Figure 1.

Behavior methods and neuron classification. (A) The 3-choice, 2-cue behavioral apparatus used in the present experiments. At the ends of the 3 platform arms are speakers and LED lights, one of which signals the reward location on a given session. Rats shuttled between the central cue zone, where cue presentation was initiated, and feeder zones, where liquid food was delivered in the case of a correct choice, and a nonlocalized error sound was presented in the case of an incorrect choice. (B) Distribution of half-amplitude (depolarization) width and half-valley (after-hyperpolarization) width of each neuron's average waveform. Neurons that had inhibitory cross-correlation profiles with other neurons (black dots) tended to cluster into a group of cells with shorter after-hyperpolarization times. All neurons in this grouping were classified as putative inhibitory cells (example in Supplementary Fig. 1, neuron A). Those with wider waveforms were classified as putative excitatory projection cells (Supplementary Fig. 1, neurons B, C, and D). Insets are example of average waveforms for putative projection and inhibitory neurons. (C) Distribution across the neuron population of 2 features calculated from each neuron's 500-ms autocorrelation: Time that the autocorrelation reached a peak, and the magnitude of the downward slope. Autocorrelations include spikes from both rest and task session epochs. Black dots represent those neurons that were determined to be inhibitory from the cross-correlations (as in B). Visual inspection revealed roughly 3 groupings, which were classified as burst firing (early peaks but rapid decay; green), regular firing (later peaks and often slower decay; orange), and fast spiking (moderately early peaks but slow decay; blue). Inset shows average autocorrelations of all cells within each group. (D) The reconstructed electrode positions of all recorded neurons overlaying coronal (left), and sagittal (right) illustrations from Paxinos and Watson (1998; plates are 3.4 mm anterior to bregma and 0.9 mm lateral, respectively). Neuron recording sites that appear to be beyond the boundaries of the medial prefrontal cortex can be explained by the limitations of projecting 3D positions onto single sections. (E) Frequency of recorded neurons along medial–lateral and dorsal–ventral axes. Each panel contains a frontal coronal plate from Paxinos and Watson, overlaid with a color map describing the number of neurons recorded relative to the number of sessions an electrode occupied the position. Hotter colors indicate a higher proportion of neurons recorded at a given location. The increased frequency of burst-firing neurons (left) at a particular medial–lateral position (bottom panel) is consistent with observations that burst-firing neurons are more frequently observed in layer IV/upper-layer V (see Discussion). Regular-firing (middle) and fast-spiking (right) neurons were observed throughout medial–lateral and dorsal–ventral positions. The 267 neurons that did not fall within the boundary classifications were distributed throughout these regions. (F) Autocorrelation features as in C, but generated from spikes during the task epoch only. Many fast-spiking neurons formed a tight cluster with peaks around 20 ms. Average autocorrelations of neurons in this group exhibited a 40- to 70-Hz (gamma-frequency) rhythm (inset).