Inductions of recoveries from injuries to the cortex: Dissociation of equipotential and regionally specific mechanisms

Abstract

Rats were first trained on a black-white discrimination problem and were then subjected to either one-quadrant or two-quadrant neocortical injuries. Following a test of retention, the subjects again sustained one-quadrant or two-quadrant lesions, always such that one quadrant remained intact. Another test of retention was administered, and then some subjects were completely decorticated by removal of the final remaining quadrant, followed by a final retention test. The effects on retention following the first-stage lesions suggested the existence of an equipotential, cortically related retrieval mechanism. The deficits in performance following secondstage injuries suggested the existence of a second, independent, posteriorly specific mechanism. The findings demonstrated that not only does interoperative retraining facilitate retrieval of preoperatively established memories, but also that injuries to any component of the cortex will induce reorganizations that will then prevent memory-retrieval deficits after any combination of subsequent injuries to the cortex. Lashley (1935), in a classical investigation, examined the effects of neocortical ablations on learning and relearning of a black-white discrimination problem. He observed that performance on the problem is cortically dependent if the cortex is intact. He also observed that rats with injuries to the visual or posterior neocortex are able to learn the task as quickly as normal animals, and that lesions of the extravisual or anterior cortex have no effect on postoperative performance of the task. Hence, he concluded that training on the problem results in the storage of a memory by the cortex, provided that the cortex is intact, but if the cortex is not intact, the trace or engram is formed by subcortical systems.Lashley's basic paradigm has since been utilized by many other students of recoveries of functions after injuries to the cerebral cortex. Several have confirmed his observation that ablations of the posterior neocortex will completely suppress postoperative performance on the black-white problem (e.g., Glendenning,

Consequences of serial cortical, hippocampal, and thalamic lesions and of different lengths of overtraining on the acquisition and retention of learning tasks.

“…Meyer, Isaac, and Maher (1958), Petrinovich and Bliss (1966), Petrinovich and Carew (1969), and Thompson (1960), the assumption was made by Finger et al (1973) that interoperative training, as opposed to “nonspecific” stimulation, is especially beneficial when large brain lesions have been performed. More recent studies are generally in line with these arguments (e.g., Cloud, Meyer, & Meyer, 1982). However, Handelmann and Olton (1981) observed that, even following restricted damage of the CA3 region of the hippocampus, the rate of recovery was increased significantly by increasing either the amount of task experience or the length of the postoperative recovery period.…”

An equipotential function of the cerebral cortex

“…Its various subregions have been shown to be involved in the performance of different kinds of functions . However, the study we describe in this report completes what we believe to be a proof of the existence of an equipotential function of the cortex in remembering (Cloud, D. R. Meyer, & P. M. Meyer, 1982; P. M. .…”

“…That is twice the cost of a onequadrant injury to the cortex, and suggests that impairments of performance of the problem are governed by a law of mass action. We have found that that law describes the impairments of bilateral anterior preparations, of subjects with either left-or right-sided hemidecortications and of subjects with an injury to one posterior quadrant and the contralateral anterior quadrant (Cloud et al, 1982; Hata, Diaz, Gibson, Jacobs, P. M. Meyer, & D. R. Meyer, 1980). The exception to the law is that rats that are prepared with first-stage ablations of both posterior quadrants require 25 trials, instead of 17, to relearn the problem after surgery (Gray & D. R. Meyer, 1981).…”

“…For example, if a rat with a one-quadrant injury is given retraining on the problem and is re-retrained after having been subjected to a second-stage injury to the cortex, it will re-relearn the problem essentially at once if it still has one posterior quadrant (Cloud et aI., 1982;Glendenning, 1972). However, if its second-stage injury completes the destruction of both posterior quadrants, the animal will re-relearn the problem in 8 mean trials (Cloud et al, 1982;Gray & D. R. Meyer, 1981). We believe that the regionally specific impairment is due to a loss of form vision (Horel, Bettinger, Royce, & D. R. Meyer, 1966;Lavond & Dewberry, 1980;Lavond, Hata, Gray, Geckler, P. M. Meyer, & D. R. Meyer, 1978), but will stress once again that the cost of that impairment is small when compared with the nonspecific costs of injuries to the posterior cortex.…”

“…The fmdings for Groups F (n=7), G (n=6), H (n= 14), and I (n=6) were also approximately the same. However, the second operations for those subjects completed the bilateral destruction of the posterior cortex, and hence they required about 8 mean trials to recover from their posteriorly specific impairments (Cloud et al, 1982;D. R. Meyer & P. M. Meyer, 1977).…”