Stimulus presentation was preceded by a Hz warning acoustic tone lasting ms. The interval between the warning tone and stimulus onset was randomized between and ms to avoid expectation. After the stimulus disappeared, SL was asked to report the stimulus feature in a two alternative forced-choice task 2AFC.
When discrimination was not possible, she was requested to guess. In order not to bias her response criterion, patient SL was informed that a stimulus was presented in each trial i. Statistical analyses are reported separately for each stimulus feature. Patient SL reported to have guessed the orientation vertical horizontal of the stimuli in almost all of the trials Given the very low number of trials responded under the aware condition i. Under the unaware condition i. Patient SL reported to have guessed the color red green of the stimuli in almost all of the trials Patient SL reported to have guessed the contrast light dark gray of the stimuli in Similarly, also under the aware condition i.
Patient SL reported to have guessed the motion direction upward downward of gratings in almost all of the trials Conversely, under the aware condition i. In the present experiment, patient SL was presented with a large variety of stimuli and asked to discriminate a stimulus feature in a 2AFC task while reporting whether or not she consciously perceived the stimulus using a binary measure. With the exception of contrast, patient SL reported to have guessed every feature in almost all the trials.
By using a binary measure to assess awareness, patient SL can, thus, be classified as a type 1 blindsight patient: visual processing in the absence of perceptual awareness. Instead of a binary measure, we used a graded measure, the PAS Overgaard et al.
Patient SL was tested also in this experiment. In addition, we collected a sample of 10 healthy volunteers aged 22—29 years as a control group. They were all right-handed and all had normal or corrected-to-normal visual acuity with no history of neurological and psychiatry disorders.
All participants signed the informed consent prior to participating in the study and they were free to withdraw at any time. Apparatus and stimuli were the same as in Experiment 1 with a few additions. In addition to stimulus feature used in Experiment 1 high discriminability stimuli , we added another condition low discriminability stimuli for each of the five features tested: orientation vertical horizontal Gabor patches.
The parameters of the low discriminability stimuli were selected on the basis of the results obtained in the group of healthy participants showing that their performance was at chance level. This additional condition served as a control condition in which chance-level performance was expected in patient SL.
Moreover, to have the possibility to calculate criterion and sensitivity measures, catch trials were added.
As in Experiment 1, patient SL was informed of the stimulus types. For both actual stimulus and catch trials, if feature discrimination was not possible, SL was asked to guess the feature of the stimulus. Both the blind and the intact visual fields were tested in SL while only the right visual field was tested in the group of healthy volunteers. The experimental procedure was the same as in Experiment 1 Figure 1C with the exception of how SL was to report awareness of the stimulus.
The actual meaning of each level was fully explained and discussed with the patient. To be certain that no misunderstanding could occur, we conducted a training session prior to the real experiment, frequently interrupting the patient to thoroughly discuss about her actual rating of the visual experience Sandberg et al. Specifically, we stressed the distinction between level of confidence as usually measured by confidence rating scales and clearness of the visual experience as measured by the PAS Sandberg et al.
As we expected that healthy controls would be able to detect the presence of the stimuli, they were not asked to rate the clarity of their visual experience but only to perform the 2AFC task with respect to the specific feature they were requested to discriminate. A total of 20 of one-tailed binomial tests five feature discriminations by two levels of discriminability for the control group and patient SL were performed.
Moreover, to detect possible response biases toward a specific level of a feature, we analyzed the binomial distribution of the responses given with catch trials. Given that with catch trials no a priori hypothesis on the direction of the difference can be made, we adopted the two-tailed binomial test to assess the presence of a response bias. The same analysis was performed on the data obtained with SL for stimuli presented in her blind field. The only difference is that for each stimulus feature, trials were classified off-line on the basis of the PAS responses, and the percent accuracy of feature discrimination was calculated for each of the four levels of the scale.
Hit rate was defined as the trials in which the stimulus was presented and the PAS response was not zero i. False alarms were defined as the trials in which the stimulus was not presented catch trials and the PAS response was not zero.
Statistical significance was measured by means of receiver operating characteristic ROC curves. As a measure of goodness-of-fit, sensitivity true positive , and specificity true negative were computed, and a ROC curve was generated. We calculated sensitivity and specificity pairs and plot sensitivity on the y -axis by 1-specificity on the x -axis in order to create the ROC curve.
The non-parametric distribution-free method to calculate the area under the curve AUC was used. AUC values range from 0. See Azzopardi and Cowey and Van den Stock et al. In the same vein, they do not show any response bias with catch trials for orientation Taken together, these results show that healthy participants performed above chance with high discriminability stimuli and at chance level with low discriminability stimuli with the exception of contrast and real motion discrimination, for which, however, a possible confound of the presence of a response bias needs to be taken into account.
As for healthy participants, these data show that in her intact visual field SL performed above chance with high discriminability stimuli and at chance level with low discriminability stimuli with the exception of contrast discrimination, for which, however, the presence of a response bias need to be considered. Statistical analysis is reported separately for each stimulus feature. She never responded 2 or 3 on the PAS, indicating lack of any clear visual experience of the stimulus.
She never responded with 2 or 3 on the PAS indicating that she did not have an almost clear or clear visual experience of the stimulus. She never responded 3 on the PAS indicating that she never had a clear visual experience of the stimulus.
She never responded 3 on the PAS, indicating that she never had a clear visual experience of the stimulus. Conversely, SL showed an opposite response bias for contrast discrimination, i. In the present experiment, patient SL was presented with the same variety of stimuli as in Experiment 1 and asked to discriminate a stimulus feature while reporting whether or not she consciously perceived the stimulus using the PAS.
Patient SL reported to have guessed the features of stimuli of high discriminability in about one-third of the trials. In contrast, she performed above chance only for contrast discrimination and when reported to have been aware, thus indicating a positive relationship between accuracy and the clarity of visual qualia.
Taken together, these data demonstrate that when using the PAS to assess awareness, patient SL cannot be classified as a type 1 blindsight patient but as a patient with conscious vision. In this study, we assessed the visual abilities within the blind field of one hemianopic patient. In two different experiments, we asked the patient to discriminate several features of visual stimuli briefly presented in her impaired visual field and to report visual awareness by means of binary yes—no response, Experiment 1 or graded four-level PAS, Experiment 2 measures.
In Experiment 1, patient SL demonstrated type 1 blindsight above-chance accuracy without acknowledged awareness for orientation, color, contrast, and real motion discrimination.
However, when asked to rate her perceptual experience with the PAS, her blindsight disappeared. In Experiment 2, indeed, she performed with above-chance accuracy for contrast discrimination only when she reported to have seen the stimuli, thus showing conscious, although degraded, vision, instead of pure blindsight.
Interestingly, in accordance with the findings by Overgaard et al. When asked to assess her awareness of the stimuli by using a binary measure, she reported a mean percentage of acknowledged awareness in only This possibility is in accord with several findings present in literature showing that visual processing within the impaired visual field is different from normal vision Azzopardi and Cowey, ; Stoerig and Barth, ; Cowey, If conscious experience is characterized by different levels of clarity, a finer scale able to better characterize subtly different perceptual experiences is preferable as it might be more suitable to disentangle genuine forms of blindsight from degraded but conscious vision.
A final important point deserving some considerations relates to the possibility for patients with a complete lesion to V1 to experience conscious visual qualia. The presence of visual qualia in patients with a lesion to V1 has two main important implications.
First, the area of the visual field represented by the lesioned portion of V1 cannot be considered as totally blind Cowey, , as conscious experience is still possible, though of a different nature of normal vision.
Second, the fact that a complete lesion to V1 does not completely abolish conscious vision, at least the crude type of consciousness sufficient to report simple visual qualia Zeki and Ffytche, , implies that V1 is not necessary for conscious vision Ffytche and Zeki, To be tenable, however, V1 lesion needs to be complete Fendrich et al.
In this paper, we studied a patient whose lesion was well documented to be complete. Indeed, fMRI evidence Celeghin et al. The exact areas that might subserve conscious visual processing in absence of V1 is still unknown.
These results suggested that the strength and the complexity of the signal carrying subliminal visual stimulus affected the extent of unconscious processing. It will be of interest for future studies to address, using neuroimaging techniques, the exact cortical processing for subliminal visual information. How to cite this article : Song, C. Unconscious processing of invisible visual stimuli. Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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