Visual Target Discrimination in Sharks — Tester and Kato 
463 
1. The neutral targets were displayed at 
both ends and the shark was allowed to ac- 
climate. 
2. The negative target was displayed at one 
end for a period of 3 min, and the shark was 
shocked each time it entered that end. 
3. After a rest period of 3-5 min, during 
which the neutral targets were displayed at 
both ends, the procedure was repeated at the 
other end of the tank. 
Each training session lasted from 36 to 45 
min with six alternating shocking and rest 
periods, or 18 min of potential exposure to 
shock. A minimum of 2 hr was allowed be- 
tween sessions. Training was continued until 
a shark displayed that it had made the required 
association, at which time a test was conducted. 
If after a reasonable number of training sessions 
the shark had not displayed signs of discrimina- 
tion, training was discontinued, usually after 
tests had been conducted. 
During test periods the negative target was 
displayed but no shock was administered when 
the shark entered the negative end. 
Both simultaneous and successive discrimina- 
tion training techniques (Sutherland, 1962) 
were employed. In the former case two neutral 
targets were presented, one above the other, 
during rest periods, one of which was replaced 
by a negative target during training. 
Criteria of Discrimination 
Abrupt changes in behavior, when occurring 
consistently with appropriate target changes, 
were considered to be end points of behavior 
indicating that the shark had made the desired 
associations, and hence discrimination between 
the negative and neutral targets. The following 
are such behavioral changes, one or several of 
which occurred with individual sharks : ( 1 ) 
head-shaking or body-quivering on facing the 
negative target for the first time at the start 
of a training period or during a test period; 
(2) a sudden swirl or an abrupt change of 
swimming pattern on presentation of the nega- 
tive target; (3) turning from the negative 
target before being shocked, either consistently 
or at least during the first few passes of each 
training period, or during test periods; (4) 
following this behavior, entering the end zone 
immediately after the negative target had been 
replaced by the neutral target; (5) sudden 
dashes into and out of the negative end zone 
immediately after the target was displayed; (6) 
dashing toward the negative target and abruptly 
turning at the line marking the entrance; (7) 
abrupt decrease in excitability when the target 
was changed to neutral, or abrupt increase in 
excitability when the negative target was pre- 
sented. 
RESULTS 
Using the criteria listed above, sharks were 
subjectively judged to have succeeded or failed 
to discriminate between negative and neutral 
targets. Results of all experiments are sum- 
marized in Table 2; the total training time 
(including shocking but not control periods) 
and number of training sessions are entered in 
Table 3. In cases of positive conclusion, the 
times and sessions represent training until dis- 
crimination was evident. In all such cases addi- 
tional training confirmed the results. 
Some sharks (marked ? in Table 2) dis- 
played good signs that they could discriminate, 
but their over-all behavior was too erratic to 
afford a firm conclusion. The implication is that 
further training might possibly have strength- 
ened the association. 
Behavior During Training 
When a shark was shocked as it entered the 
end zone displaying the negative target, its body 
twitched noticeably. Usually it would dash 
away from the end compartment (escape re- 
sponse). Often, however, depending on the 
particular shark and its experience at being 
shocked, it would continue into the end zone 
despite the shock, and would turn either at the 
end or at some intermediate point. 
During training, most sharks first developed 
an end association, i.e., after one or more shocks 
during a training period, they learned which 
end produced punishment and either avoided 
that end for the remainder of the period or, 
before penetrating it, displayed signs such as 
head-shaking which showed the end = punish- 
ment association. With most sharks, end asso- 
ciation developed into target association with 
