FUNCTIONAL POSTNATAL-DEVELOPMENT OF THE RAT PRIMARY VISUAL-CORTEX ANDTHE ROLE OF VISUAL EXPERIENCE - DARK REARING AND MONOCULAR DEPRIVATION

Postnatal development of rat visual cortical functions was studied by recording extracellularly from the primary visual cotter of 22 animals ranging in age from postnatal day 17 (P17) to P45. We found that in t he youngest animals (P17-P19) all visual cortical functions tested wer e immature. Selectivity for orientation and movement direction of visu al stimuli was almost absent, most cells received binocular input and their mean receptive field size was 5-6 times the adult size. Visual a cuity was half its adult value. These functional properties developed gradually during the following weeks and by P45 they were all adult-li ke. This functional development is affected by manipulations of the vi sual input such as dark rearing (DR) and monocular deprivation (MD). D R prevented the normal postnatal maturation of visual cortical functio ns: in P60 rats, dark reared from birth, their visual cortical functio ns resembled those of P19-P21 rats. MD from P15 to P45 resulted in a d ramatic shift of the ocular dominance distribution (ODD) in favour of the open eye and in a loss of visual acuity for the deprived eye. To d etermine the sensitive period of rat visual cortex to MD (critical per iod) we evaluated the shift in ODD of visual cortical neurones in rats that were subjected to the progressive delay of the onset of fixed MD period (10 days). Our results show that the critical period begins ar ound the end of the third postnatal week, peaks between the fourth and fifth week and starts to decline from the end of the fifth week,