Blocking Observed in Human Eyelid Conditioning

IRENE MARTIN

Psychology Department, Institute of Psychiatry, London and

ARCHIE B. LEVEY MRC Applied Psychology Unit, Cambridge

Abstract The status of blocking in human eyelid conditioning was examined in a series of four experiments modelled on rabbit nictitating membrane (NMR) procedures to ensure comparability with the animal literature. The first three employed Tones and Lights as CSs and a preliminary experiment established equivalent salience of these stimuli. The fourth employed all light conditioned stimuli (CSs). Each of the experi- ments balanced order of presentation of blocked and non-blocked stimuli in extinction; each yielded reliable evidence of blocking tested in extinction. However, the blocking effects were attenuated or abolished in those groups receiving the to-be-blocked stimulus on the first trial in extinction. The results show blocking to be a relatively weak, easily disrupted phenomenon compared with rabbit studies. They suggest that rapid re-ap- praisal of stimulus significance plays a more important role in human subjects than has been observed in animal studies.

A series of experiments examined the blocking phenomenon in human classical eyelid conditioning in an attempt to link human with animal theoretical approaches. Blocking is demonstrated if, following prior conditioning with CS (A) and a stage of compound training with CS (A + B), subsequent tests with CS (B) alone show that little or no conditioning has occurred to this component in spite of its contiguous presentation during the compound training. According to the most influential view, the Rescofla and Wagner (1972) model, blocking occurs because prior training with CS(A) allows that component to capture all the available associative strength, thus preventing any to accrue to the added element.

Human eyelid conditioning bears close resemblances to the rabbit nictitating membrane (NMR) preparation. Contiguity of CS and Unconditioned Stimulus (US) is essential, and the parameters of contiguity are remarkably similar. Definition of the human eyelid CR and its topography are close to that used in NMR studies.

Well-documented features of eyelid/NMR conditioning make it a precise tool for examin- ing associative phenomena. CR acquisition curves typically show a negatively accelerated growth function; little Unconditioned Response (UR) habituation occurs to the US; and

Address for correspondence: Irene Martin, Psychology Department, Institute of Psychiatry, de Crespigny Park, London SE5 8AF, UK.

Integrative Physiological and Behavioral Science, April-June, 1991, Vol. 26, No.2, 127-131.

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128 MARTIN AND LEVEY

controls for pseudoconditioning and sensitization are unnecessary. Thus it is relatively free of the problems attendant on other systems, particularly electrodermal responding.

The blocking procedures used in NMR studies have typically used bisensory stimuli, for example, tones and lights as CSs, and we followed this usage in our first experiments, Although Marchant and Moore (1973) obtained a highly significant effect when a CS Light was blocked, the effect was non-significant when a CS tone was blocked. They and other workers have attributed some of the apparent blocking to overshadowing effects, the CS Tone being more salient than CS Light.

To ensure subjective stimulus equivalence in intensity, a small study was conducted requiring subjects (Ss) to match a 1,000 Hz tone, varying in db level, to a light-emitting diode of 1/8" diameter. The values obtained were used in the first experiment to compare levels of conditioning that could be achieved between groups receiving (1) CS Tone, (2) CS Light and (3) random CS Tone or CS Light. No significant between groups difference in CR frequency was obtained, indicating that the Groups had conditioned to a similar level and confirming the equivalence of CS(T) and CS(L) as conditioning stimuli. These results suggested that one would be unlikely to overshadow the other, and they were used to investigate blocking in the next experiment.

Blocking Exper iment 1

In Stage 1 acquisition, Group T received CS (Tone) + US and Group L, a CS (Light) + US. In Stage 2 acquisition, Tone/Light compounds were employed. Stage 3 consisted of extinc- tion with Tone alone and Light alone trials. Each group was split into two sub-groups during extinction to counterbalance the order of presentation of the two CSs, so that the Tone was received first by half of the subjects and Light first by the other half (see Table 1).

Analyses of variance indicated no significant difference between groups conditioned to CS(T) or CS(L) in Stage 1, or between acquisition to the compound in Stage 2. Extinction data were analyzed for blocking. Between subject factors were modality of blocked CS (Tone or Light) and order of presentation in extinction (blocked first or second); the within-subject factor was the randomized blocked vs non-Nocked stimuli.

TABLE 1. Designs of Blocking Experiments.

Stage 1 Stage 2 Stage 3

Experiment 1 Acquisition (25 trials) Acquisition (25 trials) Group 1: CS(T) + US CS (T+L)+US Group 2: CS(L) + US CS (L + T) + US

Experiment 2

Extinction (20 trials) "ITLTLT[' etc. or LLTLTLL etc.

Acquisition to criterion of Acquisition (12 trials) Extinction (20 trials) 12 CRs. Group 1: CS(T) + US TLTL etc. or CS(T) + US Group 2: CS(L) + US

Group 3: CS(T+L) + US LTLT etc.

Acquisition (40 trials) (A+ C) + US (B + D) + US (20 trials of each in random order).

Experiment 3 Pre-exposure Acquisition (40 trials) (12 trials) A+ US 3 trials each B- of CS (Lights) (20 trials of each in A,B,C,D. random order).

Extinction (20 trials) CDDCD etc. or DCCDC

BLOCKING IN HUMAN CONDITIONING 129

Analysis of the 20 trial totals in extinction showed an overall main effect for blocking (F(1,22) = 4.59, p < .04) and a significant interaction of blocking with the between subject factor (F(1,22) = 9.36, p < .006) order of presentation. Figure 1 illustrates the blocking effect and the interaction.

Analysis of simple contrasts showed that the blocking effect was largely confined to those Ss who received the blocked stimulus second in extinction, that is, there was virtually no blocking effect when blocked stimuli were presented first.

Although the results support a blocking effect, there are grounds for being cautious about this interpretation. An alternative explanation might be that when the new stimulus is added in Stage 2, considerable habituation had occurred to the US, lowering its associative capacity.

Blocking Experiment 2

The next experiment explored in greater detail the nature of stimulus interaction by comparing acquisition to a compound with that to single components of the compound (see Table 1). All Ss (N=60) were trained with the same CS (Tone) in Stage 1 to a criterion of 12 CRs. Group 1 then continued with the same CS (Tone) in Stage 2, i.e., they experienced no change; Group 2 was switched to a new element (CS Light) and Group 3 (the blocking group) to a CS (Tone + Light) compound. As before, the effect of blocking in Group 3 was assessed in extinction, half the Ss receiving the blocked stimulus and half the non-blocked stimulus on the first trial.

U C

O" m L u..

L~

3 - Q -

2 = 5 "

2 = ( ~ -

1 . 5 -

1 = 0 "

0 = 5 "

77"2 / / / - - / / / / / / / / / / / / / / / / / / / / / / / / / / /

i i i

Blocked first Blocked second

B

N E]

FIG. 1. Average CR frequencies illustrating the interaction between blocking and order in extinction. Blocking Experiment 1.

130 MARTn~ AND LEVEY

At the same time, the hypothesis that differential orienting (attention) might occur to the CSs at different stages of the experiment was monitored by continuous recordings of electrodermal activity (EDA) on each trial.

The experiment had three major goals: (a) to assess the extent of conditioning to the compound CS (Tone + Light) in Group 3, the blocking group, compared with that occurring when either element alone (Groups 1 and 2) is reinforced in Stage 2; (b) to assess the blocking effect in extinction, and, (c) to examine whether there is differential orienting and attention to the two CSs.

All groups were conditioned in Stage 1 to a similar level of CR performance with the CS (tone). Mean CR frequencies in Stage 2 showed similar responding in Groups with CS(T), CS(L), and CS(T+L), analysis of variance confirming that the mean differences were not reliable. Rapid transfer of conditioning occurred to the new stimuli of Stage 2, indicating that UCS habituation did not impair conditioning to CS(Light) in Group 2.

Analysis of CR frequency over all 20 extinction trials of Group 3 (the blocking group) showed no significant effect for blocking. Individual trial analysis showed a significant effect on Trial 1; the group receiving the blocked stimulus on that trial giving fewer responses than the group given the non-blocked stimulus. This weak blocking effect is consistent with the less than asymptotic conditioning observed during the acquisition phase.

At the end of acquisition, little electrodermal responding was apparent. Although the change to non-reinforcement in extinction produced a slight increase in EDR activity, responding to CS(T) and CS(L) was approximately equivalent in Group 3, the blocking group, that is, there was no evidence of differential orienting to the two stimuli.

The results of Blocking Experiments 1 and 2 are suggestive but remain inconclusive as a demonstration of blocking. The use of bisensory CSs complicates the number of controls required, and the next experiment employed four CS Lights in a within - subject discrimina- tion paradigm, which eliminated problems of overshadowing and cross-modal transfer.

Blocking Experiment 3

This utilized a discrimination paradigm (see Table 1). To test that there was no differential responding to the four CSs, a pre-acquisition series of CS alone trials were given. Following this, Stage 1 of acquisition involved one CS+Light (A) (reinforced), and one CS-Light (B) (unreinforced), presented with equal frequency. These two stimuli were subsequently com- pounded in a second stage of acquisition with additional stimuli to make the compounds CS(A+C) and CS(B+D), both compounds being reinforced. In this design, stimulus D and the to-be-blocked stimulus C occur with equal frequency in the second stage of acquisition. Subjects were randomly allocated to two groups to counterbalance the order of presentation of C and D in extinction.

Extinction data were analyzed over the complete 20 trials. ANOVA showed a significant blocking effect and a significant interaction with the between-subject factor, order of presen- tation. Analysis of simple contrasts showed that the blocking effect was stronger in Ss who received the blocked stimulus second (see Figure 2).

These results provide clear evidence of a blocking effect, significant only when the blocked stimulus occurred second in extinction. The main significance of this experiment is that alternative explanations offered by the other experimental designs are ruled out.

We conclude that blocking can be demonstrated in human subjects, but that it is influenced by factors not usually discussed in the rabbit NMR literature. Extinction occurred extremely

BLOCKING IN HUMAN CONDITIONING 131

3 . 0 - B

2 . 5 - • N

u 2 0 - 0

1 5 - L , / / /

/ / / / / /

1 . 0 - / / / / / / ///

/ / / / / /

0 .5 / / / / / / / / / / / / / / / / / /

/ / / : J J J

Blocked f i rs t Blocked second ~G. 2. A v ~ e CR f r ~ u ~ c i ~ i l ~ s ~ m g the int~action ~tween blowing ~ d o~er in exaction.

BloWing E x ~ e m 3.

rapidly in all three experiments; blocking effects, although statistically significant, were weak relative to reported rabbit data: There were wide individual differences in performance. Humans appear to demonstrate a more general facility in utilizing predictive cues. The puzzling feature of the experiments is the interaction between order of stimulus presentation and the demonstration of blocking. If the to-be-blocked stimulus is presented first in the extinction series, blocking appears to be abolished or attenuated. If it occurs after the non-blocked stimulus, blocking is observed. A possible explanation of this effect is that rapid updating of stimulus significance occurs as a result of the saliency or surprisingness of the first extinction trial.

References

Marchant, H. G., & Moore, J. W. (1973). Blocking of the rabbit's conditioned nictitating membrane response in Kamin's two-stage paradigm. Journal of Experimental Psychology, 101, 155-158.

Rescorla, R. A., & Wagner, A. R. (1972). A theory of Pavlovian conditioning: Variations in the effectiveness of reinforcement and nonreinforcement. In A. H. Black & W. F. Prokasy (Eds.), Classical conditioning 11: Current research and theory (pp. 64-99). New York: Appleton-Century-Crofts.