These data illustrate the local cortical ensemble response to a suprathreshold (60 dB), 100 ms duration tone at the penetration site's preferred frequency. Response onset consists of an initial current sink with a robust concomitant increase in MUA in Layer 4, followed by subsequent CSD responses accompanied by less marked MUA in the supra and infragranular layers. The form of the excitatory response, initial transient with a lesser sustained component is one of the common variant tone responses observed in A1 [e.g., (O'Connell et al., 2011)]. The initial activation of Layer 4 is reflected in an LFP negativity that arises in association with the collocated current sink (“1” in Fig. 1 B, C), and with a current sink that begins slightly later in Layer 3 (“2” in Fig. 1 B, C). It is sometimes possible, as in this case, to discern an earlier negativity that arises in association with a sink/source configuration and a brief MUA burst below layer 4 (“-1” in Fig. 1 C). Modeling and physiology experiments suggest that the initial transient responses in primary sensory cortices are
