When they see, they see it almost right: normal subjective experience of detected stimuli in spatial neglect

Unilateral spatial neglect (USN) patients show reduced contrast sensitivity on their contralesional side and often miss their non-salient stimuli. What their subjective experience is when successfully reporting a stimulus remains unclear. Here, we report that despite large contrast sensitivity differences between the sides, the relative attenuation in perceived contrast measured in a contrast-matching task was small. This was true even at threshold levels where the patients missed up to 40% of the contralesional target patches, in contrast to a 100% detection rate on their ipsilesional side. When the misses were counted as zero perceived contrast events, the attenuation in perceived contrast was less than half of the sensitivity loss. When the misses were ignored, there was almost no attenuation in perceived contrast, implying that whenever the patients detected a target, they perceived it with the correct contrast. These findings suggest that contrast sensitivity reduction in USN is not due to attenuation occurring at a peripheral low-level processing stage. More likely it reflects a high-threshold added at a higher level of processing, which prevents sensory events from reaching conscious awareness. Hence, patients may often miss contralesional stimuli but see them in full contrast once they clear the high-level hurdle.