Table 1.

Summary of experimental groups

Group	Experiment	Receptive field size	Map expansion	Number of rats (Al sites)	Number of Al pairs
1	15 pps 9 kHz tone train + NB	↑	↑	4(244)	112
2	15 pps trains of multiple tone frequencies + NB	↑	–	6(223)	89
3	Two tone frequencies + NB	↓	–	6(209)	52
4	Multiple tone frequencies + NB	↓	–	6 (236)	105
5	Acoustically enriched environment	–	–	8(397)	159
6	Controls	–	–	18(692)	250
			Totals	482001	767

The first four groups heard tones paired with electrical activation of Nucleus Basalis (NB) three hundred times per day for four weeks (Kilgard et al., 2001). The fifth group was housed in an enriched environment described in (Engineer et al., 2004). Control rats were experimentally naïve and housed in standard laboratory conditions. Plasticity of frequency tuning (bandwidth and characteristic frequency) reported in previous studies (Kilgard and Merzenich, 1998; Kilgard et al., 2001; Engineer et al., 2004) is indicated for each group. Cortical synchrony was measured from multi-unit data recorded simultaneously from pairs of microelectrode penetrations into primary auditory cortex.