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Original Article |

Preserved Subliminal Processing and Impaired Conscious Access in Schizophrenia FREE

[+] Author Affiliations

Author Affiliations: Institut National de la Santé et de la Recherche Médicale (INSERM), Cognitive Neuroimaging Unit, Service Hospitalier Frédéric Joliot, Commissariat à l’Energie Atomique, Orsay, France (Drs Del Cul and Dehaene); Departement hospitalo-universitaire de Psychiatrie, Hôpital Albert Chenevier et Henri Mondor, Assistance Publique–Hôpitaux de Paris (Drs Del Cul and Leboyer), and Unite INSERM U 513, Neurobiologie et Psychiatrie, Hôpital Henri Mondor (Drs Del Cul and Leboyer), Creteil, France; and Collège de France, Paris, France (Dr Dehaene).


Arch Gen Psychiatry. 2006;63(12):1313-1323. doi:10.1001/archpsyc.63.12.1313.
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Background  Studies of visual backward masking have frequently revealed an elevated masking threshold in schizophrenia. This finding has frequently been interpreted as indicating a low-level visual deficit. However, more recent models suggest that masking may also involve late and higher-level integrative processes, while leaving intact early bottom-up visual processing.

Objective  To test the hypothesis that the backward-masking deficit in schizophrenia corresponds to a deficit in the late stages of conscious perception, whereas the subliminal processing of masked stimuli is fully preserved.

Design  Twenty-eight patients with schizophrenia and 28 normal control subjects performed 2 backward-masking experiments. We used Arabic digits as stimuli and varied quasi-continuously the interval with a subsequent mask, thus allowing us to progressively unmask the stimuli. We finely quantified their degree of visibility using objective and subjective measures to evaluate the threshold duration for access to consciousness. We also studied the priming effect caused by the variably masked numbers in a comparison task performed on a subsequently presented and highly visible target number.

Results  The threshold delay between the digit and mask necessary for the conscious perception of the masked stimulus was longer in patients compared with controls. This higher consciousness threshold in patients was confirmed by an objective and a subjective measure, and both measures were highly correlated for the patients and controls. However, subliminal priming of masked numbers was effective and identical in patients and controls.

Conclusions  Access to conscious report of masked stimuli is impaired in schizophrenia, whereas fast bottom-up processing of the same stimuli, as assessed by subliminal priming, is preserved. These findings suggest a high-level origin of the masking deficit in schizophrenia, although they leave open for further research its exact relation to previously identified bottom-up visual processing abnormalities.

Figures in this Article

A functional breakdown of large-scale cortical integrative processes caused by abnormal corticocortical and corticosubcortical long-range connectivity is postulated in schizophrenia.114 A crucial issue concerns whether, in addition to this deficit at the level of cognitive integration, patients with schizophrenia also have other possibly unrelated deficits of a lower and more modular nature.1522 Indeed, some experimental results arising from studies of visual backward masking have suggested a low-level visual deficit. In visual backward masking, the visibility of a briefly presented stimulus is reduced by a mask presented shortly after the stimulus.23,24 Patients with schizophrenia consistently show a deficit in the perception of backward-masked stimuli. Compared with normal control subjects, they require a longer interval between the stimulus and mask to identify the stimulus.2527

Breitmeyer and Ogmen23,24 have proposed a model in which masking depends on the interactions between transient (magnocellular) and sustained (parvocellular) channels within the early visual pathways. In that model, backward masking would occur when the transient channels of the mask interfere with the sustained channels of the stimulus and therefore interrupt the formation of the percept. Abnormal masking in schizophrenia would be linked to deficits in transient magnocellular channels.25,2832 An additional deficit in sustained channels (causing abnormal gamma-range activity) has also been proposed.33,34

More recently, however, new models of masking have appeared, according to which this phenomenon may also involve late and higher-level integrative processes.3543 Di Lollo et al44 and Enns et al,45 suggest that some forms of masking may be caused by a disruption in the integration between bottom-up inputs and a top-down attentional signal. Similarly, the global neuronal workplace model of conscious perception3941,46 postulates that conscious access is associated with recurrent interactions between distant brain areas. Top-down feedback from prefrontal and parietal areas to lower-level sensory regions would establish a self-amplified reverberant neuronal assembly associated with conscious reportability. During masking, a stimulus would fail to reach consciousness if the mask replaces the stimulus before this recurrent activity has become stable. Simulations show that this process is stochastic and may fail owing to random fluctuations in prestimulus spontaneous activity. The model therefore predicts an all-or-none, bimodal distribution of subjective visibility scores and event-related brain activation measures, which was recently observed experimentally during the attentional blink.40,47,48

The global workplace model, like most current models of masking, suggests that the initial feed-forward processing of masked stimuli can be largely intact, despite their reduced subjective visibility. This hypothesis is largely confirmed by studies of subliminal priming. In those studies, a masked shape called the prime is shown to influence the processing of a subsequent target stimulus. Behavioral and neuroimaging studies of subliminal priming suggest that the processing of stimuli made subjectively invisible by masking is extensive and can include visual recognition, but also lexical and even semantic levels.4953 In particular, Dehaene et al54 demonstrated a nonconscious motor conflict induced by masked primes during a number comparison task. Most relevant to present purposes, they showed that patients with schizophrenia showed a normal masked priming effect, but became disproportionately slower and showed an absence of conflict, relative to controls, when the primes were unmasked.55 These data, which suggested intact low-level visual processes and abnormal higher-level executive control in schizophrenia, were related to a major hypoactivation of prefrontal and anterior cingulate cortices. Dehaene et al55 tentatively suggested that the lack of top-down amplification from these areas might jointly explain the higher-level cognitive deficits and the change in the visual masking threshold in patients with schizophrenia.

The main purpose of the present study was to provide a further test of the hypothesis that the backward-masking deficit in schizophrenia corresponds to a deficit in the late stages of conscious perception. Our aim was to evaluate whether the bottom-up subliminal processing of masked stimuli is preserved by studying whether normal subliminal priming could be observed in schizophrenia. We used a new form of masking with a component of spatial attention, inspired by Vorberg et al56 and Di Lollo et al,44,45 in which the stimulus and mask shapes are not overlapping. Using Arabic digits as stimuli, we varied quasi-continuously the interval between a digit and the subsequent mask, thus allowing us to progressively unmask the stimulus. This manipulation allowed us to study the subliminal priming effect caused by these variably masked numbers, as well as their degree of visibility, and to precisely quantify the threshold duration for access to consciousness. Based on the global neuronal workplace model of conscious access, the following predictions were made: (1) Masking should affect conscious visibility in an all-or-none manner, ie, either the stimulus is fully consciously perceived or it is not perceived at all. (2) The threshold for conscious access should be higher in patients than in controls, ie, a longer interval between stimulus and mask should be necessary for them to identify the stimulus. (3) The subliminal processing of nonconsciously perceived stimuli, measured by their priming effect, should be preserved and identical in the 2 groups. Thus, we expected response times to be faster when the prime provided some valid information about the target, ie, about its exact identity (repetition priming) or about the nature of the forthcoming response (response priming). The preservation of both priming effects would constitute strong evidence in favor of preserved fast bottom-up perceptual processing in schizophrenia.

PARTICIPANTS

Twenty-eight patients with schizophrenia (mean age, 34 years; age range, 18-53 years; 9 women and 19 men) participated in the study. All were native French speakers. Patients met DSM-IV criteria for schizophrenia and were recruited from the psychiatric department of Creteil University Hospital (Assistance Publique–Hôpitaux de Paris). They had a chronic course and were stable at the time of the experiment. Twenty-two patients were treated with atypical antipsychotics and 6 with typical antipsychotics. This treatment had been unchanged for at least 3 weeks. The comparison group consisted of 28 subjects (mean age, 32 years; age range, 18-55 years; 18 men and 10 women). Comparison subjects were excluded for history of any psychotic disorder, bipolar disorder, schizotypal or paranoid personality disorder, and recurrent depression. Patients and controls with a history of brain injury, epilepsy, alcohol or other drug abuse, or other neurological or ophthalmologic disorders were excluded. Patients and controls did not differ significantly in sex, age, or level of education.

All experiments were approved by the French regional ethical committee for biomedical research (Hôpital de la Pitié Salpétrière), and subjects gave written informed consent.

STIMULI

In our material, a first number, the prime, was masked by a shape containing in its structure a second number, the target (Figure 1). We varied quasi-continuously the interval between the prime and the subsequent mask, thus allowing us to progressively unmask the stimulus digit. The delay between the onset of the prime and the onset of the mask could take 1 of 8 values (0, 16, 33, 50, 66, 83, 100, and 150 milliseconds). For the delay of 0 milliseconds, the prime and the mask had the same onset, but the mask persisted after the prime had disappeared.

Place holder to copy figure label and caption
Figure 1.

Experiment design. The prime was presented for 16 milliseconds at 1 of 4 positions (1.4° above or below and 1.4° to the right or to the left of the fixation cross). The mask (duration of presentation, 250 milliseconds) was composed of 3 letters (M, M, and E) and the target number (1° from the fixation cross). Those 4 symbols surrounded the prime number without touching it. In the first experiment, referred to as the priming experiment, subjects were asked to compare each target number with 5, pressing the right-hand key as fast as possible for numbers larger than 5 and the left-hand key for numbers smaller than 5. The second experiment aimed at measuring the consciousness threshold in 2 different ways. We measured an objective visibility threshold by examining the subjects' ability to perform the number comparison task on the prime. We also measured a subjective threshold by collecting introspective ratings of prime visibility, on a subjective continuous scale.

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The stimulus set consisted of 16 pairs of prime and target numbers. The pairs consisted of the numbers 1, 4, 6, and 9 written in Arabic format. As a consequence, the following factors could be analyzed: target distance (close to or far from 5), target size (larger or smaller than 5), response congruity (whether or not the prime and target fell on the same side of 5), and repetition (within the congruent trials, whether or not the prime and target were the same number).

PROCEDURE

One part of the experiment was dedicated to a measurement of objective and subjective thresholds. We measured an objective visibility threshold by examining subjects' ability to perform a number comparison task on the prime. We also measured a subjective threshold by collecting introspective ratings of prime visibility on a subjective continuous scale identical to the one used in previous studies of the attentional blink.40,47,48 The experiment consisted of 20 training trials followed by 20 trials for each delay, for a total of 180 randomly presented trials.

Another part evaluated subliminal and supraliminal priming. Subjects were asked to compare each target number with 5, pressing the right-hand key as fast as possible for numbers larger than 5 and the left-hand key for numbers smaller than 5. This experiment consisted of 20 training trials followed by 320 experimental trials (8 blocks of 40 trials, with 1 block for each delay). The different delays were presented in random order but in different blocks to facilitate the subject's task. If the delays had been randomly mixed, we believed it would have been too difficult for patients to avoid responding to the prime on conscious trials. Blocking helped them to learn to focus on the target and neglect the prime, regardless of its visibility. For similar reasons, the priming experiment, which was the most difficult, was always run before threshold measurement.

MEASURING THE THRESHOLD FOR ACCESS TO CONSCIOUSNESS

Figure 2 and Figure 3 show the distributions of visibility scores in each group and for each delay on prime-present trials. In both groups, we observed a bimodal repartition of scores, with a first set of responses close to maximal visibility (scale score, >75%) and a second set of responses peaking at zero visibility (score, <25%). Responses ranging from 25% to 75% were rare (<10%). Thus, in both groups, increasing delays did not lead to a progressive increase in subjective experience of prime visibility, but to a shift in the probability of reporting 1 of 2 discrete subjective states (“seen” or “not seen”).

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Figure 2.

Distribution of subjective visibility ratings in the patient group. In both the patient and control (Figure 3) groups, we observed a bimodal repartition of scores, with a first set of responses close to maximal visibility (scale score, >16) and a second set of responses peaking at zero visibility (score, <6). Responses ranging from 6 to 16 were rare (<10%). More not-seen responses were observed in patients than in controls, particularly at short delays.

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Figure 3.

Distribution of subjective visibility ratings in the control group.

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Based on this bimodal distribution, we arbitrarily defined seen trials as those in which the visibility score was above the middle of the scale (>50%). Figure 4B shows the proportion of seen trials as a function of delay. The proportion increased steadily with delay, but at a slower rate in the patients than in the controls. This pattern was evaluated with an analysis of variance (ANOVA) on the proportion of seen trials, with factors of group and delay. The proportion of seen trials was significantly higher in the controls than in the patients (F1,54 = 12.90 [P<.001]). We also found a significant delay effect (F7,378 = 270.64 [P<.001]) and a group × delay interaction (F7,378 = 8.17 [P<.001]). At all delays longer than 33 milliseconds, the proportion of seen trials was significantly lower in patients (all, P<.05).

Place holder to copy figure label and caption
Figure 4.

Objective and subjective measures of access to consciousness. A, Percentage of correct responses in the prime comparison with 5 as a function of delay. At each delay, the control subjects outperformed the patients. In controls, performance was significantly superior to chance at all of the delays; in patients, performance became superior to chance only for delays of 50 milliseconds and longer. B, Proportion of trials subjectively rated as “seen” as a function of delay. At all delays longer than 33 milliseconds, the proportion of seen trials was significantly lower in patients. In both graphs, the sigmoid curves fitting the data are represented as continuous lines. The mean objective (θo) and subjective (θs) thresholds were defined in each group as the delay for which the sigmoid curve reached its inflexion point. Error bars represent the standard error.

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A similar analysis was applied to the objective measure of prime processing. For each subject and each delay, we calculated the proportion of correct responses in the prime comparison task (Figure 4A). Again, this measure increased nonlinearly with delay, at a slower rate for the patients than for the controls. An ANOVA indicated that performance was significantly higher in the controls than in the patients (78.2% vs 59.6% correct, F1,54 = 87.53 [P<.001]). There was a main effect of delay (F7,378 = 100.63 [P<.001]) and a group × delay interaction (F7,378 = 5.02 [P<.001]). At each delay, the controls outperformed the patients (all, P<.05). In the controls, performance was significantly superior to chance at all of the delays. However, in the patients, performance became superior to chance only for delays of 50 milliseconds and longer.

In summary, objective and subjective measures of prime conscious perception indicated lower performance in the patients. To characterize for each subject a subjective and an objective threshold for access to consciousness, we used nonlinear regression to fit the curves in Figure 4 with a sigmoid defined in the following equation:

Figures

Place holder to copy figure label and caption
Figure 1.

Experiment design. The prime was presented for 16 milliseconds at 1 of 4 positions (1.4° above or below and 1.4° to the right or to the left of the fixation cross). The mask (duration of presentation, 250 milliseconds) was composed of 3 letters (M, M, and E) and the target number (1° from the fixation cross). Those 4 symbols surrounded the prime number without touching it. In the first experiment, referred to as the priming experiment, subjects were asked to compare each target number with 5, pressing the right-hand key as fast as possible for numbers larger than 5 and the left-hand key for numbers smaller than 5. The second experiment aimed at measuring the consciousness threshold in 2 different ways. We measured an objective visibility threshold by examining the subjects' ability to perform the number comparison task on the prime. We also measured a subjective threshold by collecting introspective ratings of prime visibility, on a subjective continuous scale.

Graphic Jump Location
Place holder to copy figure label and caption
Figure 2.

Distribution of subjective visibility ratings in the patient group. In both the patient and control (Figure 3) groups, we observed a bimodal repartition of scores, with a first set of responses close to maximal visibility (scale score, >16) and a second set of responses peaking at zero visibility (score, <6). Responses ranging from 6 to 16 were rare (<10%). More not-seen responses were observed in patients than in controls, particularly at short delays.

Graphic Jump Location
Place holder to copy figure label and caption
Figure 3.

Distribution of subjective visibility ratings in the control group.

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Place holder to copy figure label and caption
Figure 4.

Objective and subjective measures of access to consciousness. A, Percentage of correct responses in the prime comparison with 5 as a function of delay. At each delay, the control subjects outperformed the patients. In controls, performance was significantly superior to chance at all of the delays; in patients, performance became superior to chance only for delays of 50 milliseconds and longer. B, Proportion of trials subjectively rated as “seen” as a function of delay. At all delays longer than 33 milliseconds, the proportion of seen trials was significantly lower in patients. In both graphs, the sigmoid curves fitting the data are represented as continuous lines. The mean objective (θo) and subjective (θs) thresholds were defined in each group as the delay for which the sigmoid curve reached its inflexion point. Error bars represent the standard error.

Graphic Jump Location
Place holder to copy figure label and caption
Figure 5.

Positive correlation between objective and subjective consciousness thresholds across subjects. The 2 values were highly correlated as a whole (r2 = 0.95 [P<.001]), within the control subjects (r2 = 0.83 [P<.001]), and within the patients (r2 = 0.96 [P<.001]). In all cases, the slope did not differ from 1, and the intercept was not significant.

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Figure 6.

Measures of objective performance in prime comparison in each group for “seen” trials (conservatively defined as a subjective score >16) and “not-seen” trials (subjective score, ≤5). A, Performance at each delay and performance averaged across delays. B, Results for seen trial. C, Results for not-seen trials. The results demonstrate in control subjects a capacity for objective prime processing, even on trials subjectively rated as not seen (subliminal perception) and, conversely, in patients an invalability of objective information on some trials judged as seen (hallucinations). Error bars represent the standard error.

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Place holder to copy figure label and caption
Figure 7.

Measures of priming during the target number comparison task. A, Mean reaction time (RT) for each group, each condition of prime-target relation, and each delay. Response priming was defined as the difference in reaction time between incongruent (InCong) and congruent nonrepeated (CongNonRep) trials and repetition priming as the difference between CongNonRep and congruent repeated (CongRep) trials. Both effects were significant across and within each group without significant difference between groups. B and C, Delays were sorted into 2 categories, according to whether they fell above (B) or below (C) the previously measured consciousness thresholds. C indicates control subjects; P, patients. Error bars represent the standard error.

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