Figure 7.

Blank-subtracted, but not full, hemodynamic signal is a good proxy for local spiking responses. (a) Mean stimulus-evoked (that is, blank subtracted) hemodynamic response H_STIM averaged across contrasts for one experimental session. Inset, corresponding HRF_STIM (n = 162 trials, roughly 41 per contrast, mean R² = 0.93). The trial structure is indicated by the color bars (gray, fixate; red, stimulus; no bar, relax). (b) Mean stimulus-evoked spiking. Thin black line, mean stimulus-evoked (blank subtracted) measured spiking S_STIM. Thick black line, low pass–filtered mean stimulus-evoked spiking S_STIM. Red, spiking estimated by deconvolving H_STIM with HRF_STIM. Note the good match with measured S_STIM (R² = 0.82 with low pass–filtered S_STIM, R² = 0.64 without low-pass filtering). (c) Mean full hemodynamics H: same data as in a, but without blank subtraction. Inset, corresponding HRF_NULL. Note the poor mean R² = 0.37 compared with HRF_STIM. (d) Mean full spiking without blank subtraction. Thin black line, mean measured spiking S. Thick black line, low pass–filtered mean measured spiking S. Orange, spiking estimated by deconvolving H with HRF_NULL (R² = −0.53). Red, spiking estimated by deconvolving H with HRF_STIM (R² = −1.18; session mean spike rate added to deconvolved signals to align with measured spikes on vertical axis). Note the poor match of either estimate with measured full (low pass–filtered) spiking S. (e) Comparing R² for estimates of spiking from full and stimulus-evoked hemodynamic signals over the population. Orange, H_STIM deconvolved with HRF_STIM and H deconvolved with HRF_NULL. Red, both H_STIM and H deconvolved using HRF_STIM. Note the uniformly high R² for estimating S_STIM from H_STIM (mean R² (s.e.m.) = 0.85 (0.02), n = 34) versus low R², including many large negative values for estimating S from full H (using HRF_STIM, mean R² = −2.5 (0.9); using HRF_NULL, mean R² = −2.5 (1.0); n = 34).