Anatomical location of the macaque monkey cortical regions discussed in this article. Top row and bottom left: the prefrontal cortex (PFC) and three subregions (dorsolateral, ventrolateral and orbital PFC) referred to in this article. The combination of grey and blue represents the whole of the PFC in each case. Blue represents the subregion in question. The subregions illustrated are those used for lesion boundaries in a number of studies discussed here [36–38], and are meant to be illustrative rather than definitive. As we point out, the PFC can be divided in multiple ways. Bottom right: the red region indicates the location of inferotemporal cortex (IT) in the macaque brain. The role of the interaction between the PFC and the IT is discussed in the latter part of this article. Directional indicators: A, anterior; I, inferior; L, lateral; M, medial; P, posterior; S, superior. Abbreviations: AS, arcuate sulcus; CIN, cingulate sulcus; IOS, inferior occipital sulcus; LOS, lateral orbital sulcus; LS, lateral sulcus; MOS, medial orbital sulcus; OTS, occipitotemporal sulcus; PS, principal sulcus; ROS, rostral sulcus; RS, rhinal sulcus; STS, superior temporal sulcus.

Interaction of prefrontal cortex (PFC) and inferotemporal cortex (IT) is necessary for the processing of temporally complex events. (a) Schematic diagram of the tasks (stimuli not presented to scale). Monkeys performed two-choice concurrent visual object discriminations in both tasks, with a 2 s delay between choice and reward delivery. In task 2I (‘two-item’), this delay was filled by another visual object on the screen, whereas in task UD (‘unfilled delay’) the delay was not associated with any visual object. See Ref. [73] for further information regarding monkeys learning about groups of stimuli. (b) Comparison of performance of monkeys in these visual discrimination tasks following crossed unilateral ablations of the PFC and IT (PFCxIT). Bars represent mean errors to criterion of the group, and letters represent individual monkeys’ scores that have contributed to that mean. The same letter shows the same monkey's scores in the two tasks. Monkeys with PFCxIT were impaired relative to control monkeys (with no ablations) at the 2I task, in which the choice and intervening item had formed a temporally complex event, but were not impaired at the UD task, in which all contingencies were the same as task 2I except for the fact that there was no temporally complex element. It is notable that the control monkeys find the temporally complex task (2I) easier – the sequence element presumably helping to bridge the gap to the reward, something that does not occur in the UD task. It is the loss of this facilitation that seems to cause the impairment in the monkeys with PFC/IT ablations because these monkeys perform as if there were no sequence element. Adapted from Ref. [56].

Performance of the object-in-place scene-learning task [45] following different prefrontal cortex (PFC) lesions in the macaque monkey. (a) Example screenshots of the object-in-place scene problems. The left and right panels show two example scenes. Each scene is composed of a colored background, a random number of colored ellipse segments, and a single large alphanumeric character (e.g., P on the left panel and 8 on the right), which collectively form the background of the scene. In this task the monkey has to learn which of the two small alphanumeric characters (objects) presented within the unique background scene is paired with a reward, and which is not (eg. B versus m in the panel on the left, and I versus s in the right panel). Objects are always in the same place in the scene, and the scene is unique to a given pair of objects. Monkeys learn about multiple problems in a given session, where one problem is a scene containing two objects. The scenes greatly aid learning, and so monkeys can acquire these problems much faster than with a plain background. Initial studies of this and similar tasks revealed that it is the combination of the unique scenes with the objects placed consistently within them that drives this fast learning [45,72]. Further, the effect is common across randomly varying scenes, and so is not dependent on any particular element of a given scene. (b) Comparison of performance of monkeys following subtotal and total ablations of the PFC. The data are presented as a difference score, measured as difference in percent error between pre- and post-operative performance tests. Red lines represent group means, and individual monkey scores are presented as black symbols. Monkeys with selective ablations of dorsolateral, ventrolateral, and orbital PFC all have small difference scores, but the tests do reach significance in the case of the ventrolateral and orbital ablations (see Refs [36,37] for statistical analyses). The key contrast, however, is with monkeys with complete PFC ablation, who are extremely impaired. To test our hypothesis, we performed a one-sample t-test on these complete PFC difference scores compared to the sum of the average deficit scores for all 3 subtotal lesions. In line with our hypothesis, this analysis revealed a significant difference [t(3) = 2.707, P = 0.037 in a one-tailed test]. Thus the complete PFC impairment is significantly greater than the sum of the impairments following the three subtotal ablations. Data taken from Refs [36–38,46].