Extended Data Fig. 5: Criteria for identifying monosynaptically connected thalamocortical pairs.

Thalamocortical pairs were identified on the basis of two criteria: Criterion 1 (illustrated in c) sets a threshold for the thalamic unit spike-triggered average of the time derivative of the current recorded in L4 cortical neurons. Criterion 2 (illustrated in d) sets a threshold and a time window for the distribution of events detected in the time derivative of the L4 current around the time of the spike in the thalamic unit. Both criteria have to be satisfied for the thalamic unit and the L4 cortical neuron to be considered as a connected pair. a, Isolation of thalamic units in the dLGN. Left, first two principal components illustrating three separable clusters attributed to three independent thalamic units (units x, y and w in red, grey and blue, respectively). Right, electrophysiological recording illustrating the average spike shape recorded from seven electrodes for the three thalamic units. b, Differentiation of the current recorded in L4 neurons, the same experiment as in a. Top trace, the current recorded in the whole-cell configuration from an L4 cortical neuron (V_holding = −70 mV) during the presentation of a drifting grating (single trial). Middle trace, the temporal derivative of the above current (dI/dt). Lower, the times at which each one of the three thalamic units from a (x, red; y, grey; w, blue) fired during the same trial. c, Criterion 1. Left, spike triggered average of dI/dt of the current recorded in the L4 neuron for the three thalamic units illustrated in a. Time 0 denotes the time of the spike. Right, same spike-triggered averages shown on the left after high-pass filtering and z-scoring (see Methods). Note that only unit w (blue) crosses the 5z threshold. d, Criterion 2. Top left, seven individual time derivatives of currents (dI/dt) recorded in the L4 neuron (same as in b) aligned relative to seven spikes recorded in unit w (time 0 denotes the time of the spike). Each asterisk shows an event crossing the threshold of −36 pA ms⁻¹. Top right, same as left but represented as a heat map of the amplitude of dI/dt for 761 traces (the heat map colour scale ranges from +50 pA ms⁻¹ to −200 pA ms⁻¹). This heat map clearly illustrates an increase in event probability around 2 ms after the spike in unit w. Bottom left, the PSpTH for the events detected in the 761 traces illustrated above. The peak of the PSpTH is used to determine the latency (that is, the time interval between the spike recorded in the thalamic unit and the occurrence of a postsynaptic response detected in the L4 cortical neuron). The half width at half maximum is used to determine the jitter of that response (in this example the latency is 2 ms and the jitter is 188 microseconds). Bottom right, same as left but z-scored. The PSpTH must cross a threshold of 3.5z within 1–4 ms after the spike in the thalamic unit for the thalamic unit to be considered as synaptically connected to the L4 neuron. e, Left, unit w spike-triggered average of the response recorded in the same L4 neuron as in a. The continuous blue line represents unshuffled trials, the dotted line represents shuffled trials (see Methods). Right, the difference between the shuffled and unshuffled trials is used to isolate the uEPSC between unit w and the recorded L4 neuron.