This paper is a study of the behavioral and spatial firing correlates of neurons in the rat postsubiculum. Recordings were made from postsubicular neurons as rats moved freely throughout a cylindrical chamber, where the major cue for orientation was a white card taped to the inside wall. An automatic video/computer system monitored cell discharge while simultaneously tracking the position of 2 colored light emitting diodes (LEDs) secured to the animal's head. The animal's location was calculated from the position of one of the LEDs and head direction in the horizontal plane calculated from the relative positions of the 2 LEDs. Approximately 26% of the cells were classified as head-direction cells because they discharged as a function of the animal's head direction in the horizontal plane, independent of the animal's behavior, location, or trunk position. For each head-direction cell, vectors drawn in the direction of maximal firing were parallel throughout the recording chamber and did not converge toward a single point. Plots of firing rate versus head direction showed that each firing-rate/head-direction function was adequately described by a triangular function. Each cell's maximum firing rate occurred at only one (the preferred) head direction; firing rates at head directions on either side of the preferred direction decreased linearly with angular deviation from the preferred direction. Results from 24 head-direction cells in 7 animals showed an equal distribution of preferred firing directions over a 360 degrees angle. The peak firing rate of head- direction cells varied from 5 to 115 spikes/sec (mean: 35). The range of head-direction angles over which discharge was elevated (directional firing range) was usually about 90 degrees, with little, if any, discharge at head directions outside this range. Quantitative analysis showed the location of the animal within the cylinder had minimal effect on directional cell firing. For each head-direction cell, the preferred direction, peak firing rate, and directional firing range remained stable for days. These results identify a new cell type that signals the animal's head direction in its environment.

Navigation first requires accurate perception of one's spatial orientation within the environment, which consists of knowledge about location and directional heading. Cells within several limbic system areas of the mammalian brain discharge allocentrically as a function of the animal's directional heading, independent of the animal's location and ongoing behavior. These cells are referred to as head direction (HD) cells and are believed to encode the animal's perceived directional heading with respect to its environment. Although HD cells are found in several areas, the principal circuit for generating this signal originates in the dorsal tegmental nucleus and projects serially, with some reciprocal connections, to the lateral mammillary nucleus --> anterodorsal thalamus --> PoS, and terminates in the entorhinal cortex. HD cells receive multimodal information about landmarks and self-generated movements. Vestibular information appears critical for generating the directional signal, but motor/proprioceptive and landmark information are important for updating it.

The discharge characteristics of postsubicular head-direction cells in a fixed environment were described in the previous paper (Taube et al., 1990). This paper reports changes in the firing properties of head-direction cells following changes in the animal's environment. Head-direction cells were recorded from rats as they moved freely in a 76-cm-diameter gray cylinder. A white card, occupying 100 degrees of arc, was taped to the inside wall of the cylinder and served as the major orienting spatial cue in the animal's environment. Rotation of the cue card produced near-equal rotation in the preferred firing direction of head-direction cells, with minimal changes in peak firing rate, directional firing range, or asymmetry of the firing-rate/head-direction function. Card removal had no effect on peak firing rate or range of firing, but in 8/13 cells the preferred direction rotated by at least 24 degrees. Similarly, changing the shape of the environment to a rectangular or square enclosure caused the preferred firing direction to rotate by at least 48 degrees for 8/10 cells in the rectangle and 3/8 cells in the square, with minimal changes in the peak firing rate or directional firing range. Hand holding the animals and moving them around the cylinder had no effect on the preferred direction or firing range of the cell, but decreased the maximal firing rate in 7/9 cells. On 2 occasions, 2 head-direction cells were recorded simultaneously. The rotation of the preferred firing direction for one cell was the same as the rotation of the preferred direction for the second cell after each environmental manipulation. These results demonstrate that specific visual cues in the environment can exert control over the preferred firing direction and indicate that head-direction cell firing is not a simple sensory response to visual cues, but rather represents more abstract information concerning the animal's spatial relationship with its environment. The constancy of the angle between the preferred firing directions of pairs of simultaneously recorded head-direction cells suggests that there is a fixed mapping of the population onto direction within the environment. Thus, environmental manipulations appear to cause only a change in the reference direction, but leave all other discharge characteristics of directional cells unchanged. In the discussion, comparisons are drawn between the responses of head-direction cells and hippocampal place cells to similar environmental manipulations (Muller and Kubie, 1987), and ways in which these 2 spatial systems interact in navigation are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Previous studies have identified neurons in the postsubiculum which discharge as a function of the animal's head direction in the horizontal plane, independent of its behavior and location in the environment. Anatomical studies have shown that the postsubiculum contains reciprocal connections with the anterior thalamic nuclei (ATN). In order to determine how the head direction (HD) cell signal is processed in the brain, single-unit recordings were monitored in the ATN of freely moving rats in order to characterize their behavioral and spatial correlates. Animals were trained to retrieve food pellets thrown randomly into a cylindrical apparatus containing a single orientation cue. Single unit recordings in the ATN showed that approximately 60% of the recorded cells discharged in relation to the animal's head direction in the horizontal plane. Observation of the animal and quantitative analyses showed that HD cell firing was not dependent on the animal's behavior, trunk position, linear speed, angular head velocity, or location in the environment. Most of these cells were localized to the anterior dorsal thalamic nucleus. Each HD cell contained only one head direction at which the cell discharged maximally and the firing rate decreased linearly away from this preferred direction. The preferred firing directions from all cells recorded were distributed over a 360" range. Quantitative analysis showed that these cells contained similar discharge parameters (peak firing rate, directional firing range) to values reported previously for postsubicular HD cells (Taube et al., 1990a). Experiments involving rotation of the orientation cue showed that the preferred firing direction could be controlled by a salient visual cue. In contrast to postsubicular HD cells, passive rotation of a restrained animal showed that most ATN HD cells ceased discharging when the animal's head was oriented in the preferred direction. These findings demonstrate the presence of HD cells in the ATN and indicate the potential importance of this area for spatial navigation. The origin of the head direction signal is discussed and it is concluded that because of the presence of reciprocal connections between the postsubiculum and the ATN, further studies are required in order to deter

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The hippocampal formation is essential for forming declarative representations of the relationships among multiple stimuli. The rodent hippocampal formation, including the entorhinal cortex and subicular complex, is critical for spatial memory. Two classes of hippocampal neurons fire in relation to spatial features. Place cells collectively map spatial locations, with each cell firing only when the animal occupies that cell's "place field," a particular subregion of the larger environment. Head direction (HD) cells encode directional heading, with each HD cell firing when the rat's head is oriented in that cell's particular "preferred firing direction." Both landmarks and internal cues (e.g., vestibular, motor efference copy) influence place and HD cell activity. However, as is the case for navigation, landmarks are believed to exert greater influence over place and HD cell activity. Here we show that temporary inactivation of the vestibular system led to the disruption of location-specific firing in hippocampal place cells and direction-specific discharge of postsubicular HD cells, without altering motor function. Place and HD cell activity recovered over a time course similar to that of the restoration of vestibular function. These results indicate that vestibular signals provide an important influence over the expression of hippocampal spatial representations, and may explain the navigational deficits of humans with vestibular dysfunction.

Many neurons in the rat anterodorsal thalamus (ADN) and postsubiculum (PoS) fire selectively when the rat points its head in a specific direction in the horizontal plane, independent of the animal's location and ongoing behavior. The lateral mammillary nuclei (LMN) are interconnected with both the ADN and PoS and, therefore, are in a pivotal position to influence ADN/PoS neurophysiology. To further understand how the head direction (HD) cell signal is generated, we recorded single neurons from the LMN of freely moving rats. The majority of cells discharged as a function of one of three types of spatial correlates: (1) directional heading, (2) head pitch, or (3) angular head velocity (AHV). LMN HD cells exhibited higher peak firing rates and greater range of directional firing than that of ADN and PoS HD cells. LMN HD cells were modulated by angular head velocity, turning direction, and anticipated the rat's future HD by a greater amount of time (approximately 95 msec) than that previously reported for ADN HD cells (approximately 25 msec). Most head pitch cells discharged when the rostrocaudal axis of the rat's head was orthogonal to the horizontal plane. Head pitch cell firing was independent of the rat's location, directional heading, and its body orientation (i.e., the cell discharged whenever the rat pointed its head up, whether standing on all four limbs or rearing). AHV cells were categorized as fast or slow AHV cells depending on whether their firing rate increased or decreased in proportion to angular head velocity. These data demonstrate that LMN neurons code direction and angular motion of the head in both horizontal and vertical planes and support the hypothesis that the LMN play an important role in processing both egocentric and allocentric spatial information.

1. Recent conceptualizations of the neural systems used during navigation have classified two types of sensory information used by animals: landmark cues and internally based (idiothetic; e.g., vestibular, kinesthetic) sensory cues. Previous studies have identified neurons in the postsubiculum and the anterior thalamic nuclei that discharge as a function of the animal's head direction in the horizontal plane. The present study was designed to determine how animals use head direction (HD) cells for spatial orientation and the types of sensory cues involved. 2. HD cell activity was monitored in the postsubiculum and anterior thalamic nucleus of rats in a dual-chamber apparatus in an experiment that consisted of two phases. In the first phase, HD cell activity was monitored as an animal moved from a familiar environment to a novel environment. It was hypothesized that if HD cells were capable of using idiothetic sensory information, then the direction of maximal discharge should remain relatively unchanged as the animal moved into an environment where it was unfamiliar with the landmark cues. In the second phase, HD cells were monitored under conditions in which a conflict situation was introduced between the established landmark cues and the animal's internally generated sensory cues. 3. HD cells were initially recorded in a cylinder containing a single orientation cue (familiar environment). A door was then opened, and the rat entered a U-shaped passageway leading to a rectangular chamber containing a different prominent cue (novel environment). For most HD cells, the preferred direction remained relatively constant between the cylinder and passageway/rectangle, although many cells showed a small (6-30 degrees) shift in their preferred direction in the novel environment. This directional shift was maintained across different episodes in the passageway/rectangle. 4. Before the next session, the orientation cue in the cylinder was rotated 90 degrees, and the animal returned to the cylinder. The cell's preferred direction usually shifted between 45 and 90 degrees in the same direction. 5. The rat was then permitted to walk back through the passageway into the now-familiar rectangle. Immediately upon entering the passageway, the preferred direction returned to its original (prerotation) orientation and remained at this value while the rat was in the rectangle. When the rat was allowed to walk back into the cylinder, one of three outcomes occurred: 1) the cell's preferred direction shifted, such that it remained linked to the cylinder's rotated cue card; 2) the cell's preferred direction remained unchanged from its orientation in the rectangle; or 3) the cell's preferred direction shifted to a new value that lay between the preferred directions for the rotated cylinder condition and rectangle. 6. There was little change in the HD cell's background firing rate, peak firing rate, or directional firing range for both the novel and cue-conflict situations. 7. Simultaneous recordings from multiple cells in different session...