Perirhinal cortex ablation impairs visual object identification

Abstract

Impairments in both recognition memory and concurrent discrimination learning have been shown to follow perirhinal cortex ablation in the monkey. The pattern of these impairments is consistent with the hypothesis that the perirhinal cortex has a role in the visual identification of objects. In this study we compared the performance of a group of three cynomolgus monkeys with bilateral perirhinal cortex ablation with that of a group of three normal controls in two tasks designed to test this hypothesis more directly. In experiment 1 the subjects relearned a set of 40 familiar concurrent discrimination problems; the stimuli in each trial were digitized images of real objects presented in one of three different views. After attaining criterion they were tested on the same problems using similar, but previously unseen, views of the objects. In experiment 2 the subjects were tested on their ability to perform 10 of these familiar discriminations with each problem presented in the unfamiliar context of a digitized image of a unique complex scene. The subjects with ablations were significantly impaired on both tasks. These results demonstrate that the role of the perirhinal cortex is not restricted to memory, and they support the hypothesis that the perirhinal cortex is involved in visual object identification. We suggest that the perirhinal cortex is crucially involved in processing coherent concepts of individual objects. A deficit of this nature could underlie the pattern of impairments that follow perirhinal cortex damage in both visual object recognition memory and visual associative memory.

Fig. 1.

Five 50 μm sections of the brain are shown for each animal in the perirhinal groups (PRh1, PRh2, PRh3). From top to bottom the sections are spaced 4 mm apart running anterior to posterior through the area of the bilateral perirhinal cortex ablation. L, Lateral sulcus; S, superior temporal sulcus; A, anterior middle temporal sulcus;P, posterior middle temporal sulcus; O, occipital sulcus.

Fig. 2.

A, Shaded regionsshow the intended location and extent of the ablation of the perirhinal cortex on a schematic diagram of the ventral view of the brain with sulci and gyri in labeled regions; B, shaded regions show the extent of the actual perirhinal cortex lesions in two representative subjects (PRh1,PRh3).

Fig. 3.

Examples of how three different discrimination problems from experiment 1 appear to subjects (reproduced in gray scale) when stimuli are presented as digitized images of objects on the touchscreen. For each of three different problems the figure illustrates how the appearance of the objects changes with presentation of the objects in different views.

Fig. 4.

Examples of how three different discrimination problems from experiment 2 appear to subjects (reproduced in gray scale) when stimuli are presented in the context of digitized images of unique complex scenes. For each of three different problems the figure illustrates how the scene appears with the positions of the S+ and S− reversed.

Fig. 5.

Mean errors to criterion made by each group in stage B of experiment 1 in which the 40 concurrent discrimination learning problems that were learned to criterion in stage A were retested using new views of each of the objects in stage B. Individual scores for each subject are also plotted (triangles).CON, n = 3; PRh,n = 3.

Fig. 6.

Mean total errors to criterion made by each group in experiment 2 in which 10 well learned concurrent discrimination problems were retested with each of the 10 pairs of objects presented in a context not experienced before, that is, with each pair of objects embedded within a digitized image of an unique scene. Individual scores of each subject are also plotted (triangles).CON, n = 3; PRh,n = 3.