Cortical spreading depression and subcortical memory storage

Brain systems and long-term memory

This paper focuses mainly on those findings derived from lesion studies on the rat which help to identify ensembles of neural structures concerned with the expression of previously learned responses. At the outset, the use of the lesion method in the search for those neurological circuits underlying memory is defended. This is followed by an evaluation of neocortical and subcortical systems in long-term memory. Subsequently, a modest list of tentative functional neural "complexes" involved in the maintenance of certain classes of learned responses is given, based largely upon the author's own research. It is concluded that the key to the understanding of the neurological substrates of long-term memory lies in the identification of those subcortical sites which interact with neocortical sites in the performance of complex learned tasks. The most likely subcortical sites involved in this interaction appear to inhabit the regions of the basal ganglia, limbic midbrain area, and ventral portions of the brainstem reticular formation.

Learned taste aversion induced by cortical spreading depression

Male albino rats were cannulated and placed on a 24 hr water deprivation schedule. The animals were allowed 10 min access to water in a large animal cage for 5 days. On the sixth day of deprivation the animals were randomly divided into 6 groups and given either 12 percent KCl, 25 percent KCl, or Ringers solution applied unilaterally or bilaterally to the cortex immediately after access to 8 percent sucrose. On the seventh day of deprivation, each rat was placed in a two-choice situation with the sucrose solution and water. Only the unilateral and bilateral 12 percent KCl groups developed an aversion to the sucrose. These results indicate that CSD has aversive as well as amnesic properties, there exists a gradient of amnesia, dependent on concentration, and that the cortex is not necessary for learning a taste aversion.

Differential effects of cortical ablation and spreading depression on sensitivity to footshock: implications for the role of the cortex in learning

Rats were tested for flinch and jump responses to shock under unilateral and bilateral spreading depression (USD and BSD) and unilateral cortical ablation (UCA). Some Ss were tested with insulating boots on the left or right limbs. Under USD and UCA the limbs contralateral to the depressed or ablated hemisphere were as insensitive as when under BSD, and the ipsilateral limbs were almost as sensitive as in the normal condition. The data indicate that lack of transfer typically found in the USD transfer paradigm can be attributed to stimulus factors and not to learning or memory deficits, especially since most studies have used a shock level which does not exceed the jump threshold for a depressed hemisphere.

Lack of coincidence between neural and behavioral manifestations of cortical spreading depression

The presence of cortical spreading depression is typically inferred from the presence of hypesthesia. The electrocorticogram and slow-potential change were recorded during cortical spreading depression and it was found that hypesthesia remained long after the cortex recovered from neural depression. Hypesthesia, therefore, is an unreliable indicant of cortical spreading depression; if cortical spreading depression is used as a research tool, neural activity must be monitored. These data offer a special problem for memory transfer studies.