Figure 4. Sensory encoding and neurometric-psychometric comparisons across cortical regions. A, Distribution of single unit target and distractor d-prime values for all S1 (top, blue, n = 445 units), wMC (middle, green, n = 424 units), and ALM (bottom, yellow, n = 315 units) units. The average behavioral detection performance (behavioral d-prime) during these recording sessions is depicted by the red dashed lines (S1 = 2.5, wMC = 2.9 and ALM = 2.2). Note that S1 and wMC target d-prime values are highly positively skewed along the x-axis (target detection) but ALM units are not. B, Behavioral and neural d-prime measures across regions. Lines connecting columns within each set denote differences of statistical significance. Set 1, psychometric d-prime across all regions. Set 2, neurometric d-prime averaged across all single units within each region. Set 3, neurometric d-prime of summed spiking within each session averaged across all sessions. Set 4, neurometric d-prime of summed spiking of all units within each region. Combining units results in neurometric performance surpassing psychometric performance for S1 and wMC, but not ALM. C, Randomly selected units were added sequentially to determine the resulting d-prime values of pooled neuronal activity. Shown are the distributions from 300 iterations of each region. Increasing the number of combined units increased d-prime values, with the fastest rate of rise in wMC. D, Transformation of data in panel C, depicting the size of the neural pools achieving the corresponding d-prime values. Red arrows overlaying S1 and wMC data indicate the number of units needed to match behavioral performance. Fewer wMC units were required to match behavioral performance compared with S1 and ALM. The traces and shades in panels C, D are the mean ± SD.