Picture Encoding Speed and Mental Retardation

Using a modified Posner "encoding function" methodology, we assessed group differences in semantic encoding speed under conditions in which subjects encoded pictures of common objects to determine physical identity (PI) matches, name identity (NI) matches, and superordinate identity (SI) matches. Mildly retarded adults; Chronological Age (CA)-matched, nonretarded adults; and Mental Age (MA)-matched, nonretarded children were presented pairs of stimuli in which the two stimuli within the pairs were separated by a variable stimulus onset asyncrony (SOA). Subjects were required to determine as rapidly as possible whether or not the two stimuli matched. A subject's encoding time corresponded to the shortest SOA at which optimal response time pedormance on the matching task was achieved. The results of primary interest revealed group differences in basic encoding speed and indicated that the magnitude of the group differences increased with increases in the degree to which accessing semantic knowledge was necessary to encode the stimuli. In addition, our data suggested that differences in basic encoding speed are IQ rather than MA related, whereas the degree to which accessing semantic knowledge increases encoding times is related to MA.