Perceptual Gloss Parameters Are Encoded by Population Responses in the Monkey Inferior Temporal Cortex

Responses of an example neuron in perceptual gloss space (c/d space). A, Responses of an example neuron (Cell 1) to the 16 stimuli also illustrated in Figure 1B. Responses are depicted as raster plots and spike density functions (σ = 10 ms). A horizontal bar under the spike density function indicates the stimulus presentation period. B, Bubble plot showing the response magnitude of Cell 1 to each stimulus as the diameter of a circle. Responses to stimuli with high ρd (white), middle ρd (gray), and low ρd (black) are shown in the left, middle, and right panels, respectively. This neuron strongly responded to stimuli with large d value regardless of the c or ρd values.

Responses of example neurons with different selectivities. Rows show responses of Cell 2 (top), Cell 3 (middle), and Cell 4 (bottom); columns indicate, from left to right, responses to stimuli with high ρd (white), middle ρd (gray) and low ρd (black). Responses of Cell 2 changed along the d-axis, and this neuron preferred stimuli with large d value. Responses of Cell 3 changed along the c-axis, and this neuron preferred stimuli with large c values. Responses of Cell 4 changed in a direction intermediate between the c- and d-axes. Responses of all these neurons depended on the ρd value.

Multiple regression analysis of responses of individual neurons. A, Three-dimensional plots showing the results of multiple regression analyses for two example neurons depicted in Figure 3 (Cells 3 and 4). The actual neural responses to a stimulus set (black circles) and responses estimated based on the multiple regression analysis (asterisks on the gray plane) are shown in the three-dimensional space defined by axes representing c, d, and neural response. Results obtained with the optimal ρd are shown. The correlation coefficients between the actual and estimated responses are shown in the insets. B, Distribution of correlation coefficients between the actual and estimated responses. The horizontal axis indicates the correlation coefficient; the vertical axis indicates number of cells. Filled and open bars indicate significant and non-significant fits by the multiple regression analysis (F test, p < 0.05). C, Tuning direction in c/d space of each neuron significantly fit in the multiple regression analysis. Tuning direction was defined as the direction of the maximum slope of the c/d regression plane in the multiple regression analysis after the relative weighting between the c- and d-axes was adjusted (see Materials and Methods): if the responses increased (decreased) along the c axis, tuning direction becomes 0° (180)°, and if the responses increased (decreased) along the d axis, tuning directions becomes 90° (270°). Arrows corresponding to Cells 1–4 in Figures 2 and 3 are indicated.

Regression analysis of gloss parameters using the population activities of gloss-selective neurons. A, Relationship between the actual and estimated values of each physical parameter. The horizontal axis indicates actual values; the vertical axis indicates values estimated through regression from population responses. The black line shows the linear regression. The correlation coefficients between the actual and estimated values are shown in the insets. The top, middle, and bottom panels are for the parameters ρs, d = 1 − α and ρd, respectively. B, Relationship between the actual and estimated values of perceptual parameters of gloss. Conventions are the same as in A. The top, middle, and bottom panels are for the parameters c, d′, and , respectively. C, Three-dimensional plot showing the results of a regression analysis of three physical parameters of gloss (ρd, ρs, and d = 1 − α). Actual parameter values (blue) and values estimated through regression (red) are plotted in the three-dimensional space defined by axes representing the physical parameters. D, Three-dimensional plot showing the results of a regression analysis of three perceptual parameters of gloss (, c, and d′). Conventions are the same as in C. To aid visualization, color contours were traced by connecting the points corresponding to the minimum and maximum values of c and d in each stimulus set.