Tonic and phasic effects evoked concurrently by sensory stimuli in hippocampal units

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.

Neuronal aspects of septo-hippocampal relations

In unanaesthetized, conscious rabbits, in unstressful conditions, the neurons of the hippocampus and septum were investigated extracellularly during the presentation of a series of varied sensory stimuli. In the normal hippocampus these stimuli evoke habituating reactions of tonic (more usually, inhibitory) type in field CA3, with the addition of 'specific' patterned, and phasic reactions in field CA1. After complete septo-hippocampal disconnection the proportion of tonic (especially, of inhibitory) reactions in the hippocampus decreases. Theta bursts in the neuronal activity are absent; reactions to repeated sensory stimuli do not habituate. After lesion of the cortical perforant path to the hippocampus the majority of reactions in both fields are of tonic type. The proportion of neurons with regular theta bursts increases. Habituation is completely absent. A high correlation appears between the sensory reactions and the effects of midbrain reticular formation stimulation in the same neurons. The combination of both lesions does not significantly change the spontaneous activity of hippocampal neurons (except for the absence of the theta bursts). An increase in the level of activity of hippocampal neurons (by physostigmine), or rhythmic stimulation of the remaining synaptic systems, does not restore their rhythmic theta activity. In the septum deprived of hippocampal input the normal level of reactivity to sensory stimuli and the normal types of reaction are preserved. The proportion of neurons with theta bursts increases. The typical linear and rapid habituation of reactions disappears and is replaced by an unlimited increment in effects during repeated presentations of sensory stimuli. Discussion concerns the synchronizing and inhibitory influences of the septum on the hippocampus, and the role of the hippocampus in the organization of decremental processes (habituation) in the septum and brainstem structures.

Functional integration of the rat hippocampal tissue, transplanted into the rabbit septum

Embryonal tissue of the rat hippocampus (E17-18) was grafted into small acute cavity within the dorsal septum of adult rabbit after interruption of septo-hippocampal connections. Histological analysis (Nissl stain), performed 6-8 weeks later, revealed surviving hippocampal grafts in 14 out of 29 grafted animals (48%). Some of the grafts were displaced into the ventricle and had only minor contact with the host septum; some others, residing in the septum, were separated from the host tissue by continuous glial scar. Six grafts (33%) were morphologically well integrated with the host septum. The grafts contained mature pyramidal neurons but their organization into layer was absent. No signs of rejection were observed in the grafts, which were not subjected to microelectrode investigation in chronic conditions. The grafts, which for 5-9 successive days were penetrated by microelectrodes, were heavily infiltrated by lymphocytes and had necrotic areas. Electrophysiological testing revealed the absence of spontaneous neuronal activity, or the presence of hypersynchronous epileptiform bursts in isolated grafts. The 4 integrated grafts, investigated electrophysiologically, contained neurons with normal spontaneous activity of two types: low-frequency discharges with complex spikes and high-frequency single spikes, sometimes with short periods of weak rhythmic modulation in the theta-range. Characteristics of spontaneous activity of the xenografted hippocampus are very close to those described in the rat hippocampus in situ. Rhythmic theta-modulation of neuronal activity appeared or increased in stability after physostigmine injection to the host, after electrical stimulation of the host's midbrain reticular formation and ventral part of diagonal band, and after multimodal sensory stimulation of the host. The frequency of theta-bursts in the grafts was identical to that of neurons of the host septum (4.5-6.5 Hz). Besides the theta-modulation sensory stimuli evoked tonic suppression or activation of discharges, phasic reactions and on-effects. Electrical stimulation of lateral septum and diagonal band evoked orthodromic and antidromic discharges of the grafted cells. Normal spontaneous and evoked activity of the grafted cells deteriorated after 5-11 successive days of microelectrode investigation. It is concluded that xenografts of nervous tissue, developing without immunosuppression, have a low rate of survival and are vulnerable to immune response after mechanical interruption of integrity of the blood-brain barrier. Nevertheless their normal activity and close functional integration with the host brain, apparently depending on development of chimeric synapses, is possible.

Habituation of human limbic neuronal response to sensory stimulation

Hippocampal, parahippocampal gyrus, and amygdalar neuronal responses to visual and acoustic stimuli were analyzed during trains of several hundred stimulus repetitions as part of an investigation of sensory pathways to medial temporal lobe structures in complex-partial epilepsy patients who were being monitored with depth electrodes. Ten percent of more than 500 single and multiple units tested were responsive to simple sensory stimuli. The majority of the responsive units were recorded in the posterior parahippocampal gyrus (HG) during visual stimulation. Although neurons in pes hippocampi (PH; Ammons's horn) were also responsive to photic stimuli, no visually responsive units were found in amygdala. Very few units were responsive to acoustic stimuli, and these were found only in PH and amygdala, and not in HG. Significant trends of increase or decrease in response amplitude during trains of stimuli were found in all acoustically responsive units. Significant trends of visual response amplitude increase or decrease were found in 20% of PH units, and in 44% of HG units. Mean latencies of acoustically responsive units were longer than those of visually responsive units, and latencies of PH sensory units showing decremental response were longer than nondecremental PH units. Rate of response decrement was usually linear for acoustic responses and exponential for visual responses. The response dynamics of medial temporal lobe neurons are compared with those described in the animal limbic system and are related to habituation of human sensory evoked scalp potentials.

Mapping of sensory-responsive cells in the septal area of the rat

A study of sensory-responsive characteristics of single cells of the septal area of male rats was undertaken to determine the distribution of stimulation-induced changes in septal activity. Single cells were systematically sampled in six planes transecting the rostral-caudal axis of the septal area and were tested for responsiveness to visual, auditory, tactual, gustatory and olfactory stimuli. Approximately 50% of the sampled population of septal cells was responsive to at least one stimulus. Most of the responses of septal cells to stimulation were excitatory and occurred at the onset of stimulation. Regions of concentration of sensory-responsiveness cells in the system were found to lie in medial areas of the septum consisting of the dorsomedial portion of the lateral septal nucleus, the medial septal nucleus and the ventral portions of the medial and lateral nuclei. Visual, auditory and tactual stimuli were most effective in producing changes in spontaneous activity of units in these regions. On the basis of relationships of the medial septal region to other limbic areas having sensory-responsive properties, it is concluded that the medial septal area is one component of a limbic sensory system.

Some connections of the entorhinal (area 28) and perirhinal (area 35) cortices of the rhesus monkey. III. Efferent connections

In this investigation the efferent projections of the entorhinal and prorhinal cortices relative to their sites of termination in the hippocampus and fascia dentata were investigated in the rhesus monkey using experimental silver impregnation methods. Contrary to the often cited observations of Lorente de No, all entorhinal areas, including the laterally lying prorhinal cortex, were found to give rise to the perforant pathway, and furthermore, each cytoarchitectonically defined subarea was found to contribute a unique component. These perforant pathway components terminate in distinct regions of the dendritic zones of the fascia dentata granule cell and the hippocampal pyramidal cell. A previously undescribed projection to the prosubiculum and hippocampus has been found to originate from the prorhinal cortex which forms the medial wall of the rhinal sulcus along the lateral-most portion of the entorhinal cortex in the rhesus monkey. These results, in conjunction with our previous observations regarding differential afferents to the entorhinal cortex, indicate that specific afferent and efferent connections characterize each cytoarchitectonically definable subareas of this periallocortical region. Additionally, they indicate that the perforant pathway might be conceptualized as the final link in a multisynaptic series of connections instrumental in providing the hippocampus with potential modality specific and multimodal input.