Rats’ object-in-place encoding and the effect of fornix transection

Abstract

We studied the perception of simple computer-generated scenes by normal and fornix-transected Dark Agouti rats. In Experiment 1, the rats were rewarded for approaching trial-unique variable scenes differing from a constant scene that was the same across trials (constant-negative paradigm). The groups performed equivalently when scenes differed only in their objects or only in the occupied positions; however, when two scenes shared an object-place combination, the normal rats were more likely to see them as similar than were the fornix-transected rats. In Experiment 2, the rats learned to discriminate pairs of scenes. Again, there was no lesion effect when scenes differed by a single cue, object or position, but when the two scenes comprised the same objects interchanged in position, fornixtransected rats learned relatively easily. Fornix transection reduces rats' sensitivity to object-place combinations within scenes, consistent with D. Gaffan's account of scene memory as an animal analogue of episodic memory deficits in amnesia.

Dissociable Effects of Lesions to the Perirhinal Cortex and the Postrhinal Cortex on Memory for Context and Objects in Rats.

“…The performance of the fornix group on the object-in-context task lay between the good performance of the sham and perirhinal groups and the impaired performance of the postrhinal group. There have been previous reports of impairments in object-in-context tasks in rats following damage within the hippocampal system (Mumby et al, 2002; Simpson, Gaffan, & Eacott, 1998). There is also evidence that context can modify the output of hippocampal place cells (for a review, see Jeffery, Anderson, Hayman & Chakraborty, 2004), which may suggest a mechanism for such an effect.…”

Perirhinal cortex ablation in rats selectively impairs object identification in a simultaneous visual comparison task.

“…The objects in each pool were always from a different shape class than their corresponding constant object (e.g., the leftmost object in the constant scene of is a polygon, so its pool of objects comprised crosses, ellipses, letters, rectangles, or disks, but no polygons). Simpson and Gaffan (1999) showed that rats were especially sensitive to form differences between objects when they were from different shape classes, hence our decision to use different classes for Type O+P and O. Another reason is that the three objects within the constant scene were from different shape classes, therefore using different classes for Type O scenes would produce as much shape change as when constant objects were switched around for Type OP.…”

“…For variable types in which position differed, namely O+P and P, the position of an object in any variable scene differed by one step on the 4 × 4 grid in any direction (vertical, horizontal, diagonal) randomly from that of the same object in the constant scene, subject to the constraint that it remained within the 4 × 4 grid and did not overlap any other object. We chose this amount of position separation, on the basis of the findings of Simpson, Gaffan, and Eacott (1998), so as to be similar in discriminability to the object differences in Types O+P and O. Because there were few possible positions for any object in a variable scene, particular positions were reused across variable scenes in the same session, but specific combinations of three positions rarely recurred across scenes.…”

“…There was no error-correction procedure; the variable scene, and whether it appeared to left or right of the constant, were freshly randomized every trial. For fuller details, see Simpson et al (1998) Experiment 1.…”

Objects and Positions in Visual Scenes: Effects of Perirhinal and Postrhinal Cortex Lesions in the Rat.

“…Gaffan & Eacott, 1997;E. A. Gaffan, Eacott, & Simpson, 2000;Simpson & Gaffan, 1999;Simpson, Gaffan, & Eacott, 1998). We use them to investigate the nature and neural basis of rats' ability to encode aspects of scenes-the shapes or figures that the scenes contain, their positions within the scene, and the combinations or configurations of these elements.…”