Fig 2.

Synaptic responses evoked under control conditions. (a) Representative examples of responses after a single light pulse (473 nm, black dashed line) used to activate ChR2-expressing axon terminals of four afferent pathways to cortex (left to right): TC core, TC matrix, CC feedforward, and CC feedback. CSD colour plots are shown, where current sinks (inward-going transmembrane current) are blue (scale: –2.0 to 2.0 μA mm⁻³ for TC matrix and CC feedforward, –1.0 to 1.0 μA mm⁻³ for TC core and CC feedback) and layer boundaries are shown with horizontal dotted lines. (b) CSD signals evoked as in (a), but with a train of four light pulses (blue tick marks) delivered at 10 Hz. Signals shown were isolated from the channel exhibiting the earliest current sink in (a). Horizontal scale bars: 100 ms. Vertical scale bar represents 1.0 μA mm⁻³ for TC matrix and CC feedforward traces, and 2.0 μA mm⁻³ for TC core and CC feedback traces. (c) Whole-cell current-clamp recordings from four cortical pyramidal cells demonstrating intracellular analogues of CSDs shown in (b) (not necessarily from same experiment); excitatory postsynaptic potentials were evoked by a train of four light pulses at 10 Hz. Horizontal scale bar: 100 ms. Vertical scale bar represents 0.5 mV for TC matrix, TC core, and CC feedforward traces, and 2.0 mV for CC feedback traces. TC, thalamo-cortical; CC, cortico-cortical; CSD, current source density.