Guide Neuroimaging of Consciousness

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Neuroimaging of Consciousness : Andrea Eugenio Cavanna :

Yet, on those individual trials in which the observers produced an incorrect response, it is reasonable to claim that they did not perceive [i. Incorrect responses are a direct consequence, according to LSB, of a lack of processing of the target bottom stream in Fig. LSB accept that perceptual processing is not sufficient for conscious awareness and, hence, that there can be processed unconscious targets bottom half of top stream in Fig.

These trials are the ones that give rise to a subjective feeling of guessing. Note that in their framework the unaware processed trials are always correct because incorrect trials are no-perception trials. Put simply, for LSB only targets i. Conversely, if a target is not reported accurately it can be inferred that it was not perceptually processed.

The distinction between processed and unprocessed stimuli is, then, sharp and clear. Following this model, the only possible source of ambiguity is those unprocessed and hence unaware responses that are correct due to chance upward arrow in bottom stream on Fig. LSB suggest comparing unaware-correct chance-free and aware-correct trials to find the true neural correlates of consciousness.

We conclude that the sharp distinctions between unaware-correct by chance, unaware-correct chance-free, and aware-correct trials that their proposal requires make sense only if something like HTM is assumed. The strength of the internal response is hardly ever exactly at zero due to the presence of noise. In other words, a stimulus is hardly ever in an unprocessed state.

The signal of a target is always corrupted by noise, and therefore, performance capacity is determined by the signal-to-noise ratio of the internal response. There is no magical point below which subjects always completely fail to process the target and above which they always process it successfully.

The presentation of one of two possible stimuli evokes an internal response in the subject, falling into one of two Gaussian distributions. In each trial of a discrimination experiment subjects set a discrimination criterion solid vertical line and awareness criteria dashed lines against which they compare their internal response. Because the distributions overlap darker area , it is possible and quite common that stimulus A is mistaken for stimulus B, or vice versa. Wrongly classified trials are reported as conscious if the internal response crosses the awareness criterion on the wrong side of the discrimination criterion.

According to SDT Macmillan and Creelman, , the presentation of a stimulus A or B in a discrimination task gives rise to an internal response in the subject Fig. Subjects set a criterion against which they compare the internal response, which leads to the classification of the signal as being due to the presentation of stimulus A or B.

Because the distributions for the internal responses overlap, it is possible and quite common that stimulus A is mistaken for stimulus B, or vice versa. Additionally, trials are reported as aware when the internal perceptual response is strong enough to cross one of the outermost awareness criteria, and they are reported as unaware otherwise.

Note that this allows for aware-incorrect trials when the internal response is drawn from the wrong distribution and yet it is strong enough to cross an awareness criterion e. Insofar as SDT rejects this strict dichotomy between perfectly processed and unprocessed stimuli, it is incompatible with HTM. But why prefer one model over the other? A specific problem of HTM regarding consciousness studies is that it cannot explain the presence of incorrect trials when subjects report being aware of a target.

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According to the model as conceived by LSB, if subjects are aware of a target, it has to be because it was successfully processed. Thus, the presence of aware-incorrect trials is a problem. It is common practice in psychophysics to take into consideration lapse trials, i. In contrast, as can be noted in Fig.

These trials are classified as aware and hence, despite being incorrect, should be accounted for when looking for the NCC. What really convinced generations of psychophysicists that SDT is a superior model to HTM is the comparison of theoretical and empirical ROC receiver operating characteristic curves.

An ROC curve is a plot of hit rate against false alarm rate. Most empirical ROC curves from human subjects in visual experiments typically look like the one predicted by the SDT model, and hardly ever look like the one predicted by HTM. According to SDT, the trade-off between having more hits and false alarms when there is non-zero sensitivity is a non-linear relationship determined by the signal-to-noise ratio. A zero sensitivity scenario would yield a straight ROC line from zero to one diagonal dashed line , where one can only increase hits by increasing the same amount of false alarms.

A higher than zero signal-to-noise ratio means that the ROC curve will be curvilinear, where one can increase hits without increasing false alarms in the same proportion solid curve i. See Supplementary Material , including Matlab code, for details regarding the simulation. According to HTM, the vertical intercept is determined by the proportion of trials where the subject successfully processes the stimulus. The trade-off between hits and false alarms follows a linear relationship. We should note that in the memory literature, HTMs have enjoyed more popularity than in different perceptual modalities.

In particular, mixed models Aly and Yonelinas, ; Yonelinas and Jacoby, , where recognition responses follow HTM and familiarity responses conform to SDT, have been well received, but they have also been criticized from the perspective of SDT Wixted and Mickes, Here we are agnostic to this specific issue. We are not arguing that all HTMs are necessarily wrong. What we maintain here is that in the case of vision psychophysics, it is uncontroversial that SDT is much better supported by empirical data than HTM and that HTMs are inappropriate for conscious awareness studies.

Their inadequacy lies on how they depict the internal representation of signal and noise, heavily underestimating the role of the latter.

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How exactly might it have biased their results? We performed a computational simulation analysis to evaluate the degree of inadequacy of the correction method proposed by LSB. As any other theoretical model of perception, SDT has explanatory limits.

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For simplicity, we assumed that subjects performed a two-choice discrimination task, which is analytically more tractable than a 4-AFC task and its results are trivially generalizable. The simulation consisted on distinguishing between two stimulus alternatives A and B , and then reporting whether there was awareness of the target or not. It followed the SDT assumptions presented in section 3. The presentation of a stimulus along with noise is assumed to give rise to an internal perceptual response that varies from trial to trial and that falls into one of two Gaussian distributions depending on which stimulus was presented.

Discrimination is made by comparing the internal response to a criterion. The trial is reported as aware if the strength of the internal response crosses one of the awareness criteria. For every trial, we made the strength of the internal perceptual response correlate with a hypothetical neural response and a corresponding ERP of an arbitrary electrode site. We modeled this ERP as a sinusoidal response over time, scaling the amplitude of the ERP response by the strength of the internal perceptual response sampled from either of the Gaussian distributions Fig.

This late activity is meant to reflect activity that is specific to awareness. Activity intensity in c , the unaware-incorrect trials, is reduced compared to the higher activity in b , the unaware-correct trials. See Supplementary Material for details.

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When the internal response was strong enough to cross the awareness criteria, the model assumes a constant brain signal is added to it, which may reflect a putative processing signature of awareness. This extra cycle represents the differentiating processing uniquely associated with conscious awareness that is absent in trials without awareness Fig.

The idea is that by subtracting the unaware mean waveform from the aware mean waveform, if the unaware mean waveform is appropriately corrected for, we should be left just with activity properly related to awareness i. Despite its idealized nature, these simulations can help us determine the expected effectiveness of a performance correction method. Finding the precise timing and localization of these signatures is the goal of imagining studies looking for the NCC.

Hence, the simulations assumed the dissociated late timing for mere illustration purposes. The extra cycle associated with consciousness, then, could have been added earlier too e. Along with other simplifications e. To emphasize, we suggest this simple-minded model for ease of visualization and implementation only.

The results presented in Figures 4 and 5 were obtained after a 10,trial computer simulation see Supplementary Material for technical details; the Matlab code used for generating all the simulations is provided as part of the Supplementary Materials. Figure 4 shows the ERP average responses under the different relevant conditions. In Fig.

The unaware-correct response Fig. Hence, the influence of subtracting unaware correct-by-chance trials from aware-correct activations is only marginal. Both subtractive comparisons, namely, aware-correct minus unaware-correct Fig. Concretely, in the latter comparison Fig. An optimal analysis where only the awareness signature response remains after a subtractive comparison should cancel out the early response, leaving just the late response that is specific to awareness. As it is clear from Fig. It is evident from visual inspection that the influence of lucky responses was not sufficiently removed.

They assume that the ERP response of unaware-correct-by-chance trials looks the same as the ERP response of unaware-incorrect trials Fig.