The causal role of the intraparietal sulcus (IPS) in visual conscious experience. A TMS investigation.

It has recently been shown that TMS applied to visually responsive areas other than V1 can generate light sensations, called “phosphenes”, in absence of visual stimulation in the environment. In particular, contralateral phosphenes have been obtained by stimulating the intraparietal sulcus. However, a question that still remains open is whether parietal phosphenes are generated by the parietal cortex, independently from the contribution of occipital areas. Since the phosphene threshold represented the heart of my project, a preliminary experiment comparing the most common thresholding methods was carried out in order to assess which methods was the most reliable. We thus tested the “method of constant stimuli” (MOCS), the “Modify Binary Search” (MOBS) and the “Rapid Estimation of Phosphene Threshold” (REPT) in seventeen participants. Each of the three methods was repeated three times per participant in the same week. The data suggested using MOCS in the next experiments because, despite its length, it resulted to be consistent across days, with thresholds not changing depending on the number of administrations. On the other hand, MOBS appeared highly variable, while REPT tended to be less reliable than MOCS but still remaining unaffected by the number of administrations. The aim of the second experiment was to provide a characterization of parietal phosphenes to find possible differences in terms of phosphene threshold, eccentricity, size, vividness and brightness with those evoked by stimulation of the occipital cortex. Single-pulse magnetic stimulations were administered with a figure-of-eight coil, assisted by a neuronavigational system. Individual stimulation sites were functionally identified around P3 and O1 (10-20 system). To determine phosphene threshold the “method of constant stimuli” was used: randomly intermixed intensities were employed (ranging from 45% to 90%) and twenty stimulations were given for each output intensity. Subjects were also requested to draw the phosphenes as to obtain eccentricity and size and to rate vividness and brightness of evoked perceptions. Fourteen subjects participated in the study. Results showed that the threshold was reliably lower for occipital lobe (63.1% of maximum stimulator output, MSO) than for parietal lobe (72.4% MSO) TMS stimulation and that the two psychophysical curves had a significantly different shape, with correlating values only at the 50% threshold. In addition, brightness and eccentricity were modulated by the site of stimulation, respectively obtaining less bright phosphenes (and thus more difficult to perceive) following parietal stimulation and no changes in the location of parietal phosphenes by changing the intensity of stimulation, contrary to what happened for the occipital phosphenes. This was probably due to the coarser retinotopic organization of space within the parietal cortex. Given that the activation of V1 is generally thought to be involved in conscious perception of phosphenes, the possibility that V1 was implicated in parietal phosphenes could not be totally dismissed with the second experiment. Therefore, a hemianopic patient, who showed no residual activity in her left V1, was tested to further investigate the role of intraparietal sulcus in visual conscious experience. She crucially reported parietal phosphenes (broadly similar to those evoked in the intact hemisphere and in sighted individuals) when TMS was applied over the damaged hemisphere (P3), in absence of V1. Additionally, her threshold values were not very different from those of healthy participants. Finally, we sought to identify the cortical neural network activated while inducing phosphenes with parietal TMS. In particular, parietal phosphene perception was explored using a simultaneous combination of TMS and a fast optical imaging tool (event-related optical signal – EROS), which offer a good spatial resolution in conjunction with the ability to map out the time course of feed-forward and feedback processes. The phosphene threshold value was employed to compare phosphene awareness to no phosphene awareness under identical stimulation parameters and to investigate which regions critically determined the parietal phosphene perception. Preliminary data from five participants seemed to reveal different activation patterns depending on the presence/absence of phosphenes but, since a lot of negative activations were highlighted, these data were not easy to be interpreted, as often happens with fMRI data. Therefore, the current sample needs to be enlarged and more data to be collected before any serious inferences can be drawn. In sum, these findings, along with previous evidence, corroborated the hypothesis that the parietal lobe is able to generate conscious visual experience, independently from early visual areas, and that parietal phosphenes may have a different neural basis from those elicited in occipital lobe. Such a result, therefore, goes against theories suggesting that primary visual area is necessary for awareness (Tong, 2003).