The scholarly article that was written by Stoerig, Zontanou, and Cowey (2002) examines the issue of cortical blindness in four men and one monkey. More precisely, in the paper, Stoerig et al. (2002) “measure forced-choice localization of square-wave gratings as a function of contrast, and compare results from the patients’ absolutely and relatively blind fields” (p. 565). This study is innovative because its implications could be used by specialists to help such patients as “the very young or the severely aphasic” who cannot verbally tell whether they saw a stimulus or not (p. 565). The authors refer to the signal-detection paradigm to check the test subjects ability to determine a stimulus correctly. In addition to that, Stoerig et al. (2002) test two patients with the fields of relative blindness to learn whether stimuli reported by the participants of the experiment as barely visible are indeed targets or blank spaces.
In the text of the paper, Stoerig et al. (2002) do not outline the research question and the hypothesis. Nonetheless, from the introductory part of the article, it could be inferred that the scholars conducted experiments with the aim to answer whether there is a correspondence between verbal and non-verbal responses to a stimulus. The experiment that is described below gives a positive answer to the previously mentioned question. In other words, the researchers discovered correspondence between non-verbal and verbal responses to a stimulus.
The study was conducted with the participation of four people and one monkey. The experiment was held from 1999 to 2001 at the Institute of Experimental Psychology in Düsseldorf. The origin of the blindness of each participant is different. For example, the blindness of FS has “a retro-geniculate lesion of traumatic origin,” WF and HK have a vascular origin, and GY lost his sight at the age of 8 after a traffic accident (Stoerig et al., 2002, p. 566). Stoerig et al. (2002) use the monkey, Rosie, whose striate cortex on one side was entirely removed, as an example of someone who cannot verbally express whether he sees something or not.
The article describes the course of the experiment in detail. Still, the main implications are that the patients were sitting in front of the monitor, their heads were fixed, and they were asked to touch the monitor with the index finger whenever they believed they saw a signal. Rosie was fixed in the chair, and the CCTV monitored the movement of her eyes. For the experiment, each patient had a healthy eye covered, while the eye with a defect in the temporal hemifield was used.
First, the participants were asked to localize the stimulus grating at the position it was present, and in case of uncertainty, guess the correct position. Each answer was accompanied by feedback from the experimenters regarding the correctness or falsehood of the answer. After the participants reached the correct answers in 99% of cases, testing continued without feedback. The monkey received food as a reward for each correct answer during the experiment. Participants were then tested for their ability to detect when most of the stimuli appeared in the normal hemifield and less in the impaired field. At the same time, the participants were not anticipated about the possibility of the appearance of a stimulus in the impaired hemifield.
Localization testing has allowed researchers to reach a number of significant results. First and foremost, the experiment identified that awareness responses are not a predictor of successful stimulus localization. In particular, the researchers found no relationship between separate testing of high and low contrasts and the frequency of the awareness responses. However, when there is a gradual increase, in contrast, the number of awareness responses, as well as the performance of localization, significantly increases. In the analysis of the detection stimulus, all participants successfully coped with the task when a stimulus appeared in a normal field, while when a stimulus appeared in an impaired field, everyone signaled a lack of stimulus. Participants’ responses in the detection test depended on both contrast and the likelihood of a stimulus appearing in a particular field. While the human subjects performed roughly the same on both tests, Rosie did much worse on the detection task than on the localization task.
These video records enabled researchers to control the non-verbal response of the monkey to a signal on the screen. The scholars gathered approximately 200 responses from each member of the experiment. Afterwards, the software analyzed the reactions of four men and Rosie and compared them to the correct answers. For the analysis of human responses, the distribution of correct and false answers was assessed using the χ² test; standard errors were estimated using the binomial distribution. The scholars then employed the SPSS statistical package to calculate Pearson’s correlation coefficients and P-values.
The methods described above enabled Stoerig et al. (2002) to conclude that there is a “perfect correspondence between verbal and non-verbal aware responses in the absolute defects” (p. 571). This finding, in turn, proves that the aforementioned signal detection paradigm indeed reflects “complete loss of conscious vision that characterizes a field of absolute cortical blindness” (Stoerig et al., 2002, p. 571). Even though the authors raise several essential questions on the quality of Rosie’s blindness and the existence of conscious vision in Rosie’s normal hemifield, they still made a significant step towards detecting blindness in patients via non-verbal means.
Reference
Stoerig, P., Zontanou, A., & Cowey, A. (2002). Aware or unaware: Assessment of cortical blindness in four men and a monkey.Cerebral Cortex, 12(6), 565-574.