Abstract A visual stimulus flashed sequentially in neighbouring locations elicits a vivid perception of apparent motion. This perception is correlated with the activity of motion selective neurons in area MT that receive direct inputs from area V1. In man, speed discrimination of apparent motion sequences shows that a Gabor patch collinear to the motion path appears much faster than a Gabor patch at an angle to it at high (>40°/s), but not at low (<12°/s), speed (Lorenceau & al., 1998). We propose that this perceptual bias reflects temporal constraints imposed by horizontal connectivity dynamics in V1. A model, based on the finding that intracortical activity propagates slowly through long-range horizontal connections (Bringuier et al, 1999), predicts that, at high speeds, the phase advance in the evoked firing of V1 neurons resulting from optimized synaptic summation biases the read-out of the spatiotemporal correlation performed by MT-like speed selective cells. To test this prediction, intracellular recordings were performed in cat area 17 with apparent motion sequences of co-aligned or parallel Gabor stimuli, sequentially flashed from surround to center of the recorded discharge field, and from center to surround. Physical speed was adjusted to explore different phase lags (+/- 100 ms) between the feedforward (central stimulus alone) and the horizontal wave (peripheral stimuli alone). Decreased response latency and increased response amplitude to the central test stimulus was observed when 1) the two synaptic waves were in phase, 2) the motion axis was the orientation axis of the recorded cell, 3) the stimuli were collinear to the motion axis, 4) the sequence proceeded from surround to center, and 5) the contrast of the center stimulus was low. These data confirm the prediction of the model and suggest that modulation of firing latencies evoked by apparent motion sequences in V1 cells constitutes the synaptic basis of the observed perceptual speed bias.