Biomedical subjects

P A Hetherington

Publications and source records attributed to P A Hetherington.

5 recordsLinked to original sources

PubMed1997

Hippocampal place fields are altered by the removal of single visual cues in a distance-dependent manner.

Hippocampal CA3 cells were recorded in male Long-Evans rats that explored a square recording chamber. Three of the 4 chamber walls held a rectangular cue card, each of different size. Rotating the set of cue cards rotated the location of the place fields. Place fields were common close to the walls of the recording chamber, particularly the walls with cues. When single cues were removed, the spatial information content decreased but returned to baseline levels when the cue was replaced. When a cue near a place field was removed, the place field firing rate and area decreased; when a distant cue was removed, firing rate and area increased. Thus, removing single visual cues predictably and reversibly altered hippocampal place fields. Together, the results suggest that hippocampal neurons may optimize the encoding of visual information and are consistent with a distance-encoding hypothesis of CA3 network function.

Animals↗

PubMed1994

Ipsilateral associational pathway in the dentate gyrus: an excitatory feedback system that supports N-methyl-D-aspartate-dependent long-term potentiation.

Axons from granule cells in the dentate gyrus of the rat hippocampus project to cells in the hilar region, including mossy cells, which project along the longitudinal axis of the hippocampus and synapse in the inner (proximal) one-third of the molecular layer of the dentate gyrus. To study this feedback system, multiple recording electrodes were located along the longitudinal (septo-temporal) axis in the dorsal leaf of the dentate gyrus in urethane-anesthetized rats. Single pulse electrical stimuli delivered to the hilar region evoked negative-going, monosynaptic field potentials that were largest in the inner one-third of the molecular layer (commissural zone). These evoked field potentials (EFPs) were recorded simultaneously at three to five locations. The latency to onset and peak amplitude of the EFP varied linearly with distance from point of stimulation, and EFPs were elicited in both directions along the longitudinal axis. The transmission speed was estimated to be 1.4 m/s. Tetanic stimulation of the hilar region potentiated the EFP slopes (mean = 26%). Potentiation lasted at least 2 hours and was specific to responses from the tetanized stimulating electrode; the responses to other stimulating electrodes in the hilus and the angular bundle of the perforant path changed less than 4%. Combined stimulation of the hilus and the medial perforant path increased the magnitude of recorded field potentials and population spikes, demonstrating that both pathways are excitatory. NMDA antagonist NPC-17742 blocked potentiation of EFP slopes in both the medial perforant path and hilus pathways. The results suggest that the ipsilateral associational system of the dentate gyrus is excitatory and capable of supporting long-lasting NMDA-dependent, synapse-specific plasticity.

Afferent Pathways↗

PubMed1993

A simple network model simulates hippocampal place fields: parametric analyses and physiological predictions.

Hippocampal place cells may be the computational units of a neuronal cognitive mapping system. A network model trained to compute locations from distal cues simulated the defining properties of hippocampal place cells (i.e., place-specific activation). The model produced units with detailed properties of place cells, including multiple subfields, "silent" and "noisy" cells, fields that persisted after cue removal, and groups of simulated fields that overlapped in multiple clusters. Quantitative variants of the model showed that different properties of the fields were influenced by the complexity of the visual input (the number of spatial cues), the available computational resources (the number of hidden units), and the output encoding used to represent location. The simulations provide a framework for testing relationships between place field properties, variations in spatial environments, and the integrity of the hippocampal system.

Animals↗

PubMed1993

A simple network model simulates hippocampal place fields: II. Computing goal-directed trajectories and memory fields.

Place cells have been described as the computational elements of a neuronal cognitive mapping system that encodes and stores relationships among spatial stimuli (O'Keefe & Nadel, 1978). Furthermore, place cells seem to encode remembered locations because neural activity is maintained when the visual stimuli that influence place field location are vastly degraded, such as when cues are removed or the lights are turned off (O'Keefe & Speakman, 1987; Quirk, Muller, & Kubie, 1990). A feed-forward network model that mapped visual input onto a representation of location simulated some basic properties of hippocampal place fields, including resistance to disruption after partial cue removal (Shapiro & Hetherington, 1993). However, the stimulated place fields required visual input for their activation. We now report that a network that incorporates feedback (a) computed correct trajectories toward simulated goals and (b) simulated place fields that persist in the absence of visual input. The simulation suggests that feedback properties can provide a computational account of O'Keefe and Speakman's data.

Animals↗

PubMedDate not supplied

Receptive field and orientation scatter studied by tetrode recordings in cat area 17.

The receptive-field positions and orientation preferences of neurons occupying the same tangential location in visual cortex are thought to be similar but to have an associated random scatter. However, previous estimates of this scatter may have been inflated by the use of subjective plotting methods, sequential recording of single units, and residual eye movements. Here we report measurements of receptive-field position and orientation scatter in cat area 17 made with tetrodes, which were able to simultaneously isolate and record up to 11 nearby neurons (ensembles). We studied 355 units at 72 sites with moving light and dark bars. Receptive-field sizes and positions were estimated by least-squares fitting of Gaussians to response profiles. We found that receptive-field position scatter was about half of the ensemble average receptive-field size. We confirmed previous estimates of orientation scatter, but calculations suggested that much of it may be accounted for by anatomical scatter in the positions of recorded neurons relative to the tetrode in a smooth map. Orientation tuning width was positively correlated with the degree of orientation scatter. Scatter was not independent in the two eyes: deviations from the local mean for both preferred orientation and receptive-field position were correlated although a significant amount of residual inter-ocular orientation and receptive-field position scatter was present. We conclude that cortical maps of orientation and receptive-field position are more ordered than was previously thought, and that random scatter in receptive-field positions makes a relatively small contribution to cortical point image size.

Animals↗