Figure 1.
Correlation analysis reveals that responses in many visual cortical areas are insensitive to time reversals: illustration from area MT+. A, Representative frames from the silent films (taken from City Lights). Observers viewed two presentations of each original movie (forward) and each time-reversed movie (backward). B, Average time courses (8 observers) sampled from MT+ for the two forward presentations (F1, F2) of the silent films. C, The average time courses in MT+ for the two backward presentations (B1, B2). D, Superimposed traces of the time-reversed version of the backward time course (rB) and the forward time course (F). Both time courses were shifted (Δt = 5 s) to correct for hemodynamic delay in the fMRI responses. E, Cross-correlations: forward 1 versus forward 2 (CF1:F2, black); backward 1 versus backward 2 (CB1:B2, red); forward versus backward (CF:B, blue); reversed backward versus forward (CrB:F, green). The correlation was computed at different lags (2 s intervals); the peak at lag 0 indicates reproducible responses that were time-locked to the movies. CF:B, which is an estimate of the arbitrary correlation values that can be expected from such complex stimuli, is much lower than the other three cross-correlations. Error bars represent the SEM across nonoverlapping movie segments.