns fell upon different retinae as when stationary and mobile patterns were projected upon the same retina. It proved impossible to account for these results as the consequence of either inadequate light-adaptation or inadequate retinal stability of the stationary pattern. The results show that the steady discharge of retinal ganglion cells which is continually related to stationary retinal illumination (Kuffier, Fitzhugh & Barlow 1956) reaches the cerebral cortex. This input to the cortex is too weak to drive cortical neurons, but is sufficient to modify their responses to more powerful stimuli. If properties of the human visual system are similar to those described here for the cat, some aspects of stabilized retinal images as observed by human subjects could be explained.

Point-sources of light (dots) were exposed for 10 to 50 msec, before five dark-adapted subjects in a dimly illuminated room. During voluntary fixation with one eye, the target was exposed some 10° on the nasal side of the optic axis. The intensity X duration of all targets was 2 X threshold and they consisted of either a single dot, or a pair of dots separated by a distance that was less than that required for two-point discrimination. In two-thirds of trials both the single-dot and the two-dot targets were perceived as short thin lines of various orientation. Although individual percepts were unpredictable, there was a preferred or most likely orientation for responses to the single-dot target; this was near to the horizontal for all five subjects. There was no significant difference between the preferred orientations for single-dot targets tested at sites more than 1° apart in the visual field. When two single-dot targets, separated by about 1°, were exposed simultaneously, the orientations of the perceived lines sometimes differed by as much as 80°; occasionally, one target was reported as a dot while the other was seen as a thin line. If the single-dot was briefly exposed between two continuously visible and parallel straight lines, the target usually appeared as a thin line, parallel to the framing lines. Some of these results appear to be consistent with the hypothesis that the human visual cortex, like that of the cat and monkey, contains neurones that are orientation specific.

Brain ReflexesEdited by E. A. Asratyan. (Progress in Brain Research, Vol. 22.) Pp. xv + 600. (Amsterdam, London and New York: Elsevier Publishing Company, 1968.) 255s.

Sensory thresholds have been measured in human subjects while breathing 25 per cent nitrous oxide in oxygen.

Human subjects and cats breathing a weak mixture of nitrous oxide (10–40 per cent) in oxygen have been observed.

It is possible to maintain active mammalian cardiac and skeletal muscle in good condition for 3 or 4 hours by the intravascular perfusion of these tissues with warm, moist oxygen containing 5% carbon dioxide. Some of the uses of this perfusion technique are discussed.

The Organization of the Cerebral Cortex By Dr. D. A. Sholl. Pp. xvi + 125. (London: Methuen and Co., Ltd.; New York: John Wiley and Sons, Inc., 1956.) 18s. net. Studies on the Cerebral Cortex (Limbic Structures) By Santiago Ramón y Cajal. (Translated from the Spanish by Lisbeth M. Kraft.) Pp. xi + 179. (London: Lloyd-Luke (Medical Books), Ltd., 1955.) 27s. 6d. net.