PMC

(A–D) Examples of responses from four different neurons. Some responded to all trials containing facial features (A), whereas others increased their firing rates only to whole faces (B–D). Each of the units is from a different recording session, and for each the raster (top) and the poststimulus time histogram (bottom) are shown with color coding as indicated. Trials are aligned to scramble onset (light gray, on average 1 s, variable duration) and face stimulus onset (dark gray, fixed 500 ms duration). Trials within each stimulus category are sorted according to reaction time (magenta line).
(E) Waveforms for each unit shown in (A)–(D). shows the rasters for the units shown in (C) and (D).

(A) Mean response of all whole-face (WF)-selective units that increased their spike rate for WFs compared to bubble trials (n = 32 units, ±SEM, normalized to average response to WFs for each unit separately). Asterisk indicates a significant difference between the response to WFs and bubble trials (p < 0.05, two-tailed t test, Bonferroni-corrected for 14 comparisons). The response to eye and mouth regions of interest (ROIs) that was used for selecting the units is shown but not used for statistics.
(B) Histogram of firing rates before and after scramble onset as well as for WFs (n = 32 units). Mean rates were 1.4 ± 0.24 Hz, 1.7 ± 0.3 Hz, and 2.2 ± 0.4 Hz, respectively.
(C) Histogram of the whole-face index (WFI) for all recorded units (n = 185), according to whether the unit was classified as a non-WF-selective (top) or WF-selective (bottom) unit. The WFI was calculated as the baseline-normalized difference in response to whole faces compared to bubbles (which was independent of how we classified units as WF selective or non-WF selective).
(D) Distributions (plotted as cumulative distributions) of the WFI across the entire population for both WF- and non-WF-selective units (n = 36 and n = 149, respectively), calculated for both ROI trials (bold lines) and bubble trials (dashed lines). The two WFI populations for bubble trials were significantly different (p < 1e-9, two-tailed Kolmogorov-Smirnov test). Note the similarity of the distributions for cutouts (bold lines) and bubble trials (dashed), indicating that the response to both is very similar.

(A) Timeline of stimulus presentation (top). Immediately preceding the target image, a scrambled version was presented for a variable time between 0.8 and 1.2 s. Next, the target image was presented for 500 ms and showed either a fearful (50%) or happy (50%) expression. Subjects indicated whether the presented face was happy or fearful by a button press. The target presentation was followed by a variable delay. Target images and associated color code used to identify trial types in later figures are shown (bottom).
(B and C) Behavioral performances from the patients.
(B) Learning curve (top) and reaction time (bottom) (n = 11 and 12 sessions, respectively, mean ± standard error of the mean [SEM]). The inset shows example stimuli for 20, 30, and 40 bubbles revealed. Patients completed on average a total of 421 bubble trials, and the average number of bubbles required ranged from 100 at the beginning to 19.4 ± 7.9 on the last trial (n = 11 sessions, ±standard deviation [SD]; one session omitted here because the learning algorithm was disabled as a control, see results). The average reaction time was fastest for whole faces and significantly faster for whole faces than bubble trials (897 ± 32 ms versus 1,072 ± 67 ms, p < 0.05, n = 12 sessions, relative to stimulus onset).
(C) Behavioral classification image (n = 12 sessions). Color code is the z scored correlation between the presence or absence of a particular region of the face and behavioral performance: the eye and mouth regions conveyed the most information, as described previously []. See for further analyses of behavioral performance.

The response to partially revealed faces did not predict the response to whole faces.
(A) Example stimuli for 60% and 80% of the ROIs revealed.
(B1–B7) Example neurons. Normalized response as a function of how much of the eye and mouth ROIs were revealed (B1–B6 increase rate for WFs, B7 decreases) is shown. Only data points to which at least ten trials contributed are shown (bin width is 15%, steps of 5%). Most neurons showed a nonlinear response profile when comparing bubbles and WFs: those that decreased their response with more bubbles increased it to WFs and conversely. The slope of the regression of percentage of ROI on spike rate was significant for units B1, B5, and B6. Different portions of the x axis are plotted for different neurons, because the patients were shown different densities of bubbles contingent on their different performance accuracies (unit B4 is from the patient who was shown a very high density of bubbles for comparison; cf. for further details).
(C) Population responses of all WF-selective neurons that increase their response to WFs (n = 32 units, 5,686 trials). The slope was −0.18 and significantly negative (linear regression, p < 1e-11) and the last and first data points are significantly different (two-sided t test, p < 1e-5). We verified the statistical significance of the slope across the units that contributed to this population response using a bootstrap statistic over trials and units (right side, average slope −0.18 ± 0.05 and −0.17 ± 0.13, respectively; red line indicates observed value). For the first half (5%–40%) the slope was −0.07 (p = 0.27, not significant [NS]), and for the second half (45%–90%) it was −0.44 (p < 1e-8).
(D) Population response for non-WF-selective units (n = 149, 15,922 trials). The slope was not significantly different from zero for the curve shown (p = 0.62) and across the population over trials and units (bootstrap statistic; right side, average slope 0.01 ±0.04 and 0.01 ±0.07, respectively). The last and first data points were not significantly different.
(E) Response for WF-selective (red) and non-WF-selective units (blue) is not a function of contrast threshold (the contrast threshold is a model-derived index of the visibility of the stimuli). The response of the control trials for eye and mouth cutouts used for selecting the units are shown in (C) and (D) for comparison. Errors are ±SEM over trials. See also .