Lights from the Dark: Neural Responses from a Blind Visual Hemifield

Abstract

Here we present evidence that a hemianopic patient with a lesion of the left primary visual cortex (V1) showed an unconscious above-chance orientation discrimination with moving rather than static visual gratings presented to the blind hemifield. The patient did not report any perceptual experience of the stimulus features except for a feeling that something appeared in the blind hemifield. Interestingly, in the lesioned left hemisphere, following stimulus presentation to the blind hemifield, we found an event-related potential (ERP) N1 component at a post-stimulus onset latency of 180–260 ms and a source generator in the left BA 19. In contrast, we did not find evidence of the early visual components C1 and P1 and of the later component P300. A positive component (P2a) was recorded between 250 and 320 ms after stimulus onset frontally in both hemispheres. Finally, in the time range 320–440 ms there was a negative peak in right posterior electrodes that was present only for the moving condition. In sum, there were two noteworthy results: Behaviorally, we found evidence of above chance unconscious (blindsight) orientation discrimination with moving but not static stimuli. Physiologically, in contrast to previous studies, we found reliable ERP components elicited by stimuli presented to the blind hemifield at various electrode locations and latencies that are likely to index either the perceptual report of the patient (N1 and P2a) or, the above-chance unconscious performance with moving stimuli as is the case of the posterior ERP negative component. This late component can be considered as the neural correlate of a kind of blindsight enabling feature discrimination only when stimuli are moving and that is subserved by the intact right hemisphere through interhemispheric transfer.

On the “blindness” of blindsight: What is the evidence for phenomenal awareness in the absence of primary visual cortex (V1)?

“…Evidence of residual vision has also been reported with other stimulus features. For example, hemianopic patients, with or without blindsight, have been found to be aware of high contrast, low frequency, high displacement and to a lesser extent colour of stimuli presented in the blind field (Barbur et al, 1999;Bollini et al, 2017;Kentridge et al, 1999Kentridge et al, , 1997Kleiser et al, 2001;Mazzi et al, 2016;Sahraie et al, 2010Sahraie et al, , 1997Weiskrantz et al, 1995). However, as with GY, these seem to reflect blindsight Type 2 rather than phenomenal awareness (Barbur et al, 1999;Sahraie et al, 2010;Sahraie et al, 1997).…”

The Glamor of Old-Style Single-Case Studies in the Neuroimaging Era: Insights From a Patient With Hemianopia

“…Over the last few years, we have extensively tested one hemianopic patient, SL, using several methodologies, ranging from pure behavioral measures (Celeghin et al, 2014, 2015; Mazzi et al, 2016) to EEG/ERPs (Bollini et al, 2017; Sanchez-Lopez et al, 2017; Mazzi et al, 2018b), TMS (Mazzi et al, 2014), TMS-EEG co-registration (Bagattini et al, 2015), and, very recently, fast near-infrared optical signal (Mazzi et al, 2018a).…”

“…If V1 and feedback to it are necessary for awareness to emerge, as stated in Lamme's model, a complete lesion to V1 should prevent SL from having perceptual awareness in her blind field. In this respect, SL, tested with a broad variety of stimulus features (Mazzi et al, 2016; Bollini et al, 2017) reported some visual conscious experience of all kind of stimuli presented contralaterally to her lesion in a considerable number of trials (see Mazzi et al, 2017b for a review of older pieces of evidence on conscious experience within the “blind” field of hemianopic patients). Importantly, she could grade conscious visual experience within her “blind” field and the corresponding ERPs revealed differential neural activity with respect to when stimuli remained undetected (Bollini et al, 2017; Sanchez-Lopez et al, 2017; Mazzi et al, 2018b) 1 .…”

“…In this respect, SL, tested with a broad variety of stimulus features (Mazzi et al, 2016; Bollini et al, 2017) reported some visual conscious experience of all kind of stimuli presented contralaterally to her lesion in a considerable number of trials (see Mazzi et al, 2017b for a review of older pieces of evidence on conscious experience within the “blind” field of hemianopic patients). Importantly, she could grade conscious visual experience within her “blind” field and the corresponding ERPs revealed differential neural activity with respect to when stimuli remained undetected (Bollini et al, 2017; Sanchez-Lopez et al, 2017; Mazzi et al, 2018b) 1 . Moreover, her electrophysiological data were similar, both in latency and topography, to those observed with healthy participants using stimuli at detection threshold level (Tagliabue et al, 2016), thus suggesting a normal pattern of neural activity even in the absence of a functioning V1.…”

Functional interactions in patients with hemianopia: A graph theory-based connectivity study of resting fMRI signal

“…This phenomenon has been termed "blindsight" that is, the ability to detect, localize or discriminate visual stimuli without being aware of them [3]. Over the past 30 years, task-related brain activity [4] and electrophysiological signals [5] have been recorded during visual stimulus presentation to the blind hemifield of these patients with the aim of exploring the neural mechanisms of unconscious vision. In these experiments, patients are asked to give a response even if they cannot consciously perceive anything in the blind visual field.…”