مشاهدات در رابطه حافظه صریح و ضمنی کلامی و تراکم سلول های عصبی در هیپوکامپ

The relationship between neuronal density and verbal memory in left and right hippocampal subfields was investigated in patients who underwent surgery for alleviation of temporal lobe epilepsy. The surgery consisted of unilateral partial removal of the hippocampus along with the anterior temporal lobe and amygdala. Study 1 looked at post-surgical explicit vs implicit verbal memory for lists of words while Study 2 looked at pre- and post-surgical explicit memory for word pairs. Left subfield CA1 appeared to be the most consistently involved in explicit and implicit memory. The results of the two studies confirm presence of hemispheric asymmetry in verbal memory. The notion that hippocampal control of memory is most apparent in post-surgical performance is discussed.

1.1. Sperryand Cajal Around the turn of the 20th century, the eminent neuroanatomist Santiago Ramon y Cajal (Nobel Laureate, 1906) proposed the notion of regularity in neuronal connectivity in the brain. Axons connect with each other according to a predetermined specific plan and this through chemical signals emanating from the tips of the axons themselves. Much later, in the 1940’s, Roger W. Sperry proposed the notion of chemoaffinity to explain how developing axons in one part of the nervous system ‘‘know’’ where their final target is located. He conducted a series of elegant, by now classic, critical animal experiments which illustrated the notion of specificity in neuronal connectivity. Sperry admired Cajal greatly; he remarked that ‘‘he was the greatest neuroscientist of this century’’. Despite his prodigious output, Cajal did not pay much attention to left–right differences in the brain nor to a huge bundle of fibers, the corpus callosum, that connected the two halves of the cerebrum. The left and right sides were the main focus of Sperry’s work with cats, monkeys, and neurosurgical patients 47 and 48; his findings on functional brain asymmetries in commissurotomy patients (‘‘split-brain’’) won him a Nobel Prize in 1981. A world-wide scientific interest in brain asymmetry was sparked by the work that he began and the observations on hippocampal asymmetries described here reflect one of the outcomes of this interest. Ramon y Cajal conducted many anatomical investigations of the hippocampus and provided some of the most detailed descriptions of this structure to date [37]. His specimens were mostly (though not exclusively) from the mouse and rabbit which are now known to have somewhat different anatomical arrangements in the hippocampus than humans, especially with regards to commissural connection between the hippocampi in the two sides 3, 4, 13 and 36. The hippocampal commissure appears to have grown relatively smaller in evolution as one moves from rats, to cats, to monkeys, to humans 40, 58 and 59. From a phylogenetic perspective alone, as hemispheric specialization evolved the size of the corpus callosum grew; there may have also been a progression toward somewhat less direct communication between the left and right hippocampi but with increased ipsilateral structural\functional connections between hippocampus and neocortex [40]. Such an evolutionary trend would support the notion that human memory systems in the two sides are wired up differently to support separate but complementary functional specialization in the hemispheres [61].