Neuronal plasticity in the afferent visual system - II. Conditioned modifications in geniculate neurons

The susceptability to, plastic changes of single units along the afferent visual pathway (optic tract/OT/, dorsal lateral geniculate body /LGB/) was investigated by means of classical conditioning procedures. These consisted of pairing either two light stimuli in different parts of the receptive field or pairing a light-spot with a non-visual stimulus. OT and LGB units were classified into dominant tonic and phasic groups on the basis of their response pattern to illumination of the receptive field center. 1) No consistent, reproducible modification was found in OT units (either in the tonic or in the phasic group) and in most LGB units with dominant tonic discharge pattern. 2) One-third of phasic LGB units (11% of the whole sample) with large, eccentric receptive fields exhibited consistent modifications of the response pattern during conditioning which persisted in some units for several minutes, in other units only for 20-30 seconds. 3) It has been suggested that the phasic (transient or Y-) system of the LGB incorporates plastic elements with a capability of short-term information storage.