Detecting changes in scenes: the hippocampus is critical for strength-based perception

Abstract

Recent findings have ignited a controversy over whether the hippocampus is critical for visual perception as well as memory. Some studies have shown that hippocampal damage impairs perception of scenes, but others found no evidence for hippocampal involvement. These studies measured perception as a unitary phenomenon, but recent findings indicate that perceptual discriminations can be based on two kinds of information: states of perceiving local differences or global strength of relational match. In the current study, we separated state- and strength-based perception using a change detection paradigm with scenes. Patients with selective hippocampal damage exhibited significant reductions in strength-based perception but showed spared state-based responses. In a follow-up neuroimaging study, hippocampal activation linearly tracked confidence in strength-based perception, and was not differentially associated with state-based responses. The hippocampus therefore plays a selective role in perception, contributing high-resolution strength information possibly through its role in the representation of relational information.

Figure 1

MRI scans for patients with selective hippocampal damage and a healthy control.

Figure 2

Hippocampal damage impairs perceptual judgments based on strength-based relational match, but not those based on discrete states of identifying local differences. (A) Scenes were presented simultaneously for 1.5 seconds, then replaced by a 1–6 scale for a self-paced confidence judgment. The manipulations consisted of contracting or expanding the scenes, keeping the size of the images the same. This changes the configural/relational information within the scenes without adding or removing any objects. In the example shown here, the windows near the center of the building are closer together in the scene on the left than the scene on the right. (B) Aggregate ROCs suggest an impairment in strength-based perception (reduced curvilinearity) in the patients compared to controls, whereas state-based perception (upper x-intercept) is unaffected. (C) Parameter estimates confirm that patients are selectively impaired in strength-based perception; state-based perception is intact. Data for individual patients are overlaid on the patient average; filled shapes are hippocampal patients, open shapes are patients with left (circle) or right (diamond) MTL damage. State- and strength-based perception are on different scales (probability and d′, respectively). Error bars depict +/− 1 SEM.

Figure 3

The hippocampus continuously tracks the strength of perception. The left posterior hippocampus (A) was more active on correct ‘different’ than correct ‘same’ scene trials (B). Activation tracked the strength of evidence, with increasing activation with increased confidence in difference (C). MNI coordinates for peak voxel : −15, −34, 3. Error bars depict +/− 1 SEM. Also see Figure S2.