Table 2.

Categorization of units based on firing properties and field locations

Firing location	Number of units
Delay zone	45
Stem	87
Return arms	35
Reward arms	17
Pedestal/ramp	58
Sparse firing	87
Interneurons	11

Sparse firing neurons emitted <100 spikes in a session. Pyramidal cells were distinguished from interneurons based on the shape of the interspike interval histogram, waveform shape, and firing rate (see Materials and Methods).