Figure 2.

Representational geometries visualized by arranging the stimuli in two dimensions. An intuitive way of jointly visualizing the many pairwise high-dimensional distances that characterize a representation is to arrange the stimuli in two dimensions. This figure shows the stimuli presented in four studies (panels A–D) arranged in 2D such that stimuli eliciting similar response patterns are placed together and stimuli eliciting different response patterns are placed apart. (A) Hegdé and Van Essen investigated early visual neuronal population representations of grayscale symbols and patterns [32]. The stimuli are colored here according to the three clusters they formed in V2. Reproduced, with permission, from [32]. (B) Brouwer and Heeger found that the representation of colors (as shown) reflects perceptual color space (connection lines) in V4, but not in V1, despite high within-color clustering in V1, indicating color decodability [38]. Analyses were based on fMRI response patterns. Reproduced, with permission, from [38]. (C) Op de Beeck et al. studied the representation of parameterized shapes in monkey inferior temporal (IT) neurons and its reflection in the animals’ behavioral judgments [13]. Shape parameters were smoothly reflected in both the IT representation (gray) and behavioral judgments (black), whose independently performed multidimensional scaling (MDS) arrangements are superimposed here for comparison. Reproduced, with permission, from [13]. (D) Edelman et al. investigated the representation of shaded renderings of 3D models of animals and vehicles in human visual cortex with fMRI [11]. Similarity judgments and fMRI activity patterns in lateral occipital (LO) cortex reflected categorical divisions. Reproduced, with permission, from [11]. (E) Kriegeskorte et al. examined the representation of face and house images along stages of the ventral stream 15, 63. The fMRI patterns from early visual cortex (EVC) significantly discriminated (red lines) all physically dissimilar images; the fusiform face area discriminated the two categories; and an anterior IT (aIT) face-identity region discriminated the physically similar individual faces. Reproduced, with permission, from 15, 63. The arrangements in all panels were computed from the response pattern dissimilarities by MDS, except for (B), where the space spanned by the first two principal components is shown.