Fig. 2
From: Neuronal allocation and sparse coding of episodic memories in the human hippocampus
Empirical quantile-quantile (Q–Q) plots of the normalized spike count distributions at retrieval partitioned by relative excitability at encoding. The target-by-neuron response distribution (y-axis) vs. the foil-by-neuron response distributions (x-axis) was compared for items categorized by their pattern of spiking activity at encoding for the hippocampus (a) and the amygdala (b). As in Fig. 1, the distributions consisted of normalized spike counts at retrieval from each neuron in response to each trial across all patients and sessions. Targets were divided into distinct subsets based on their relative excitability during encoding (spiking activity directly before and during stimulus presentation): Low–High,  High–High, High–Low, and Low–Low. For each category, the foil-by-neuron response distribution (x-axis) was compared to the corresponding target-by-neuron response distribution in the hippocampus and amygdala. The predicted sparse coding signal (nonlinear deflection toward the y-axis) was selectively detected in the hippocampus for items at retrieval with a relative increase in excitability at encoding (a: Low-High, 43,577 recordings) and was not present for the amygdala (b: Low-High, 59,661 recordings). The deflection was not present for any other pattern of firing at encoding in the hippocampus and amygdala (see Supplementary Fig. S1). The skewness values between the two distributions (Low-High targets vs. foils) differed significantly between brain regions (hippocampus vs. amygdala), with a significant difference observed in the hippocampus only (B = 10,000, p < 0.0125, Bonferroni-corrected, two-tailed). Refer to Table 2 for detailed statistical reporting. Supplementary Fig. S2 reports the corresponding Q-Q plots with the top 0.25% of both the target-by-neuron and foil-by-neuron distributions removed. The deflection observed in panel (a) disappeared after removing the top 0.25% of data, indicating that relatively few hippocampal neurons fired strongly in response to targets with a relative increase in excitability at encoding compared to foil items. **p < 0.01.
