Mechanisms for limbic modification of cerebellar and cortical afferent information

The hearing hippocampus

The hippocampus has a well-established role in spatial and episodic memory but a broader function has been proposed including aspects of perception and relational processing. Neural bases of sound analysis have been described in the pathway to auditory cortex, but wider networks supporting auditory cognition are still being established. We review what is known about the role of the hippocampus in processing auditory information, and how the hippocampus itself is shaped by sound. In examining imaging, recording, and lesion studies in species from rodents to humans, we uncover a hierarchy of hippocampal responses to sound including during passive exposure, active listening, and the learning of associations between sounds and other stimuli. We describe how the hippocampus' connectivity and computational architecture allow it to track and manipulate auditory information - whether in the form of speech, music, or environmental, emotional, or phantom sounds. Functional and structural correlates of auditory experience are also identified. The extent of auditory-hippocampal interactions is consistent with the view that the hippocampus makes broad contributions to perception and cognition, beyond spatial and episodic memory. More deeply understanding these interactions may unlock applications including entraining hippocampal rhythms to support cognition, and intervening in links between hearing loss and dementia.

Exploring the significance of morphological diversity for cerebellar granule cell excitability

The relatively simple and compact morphology of cerebellar granule cells (CGCs) has led to the view that heterogeneity in CGC shape has negligible impact upon the integration of mossy fibre (MF) information. Following electrophysiological recording, 3D models were constructed from high-resolution imaging data to identify morphological features that could influence the coding of MF input patterns by adult CGCs. Quantification of MF and CGC morphology provided evidence that CGCs could be connected to the multiple rosettes that arise from a single MF input. Predictions from our computational models propose that MF inputs could be more densely encoded within the CGC layer than previous models suggest. Moreover, those MF signals arriving onto the dendrite closest to the axon will generate greater CGC excitation. However, the impact of this morphological variability on MF input selectivity will be attenuated by high levels of CGC inhibition providing further flexibility to the MF → CGC pathway. These features could be particularly important when considering the integration of multimodal MF sensory input by individual CGCs.

Redefining the cerebellar cortex as an assembly of non-uniform Purkinje cell microcircuits

The adult mammalian cerebellar cortex is generally assumed to have a uniform cytoarchitecture. Differences in cerebellar function are thought to arise primarily through distinct patterns of input and output connectivity rather than as a result of variations in cortical microcircuitry. However, evidence from anatomical, physiological and genetic studies is increasingly challenging this orthodoxy, and there are now various lines of evidence indicating that the cerebellar cortex is not uniform. Here, we develop the hypothesis that regional differences in properties of cerebellar cortical microcircuits lead to important differences in information processing.

Relationship between complex and simple spike activity in macaque caudal vermis during three-dimensional vestibular stimulation

Lobules 10 and 9 in the caudal posterior vermis [also known as nodulus and uvula (NU)] are thought important for spatial orientation and balance. Here, we characterize complex spike (CS) and simple spike (SS) activity in response to three-dimensional vestibular stimulation. The strongest modulation was seen during translation (CS: 12.8 +/- 1.5, SS: 287.0 +/- 23.2 spikes/s/G, 0.5 Hz). Preferred directions tended to cluster along the cardinal axes (lateral, fore-aft, vertical) for CSs and along the semicircular canal axes for SSs. Most notably, the preferred directions for CS/SS pairs arising from the same Purkinje cells were rarely aligned. During 0.5 Hz pitch/roll tilt, only about a third of CSs had significant modulation. Thus, most CSs correlated best with inertial rather than net linear acceleration. By comparison, all SSs were selective for translation and ignored changes in spatial orientation relative to gravity. Like SSs, tilt modulation of CSs increased at lower frequencies. CSs and SSs had similar response dynamics, responding to linear velocity during translation and angular position during tilt. The most salient finding is that CSs did not always modulate out-of-phase with SSs. The CS/SS phase difference varied broadly among Purkinje cells, yet for each cell it was precisely matched for the otolith-driven and canal-driven components of the response. These findings illustrate a spatiotemporal mismatch between CS/SS pairs and provide the first comprehensive description of the macaque NU, an important step toward understanding how CSs and SSs interact during complex movements and spatial disorientation.

What makes us human? A biased view from the perspective of comparative embryology and mouse genetics

For a neurobiologist, the core of human nature is the human cerebral cortex, especially the prefrontal areas, and the question "what makes us human?" translates into studies of the development and evolution of the human cerebral cortex, a clear oversimplification. In this comment, after pointing out this oversimplification, I would like to show that it is impossible to understand our cerebral cortex if we focus too narrowly on it. Like other organs, our cortex evolved from that in stem amniotes, and it still bears marks of that ancestry. More comparative studies of brain development are clearly needed if we want to understand our brain in its historical context. Similarly, comparative genomics is a superb tool to help us understand evolution, but again, studies should not be limited to mammals or to comparisons between human and chimpanzee, and more resources should be invested in investigation of many vertebrate phyla. Finally, the most widely used rodent models for studies of cortical development are of obvious interest but they cannot be considered models of a "stem cortex" from which the human type evolved. It remains of paramount importance to study cortical development directly in other species, particularly in primate models, and, whenever ethically justifiable, in human.

Activation of mossy and climbing fiber pathways to the cerebellar cortex by stimulation of the fornix in the rat

The fornix of the rat was electrically stimulated with bipolar concentric electrodes to determine the properties of single unit responses in Purkinje cells of the cerebellar cortex. Both climbing (CF) and mossy fiber (MF) pathways were activated by fornix stimulation. MF responses were indicated by single or double spike responses appearing at latencies of 5--10 ms. The MF spike responses, as quantified by histogram analysis, were further identified by appearance of graded responses with increasing stimulus strength and by following at frequencies up to and greater than 20/s. CF responses were identified by characteristic complex all-or-none burst responses with latencies usually between 10 and 20 ms and with following frequencies at no faster than 10/s. Experiments which involved movement of the stimulating electrode and production of lesions around it established that the activated fiber system was within the dorsal fornix and not in adjacent areas. The results indicate that hippocampal and other limbic areas can influence the cerebellar cortex by direct mossy and climbing fiber pathways, as has been demonstrated for other afferents. It is further suggested that motor patterns linked to hippocampal activity may be regulated by this system.

[Klüver-Bucy syndrome and primitive motor reflexes after heavy brain trauma (author's transl)]

A 16-year-old physically and mentally retarded male developed a complete Klüver-Bucy syndrome similar to that demonstrated in animal experiments and characterized by so-called psychic blindness, oral tendencies, hypermetamorphosis, emotional disturbance, and hypersexuality after severe brain trauma with loss of consciousness and a transitory hemiplegia. In addition, transitory oral and grasp reflexes were observed. Despite the severity of the clinical symptoms, the patient was able to realize and report on his subjective experience of the reflex and on the instinct-motivated behavior associated with the Klüver-Bucy syndrome. In the case described, two basic behavior patterns could be differentiated, each influencing drive and the abnormally increased sexuality. The first phase of the illness was dominated by relatively constant automatic reactions. These had the character of pre-determined instinctive behavior and were more or less emotionally neutral. In the second phase of the illness, the instinctive behavior became less evident and the patient was able to respond more normally to his sensory-motor impulses. Also, emotional experience such as frustration, pleasure, and displeasure regained their meaning for the patient.

A hypothesis concerning the structural relationship between memory and emotion

Limbic and temporal lobe structures have long been implicated as the anatomic substratum for emotion. Recent investigations have localized the processing of short term memory to a virtually identical anatomical locus. It would seem logical that the same structures subserve both functions if one assumes that the strength of the memory trace is dependent on the emotional value of the information for the organism. Under this hypothesis, the hippocampus emotionally weights sensory data as a preliminary to incorporation of such material into long term memory. It is not necessary, nor is it likely, that long term memory reside within the temporal lobe. Previous associations stored in remote memory can be accessed by the temporal lobe and these may serve to modulate the weighting of the present stimulus. Some of the functions of the temporal lobe can be modeled by a simple electronic filter; the addition of a single feedback loop to this system can account for other properties inherent in the hypothesis. The anatomical connections of the system would permit it to be an early and efficient repressor, in the pyschodynamic sense.

Cerebellar control of basal forebrain seizures: amygdala and hippocampus

Electroamygdalagrams and electrohippocampalgrams of the cat and monkey were studied before, during, and after electrically induced afterdischarges. Cerebellar stimulation, particularly of midline cortex, shortened or terminated afterdischarges. Prestimulation of the vermis suppressed afterdischarges for as long as 5 min. Excitation of nucleus fastigii prolonged afterdischarges, and legions of n. fastigii abolished the effects of stimulation of the vermis. Cooling of the vermis prolonged afterdischarges, an effect reversed by warming. Pretreatment with 6-OH dopamine to induce chemical lesions in the catecholamine system reduced the effectiveness of cerebellar stimulation. The findings indicate that the cerebellar cortex can exert a tonic suppressor (inhibitory?) INFLUENCE ON THE AMYgdala and hippocampus by way of n. fastigii. Evoked responses demonstrated cerebellar projection to the amygdala and the hippocampus. The evoked potentials were small with a short latency (4 to 10 msec to peak) and larger with a long latency (20 to 30 msec to peak) wave. Return projection to the cerebellum was shown, the small evoked potential having a latency of 6 to 12 msec and the larger one of 20 to 30 msec.

Effects of hippocampal afterdischarges on Purkinje cell activity

Hippocampal afterdischarges can induce increased Purkinje cell activity that characteristically shows a pronounced acceleration during the period of the afterdischarges. Increased Purkinje cell activity may continue for many seconds after termination of the hippocampal activity. Typical frequency histograms are presented. A short period of hippocampal afterdischarges (under 25 sec) may occur with few if any alterations in cerebral activity and vice versa. However, such afterdischarges can produce changes in cerebellar activity as shown by enhanced Purkinje cell activity. These data support the view that the nucleus tegmenti pontis may be prominently involved in relaying hippocampal afterdischarges to the cerebellum whereas much of the neocortical seizure activity is relayed trough the pons, inferior olive, and lateral reticular nucleus, rather than through nucleus tegmenti pontis. EEG tracings taken from these areas illustrate the findings.

Operant controlled neural event: formal and systematic approach to electrical coding of behavior in brain

Traditional studies of electrophysiological correlates of behavior contain inherent high variability resulting from the arbitrary choice of behaviors, brain locations, and wave parameters. The operant control of neural events is a formal and systematic approach to the study of prespecified parameters and components of brain activity as they encode behaviors. Two studies in which the electrical activity of brain was the criterion for reinforcement demonstrate the acquisition, under such operant control, of two mutually exclusive behaviors or states which selectively alter evoked potential components.