Heritability of IQ by social class: evidence inconclusive.

.025), while the responses of the sham-operated were not significantly changed. After the operation the scores of the deganglionated group were significantly lower than the scores of the sham-operated group (P = .013). These results suggest that the siphon withdrawal component of the defensive reflex is mediated primarily by the central nervous system. This does not mean that all siphon withdrawal is centrally mediated. In the absence of the abdominal ganglion, the peripheral plexus is still able to move the siphon when it is directly stimulated. However, deganglionated animals are unable to perform an integrated withdrawal reflex that removes the siphon from view. Since the siphon can move in the deganglionated animal, we sought to determine whether the sensitization of the reflex might be due to the facilitation of the relatively small peripherally mediated siphon movements. Twenty-four animals were matched as in previous experiments and all animals were given sensitization training (four shocks per day for 4 days) beginning 6 days after the habituation session. On the day after the last shock, half of the animals were deganglionated and the other half were sham-operated. All animals were given a 1-day postoperative retention test using a blind procedure (Table lb). The deganglionated animals failed to show any effect of the previous sensitization training. In fact, these animals showed significantly less siphon withdrawal than they did previously (P = .025) (Table lb). By contrast, the sham-operated animals showed significant sensitization (P = .005) despite the intervening anesthetic and surgical procedure. Two days after the last shock, the median value of the sham-operated group was about 440 percent of the presensitization value. These results suggest that long-term sensitization of siphon withdrawal is centrally mediated. The demonstration of long-term sensitization of defensive siphon withdrawal in Aplysia is a further extension of the shortand long-term forms of behavioral modifications that this simple reflex can undergo. Whereas habituation is perhaps the most elementary behavioral modification, sensitization is more complex because it involves changes in one reflex pathway as a result of activity in another one. As Groves and Thompson (14) have pointed out, sensitization represents an aspect of arousal, or a "state